Current Status of 4D Printing Technology and The

Virtual and Physical Prototyping

ISSN: 1745-2759 (Print) 1745-2767 (Online) Journal homepage: http://www.tandfonline.com/loi/nvpp20

Current status of 4D printing technology and the potential of light-reactive smart materials as 4D printable materials Steven K. Leist & Jack Zhou To cite this article: Steven K. Leist & Jack Zhou (2016): Current status of 4D printing technology and the potential of light-reactive smart materials as 4D printable materials, Virtual and Physical Prototyping, DOI: 10.1080/17452759.2016.1198630 To link to this article: http://dx.doi.org/10.1080/17452759.2016.1198630

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Date: 05 July 2016, At: 00:43

VIRTUAL AND PHYSICAL PROTOTYPING, 2016 http://dx.doi.org/10.1080/17452759.2016.1198630

Current status of 4D printing technology and the potential of light-reactive smart materials as 4D printable materials Steven K. Leist and Jack Zhou


Mechanical Engineering and Mechanics Department, Drexel University, Philadelphia, PA, USA ABSTRACT

ARTICLE HISTORY

The onset of multi-material 3D printing and the combination of smart materials into the printable material has led to the development of an exciting new technology called 4D printing. This paper will introduce the background and development into 4D printing, discuss water reactive 4D printing methods and temperature reactive 4D printing, modelling and simulation software, and future applications of this new technology. Smart materials that react to different external stimuli are described, along with the benefits of these smart materials and their potential use in 4D printing applications; specifically, existing light-reactive smart materials. 4D printing has the prospective to simplify the design and manufacturing of different products and the potential of automating actuation devices that naturally react to their environment without the need for human interaction, batteries, processors, sensors, and motors.

Received 12 April 2016 Accepted 1 June 2016

Introduction Additive manufacturing is a rapidly growing industry that has become a multibillion-dollar business since its creation in the 1980s. Research has shown that the 3D printing sector will continue to grow economically and access further industries in the next 10 years (Gebler et al. 2014, Kietzmann et al. 2015, Lu et al. 2015, Rayna and Striukova 2016). 3D printers have the capabilities of printing multiple hard and soft materials (plastics, resins, metals, polymers, food, and biomaterials), highly accurate models, and complex designs. The decreasing price of 3D printers is providing more people with access to this technology, which is creating a community of designers, engineers, and hobbyist focused in the additive manufacturing sector. 3D printers are no longer an industrial or academic research tool, but is now a commercial product that can be found in homes. 3D modelling software provides users the ability to create custom designs or download premade designs from other users, due to the Internet of Things, and print these designs from their personal 3D printers. Although 3D printing has many benefits, it still suffers from rigid and static parts that cannot actuate or transform shape right off the print bed. If users desire to make moving parts, such as hinges or actuators, they must assemble multiple parts together after prints. Post processing of 3D printed parts can still be tedious and time consuming just like machined parts. The 3D printer’s print bed size is another issue because it limits the CONTACT Steven K. Leist

[email protected]

© 2016 Informa UK Limited, trading as Taylor & Francis Group

KEYWORDS

4D printing; smart materials; 3-dimensional printing; rapid prototyping; 3D printing; additive manufacturing

number of parts and size of parts that can be printed in one iteration. One solution to these problems is to use a material that can dynamically change shape over time when exposed to an external stimulus after it has been 3D printed.

4D printing and self-assembly There have been many advances in 3D printing technology, namely, the creation of multi-material printing and development of new printable materials. Some researchers are 3D printing materials that can change shape over time. These materials are called smart materials (Ge et al. 2013, Campbell et al. 2014a, Raviv et al. 2014, Tibbits 2014, Zhou et al. 2015). Rigid materials can be 3D printed along with smart materials to create specific areas of a part that act as joints and hinges for bending. This process of 3D printing parts that change shape over time when exposed to an external energy has been termed 4D printing by Skylar Tibbits from Massachusetts Institute of Technology (MIT) (Ge et al. 2013, Campbell et al. 2014b, Raviv et al. 2014, Tibbits 2014, Zhou et al. 2015). He argues that construction must be made smarter and solve the problems of wasting large amounts of energy, materials, money, and time for building. These issues can be solved using design programs and software to embed information into the materials that makes the material and construction more accurate. At the beginning, Tibbits proposed a process called self-


