Advancing Exoskeleton Use Post Stroke: Developing and Optimizing a Soft Biplanar Ankle Exoskeleton

QC 250508

Abstract

Stroke is a leading cause of long-term disability worldwide. Dropfoot gait, or inability to adequately lift, advance, and land on the foot on the more impaired side, is one of the most common gait impairments following a stroke, and often results in reduced mobility and increased fall risk, severely impacting independence and quality of life. Wearable robotics and exoskeletons have been widely explored for their potential in physical rehabilitation and mobility assistance for individuals with motor disorders. However, despite their promise, few existing systems have demonstrated convincing evidence for use. The specific exoskeleton performance challenges in assisting dropfoot gait that are addressed in this compilation thesis are 1) to simultaneously control both sagittal plane ankle dorsiflexion and frontal plane ankle inversion/eversion motions, and 2) to identify control strategies that are individualized based on gait impairments and subjective preferences. 

Specifically, the aims of the thesis were to design an ankle exoskeleton that is capable of providing biplanar assistance and meets both biomechanical and feasibility requirements, to develop a customized control framework that optimizes multiple performance metrics, and to identify subject-specific assistive parameters that improve gait metrics while aligning with users' subjective assessments as well.

The first two studies focus on the development and feasibility of a soft ankle exoskeleton designed to provide  assistance in both ankle dorsiflexion and eversion, via  cable-driven mechanisms and compliant materials. Initial testing confirmed the exoskeleton's ability to effectively guide ankle joint motion in both planes with minimal resistance. In the second study, the device's feasibility was evaluated in a pilot group of persons with dropfoot gait following a stroke. Improvements in key gait parameters, along with positive user assessment on the device's comfort, usability, and perceived effectiveness, encourage further application of the device in persons in a chronic post-stroke phase.

The third and fourth studies focus on developing personalized exoskeleton control strategies, specifically a multi-objective human-in-the-loop optimization framework that evaluates individual responses to various assistive profiles, then identifies assistive profiles tailored to each individual's gait impairments. The framework was constructed to simultaneously optimize two objectives that describe gait quality. This approach yielded not just one, but a group of good solutions that improve both gait metrics to varying degrees, among which solutions can be selected based on context and preference. The framework was developed and tested on a group of non-disabled subjects with a simulated dropfoot impairment in the third study and on a pilot group of persons with dropfoot following a stroke in the fourth study. In the fourth study, the personalization framework was further advanced by incorporating user preferences, thereby incorporating both objective gait quality metrics and subjective preference in identifying optimal exoskeleton assistance.

This thesis advances the application of exoskeletons for individuals post-stroke by addressing both hardware design and personalized control strategies. The findings highlight the potential of the developed ankle exoskeleton to enhance mobility in this population and underscore the importance of individualized assistance to meet diverse user needs. Together, the exoskeleton design and individualized control framework offer a valuable foundation for future research and practical implementation of assistive technologies.

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