3D Printing of Hydrogels for Stretchable Ionotronic Devices

Abstract

In the booming development of flexible electronics represented by electronic skins, soft robots, and human-machine interfaces, 3D printing of hydrogels, an approach used by the biofabrication community, is drawing attention from researchers working on hydrogel-based stretchable ionotronic devices. Such devices can greatly benefit from the excellent patterning capability of 3D printing in three dimensions, as well as the free design complexity and easy upscale potential. Compared to the advanced stage of 3D bioprinting, 3D printing of hydrogel ionotronic devices is in its infancy due to the difficulty in balancing printability, ionic conductivity, shape fidelity, stretchability, and other functionalities. In this review, a guideline is provided on how to utilize the power of 3D printing in building high-performance hydrogelbased stretchable ionotronic devices mainly from a materials' point of view, highlighting the systematic approach to balancing the printability, printing quality, and performance of printed devices. Various 3D printing methods for hydrogels are introduced, and then the ink design principles, balancing printing quality, printed functions, such as elastic conductivity, self-healing ability, and device (e.g., flexible sensors, shape-morphing actuators, soft robots, electroluminescent devices, and electrochemical biosensors) performances are discussed. In conclusion, perspectives on the future directions of this exciting field are presented.

Patterning meets gels: Advances in engineering functional gels at micro/nanoscales for soft devices

“…the feature size of a few hundred micrometers in general, depending on the nozzle size and appropriate rheology modifiers. 28,32 Inkjet printing uses piezoelectrically controlled head to produce microdroplets and affords arbitrary patterning with a higher resolution of tens of micrometers, or even down to sub-10-μm scale assisted by electrohydrodynamic jet printing. 33 In addition, tipbased ink printing such as dip-pen nanolithography and polymer pen lithography also allows facile generation of submicron gel patterns (Figure 2, middle).…”

“…For patterning in a direct‐write manner, the inks can be precisely delivered to the desired region without affecting the background. Extrusion of viscoelastic inks has been extensively exploited for 3D printing of hydrogels with the feature size of a few hundred micrometers in general, depending on the nozzle size and appropriate rheology modifiers 28,32 . Inkjet printing uses piezoelectrically controlled head to produce microdroplets and affords arbitrary patterning with a higher resolution of tens of micrometers, or even down to sub‐10‐μm scale assisted by electrohydrodynamic jet printing 33 .…”

“…Owing to the versatility of gel networks, engineering functional gel materials on multi-scales have been reviewed recently from various aspects, highlighting the role of hydrogels and 3D printing in stretchable ionotronics, bioelectronics and biomedical engineering (e.g., bioprinting, cell study). 6,7,28,29 Given the rapid evolution of this field, herein we intend to depict emerging trends from the recent 5-year research updates of tailoring the function gels at micro/nanoscales with patterning tools, with the focus on applications in soft electronic and optical devices. As illustrated in Figure 1B, based on the role of patterning, we begin with introducing the generation of micropatterned gel building blocks via inkand light-based approaches, which act as the vital elements in microscale or high-density integrated devices.…”

Digital light processing 3D printing of hydrogels: a minireview

“…2,3 Owing to the dimensions and flexibility in endowing materials with micro-to nano-scale structural architectures and new functionalities beyond those of their bulky counterparts, 4 3D printing has received growing attention and will continue to favor broad applications including biomedical devices, intelligent systems, energy devices, and soft robotics. [5][6][7][8] With the continuous in-depth research on the forming mechanisms and material systems, various 3D printing technologies have been developed in recent years, including fused deposition modelling (FDM), 9,10 direct ink writing (DIW), [11][12][13] direct inkjet printing (DIP), 14,15 selective laser sintering (SLS), 16,17 stereolithography apparatus (SLA), 18 digital light processing (DLP) 13,19 and so on. [20][21][22] Among these printing technologies, DLP belonging to light-based printing utilizes projection light to polymerize precursors to obtain pre-designed architectures.…”

In situ 3D printing of liquid metal-hydrogel hybrid for multifunctional soft bioelectronics and devices

“…Owning to the huge prospect and growing market for wearable and implantable electronic devices, research on soft bioelectronics and bio-interface is gaining increasing attention from multidisciplinary elds including material science [1][2][3][4][5] , neural science 6 , tissue engineering 7,8 and wound healing 9 and etc. Among which, hydrogel-based bioelectronics and devices are emerging with the intrinsic advantages such as the high water-content, high biocompatibility, ionic conductivity, and excellent compliance to skin and tissues 5,10 . For instances, mechanically-matched hydrogel coatings can improve the biocompatibility by remarkably reducing the foreign body sensation 11 and immune responses 12 .…”