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S. K. LEIST AND J. ZHOU

assembly, which uses small units that form larger structures when exposed to external energy (Tibbits 2012). Self-assembly may be applied to 1D strands forming organised 2D and 3D shapes, along with 2D planar shapes forming 3D objects. Self-assembly may not be efficient for every purpose, which means different sectors and applications must be identified that benefit most from self-assembly (Tibbits 2011). Tibbits proposed his concept of Logic Matter, materials with programming built into themselves that could be applied to the self-assembly of complex structures (Tibbits 2010). The research is based on a combination of computer science and architectural design, which uses three concepts of construction information input: user input, environmental input, and material input (Tibbits and Cheung 2012). Tibbits proposed that materials could be implanted with information storage that could receive an input, analyse the information, and aid in the assembly of a structure. This process is demonstrated using geometrical constraints on the smallest assembly units to form larger structures when exposed to random mechanical energy and vibration (Figure 1). Tibbits states that realistic building materials such as metals, plastics, and wood could be imbued with these intelligent properties and he uses the expansion and contraction of wood when exposed to moisture as an example of activating wood’s Logic Matter. This led to the developments of Tibbits’ 4D printing research using water absorbable materials that can be 3D printed, which is discussed later in the paper. Now, other researchers are experimenting with different materials that react to different activation stimuli to use in 4D printing. Some of these researchers are using

Figure 1. Skylar Tibbits SAL’s design for disassembled building units possessing unique geometric constraints that form two separate spheres when induced with random mechanical energy. Adapted with permission from SAL, ‘Chiral Self-Assembly’, S. Tibbits, A. Olson, and Autodesk Inc.

different 3D printing methodologies compared to Tibbits, in order to produce their 4D printed products. 4D printing opens up many new design techniques and applications that were not possible with 3D printing technology. A market research report predicts that the 4D printing industry could be worth $63.00 million in 2019 and $555.60 million in 2025 (Markets and Markets 2015). The market report predicts that 4D printing may find applications in the automotive, textiles, construction, healthcare, utility, aerospace, and military industries. It is predicted the defense and military sector would have the largest share, followed by the aerospace industry. These are just a minor number of application ideas for 4D printing that reflect a number of 3D printing methodologies and activation energies. This paper presents the current research focused on 4D printing methods, smart materials that allow for its activation, and the future applications for this exciting innovative technology.

Smart materials Smart materials are a class of materials that produce a change in shape or property (rigidity, colour, texture, transparency, volume) when exposed to an external stimulus. Shape memory alloys (SMAs) are a popular smart material that possess two phases: martensite phase (low temperature) and austenite phase (high temperature). These two phases allow SMAs to alter shape and return to its original shape when exposed to high temperatures (Brinson et al. 1996, Auricchio et al. 2014). SMAs are bent into a desired shape then annealed at temperatures above its transition temperature to retain a permanent shape. After annealing, the SMA can be deformed into a temporary custom shape. When the SMA is heated above the transition temperature it returns to its permanent shape. Nitinol, a SMA with a composition of nickel and titanium developed at the Naval Ordinance Lab, has found popularity in the automotive (Jani et al. 2014), aerospace (Humbeeck 1999), biomedical (Morgan 2004), robotics (Wu and Schetky 2000, Mazzolai et al. 2012, Jani et al. 2013, Cianchetti et al. 2014, Laschi and Cianchetti 2014), and soft actuation industries (Figure 2) (Cianchetti et al. 2014). Another class of smart materials that are gaining popularity are shape memory polymers (SMPs). SMPs possess the ability to remember a permanent shape and transform to a temporary shape when exposed to a number of external stimuli such as temperature (Figure 3) (Srivastava et al. 2010, Yang et al. 2014, Zarek et al. 2015), pressure (Ramuz et al. 2012, Tee et al. 2012), water (Yang et al. 2006), pH-levels (Qiu and Park 2012), magnetism (Leng et al. 2011, Zhao

VIRTUAL AND PHYSICAL PROTOTYPING


Figure 2. A bioinspired robotic tentacle that uses SMA springs for actuation. Adapted with permission from Cianchetti et al. (2014) under Multidisciplinary Digital Publishing Institute (MDPI).

et al. 2013), or light (Lendlein et al. 2005, Habault et al. 2012, White 2012, Zhang 2016). SMP materials could create products that react to their environment automatically without the need for complex, heavy, and expensive electronic actuation systems. Some other smart materials, called self-healing materials, that possess the ability to react to external stimulus and repair themselves, which may prove useful for devices exposed to extreme environments(Hager

