1. Introduction
Traditional mechanical devices are mainly powered by electric motors or engines, which can provide various driving forces to mechanical devices or robots through various sophisticated forms of mechanical transmission. In the production of microrobots, various physical and chemical fields are needed to accomplish microscale motion and deformation [1]. By responding to various stimuli, such as temperature [2], light [3], pH [4], humidity [5], magnetic fields [6,7,8,9], etc., the robot can be provided with energy for movement, and certain motions can be accomplished. Soft actuators have received increasing attention in the study of bionic robots [10,11,12]. In nature, many organisms rely on soft tissues to achieve flexible drives to perform complex and precise movements. Studying biological brake systems and learning from nature may inspire the generation of new mechanisms and structures for the operation of flexible brakes.
With the help of the development of functional materials and the rational design of mechanical structures, it is possible to imitate the flexible movement behavior of some living organisms in nature [11,13,14]. Macroscopic aspects are commonly studied with pneumatic manipulators [10]. Microscopic aspects are also an effective form of flexible actuation by directly incorporating living biological components, such as cardiomyocytes, in abiotic structures [15]. Bingzhe Xu et al. created a bio-hybrid planar swimming robot by growing a cardiac microstructure on a bio-affinity material and using it as a tail fin to mimic the swimming of a whale [16]. Polymer hydrogels that can respond to external stimuli can also be used as actuators in applications such as microrobotics [17,18,19]. One of the most widely used temperature-responsive hydrogels is PNIPAM hydrogel, which has a low critical solution temperature (LCST) of approximately 32 °C [20,21,22,23]. The N-isopropylacrylamide molecule has both a hydrophilic amide group and a hydrophobic isopropyl group. When the temperature is lower than LCST, the hydrogel exhibits hydrophilicity due to increased intermolecular forces between polymer chains and water molecules, and the hydrogel absorbs water and swells. When the temperature is higher than LCST, PNIPAM shifts from hydrophilic state to hydrophobic state due to the weakening of the interaction between polymer and water molecules, and the hydrogel expels water and shrinks in volume. Temperature can affect the contraction and expansion of the volume of temperature-sensitive hydrogels, and its characteristics...
