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Additive Manufacturing: A Comprehensive Review[

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Longfei Zhou

Longfei Zhou

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Jenna Miller

Jenna Miller

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Jeremiah Vezza

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Maksim Mayster

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Abstract and Figures

Additive manufacturing has revolutionized manufacturing across a spectrum of industries by enabling the production of complex geometries with unparalleled customization and reduced waste. Beginning as a rapid prototyping tool, additive manufacturing has matured into a comprehensive manufacturing solution, embracing a wide range of materials, such as polymers, metals, ceramics, and composites. This paper delves into the workflow of additive manufacturing, encompassing design, modeling, slicing, printing, and post-processing. Various additive manufacturing technologies are explored, including material extrusion, VAT polymerization, material jetting, binder jetting, selective laser sintering, selective laser melting, direct metal laser sintering, electron beam melting, multi-jet fusion, direct energy deposition, carbon fiber reinforced, laminated object manufacturing, and more, discussing their principles, advantages, disadvantages, material compatibilities, applications, and developing trends. Additionally, the future of additive manufacturing is projected, highlighting potential advancements in 3D bioprinting, 3D food printing, large-scale 3D printing, 4D printing, and AI-based additive manufacturing. This comprehensive survey aims to underscore the transformative impact of additive manufacturing on global manufacturing, emphasizing ongoing challenges and the promising horizon of innovations that could further elevate its role in the manufacturing revolution.

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Available via license: CC BY 4.0

Citation: Zhou, L.; Miller, J.; Vezza, J.;

Mayster, M.; Raffay, M.; Justice, Q.;

Al Tamimi, Z.; Hansotte, G.; Sunkara,

L.D.; Bernat, J. Additive

Manufacturing: A Comprehensive

Review. Sensors 2024,24, 2668.

https://doi.org/10.3390/s24092668

Academic Editor: Shah Nawaz

Burokur

Received: 1 March 2024

Revised: 19 April 2024

Accepted: 20 April 2024

Published: 23 April 2024

Licensee MDPI, Basel, Switzerland.

This article is an open access article

conditions of the Creative Commons

Attribution (CC BY) license (https://

creativecommons.org/licenses/by/

4.0/).

sensors

Review

Additive Manufacturing: A Comprehensive Review

Longfei Zhou *, Jenna Miller, Jeremiah Vezza, Maksim Mayster, Muhammad Raffay, Quentin Justice,

Zainab Al Tamimi, Gavyn Hansotte, Lavanya Devi Sunkara and Jessica Bernat

Department of Biomedical, Industrial and Systems Engineering, School of Engineering and Computing,

College of Engineering and Business, Gannon University, Erie, PA 16541, USA

*Correspondence: zhou009@gannon.edu; Tel.: +1-814-871-5491

Abstract: Additive manufacturing has revolutionized manufacturing across a spectrum of industries

by enabling the production of complex geometries with unparalleled customization and reduced

waste. Beginning as a rapid prototyping tool, additive manufacturing has matured into a compre-

hensive manufacturing solution, embracing a wide range of materials, such as polymers, metals,

ceramics, and composites. This paper delves into the workflow of additive manufacturing, encom-

passing design, modeling, slicing, printing, and post-processing. Various additive manufacturing

technologies are explored, including material extrusion, VAT polymerization, material jetting, binder

jetting, selective laser sintering, selective laser melting, direct metal laser sintering, electron beam

melting, multi-jet fusion, direct energy deposition, carbon fiber reinforced, laminated object manufac-

turing, and more, discussing their principles, advantages, disadvantages, material compatibilities,

applications, and developing trends. Additionally, the future of additive manufacturing is projected,

highlighting potential advancements in 3D bioprinting, 3D food printing, large-scale 3D printing,

4D printing, and AI-based additive manufacturing. This comprehensive survey aims to underscore

the transformative impact of additive manufacturing on global manufacturing, emphasizing ongo-

ing challenges and the promising horizon of innovations that could further elevate its role in the

manufacturing revolution.

Keywords: additive manufacturing; 3D printing; material extrusion; fused deposition modeling; VAT

polymerization; binder jetting; material jetting; power bed fusion; CAD; review

1. Introduction

Additive manufacturing (AM), widely recognized as 3D printing, has undergone a

significant evolution since its inception in the 1980s, transitioning from a rapid prototyping

tool to a viable manufacturing method for a broad spectrum of applications. The tech-

nology’s advancement is marked by innovations in materials, technology, and software,

propelling AM into industries such as aerospace, automotive, healthcare, and fashion [

1

].

Today, 3D printing is celebrated for its unparalleled customization capabilities, reduced

waste, and the ability to fabricate complex geometries that are difficult or impossible to

achieve with traditional manufacturing methods [

2

]. This evolution has transformed AM

into a sophisticated array of processes that cater to a diverse range of materials, including

polymers, metals, ceramics, and composites, driving innovations across numerous sectors.

As AM continues to advance, it is set to revolutionize production processes, supply chains,

and product design worldwide, marking its significance as a transformative technology in

the manufacturing realm and outlining a future where on-demand customized production

can become a reality across industries.

AM distinguishes itself from traditional formative and subtractive methods through

its capacity for complex geometries and customization with minimal waste, making it ideal

for prototypes and small production runs. Figure 1shows the comparison of cost per part

over different numbers of parts between AM and traditional manufacturing methods [

3

].

Sensors 2024,24, 2668. https://doi.org/10.3390/s24092668 https://www.mdpi.com/journal/sensors

Sensors 2024,24, 2668 2 of 44

However, AM faces limitations in material variety, part size, and requires significant post-

processing, potentially leading to anisotropic mechanical properties. In contrast, traditional

manufacturing excels in mass production efficiency, offering superior mechanical strength,

isotropy, and surface finish due to a wider range of materials and established processes.

While AM can reduce initial costs by eliminating the need for molds or tooling, its cost

advantages diminish with scale, unlike traditional methods that benefit from economies

of scale. Thus, the choice between AM and traditional methods hinges on the project’s

specific demands regarding complexity, volume, and cost efficiency.

Figure 1. Cost comparison between formative, subtractive, and additive manufacturing [3].

deposition modeling (FDM), developed by Stratasys in the late 1980s, is one of the most

widely used AM technologies. It works by extruding thermoplastic filaments through a

heated nozzle, laying down material layer by layer to construct an object. Recent advance-

ments have focused on enhancing the mechanical properties of FDM-printed parts through

the incorporation of reinforced composite filaments and optimizing process parameters

for better surface finish and dimensional accuracy [

4

]. Stereolithography (SLA), patented

by 3D Systems in 1986, was the first commercial AM process. It uses an ultraviolet laser

to cure photosensitive resins in a layer-by-layer fashion. Technological advancements

in SLA have been directed towards improving the speed and resolution of the printing

process, enabling the production of parts with high detail and smooth surface finishes [

5

].

Digital light processing (DLP) technology, similar to SLA, uses a digital light projector to

cure photosensitive resins. It stands out for its high printing speed and ability to produce

parts with fine details. Advances in DLP technology include the development of new

resin formulations that expand the mechanical and thermal properties of printed objects,

enabling their use in more demanding applications [1]. Selective laser sintering (SLS) and

direct metal laser sintering (DMLS) are powder bed fusion technologies that use lasers to

sinter powdered materials, binding them together to form a solid structure. These technolo-

gies have seen significant development in the variety of materials that can be processed,

including metals, polymers, and ceramics, allowing for the production of functional parts

with complex geometries. Recent research has focused on optimizing process parameters

and post-processing techniques to improve the mechanical properties and surface quality

of printed parts [

6

]. Material jetting works similarly to inkjet printing, where droplets of

material are selectively deposited and cured. Binder jetting involves depositing a liquid

binding agent onto layers of powder material. Both technologies have advanced in terms

of material diversity and printing resolution. Recent innovations aim at expanding the

range of applications by developing new materials with enhanced properties and reducing

production costs [7].

Sensors 2024,24, 2668 3 of 44

The future of AM is poised for growth along several vectors, including the develop-

ment of new materials, process innovations, and the integration of AM technologies into

traditional manufacturing lines. Significant research is focused on scalable AM techniques

for large structures, bioprinting for medical applications, and the integration of artificial

intelligence to optimize printing processes and material properties [

8

]. AM continues

to push the boundaries of what is possible in manufacturing, driving towards a future

where customized production on-demand can become a reality across industries. As the

properties, its role in the next manufacturing revolution becomes increasingly significant.

Despite the remarkable achievements of 3D printing technology, encompassing a wide

range of industries from healthcare to aerospace with its innovative applications, it contin-

ues to face significant limitations that highlight its future development potential. One of the

primary challenges is the speed of printing, which, despite improvements, remains slow for

mass production applications, limiting its utility to prototyping and small-scale manufactur-

ing [

9

]. Material limitations also pose a significant barrier. The range of materials suitable

for 3D printing is expanding, but still lacks the diversity and performance characteristics

of those used in traditional manufacturing processes. Furthermore, the resolution and

surface finish of printed objects often require extensive post-processing to meet industry

standards, adding time and cost to the manufacturing process. Additionally, the cost of 3D

printers and materials can be prohibitively high for widespread adoption, particularly in

developing countries. There are also challenges related to intellectual property rights and

10

]. Look-

ing forward, the evolution of 3D printing technology promises advancements in printing

speed, material diversity, and cost efficiency. Innovations such as and enhanced precision

are on the horizon, aiming to overcome current limitations and expand the technology’s

applicability across more sectors, paving the way for a future where 3D printing plays a

pivotal role in global manufacturing and beyond.

In this paper, the main workflow of AM is first discussed, including design and

modeling, conversion to STL, slicing, 3D printing, and post-processing. Then, different

types of AM technologies are introduced, including material extrusion, VAT polymerization,

material jetting, binder jetting, selective laser sintering, selective laser melting, direct

metal laser sintering, electron beam melting, multi-jet fusion, direct energy deposition,

carbon fiber reinforced, laminated object manufacturing, etc. The principle of operation,

advantages and disadvantages, materials, main applications, and developing trends of

each type of AM technology are discussed in detail. Finally, the challenges, development

trends, and potential innovations of AM are analyzed, including important topics such

as 3D bioprinting, 3D food printing, large-scale 3D printing, 4D printing, and AI-based

AM technologies, as well as new materials, quality control, post-processing innovations,

standardization, regulatory, circular economy, and sustainability.

2. Additive Manufacturing Process

Additive manufacturing (AM), commonly known as 3D printing, has revolutionized

the way objects are created, from simple models to complex structures used in various

industries. The process involves adding material layer by layer to create objects from

3D model data, contrary to traditional subtractive manufacturing methods. The overall

pipeline of AM encompasses several critical steps, each contributing to the final product’s

accuracy, quality, and functionality. Each step in the AM pipeline is vital for ensuring

that the final product meets the desired specifications and quality standards. Advances

in software, materials, and printing technologies continue to expand the possibilities of

what can be achieved with 3D printing, making it an increasingly integral part of modern

manufacturing processes Figure 2. The overall workflow of the additive manufacturing

process is given below.

Sensors 2024,24, 2668 4 of 44

3D Model STL File G-code Print

Design &

Modeling Convert to STL Slicing

3D Printing

& Post-processing

Figure 2. Overall workflow of the additive manufacturing process.

2.1. Design and Modeling

The design phase marks the beginning of the AM process, where the creation of a 3D

model is pivotal. Utilizing Computer-Aided Design (CAD) software tools, designers and

engineers can craft detailed and complex digital representations of the desired object. This

stage is characterized by meticulous planning of each aspect of the model, ensuring that

every curve, edge, and dimension aligns perfectly with the final product’s requirements.

A number of critical factors influence the design process.

Design Complexity and Material Considerations: CAD software facilitates the creation

of intricate designs that are often impossible to achieve through traditional manufacturing

methods. This complexity enables the exploration of new design paradigms and function-

alities. Concurrently, the choice of printing material plays a significant role, as different

materials offer varying properties, such as durability, flexibility, and thermal resistance.

These properties must be aligned with the object’s intended application, dictating specific

design adjustments to cater to the chosen material’s strengths and weaknesses.

Technology Limitations and Object Orientation: The capabilities and limitations of

the selected 3D printing technology significantly impact the design process. Technologies

such as fused deposition modeling (FDM), stereolithography (SLA), and selective laser

sintering (SLS) each have unique specifications regarding resolution, accuracy, and the

ability to create complex structures. Moreover, the size of the print bed limits the max-

imum dimensions of the object, potentially necessitating the division of larger objects

into smaller, assembleable parts. The orientation of the model during printing also re-

quires careful consideration to optimize the print’s quality, strength, and the necessity for

support structures.

Optimization Strategies: Design optimization for AM can take several forms, including

topology optimization for material efficiency and structural integrity, and strategic orienta-

tion to reduce the need for support structures and improve surface finish. Techniques such

as hollowing are employed to minimize material use and print time without compromising

the object’s durability.

The design stage, therefore, is not just about creativity but also about a deep under-

standing of the chosen material, printing technology, and the practical applications of the

final product.

2.2. Conversion to STL

Once the 3D model is complete, it is exported as an STL file. The conversion of 3D

models into stereolithography (STL) files represents a crucial step in the pipeline of 3D

printing, serving as the bridge between digital design and physical realization. This process

involves transforming a 3D design into a format that 3D printers can interpret and use to

build objects layer by layer.

The STL format simplifies the model by breaking down its surface into a series of

triangles, also known as tessellation. Each triangle is described by the coordinates of its

Sensors 2024,24, 2668 5 of 44

vertices and the direction of its normal (an outward-facing vector perpendicular to the

triangle’s surface). This simplification is crucial because it translates complex geometrical

shapes into a uniform language of triangles that 3D printers can understand.

STL files contain many important characteristics such as resolution and manifoldness.

The resolution of an STL file depends on the size of the triangles. Smaller triangles result in

higher resolution, producing more detailed prints, but at the cost of increasing the file size.

Besides, the STL model must also be “watertight”, meaning it should not have any gaps or

holes, and all normals should point outward. Non-manifold models can lead to printing

errors. After exporting to STL, the file undergoes a slicing process, where specialized

software further translates the STL file into G-code. Before slicing, users may adjust the

model’s orientation, scale, and other parameters to optimize printing.

There are some potential issues in the STL files. STL files can sometimes contain

errors such as reversed normals or non-manifold edges. Many slicing programs include

repair functions to automatically fix these issues. To reduce printing time and material

use, designers might optimize their models for the STL format by minimizing overhangs

and supports.

Converting 3D models into STL files is a fundamental step that transforms intri-

cate digital designs into a language that 3D printers can effectively interpret, setting the

stage for the AM process. This conversion encapsulates the transition from conceptual

design to physical object, embodying the core of what makes 3D printing a revolutionary

manufacturing technique.

2.3. Slicing

Once the STL file is generated, the next essential step is slicing, which is a pivotal

process where specialized software, often referred to as a slicer, transforms the STL file into

a series of thin, horizontal layers. This conversion is critical because it breaks down the

complex geometry of the model into a stack of manageable, printable layers, effectively

translating the digital design into a physical object. The slicing software plays a multifaceted

role in the 3D printing process.

Generation of G-code: The primary output of the slicing process is the generation of

G-code, a universally recognized programming language for Computer Numerical Control

(CNC) machinery, including 3D printers. G-code is meticulous and detailed, containing

precise instructions that direct the printer’s movements, such as the path it should follow,

the speed of the extrusion head or laser, and where to start and stop extruding material.

This code is what makes it possible for the 3D printer to materialize the digital model into

a tangible form, layer by layer.

Customization of Print Parameters: Slicing software offers users the ability to cus-

tomize various print parameters, which significantly impact the quality, strength, and ap-

pearance of the final print. These parameters include the layer height, which determines

the print’s resolution; the fill density, affecting the object’s solidity and weight; and the

necessity for support structures, which are crucial for printing overhangs and undercuts

without deformation. Adjusting the print speed can also influence the print’s quality,

with slower speeds generally resulting in higher-quality finishes.