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et al. 2010, Hager et al. 2015). SMPs do have their drawbacks compared to SMAs; they possess low strength, low moduli, and low operating temperature; however, they are cheaper, possess high strain recovery, low density, biocompatibility, and biodegradability when compared to SMAs (Khoo et al. 2015). One major benefit of SMPs is their 3D printability. Most methods of 4D printing use an Objet500 Connex3 Polyjet printer from Stratasys to dispense the SMP(Ge et al. 2013, Ge et al. 2014; Tibbits 2014). Polyjet printing dispenses the liquid material on a layer-by-layer basis and cures each layer using UV light. Polyjet printers have the ability to print multiple materials simultaneously that can be either rigid or elastic. Researchers can take advantage of the Connex3 printer’s ability to dispense multiple materials to print the static plastic material that acts as the structure of a part combined with the pre-programmed shape memory material for bending and twisting motion. Researchers at Harvard’s School of Engineering and Applied Sciences (SEAS) are capable of 3D printing a single smart material using a syringe nozzle and photopolymerisation of their smart material for their 4D printing research (Sydney Gladman et al. 2016). Another method of photopolymerisation in 4D printing uses stereolithography (SLA) 3D printers to cure the smart material from a pool of resin (Zarek et al. 2015).

Figure 3. Examples of a heat reactive SMP that reverts to its original shape when exposed to 70°C for (a) a cardiovascular stent, (b) Eiffel Tower, (c) and a bird. Adapted with permission from Zarek et al. (2015) under the John Wiley and Sons License.

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Figure 4. (a) A 3D printed 1D strand that transforms into the letters ‘MIT’ when exposed to water. (b) Flat 2D plate folding into a cube when submerged in water. Adapted with permission from Tibbits (2014) under the John Wiley and Sons License (c) 3D printed 2D flat object that transforms into a 3D Octahedron. Adapted with permission from SAL, ‘4D Printing: Self-Folding Truncated Octahedron’, SAL, Stratasys ltd., and Autodesk Inc. (d) Fused deposition modelling (FDM) printed wood based material that folds when submerged in water. Adapted with permission from SAL, ‘Programmable Wood’, SAL, C. Guberan, E. Demaine, Autodesk Inc., and Institute of Computational Design, University of Stuttgart.

Smart materials are essential to the development of 4D printing research. There are many smart materials in development; however, not every smart material can be 3D printed. Also, smart materials do not need to possess shape change in order to be important to 3D printing research. Materials that possess the ability to change colour, hardness, or transparency may become important in camouflage technology, signalling for users, detecting foreign substances, and biomedical applications (Lendlein 2010). The different types of 4D printing activation methods will increase as more and more smart materials are investigated.

Water activated 4D printing At the helm of 4D printing research is Skylar Tibbits, the director of the Self-Assembly Lab (SAL) at MIT. Before 4D printing, Tibbits was focused in the areas of programmable matter, adaptive materials, and self-assembly inspired by inserting programming into physical materials (Tibbits and Cheung 2012). One of SAL’s first prototypes was a 1D strand made of plastic units consisting of hinges and pins. Each hinge is designed to bend in a specific direction and connected to each other to form long chains. These plastic 1D chains transform into 3D shapes when given random mechanical energy because of the material’s embedded bending limitations. Other versions of self-assembly used combinations of wires, magnets, and embedded bending angles to form 3D objects from 2D planar shapes or assemble a 3D object from dissembled pieces (Ge et al. 2013, Campbell et al. 2014a, Raviv et al. 2014, Tibbits 2014, Zhou et al.

2015). This research assisted in the development of 4D printing by expanding this research to 3D printing a multi-material 1D strand that could form into a cube or the 2D letters ‘MIT’ (Figure 4(a)) (Tibbits 2014). Specific areas of the strand are made of rigid plastic and the hinges that cause the bending are made of the water activated smart material. Tibbits along with Stratasys are 3D printing smart materials that expand when exposed to moisture (hygroscopic). Researchers can control the bending angle and direction of the expanding material by placing rigid materials in select areas that prevent the hygroscopic material from expanding in unintended directions. The material is a hydrophilic polymer that can expand 150% when exposed to water. The multi-material printing uses a static rigid material as the framework of the object and the smart material as the programming of the object. Specially designed hinges use a combination of rigid material and hygroscopic material to induce the transformations. When the entire part is submerged in water, only the areas with hygroscopic material will actuate. These hinges can be placed in strategic location on a 1D strand or flat 2D plane to form different shapes such as an octagon or cube (Tibbits 2014) (Figure 4(b) and 4(c)). The technique could also be applied to transforming a 3D shape to a different 3D shape or dissemble into a flat compact shape (Figure 4(d)). SAL proved that 4D printing eliminates the need for wires, motors, and on-board power to make objects move right off the print bed. Actuated objects such as micro robots could become lighter and simpler, which reduces number of chances to fail. Along with 4D



