Bioinspired gradient structured soft actuators: From fabrication to application

The gradient structure inherent in the biological system plays a key role in facilitating efficient and precise actuation. To date, bioinspired gradient structured soft actuators have a groundbreaking impact in many fields such as soft robotics, flexible electronics, and biomedical devices. Bioinspired gradient structured soft actuators overcome complex control of homogeneous actuators and delamination of bilayer actuators. Meanwhile, they can achieve integrated, smooth continuous changes, fast response actuation/recovery, and high robustness during frequent deformations under various external stimuli such as temperature, pH, NIR light, humidity and chemicals. In this review, we mainly focus on recent advances in bioinspired gradient structured soft actuators. First, we briefly present the synthetic materials of gradient structured soft actuators including hydrogels, carbon-based materials, shape memory polymers (SMPs), and liquid crystal polymers (LCPs). Then, we focus on summarizing and comparing five formation mechanisms of gradient structured soft actuators such as UV induction, electric/magnetic field induction, infiltration, wettability and 4D printing. Next, we introduce the representative applications of gradient structured soft actuators, such as smart grippers, bionic soft robots, on–off switches, and flexible electronics. Finally, we have a deep discussion on the existing challenges and future perspectives. The review provides guidance for the design of bioinspired gradient structured soft actuators, which would promote the further development of integrated advanced materials.