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REVIEW

4D Printing Technology: A ReviewJin Choi,1,2, * O-Chang Kwon, 1,3, * Wonjin Jo, 1 Heon Ju Lee, 1 and Myoung-Woon Moon 1

Abstract

3D printing has been recognized as a disruptive technology for future advanced manufacturing systems. With agreat potential to change everything from our daily lives to the global economy, signicant advances in 3Dprinting technology have been made with respect to materials, printers, and processes. In this context, although

similar to 3D printing technology, 4D printing technology adds the fourth dimension of time. 4D printing allowsa printed structure to change its form or function with time in response to stimuli such as pressure, temperature,wind, water, and light. Recently, rapid advances in printing processes and materials development for 3Dprinting have allowed the printing of smart materials or multimaterials designed to change function or shape. Inthis review, we rst compare the similarities and differences between 3D printing and 4D printing. We thenlook into the main factors composing 4D printing technology such as smart materials and designs. Finally, wesummarize the current applications of 4D printing.

Introduction

Additive manufacturing , more popularly known as 3-dimensional (3D) printing technology, has been developedfor more than 30 years. Recently, 3D printing has been rec-ognized as a disruptive technology for future advancedmanufacturing systems. With a great potential to changeeverything from our daily lives to the global economy, sig-nicant advances in 3D printing technology have been madewith respect to materials, printers, and processes. 1,2 In thiscontext, an innovative concept of printing technology knownas 4D printing technology has been developed. The term ‘‘4Dprinting’’ was introduced by Skylar Tibbits in his TED con-ference talk. 3 Although similar to 3D printing, 4D printingtechnology involves thefourth dimensionof time in additiontothe 3D space coordinates. Therefore, one can regard 4Dprinting as giving the printed structure the ability to change itsform or function with time ( t ) under stimuli such as pressure,temperature, wind, water, or light.Figure1 depicts a schematicof the 1-, 2-, 3-, and 4D concepts. The concepts of 1-, 2-, and3D represent line, plane, and 3D space structures, respectively.For 4D, the concept of changes in the 3D structure ( x, y, z) withrespect to time ( t ) is added, as indicated by curved arrows. The3D printed structure can change its color, shape, function, orother characteristics in response to stimuli suchas temperature,water, ultraviolet (UV) rays, or magnetic energy. 4–7 In this

review, we rst compare the similarities and differences be-tween 3D printing and 4D printing. We then discuss the mainfactors and recent trends in 4D printing technology such assmart materials and designs. Finally, we discuss the currentapplications of 4D printing.

3D printing is dened as an additive manufacturingmethod for the fabrication of 3D structures by layering ma-terials depending on a predetermined design. However, thereis currently a tendency to use 3D printing as a representativeterm for all additive manufacturing technologies. 3D printingtechnology is a convergence technology that uses materials,designs, and 3D printers for certain applications since it wasrst described in 1984. 8 Recently, many countries have de-clared 3D printing technology to be an innovative productionmanufacturing technology leading the global megatrend for themanufacturing industry. Open-source projects such as RepRapin the United Kingdom began after the expiration of major 3Dprinting patents owned by Stratasys Inc. and 3D Systems Inc.,leadingto the sharing of important technical data for printerandprocess creation. Such open-source projects also led to theexplosive development and application of 3D printing tech-nology to diverse industries, suchas sports, culture, electronics,and aerospace. Recently, innovative advances have been madein 3D printing technology with regard to printing processes andmaterials. Carbon 3D Inc. recently announced a new con-tinuous liquid interface production method that can print an

1 3D Printing Group, Computational Science Research Center, Korea Institute of Science and Technology, Seoul, Republic of Korea.2 Department of Multimedia Science, Sookmyung Women’s University, Seoul, Republic of Korea.3 Department of Mechanical Systems Engineering, Hansung University, Seoul, Republic of Korea.*These two authors contributed equally to this work.

