Direct Ink Writing Based 4D Printing of Materials and Their Applications

Abstract

4D printing has attracted academic interest in the recent years because it endows static printed structures with dynamic properties with the change of time. The shapes, functionalities, or properties of the 4D printed objects could alter under various stimuli such as heat, light, electric, and magnetic field. Briefly, 4D printing is the development of 3D printing with the fourth dimension of time. Among the fabrication techniques that have been employed for 4D printing, the direct ink writing technique shows superiority due to its open source for various types of materials. Herein, the state-of-the-art achievements about the topic of 4D printing through direct ink writing are summarized. The types of materials, printing strategies, actuated methods, and their potential applications are discussed in detail. To date, most efforts have been devoted to shape-shifting materials, including shape memory polymers, hydrogels, and liquid crystal elastomers, showing great prospects in areas ranging from the biomedical field to robotics. Finally, the current challenges and outlook toward 4D printing based on direct ink writing are also pointed out to leave open a significant space for future innovation.

Keywords: 4D printing; direct ink writing; hydrogels; liquid crystal elastomers; shape memory polymers.

Figure 1

Brief summary of 4D printing by direct ink writing. a) Schematic illustration of direct ink writing. Reproduced with permission.^{[ 150 ]} Copyright 2006, Wiley‐VCH. b) The important elements evolved in 4D printing by direct ink writing. The actuation methods: heat, electric field, magnetic field, light, and liquid. Major categories of materials: shape memory polymers, liquid crystal elastomers, hydrogels, and ferromagnetic domains. Representative functionality (reproduced with permission.^{[ 137 ]} Copyright 2017, Royal Society of Chemistry) and applications: tissue engineering (reproduced with permission.^{[ 94 ]} Copyright 2019, Wiley‐VCH), biomedicine (reproduced with permission.^{[ 16 ]} Copyright 2017, American Chemical Society), electronics (reproduced with permission.^{[ 60 ]} Copyright 2019, Elsevier), smart grippers (reproduced with permission.^{[ 84 ]} Copyright 2019, Wiley‐VCH), and soft robotics (reproduced with permission.^{[ 11 ]} Copyright 2018, Nature Publishing Group).

Figure 2

4D printing of thermoplastic and partially crosslinking SMPs via DIW. a) Schematic diagram of constructing 3D structures using PLA‐based inks layer‐by‐layer under UV irradiation. b) Self‐expanding behavior of printed PLA/Fe₃O₄ scaffold in response to a 30 kHz alternating magnetic field and the schematic diagram to support a narrow vessel. a,b) Reproduced with permission.^{[ 16 ]} Copyright 2017, American Chemical Society. c) Optical and SEM images of a freeform 3D printed spiral using PLA/Ag@CNFs ink. d) High electrical conductivity of PLA/Ag@CNFs nanocomposites as the printed components lighted up an LED light under a 2.5 V voltage. c,d) Reproduced with permission.^{[ 56 ]} Copyright 2017, American Chemical Society. e) Electro‐responsive shape transformation of a U‐shaped scaffold printed by PLMC/CNT ink under 25 V within 16 s. Reproduced with permission.^{[ 60 ]} Copyright 2019, Elsevier.

Figure 3

4D printing of thermoset SMPs via DIW. a) Shape programming and self‐folding transformation of a box‐like scaffold printed by ESBO/BFDGE/CNFs ink under heat stimulus (scale bars: 1 cm). Reproduced with permission.^{[ 10 ]} Copyright 2016, Nature Publishing Group. b) Schematic diagram of printing tough and isotropic epoxy assisted by UV light and post‐thermal curing. Reproduced with permission.^{[ 64 ]} Copyright 2018, Royal Society of Chemistry. c) Thermal‐responsive shape change of printed vase and toy based on semi‐IPN elastomer (scale bars: 6 mm). Reproduced with permission.^{[ 65 ]} Copyright 2018, American Chemical Society. d) Synthesis procedure of PDAPU. e) Representative complex structures printed by PDAPU (scale bar: 1 cm). d,e) Reproduced with permission.^{[ 66 ]} Copyright 2019, Royal Society of Chemistry. f) Schematic printing process of graded multimaterial assisted by dynamic photomask. g) Design of a periodic structure with three graded regions and its compressive deformation (scale bars: 10 mm). h) Sequential shape recovery process of an auxetic lattice structure within 120 s at 26 °C (scale bars: 10 mm). f–h) Reproduced with permission.^{[ 68 ]} Copyright 2019, Wiley‐VCH.

