Soft Robotic Engines with Non-Reciprocal Motion by Physical Intelligence

Movement is important for living organisms to access food, find habitat or escape predators. In the microscopic world, cell crawling and vesicle transport on microtubules rely on the movement of molecular motors. Common to all scales is the unifying principle of breaking symmetry to accumulate work through non-reciprocal motion trajectories. Inspired by nature, the field of soft robotics has developed different actuators that respond to the environment and manipulate objects. However, these actuators typically exhibit reciprocal motion trajectories, where work done in one direction is negated in the return motion. This critically prevents work accumulation in cyclic operation. Here, we introduce a radically simple and universally applicable concept for hydrogel engine systems that overcomes this limitation and achieves non-reciprocal forward and backward motion trajectories through hard-coded kinetic asymmetry in swelling and deswelling transitions. This enables continuous extraction of mechanical work in cyclic operation using a single, uniformly applied stimulus, without the need for complex, externally orchestrated control methods. Our concept leverages physical intelligence in the context of a material-embodied ratchet mechanism, which is independent of scale and geometry, and generalizable to many stimuli. The hydrogel engine system has been successfully implemented in various soft robotic devices, serving as artificial cilia for fluid pumping and conveyor belts for object transport. Starting from thermoresponsive macroscopic engines, the concept is generalized to smaller sizes by microscale printing and to other stimuli. Our designs challenge the field of soft robotics to transcend chemical complexity, and rather harness physical intelligence to achieve new behavior.