Soft Robotic Haptic Interface with Variable Stiffness for Rehabilitation of Neurologically Impaired Hand Function

Abstract

The human hand comprises complex sensorimotor functions that can be impaired by neurological diseases and traumatic injuries. Effective rehabilitation can bring the impaired hand back to a functional state because of the plasticity of the central nervous system to relearn and remodel the lost synapses in the brain. Current rehabilitation therapies focus on strengthening motor skills, such as grasping, employ multiple objects of varying stiffness so that affected persons can experience a wide range of strength training. These devices have limited range of stiffness due to the rigid mechanisms employed in their variable stiffness actuators. This paper presents a novel soft robotic haptic device for neuromuscular rehabilitation of the hand, which is designed to offer adjustable stiffness and can be utilized in both clinical and home settings. The device eliminates the need for multiple objects by employing a pneumatic soft structure made with highly compliant materials that act as the actuator of the haptic interface. It is made with interchangeable sleeves that can be customized to include materials of varying stiffness to increase the upper limit of the stiffness range. The device is fabricated using existing 3D printing technologies, and polymer molding and casting techniques, thus keeping the cost low and throughput high. The haptic interface is linked to either an open-loop system that allows for an increased pressure during usage or closed-loop system that provides pressure regulation in accordance to the stiffness the user specifies. Preliminary evaluation is performed to characterize the effective controllable region of variance in stiffness. It was found that the region of controllable stiffness was between points 3 and 7, where the stiffness appeared to plateau with each increase in pressure. The two control systems are tested to derive relationships between internal pressure, grasping force exertion on the surface, and displacement using multiple probing points on the haptic device. Additional quantitative evaluation is performed with study participants and juxtaposed to a qualitative analysis to ensure adequate perception in compliance variance. The qualitative evaluation showed that greater than 60% of the trials resulted in the correct perception of stiffness in the haptic device.

Haptic Perception, Mechanics, and Material Technologies for Virtual Reality

“…Emerging material technologies can provide new functionalities, including mechanical stretchability, conformity, or transparency, and can provide new means of transduction for sensing and actuation, that are different from those that can be supplied conventional rigid or even flexible materials. [ 88,89 ] Some of these advances may also hold the potential to one day meet the challenging requirements of engineering high‐fidelity haptic devices for virtual reality. Another reason these trends have attracted attention for haptics is due to the biomechanical characteristics of the skin and other soft tissues.…”

Characterization of Mechanical and Dielectric Properties of Silicone Rubber

“…One of the applications for which silicone rubber is commonly used is in medical implantable devices, such as defibrillators, heart pumps, and surgical reconstructive components [ 3 , 6 , 7 , 8 ], as well as rehabilitation devices such as soft robotic systems [ 9 , 10 , 11 ].…”

Modal Frequency Analyses of the Variable Stiffness Mechanism Design of the Soft Robotic System

“…Variable stiffness structure designs applying smart materials and conductive polyurethane through position feedback [11]. Rehabilitation robots for impaired hand function are investigated for the soft haptic interface of pneumatically actuated robots [12].…”