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object 100 times faster than existing methods by creating anoxygen depletion zone in liquid resins. 9 New or smart ma-terials and multimaterial composites have been also intro-duced with dramatic improvements in performance. Forcertain functional applications, many efforts have focused onthe fabrication of new materials with desired functionality bymixing nanomaterials such as graphene, carbon nanotubes,and functionalized nanoparticles or biomaterials with exist-

ing 3D printing materials.10,11

4D Printing

While 3D printing technology has been used to make staticstructures from digital data in 3D coordinates, 4D printingadds the concept of change in the printed conguration overtime, dependent on environmental stimuli. The 3D and 4Dprinting processes are nearly equivalent, with printing be-ginning with product design in 3D modeling programs suchas computer-aided design followed by printing the designwith a 3D printer. Smart design and smart materials are thekey differences of 4D printing compared to 3D printing, as4D printed structures may transform in shape or function(Table 1). Therefore, the design of 4D printed structuresshould be fully preprogrammed in detail by accounting forany anticipated time-dependent deformations of objects. 12

Another core aspect of 4D printing technology is smart ma-terials, which can become more expandable, exible, or de-

formable in response to applied stimuli. Lastly, to implement4D structures by combining design with smart materials, new4D printer concepts should be developed, or existing highlyfunctional 3D printers should be improved.

4D printing materials

To construct 3D structures, materials such as plastic, me-tal, and ceramics are widely used as 3D printing materials.However, most of these materials are not applicable to 4Dprinting because of their lack of reaction to external stimuli.Recently, more materials with functional properties havebeen printed using 3D printers by adjusting or modifyingprocess parameters such as nozzle characteristics, tempera-ture, and printing environment. Therefore, the proper choiceof materials is important for 4D printing.

Recent developments have yielded several smart materi-als for functional 3D printing or 4D printing, including ma-terials that self-assemble in response to temperature, UV rays,self-degradation, or water absorption. 8,13–19 One recently in-troduced 4D printing material utilizes water absorption cap-abilities to produce a 4D printed structure. A research group atMassachusetts Institute of Technology (MIT) has printedmultimaterials that change shape underwater. 16 The groupused two different materials with different porosities and waterabsorption capacities to print bimaterial structures. Thesestructures have a porous water-absorbing material on one side

FIG. 1. Schematic of 1-, 2-, 3-, and 4D concepts. A 4D structure is a structure ( x, y, z) made by 3D changes over time ( t ).Arrows indicate the direction of change with respect to time.

Table 1. Simple Comparison Between 3D and 4D Printing Technology

3D printing 4D printing

Materials ThermoplasticsMetalsCeramicsBiomaterials or nanomaterials

Self-assembled materialsMultimaterialsDesigned materials

Examples: shape memory alloy/polymer,self-degradation/deformation materials, temperature-or UV-driven materials

Design 3D digital information (scanning, drawing) 3D digital information for change (deformation)Printer 3D printer

Examples: stereolithographyapparatus, material extrusion,and selective laser sintering

Smart 3D printerExamples: modied nozzle, binder, and laser

Multimaterial 3D printerExamples: solid/liquid, solid/solid, gradient materials,

and nanocompositesChange As printed Changed after printing in shape, color, function, etc.Application Jewelry, toys, fashion, entertainment,

automobile, aerospace,defense, and bio/medical devices

Dynamically changing conguration for all applicationsby 3D printing

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and a rigid waterproof material on the opposite side. Afterprinting, the structure was inserted into a water bath, leading towater absorption on both sides. The water-absorbing side in-creased in volume because of water absorption, whereas theother side remained unchanged, resulting in bending towardthe rigid side as shown in Figure 2a. In the water bath, astraight-line structure with programmed hinges could be de-formed into a 3D conguration over time. This approach,

which involves combining materials with different porositiesor structures into one printed structure, originates from themultimaterial printing method.

