Adaptive ankle–foot orthoses based on superelasticity of shape memory alloys

This article presents two innovative adaptive solutions for the ankle–foot orthosis based on mechanical and structural stiffness control of shape memory alloys. These concepts address gait abnormality in drop foot patients for various walking conditions such as different walking speeds. In the first design, a superelastic rod provides variable torsional stiffness that is adjusted by a controlled axial load. In the second design, the active length of superelastic hinge is adjusted in order to control the bending stiffness of the element. By adjusting the stiffness, variable level of compliance is achieved at the ankle. In both concepts, during powered plantarflexion in the stance phase of the gait, energy is stored in the shape memory alloy element. Release of this energy through superelasticity enables the ankle–foot orthosis to provide the desired controlled dorsiflexion motion in the sagittal plane and to raise the foot during the swing phase of the gait. The ultimate goal is to assist the patients in achieving a more natural gait and to prevent muscle atrophy. For the presented designs, numerical simulations are carried out to evaluate the stiffness properties of the active component under different gait speeds. To this end, experimental data of human gait are used to calculate the variation in ankle stiffness. The superelastic elements mimic the experimental ankle stiffness profiles.