Figure 4

4D printing of LCEs via DIW. a) Alignment of LCE chains in the printing direction during DIW process. b) Design of printing paths in different layers to control shape transformation of c) twisted helical ribbon structures. a–c) Reproduced with permission.^{[ 80 ]} Copyright 2017, American Chemical Society. d) Synthesize procedure of LCE with modified molar ratio and schematic change of LCE molecules during printing. e) Representative planar‐to‐3D (scale bars: 1 mm) and 3D‐to‐3D′ shape transformations (scale bars: 5 mm). d,e) Reproduced with permission.^{[ 81 ]} Copyright 2018, Wiley‐VCH. f) Design of asymmetric structure of LCE/PDMS and the use of it to alter focusing properties due to contraction of LCE in elevated temperatures. Reproduced with permission.^{[ 82 ]} Copyright 2018, Wiley‐VCH. g) The utilization of printing two LCE inks with different T _NIs at different locations to realize sequential shape change from disk to cone (scale bars: 5 mm). Reproduced with permission.^{[ 84 ]} Copyright 2019, Wiley‐VCH. h) Fabrication procedure of a long LCE fiber in three steps. Reproduced with permission.^{[ 85 ]} Copyright 2019, American Chemical Society. i) Sequential shape change of a manually folded 3D printed triangulated structure upon staged heating (scale bars: 1 cm). Reproduced with permission.^{[ 86 ]} Copyright 2019, American Association for the Advancement of Science.

Figure 5

4D printing of water‐ and thermal‐responsive hydrogels via DIW. a) 4D printed flower mimicked biomimetic shape transformation based on anisotropic alignment of fibrils within the structure (scale bars: 5 mm, inset: 2.5 mm). Reproduced with permission.^{[ 92 ]} Copyright 2016, Nature Publishing Group. b) Folding and unfolding behavior of printed tube under the absence and presence of Ca²⁺ ions. Reproduced with permission.^{[ 94 ]} Copyright 2017, Wiley‐VCH. c) Fabrication procedure of polyrotaxane‐based monoliths with inter‐ring hydrogen‐bonding. Reproduced with permission.^{[ 95 ]} Copyright 2017, Wiley‐VCH. d) Thermal‐responsive poly(N‐isopropylacrylamide)‐based hydrogel valve to control open and closure at different temperatures. Reproduced with permission.^{[ 99 ]} Copyright 2015, Cambridge University Press. e) Structural design of a skin‐like sensor. Reproduced with permission.^{[ 100 ]} Copyright 2017, Royal Society of Chemistry. f) Folding and defolding behavior of a printed hydrogel cubic box upon cooling and heating in hydrated state. Reproduced with permission.^{[ 101 ]} Copyright 2017, Wiley‐VCH. g) Shape transformation of a printed whale‐like hydrogel composite upon thermal stimulus. Reproduced with permission.^{[ 102 ]} Copyright 2018, American Chemical Society. h) Shape‐morphing process of multilayer hydrogel from a C‐shaped structure to a 3D spring at 48 °C. Reproduced with permission.^{[ 103 ]} Copyright 2019, MDPI.

