Obtaining skin contact force information from implanted nerve cuff recording electrodes

An investigation was conducted to determine how the electrical activity in a cutaneous nerve relates to the force applied on glabrous skin. Nerve cuff electrodes implanted on the tibial nerve of cats were used to record the electroneurogram (ENG) when the footpad was probed by a servo-controlled motor. The ENG signal was nonlinearly dependent on the applied force. It saturated for high forces, adapted when constant forces were applied, and was very sensitive to slippage. Linear system identification techniques provided estimates of force amplitude and frequency. Improved estimates were obtained using a real-time, analog, nonlinear filter that included proportional integral and exponential terms. Accurate estimations of input force could best be made if the force was not constant but varied in the 0.1-5.0-Hz range. Cuff electrodes implanted on nerves supplying glabrous skin of the hands or feet may provide feedback suitable, for example, for the closed-loop control of functional electrical simulation of paralyzed muscles in spinal cord injured patients