Optimization for Material and Printer: Different materials and printers have unique

capabilities and limitations. The slicing stage allows for optimization tailored to the specific

characteristics of the material (such as temperature sensitivity, shrinkage, and required

post-processing) and the printer’s specifications (such as precision, maximum print size,

and compatible materials). This optimization ensures the final print meets the desired

criteria for functionality and aesthetics.

Simulation and Troubleshooting: Many slicing programs provide simulation tools

that preview the printing process, offering valuable insights into potential issues, such as

insufficient supports, overly thin walls, or areas prone to warping. This preview enables

users to make informed adjustments before committing to the actual print, saving time,

materials, and effort by reducing the likelihood of print failures.

Sensors 2024,24, 2668 6 of 44

After adjusting these parameters and settings, the slicer recompiles the model into

an optimized G-code file, ready to be sent to the printer. This step marks the transition

from a digital design to a set of actionable instructions tailored to the unique characteristics

of the 3D printer and the material selected. The slicing process, therefore, is not merely

a conversion but a critical phase of preparation that bridges digital modeling with the

physical act of printing, ensuring that the envisioned design materializes accurately and

efficiently in the real world.

2.4. Preparation, Printing, and Post-Processing

With the G-code prepared, the stage is set for the 3D printer to initiate the manufac-

turing process. Preparing a 3D printer for operation involves a series of essential steps to

ensure optimal performance and print quality. Initially, the material installation is carried

out, which varies depending on the type of printer; FDM printers use a filament that must

be loaded into the extruder, while SLA printers require the pouring of liquid resin into the

resin tank. Following this, leveling the print bed is crucial to guarantee the proper adhesion

of the first layer, a step that may involve manual adjustments or rely on auto-leveling fea-

tures depending on the printer model. The final preparatory step is pre-heating the printer

to the appropriate temperatures for both the print bed and the extruder in the case of FDM

printers, ensuring materials are at the optimal temperature for printing. This pre-heating

process aids in material adhesion and reduces issues such as warping, thus contributing

to the overall quality of the print. These preparatory steps, from material installation to

pre-heating, are critical for achieving successful printing outcomes and maintaining the

printer’s longevity and safety.

As the printing begins, the printer’s nozzle, or in some technologies, the laser or

projector, precisely follows the paths dictated by the G-code. This path guides the deposition

of material, constructing the object in a successive layering approach. The choice of material

is crucial and is selected based on the object’s intended use, ranging from thermoplastics,

photopolymers, and metals to more specialized materials, such as ceramics and composite

filaments. Each layer solidifies upon deposition or through subsequent curing, gradually

forming the final object with a high degree of accuracy and complexity. More details of

different 3D printing technologies are discussed in Section 3.

The completion of the printing phase marks the beginning of another essential phase:

post-processing. This stage is vital for enhancing the physical properties and aesthetic quali-

ties of the printed object. Common post-processing methods include the removal of support

structures, which are often necessary to prevent the collapse of overhanging features during

printing. Surface finishing techniques, such as sanding, polishing, or chemical bathing,

are applied to improve smoothness and appearance. For resin-based prints, curing under

UV light is a critical step to achieve full strength and stability. Additionally, objects may

undergo secondary processes, such as painting, coating, or metal plating, to meet specific

functional or aesthetic requirements. The necessity and complexity of post-processing vary

significantly with the printing technology used and the end-use of the object. For instance,

metal prints might require stress-relief heat treatments and machining to achieve precise

tolerances, while plastic prints might need minimal finishing. Despite the advancements in

3D printing technologies, post-processing remains an indispensable step in many cases,

affecting the overall time and cost of production. The ongoing development in AM aims

to reduce the dependency on extensive post-processing, striving for a future where prints

emerge from the printer ready for immediate use, further streamlining the manufacturing

process and expanding the applicability of 3D printing across industries.

3. Additive Manufacturing Technologies

In this section, different types of AM technologies are discussed, including their

principle of operation, advantages and limitations, common materials, main applications,

and development trends. Table 1compares the advantages, limitations, and common

Sensors 2024,24, 2668 7 of 44

materials between these different types of AM technologies. More details are discussed one

by one later in this section.

Table 1. AM technology comparison.

AM Tech. Advantages Limitations Common Materials

FDM

Low price

Speed efficiency

Low maintenance

Requires high temperatures

Requires supports

Inability for certain geometries

PLA

ABS

PETG

VP Exceptional details and surface

Ideal for intricate features

Limited build volume

Shrinkage and warping

Toxicity and environmental concerns

Photocurable resins

Waxes

Ceramics

MJ

Exceptional resolution and details

Multi-material and full-color

Wide array of materials

High costs

Restricted build volume

Low printing speed

Photopolymers

Thermoplastic polymers

Metal powders

BJ Full-color

Various materials

Low-density parts

Labor-intensive

Complex post-processing

PVP

PVA

PAA

SLS

Support free

Isotropic final products

Complex geometries

Porosity, shrinkage, and impurities

Poor surface quality

Post-processing for final appearance

Plastics

Composites

Ceramics

SLM

Binder-free

Often faster than SLS

High powder recyclability

High costs

Less material flexibility

Support structures and inert gas

Titanium alloys

Stainless steels

Aluminum alloys

DMLS

Support-free

Various materials of metal alloy

High power recyclability

High costs

High porosity

Limited build volume

Stainless steels

Aluminum

Titanium

EBM

Excellent material properties

Processing reactive metals

Fast and efficient process

High equipment costs

Additional post-processing

Limited selection of materials

Titanium alloys

Nickel-based superalloys

Cobalt–chrome alloys

MJF

High speed and efficiency

Less post-processing

Multiple colors

High ductility

Material limitations

Specific use cases

Polyamides

Thermoplastic Polyurethanes

Polypropylene

DED

Minimizes waste

Part repair and modification

Efficiency for larger components

Post-processing for a smooth finish

Low precision

Stainless steel

Titanium alloys

Nickel-based alloys

CFR Strength and lightness

Customizable fiber orientation High costs

Limited material compatibility

Nylon + Carbon fiber

PEEK + Carbon fiber

ABS + Carbon fiber

LOM

Cost-effectiveness and high speed

Environmental friendliness

Large part creation

Low dimensional accuracy and strength

Post-processing for surface finish

Limited material range for composites

Paper

Metal foils

Plastics

Note: A list of abbreviations is in the Abbreviations section at the end of the manuscript.

3.1. Material Extrusion

Three-dimensional printing has become a more common way to produce materials

at a reduced cost, time, and expense. One common type is Material Extrusion (ME), also

known as fused deposition modeling (FDM). The main premise of FDM printers involves

the creation of parts by extruding thermoplastics layer by layer as opposed to subtractive

manufacturing [11].

Sensors 2024,24, 2668 8 of 44

3.1.1. Principle of Operation of FDM

The production of each printer, in this case, FDM, is unique. Prior to each print, print

orientation and the raster design angle must be selected to decrease the strain on the created

object [

12

]. PLA is the typical material deposited to create each object. The structure of

typical FDM printers is shown in Figure 3. As printing begins, two different materials are

extruded through the extrusion head, i.e., one to create the desired part and the second to

create supports for the part. The thermoplastics are heated through the extrusion heads at

a temperature between 190 and 230 degrees Celsius and are then printed in strands layer

by layer. As they are heated, they turn to liquid but are immediately cooled after being

printed and fuse with the layer beneath them [

13

]. As the target part is made, the support

material and part material are alternated between per layer. The initial layer begins on

the base plate and is then built upon as the material deposited onto each layer [

14

]. Since

the build is three-dimensional, the x, y, and z dimensions each have a motor to control

movement on each axis. This leads to a bottom-up approach in FDM. The completed part

can be removed from the printer by holding the part and twisting the tray. This will release

the part and the supports can be broken or cut off.

Figure 3. Schematic diagram of FDM 3D printing technology [

15

]. The picture on the left shows the

whole structure of typical FDM printers where parts (1) to (7) are x-axis motor, z-axis motor, y-axis

motor, hot nozzle, printing bad, controller display board, and filaments, respectively. The picture on

the right shows more details of the printing nozzle where (7A) is the feed filament, and (4A), (4B),

and (4C) are heating wires, Hotend, and extruded materials, respectively.

3.1.2. Advantages and Limitations of FDM

The main advantages of FDM printing are the low price, their ability to be reproduced,

their speed efficiency, and low maintenance [

16

]. However, there are also many different

limitations facing FDM printers. This type of printer typically requires a higher temperature

and supports for angles greater than 45 degrees [

17

]. These supports lead to materials being

wasted and an increased production time. One of the biggest limitations of FDM is its

inability to produce a part regardless of the geometry. These limitations will be discussed

in a later section of this review.

3.1.3. Materials of FDM

FDM primarily employs thermoplastics for their advantageous properties of being

easily heated and reshaped, catering to a wide range of applications. Among the materials

commonly utilized in FDM, polylactic acid (PLA) stands out for its unique combination of

Sensors 2024,24, 2668 9 of 44

being a thermoplastic that is also biodegradable, making it an environmentally friendly

option. This characteristic, coupled with its safety for use in sensitive environments

such as hospitals and food packaging, underscores its popularity in FDM applications.

Besides PLA, FDM technology often uses materials such as acrylonitrile butadiene styrene

(ABS), polyethylene terephthalate glycol (PETG), and various polyolefins [

18

]. These

materials are chosen for their robustness and flexibility, although they initially present

more significant mechanical constraints compared to PLA. These limitations can affect the

durability and functional applications of printed objects, necessitating careful consideration

in material selection based on the specific requirements of the intended application.

High-performance thermoplastics such as polyether ether ketone (PEEK) and polyether-

imide (PEI) are valued for their exceptional mechanical properties and resistance to high

temperatures. These materials are essential in applications demanding durability, chemical

resistance, and thermal stability, such as in aerospace, automotive, and medical industries.

However, their use in FDM requires specialized printing equipment capable of achiev-

ing and maintaining high extrusion temperatures—typically above 350 °C for PEEK and

around 340 °C to 360 °C for Ultem. Additionally, a heated print bed and chamber are

crucial to ensure proper adhesion and minimize warping, as these materials are prone to

contraction upon cooling. The high processing temperatures also necessitate advanced

cooling systems and thermal management strategies to prevent material degradation and

ensure dimensional accuracy.

3.1.4. Applications of FDM

FDM technology finds application across a broad spectrum of industries, significantly

impacting fields from healthcare to automotive and even consumer goods. Within the med-

ical sector, FDM plays a crucial role in enhancing surgical procedures and medical training

by facilitating the creation of precise anatomical models. These models are invaluable

for surgical preparation and educational purposes, offering a high degree of anatomical

accuracy [

19

]. Furthermore, FDM’s versatility allows for its use in developing specialized

medical devices, such as dialysis catheters, innovative drug delivery systems, and scaf-

folds for tissue engineering, which are pivotal in regenerative medicine practices [

20

–

22

].

In the automotive industry, FDM’s contribution extends to the production of durable and

lightweight components such as window car holders, showcasing the technology’s ability

to produce parts with complex geometries and tailored mechanical properties at a reduced

cost and turnaround time [23]. Beyond professional applications, FDM’s accessibility and

affordability have made it a favorite among hobbyists and educators. It serves as a tool

for recreational projects, educational models, and prototype development, underlining its

versatility and the broad appeal of 3D printing technologies. This wide-ranging utility of

FDM underscores its transformative potential across various sectors, from revolutionizing

medical practices to enhancing manufacturing processes and promoting innovation at the

consumer level.

3.1.5. Developing Trends of FDM

As AM becomes more common, advancements are being made. Specifically, FDM

has begun to implement five-axis 3D printers. This reduces overhanging structures and

supports needed, which overall reduces the material and cost for each print. The two

additional axes allow for a rotation during printing. This ensures a more detailed print

with less required support [

24

]. Since AM is increasing in popularity advancements are

needed to keep costs low and increase efficiency. Fused deposition modeling is a reliable

method with a variety of uses and is a feasible solution to the production of a variety of

products. Although there are many types of 3D printing, FDM should not be overlooked.

The innovation of filament core reinforcement with glass, carbon, or basalt fibers

in FDM significantly enhances the mechanical properties of printed parts. These fibers,

embedded within the core of thermoplastic filaments, provide improved strength, stiff-

ness, and thermal stability compared to standard thermoplastics. Carbon fiber-reinforced

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filaments, for instance, offer superior strength-to-weight ratios and dimensional stability,

making them ideal for aerospace, automotive, and industrial applications. Glass fiber rein-

forcement improves impact resistance and tensile strength, whereas basalt fibers contribute

to excellent thermal and chemical resistance.

Simultaneously, nanotechnology trends in FDM are pushing the boundaries of material

performance further. The incorporation of nanoparticles into filaments, such as carbon

nanotubes or graphene, enhances electrical conductivity, heat dissipation, and mechanical

properties at the nanoscale, opening new avenues for functional and structural applications.

These nanocomposites can be tailored for specific uses, ranging from wearable electronics

with conductive properties to components requiring enhanced thermal management or

mechanical performance.

Both fiber reinforcement and nanotechnology trends in FDM represent a leap towards

manufacturing more durable, functional, and customized 3D printed products, bridging

the gap between prototyping and end-use manufacturing.

3.2. VAT Polymerization

Generally, the VAT polymerization (VP) technique uses a light source to cure a photopoly-

mer resin contained within a reservoir [

25

]. There are three types of VP—stereolithography,

digital light processing, and continuous digital light processing.

3.2.1. Principal of Operation of VPs

Of these three forms of VP, stereolithography (SLA) is distinct from the two similar

techniques of regular DLP and continuous DLP (cDLP). Therefore, SLA will be discussed

first, followed by both DLPs. Stereolithography utilizes a laser to cure a photopolymer resin

onto a build plate in the desired geometry. The laser traces the resin within the reservoir

as required per the guidance of a mirror. SLA printers are either top-down or bottom-up

oriented. For top-down printers, the build plate starts at the top of the reservoir, the laser

cures the layer as specified by the geometry code, and then the build plate moves down

corresponding to one increment of layer thickness; thus, each successive layer is above the

previous. On the other hand, the build plate of bottom-up printers starts at the bottom

of the resin reservoir. Between each curing, the build plate moves up one thickness layer,

resulting in each successive layer being below the prior [

26

]. Parameters that influence

which printer orientation is used to print an object includes product size and complexity,

time, reliability, etc. Digital light processing is similar to SLA in that a photopolymer resin

is being cured; however, this method utilizes a digital light projector for curing. This digital

light projector cures the complete layer simultaneously rather than tracing. Because the

curer is a digital source, the print will exhibit square-like subunits composing each layer;

these are square light pixels called “voxels” [

27

]. Despite these differences, DLP printers can

also be oriented top-down or bottom-up with the same operation. The distinction between

DLP and cDLP is the movement of the build plate along the z-axis. For DLP, the build plate

moves one layer thickness per cure, while the cDLP build plate continuously moves along

the z-axis to accommodate continuous curing, which results in shorter build times [

28

].

Figure 4shows the schematic of the SLA 3D printing process and a print sample. Note that

a schematic for cDLP is not included because the difference in build plate motion is not

visible in an image. Furthermore, two images are provided for SLA to display top-down

versus bottom-up printer orientation, but this is applicable to DLP as well. Figure 5shows

the different lighting technologies between SLA and DLP. The can be seen that a laser cures

the resin by tracing a path in SLA while a voxel grid is illuminated to cure resin a full layer

at a time.

Sensors 2024,24, 2668 11 of 44

Figure 4. Schematic of the SLA 3D printing process and a print sample [29].

Figure 5. Different lighting technologies between SLA and DLP [29].

