Recent advancements in soft ankle/knee exoskeletons technologies: systems, actuation and control

Exoskeletons are widely used for both rehabilitation purposes and physiotherapy applications. The continuously increasing elderly population and the number of people needing assistance due to certain illnesses or accidents demand more of such effective wearable devices. Researchers are increasingly focusing on developing exoskeletons for specific areas of the human body consequently making the device affordable and less complex. Recent times have shown a transition towards the use of soft materials in constructing exoskeletons. This is because of its superior flexibility, compactness, lightweight, variability and ability to work in parallel to human muscle fibre. The objective of this paper is to give both qualitative and quantitative information on the suitability of different soft smart materials in constructing lower limb exoskeletons, especially, ankle and knee joints. The various actuation means using soft materials, their unique characteristics, performance, and the various control methods employed are discussed in this paper. Factors such as durability, fatigue life, actuation force, displacement achieved, feedback delays, ease of usage, ergonomics, and cost affect the performance and functionality of the exoskeleton. Even though there have been a lot of developments in soft exoskeletons, further improvements are still necessary for their implementation on a commercial scale. The merits and limitations seen with recently developed soft ankle/knee exoskeletons are presented and the possible prospects to improve their effectiveness are also highlighted in this paper.