Abstract: Soft robots have unique advantages over traditional rigid robots and have broad application prospects in many fi elds. To 
expand their bioinspired applications, we propose a novel Soft Pneumatic Actuator (SPA) associated with spiral confi guration inspired by the structure and unwinding motion of the seahorse tail. Diff erent from bending motion of common soft 
actuators, the spiral SPA can generate unwinding motion as input air pressure increases. First, to explore the eff ect of diff erent initial spiral types on unwinding performance, three typical spiral SPAs are designed and simulated while keeping the 
outside arc of actuator body constant. Second, a static model of the spiral SPA is established by combining the hyperelastic 
material model, geometric relationships, and virtual work principle. To improve model accuracy, two geometric correction 
parameters are employed and their physical signifi cance is analyzed by fi nite element simulations. Third, a prototype of the 
logarithmic spiral SPA (Log_spiral SPA) is fabricated and a Fiber Bragg Grating (FBG) sensor array is designed to detect and 
reconstruct unwinding shapes of the prototype. Finally, the unwinding performance, static model and output force capability 
of the prototype are tested and verifi ed. Furthermore, we discuss prospects for this novel spiral SPA and test its practical 
applications in inchworm-like motion, assisting fi nger rehabilitation and object capture.

Key words: Soft robotics, Spiral, Soft actuator, Virtual work principle, Fiber Bragg grating