4D printing of biomimetic anisotropic self-sensing hydrogel actuators

Organisms in nature, from plants, insects to mammals, have anisotropic and regularly arranged tissue structures, which are key to rapid and precise response to environmental stimuli. In this study, nature-inspired anisotropic hydrogel actuators are fabricated by 4D printing thermo-responsive composites of short carbon fibers (SCFs) and poly(N-isopropylacrylamide) (PNIPAM). The SCFs could be well aligned and programmed in hydrogel matrix, resulting in diverse precisely controlled anisotropic structures. The intrinsic anisotropy causes unbalanced swelling and shrinking upon thermo-stimulation, leading to reversible biomimetic shape morphing. Finite element analysis (FEA) simulations are used to estimate the internal anisotropic stress distribution and structure evolution. Biomimetic anisotropic butterfly-like, flytrap-like and starfish-like hydrogel actuators are fabricated by 4D printing. On the other hand, the anisotropic SCFs/PNIPAM hydrogels are conductive and could be used as wearable strain sensors with high sensitivity. The conductive anisotropic hydrogels can generate sensing signals when they are motivated by temperature changes. This self-monitoring soft actuator is promising for the development of soft robots with stimulation-sensing-actuation feedback loop, which has broad application prospects in artificial intelligent devices.