4D Printing of Shape‐Memory Hydrogels for Soft‐Robotic Functions

Abstract

a new active dimension of "time" has evolved, leading to the new concept of 4D printing, which refers to the ability of 3D printed structures to actively transform over time in response to environmental stimuli. [5] Smart or stimuliresponsive materials have the unique ability to return from a temporary deformed state, induced by heat, light, pH, ultrasound, chemical substances, [6][7][8][9][10][11][12][13] etc., to their permanent, i.e., original, shape, thus exhibiting advantages for applications in numerous sectors, such as sensors and actuators, [14] tissue engineering, [15] bio-separation devices, and controlled drug delivery. [16][17][18][19][20][21] To date, two main types of materials have been considered to realize 4D printing: shape memory polymers (SMPs) and hydrogels. SMP-based 4D printing offers structural modification and recovery in response to temperature, which are established through complex functionalities of multiple or reversible shape switching, and such printing may provide inspiration for the molecular architecture of shape memory hydrogels (SMHs). However, SMPs cannot completely replace hydrophilic soft materials due to the limitations arising from their sustainability in wet environments, rigidity, material permeability, and biological compatibility. [22] Therefore, mechanically active, self-shaping hydrogels that undergo desired, programmable 3D shape transformations and execute mechanical tasks as soft robots under an external trigger have recently attracted growing interest. The use of a hydrogel system in soft robotic counterparts offers distinct advantages: simple designs, low cost, processability at low temperatures and in aqueous environments, and the possibility to mimic human functionality. [23,24] Directed movement of hydrogels can be obtained by expansion/contraction, for example, by isotropic volume expansion or shrinkage of homogeneous hydrogels or by the bending/unbending approach, which represents an anisotropic deformation and often involves fabrication of a hydrogel structure with two layers with different swellability values. [25][26][27][28][29] The first hydrogel-based bilayer actuation system composed of pNIPAM and acrylamide, obtained through conventional mold techniques, was demonstrated by Hu et al.; [25] after that, a range of self-assembled, origami-inspired structures were reported, [27,[30][31][32][33][34][35][36][37][38] but only a few works successfully realized 4D printing with hydrogels. [28,29] Some noteworthy Hydrogel actuators with soft-robotic functions and biomimetic advanced materials with facile and programmable fabrication processes remain scarce. A novel approach to fabricating a shape-memory-hydrogel-(SMG)-based bilayer system using 3D printing to yield a soft actuator responsive to the methodical application of swelling and heat is introduced. Each layer of the bilayer is composed of poly(N,N-dimethyl acrylamide-co-stearyl acrylate) (P(DMAAm-co-SA))-based hydrogels with different concentrations of the crystalline monomer SA within the SMG network...

4D Printed Hydrogels: Fabrication, Materials, and Applications

“…[ 108–110 ] Shiblee et al fabricated a gripper made of bilayer structures of shape‐memory hydrogels printed by a customized stereolithographic printer. [ 59 ] The bilayer was composed of poly( N , N ‐dimethyl acrylamide‐ co ‐stearyl acrylate) [P(DMAAm‐ co ‐SA)]‐based hydrogels of various monomer concentrations of hydrophobic crystalline stearyl acrylate (SA). The low SA concentration side (namely, the “SMG90‐SA10” side) was the active layer for the actuation.…”

“…Gripper: Reproduced with permission. [ 59 ] Copyright 2019, Wiley‐VCH. Drug release: Reproduced with permission.…”

Flexible and Conductive 3D Printable Polyvinylidene Fluoride and Poly(N,N‐dimethylacrylamide) Based Gel Polymer Electrolytes

“…3D printing systems have made it possible to make flexible, gel like structures to be free formed in customized sizes and shapes and thus offer numerous scopes in applied sectors. [ 19 ] Previously, our research group has successfully fabricated various types of 3D printed soft materials using customized 3D printers such as inter crosslinking network gels for gel phantom model, [ 20 ] highly robust thermoresponsive shape memory gels for soft robotics, [ 21,22 ] and ionic liquid‐based conductive gels and nanocomposites for electrochemical devices through a moldless process. [ 23,24 ]…”

A Self-Deformation Robot Design Incorporating Bending-Type Pneumatic Artificial Muscles

“…Configuration-wise, these robots are normally driven by soft actuators made of soft materials such as rubber, gel, or a polymer [13,14]. Though flexible actuators tend to be less powerful, accurate, and responsive than high-stiffness actuators, their flexibility offers advantages in terms of environmental adaptability.…”