Date Published: January 15, 2019
Publisher: John Wiley and Sons Inc.
Author(s): Xiaoxia Le, Wei Lu, Jiawei Zhang, Tao Chen.
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.
Nature is a perpetual source to inspire the development of artificial intelligent materials that can adapt and actuate in response to external stimuli. In particular, living creatures from unicellular organisms to humans could generate movements upon environmental stimuli. The actions of animals are normally based on the contraction of certain muscles, while the motions of plants are mainly driven by water absorption or dehydration of cells. For instance, pinecone scales are formed by two kinds of tissues with different orientation of cellulose fibrils, the inhomogeneous local swelling/shrinking in humid environments will trigger the opening or closing of pinecone (Figure1a).1 The same mechanism was found to contribute to the bending of wheat awns.2 Different from pinecone that needs to exchange water with the environment, Mimosa3, 4 and Venus Flytrap5 could generate motions by a redistribution of water inside their tissues (Figure 1c,d).
The driving force of polymeric hydrogel actuators is uptake and release of water. If the hydrogel actuators have isotropic structures, only simple homogeneous swelling/shrinking could be achieved under uniform stimuli, which limits their further applications. With the development of hydrogel actuators, complex deformations/movements have been explored to expand their potential applications. The current investigated approaches to realize complex deformations/movements can be divided into two main categories: 1) imposing external nonuniform stimuli such as electric field or local light irradiation onto isotropic hydrogels29, 30, 31 and 2) preparation of internal anisotropic hydrogels.32 Because of the difficulty of applying nonuniform external stimuli precisely onto hydrogel actuators, the fabrication of anisotropic structures has become more and more popular to achieve complex deformations. These hydrogel actuators could provide various complex deformations/movements upon uniform stimuli due to the heterogeneous responsiveness of inhomogeneous properties. In this review, we will focus on hydrogel actuators with anisotropic structures.
It is a universal principle that the potential applications of materials are predominantly determined by properties, which are essentially determined by structures. Therefore, the design and fabrication of anisotropic structures is fundamental for the development of hydrogel actuators. Up to now, various inhomogeneous structures including bilayer structures,33, 34, 35 gradient structures,36, 37, 38 patterned structures,39, 40, 41, 42 oriented structures,43, 44, 45 and some others46, 47 have been explored (Figure2). In this section, we are going to discuss the diverse anisotropic structures of hydrogel actuators.
As mentioned above, the potential applications of hydrogel actuators are closely related to their reversible shape transforming performances. Though still in the conceptual stage, hydrogel actuators have been designed as grippers,12, 49 walkers,29, 71 swimmers,36 artificial muscles,72, 73 valves,74, 75 and so on to realize corresponding functions such as grasping, transporting, and releasing objects, walking or swimming, lifting, and controlling the flow flux. Recently, the most popular applications of hydrogel actuators are as grippers, walkers, and swimmers. In this section, we will discuss the potential applications of anisotropic hydrogel actuators.
Most of the hydrogel actuators only have shape deformation behaviors, in order to develop the potential applications of hydrogel actuators, recent attention has been paid to integrate some other functions such as fluorescence and shape memory into hydrogel actuators.
In the present review, we have presented the recent progress in the field of anisotropic polymeric hydrogel as biomimetic soft actuators. It is clear that through constructing bilayer, gradient, patterned, oriented, as well as other anisotropic structures, various deformations including bending, twisting, and 3D complex shape transformations could be achieved, leading to potential applications as grippers, walkers, swimmers, biomimetic devices, or valves. Moreover, some other functions such as fluorescence and shape memory could be integrated into hydrogel actuators, multifunctional hydrogel actuators have thus been developed to expand the potential applications of hydrogel actuators. However, in spite of the promentioned attractive achievements of hydrogel actuators, there remain challenges and opportunities in this field.
The authors declare no conflict of interest.