Temperature-triggered inflatable hydrogel muscles with snap-through instability for untethered robots.

Pneumatic artificial muscles have been widely used in the field of robotics because of their large output force and fast actuation, however, the accompanying bulky compressors and pumps limit their miniaturized applications. Despite current untethered pneumatic artificial muscles can be driven by adjusting the internal pressure, it is challenging to structurally mimic natural muscles with high water content. Here, we propose untethered pneumatic artificial muscles comprising a hydrogel actuator with snap-through instability and an air storage chamber. These hydrogel actuators can realize the conversion from hydrophobic association of octyl acrylate moieties to host-guest interaction between β-cyclodextrin and octyl acrylate under thermal stimuli, leading to the decrease of their moduli. The inflated hydrogel actuators exhibit rapid actuation with a radial expansion speed of 200% s(-1), which are powered by snap-through instability, thermal expansion of the gas inside the hydrogel actuator, and evaporation of water on its internal surface. With the pneumatic artificial muscles miniaturized, we demonstrate diving and rolling robots, exemplifying bionic robots able to adapt to and modify the environment. We expect that the design of hydrogel actuator in miniaturized pneumatic artificial muscles will facilitate rapid locomotion for future bionic robotic platforms.