Recent Progress in Biomimetic Anisotropic Hydrogel Actuators

Abstract

Polymeric hydrogel actuators refer to intelligent stimuli-responsive hydrogels that could reversibly deform upon the trigger of various external stimuli. They have thus aroused tremendous attention and shown promising applications in many fields including soft robots, artificial muscles, valves, and so on. After a brief introduction of the driving forces that contribute to the movement of living creatures, an overview of the design principles and development history of hydrogel actuators is provided, then the diverse anisotropic structures of hydrogel actuators are summarized, presenting the promising applications of hydrogel actuators, and highlighting the development of multifunctional hydrogel actuators. Finally, the existing challenges and future perspectives of this exciting field are discussed.

Keywords: anisotropic structures; biomimetic actuation; hydrogel actuators; stimuli‐responsive hydrogels.

Figure 1

The design of hydrogel actuators is inspired by the movements of natural plants, such as a) the opening of a pinecone (adapted with permission.1 Copyright 2012, Annual Reviews), b) the bending of wheat awns upon humidity stimulus (adapted with permission.2 Copyright 2007, AAAS), c) the droop of Mimosa (adapted from with permission.4 Copyright 2018, Royal Society of Chemistry), and d) the closure of Venus flytrap triggered by touching (reproduced with permission.5 Copyright 2007, John Wiley and Sons).

Figure 2

The cartoon figures of various anisotropic structures, including bilayer structure, gradient structure, patterned structure, and oriented structure.

Figure 3

a) A bilayer hydrogel consisting of a PNIPAM layer and a P(AAc‐co‐AAm) layer is fabricated, the PNIPAM layer would release water with increasing temperature, while the P(AAc‐co‐AAm) layer would uptake water with increasing temperature, and the bilayer hydrogel would bend as a result. b) Closing and reopening of a hydrogel flower at 40 and 15 °C in water. c) Closing and reopening of a hydrogel flower at 40 and 15 °C in paraffin. d) The hydrogel gripper grasps a heated metal wire, lifts it up, and releases it when brought in contact with an ice‐cold surface. Adapted with permission.4 Copyright 2018, Royal Society of Chemistry.

Figure 4

a) Chemical structures of RH and NRG hydrogels. b) Reversible bidirectional bending performance of a bilayer hydrogel assembled from the RH and NRG hydrogels. c) Enhanced responsive shape changing behavior of a bilayer hydrogel. Reproduced from with permission.54 Copyright 2014, John Wiley and Sons.

Figure 5

Scheme illustrating the hydrothermal synthesis of gradient porous hydrogel containing NIPAM and 4HBA. Reproduced with permission.36 Copyright 2015, John Wiley and Sons.

Figure 6

a) Stages of twisting of seed pods. Reproduced with permission.58 Copyright 2011, John Wiley and Sons. b) Images of the helices generated by anisotropic hydrogel sheets with photolithographed patterns. Reproduced with permission.41 Copyright 2013, Nature Publishing Group. c) Multiresponsive 3D complex deformations of anisotropic hydrogel under the stimulus of pH, IS, NIR light, and temperature, respectively. Reproduced with permission.12 Copyright 2016, John Wiley and Sons.

Figure 7

a) Illustration of the control of cross‐linking degree via digital light exposure, the obtained honeycomb‐shaped domes and 3D theater. Reproduced with permission.39 Copyright 2017, John Wiley and Sons. b) Cross‐linking between ferric ions and the carboxyl groups of hydrogel, the deformation of a hydrogel sheet with patterns of ferric ions. Adapted with permission.60 Copyright 2017, John Wiley and Sons.

Figure 8

a) 3D printed hydrogel actuators with oriented cellulose nanofibrils. Reproduced with permission.46 Copyright 2016, Nature Publishing Group. b) Unidirectional procession of an L‐shaped PNIPAM‐based hydrogel actuator with cofacially aligned TiNS. Reproduced with permission.43 Copyright 2015, Nature Publishing Group.

Figure 9

Building blocks hydrogel with shape deformation behavior. Reproduced with permission.54 Copyright 2014, John Wiley and Sons.

Figure 10

a) Transportation of a hydrogel gripper by a moving pearl. Reproduced with permission.49 Copyright 2015, John Wiley and Sons. b) The grabbing process of a multiresponsive hydrogel actuator under the stimuli of NIR, IS, and temperature. Reproduced with permission.12 Copyright 2016, John Wiley and Sons.

Figure 11

a) The walking performance of an electroactuated hydrogel walker. Reproduced with permission.29 Copyright 2014, Royal Society of Chemistry. b) Hydrogel walkers loaded with different weights. Reproduced with permission.76 Copyright 2015, Nature Publishing Group. c) Hydrogel swimmer driven by laser irradiation. Reproduced with permission.36 Copyright 2015, John Wiley and Sons.

Figure 12

Reversible snapping hydrogel assembly and the corresponding height in the process of transformation. Reproduced with permission.56 Copyright 2016, Royal Society of Chemistry.

Figure 13

a) pH‐sensitive hydrogel bistrip valve. Reproduced with permission.76 Copyright 2015, American Institute of Physics. b) PNIPAM‐based hydrogel with anisotropic shrinkage behavior as a valve. Reproduced with permission.75 Copyright 2012, Royal Society of Chemistry.

Figure 14

a) Illustration of the fabrication of bilayer hydrogel actuators with fluorescence color‐changing behaviors. b) The unfolding of the hydrogel actuator via increasing temperature and the change of fluorescence color upon the trigger of pH. c) The closing of the hydrogel actuator under the stimulus of temperature. Reproduced with permission.57 Copyright 2018, John Wiley and Sons.

Figure 15

a) Illustration of hydrogel actuators with shape memory functions. b) The combination of actuating and shape deformation behaviors. Reproduced from with permission.78 Copyright 2018, Royal Society of Chemistry.