printing plastics, SAL is experimenting with other 3D printable material such as wood, textiles, and carbon fibre combined with smart materials to create actuated objects (Tibbits 2014, Ward 2014). This expansion of materials allows for possibilities of 4D printing furniture, automotive and aviation parts, and smart clothing that reacts to the user’s body or environment. Recently, researchers at Harvard’s SEAS have created a 4D printing system inspired by biomimetics (Sydney Gladman et al. 2016). A biocompatible hydrogel composite ink that is composed of stiff cellulose fibrils embedded in a soft acrylamide matrix that swells when immersed in water. The fibrils act as stiff filaments that creates anisotropic swelling in the soft matrix, similar to the cell wall swelling found in plants (Burgert and Fratzl 2009). The material is 3D printed at ambient temperature and physically crosslinked using UV polymerisation. The anisotropic orientation is created when the composite ink is extruded through the 3D printer nozzle and causes shear alignment of the fibrils. The composite ink swells along the longitudinal direction of the 3D printed filament or in the direction of the printing path. The bending can be controlled when the material is 3D printed in a bilayer architecture. Researchers found that there is no observable actuation if a flat 2D pattern is 3D printed using the composite ink without the fibrils; however, when the fibrils are included in the ink then twisting or bending can be observed. The researchers were able to create the printing path using 3D modelling software and simulate the bending mechanics of the component. Different printing path geometries are 3D printed such as spirals, grid, and combination of spirals and grid patterns that produced negative Gaussian curvature, positive Gaussian curvature, and a combination of positive and negative Gaussian curvature, respectively. Orientation of the print path (90° and 45°) induces different shapes such as cylindrical curving and twisting if the bilayer is printed in a grid pattern (Figure 5). After printing, the component can be submerged in water and take minutes to transform; however, this shape change is not reversible. The 4D printed hygroscopic parts can display dual shape change when it is exposed to different temperatures of water (Room temperature water or 50°C) by adding temperature sensitive poly (N-isopropylacrylamide). SEAS 4D printing system provides a method of creating 3D printed parts that change shape in water immersion using a single material. Their modelling and simulation program allow designers to create products that can actuate and change shape based on material and printing properties: filament diameter, spacing between print paths, and path orientation. The technique opens up the

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Figure 5. Folding behaviour of a 4D printed water activated hydrogel composite that displays different folding behaviours depending on the geometry of the 3D printed path. Adapted by permission from Sydney Gladman et al. (2016). under the Nature Publishing Group License

possibilities for 4D printing to access the biomedical market because of the biocompatible material. Cells could be seeded into more complex scaffold designs, better strategies for drug delivery systems, and improvement of minimally invasive medical devices. Hygroscopic 4D printing shows how objects can react intelligently to their environment without the need for human interaction. Intelligent sensors or structures could be deployed in extreme environments that prove difficult for human exploration such as deep sea exploration. The hygroscopic polymers can be made of soft materials that may prove useful when handling biological substance such as tissue, organs, or live flora and fauna. Self-assembly shelters, structures, and barriers could be deployed in areas of natural disasters to protect victims and refugees. These shelters can be transported in compact flat shapes to take advantage of the shipping volume, then transform into their ultimate form at their final destination.

Heat and stress activated 4D printing An advantageous activation method for 4D printing is the use of high temperatures to trigger the smart material for shape change purposes and the concept of self-assembling origami. Researchers at the University of Colorado, Boulder have created a 4D printing technique that uses a combination of heat and stress to activate a shape memory composite to bend at different rates and directions depending on the design of the hinges. Glassy polymers in the form of fibres exhibit shape memory effects (SMEs) when heated above their glass transition temperature (Tg) and are 3D printed within an elastomeric matrix (Figure 6(a) and 6(b)). This

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Figure 6. (a) The PAC architecture made up of a top layer pure elastomeric matrix material and bottom layer of glassy polymer and elastomeric matrix material. (b) The process of activating the PAC by heating the material, applying a load to the material, allowing it to cool, and releasing the stress to allow the material to bend. Reheating the PAC allows the material to return to its original flat shape. Adapted with permission from Ge et al. (2013) under the AIP Publishing LLC License.

combination of elastomer and glassy polymer fibres creates a soft composite, which the team has named printed active composites (PACs) (Ge et al. 2013). The volume and orientation of the glassy polymer fibres inside the polymer matrix can be specified using CAD software. These characteristics are important design parameters for the PAC since they can determine bending angle, speed, storage modulus, strain, and fixity. Researchers 3D printed a double layer composite, which specified the top layer as strictly elastomeric material and the bottom layer as a combination of glassy polymer and elastomeric material. Researchers tested different volume fractions of the glassy polymer and elastomer material (0.10, 0.28, 0.40, and 0.60) and different fibre orientations (0°, 15°, 30°, 45°, 60°, 75°, and 90°) along the load direction. They found fixity increases with increasing volume fraction because of the stiffness inside the composite and higher storage modulus with 0° fibre orientation. Different geometric transformations are created depending on the fibre orientation and location of the fibre such as bending, rolling, twisting, and wave forms (Figure 7(a) and 7(b)).

