Design and Prototype of Supernumerary Robotic Finger (SRF) Inspired by Fin Ray® Effect for Patients Suffering from Sensorimotor Hand Impairment

In this paper, we present the design and prototype of a wearable supernumerary robotic finger, inspired by Fin $\mathbf{Ray}\bigcirc\!\!\!\!\!\!\mathrm{R}$ , an underactuated device inspired by the physiology of fish fins. The application we propose has been designed for patients suffering from sensorimotor hand impairment, to compensate their missing grasping abilities. In this context Fin $\mathbf{Ray}\bigcirc\!\!\!\!\!\!\mathrm{R}$ effect based closed-chain structure, effectively exploiting underactuation and compliance, are an interesting solution to meet the ergonomics and functional requirements, and to get a light but robust wearable device. The finger is actuated through a single linear actuator and it has a complaint structure with stiff crossbeams that buckle and deform in to conform around objects. We performed Finite Element Modeling (FEM) simulations to compare the soft-rigid structure with the Fin $\mathbf{Ray}\bigcirc\!\!\!\!\!\!\mathrm{R}$ effect based closed-chain structure. We performed a set of tests to exploit the device potentialities in grasp compensation tasks through qualitative experiments based on activity daily living (ADL). Results showed that proposed robotic device can improve the autonomy of patients suffering from sensorimotor hand impairment in ADL and allow them to complete tasks which otherwise are impossible to perform.