3.2.2. Advantages and Limitations of VP

VAT polymerization, which encompasses technologies such as SLA and DLP, offers

several significant advantages in the realm of AM. One of its primary benefits is the ability to

produce parts with exceptional detail and surface finish, surpassing most other 3D printing

technologies. This precision makes it an ideal choice for applications requiring intricate

features, such as jewelry, dental applications, and highly detailed prototypes. Moreover,

the variety of resins available for VAT polymerization technologies enables the creation of

parts with specific mechanical, thermal, and optical properties. This versatility supports

a wide range of applications, from functional prototypes to end-use parts in various

industries. The technology also allows for relatively fast printing speeds, especially in the

case of DLP, which can cure entire layers simultaneously [

30

]. Additionally, the support

structures required for overhanging parts are generally less extensive than those needed

for extrusion-based processes, simplifying post-processing steps.

Despite its advantages, VAT polymerization also has several limitations that can affect

its applicability for certain projects. One of the primary drawbacks is the limited build

volume, which can restrict the size of parts that can be produced. This makes it less

suitable for applications requiring large components or high-volume production runs.

Additionally, the materials used in VAT polymerization, typically photopolymer resins,

can be more expensive than those used in other forms of 3D printing, such as filament for

FDM. The resins can also be brittle and may not possess the same strength or durability

as materials used in other manufacturing processes, limiting their use in functional parts

that require high mechanical strength [

31

]. Furthermore, the parts produced by VAT

polymerization may suffer from shrinkage and warping due to the curing process, which

can affect dimensional accuracy and require additional post-processing to achieve the

desired tolerances. Lastly, working with photopolymer resins requires careful handling

due to their potential toxicity and the need for proper disposal, posing environmental and

safety concerns.

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3.2.3. Materials of VP

Most commonly, VAT polymerization is used with a photocurable resin. These resins

can range from standard resins with average mechanical properties for general prototyping,

to structural, tough and durable, or elastic resins depending on usage or application. Due

to the variability of polymers, resins come in many color options, as opposed to other

printing materials, such as metals. More advanced materials that can also be used include

ceramics or waxes, biocompatible resins, and bioinks; these are typically seen in biomedical

applications [

32

]. Generally, as material quality/complexity increases, so does processing

time and costs (both raw materials and equipment).

3.2.4. Applications of VP

Due to the versatility of 3D printing, there are endless applications in which VAT

polymerization can be applied. Both SLA and DLP are utilized over a wide range of

fields including manufacturing, research development, and medicine, to name a few.

In research and development, these methods can quickly create products to test form, fit,

and function or bring vision to a project through prototyping without a lot of time or

financial investment [

33

]. Minimal time and financial obligations are also an incentive for

manufacturers, particularly in areas of investment castings for metallurgy, because complex,

dimensionally accurate molds and cores can be developed, which were otherwise made less

accurately through hand carvings or FDM [

34

]. The revolution of 3D printing and market

demand has also made utilizing this equipment for personal entertainment possible.

3.2.5. Developing Trends of VP

One of the greatest selling points to a solution in healthcare is its ability to impact a

large range of patients. Because each human is unique, the more customizable a solution,

the greater chances for success for a given patient. This makes VAT polymerization a great

method for creating medical products. Recently, VAT (more specifically SLA) has been

under development for its use and effectiveness with pharmaceuticals. Due to its high

accuracy, resolution, speed, and materials, this method is thought to be up and coming for

development of micro-scale, yet high concentration drug delivery systems in areas such as

pain or disease management [35].

Besides, the biocompatibility of materials used in VP is a critical consideration, espe-

cially for applications in the medical and dental fields, where direct or indirect contact with

the human body is frequent. Biocompatible photopolymer resins have been developed to

meet stringent regulatory standards for medical devices and implants. These materials

undergo rigorous testing to ensure they do not elicit an adverse reaction from biological

tissue. This includes assessments for cytotoxicity, irritation, sensitization, and, in some

cases, longer-term biostability when intended for implantation [

36

]. The advantage of using

VAT polymerization for producing biocompatible parts lies in its ability to create complex,

high-resolution structures that can match the specific anatomical features of patients, es-

sential for custom implants, dental restorations, and surgical guides. Furthermore, recent

advancements have led to the development of resins that not only meet biocompatibility

requirements but also possess properties such as improved mechanical strength, flexibility,

and thermal stability, expanding their use in creating functional medical devices [37].

It is also worth noting that the biocompatibility of a printed part is not solely depen-

dent on the material, but also on the printing process and post-processing steps. Proper

cleaning and curing are essential to remove uncured resin and achieve the desired material

properties, including biocompatibility. In some cases, additional sterilization is required

before clinical use. As technology and materials evolve, the range of biocompatible resins

and their applications in healthcare continues to expand, offering promising opportunities

for personalized medicine and advanced medical treatments.

Sensors 2024,24, 2668 13 of 44

3.3. Material Jetting

Material Jetting (MJ) combines the precision of resin 3D printing with the speed of

filament printing, producing parts with realistic colors and textures, primarily aimed at

professionals in industries such as automaking, design, healthcare, and product manu-

facturing. Two major brands, Stratasys and 3D Systems, dominate the production of MJ

devices. This technology now extends to diverse applications, from biocompatible dental

molds to high-speed manufacturing tooling. Three primary MJ types include PolyJet by

Stratasys, Nano Functional Technology using solid nanoparticles, and Drop on Demand

(DOD) for thick fluid materials, particularly useful in creating wax models for investment

casting in jewelry manufacturing.

3.3.1. Principle of Operation of MJ

Material Jetting (MJ) operates akin to a two-dimensional inkjet printer, but instead of

ink, it sprays material onto the construction platform using either a continuous or DOD

approach. An illustration of how the Stratasys J55 (Stratasys, Ltd., Eden Prairie, MN, USA;

Rehovot, Israel) dispenses print material is shown in Figure 6[

38

]. The process involves

layer-by-layer construction as the material solidifies upon the substrate, and the print head,

equipped with nozzles, moves horizontally across the build platform. The complexity of

these machines varies, affecting the deposition methods and the materials used. The limited

material options include polymers and waxes due to their viscosity and suitability for

forming droplets.

Figure 6. An illustration of how the Stratasys J55 dispenses print material [38].

The core components of an MJ system comprise the print head, responsible for precise

material deposition; the build platform, where the object gradually forms layer by layer;

and a UV or heat source to initiate material solidification. This layer-by-layer printing

process allows for intricate shapes and structures. Support structures may be generated

as needed and removed post-printing. After completion, post-processing steps might be

necessary, including support removal, surface finishing, and additional curing for certain

materials. MJ ensures high-resolution and precision in printing, often accompanied by

quality control measures to validate the final product against specifications.

In MJ’s operation, the material is heated to an optimal viscosity before tiny beads of

photopolymer are deposited by the print head. UV light solidifies the material, forming

each layer as the build platform gradually descends. Unlike many other 3D printing

methods, MJ employs a line-wise material deposition system, enabling multiple heads

to allocate different materials for multi-material printing or dispensing soluble support

structures. Despite similarities to SLA in utilizing photo-polymerization, MJ-printed

parts achieve desired properties without additional post-curing due to the minute layer

thickness employed.

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3.3.2. Advantages and Limitations of MJ

Material jetting stands out for its exceptional resolution and intricate detailing, apt

for crafting complex geometric shapes with fine features. Capable of achieving layer

thicknesses as minimal as 16 microns, it delivers remarkably accurate and smooth surfaces.

Additionally, the technology enables multi-material printing, allowing simultaneous use of

various materials, encompassing different colors, rigid and flexible materials, or materials

with differing properties. This versatility facilitates the production of parts with diverse

characteristics within a single print job. Certain material jetting systems excel in full-

color printing, ideal for crafting vibrant prototypes, architectural models, and consumer

products, enhancing realism in aesthetics. Known for its relatively high speed, particularly

in producing small and intricate parts, material jetting serves as a favorable option for

rapid prototyping and small-batch production. Moreover, it minimizes or eliminates the

need for extensive support structures, as support materials can be easily removed during

post-processing, reducing laborious support removal tasks. Embracing a wide array of

compatible materials, from photopolymers to thermoplastics, ceramics, and even select

metals, material jetting broadens its potential applications across various industries.

Material Jetting encounters several limitations, starting with its high costs, encompass-

ing substantial initial investments and ongoing operational expenses. Moreover, the ma-

terials employed can be particularly pricey, especially for full-color printing applications.

Another constraint lies in the restricted build volume of material jetting machines, render-

ing them inadequate for large-scale production or crafting sizable parts. Post-processing

becomes a requisite, involving the removal of support materials, surface refinement through

sanding, and sometimes the application of additional coatings, consequently elongating

production times. Material considerations pose a challenge as well, given the technology’s

limitation to specific compatible materials, despite an expanding range, potentially falling

short for certain applications. Additionally, the trade-off between resolution and speed

presents a hurdle, as achieving high resolution often demands slower print speeds, posing

a challenge in striking a balance between these parameters. Material waste emerges as

another concern, particularly in discarding support materials, leading to increased expenses

and environmental concerns. Lastly, the complexity involved in operating and maintaining

material jetting machines necessitates skilled personnel and regular upkeep, adding to the

operational intricacies.

3.3.3. Materials of MJ

In material jetting, a diverse range of materials have been utilized, including plastics,

polymers, and, intriguingly, some metals. Each material boasts unique properties, signifi-

cantly influencing both the printing process and the final attributes of the printed object.

Typically, these materials exist in fluid or semi-fluid states, easily dispensed through a print

head. Commonly used materials in material jetting can be categorized into several types,

each demonstrating distinct impacts on the printing process [39].

Polymers represent a substantial portion of material choices in material jetting. Among

them, photopolymers reign as the most prevalent. These liquid resins solidify upon expo-

sure to ultraviolet (UV) light, recognized for their remarkable high resolution and capacity

for intricate detailing, making them ideal for producing complex objects. Additionally,

certain material jetting systems utilize thermoplastic polymers like PLA, ABS, and PETG.

These materials, heated and extruded through the print head, enable layer-by-layer deposi-

tion and solidification.

Metal material jetting, while less common, stands as a more advanced process. It

involves jetting metal powders encased in a polymer matrix or liquid binder. Subsequently,

parts undergo sintering or debonding processes to eliminate the binder and fuse the metal

particles, resulting in solid metal parts. This method serves applications necessitating high

strength and metallic properties.

Ceramic material jetting parallels metal material jetting in approach. Ceramic powders,

mixed with a binder, create a printable slurry. Post-printing, the binder is removed, and the

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ceramic part is often sintered at elevated temperatures to attain desired mechanical and ther-

mal properties. Such material jetting is employed in scenarios requiring high-temperature

resistance and electrical insulation.

Composite materials are also compatible with material jetting, involving the mixing

of two or more materials during printing. For instance, carbon-fiber-reinforced polymers

yield lightweight, robust parts with enhanced mechanical properties.

The selection of materials impacts the printing process in multifaceted ways. First,

diverse materials possess distinct optimal printing parameters—such as print speed, layer

height, and curing times—necessitating printer configuration tailored to each material. Sec-

ond, material choice affects the achievable resolution and detailing capabilities, with pho-

topolymers excelling in intricate object production. Third, the mechanical properties,

including strength and thermal resistance, vary among materials, requiring alignment with

the desired final product properties. Furthermore, post-processing steps—such as sintering,

curing, or debonding—may be material-dependent, impacting the complexity and cost of

finalizing the product. Finally, specific application needs, such as electrical conductivity,

mandate careful material selection tailored to meet these requirements.

3.3.4. Applications of MJ

Material jetting has become a versatile solution applied across various real-world

domains due to its capacity for producing high-resolution, multi-material, and multi-

color components. Its practical utilization spans diverse industries. For instance, in the

domain of prototyping and product development, material jetting plays a pivotal role,

especially within sectors like automotive, aerospace, and consumer electronics. Engineers

and designers harness this technology for swift prototyping, enabling rapid iterations and

efficient design testing before moving into large-scale production. Similarly, in the sphere

of dental and medical devices, material jetting showcases its significance by crafting precise

and tailored models, orthodontic devices, hearing aids, and surgical guides. Its ability to

create intricate, patient-specific parts proves indispensable in these critical applications.

The aerospace industry heavily relies on material jetting to fabricate lightweight, high-

performance components such as air ducts, interior panels, and even small satellite parts.

Architects and construction professionals leverage material jetting to develop intricate

architectural models and prototypes, aiding in effective visualization and communication of

design concepts. Additionally, the jewelry and fashion sectors benefit from the technology,

utilizing its capabilities to produce finely detailed accessories and jewelry, often integrating

a myriad of colors and materials.

Moreover, high-end consumer electronics, including custom smartphone cases, tap

into material jetting for its precision and capacity to amalgamate various materials within a

single print. Educational institutions and research labs also find value in material jetting,

employing it for both educational purposes and research exploration into 3D printing tech-

nology and its expansive potential. Furthermore, the shoe and orthopedic industries deploy

material jetting to manufacture custom insoles and orthotic inserts, precisely conforming to

the wearer’s foot shape for superior comfort and support.

An emerging frontier, 3D food printing recognizes the potential of material jetting in

creating intricate and decorative food items or customizing food textures. This indicates its

prospective utility in culinary and confectionery applications, potentially revolutionizing

the way food is presented and experienced.

3.3.5. Developing Trends of MJ

Material jetting stands as an established 3D printing technology, continuously evolving

through ongoing developments and applications. Its growth likely involves embracing

new innovations and trends that further enhance its capabilities. Notable advancements

within material jetting encompass a range of significant developments.

Firstly, the exploration of hybrid systems integrates MJ with other 3D printing tech-

nologies such as FDM and SLA. This approach aims to amalgamate the strengths of

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multiple technologies within a single print job, potentially unlocking new possibilities and

diversifying the applications of material jetting.

Secondly, the technology’s inherent strengths lie in its high resolution and precision.

Material jetting excels in creating highly precise 3D-printed objects by jetting tiny droplets

of photopolymer material, cured subsequently with UV light. Such precision makes it an

ideal choice for applications requiring intricate details and fine surface finishes, particularly

evident in industries such as healthcare and prototyping.

Lastly, the influence of MJ expands into multi-material printing capabilities. This

unique feature allows for the simultaneous use of diverse materials within a single print

job, offering substantial value in crafting objects with varying properties, colors, and func-

tionalities [

40

]. Industries such as automotive, aerospace, and healthcare benefit immensely

from this versatile capacity, enabling the creation of complex, multi-functional components.

Additionally, ongoing advancements in material properties, along with a focus on sustain-

ability, further drive the technology’s evolution toward broader applications and more

eco-friendly practices.

3.4. Binder Jetting

Binder jetting (BJ) was developed at MIT in the 1990s. The printing process of BJ

consists of a binder being printed onto a bed of powder to form the cross sections of a print,

similar to powder bed fusion printing [41].

3.4.1. Principle of Operation of BJ

The BJ technology employs a precise and controlled process where a printer nozzle

systematically traverses a powder bed, depositing a liquid binder to form the desired cross-

sectional shape for each successive layer. Following the application of the binder, the system

lays down an additional layer of powder. This ensures the freshly applied binder acts as

an adhesive, effectively bonding the powder particles to form a solid layer [

42

]. The size

of the liquid binder droplets is meticulously controlled, typically less than 100 microns in

diameter, to ensure high precision and detail in the final product. After the binder is applied,

the entire print bed undergoes a curing process, either under a heat lamp or within a furnace.

This step is crucial for activating the binder and solidifying the powder, thereby imbuing

the emerging object with the necessary mechanical strength [

43

]. Following this, the process

of powder layering and binder application is repeated, layer by layer, until the object is

fully formed. The interim product of this process, known as the "green body", signifies the

nascent stage of the printed object. It has achieved its geometric specifications but requires

further post-processing to enhance its structural integrity and surface finish. Through

this intricate layer-by-layer approach, binder jetting technology facilitates the creation of

complex parts with a high degree of accuracy and detail. One of the most common BJ

technologies is ColorJet Printing (CJP). Figure 7shows the principle of operation of the BJ

technology [44].