The researchers were successful in implementing their 4D printing techniques for the creation of self-assembling origami structures: a box, pyramid, a three hinged airplane, and a five hinge airplane. A box with the six rigid sides connected by PAC hinges is 3D printed in a flat shape. The PAC hinges are pre-programmed with definitive bending angles using the relation of PAC hinge length and applied strain to determine the bending angle. The walls of the box should bend at a 90°, so the hinges required a strain of 20% to be applied while the material is heated above its glass transition temperature (Tg), cooled, then releasing the prestrain. The component managed to form the box shape with minor irregularities due to inconsistencies when straining the material (Figure 8(a) and 8(b)) (Mao et al. 2015). Similar methods are used to create self-assembling and disassembling trestles, but with different geometrical parameters for the PAC lamina and matrix laminates (Ge et al. 2014, Wu et al. 2016). Researchers found that the inclusion of smart materials in their 3D printing components could save time and material. 3D printing a 20 mm × 20 mm × 20 mm hollow

Figure 7. (a) Initial flat shape of PAC without heat activation. (b) Flat PAC material displays bending, twisting, and wavy shapes when heat and stress is applied depending on the orientation of the PAC fibre. Adapted with permission from Ge et al. (2013) under the AIP Publishing LLC License.


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Figure 8. (a) Initial 3D printed flat surface that (b) folds into a 3D box when heat and stress is applied. Adapted with permission from Mao et al. (2015) under the Nature Publishing Group. (c) A flat 3D printed trestle that contains two fibers with different glass transition temperatures. (d) Trestle possesses the ability to assemble when exposed to hot water and disassemble if the temperature of the water is increased (Wu et al. 2016). Adapted with permission from Wu et al. (2016) under the Nature Publishing Group.

cube with a wall thickness of 1 mm could take 10 minutes when printed in a flat 2D shape then activated to become a 3D cube. However, direct 3D printing a cube with the same dimensions takes 3 hours to print and post processing of removing support material can take numerous hours (Ge et al. 2014). Although, heat-activated 4D printing requires additional involvement using pre-strain to change shape, it reduces the post processing time when compared to typical 3D printing methods.

3D printing of SMAs SMAs are a popular smart material in industry and research; however, the production of complex SMA designs proves to be difficult. One group combines polyjet printing with SMA wire (Meisel et al. 2014), while some other groups have used selective laser melting (SLM) to create complex models using Nickel–Titanium (NiTi) SMA (Shishkovsky et al. 2012, Haberland et al. 2014, Walker et al. 2016). The SLM process produces parts on a layer-by-layer basis by melting powder using a laser. In these cases, the powder is the NiTi SMA powder. The desired complex shapes can be designed using CAD software then sent to the SLM printer for processing. A layer of powder is dispensed and flattened with a roller or wiper. The material is bonded by melting each layer of powder to the previous layer using the laser that traces the design of sliced object. It should be noted that the 3D printing of SMAs has not been defined in

4D printing, but it has been included in the paper because it outlines the similar process of 3D printing a smart material that can change shape over time when exposed to an external stimuli. One research group has demonstrated two-way shape memory in a 3D printed SMA using the two-way SME (Clare et al. 2008). The process involves lowering the temperature of the 3D printed SMA cantilever below its martensite phase and deforming the cantilever. Next, the cantilever is heated above its austenite phase and deformed. The annealing process is repeated until the SME is observable in the cantilever. The cantilever will deform into its martensite shape when the temperature falls below the martensite temperature (Figure 9(a)) and transform into a different shape when the temperature is raised above its austenite temperature (Figure 9(b)) (Clare et al. 2008). There are some issues with 3D printing SMAs. NiTi processing is prone to oxidation and requires a minimal oxygen environment, which is typically processed in an argon atmosphere. Also, it is difficult to prevent impurities in the SMA parts and adjusting for ideal SLM printing properties. SMAs do have their benefits. Researchers have shown that SLM produced NiTi parts possess favourable biocompatibility properties (Habijan et al. 2013). SMA 3D printing may prove useful for designing stents and scaffolds with various permeability. 3D printing of SMAs has provided an option for researchers to design more complex SMA parts, which may lead to more applications and industries.

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Figure 9. (a) A SLM NiTi cantilever beam bending upwards in the low temperature martensite phase and (b) transforms to a straight state in the high temperature austenite phase. Adapted with permission from Clare et al. (2008) under the Springer License.


