Fabrication and Characterization of a Passive Variable Stiffness Joint based on Shear Thickening Fluids

In soft robotics, variable stiffening is the key to taking full advantage of properties such as compliance, manip-ulability and deformability. However, many variable stiffness actuators and mechanisms which have been produced so far to control these properties of soft robots are slow, bulky, or require additional complex actuators. This paper presents a novel passive soft joint based upon the intrinsic non-Newtonian behavior of Shear Thickening Fluids (STFs). The joint stiffness is varied by changing the speed at which it is actuated meaning that is it is for use in dynamic systems and its stiffness cannot be varied at rest. The joints fabricated for testing have a simple cylindrical structure comprised of a soft silicone shell filled with a STF. Three prototypes with lengths of 20, 40 and 60mm were produced for experimental validation. We characterize the behavior of the joints in compression, expansion and bending, yielding a stiffness variation of more than 5x based on actuation speed in compression testing. This paper is the first step in producing a new category of variable stiffening mechanisms based on STFs which can be incorporated into soft robots without the need for additional actuation. It is envisaged that this new soft joint will find applications in soft manipulators and wearable devices.