A soft pneumatic actuator as a haptic wearable device for upper limb amputees: Toward a soft robotic liner

Object manipulation and fluid, goal-directed, movements require sensory information for effective execution. Amputees lose this intrinsic feedback when controlling their artificial limbs and must instead rely on visual information to compensate. Here, we describe an application for providing touch and kinesthetic information to amputees. We report on a soft robotic pneumatic actuator approach that can be incorporated into a prosthetic limb as the silicone suction socket liner itself. This approach alleviates many problems inherent to rigid tactors such as poor trim lines from external mounting, electromyography (EMG) signal contamination, and loss of limb fixation suction due to holes in the liner to pass touch and vibration to the residual limb. We analyzed two soft materials and different chamber geometries to generate a prototype. We characterized the static and dynamic properties of this prototype during operation obtaining a maximum force of 12.5 N at 70 kPa, free displacement of 4.5 mm at 50 kPa, and a bandwidth near 70 Hz. We presented an analytical model that fits well with the experimental data and provided a comparison between this soft pneumatic actuator and other rigid tactor devices. The results of testing the prototype in able-bodied participants and one amputee individual demonstrated that this soft pneumatic actuator achieved good performance at frequencies of 5 and 70 Hz at 60 kPa. In sequential days of training with the prototype participants reported perceptions of wrist flexion/extension and demonstrated learned associations between tapping and hand closing.