3.4.2. Advantages and Limitations of BJ

The BJ technology offers several compelling advantages compared to alternative

AM techniques, making it an attractive option for a range of applications. Notably, BJ

is recognized for its cost-effectiveness, which stems from the efficient use of materials

and the process’s scalability. Additionally, it minimizes the occurrence of heat-induced

distortions and defects that are more common in methods involving high-temperature

processes. A unique feature of BJ is its capability to produce parts in multiple colors

by utilizing different colored binders, enhancing the aesthetic appeal and functionality

of printed objects for applications such as prototypes and functional parts. Moreover,

the versatility of BJ in printing with a diverse array of materials—from metals to ceramics

and polymers—broadens its applicability across various industries [45].

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Figure 7. Schematic diagram of binder jetting 3D printing technology: (a) the BJ system; (b) pow-

der/binder interaction between adjacent layers [44].

However, the BJ technique is not without its limitations. One of the main challenges is

achieving high-density parts, as the process tends to produce objects with lower density

due to the use of larger particle sizes in the powder [

46

]. The post-processing stage of BJ is

also more labor-intensive and complex, involving multiple steps that may include curing,

infiltration, and sintering to enhance the mechanical properties and density of the printed

objects. Additionally, printed parts are susceptible to deformation during the removal of

supports or during post-processing. Manual intervention is often required to manage and

finish the prints, adding to the labor cost and time [

47

]. Another concern is the residue left

by the binders, which can affect the surface finish and may require further post-treatment

to remove. Lastly, the initial mechanical properties of the green bodies—the objects in their

nascent stage post-printing—tend to be poor, necessitating additional processes to achieve

the desired strength and durability [48].

3.4.3. Materials of BJ

The BJ technology exhibits versatility in working with a wide array of materials,

including polymers, metals, and ceramics, underscoring its adaptability across various

manufacturing domains [

49

]. The selection of binder agents, predominantly composed

of organic polymer-based materials, is critical and is primarily influenced by the binder

solution’s wettability and bindability characteristics. This selection process is vital, as each

binding agent possesses unique properties tailored to the specific type of powder it aims

to bind. Commonly used polymers in this context include polyvinyl pyrrolidone (PVP),

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polyvinyl alcohol (PVA), and polyacrylic acid (PAA), each offering distinct advantages and

considerations depending on the application and material compatibility [48].

3.4.4. Applications of BJ

The diverse material compatibility of BJ technology enables its application across a

multitude of sectors, significantly broadening its utility beyond conventional manufac-

turing paradigms. In the pharmaceutical domain, BJ technology is revolutionizing the

production of oral medications, offering customized dosages and release profiles, which

could lead to more personalized medicine practices [

50

]. Additionally, in dental medicine,

the precision and adaptability of BJ facilitate the creation of ceramic dental prostheses,

promising significant advancements in dental restoration and cosmetic dentistry [

51

]. Be-

yond medical applications, BJ’s capacity to work with metals, including 316L stainless

steel, opens new avenues in manufacturing complex, high-strength components with appli-

cations ranging from aerospace to automotive industries, underscoring the technology’s

versatility and potential to innovate across various manufacturing landscapes [

52

]. This

breadth of application showcases the transformative potential of BJ technology in leading

the next wave of manufacturing innovation, pushing the boundaries of what is possible in

both medical and industrial applications.

3.4.5. Developing Trends of BJ

Currently, the main goal with binder jetting is to find a way to eliminate the porosity

that comes with the prints to make it a more viable option. One of the more recent

developments and applications would be printing an oral medicine using binder jetting.

For example, BJ is being used to print Spritam, which is a medicine used to treat seizures [

50

].

This advancement is very important because to be able to mass produce medicines of

all types makes medicines more available to more people and lowers the cost of those

medications, making it very relevant in today’s world.

3.5. Selective Laser Sintering

Selective laser sintering (SLS) technology stands out in 3D printing due to its ability

to utilize a variety of materials and produce complex, functional parts with high accuracy.

Unlike traditional 3D printing methods that rely on layers of material being deposited or

cured, SLS employs a laser to selectively fuse powdered materials, eliminating the need for

support structures and allowing for greater design freedom. Its versatility in material use,

lack of need for support structures, and capability to create intricate, fully functional parts

set SLS apart from other conventional 3D printing technologies.

3.5.1. Principle of Operation of SLS

SLS technology utilizes lasers as the heat source and is a form of powder bed fusion.

This technology is an effective method for rapid prototyping and forms a more solid layer

of material with the high-density laser onto the bed of powder. This powder can be a

variety of different materials and will be discussed further in the materials portion of this

report. By printing parts one layer at a time and then building upon the previous layers,

it is capable of rapidly producing 3D physical parts directly from a user-designed 3D

model [

53

]. There are two classifications of SLS, including direct and indirect. These are

based on the mechanism present in the specific SLS printer used [

54

]. A standard design

for an SLS printer can be seen in a schematic form on Figure 8[55].

The process follows these steps: the powder is filled, and the feed container is con-

nected to the build cylinder, next a protective gas is passed into the forming room to reduce

oxygen content, then the roller moves a thin layer of the powder over the build plate,

and finally the laser will scan the powder surface and form the appropriate layers [53].

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Figure 8. Schematic of SLS 3D printers [55].

3.5.2. Advantages and Limitations of SLS

The thermal and fluid behaviors of the layers during typical AM processes are essential

to the proper adhesion between layers for the final product to be a strong and solid part.

For SLS, this is a very good advantage, since the layer binding is very precise because of the

use of a laser and results in the final product being almost isotropic [

56

]. This also allows

SLS to be used for very complex component designs while having superior production

time when compared with traditional manufacturing processes for the same designs, some

of which would be unable to be used without AM. However, some disadvantages come

from using powdered materials, which include porosity, shrinkage, impurities, and poor

surface quality. Most parts will require post-processing for a clean final product [54].

3.5.3. Materials of SLS

SLS is readily capable of producing 3D components utilizing a few materials, including

plastics, composites, and ceramics [

57

]. Mainly, SLS is used for precise polymer production

and is common in many industries for precise and rapid prototyping of more expensive

components. The most commonly used polymer in SLS is nylon. In recent years, improve-

ments to the post-processing of ceramic SLS components have made a viable final product.

These improve some of the flaws originally found with the low density and strength of the

original ceramic productions [58].

3.5.4. Applications of SLS

With the rapid manufacturing time that SLS provides, it is a resourceful way to both

prototype and produce fully functional polymer and ceramic parts that can be used in a

variety of industrial applications. The first of which in this discussion involves the use

of SLS in health fields. By using pharmaceutical powders, researchers have been able to

create orally consumed medicines with different release times and could be an alternative

in production in the short term [

59

]. The printing of pills is a very promising technique

for the development of more patient-tailored medications to get the proper dosage rather

than the standard sizes that currently exist [

60

]. SLS is also a beneficial technology with a

variety of applications in aerospace as well. Mainly for the manufacturing of noncritical

components as well as rapid prototyping of nonfunctional components. There are also

instances where SLS is used as a functionally graded material due to the ability to produce

SLS components with different crystallinities for different applications [61].

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3.5.5. Developing Trends of SLS

There have been recent developments in the creation of flexible electronics using SLS.

The precise nature of the laser in the device allows it to be used in combination with metal

nanoparticles for the creation of micropatterns, which can be used in small electrical devices

as they take up minimal space [62].

In the SLS process, defects arise from the creation of gas bubbles due to overheating

and trapped surrounding gasses. Artificial intelligence (AI) models are currently being

developed to control the SLS device and decrease the amount of imperfections in the

condition in which a defect would result from [63].

3.6. Selective Laser Melting

Selective laser melting (SLM), also known as laser powder bed fusion (LPBF), is a

type of powder bed fusion (PBF) printer that melts and fuses material powder together in

layers [

64

]. Figure 9represents the general components in a schematic found in all types of

PBF printers. A heat source, either a laser or an electron beam, is used to fuse the first layer,

that is the first cross-section of the part, covering the build platform. Then, the printing bed

lowers by a layer for new powder to be spread over the previous using a roller or blade,

which is often vibrated to promote a more even powder distribution, and the cycle repeats

until the part is completed [65].

Figure 9. Schematic of powder bed fusion [65].

3.6.1. Principle of Operation of SLM

SLM follows the same process as PBF from beginning to end in layer-by-layer con-

struction, but only takes metals, typically pure, and specifically uses a high powered laser

to completely fuse the powder to print the part. The SLM process begins with a fresh layer

of metallic powder deposited onto the print bed using a roller or blade provided by the

powder reservoir and any excess is caught by the overflow container. The building platform

or print bed is constrained to a translation in the

z

-axis. The laser then activates. The beam

is controlled by a scanner system, constrained to

x

–

y

rotation, which heats the powder

bed to slightly above melting point to create a melt pool and draw the desired shape of

the slicing plane. The molten metal then cools rapidly and solidifies, resulting in fused

tracks. The stage starting when the laser interacts with the powder until the end of the

molten metal’s solidification is called the Laser Powder Melt Pool (LPMP). After scanning

the cross-section, the building platform lowers by an amount equal to the next layer’s

thickness and the process repeats until the printed part is complete. After having fully

Sensors 2024,24, 2668 21 of 44

cooled, the building platform is then raised to reveal the print and reduce lead time by

freeing it from most of the powder [

66

]. During post-processing, the remaining unfused

powder is retrieved and can be reused again and again [

67

]. Support structures of the

recovered part are removed, and the part is cleaned with isopropyl alcohol or processed

with CNC work to improve surface finish [68].

SLM parameters are controlled to minimize build time while conserving product

quality. Scanning speed, hatch speed, layer thickness, and laser power are some of the

most important to SLM [

69

]. The melting temperature the laser induces in the powder is

controlled by laser parameters such as incident energy density (J/cm

2

) and laser power.

Powder characteristics, including morphology, size, and apparent density, are important

considerations in micro-structure parts. The typical layer thickness of commercial SLM

systems is 20–100

µ

m, with particle size ranging from 20 to 50

µ

m [

70

]. The typical

tolerance is 0.2% with a lower limit of

±

0.1 (

±

0.003

′′

). The best materials for SLM are single-

component metals including titanium, aluminum, and steel [

71

]. Melting metal can require

temperatures as high as 1600 °C, which creates destabilization in the microscopic level.

Therefore, support structures are needed due to the high residual stress and to mitigate the

distortion. SLM is also widely used with cobalt-chromium, titanium alloys, nickel alloys,

iron-based alloys, aluminum alloys, niobium alloys, refractory alloys, amorphous alloys,

and super alloys [

72

]. There is a reason why SLM favors homogenous, elemental metals.

While SLM has been used with alloys such as cobalt chromium, it is more feasible with

monometallic powders because of the singular melting point [

73

]. Complete melting results

in full fusion of the particles but sintering, in comparison, does not result in fully fused parts

since the material remains slightly under the melting point [

74

]. Therefore, the mechanical

properties of an SLM print are better because of increased particle merging and fuller

density. The surface finish of SLM is also better [

75

]. Overall, SLM works well with pure

metals and certain alloys, but will struggle to combine and process a heterogenous metal

powder mixture.

3.6.2. Advantages and Limitations of SLM

SLM, similar to other AM technologies, benefits from a shorter build time than tra-

ditional machining process. This is especially powerful when coupled with the fact that

the process results in a fully functional object because of its high forming accuracy, net-

shape ability, and high tolerancing as well as its ability to fabricate complex shapes [

76

].

The advantages of SLM are highly attributed to its ability to melt and fully fuse powder

that results in fully dense parts—up to 100% density which allows it to maximize tensile

strength (unlike sintering) to its full potential and allowing it to create light, but strong parts

using metals such as aluminum. SLM is also suitable for visual models and prototypes.

There is no need for binders and fluxing agents in the metallic powder [

77

]. The SLM pro-

cess is often faster than selective laser sintering (SLS). One boon of SLM is the recyclability

of its unmelted powder, with up to 99% powder recovery, that can virtually always be

reused. SLA can produce functional, complex, net-shaped, and fully dense parts, but still

struggles with limitations. The extreme heat is, again, a culprit of many problems seen with

the nature of having to fully melt metal such as warping. It is a high-energy process that

relies on temperature gradients to fuse particles [

68

], which requires higher energy costs

and has been found to be less energy efficient than SLS by upwards of 10–20%. One paper

on the quality control of SLM gives an in-depth review of the major factors that contribute

to the formation of internal defects in SLM printed objects during the LPMP stage. SLM

fusion, gas pores and micro-cracks, which are promoted by the balling, spatter, and keyhole

phenomena. Defect formation can be suppressed by adjusting process parameters to the

knowledge of the formation mechanism of each problematic phenomenon. SLM’s material

flexibility, as addressed, is minimal because of its bias for single-component metals. While

powder can act as an integrated support material in PBF, this does not apply to SLM because

of its need for support structures due to the problems that arise from high temperatures.

Sensors 2024,24, 2668 22 of 44

Furthermore, the build chamber must be filled with inert gas [

78

]. There is also a size

limitation to SLA, with a max part size of about 280 ×280 ×325 mm3.

3.6.3. Materials of SLM

The main materials utilized in SLM include titanium alloys (notably Ti6Al4V) for their

exceptional strength-to-weight ratio and biocompatibility, making them ideal for aerospace

and medical applications. Stainless steels (such as 316L and 304) are also widely used due to

their corrosion resistance and mechanical properties, suitable for a wide range of industrial

applications. Aluminum alloys, such as AlSi10Mg, offer advantages in automotive and

aerospace parts because of their lightweight and good thermal properties. Cobalt–chrome

alloys are chosen for high wear resistance in medical implants and aerospace components.

Nickel-based superalloys, such as Inconel 625 and 718, are selected for their excellent

strength and thermal resistance, crucial for applications in harsh environments. Finally, tool

steels (e.g., H13, D2) are used for creating durable tooling components. Despite the broad

utility of these materials, challenges such as controlling residual stresses, porosity, thermal

crack, and ensuring material properties consistency remain key research and development

focuses within the field.

3.6.4. Applications of SLM

SLM sees application in the jewelry, dental, aerospace, automobile, and medical

industries [

79

]. Its demand in high-tech areas and industrial interest stems from its ability to

manufacture fully functional parts with excellent mechanical properties and high geometric

complexity. Recently, SLM has now been able to open its doors to ceramics, such as

alumina and zirconia, and to gradient materials. In 2010, Hagedorn was able to develop

technology that completely laser melts pure ceramic powder and manufactures net-shaped

specimens with almost 100% densities without requiring post-processing to achieve it [

76

].

AM technologies have been used by surgeons and material scientists to create patient-

specific medical devices of any geometry and sometimes within a day [

80

]. Furthermore,

while an even more recent paper demonstrates the success of an SLA created hip implant,

the reliability of these biomedical parts needs to be investigated, especially the powder

materials themselves, the corrosion behavior, and fatigue properties [81].

3.6.5. Development Trends of SLM

SLM technology has seen significant advancements and developing trends aimed

at overcoming its initial limitations and broadening its application spectrum. One of

the foremost trends is the exploration and development of new material systems, includ-

ing high-entropy alloys and functionally graded materials, to exploit unique property

combinations for tailored applications [

82

]. Concurrently, there is a focus on optimizing

process parameters through machine learning and AI to enhance part quality, reduce de-

fects, and improve material properties predictively. Multi-laser systems have emerged to

increase build rates and improve the efficiency of the SLM process, addressing produc-

tivity concerns for industrial-scale production. The integration of in situ monitoring and

control systems using sensors and real-time data analysis aims to achieve consistent and

reliable part quality by detecting and correcting process anomalies as they occur. Lastly,

efforts towards achieving sustainability in SLM involve recycling of powder materials

and energy efficiency improvements, crucial for minimizing the environmental impact of

manufacturing processes. These trends underscore the dynamic evolution of SLM, pushing

towards broader industrial adoption and the creation of more complex, high-performance

components across sectors.

