Polymeric Pressure Cushions for Potential Applications on Forearm Robotic Orthoses

Robot devices for stroke rehabilitation measure the interaction forces between users and the structure of the orthosis through load cells. Although these load cells are well-suited for stationary robotic devices in hospitals, they do not easily allow for the development of affordable wearable orthoses that can assist in daily living. When load cells are attached onto a robotic orthosis, they neither conform to the shape of the user’s body nor directly measure the applied forces at the contact point between the user and the orthosis. A polymeric cushion containing atmospheric air was developed as an alternative technology for measuring forces. A finite element model (FEM) of the polymeric cushion was made to simulate air pressure changes inside the polymeric cushion from applied forces. The polymeric cushions were fabricated entirely of Poly(dimethylsiloxane) (PDMS), making them biocompatible, flexible, and free of electrically conductive materials. An air pressure sensor attached to the tube of the polymeric cushion measured the air pressure and converted it into an electrical signal to be processed by a data acquisition board (DAQ). A test bench setup was made to characterize the relationship between the air pressure and applied force from each polymeric cushion, where a linear stage applied a setpoint force onto the cushion with an aluminum flat plate and a spherical glass tube. The characterization results of the experimental test bench setup were compared to the FEM results. Six polymeric cushions were mounted onto a wrist brace exoskeleton, where a LabVIEW program was written to record specific combinations of pressure sensors and measure the pronation/supination torque of the forearm (rotation), flexion/extension force of the elbow (up/down), and the internal/external rotation of the shoulder (left/right) at the forearm. These measured force values from the polymeric cushions were compared to the measured values of a torque sensor and load cell. The potential suitability of polymeric cushions for the measurement of isometric forces on an orthoses, is compared to the abilities of exoskeleton devices which involve the motions tested in this study using the wrist brace exoskeleton.