Hydrogel‐Based Artificial Muscles: Overview and Recent Progress

Abstract

Artificial muscles are promising as an intelligent material that can replace conventional power systems to reproduce the motion of a living organism. Among various building materials, hydrogels have great advantages as artificial muscles, such as responsiveness to a wide range of stimuli, large volume deformation, and sensitivity. However, conventional hydrogel actuators that generate only isotropic volume change in response to an external stimulus cannot be considered as artificial muscles because complex deformations are not possible. Instead, programmed complex deformations originated from a rationally designed anisotropic structure are desired in artificial muscles. Herein, recent approaches for constructing stimuli‐responsive hydrogels with an anisotropic structure, thereby enabling a directional complex motion to mimic a muscle are reviewed. First, stimuli‐responsive shape‐deforming hydrogels as a main building matrix are categorized according to stimulating external signals. The representative methods for modulating the isotropic structure of a hydrogel into various anisotropic structures, such as multilayer structure, linearly oriented structure, and patterned structure are discussed. Finally, the recent strategies to achieve muscle‐like motion of hydrogels by combining stimuli responsiveness and anisotropic structures for translation from elementary contraction and expansion to programmed deformations, such as multidirectional bending, directionally amplified deformation, and compositive programmed motion, are covered.

Actuation of Three‐Dimensional‐Printed Nanocolloidal Hydrogel with Structural Anisotropy

“…While liquid crystalline elastomeric actuators have attracted strong interest, the search continues for new actuating materials that are cost-effective, biocompatible, sustainable, and ideally, multifunctional. [20,25] In the present work, we report a 3D-printed single-layer nanocolloidal hydrogel actuator that undergoes stimulus-triggered shape transformation solely due to structural anisotropy. The hydrogel was formed from rod-like cellulose nanocrystals (CNCs) and methacryloyl gelatin (GelMA).…”

“…Since extrusion-based 3D printing induces uniaxial orientation of Al 2 O 3 nanofibers, [26] metal nanorods, [27] carbon nanotubes, [28] cellulose nanofibers, [29] and CNCs, [30,31] as well as other types of high aspect ratio particles, our work paves the way for the fabrication of actuators using nanocolloidal inks. Actuating hydrogels can be used in the design of drug delivery systems, [32] soft robotics, [33,34] artificial muscles, [20] microfluidic devices, [35] and sensors. [36]…”

Intelligent Polymer‐Based Bioinspired Actuators: From Monofunction to Multifunction

“…In this regard, stimuli–responsive materials are excellent candidates due to their capabilities of changing conformations and physicochemical properties in responding to external stimuli. [ 12 ] Representative stimuli–responsive materials for fabricating intelligent bioinspired actuators include vapomechanically responsive polymers ( Figure a), [ 13 ] liquid crystal elastomers (LCEs) [ 14 ] (Figure 1b), stimuli–responsive hydrogels [ 15 ] (Figure 1c), shape memory polymers (SMPs) [ 16 ] (Figure 1d), and electroactive polymers (EAPs) and their composites [ 17 ] (Figure 1e).…”

Liquid Crystal Polymer‐Based Soft Robots

“…[11][12][13][14][15][16][17] Recent research results indicate that stimuli-responsive soft matters are excellent candidates for making soft robotics, because they offer great potential to integrate sensors, actuators, and control systems into the millimeter-or even micrometer-scale soft supportive bodies. [18][19][20][21][22] Among the variety of soft materials explored so far, liquid crystal polymers, in the form of liquid crystal networks or elastomers (abbreviated as LCNs hereafter) have gained much attention for flexible actuator and soft robotic applications thanks to their unique properties such as rapid stimuli responsiveness, anisotropic deformation, excellent elasticity and flexibility as well as order-disorder phase-transition-induced shape change. [23][24][25] LCNs are usually prepared by first aligning the rod-like mesogens uniaxially or organizing the LC director orientation into specific 2D or 3D profiles, through mechanical stretching, surface effects, and electric or magnetic fields, and then carrying out polymer chain crosslinking to retain the shape of LCN in the oriented state.…”