Figure 6

4D printing of photo‐ and other responsive hydrogels via DIW. a) Schematic diagram of 3D printing of poly(N‐isopropylacrylamide)‐based hydrogels/MWCNTs on PDMS and the composition of composite hydrogel ink. Reproduced with permission.^{[ 104 ]} Copyright 2019, American Chemical Society. b) Comparison of diameter shrinkage in photoresponsive hydrogel and hydrogel composite upon NIR irradiation. Reproduced with permission.^{[ 105 ]} Copyright 2017, American Chemical Society. c) Photoresponsive shape change of a printed flower consisting of F127DA/PLGA/GO hydrogel composite within 240 s (scale bars: 5 mm). Reproduced with permission.^{[ 107 ]} Copyright 2019, Elsevier. d) The saddle‐like shape change of a biphasic structure with different heights consisting of alginate and alginate/PDA composite (scale bar: 5 mm). Reproduced with permission.^{[ 108 ]} Copyright 2019, IOP Publishing. e) Magnetic‐responsive shape change of a printed seajelly‐like structure (scale bar: 15 mm). Reproduced with permission.^{[ 109 ]} Copyright 2019, Wiley‐VCH. f) Shape transformation of multistimuli‐responsive hydrogels in the shape of a printed octopus under a magnetic field, and g) different shape changes of printed leaves with different printing paths under heat stimulus. f,g) Reproduced with permission.^{[ 112 ]} Copyright 2019, Wiley‐VCH. h) Schematic diagram of printing programmed cells and chemicals within a scaffold. Reproduced with permission.^{[ 114 ]} Copyright 2018, Wiley‐VCH.

Figure 7

4D printing of elastomer‐derived ceramic and ferromagnetic domains via DIW. a) One method to construct 4D ceramics by printing Miura‐ori pattern and flexible joints on a prestretched elastomeric substrate followed by releasing external force to trigger bucking (scale bars: 1 cm). b) FEA analysis of the other method to construct 4D ceramics by designing the directional relationship between printed filaments and prestretched substrate. a,b) Reproduced with permission.^{[ 119 ]} Copyright 2018, American Association for the Advancement of Science. c) Structural design of multimaterial in both composition and location. d) Printed multimaterial and characterization of alignment of magnetized particles in targeted area (scale bar: 5 mm). c,d) Reproduced with permission.^{[ 120 ]} Copyright 2015, Nature Publishing Group. e) Schematic illustration of oriented ferromagnetic particles during printing under a magnetic field with programmed magnetic polarity. f) Shape transformation of auxetic behavior within 0.5 s under a magnetic field of 200 mT. e,f) Reproduced with permission.^{[ 11 ]} Copyright 2018, Nature Publishing Group.

Figure 8

4D printing of other types of materials via DIW. a) Schematic mechanism of transformation from a 2D planar to 3D bending shape. b) Fabrication procedure to construct a cubic box in three steps. a,b) Reproduced with permission.^{[ 123 ]} Copyright 2019, Wiley‐VCH. c) Optical images of 3D printed VO₂/PDMS THz photonic crystals with a woodpile structure. Reproduced with permission.^{[ 124 ]} Copyright 2019, Royal Society of Chemistry. d) Schematic diagram of thermal‐responsive supercapacitor consisting of SMA substrate, rGO/CNT electrode, and LiCL electrolyte. Reproduced with permission.^{[ 125 ]} Copyright 2019, Wiley‐VCH. e) Schematic graph of color and shape transformation of CuSO₄/PLGA scaffold in response to ethanol and water, respectively. f) Transformation of printed circular scaffold after printing, washing, and salt‐leaching in sequence. e,f) Reproduced with permission.^{[ 126 ]} Copyright 2018, Elsevier. g) Multiplex bilayer lattice design consisting of four PDMS‐based materials, multimaterial 4D printing process, and printed lattice of a planar human face and the morphing curved shape under heat stimulus. Reproduced with permission.^{[ 131 ]} Copyright 2019, National Academy of Sciences.

Figure 9

4D printing via DIW for self‐healing functionality. a) Shape memory assisted self‐healing process of a damaged spiral by heating above the melting temperature of PCL. Reproduced with permission.^{[ 65 ]} Copyright 2018, American Chemical Society. b) In situ self‐healing of a printed spider‐like structure under NIR irradiation at damaged location and the enlarged optical image of damaged crack (scale bar: 1 cm). Reproduced with permission.^{[ 66 ]} Copyright 2019, Royal Society of Chemistry. c) Recycling process of vitrimer epoxy and the comparison of a printed octopus after recycling and self‐healing (scale bar: 8 mm). Reproduced with permission.^{[ 137 ]} Copyright 2017, Royal Society of Chemistry.