3.7. Direct Metal Laser Sintering

Direct metal laser sintering (DMLS) is another PBF process that is similar to SLM but

only takes metal alloys or combined powder metals. DMLS was evolved from SLS and uses

a laser beam to condense metal powder without binders, creating a high-density product.

Sensors 2024,24, 2668 23 of 44

It can produce sophisticated components with complex geometries that casting, machining,

and forming cannot because to their limitations [83].

3.7.1. Principle of Operation of DMLS

DMLS feeds and deposits metallic powders using a core concept. It uses lasers

to overlay metallic powders to make functioning parts. This method has three phases

including powder delivery, deposition, and waste storage. In the powder delivery stage,

a feeder with a roller and rake precisely applies material with the proper thickness to

the preceding layer, as shown in Figure 10 during deposition. The deposition stage fuses

metallic powder with the underlying layer by precisely moving the laser source [

84

].

Overhanging structures in fused layers are supported by powder not fused by high-energy

sources [85]. In the waste storage stage, powder not used in deposition is stored for reuse.

Looping through these phases creates a 3D component. The sintering mechanism uses heat

to fuse powder particles into a dense component [86].

Figure 10. Schematic of direct metal laser sintering [87].

3.7.2. Advantages and Limitations of DMLS

AM through DMLS stands out as a support-free technique that enhances its capacity

to craft intricate pieces with precise dimensions [

88

]. This process facilitates the direct

printing of metal components or prototypes using a diverse range of metal alloys while up-

holding their inherent qualities [

89

]. Moreover, the innovative feature of recycling unused

metal powder post-printing elevates productivity in creating prototypes and components,

concurrently curbing material costs and waste [90].

However, the utilization of DMLS in 3D projects entails considerable expenses, con-

stituting one of the pricier avenues within AM due to machine and material costs [

91

].

Additionally, the nature of DMLS tends to produce components with higher porosity com-

pared to traditional metal AM methods, albeit engineers have a degree of control over

porosity during the printing process. Furthermore, when considering 3D printing projects

constrained by build volume, engineers opt for the most suitable technology, recognizing

DMLS as a choice for low-volume metal additive applications [92].

3.7.3. Materials of DMLS

DMLS technology has applications in the construction of components utilizing a

range of materials [

93

]. The materials that can be used in this technique are ABS nylon

filled with glass and polymers of polycarbonate. In addition, DMLS is capable of working

with a variety of metallic materials, such as copper, low-carbon steels, superalloys, and

stainless steels. DMLS makes it easier to manufacture additive metal parts by providing

Sensors 2024,24, 2668 24 of 44

compatibility with a wide variety of metals, including aluminum, titanium, steel, stainless

steel, cobalt chrome, nickel alloys, and precious metals.

3.7.4. Applications of DMLS

The manufacture of high-performance components is one of the primary applications

for DMLS, which has seen widespread adoption in the aerospace and automotive industries.

Additionally, it has applications in a wide variety of other industries, such as functional

prototypes, tooling, and medical prostheses, to name a few. In addition, the use of DMLS

in the process of manufacturing orthopedic implants for dogs is quickly becoming more

commonplace. This broadens its uses beyond the realm of traditional implants to include

individualized prostheses and bone replacements in veterinary medicine as well as other

fields [94].

3.7.5. Developing Trends of DMLS

In order to overcome the surface accuracy issues, a technique of optimization that is

based on two criteria is presented. This strategy places an emphasis on optimizing certain

subprocesses, such as the orientation of the component, the determination of the layer

thickness, and the directions of the laser scanning, with the twin goals of achieving: (a) the

shortest possible production time and (b) the fewest possible surface flaws. In addition

to this, when it comes to the optimization process, the model takes into consideration the

impacts that are caused by the shrinking of the material [95].

3.8. Electron Beam Melting

Electron beam melting (EBM) is an AM technique that uses a high-energy electron

beam to fuse metal powder particles layer by layer to build complex parts.

3.8.1. Principle of Operation of EBM

The process occurs inside a vacuum chamber to prevent oxidation of the materials and

to maintain the integrity of the electron beam. A high-voltage electron beam is generated

and focused onto a thin layer of metal powder, selectively melting the powder according to

the digital design of the part. After one layer is melted and solidified, the build platform

is lowered, and a new layer of powder is applied. This process repeats until the part is

fully constructed [

96

]. The energy source, an electron beam, allows for rapid melting and

solidification, making EBM distinct in its speed and the quality of the parts produced.

3.8.2. Advantages and Limitations of EBM

EBM technology offers several benefits, including the ability to produce parts with

complex geometries that are difficult or impossible to achieve with traditional manufactur-

ing methods. It provides excellent material properties, comparable to wrought metals, due

to the rapid cooling rates and high-temperature processing, which results in fine microstruc-

tures. EBM can process reactive metals such as titanium and its alloys in a vacuum, which

is essential for aerospace and medical applications. Moreover, the process is relatively fast

and efficient, with minimal material waste.

EBM also has its drawbacks. The requirement for a vacuum environment and the use

of electron beam technology contribute to high equipment costs. The surface finish of EBM-

produced parts is often rougher compared to other AM technologies, which may require

additional post-processing. The selection of materials compatible with EBM is currently

more limited than for other AM methods. Additionally, the layer-by-layer construction can

introduce residual stresses, requiring heat treatment or other stress-relief processes.

3.8.3. Materials of EBM

EBM technology primarily processes metals, with a focus on high-value, high-performance

materials. Titanium alloys, such as Ti6Al4V, are among the most commonly used materials

due to their strength, lightweight, and biocompatibility, making them ideal for aerospace

Sensors 2024,24, 2668 25 of 44

and medical implants [

97

]. Nickel-based superalloys, such as Inconel 718, are used for

applications requiring high strength and corrosion resistance at elevated temperatures.

Cobalt–chrome alloys are also processed for their wear resistance and biocompatibility

in medical prostheses. The general requirement for EBM precursor powder is stringent,

emphasizing the need for high purity and uniform particle size to ensure consistent melting

and solidification properties. This is crucial in metallic 3D printing, where the choice

and quality of the powder significantly affect the final product’s mechanical properties

and surface finish. Recent research is expanding the range of materials suitable for EBM,

including refractory metals and high-entropy alloys, to broaden its application base [98].

3.8.4. Applications of EBM

EBM has found applications in industries where the properties of metal parts are

critical, and complexity is valued. In aerospace, EBM is used to manufacture lightweight

structural components and complex engine parts that benefit from the weight reduction

without compromising strength. The medical field uses EBM to create custom implants

and prosthetic devices, taking advantage of the ability to produce porous structures that

encourage bone ingrowth. The energy sector benefits from the production of durable parts

for high-temperature applications. As the technology matures, its applications continue

to expand into areas such as automotive and tooling, where customization and material

properties are key considerations.

3.8.5. Developing Trends of EBM

The future of EBM technology is focused on overcoming its current limitations while

expanding its capabilities and applications. Research is underway to improve surface finish

and reduce residual stresses, potentially broadening the range of materials that can be

effectively processed. Advances in machine learning and artificial intelligence are being

applied to optimize process parameters and predict outcomes, enhancing efficiency and

part quality. There is also a trend towards multi-material processing, allowing for parts

with graded properties or complex material systems. Sustainability efforts are focusing on

reducing energy consumption and increasing material utilization rates. As the technology

progresses, EBM is poised to play a significant role in the future of AM, with ongoing

developments aimed at increasing its accessibility, versatility, and efficiency.

3.9. Multi-Jet Fusion

Multi-jet fusion (MJF) is a very innovative AM method created by the company

Hewlett-Packard [

99

]. This type of AM method falls into the powder bed fusion family of

printers alongside technologies such as SLS, DMLS/SLM, and EBM [100].

3.9.1. Principle of Operation of MJF

MJF printers’ function in ways similar to most other 3D printing technologies but

also has its own unique functions. MJF uses an array of inkjets to lay a layer of detailing

agents one layer at a time to be fused in a bed of powdered material. These layers of

detailing agents fuse together by use of a heating element [

101

]. This heating element

functions by using an infrared lab that fuses materials in the same vein as the feed material.

These various materials are thermoplastic polymers and are commonly used in such

applications [

102

]. The step-by-step process of how a MJF printer operates is shown in

Figure 11 [

103

]. Here, a material coating is applied that acts as the base layer for printing.

Next, an agent is applied to the desired places in the print. Energy is then applied using

the infrared heating element mentioned above, then the final fusion takes place, and the

print is ready.

Sensors 2024,24, 2668 26 of 44

Figure 11. Schematic diagram of multi-jet fusion (MJF) [103].

3.9.2. Advantages and Limitations of MJF

When it comes to the MJF technology, it offers relatively low machining time with

similar part properties with minimal post processing required after the fact [

101

]. Some

studies, such as the AAPM study below, have reported MJF to be up to about 10 times faster

than standard FDM printing for mass production of various parts [

104

]. When it comes to

how the materials perform, MJF-printed designs often have a more ductile nature to them,

which could be a desired material property for some applications but also a limitation in

other applications depending on needs [

105

]. An MDPI study even found MJF printing to

have an average printing cost of about half of other similar products. A wonderful photo

can be seen below in Figure 12 of the various colors that MJF can print as well as the types

of projects that can be printed out of MJF [

106

]. One of the MJF’s main limitations is its

material limitations, because the ductility of MJF parts, while beneficial in some cases, may

not be suitable for applications requiring higher material stiffness or strength. Besides,

the advantages of MJF, such as its speed and cost-effectiveness, are maximized in particular

scenarios, such as mass production, which may not apply to all projects.

Figure 12. Sample 3D-printed part through the MJF technology by HP [106].

3.9.3. Materials of MJF

MJF technology employs an extensive selection of powdered materials as the founda-

tion for its printing process. These materials are meticulously fused by the application of

two distinct types of binder fluids: a fusing agent and a detailing agent. The detailing agent,

often incorporating a cyan, magenta, yellow, and key (CMYK) color scheme, plays a crucial

role in enabling the production of parts in a vast spectrum of colors, adding to the versatility

and aesthetic appeal of the printed objects. This capability was exemplified in Figure 12,

showcasing the technology’s ability to achieve detailed and vibrant colorations in printed

parts. The fusion of these agents with the base materials under precise conditions not only

ensures the structural integrity of the components but also opens up new possibilities for

customization and design innovation in AM [107].

3.9.4. Applications of MJF

The potential applications of MJF go far and wide, but in part to being a bit more

energy and resource efficient than SLS, MJF can be more attractive to industries that require

Sensors 2024,24, 2668 27 of 44

more energy and resource efficiency [

108

]. MJF printed parts are said to have significant

vibration isolation characteristics, especially between the ranges of 10–30 Hz, which means

that MJF parts could be desired for various applications that require that specific kind of

isolation within that range [109].

3.9.5. Developing Trends of MJF

Some common developing trends in the world of MJD 3D manufacturing would be

to use MJF parts in mechanical parts/devices along with biometrical lattices, structures,

and other medical and orthotics applications such as prosthetics. The MJF technology was

also found to have a potential in creating mechanical tools and well as in devices that are

required to be fluid-tight [

110

]. These applications are only scratching the surface as to

what this technology has to offer. The future of MJF is bright and shows potential to take

a significant part of certain industries in the future with its abilities to print efficiently as

well as print parts with certain desirable material properties. MJF, as most 3D printing

technologies, is still more or less in its infant stage and could see some major growth in the

coming years.

3.10. Direct Energy Deposition

Direct energy deposition (DED) is an advanced AM technology that involves the use

of focused thermal energy—such as a laser, electron beam, or plasma arc—to fuse materials

by melting as they are being deposited. DED is distinguished by its ability to create high-

of material efficiency, design flexibility, and the ability to repair or add material to existing

components. This report explores the principle of operation, advantages and limitations,

materials, applications, and developing trends of DED technology.

3.10.1. Principle of Operation of DED

DED technology operates by focusing thermal energy to melt a material—typically

metal powder or wire—as it is being deposited onto a substrate or part surface. The material

is delivered through a nozzle that moves in multiple axes, allowing for the construction of

complex geometries. Figure 13 shows the principle of operation of two different types of

DED technologies [111].

Figure 13. Schematic of directed energy deposition (DED): (a) Powder DED (laser source); (b) Wire

DED (E-beam source) [111].

The process is typically controlled by a CAD file, which guides the nozzle’s path to

create the desired shape layer by layer. The precise application of energy and material

results in parts that are fully dense and strongly bonded to the substrate [112].

Sensors 2024,24, 2668 28 of 44

3.10.2. Advantages and Limitations of DED

There are several advantages of DED. DED minimizes waste by depositing material

only where needed. DED also enables the creation of complex geometries that are difficult

or impossible to achieve with traditional manufacturing methods. DED can be used to

add material to existing parts, allowing for the repair of worn or damaged components,

or the modification of parts to improve performance. DED is capable of processing a wide

range of materials, including difficult-to-process alloys [

113

]. For certain applications, DED

can be faster than other AM technologies, especially when producing large or medium-

sized components.

DED has some limitations, such as rough surface finish and low precision. DED often

requires post-processing to achieve a smooth surface finish, as the parts may have a rough

texture directly after manufacture. While suitable for many applications, DED may not

achieve the fine detail or dimensional accuracy of some other AM processes.

3.10.3. Materials of DED

DED technology boasts a remarkable capacity to handle an expansive range of ma-

terials, making it a versatile tool in the AM landscape. This technology is proficient in

processing a wide spectrum of metals, such as stainless steel, titanium alloys, nickel-based

alloys, aluminum, and cobalt–chrome, catering to the demands of various industrial ap-

plications. Beyond metals, DED also extends its capabilities to ceramics and composite

materials, although these applications are less widespread [

114

]. For DED technology,

flowability, and purity to ensure efficient deposition and optimal material properties in

the final product. The precision in powder characteristics is vital to achieving the desired

material behavior during the melting and solidification processes, which is instrumental in

extending DED’s application in manufacturing complex and custom fabrications across

various industries. Recent advancements in DED technology are continually broadening

the scope of its material compatibility, paving the way for its application in more specialized

fields. These developments not only enhance the utility of DED, but also underscore its

potential to revolutionize manufacturing processes by accommodating a diverse array of

materials for custom and complex fabrications [115].

3.10.4. Applications of DED

DED technology is widely applied in several key industries, demonstrating its versatil-

ity and effectiveness. In the aerospace sector, it is utilized for manufacturing and repairing

vital components such as turbine blades and structural parts, underscoring its importance

in maintaining the reliability and performance of aircraft. The energy industry benefits

from DED through the repair of parts used in oil and gas exploration as well as in power

generation equipment, highlighting its role in sustaining critical infrastructure. In the

medical field, DED’s ability to produce customized implants and prosthetics showcases its

potential in personalized medicine, offering solutions tailored to individual patient needs.

The tooling industry uses DED to add features or coatings to tools, dies, and molds, en-

hancing their functionality and lifespan. Additionally, its flexibility in processing different

materials makes DED invaluable for research and development purposes, particularly for

prototyping and material development, thereby fueling innovation across various sectors.

3.10.5. Developing Trends of DED

The evolving landscape of DED technology is marked by several promising develop-

ments that are set to enhance its capabilities and broaden its applications across industries.

Hybrid manufacturing emerges as a notable trend, integrating DED with traditional sub-

tractive manufacturing within a single setup to harness the combined strengths of both

methods, optimizing the production process for efficiency and precision. Concurrently,

advancements in DED are paving the way for multi-material printing, enabling the cre-

ation of components with multiple materials in a single build. This innovation introduces

Sensors 2024,24, 2668 29 of 44

the potential for parts with functionally graded materials and localized property varia-

tions, expanding the design possibilities and functional capabilities of manufactured parts.

