Variable stiffness prosthetic foot based on rheology properties of shear thickening fluid

The unique rheological properties of discontinuously shear thickening fluids (STF) have been employed in various engineering applications in recent years. In a commercial aspect, this has most notably been body armor and protective equipment, but also specialized smart structures in damping and force-coupling applications. The topic of this work is the application of STF in an articulating prosthetic foot for adaptable force response. Connected in series and parallel to a spring system, a STF based element can be used to affect the force transfer within the system, and thereby, influence the stiffness of the prosthetic foot dynamically over the gait cycle. The device described, prototyped and tested in this work, is a STF filled piston/cylinder design. The objective is a velocity dependent force response over the translational motion. Ranging from dampened, compliant deflection at low velocity, to a more efficient force transfer (coupling) for energy storage and return in the spring system, at higher speed. The rapid viscosity increase in the STF at a critical shear rate is used to approach a stepwise force response, thereby enabling adaptive response of the foot for different load rates. The adaptive response results in a greater range of motion with easier rollover for slow movement, for instance standing up from a seated position and adaptation to inclined surfaces, without sacrificing the energy return favorable for normal walking.