Multimaterials or functionally graded materials for 3Dprinting were reported by Oxman, who mimicked the cellularstructure of materials that dynamically change in shape. 4

Oxman introduced a multimaterial printer using cement andconcrete foam along with software for variable propertymodeling. Porosity was controlled by changing the ratio of aluminum to cement. This process has the ability to dy-namically mix, grade, and vary the ratios of materials toproduce functional components with continuous gradients.Such components can be highly optimized for performance,efciently use materials, reduce waste, and possess highly

customizable features with added functionalities.Temperature-responsive materials such as shape memory

polymers and alloys are well-known 4D materials because of their ability to contract or expand in response to changes intemperature. When the temperature of a shape memorypolymer is increased above the critical temperature for shapechange, the deformed structure returns to its original struc-ture. As shown in Figure 2b, shape memory polymers can beutilized as lamentous material in the material extrusion(ME) method to print articial structures that can be refoldedfrom their unfolded states simply by changing the tempera-ture. Figure 2c shows a functional 3D robotic device that canself-fold from a single planar material into a 3D structure. 6

These devices have several functions, including temperature-

activated self-folding, actuation controlled by an externalmagnetic eld, and degradation in liquids such as acetoneor water.

Photo- or light-responsive materials can also serve as 4Dprinting materials, as a color reaction in a 3D printed object

can be triggered by UV irradiation or sunlight. 20,21 UV-responsive polymer chains that include azo compounds candeform as shown in Figure 3. UV light energy induces thedeformation of the polymer chain structure, triggering a colorchange from white to purple. This color change is caused by ashift in the polymeric chain from the ordered nematic phaseto the disordered phase. 22 When maintained in a dark envi-ronment, the color of the printed object returns to the original

white. Instead of color, the shapes or surface patterns of somelight-responsive materials have been known to change viasimilar mechanisms. These light-responsive materials can beused in shopping bag packaging, aerospace structures, pho-tovoltaics, and biomedical devices. 22 UV-responsive materi-als are available as ME laments, and promising applicationsof UV-responsive materials exist in areas such as the fashionand entertainment industries.

Biomaterials are a major class of smart materials for4D printing. Functional materials capable of autonomouslydegrading in the body were studied in late 1990 with 3Dprinting technology. 23 Because the human body is composedof dynamically and continuously moving systems, each partis exposed to a unique environment and must therefore re-

spond over time to differences in conditions such as tem-perature or body uid. Therefore, printed body parts orstructures should have dynamic functional behaviors foruse in vivo . In this regard, biocompatible materials shoulddegrade in the body environment within a certain periodof time. Temperature-, water-, or UV-responsive reactionsoccur in the aforementioned materials over periods rangingfrom a few seconds to days in the body, whereas the bio-materials may take several years to degrade completely in auid environment. Well-known self-degrading materials arepolylactic acid (PLA) and poly-caprolactone (PCL). PLA isthe major material for the ME method, and PCL is the majormaterial for the selective laser sintering methods. 24,25 Bothmaterials have been reported to degrade over a period of

several years until the polymer chain has completely dis-solved in body uid. 25,26

However, these approaches did not consider any time-dependent changes except for degradation. Morrison et al.created a supporting structure using the stereolithography

FIG. 2. Examples of 4D printing technology. (a) Transformation of a structure from 1D to 3D with water absorption materials,printedby Massachusetts Institute of Technology (MIT). Reprinted withpermissionfromTibbits. 14 Copyright ª 2014JohnWiley& Sons, Ltd. (b) Temperature-responsive design of articial hands by Korea Institute of Science and Technology. (c) Functional3D device fabricated by MIT. Reprinted with permission from Miyashita et al. 6 Copyright ª 2015, IEEE.

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(SL) apparatus method with liquid PCL for infants with se-vere tracheobronchomalacia to prevent airway collapse dur-ing normal breathing. 27 PCL is known as a biodegradable andbiocompatible material. 28 While Morrison et al. printed thesupporting structure with a customized design for patientsless than 1 year old, the splint accommodated changes in

airway size by expanding as the patient grew for 3 years asshown in Figure 4. After 3 years, the materials were com-pletely removed from the body and the fully grown airwayswere able to function unaided. 29,30 This example demon-strates that, in combination with 3D printing technologyand designs for changes in shape and material properties,

FIG. 3. Printing examples using a photoresponsive material: (a) cat, (b) ower, and (c) schematic description of changesin the polymer microstructure in response to light.