Furthermore, the integration of real-time monitoring and adaptive control systems is revo-

lutionizing the DED process, significantly improving part quality and process reliability

through precise control and adjustments during fabrication. Complementing these techno-

logical strides, the development of sophisticated software and simulation tools is crucial,

offering predictive insights and optimization strategies for material properties and process

parameters, thereby minimizing the reliance on trial and error and enhancing the overall

quality of the final products. Collectively, these developments signify a significant leap

forward for DED technology, showcasing its growing importance in the AM domain and

its potential to revolutionize manufacturing practices across a diverse array of sectors by

offering unmatched material efficiency, design flexibility, and the capability to repair or

augment existing components.

3.11. Carbon Fiber Reinforcement

Carbon fiber reinforcement (CFR) technology combines carbon fiber with polymer

matrixes to produce composite materials that are exceptionally strong and lightweight.

3.11.1. Principle of Operation of CFR

The principle of operation involves the selective layer-by-layer deposition of carbon

fiber-infused filaments, which are then melted and fused together using a heated nozzle

in a process similar to FDM. This technique allows for the precise alignment of carbon

fibers, optimizing the strength-to-weight ratio of the final product [

116

]. Advanced variants

of this technology can also embed continuous carbon fiber strands into a base material,

significantly enhancing the mechanical properties of the printed object. Figure 14 shows

3D-printed bicycle lugs reinforced with continuous carbon fiber [117].

Figure 14. Three-dimensionally-printed bicycle lugs reinforced with continuous carbon fiber [117].

3.11.2. Advantages and Limitations of CFR

CFR offers several compelling advantages, including the production of parts that are

significantly stronger and lighter than those made from conventional materials, making it

ideal for high-performance applications. The technology allows for complex geometries that

are difficult to achieve with traditional composite manufacturing techniques, along with

reduced waste and shorter production cycles [

118

]. Additionally, the ability to customize

the fiber orientation provides unparalleled control over the mechanical properties of the

final part.

Despite its benefits, CFR technology faces limitations such as higher material and

operational costs compared to standard AM processes. The range of materials compatible

with carbon fibers is also limited, primarily to certain thermoplastics and thermosetting

polymers. Moreover, the technology requires specialized equipment and expertise to ensure

the optimal placement of fibers and to manage the thermal stresses during printing, which

can affect the dimensional accuracy and surface finish of the parts.

Sensors 2024,24, 2668 30 of 44

3.11.3. Materials of CFR

In CFR, the choice of materials plays a pivotal role in achieving the desired mechanical

and thermal properties of the final product. The base matrix typically involves high-

performance thermoplastic polymers such as nylon, PEEK, and ABS. These polymers are

chosen for their excellent balance of strength, durability, and ease of processing. The re-

inforcement comes in the form of short or continuous strands of carbon fiber, which are

embedded into the polymer matrix to enhance the composite material’s overall proper-

ties [

119

]. The inclusion of carbon fiber, particularly in a continuous form, significantly ele-

vates the mechanical performance of the composite. Continuous carbon fiber reinforcement

leads to a marked increase in tensile strength and rigidity, which are critical for components

subjected to high stress or load. Moreover, these carbon fiber-reinforced materials exhibit

superior thermal stability, maintaining their integrity and performance over a wide tem-

perature range. This combination of high strength, stiffness, and thermal resistance makes

CFR composites ideal for use in advanced engineering applications, including aerospace,

automotive, and high-performance sporting equipment, where materials must withstand

rigorous operational conditions while minimizing weight and maximizing performance.

3.11.4. Applications of CFR

CFR is increasingly employed in industries where strength-to-weight ratio is critical,

such as aerospace, automotive, and sporting goods. In aerospace, it is used for producing

lightweight structural components and fixtures. Automotive applications include the

fabrication of high-performance parts such as gears, brackets, and structural components

that benefit from the reduced weight without sacrificing strength. In the sporting goods

industry, CFR technology is used to create equipment such as bicycles, racquets, and drones,

offering enhanced performance characteristics.

3.11.5. Developing Trends of CFR

The future of CFR is driven by ongoing innovations aimed at expanding its applica-

tions and improving performance. One key trend is the development of new composite

materials that offer higher thermal and chemical resistance, opening up new industrial

applications. Another area of focus is the improvement in printing technologies to enable

the use of continuous carbon fiber reinforcement over larger areas and more complex

geometries, further enhancing the mechanical properties of printed parts. Additionally,

advancements in simulation and modeling software are improving the predictability and

optimization of fiber orientations, reducing the need for physical prototypes and accelerat-

ing the design-to-production cycle. As these trends progress, CFR is set to revolutionize

industries by providing lighter, stronger, and more customizable components.

3.12. Laminated Object Manufacturing

Laminated Object Manufacturing (LOM) involves the layer-by-layer bonding of sheet

materials, which are precisely cut to shape and then bonded together to form a three-

dimensional object.

3.12.1. Principle of Operation of LOM

The process of LOM starts with a roll or a stack of sheets made of metal, paper,

or plastic. Each sheet is affixed to the layer beneath using an adhesive, welding, or another

bonding technique. A laser or knife then precisely cuts the outline and internal features of

the current layer based on the digital model. This process repeats, stacking and shaping

layers until the desired 3D object is fully formed. Excess material from each layer acts as

support for the object during the build and can be removed afterward [120].

3.12.2. Advantages and Limitations of LOM

LOM offers several advantages, including the ability to use a wide range of materials,

from paper and polymers to metals, making it versatile across various applications. It is

Sensors 2024,24, 2668 31 of 44

cost-effective, especially when using inexpensive materials such as paper, and the process

can be faster than other AM techniques for certain geometries and materials. Additionally,

the waste material can often be recycled, especially when paper is used, making it an

environmentally friendly option. The technology is also beneficial for creating large parts

and offers unique aesthetic finishes, particularly with paper-based laminates [121].

However, the technology faces limitations, including lower dimensional accuracy

and part strength compared to other AM methods, due to the layer bonding mechanism.

posite or functionally graded materials. Furthermore, the post-processing required to

remove excess material and achieve the final part geometry can be labor-intensive and time-

consuming. The surface finish may also be rough, requiring additional post-processing to

smooth out.

3.12.3. Materials of LOM

LOM can process a diverse array of materials, including paper, metal foils, various

plastics, ceramic-based materials [

122

], and composite materials [

123

]. Paper is commonly

used for conceptual models and visual prototypes due to its low cost and ease of handling.

Metal foils allow for the creation of more durable and functional prototypes or parts,

though they require stronger bonding techniques such as ultrasonic welding. Plastics

offer a balance between durability and ease of processing, suitable for both prototypes

and functional parts. The choice of material significantly influences the application and

performance of the finished product.

3.12.4. Applications of LOM

The applications of LOM are varied and span several industries. In prototyping,

cost-effectiveness and rapid turnaround. Metal and plastic laminates are used in the

automotive, aerospace, and electronics industries for creating functional prototypes, tooling,

and even end-use parts that benefit from the layered aesthetic or specific material properties.

Additionally, the education and architectural sectors utilize LOM for creating detailed scale

models and teaching aids, capitalizing on its ability to produce large objects at relatively

low costs.

3.12.5. Developing Trends of LOM

The field of LOM is witnessing several developing trends aimed at overcoming its

limitations and broadening its application scope. One significant trend is the integration of

advanced materials, including composites and functionally graded materials, to enhance

part strength and functionality. There is also a growing focus on improving bonding tech-

niques to increase durability and dimensional accuracy. Innovations in cutting technology,

such as more precise lasers and knives, are enabling finer details and better surface finishes.

Additionally, the development of automated post-processing solutions is aiming to reduce

the time and labor associated with removing excess material. As these trends evolve, LOM

is expected to become more competitive with other AM technologies, offering unique

advantages in cost, speed, and material diversity.

4. Developing Trends of Additive Manufacturing

AM is evolving rapidly, with key trends shaping its future. Figure 15 shows the devel-

oping trends of AM technologies. 3D bioprinting is advancing the fabrication of complex

biological structures, promising revolutionary healthcare solutions. Three-dimensional

food printing is transforming culinary arts, allowing for customized nutrition and intricate

food designs, merging gastronomy with technology for personalized eating experiences.

Meanwhile, large-scale 3D printing is pushing the boundaries of construction and industrial

production, enabling the creation of large structures and components with unprecedented

speed and efficiency. 4D printing introduces time as a dimension, enabling printed objects

Sensors 2024,24, 2668 32 of 44

to change shape or function in response to stimuli, offering innovative applications in smart

materials and structures. AI-based 3D printing is optimizing the manufacturing process,

enhancing design, predicting material behavior, and improving the quality and efficiency of

printed objects, marking a significant leap toward intelligent and autonomous production

systems. There are also other directions, such as multi-material and new materials.

(a) 3D bioprinting (b) 3D food printing

(c) Large-scale 3D printing (d) 4D printing

Figure 15. Developing trends of additive manufacturing technologies: (a) a 3D-bioprinted human

tissue (an ear) with biodegradable plastic scaffolding using an integrated tissue and organ printing

system (ITOP) by the Wake Forest Institute for Regenerative Medicine [

124

]; (b) a 3D-printed pizza

by Beehex through a fund from NASA [

125

]; (c) the Crane Wasp 3D printer, created to print homes

using local materials [

126

]; (d) a ‘smart’ self-folding material that can transform shape, printed by

MIT’s Self-Assembly Lab [127].

4.1. 3D Bioprinting

Biological additive manufacturing, also known as biological fabrication, integrates

principles of biology, chemistry, and engineering to fabricate biological structures and

materials via AM techniques [

128

]. This innovative field leverages the precision and

versatility of 3D printing technologies to create complex biological constructs, such as

tissues, organs, and biocompatible materials, layer by layer [

129

,

130

]. State-of-the-art

advancements include the development of sophisticated bioprinting methods capable of

handling living cells, bioinks composed of natural or synthetic biomaterials that support

cell growth and differentiation, and the creation of vascularized tissues that mimic natural

blood vessels for improved survival and function of fabricated tissues. Challenges remain,

including the replication of the complex microarchitecture of native tissues, ensuring long-

Sensors 2024,24, 2668 33 of 44

term viability and integration of printed constructs within the body, and scaling up the

technology for widespread clinical application [131].

Significant efforts are underway to overcome these hurdles, with ongoing research

focusing on improving the resolution and fidelity of bioprinted structures, developing new

bioink formulations, and enhancing the mechanical and functional properties of printed

tissues. Figure 15a shows a 3D-bioprinted human tissue (an ear) with biodegradable plastic

scaffolding using an integrated tissue and organ printing system (ITOP) by the Wake

Forest Institute for Regenerative Medicine [

124

]. Besides, bioinspired AM is a cutting-

edge convergence of biology, material science, and advanced manufacturing techniques.

This innovative approach draws inspiration from the natural world, aiming to mimic

the complex structures and functionalities observed in biological systems through AM

processes [132].

4.2. 3D Food Printing

Three-dimensional food printing represents a pioneering intersection of technology

and gastronomy, offering personalized nutrition, intricate edible designs, and the potential

for sustainable food production. State-of-the-art trends in this domain involve the use

of a variety of edible materials, including proteins, carbohydrates, and fats, which are

extruded layer by layer to create complex and customized food items. This technology

not only allows for the customization of nutritional content to meet individual dietary

needs, but also introduces novel textures and forms in culinary arts, potentially reducing

food waste by utilizing ingredients more efficiently. Despite its innovative prospects, 3D

food printing faces significant challenges, such as the limited range of printable food

materials, slow printing speeds compared to conventional cooking methods, and the need

for stringent food safety standards. Additionally, the acceptance of 3D-printed foods by

consumers and the integration of such technologies into commercial kitchens pose further

hurdles. Ongoing research and development aim to overcome these obstacles by enhancing

printer technology, expanding the variety of printable food materials, and improving the

scalability of 3D food printing processes [

125

]. Figure 15b shows a 3D-printed pizza by

Beehex through a fund from NASA [125].

Optimizing the rheological properties of 3D printing food products presents unique

challenges, as these properties are crucial for both the printability of food pastes and

the quality of the final product. Rheological properties, including viscosity, yield stress,

and thixotropy, must be carefully balanced to ensure that food materials can be extruded

smoothly through the printing nozzle and maintain their shape once deposited. Some

challenges in optimizing these properties are discussed as follows.

•

Complexity of Food Materials: Food materials are inherently complex and can exhibit

non-Newtonian behavior, making it challenging to predict how they will behave under

the shear rates applied during 3D printing. Ingredients can interact in unpredictable

ways, affecting their rheological properties.

•

Variability of Ingredients: Natural variability in food ingredients due to factors such

as origin, season, and processing can lead to fluctuations in the rheological properties

of the food paste, affecting consistency and printability.

•

Sensitivity to Processing Conditions: The rheological properties of food materials can

be highly sensitive to temperature, pH, and the presence of air bubbles, among other

factors. Maintaining optimal conditions throughout the printing process to ensure

consistent properties is challenging.

•

Balancing Printability with Nutritional and Sensory Qualities: Achieving the right

rheological properties for printing often requires the addition of modifiers such as

hydrocolloids or emulsifiers. However, these adjustments can impact the nutritional

value, taste, and texture of the final product, potentially compromising consumer

acceptance.

Sensors 2024,24, 2668 34 of 44

•

Post-Processing Changes: Foods can undergo changes in structure and rheology after

printing or during cooking or cooling processes. Predicting and controlling these

changes to ensure the final product meets desired quality standards is difficult.

•

Equipment Limitations: The performance and characteristics of 3D food printers,

especially the extrusion system, can limit the range of rheological properties that

can be effectively printed. Printers with enhanced capabilities can be expensive and

less accessible.

Addressing these limitations requires a multidisciplinary approach, combining food

science, material science, and engineering. Advances in ingredient technology, precise

control systems for 3D printers, and a deeper understanding of the rheology of complex

food systems are essential for overcoming these challenges and unlocking the full potential

of 3D food printing.

4.3. Large-Scale 3D Printing

Large-scale additive manufacturing has emerged as a groundbreaking technology, sig-

nificantly impacting the construction, aerospace, and manufacturing sectors by enabling the

direct fabrication of large structures, components, and parts. State-of-the-art advancements

include the development of large-format 3D printers capable of printing entire buildings or

large aerospace components from specialized materials, ranging from concrete and metals

to composite polymers. This technology offers the potential for increased customization,

reduced waste, and accelerated construction times [

133

]. However, challenges persist,

including the need for improved mechanical properties of printed materials, the scaling

up of printing processes without compromising precision or material properties, and the

integration of large-scale 3D printing into existing manufacturing and construction ecosys-

tems. Additionally, there are logistical challenges in transporting and operating large-scale

ability of the materials used [

134

]. Ongoing research and development aim to address these

issues by enhancing material formulations, printing technologies, and process efficiencies,

paving the way for broader adoption and innovation in large-scale AM. Figure 15c shows

the Crane Wasp 3D printer, created to print homes using local materials [126].

Integrating structural reinforcement into large-scale 3D printed components presents

significant challenges that necessitate meticulous attention to the material and process intri-

cacies. The mechanical strength and durability of printed structures, especially in sectors

demanding high reliability such as aerospace and construction, require advancements in

reinforcement techniques and materials. Moreover, the absence of standardized guidelines

and regulations specifically tailored to large-scale AM poses a hurdle to ensuring uniform

quality, safety, and performance across the industry. Establishing comprehensive standards

and regulatory frameworks is essential for fostering innovation, ensuring structural in-

tegrity, and promoting widespread acceptance and integration of large-scale 3D printing

technologies into traditional manufacturing and construction practices.