Light-activated SMPs Light is an effective activation technique because it is an abundant source of energy, wireless, and controllable. However, it can be difficult to transform light energy into mechanical energy for use in SMPs. Light-activated SMPs have been used in areas of self-assembly structures, complex folding methods, transformative surface deformations (Ubukata et al. 2007, Kravchenko et al. 2011), UV sensor and filters (Kim et al. 2010a, 2010b), and soft robotics. Some 4D printing research that uses light-activated materials actually uses the heat from the light source to activate the shape change properties. Examples of light-activated SMP research and their future potential in 4D printing research will be discussed. Origami packing is a complex folding method that is beneficial for space saving solutions and compact transportation of products (Dubey and Dai 2006, 2012, An et al. 2013, Miyashita et al. 2015). A method, similar to microfabrication techniques, uses polymer sheets that are stretched and covered with a photomask with the design of the bending hinges of the origami shape (Ryu et al. 2012). The sheet is irradiated with light on the photomask side in order to generate stress relaxation through the thickness of the exposed polymer sheet. The photomask and strain are removed, and the desired shape of the product is cut out of the polymer sheet. Once the strain is removed, the hinges will bend in order to reach mechanical equilibrium. The process presents a simpler activation method for smart materials because it uses a single material, a hinge with no moving parts, no additional wires, and no communication system. Another method of light-activated smart polymers uses pre-stressed polystyrene films with black ink printed on areas of the sheet designated as hinges (Liu et al. 2012; Lee et al. 2015). The method uses an infrared

light to heat the material above its glass transition temperature (Tg); the areas with black ink heat up quicker and bend faster than the remaining material. Ink printed on top of the sheets bends towards the light direction and ink printed on the bottom of a transparent sheet bends away from the light. Altering the ink geometry, ink width and number of printed lines produces different bending angles, bending times, and light intensity requirements. For example, a polystyrene film can be cut into the shape of a flat unassembled cube. Next, the hinges are printed using black ink from an inkjet printer. Objects, such as a grain of rice, can be placed into the centre of the unassembled cube (Figure 10(a)). The hinges of the flat film can be bent to form a cube when the film is heated above its Tg using a light source (Figure 10(b)). The 3D shape can transform back into a flat sheet when the entire polystyrene film is heated above its Tg (Figure 10(c)) (Liu et al. 2012). This research does not represent traditional 3D printing since the addition of material layer-by-layer is not applied and the only printing involved is inkjet printing black ink onto the surface of the polystyrene film. Other methods of light-activated SMPs use photochromophores such as azobenzene and spiropyrans that react to light through photoisomerisation. Lightactivated SMPs have been used in the areas of microrobotics and soft robotics because of their ability for quick shape change and reversibility. One area of robotics that benefits from the use of photo-activated SMPs is microswimming robots (Camacho-Lopez et al. 2004, Milam et al. 2010). SMPs simplify robotics by eliminating the need for on-board power systems, processors, and motors that are sensitive to moisture and may damage if exposed to water. Also, the removal of these devices reduces the weight on board the robots, which means the size of the robots can be reduced and potentially used within the organic body. Researchers have


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Figure 10. A polystyrene film in the cut-out shape of a box with inkjet printed black hinges that absorb heat faster than the remaining material and capable of folding into a cube for encapsulation. (a) A grain of rice is placed in the centre of the polystyrene flat box. (b) The rice can be encapsulated upon heating the polystyrene film with a light source and folding the hinges mechanically. (c) The polystyrene box unfolds when reheated above Tg using a light source. Adapted with permission from Liu et al. (2012) 1764–1769 under the Royal Society of Chemistry Publishing Contracts & Copyright Executive.

created a swimming flexible microrobot that mimics the movement of a flagellum to drive itself (Huang et al. 2015). The microbot uses a light-driven liquid crystal film (LDLCF) composed of azobenzene chromophores. The film bends when exposed to UV light and returns to its original shape when exposed to visible white light. The microrobot is composed of a head, LDLCF, and flexible tail. The head of the robot may also be attached to a LDLCF gripper that can open when exposed to UV light and close when exposed to visible light. The possibilities of 3D printing complex networks of light-reactive shape memory material that can be lined with fibre optic wires to induce shape change in a compact and clean system. The use of light-activated SMPs may be the next step in 4D printing research. There are many projects on lightactivated SMPs, but the 3D printability of these polymers has yet to be seen (Lendlein et al. 2005, Chen et al. 2010, Lee et al. 2011, 2012, Mahimwalla et al. 2012, White 2012, Kuksenok and Balazs 2015, Zhang 2016). Introducing 3D printing allows for complicated and accurate hinge and actuator designs to be created, which allows for more complicated movements. Also, different multipart products can be printed in a single iteration, instead of assembling multiple materials and parts manually. Light-activated SMPs can induce shape change without applied pre-strain and can bend in different directions depending on the direction of light or different wavelength of light (Lee et al. 2012). The reversibility of these polymers allows for the assembly and disassembly of 4D printed products, but it also allows for the creation of motors for generation of energy and heat (An et al. 2013, Iqbal and Samiullah 2013). Although some 4D printing materials already induce reversible shape change, they require human interaction to pre-strain