FIG. 4. (a) Digital Imaging and Communications in Medicine images of a patient’s computed tomography scan used togenerate a 3D model of the patient’s airway via segmentation in Mimics. (b) Stereolithography representation of atracheobronchial splint demonstrating the bounded design parameters of the device. (c) Final 3D printed poly-caprolactonetracheobronchial splint. (d) Mechanism of action of the tracheobronchial splint in treating tracheobronchial collapse intracheobronchomalacia. Reprinted with permission from Morrison et al. 27 Copyright ª 2015, The American Association forthe Advancement of Science.

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4D biomaterials are extremely useful and promising formedicine.

4D printing can be useful for a wide variety of healthcareapplications, ranging from nanoparticle design to tissueengineering to the manufacture of self-assembling human-scale biomaterials. 31–33 Organovo Holdings Inc., U.S.A.,has been involved in several bioprinting projects focused onthe development of functional human tissues. 34 This com-pany is developing an articial human liver using 4Dprinting technology. The potential of this technology tocreate programmable biological materials with changeableshapes and properties can be a foundation for enabling smartpharmacology, personalized medicine, and programmablecells and tissues that can precisely target treatments fordiseases. 35,36

Smart design

In addition to smart materials, one of the core techniquesfor 4D printing is the design of materials for structuralchange. Although the smart material itself plays a pivotal rolein transforming a printed object into another shape or con-guration, sophisticated design based on a rigorous under-standing of mechanisms, predicted behaviors, and requiredparameters should be performed to achieve controllable re-sults. 37,38 The powerful advantage of 3D printing technologyis the capability to create complex 3D shapes with variedmaterial distributions through spatial arrangement. By de-signing the orientation and location of smart materials such asshape memory polymer bers within composite materials, wecan facilitate morphological changes in response to external

FIG. 5. 4D printing with a shape memory polymer. (a) Schematic of the folding mechanism and (b) representative imagesfor folding by heat. Reprinted with permission from Ge et al. 39 Copyright ª 2013, AIP Publishing LLC.

FIG. 6. (a) Left: rendered illustration of the primitive folding. This design is composed of bars and disks. The disks in thecenter act as stoppers. By adjusting the distances between the stoppers, it is possible to set the nal folding angle. Right:video frames of the fabricated primitive folding in water over time. (b) Fabricating a time-varying curve. From left to rightand top to bottom, the curve deforms over time into a different shape. Reprinted with permission from Raviv et al. 40

Copyright ª 2014, rights managed by Nature Publishing Group.




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stimuli. For example, Ge et al. investigated the design vari-ables that are important for creating a laminated architec-ture. 39 They examined various ber orientations and volumefractions as well as the magnitude of the curvature as afunction of the composite geometry, applied mechanicalload, and thermal history. A two-layer laminate consisting of one lamina layer with bers at a prescribed orientation andone layer of pure matrix material was constructed (Figure 5a).

When the samples were heated, the printed two-layer lami-nates transformed into bent, coiled, and twisted strips; foldedshapes; and complex contoured shapes with nonuniform andspatially varying curvatures depending on each sample’sprescribed ber architecture (Figure 5b).