4.4. 4D Printing

Four-dimensional printing, an innovation extending beyond traditional 3D printing,

incorporates the dimension of time to create objects that can change shape, properties,

or functionality in response to external stimuli such as temperature, light, moisture, or mag-

netic fields. This dynamic technology leverages smart materials, including shape-memory

polymers and responsive hydrogels, to fabricate objects with pre-programmed transforma-

tions [

135

]. Shape memory alloys (SMAs) represent a pioneering class of smart materials

in 4D printing, offering distinct advantages due to their ability to return to a pre-defined

shape when subjected to an appropriate thermal stimulus. These materials, including

nickel–titanium alloys, can undergo phase transitions that enable them to remember and

revert to their original shapes upon heating [

136

]. Their integration into 4D printing opens

avenues for creating complex, self-adjusting structures and devices that can react to tem-

perature changes with high precision. The exploration of SMAs in 4D printing highlights

Sensors 2024,24, 2668 35 of 44

the potential for advanced applications in fields requiring adaptive materials, such as

responsive aerospace components and medical devices that can conform to anatomical

changes, further pushing the boundaries of what is achievable with AM technologies.

State-of-the-art developments in 4D printing have led to potential applications in

various fields, including biomedical devices that adapt to bodily changes, self-assembling

structures for aerospace and construction, and smart textiles that adjust to environmental

conditions [

137

]. Despite its promise, 4D printing faces challenges, including the limited

range of smart materials available, the need for precise control over material properties to

perform intended functions under real-world conditions. Ongoing research aims to address

these challenges by developing new materials, refining AM techniques, and enhancing

computational design tools to accurately predict and program the dynamic behavior of

printed objects. Figure 15d shows a ‘smart’ self-folding material that can transform shape,

printed by MIT’s Self-Assembly Lab [127].

4.5. AI-Based Additive Manufacturing

AI-based additive manufacturing integrates advanced technologies such as AI, ma-

chine learning, and the Internet of Things (IoT) with traditional 3D printing processes to

enhance efficiency, quality, and customization. This intelligent approach enables real-time

monitoring and control of the printing process, predictive maintenance of equipment,

and dynamic adjustment of printing parameters for optimal results [

138

]. State-of-the-art

trends in AI-based AM involve the use of AI algorithms to predict material properties and

outcomes, IoT for seamless integration of AM systems into broader production networks,

and digital twin technologies for virtual simulation and optimization of the manufactur-

ing process [

139

]. Challenges facing AI-based AM include ensuring data security and

the resistance to adopting new manufacturing paradigms within established industries.

Furthermore, there is a continuous need for skilled workers capable of operating these

advanced systems and for ongoing development to reduce costs and improve the scalability

of smart AM solutions. Research and development efforts are focused on addressing these

challenges, aiming to unlock the full potential of AI-based AM across various sectors. AI

and machine learning algorithms optimize printing parameters in real time, enhancing

quality, reducing material waste, and improving the mechanical properties of printed

parts. Simulation software allows for the predictive modeling of material behavior and the

optimization of designs before printing, saving time and resources.

4.6. Innovations in Materials, Quality Control, and Post-Processing

The ability to print with multiple materials simultaneously or to combine different AM

processes in a single machine (hybrid printing) is a significant area of development. This

advancement enables the creation of parts with varied properties (e.g., rigid and flexible,

conductive and insulative) within a single build process, expanding the functional capabili-

ties of AM parts. Hybrid systems that integrate subtractive processes (e.g., CNC milling)

with additive processes improve surface finishes and dimensional accuracy. The exploration

and development of new materials specifically designed for AM processes are expanding

the capabilities and applications of AM. This includes the creation of high-performance

polymers, metals, ceramics, and composite materials. Material innovation also focuses on

sustainability, with an increasing emphasis on recyclable and biobased materials.

Improvements in build size and printing speed are crucial for the broader adoption

of AM across industries. Developments in this area include the creation of larger print

beds, faster printing techniques, and more efficient post-processing methods. These im-

provements aim to make AM viable for mass production and large-scale applications,

moving beyond prototyping and small-batch production. Enhancements in the precision

and repeatability of AM processes are vital for meeting industrial quality standards. This

involves the development of more sophisticated monitoring and control systems that can

Sensors 2024,24, 2668 36 of 44

detect and correct errors in real time during the printing process. Such systems rely on

sensors and machine vision technologies to ensure parts are built to specification, reducing

failure rates and improving reliability.

Advancements in post-processing are crucial for advancing AM from a specialized

to a mainstream production technique, especially important in sectors such as aerospace

and healthcare. Automated support removal, using mechanical, chemical, or thermal

methods, now allows for efficient and precise part cleaning. Surface finishing technologies,

such as chemical vapor smoothing and electro-polishing, are reducing manual work and

improving both the looks and function of 3D printed parts by minimizing surface roughness.

Furthermore, techniques such as hot isostatic pressing (HIP) and annealing are being

automated to enhance the mechanical properties of 3D printed parts, addressing common

issues such as stress and porosity, thereby making AM more viable for a wide range

of applications.

4.7. Standardization and Regulatory

Addressing standardization and regulatory challenges in AM is crucial for unlocking

its full potential across various industries. The absence of universally accepted standards

and clear regulatory guidelines currently presents significant obstacles to the widespread

adoption of AM technologies. These challenges stem from the innovative nature of AM,

production techniques, and product properties. Standardization efforts are essential for

ensuring consistency, safety, and quality in AM-produced goods. Without standardized

protocols, comparing and validating the performance of products made through AM

across different sectors becomes problematic. This lack of standardization also hampers

material development, process optimization, and the sharing of knowledge within the AM

community, ultimately slowing down innovation. On the regulatory front, establishing

clear guidelines is necessary to address safety concerns and intellectual property issues,

which are paramount for consumer and industry confidence. For instance, in highly

regulated sectors such as aerospace, automotive, and medical devices, manufacturers must

navigate complex certification processes that are not yet fully adapted to the specifics of

AM technologies. This adaptation is essential for ensuring that AM products meet stringent

safety and performance requirements.

Moreover, there is a pressing need for regulations that encompass the entire lifecycle

of AM products, from design and production to use and end-of-life disposal. Such compre-

hensive regulatory frameworks could address environmental concerns, including energy

consumption and material waste, promoting sustainability in AM practices. Progress

in overcoming these standardization and regulatory hurdles is being made through the

collaboration of industry stakeholders, research institutions, and regulatory bodies. These

efforts include the development of industry-specific standards, the creation of certifica-

tion pathways for AM materials and processes, and the initiation of dialogue on ethical

considerations and intellectual property rights in the context of AM.

In conclusion, tackling the challenges of standardization and regulation is key to

facilitating the broader adoption of AM technologies. By establishing clear, universally

accepted standards and regulatory frameworks, the AM industry can ensure product safety

and quality, foster innovation, and build the trust of consumers and industries alike, paving

the way for a new era of manufacturing.

4.8. Circular Economy and Sustainability

The economic analysis of AM processes reveals a complex landscape of costs, benefits,

production efficiencies, and return on investment (ROI), varying significantly across differ-

ent AM technologies and application contexts. Understanding these financial implications

is crucial for businesses considering adopting AM processes.

Initial costs for AM include the purchase of printers, materials, and software, alongside

operational costs such as energy consumption, maintenance, and labor. Advanced AM

Sensors 2024,24, 2668 37 of 44

systems, especially those capable of metal printing, can require substantial initial invest-

ment. Material costs can also be high, particularly for specialized or high-quality inputs.

Training and integrating AM into existing production workflows can add to the initial

expenses. AM offers several financial benefits, including reduced material waste, lower in-

ventory and storage costs, and the ability to produce complex designs without a significant

increase in cost. For customized or low-volume production, AM can significantly reduce

the cost per unit compared to traditional manufacturing methods. Technology also enables

faster product development cycles, reducing time to market and potentially increasing

competitive advantage. AM can enhance production efficiency by consolidating parts,

thereby reducing assembly time and costs. It allows for just-in-time production, minimizing

inventory levels and associated costs. However, the production speed of AM may be slower

compared to traditional manufacturing methods for high-volume production, potentially

offsetting some efficiency gains unless combined with other manufacturing processes in a

hybrid approach.

The return on investment (ROI) from different AM technologies can vary. FDM is often

used for prototyping due to its lower cost, FDM can offer a quick ROI in environments

where design flexibility and rapid prototyping are valued. SLS and DMLS are more expen-

sive but can produce functional parts with properties close to traditionally manufactured

items. They are more likely to be used in final product manufacturing, potentially offering a

higher ROI by enabling the production of complex, high-value items. SLA and MJ are ideal

for producing high-detail parts with smooth surface finishes, SLA can be cost-effective for

applications requiring precise geometries and aesthetics, such as dental or medical models.

The ROI also depends on the industry. In aerospace and medical sectors, where the cost

of failure is high, and the value of customized, complex parts is significant, the ROI can

be substantial. In contrast, for industries focused on high-volume, low-margin products,

integrating AM may require a more strategic approach to realize a positive ROI.

The financial implications of adopting AM are influenced by a myriad of factors,

including the type of AM technology, the scale of adoption, the specific application, and the

industry. While the initial costs can be high, the benefits of customization, reduced lead

times, and efficiency improvements present a compelling case for AM in many scenarios.

A thorough cost–benefit analysis, considering both direct and indirect financial impacts, is

essential for businesses to make informed decisions about investing in AM technologies.

The strategic integration of AM can offer competitive advantages, ultimately leading to a

positive return on investment.

The development of AM technologies is increasingly aligned with the principles of

the circular economy. This includes the use of recycled materials in the printing process,

the development of recyclable or biodegradable materials, and the potential for AM to

reduce waste through optimized design and on-demand production. The direction of

development in AM technologies and processes reflects a push towards greater integration

with digital technologies, enhanced material capabilities, and a focus on sustainability.

These trends aim to solidify AM’s role in the future of manufacturing, offering unparalleled

flexibility, efficiency, and innovation across industries.

5. Conclusions

This study has provided an extensive overview of the AM landscape, reflecting on its

historical evolution, current state, and future prospects. AM technologies, characterized

by their versatility and innovation, offer significant advantages across various sectors,

including aerospace, healthcare, automotive, and fashion. The exploration of different AM

technologies reveals a nuanced understanding of their operational mechanisms, material

requirements, and application potentials. Despite the transformative impacts of AM, chal-

lenges such as printing speed, material diversity, cost, and post-processing requirements

persist, indicating areas for future improvement and research.

Future developments in AM are poised to address these challenges through advance-

ments in material science, printing technologies, and process optimization. The integration

Sensors 2024,24, 2668 38 of 44

of artificial intelligence and the exploration of new domains such as 3D bioprinting, 3D

food printing, and large-scale 3D printing are expected to further enhance the efficiency,

accessibility, and applicability of 3D printing technologies. Moreover, the potential for multi-

material printing and improved precision offers a glimpse into a future where 3D printing

could seamlessly blend into traditional manufacturing processes, offering customized and

on-demand production capabilities that were previously unattainable. Standardization,

regulation, circular economy, and sustainability also need future efforts and studies.

In conclusion, AM stands at the cusp of a new era in manufacturing, with the potential

to fundamentally alter how products are designed, produced, and distributed. As the

technology continues to evolve, overcoming its current limitations, its integration into

the broader manufacturing ecosystem will likely accelerate, heralding a future where

AM plays a central role in the next manufacturing revolution. The ongoing research and

development within this field are critical, not only for advancing the technology itself but

also for realizing the full spectrum of its applications across industries worldwide.

Author Contributions: Conceptualization, L.Z.; methodology, L.Z.; formal analysis, L.Z.; investiga-

tion, L.Z., J.M., J.V., M.M., Z.A.T., J.B., Q.J., G.H., M.R. and L.D.S.; writing—original draft preparation,

L.Z.; writing—review and editing, L.Z.; visualization, L.Z.; supervision, L.Z.; project administration,

L.Z. All authors have read and agreed to the published version of the manuscript.

Funding: This research was funded by the Partnerships for Opportunity and Workforce and Eco-

nomic Revitalization (POWER) Initiative grant (grant number: PW-20426-IM-21) awarded by the

Appalachian Regional Commission (ARC), USA.

Conflicts of Interest: The authors declare no conflicts of interest.

Abbreviations

The following abbreviations are used in this manuscript:

AM Additive manufacturing

ME Material extrusion

FDM Fused deposition modeling

PEEK Polyether ether ketone

PEI Polyetherimide

VP VAT polymerization

SLA Stereolithography

DLP Digital light processing

cDLP Continuous digital light processing

SLS Selective laser sintering

DMLS Direct metal laser sintering

CAD Computer-aided design

STL Stereolithography

CNC Computer numerical control

PLA Polylactic acid

ABS Acrylonitrile butadiene styrene

MJ Material jetting

DOD Drop-on-demand

UV Ultraviolet

PETG Polyethylene terephthalate glycol

FFF Fused filament fabrication

BJ Binder jetting

CJP Color jet printing

PVP Polyvinyl pyrrolidone

PVA Polyvinyl alcohol

PAA Polyacrylic acid

SMA Shape memory alloy

AI Artificial intelligence

Sensors 2024,24, 2668 39 of 44

SLM Selective laser melting

LPBF Laser powder bed fusion

PBF Powder bed fusion

LPMP Laser powder melt pool

EBM Electron beam melting

MJF Multi-jet fusion

CMYK Cyan, magenta, yellow, and key

DED Direct energy deposition

CFR Carbon fiber reinforcement

LOM Laminated object manufacturing

ITOP Integrated tissue and organ printing system

IoT Internet of things

HIP Hot isostatic pressing

ROI Return on investment

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... Additive manufacturing (AM) which also includes selective laser melting (SLM), fused deposition modelling (FDM), directed energy deposition (DED) and binder jetting has become one of the most radical technologies of production in the modern age [5,==6]==. Its inherent benefits of geometric freedom, material efficiency, and close to zero tooling overhead make it especially subject to optimization by AI....

Artificial intelligence in additive and smart manufacturing: a critical review of design, process optimization, and quality control

Article

Full-text available

• Sep 2026

The rapid adoption of artificial intelligence (AI) in the production sector has triggered a revolutionary change in the design, manufacturing, monitoring, and optimization. The paper is a critical analysis of AI-based additive manufacturing (AM) and smart manufacturing systems with an emphasis on design intelligence and process optimization, real-time quality control, and sustainable production. Based on recent literature, this paper considers the major AI methods, such as machine learning (ML), deep learning (DL), reinforcement learning (RL), and physics-informed neural networks (PINNs) throughout the manufacturing lifecycle. The results show that AI can greatly decrease the time spent on design iterations, increase the accuracy of predictions of process parameters, and allow in-situ defect detection with high accuracy. Moreover, AI helps to ensure predictive maintenance strategies, reduce the number of unforeseen failures and enhance operational efficiency. The interplay of AI and digital twin technology, Industrial Internet of Things (IIoT), edge computing, and big data analytics is cited as a pillar of Industry 4.0 and the future Industry 5.0 paradigm. Regardless of these developments, the issues of data sparsity, the applicability of the model, computational expense, and interpretability are the key obstacles to large-scale industrial adoption. The value of human-AI cooperation and the necessity to have explainable and reliable AI systems in critical safety areas are also emphasized in the study. Lastly, the review provides future research directions, which focus on multi-modal AI, large-scale manufacturing models, and sustainability-based optimization frameworks. In general, the article is a thorough synthesis of the recent developments and realistic directions of introducing AI-enabled smart manufacturing.

... Combining digital methods with new materials is changing how personal and vehicle armor is developed, making the process faster and smarter. ==115== In summary, ballistic materials are evolving quickly thanks to advances in smart technologies, new materials, and sustainable design. Innovations now include nanotech composites, shearthickening fluids, 3D textiles, and sensor-enabled fabrics....