the materials, activate the materials with an external energy, then release the strain for transformation; lightactivated 4D printing could remove this need and fully automate shape transformations without further human interaction.

4D printing software The SAL uses a research development program called Project Cyborg by Autodesk Research to simulate the 4D printed object’s bending algorithm. Originally, Tibbits used an algorithm that carries out complex self-assembly structures using one chain of feedbacks. The single chain can be created to meet a final destination over any curve, surface, or volume; however, this does not mean the chain can form any single path because that chain cannot intersect itself or it will become entangled (Tibbits 2011). Now, Project Cyborg allows for specific areas of a part, such as joints and hinges, can be assigned with the smart material properties and provide a visualisation of the object’s bending process. This allows users to predict if folds will block another fold’s path or improve bending efficiency. Designers can make their changes before 3D printing their prototypes, which saves materials and time. Another 4D printing software, Kinematics by Nervous System, takes custom objects and folds the design into a compact shape that can be 3D printed in a single run. The software has been used for 4D printing clothing because of its advanced folding algorithm. First, a person’s body is 3D scanned then imported into the program. Next, the user can alter the clothing by changing the clothing’s shape and pattern. The custom designed clothing can be draped to show what it will


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look like on the individual or folded into a compact profile. That piece of clothing is 3D printed with the proportions specific to that individual. The object is made of small triangular hinged fragments that allow for movement and form (Figure 11(a)). Typically, a dress is larger than the print bed of most 3D printers and would require multiple prints in order to be pieced together. Kinematics allows the dress to be folded in a way that models the behaviour of how a person would fold the dress to fit the print volume of the 3D printer (Figure 11(b)). Project Cyborg and Kinematics show that there is another important area of 4D printing industry, 4D printing software, which requires further research and development. Different algorithms may need to be developed for different activation methods of the smart material and the effects of different 3D printing techniques (fused deposition modelling (FDM), Stereolithography, select laser sintering (SLS), Polyjet) need to be analysed for each smart material’s activation method. Modelling and simulation programs for fused deposition modelling and 3D printing algorithms may need to be altered because the printing geometry, speed, direction, and shear stress on the filament may affect the anisotropic nature of different smart materials (Duigou et al. 2016). Programmers will need to decide if they include access to all activation methods of a 4D printed part in the software or will different activation methods pertain to specific software.

Future of 4D printing and applications A number of 4D printing methods and potential 4D printable smart material have been described in this paper. Polyjet printing and syringe printing are the most popular forms of 4D printing, but other 3D printing technologies such as FDM, SLS, and stereolithography are

future processes that could be used in 4D printing technology; however, some 4D printing technologies may require multi-materials and multiple nozzles, which limits what 3D printing methods can be used. Exploring different printing methods may allow for different smart materials to be 3D printed that are stronger, lighter, induce different property changes, and react to different stimuli. 4D printing may be applied to textiles and camouflage technology by altering not just colour and patterns, but also altering the texture of the surface. Octopi, cuttlefish, and other cephalopods use their visual senses to detect their environment and camouflage themselves according to their surroundings (Hanlon 2007, Barbosa et al. 2008, Hanlon et al. 2010). Smart materials could be 3D printed onto textiles and alter the shape and rigidity of the clothing to give a textured look in changing environments. Camouflage could react to different weather effects and change dynamically according to the environment. Clothing could react to the environment’s temperature or the wearer’s body temperature allowing ventilation or insulation for dynamic cooling and heating. There is the possibility of extending the longevity of parts that experience a higher probability of damage by 3D printing specific areas with a class of smart materials called self-healing polymers, which can restore damaged and cracked areas when exposed to external stimuli (Hager et al. 2010, Habault et al. 2012). Researchers have proven that 3D printable materials such as Acrylonitrile butadiene styrene (ABS), polylactic acid (PLA), and nylon can be 3D printed on the surface of different textiles: polyester, nylon, cotton, polypropylene, and polywool (Pei et al. 2015). Other possibilities are 4D printing textile applications would be embedding the smart material into the fibre and 3D printing the fibre into desired shapes. Layers of cloth with adhesive are

