Design and Experimental Evaluation of Passive Ankle-Foot Orthoses Hinges Using Superelastic NiTi Shape Memory Alloy

The mechanical characterization of ankle foot orthoses (AFOs) equipped with shape memory alloy (SMA) hinges was investigated experimentally. The hinges were made of heavily cold-worked, superelastic NiTi alloy wire loops, which had been subjected to heat treatment at various temperatures and durations. The effects of intensity and duration of heat treatment of the hinge and experiment loading rate (speed) on the stiffness of the AFO were studied. The peak resistance and stiffness of the AFO with an SMA hinge decreased with increasing the hinge's heat treatment temperature and/or duration. Raising the heat treatment temperature from 350 to 450 °C decreased the peak resistance load of the AFO by up to 27%. Increasing the loading rate from 1 to 20 mm/s in AFOs resulted in up to 46% increase in average peak load and stiffness. The results indicate that producing AFO hinges from heavily cold-worked superelastic SMA followed by an optimum heat treatment could be a good alternative to producing AFOs that meet patent-specific resistance torque needs.