Figure 10

4D printing via DIW for biomedical applications. a) Programming and shape recovering of a printed hollow tube to simulate vascular repair (scale bars: 2 mm). Reproduced with permission.^{[ 65 ]} Copyright 2019, American Chemical Society. b) Fluorescence microscopy images of the printed cell‐laden hydrogel tubes during cell culture in 7 days. The first, second, and third rows corresponded to live, dead, and all cells in the printed structure, respectively. Reproduced with permission.^{[ 94 ]} Copyright 2019, Wiley‐VCH. c) Fluorescence microscopy images of the printed PEGDA/carbomer hydrogel during cell culture within 48 h (scale bars: 100 µm). d) Calculated cell viability of PEGDA/carbomer hydrogel. c,d) Reproduced with permission.^{[ 112 ]} Copyright 2019, Wiley‐VCH.

Figure 11

4D printing via DIW for electronics. a) Schematic illustration of a shape‐changing sensor with high detecting precision when the liquid level decreased. b) Comparison of relative resistance change between flat and folded sensors immersed in ethanol. a,b) Reproduced with permission.^{[ 60 ]} Copyright 2019, Elsevier. c) Printed electrical component stretched upon heating at 85 °C and lighted up a LED light. Reproduced with permission.^{[ 10 ]} Copyright 2016, Nature Publishing Group. d) Printed Ag NPs/elastomer as a flex sensor to detect the motion of finger bending and the relative resistance change during four cycles. Reproduced with permission.^{[ 127 ]} Copyright 2017, IOP Publishing. e) Programming of printed ferromagnetic domains in response to different magnetic directions to trigger different circuits. Reproduced with permission.^{[ 11 ]} Copyright 2018, Nature Publishing Group. f) THz transmittance spectra of 3D photonic crystals at different temperatures. Reproduced with permission.^{[ 124 ]} Copyright 2019, Royal Society of Chemistry. g,h) Schematic and experimental shape change with a bending angle of the printed supercapacitor upon heat stimulus. Reproduced with permission.^{[ 125 ]} Copyright 2019, Wiley‐VCH. i) The printed multilevel triboelectric nanogenerator was used as an energy harvester to supply power for LEDs under sequential compressive modes. Reproduced with permission.^{[ 68 ]} Copyright 2019, Wiley‐VCH.

Figure 12

4D printing via DIW for soft robotics and actuators. a) The shape change of a printed SMPC gripper lifting a bolt under a 1 V voltage. Reproduced with permission.^{[ 56 ]} Copyright 2019, American Chemical Society. b) Two‐way shape change of a LCE gripper when the electric current was on and off. Reproduced with permission.^{[ 83 ]} Copyright 2018, IOP Publishing. c) Grabbing behavior at low temperature and not grabbing behavior at high temperature by programming LCEs with different T _NIs with different printing directions (scale bars: 10 mm). Reproduced with permission.^{[ 84 ]} Copyright 2019, Wiley‐VCH. d) Twining printed LCE fibers together to mimic the bending motion of bicep muscle fibers to grab objects upon heat stimulus. Reproduced with permission.^{[ 85 ]} Copyright 2019, American Chemical Society. e) Biomimetic bending and anchoring behavior of an artificial tendril consisting of photoresponsive hydrogel/PDMS. The light source moved from the base part to tip part of the structure successively to actuate the shape change (scale bars: 10 mm). Reproduced with permission.^{[ 104 ]} Copyright 2019, American Chemical Society. f) Mechanical fastening behavior of magnetic‐assisted multimaterials upon liquid immersion (top: as‐printed valve; bottom: immersed in ethyl acetate. Scale bars: 15 mm). Reproduced with permission.^{[ 120 ]} Copyright 2015, Nature Publishing Group. g) Printed water‐responsive hydrogel converted chemical energy to mechanical work, lifting a coin upward. Reproduced with permission.^{[ 95 ]} Copyright 2017, Wiley‐VCH. h) Printed ferromagnetic domains carried and released a pill under a rotating magnetic field. Reproduced with permission.^{[ 11 ]} Copyright 2018, Nature Publishing Group. i) Mechanically and pneumatically actuated gripper based on prestretched 2D elastomers. Reproduced with permission.^{[ 123 ]} Copyright 2019, Wiley‐VCH.

Figure 13

The summary statistics of publications about 4D printing by DIW. a) Numbers of publications versus year. b) Pie chart about categories of 4D printed materials by DIW.