Multifunctional Position and Stiffness Control of Soft Fluidic Actuators Using Supercooled Liquid

Soft fluidic actuators (SFAs) are widely adopted for soft robotic applications, for example, for the manipulation of delicate objects of various geometries. However, the innate compliance of SFAs prevents them from sustaining large forces when needed. Existing methods to achieve variable stiffness in SFAs necessitate complex mechanisms that require additional control inputs and often limit the flexibility of the actuators. This work explores the use of supercooled sodium acetate trihydrate (SAT) solution as a multifunctional working fluid for SFAs to allow independent control of position and stiffness; the solution is used to both inflate the elastomer chambers while in liquid state and to achieve a dramatic increase in stiffness with rapid solidification. The mechanical properties of crystallized SAT samples in flexible membranes and with embedded materials for reinforcement are first investigated. SFAs that use the SAT solution to greatly increase their stiffness (up to 13 times) are then tested. Furthermore, the use of supercooled SAT is demonstrated to achieve both manipulation and stiffness change with a single control input. The proposed approach is a new way to combine the high manipulability of SFAs with stiffness tunability and opens up new applications for soft robots and manipulators.