Raviv et al. fabricated self-evolving structures with a varietyof highly specic joint designs for folding, curling, twisting,linear expansion, and shrinkage as well as other transforma-tions in the presence of water. 40 They used a computationaldesign approach to characterize topology transformations of the various self-evolving structures. As a design step, thegroupmodeled three primitives: a linear stretching primitive, a ringstretching primitive, and a folding primitive. Two differentmodels were created for stretching, and another model was

created for folding.The length of the linear stretching primitivecould be controlled over time. By adjusting the ratio of ex-panding material in the middle to the rigid disks as shown inFigure 6a, they were able to change the length of stretchingand the percentage of linear expansion in the joint. The ringstretching primitive was based on expansion of the ring-likeshape into a bar. Because the inner and outer rings wereprinted with different materials, the inner ring expanded andforced the structure to deform into a bar shape once thestructure was submerged in water. By controlling the radius

of the ring, They were able to adjust the stretching length of the structure (Figure 6b). In the folding primitive, the rigiddisks in the center acted as stoppers. By adjusting the dis-tances between the stoppers, the desired folding angle couldbe created.

The importanceof smart design in 4D printed objects is clearin the fabrication of self-assembling origami, where a at sheetautomatically folds into a complicated 3D component. 5 Ge

et al. extended theconcept of self-assembling origami by usingspatial variations in the material composites to control shapedeformation in an origami structure. 39 They also fabricated aself-folding and self-opening box with two-layer printed activecomposites as hinges connecting six inactive plates of a stiff plastic as shown in Figure 7a. Ge et al. developed a theoreticalmodel that considered design parameters and the complexthermomechanics of the composite hinge structures to fab-ricate various active origami structures. 5 More specically,the group focused on characterizing hinge behavior with re-spect to the hinge bending angle as a function of geometricparameters (hinge length and laminate/lamina conguration),thermomechanical loading parameters (stretching strain),and programming parameters (deformations and heating/

cooling temperatures).41

Using this model, Ge et al. couldactuate the hinges created from composites with polymerbers, making the hinges fold to a prescribed angle. Finally,the group created a number of active origami components,including a box, a pyramid, and two origami airplanes basedon different design parameters. They demonstrated that thefolding of the printed composite hinges depended on thematerial properties of the polymers (including the shapememory behavior of the bers), the lamina and laminate ar-chitecture, and the thermomechanical loading prole.

FIG. 7. (a) Folding processes of cubes printed with a composite material with a hinge made of shape memory polymer.Reprinted with permission from Ge et al. 39 Copyright ª 2013, AIP Publishing LLC. (b) Folding processes of cubes printedwith a single shape memory material. (c) Hinge design of a heat-induced folding cube made from a single material.

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We have printed a hexahedral planar structure using ME3D printer with a thermal polyurethane (TPU), which is ashape memory polymer lament. While all of the parts canbe printed with TPU, we designed hinges with differentthicknesses to allow for the proper deformation and bendingof each arm as shown in Figure 7b and c. We also connectedthe six faces by inserting TPU lines through predesignedholes to improve shrinkage strain as temperature was in-

creased.

Printers for 4D printing

In general, conventional 3D printing materials such asPLA or acrylonitrile butadiene styrene (ABS) are optimizedfor the printing parameters (e.g., temperature and nozzledesign) that are already preset in each 3D printer. Smartmaterials with specic functionalities or multicomponentmaterials may cause problems in current 3D printers, as thesematerials may become agglomerated, clogged, or resolvedduring the printing process. Therefore, several techniqueshave been adopted for 4D printers. We used a printer with acoated nozzle tip that was adapted for stable printing of TPU

with the ME method. This printer also has a heating bed forproper heat circulation during the printing process. BecauseTPU has a high thermal expansion coefcient and com-presses in the nozzle when heated, the printing nozzle easilybecomes clogged. In addition, the molten TPU may ow overcold end regions, leading to poor adhesion between layers orpores in the printed line. To suppress the overow of moltenmaterials and to reduce friction, the TPU printer nozzle iscoated with polytetrauoroethylene and has a barrel that is1.2–1.5 times longer than the typical nozzles used for PLA orABS. In addition, the heating device is placed close to thenozzle to minimize heat loss.

