Variable Stiffness Orthosis
Motivation
The design of the Variable Stiffness Orthosis (VSO) was motivated by the minimal change in daily-use orthoses over the last few centuries. There is a sharp contrast between the functionality of the passive orthoses used in daily life and the powered exoskeletons developed in research, because research continues to advance without a matching rate of commercial translation. To date there are no powered AFOs available for daily use, stemming from challenges with reliability, safety, weight, noise, and cost. This work aims to bridge the gap between existing daily-use orthoses and the long-term vision of greater technology to foster mobility.
Approach
The Variable Stiffness Orthosis (VSO) has a customizable cam-based transmission adapted from the Variable Stiffness Prosthetic Ankle. The VSO’s cam transmission can be designed to specify any torque-angle function and can be quickly swapped out to accommodate different users. The device can also change the magnitude of its overall stiffness in real time by changing the support condition on the leaf spring to accommodate different ambulation tasks.
Dynamometer testing demonstrated that the VSO can exhibit six behaviors previously not possible for unpowered orthoses, while operating at 150% the energy efficiency of other AFOs. These include: 1) customizable, nonlinear torque-angle relationships, 2) step-to-step adjustment of ankle stiffness ranging from the softest to stiffest commercially available AFOs, 3) negative stiffness, 4) extreme stiffness greater than any of the constituent springs, 5) changing equilibrium angle, and 6) exchanging energy between different phases of gait. Many of these behaviors are typically attributed to powered exoskeletons, showcasing the potential for versatile passive mechanisms to blur the line between passive and powered behavior.
A decoupled energy storage and return module can also be used in the Variable Stiffness Orthosis in order to recycle energy from one phase of gait for use in another.
Research
Testing on two participants with and without sciatic nerve injury (SNI), showed that both participants had activity-dependent stiffness preferences that spanned a large stiffness range. Thus, prescribing an AFO with a fixed stiffness necessitates a compromise that is undesirable to the user. The maximum stiffness preferred by both participants was 260-800% stiffer than their minimum preferred stiffness, which is much greater than the minimum stiffness change of 12% that people can reliably perceive. Preliminary results showed that using the VSO led to reduced foot drop, increased self-selected speed, increased total ankle moments, reduced biological moments, reduced toe striking, and reduced steppage in the participant with SNI compared to daily-use AFO and shoes-only conditions. These results are promising because the participant with SNI is at the lower end of the weight range for which the VSO was designed, meaning they experienced increased relative mass compared to heavier participants. While this paper centers on the design contribution, a robust characterization is presented (both benchtop and with two users).
Contributors: Nikko Van Crey, Emily Bywater, David Lam, Varun Shetty, Hemanth Aroumougam, Elliott Rouse
Publications
Van Crey, N. Lam, D.J., Bywater, E.A., Shepherd, M., and Rouse, E.J. “The Variable Stiffness Orthosis: Customizable Mechanics for Assistance and Rehabilitation.” Authorea Preprints (2024).
Bywater, Emily A., Nikko Van Crey, and Elliott J. Rouse. “Optimizing the Mechanics of a Variable-Stiffness Orthosis With Energy Recycling to Mitigate Foot Drop.” IEEE Transactions on Medical Robotics and Bionics (2024).
van Noort, L., Van Crey, N., Rouse, E.J., Martínez-Caballero, I., van Asseldonk, E.H.F., and Bayón, C. “A usability study on the inGAIT-VSO: effects of a variable-stiffness ankle-foot orthosis on the walking performance of children with cerebral palsy.” Journal of neuroengineering and rehabilitation 21.1 (2024): 132.
Bayón, C., Van Crey, N., Rocon, E., Rouse, E., and van Asseldonk, E. “Comparison of Two Design Principles of Unpowered Ankle-Foot Orthoses for Supporting Push-Off: A Case Study.” 2023 International Conference on Rehabilitation Robotics (ICORR). IEEE, 2023.
Van Crey, N. Cavallin, M., Shepherd, M., and Rouse, E.J. “Design of a quasi-passive ankle-foot orthosis with customizable, variable stiffness.” 2023 International Conference on Rehabilitation Robotics (ICORR). IEEE, 2023.