ADVANCED BALLISTIC MATERIALS: A COMPREHENSIVE REVIEW OF NATURAL AND SYNTHETIC FIBER COMPOSITES FOR BODY ARMOR APPLICATIONS

Article

• Aug 2026
• CELL CHEM TECHNOL

Body armor plays a crucial role in modern defense and law enforcement, offering lifesaving protection against ballistic threats. With the growing complexity of threats and the demand for improved wearer mobility, research has shifted toward optimizing materials for higher strength-to-weight ratios, enhanced energy absorption, and flexibility. This review comprehensively examines advancements in both natural and synthetic fibers used in soft and hard ballistic armor systems. Focus is given to their mechanical properties, structural configurations, and the incorporation of novel technologies, such as hybrid composites. The role of fabric architecture, surface treatments, and fiber-matrix adhesion in enhancing energy absorption is also discussed. The article concludes with future directions for the development of multifunctional, lightweight, and eco-friendly ballistic protection systems using emerging material technologies.

Fused deposition modeling–subtractive hybrid manufacturing: process integration, system configurations, and performance trade-offs

Article

Full-text available

• Sep 2026
• INT J ADV MANUF TECH

The introduction of Fused Deposition Modeling (FDM)-subtractive manufacturing hybrid techniques represents one possible avenue toward sustainability through manufacturing systems. The advantage offered by the use of FDM is the capability of fabricating complex geometries; however, components manufactured via the use of FDM present poor surface finish and have anisotropic characteristics. On the other hand, the advantage provided by subtractive machining is the high precision in terms of dimensional accuracy and surface finish. Unfortunately, this technology suffers from the drawback of wasting material. As such, the present study will provide an overview of the current status of FDM-subtractive hybrid manufacturing systems along with the advantages and disadvantages of these manufacturing processes. The comparison presented shows the improvement offered by the hybridization of the two processes in terms of dimensional accuracy and functional performance, along with surface finish of manufactured parts. In addition, it was found that these systems can overcome limitations concerning material utilization efficiency and process efficiencies. However, the degree of improvement heavily depends on process sequencing, machine arrangement and material compatibility. The current limitations observed include the lack of multi-material platform for both FDM and subtractive technologies, the limitation in the use of hybrid capabilities of machines within CAM software packages, and the inadequate application of integrated process planning and adaptive controls. Such limitations need to be overcome for further development of hybrid manufacturing systems.

Effect of Immediate Thermal Exposure on the Surface Mechanical Performance of Polyurethane-Coated Oak Wood and Visible 3D-Printed Furniture Components

Article

• Sep 2026

A polyurethane coating represents the conventional solution for protecting and finishing visible wooden furniture surfaces. However, the increasing use of additive manufacturing has introduced visible 3D-printed polymer components whose surface mechanical performance is relevant to their application in furniture. This study investigated the effect of short-term exposure to 60 °C for 1 h on the surface mechanical properties of a pigmented polyurethane coating applied to oak wood and 3D-printed PLA, ABS-T and PET-G components. Specimens were evaluated under laboratory conditions (20 °C) and immediately after exposure to 60 °C, while their surfaces remained at an elevated temperature. Impact resistance, abrasion resistance, and scratch resistance using a tungsten carbide tip were determined according to the relevant standards, and the surface damage was assessed by visual inspection and digital microscopy. The polyurethane coating exhibited the smallest impact indentation diameter but showed earlier crack initiation and lower abrasion resistance than the 3D-printed polymers. Among the investigated polymers, ABS-T provided the most balanced combination of impact resistance, abrasion resistance and surface hardness, whereas PLA exhibited the greatest dimensional changes after exposure to 60 °C. Microscopic analysis revealed surface defects that were not detectable by visual inspection, demonstrating the value of digital microscopy for detecting subtle surface damage. The results indicate that ABS-T is a promising material for visible furniture components exposed to short-term elevated temperatures, while PET-G should be used with caution in applications exposed to radiant heat or direct sunlight.

Caracterização microestrutural em pré-formas de aço inoxidável 316L-Si obtidas através de manufatura aditiva por deposição a arco via GMAW-pulsado

Conference Paper

Full-text available

• Sep 2026

Entre as tecnologias de manufatura para metais, a manufatura aditiva por deposição a arco (WAAM) tem ganhado destaque por permitir a fabricação de geometrias complexas, pelas altas taxas de deposição, pela facilidade em fabricar peças de grandes dimensões e pelo baixo custo dos equipamentos. O aço inoxidável 316L é um dos materiais mais empregados na WAAM, devido à sua boa soldabilidade, resistência à corrosão, ductilidade e tenacidade. Contudo, diversos desafios associados a esse método de fabricação ainda limitam a difusão da WAAM na indústria, entre eles: a concentração da transferência de calor ao longo da direção de construção; a formação de porosidade; as tensões residuais causadas pela oscilação térmica; e a anisotropia induzida por gradientes térmicos. Os efeitos dos parâmetros de soldagem na microestrutura do aço 316L têm sido amplamente estudados nos últimos anos, destacando-se a formação da fase metaestável ferrita-δ e a influência da taxa de resfriamento em sua morfologia. Embora alguns estudos tenham demonstrado a inflência dos aspectos térmicos da WAAM na microestrurura de aços inoxidáveis 316L, ainda há uma lacuna na compreensão da heterogeneidade microestrurural ao longo das camadas em processos baseados em soldagem a arco pulsado com eletrodo consumível (GMAW-P). Motivado por essas lacunas na literatura, este estudo tem como objetivo investigar o impacto das variações térmicas na evolução microestrutural e dureza ao longo das camadas de pré-formas de aço inoxidável 316L-Si manufaturadas por WAAM, especificamente pelo processo de soldagem GMAWpulsado. Foram retiradas amostras para análise metalográfica por microscopia óptica, para medição do teor de ferrita- δ e espaçamento dos braços dendríticos secundários (SDAS) através do software ImageJ® e para ensaio de microdureza Vickers. As análises microestruturais revelaram teores de ferrita-δ menores nas primeiras camadas depositadas em razão do histórico térmico, bem como espaçamentos de braços dendríticos secundários maiores e taxas de resfriamento menores nas camadas superiores. Os ensaios de microdureza Vickers revelaram uma diminuição da dureza nas camadas superiores em função da microestrutura grosseira formada sob menores taxas de resfriamento.

Natural Fiber-Reinforced Polylactic Acid Composites: 3D Printing, Life Cycle Assessment, and Applications

Article

• Sep 2026

Natural fiber-reinforced biopolymer composites are an attractive class of materials owing to their renewability, sustainability, and eco-friendliness. Meanwhile, fused deposition modeling (FDM) is transforming the manufacturing industry and gaining popularity for fabricating intricate and customized geometries that are difficult to produce with traditional techniques. Recently, FDM printing of natural fiber-reinforced PLA composites (NFRPCs) has emerged as a promising approach for developing sustainable green composites with potential application across diverse fields. This review comprehensively examines recent advances in the development of FDM-printed NFRPCs, with particular emphasis on mechanical and tribological properties. In addition, the review examines life cycle assessment (LCA) studies to evaluate the environmental sustainability of FDM-printed NFRPCs and identifies important gaps that require further investigation. Challenges associated with the processing of NFRPCs, including printability, interfacial adhesion, and optimal printing parameters, are also highlighted. Finally, future research directions are proposed to facilitate the development of high-performance, printable, and environmentally sustainable NFRPCs.

WLAM of inconel 625 and stainless steel 316 bimetallic structures: an investigation on microstructure, mechanical properties, environment stability and corrosion performance

Article

Full-text available

• Sep 2026
• INT J ADV MANUF TECH

Wire-feed laser additive manufacturing (WLAM) offers significant potential for producing bimetallic structures (BMs) by combining the unique properties of dissimilar alloys. This study investigates the microstructural evolution, mechanical properties, and environmental performance of a BM composed of Inconel 625 (IN625) and Stainless Steel 316 (SS316) fabricated using a WLAM system. Monolithic IN625, monolithic SS316, and a 50/50 IN625-SS316 BM were characterized using optical microscopy, scanning electron microscopy (SEM), X-ray diffraction (XRD), and mechanical testing. Microstructural analysis revealed a smooth transition zone with good metallurgical bonding, exhibiting epitaxial grain growth in the form of columnar dendrites and equiaxed grains with effective elemental mixing and no sharp discontinuities. XRD confirmed the stability of face-centered cubic (FCC) phases throughout the bimetallic structure, with no detectable peaks corresponding to brittle secondary phases. Mechanical testing showed that the BM exhibited intermediate properties, with a UTS of ~ 506 MPa and an average hardness of ~ 145 HV, corresponding to 27.5% lower strength and 18.5% lower hardness than IN625, but 4.3% higher strength and 10.7% higher hardness than SS316. High-temperature oxidation tests at 400 °C, 600 °C, and 900 °C showed intermediate oxidation resistance in the BM, associated with the formation of Cr and Fe-rich oxides. Electrochemical testing in 3.5 wt% NaCl showed corrosion rates of ~ 0.178, ~ 0.879, and ~ 0.353 mils per year (mpy) for IN625, SS316, and the 50/50 BM, respectively. The BM exhibited intermediate corrosion resistance, with a corrosion rate approximately 59.8% lower than SS316 but 98.3% higher than IN625.This intermediate behavior is attributed to the combined electrochemical response of the corrosion resistant IN625 region and the comparatively more susceptible SS316 region in the bimetallic structure.

Measurement of Optimum Laser Energy Required to 3d Print Continuous Fiber Reinforced Composites Using Photo-Curable Thermoset Resin

Conference Paper

Full-text available

• Feb 2024

Continuous carbon fiber (CCF) reinforcement has garnered significant attention in 3D printing due to its ability to impart exceptional mechanical performance characteristics to polymer composites. Light-assisted 3D printing of thermoset resin with CCF reinforcement enables the scalable production of complex structures using additive manufacturing. However, the opaqueness of carbon fiber poses challenges to the curing process of thermoset resins during printing with CCF reinforcement. Therefore, knowledge on the minimum light energy required to continue printing while retaining the desired print shape is crucial for designing energy-efficient and successful manufacturing systems. The primary objective of this study was to measure and compare the minimum energy required to continue light-assisted 3D printing of thermoset resin with CCF reinforcement. The study specifically investigated the minimum light energy needed for printing with both an acrylate-based thermoset resin and an epoxy-acrylate resin. Optimized laser irradiation with the appropriate light energy is imperative for achieving scalable production of CCF reinforced composites with minimum energy consumption.

A Review of the Metal Additive Manufacturing Processes

Article

Full-text available

• Dec 2023

Metal additive manufacturing (AM) is a layer-by-layer process that makes the direct manufacturing of various industrial parts possible. This method facilitates the design and fabrication of complex industrial, advanced, and fine parts that are used in different industry sectors, such as aerospace, medicine, turbines, and jewelry, where the utilization of other fabrication techniques is difficult or impossible. This method is advantageous in terms of dimensional accuracy and fabrication speed. However, the parts fabricated by this method may suffer from faults such as anisotropy, micro-porosity, and defective joints. Metals like titanium, aluminum, stainless steels, superalloys, etc., have been used—in the form of powder or wire—as feed materials in the additive manufacturing of various parts. The main criterion that distinguishes different additive manufacturing processes from each other is the deposition method. With regard to this criterion, AM processes can be divided into four classes: local melting, sintering, sheet forming, and electrochemical methods. Parameters affecting the properties of the additive-manufactured part and the defects associated with an AM process determine the method by which a certain part should be manufactured. This study is a survey of different additive manufacturing processes, their mechanisms, capabilities, shortcomings, and the general properties of the parts manufactured by them.

4D printing of recoverable buckling-induced architected iron-based shape memory alloys

Article

Full-text available

• Aug 2023

A R T I C L E I N F O Keywords: Architected materials Fe-based shape memory alloys Energy absorption Shape recovery Energy dissipation Snap-through Laser powder bed fusion 3D and 4D metal printing A B S T R A C T Architected materials exhibit extraordinary properties in comparison with conventional materials and structures, resulting in additional functionality and efficiency by engineering the geometry in harmony with the base material. Buckling-induced architected materials (BIAMs) are a class of architected materials that exhibit a significant potential to absorb and dissipate energy owing to their local instabilities. Previous studies have shown a trade-off between energy dissipation and geometrical recoverability in metallic BIAM, which limits their use in applications that require both of these features. This study, for the first time, presents 4D printing of buckling-induced architected iron-based shape memory alloys (BIA Fe-SMAs) using laser powder bed fusion (LPBF). The results show that 4D printing of BIA Fe-SMAs can offer both energy dissipation and geometrical recoverability (i. e., recentring). The study was conducted on two different alloy compositions of Fe-17Mn-5Si-10Cr-4Ni. Quasi-static cyclic tests were performed on the two BIA Fe-SMAs, and the samples were subsequently heated to 200 • C to activate the shape memory effect (SME) of the base material. The samples could recover the residual deformations accumulated during the cyclic load owing to the SME of the base material, which led to shape-recovery ratios of 96.8 and 98.7% for the studied BIA Fe-SMAs. The results of this study demonstrate that 4D printing of BIA Fe-SMAs can yield an enhanced multi-functional behavior by combining the material's inherent functional behavior with the functionalities of the architected structure. Notably, BIA Fe-SMA samples could reconfigure their initial shape without damage after densification, which sets them apart from conventional crushable lattices.

A Review on the Metal Additive Manufacturing Processes

Preprint

Full-text available

• Aug 2023

In this article, metallic additive manufacturing (AM) processes were classified and demonstrated. AM technology can be applied to a wide range of industrial areas because of its great feasibility in the design and manufacturing of various complex parts. The main factor that distinguishes various metallic AM processes from each other is the deposition method. Deposited metal can be imported in the form of melt or semi-solid. AM technology also covers a wide range of materials like aluminum, titanium, and many other alloys. Some of the common applications of metallic AM processes are the quick manufacturing of the parts which utilized in various industries like medical, automotive, aerospace, and jewelry industries. AM can help to fabricate parts that are impossible to be manufactured by other processes.

New Trends in 4D Printing: A Critical Review

Article

Full-text available

• Jun 2023

In a variety of industries, Additive Manufacturing has revolutionized the whole design–fabrication cycle. Traditional 3D printing is typically employed to produce static components, which are not able to fulfill dynamic structural requirements and are inappropriate for applications such as soft grippers, self-assembly systems, and smart actuators. To address this limitation, an innovative technology has emerged, known as “4D printing”. It processes smart materials by using 3D printing for fabricating smart structures that can be reconfigured by applying different inputs, such as heat, humidity, magnetism, electricity, light, etc. At present, 4D printing is still a growing technology, and it presents numerous challenges regarding materials, design, simulation, fabrication processes, applied strategies, and reversibility. In this work a critical review of 4D printing technologies, materials, and applications is provided.

Bio-Inspired Additive Manufacturing of Hierarchical Materials: From Bio-Structures to Functions

Article

Full-text available

• Jun 2023

Throughout billions of years, biological systems have evolved sophisticated, multiscale hierarchical structures to adapt to changing environments. Biomaterials are synthesized under mild conditions through a bottom-up self-assembly process, utilizing substances from the surrounding environment, and meanwhile are regulated by genes and proteins. Additive manufacturing, which mimics this natural process, provides a promising approach to developing new materials with advantageous properties similar to natural biological materials. This review presents an overview of natural biomaterials, emphasizing their chemical and structural compositions at various scales, from the nanoscale to the macroscale, and the key mechanisms underlying their properties. Additionally, this review describes the designs, preparations, and applications of bio-inspired multifunctional materials produced through additive manufacturing at different scales, including nano, micro, micro-macro, and macro levels. The review highlights the potential of bio-inspired additive manufacturing to develop new functional materials and insights into future directions and prospects in this field. By summarizing the characteristics of natural biomaterials and their synthetic counterparts, this review inspires the development of new materials that can be utilized in various applications.