The printing of multimaterial components is a key factorfor the 4D printing of structures with adaptability and desired

functionalities. Multimaterial printers may allow printedstructures to have colors, shapes, or electronic properties thatchange in response to UV rays, light, heat, or water. Multi-material printers can print bimaterial structures or function-ally graded structures by mixing two or three differentmaterials within one printed structure. Several printers havealready been developed for multimaterial printing. Lopeset al. performed discrete multimaterial fabrication and pro-duced functional electroactive polymer actuators via non-heated ME. Lopes et al. also produced biomedical scaffoldsand 3D structural electronics via a hybrid manufacturing sys-tem that integrated SL and direct print (DP) technologies tofabricate 3D structures with embedded electronic circuits. 42,43

A hybrid SL/DP system was designed and developed using a3D Systems SL 250/50 machine and an nScrypt micro-dispensing pump integrated with the SL machine via orthog-onally aligned linear translation stages.

A corresponding manufacturing process was also developedusing this system to fabricate 2D and 3D monolithic structureswith embedded electronic circuits. The process involved partdesign, process planning, integrated manufacturing (includingmultiple starts and stops of both SL and DP and multipleintermediate processes), and postprocessing. SL providedsubstrate/mechanical structure manufacturing, and intercon-nections were achieved using DP of conductive inks. Simplefunctional demonstrations involving 2D and 3D circuit de-

signs were accomplished. 44 Espalin et al. introduced the useof professional-grade ME systems for discrete multimaterialfabrication. 24 A multimaterial, multitechnology ME systemwas developed and constructed to enable the production of parts using either discrete multimaterials or build processvariations (variable layer thickness and road width). Twolegacy ME machines were modied and installedonto a singlemanufacturing system to allow strategic, spatially controlled

thermoplastic deposition of multiple materials with multipleextrusion nozzles during the same build. This automatedprocess was enabled by a build platform attached to a pneu-matic slide that moved the platform between the two MEsystems, an overall control system, a central PC, a customprogram (FDMotion), and a graphic user interface. Contourand raster road widths are parameters that can be selectedfrom a certain range using the ME part preparation software,and these road widths are controlled by the ME machineduring the manufacturing process by feeding more or lessmaterial through the nozzle for a given extrusion head speed.

Summary and Perspective

The rapid development of materials, design, and printersfor 3D printing gave birth to the 4D printing concept. Re-cently, 4D printing has been gaining attention because 4Dprinted structures have the capability to change in form orfunction over time in response to stimuli such as pressure,temperature, wind, water, and light. 4D printing technology,which uses smart materials, designs to forecast changeprocesses, and smart printing, can be applied to variouselds from simple changes to bioprintings for organisms. 45

The U.S. Army has already tried to adapt this technology toproduce camouage textiles that help soldiers hide in cer-tain environments by bending the light reected from theclothing. 46 4D printing can also be used to create systemsfor the International Space Station that can be transported

easier than existing systems produced with conventionalmethods.Because 4D printing and 3D printing are different from

conventional manufacturing technology, these new technol-ogies can reduce the manufacturing time and human laborrequired to assemble machines or goods. Furthermore, 4Dprinting can reduce the time and labor required for logistics,transportation charges, and the volume of goods that must betransported. Finally, 4D printing can address the consumerneed for personal designs or options on consumer goodsitems such as smart phones or watches.

Acknowledgments

This work was supported by a Korea Institute of Scienceand Technology internal project. The authors acknowledgesupport from theMinistry of Culture, Sports and Tourism andthe Korea Creative Content Agency in the Culture Technol-ogy Research & Development Program. The authors alsoacknowledge support from the Center for Advanced Meta-Materials (CAMM) funded by the Ministry of Science, ICT,and Future Planning as a Global Frontier Project (CAMM-No. 2014063701).

Author Disclosure Statement

No competing nancial interests exist.


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Address correspondence to: Myoung-Woon Moon

3D Printing GroupComputational Science Center

Korea Institute of Science and Technology Hwarangno 14-gil 5, Seongbuk-gu

136-791 Seoul Republic of Korea

E-mail: [email protected]


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