Heimir Tryggvason

This paper presents a novel design of a prosthetic foot that features adaptable stiffness that changes according to the speed of ankle motion. The motivation is the natural graduation in stiffness of a biological ankle over a range of ambulation tasks. The device stiffness depends on rate of movement, ranging from a dissipating support at very slow walking speed, to efficient energy storage and return at normal walking speed. The objective here is to design a prosthetic foot that provides a compliant support for slow ambulation, without sacrificing the spring-like energy return beneficial in normal walking. The design is a modification of a commercially available foot and employs material properties to provide a change in stiffness. The velocity dependent properties of a non-Newtonian working fluid provide the rate adaptability. Material properties of components allow for a geometry shift that results in a coupling action, affecting the stiffness of the overall system. The function of an adaptive coupling was tested in linear motion. A prototype prosthetic foot was built, and the speed dependent stiffness measured mechanically. Furthermore, the prototype was tested by a user and body kinematics measured in gait analysis for varying walking speed, comparing the prototype to the original foot model (non-modified). Mechanical evaluation of stiffness shows increase in stiffness of about 60% over the test range and 10% increase between slow and normal walking speed in user testing.

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.

Different tasks and conditions in gait call for different stiffness of prosthetic foot devices. The following work presents a case study on design modifications of a prosthetic foot, aimed at variable stiffness of the device. The objective is a proof-of-concept, achieved by simulating the modifications using finite element modeling. Design changes include the addition of a controlled damping element, connected both in parallel and series to a system of springs. The aim is to change the stiffness of the device under dynamic loading, by applying a high damping constant, approaching force coupling for the given boundary conditions. The dynamic modelling simulates mechanical test methods used to measure load response in full roll-over of prosthetic feet. Activation of the element during loading of the foot justifies the damped effect. As damping is in contrast to the main design objectives of energy return in prosthetic feet, it is considered important to quantify the dissipated energy in such an element. Our design case shows that the introduction of a damping element, with a high damping constant, can increase the overall rotational stiffness of the device by 50%. Given a large enough damping coefficient, the energy dissipation in the active element is about 20% of maximum strain energy.