Figure 11. (a) Triangular hinged fragments that allow for flexibility and movement in the 3D printed structure from the linking of small rigid components. (b) A simulation of the Kinematics dressed folded and compressed into a compact form that will fit the build volume of a 3D printer. Adapted with permission from Nervous System (http://nervo.us).



dispensed on top of each other on a layer-by-layer basis. Each layer of the cloth is cut with a laser in the shape of the object. The outer layer of the cloth can be removed when the printing process is completed (Peng et al. 2015). Another method is 4D printing intelligent parts and integrating them with textiles after post processing (Mikkonen et al. 2013). Initially, smart textiles may find applications in the medical field, space exploration, and military; however, once these applications are concrete then they may transfer over to the sports and casual fashion industries (Langenhove and Hertleer 2004). The advanced manufacturing sector could see major benefits of using 4D printing by reducing the number of assembly parts, reducing required resources, and easier transportation of products. Parts can be 3D printed in flat compact shapes for easy transportation and shipping; next, a source of energy could transform the object into a desk, chair, or storage unit when it reaches its final destination. This process may be able to work reversely; if users are relocating, the product could be activated to change into a more compact shape for easier transportation. Areas with extreme environmental conditions such as space exploration or natural disaster areas could benefit from the automated transformation of 4D printed products. Areas that require quick and temporary assistance could deploy compact objects that transform into shelters and protect refugees or evacuees from the elements. Another potential future application of 4D printing may be applied to manufacturing in space. NASA has performed 3D printing on the International Space Station with the help of Made in Space, Inc. Cubesats could be 3D printed and deployed from a space station, which reduces the resources needed for rockets, fuel, and scheduling time of cubesat launches could be reduced. The walls or frame of the cubesat could be 3D printed in a flat shape and activated with light to transform into its 3D shape. One issue with using smart materials in these devices is how to prevent the 4D printed products from reactivating to external stimuli, since space can have vastly different temperature and light environments. However, users could take advantage of the light-reactive smart materials and create solar panels that constantly track the movement of sunlight, providing constant power and potentially reducing the size and power requirements of on-board batteries. An application of additive manufacturing gaining popularity is 3D bioprinting. Bioprinting is the deposition of biological material using additive manufacturing technologies. There are three main methods of bioprinting: inkjet, micro-extrusion, and laser assisted (Murphy and Atala 2014). Cell diffusion and cell density throughout

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the volume of the scaffolds remains inconsistent and limited near the centre of the scaffold (Kim et al. 1998, Solchaga et al. 2006, Chan and Leong 2008, Melchels et al. 2010, 2011). One solution is the 4D printing of scaffolds in a flat plane, seed the cells, and transform the scaffold into its final shape. First, the scaffold is printed in a flat 2D shape, then the cells are seed onto the surface of the scaffold in culture medium. An external stimulus causes the 2D scaffold to sequentially fold into its 3D shape. 3D printing of an thermal sensitive SMP that can fold in sequence has been proven and could be applied to scaffold design (Lendlein et al. 2005, Chen et al. 2010, Lee et al. 2011, 2012, Mahimwalla et al. 2012, White 2012, Kuksenok and Balazs 2015, Mao et al. 2015, Zhang 2016). One issue of combining 4D printing and bioprinting is the biological compatibility of many different SMPs has not been tested. Also, their reaction stimuli that causes their shape change would have to be safe for the biological body and easily controllable. 4D bioprinting could become a major advancement in the design and application of bio-scaffolds and biomedical devices.

Conclusion Additive manufacturing is still a growing industry; new materials, printing methods, software, and machines are constantly being developed and improved, which makes 4D printing a more realistic and accessible industry. 4D printing may promote the use of 3D printers for creating final products instead of prototypes. 4D printing has introduced new design techniques that may reduce the amount of energy, materials, time, and money to create products. The point of 4D printing is to simplify the design and manufacturing process that allows for complicated designs and actuated products to be created from just the base materials. Light-reactive 3D printable smart materials may be the future of 4D printing technology; specifically, photochemical reactions and not temperature reaction due to light, which possess the ability for fast reactions and reverse shape change. The future of 4D printing lies in the controllability of the assembly and disassembly of printed products, automating the reactions of 4D printing products (removing the human interaction), expanding the activation stimuli for 4D printable smart materials, and creating software that models and simulates shape change behaviour. Once these challenges are solved, this exciting and new technology will hopefully simplify and support countless people’s lives in many different applications and industries.

Disclosure statement No potential conflict of interest was reported by the author.

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Funding This work was supported by the Division of Civil, Mechanical and Manufacturing Innovation [grant number 1538318].


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