Paired Fluid‐Mediated Bidirectional SMA Actuators for Soft Robots

Abstract Shape memory alloys (SMAs) are promising materials for soft robotic actuators due to their high energy density and compact form. However, conventional SMA‐based systems are inherently limited in achieving effective bidirectional motion. To address this challenge, Paired Fluid‐Mediated Bidirectional Shape Memory Alloy (Paired‐FBSMA) actuators are proposed, which couple thermally driven SMA springs with fluid‐mediated force transmission. Each FBSMA unit consists of a fluid‐filled bellows actuated by two parallel SMA springs, and two such units are interconnected to enable reciprocal fluid exchange and active bidirectional actuation. This design supports modular and flexible integration into robotic systems while enhancing actuation efficiency. By tuning the water‐air ratio inside the bellows, both output force and displacement stroke can be effectively modulated, with higher water content yielding greater recovery forces. Extensive experimental validation demonstrates that Paired‐FBSMA actuators outperform conventional SMA‐only and SMA‐bellows configurations in both stroke and force generation. The actuators are implemented in a soft crawling robot and a two‐degree‐of‐freedom joint mechanism, achieving stable crawling locomotion and repeatable reciprocating motion. These results highlight the versatility, tunability, and strong potential of the Paired‐FBSMA actuators for integration into next‐generation soft robotic systems requiring compact, distributed, and high‐output actuation.