Soft Ankle-Foot Exoskeleton for Rehabilitation: A Systematic Review of Actuation, Sensing, Mechanical Design, and Control Strategy

Robot-assisted rehabilitation therapy has become a mainstream trend for the treatment of stroke patients. It can not only relieve physiotherapists from heavy physical duties, but also provide patients with effective ankle-foot rehabilitation and walking assistance. Soft ankle-foot exoskeletons have rapidly evolved in the last decade. This article presents a compressive review of soft ankle-foot exoskeletons in terms of robot actuation, wearable sensor, mechanical design, and control strategy. Representative commercial and laboratory ankle-foot exoskeletons are demonstrated. Special attention is paid to the emerging soft actuators, wearable sensing techniques, and human-in-the-loop and hierarchical control methods. Finally, essential challenges and possible future directions are also analyzed and highlighted in this paper, which can provide reliable guidance on the development of next-generation soft ankle-foot exoskeletons.

I. Introduction

According to World Health Organization (WHO), the proportion of the world’s population over 60 can rise from 12% to 22% between 2015 to 2050. Meanwhile, about 15% of the world’s population experience some form of disability, of whom 2-4% suffer significant functional difficulties [1]. Due to the aged society and increasing disabled population, ankle joint injury can be easily caused by external forces or nervous system diseases, and long-term abnormal gait patterns may lead to increasing energy consumption and secondary injury [2]. Therefore, it is imperative for patients to improve motor function and mobility [3], [4]. Rehabilitation through active exploration and movement can promote the immediate and dramatic increase in new neural connections associated with the injured area, which effectively promotes neural circuit remodeling [5], [6]. Robotic exoskeletons have the potential to maximize neural recovery through repetitive passive training, resistance training, and isometric training [7], [8], which play a positive role in nerve remodeling through function-oriented exercise [9].