Development of capacitive pure bending strain sensor for wireless spinal fusion monitoring

A MEMS (microelectromechanical system) capacitive-based pure bending strain sensor is presented for use in spinal fusion monitoring. The sensor is designed to interface with a telemetry system that does not require a battery and contained in a housing that is attached to spinal fusion rods. The cantilever structure of the sensor is composed of two parallel plates with a narrow gap and a conjoint end. Nine permutations of the design with different metal coverage areas (14 mm2, 9.3 mm2 and 4.7 mm2) and gaps (3 μm, 6 μm and 7.4 μm) were examined. The nominal capacitance ranges from 7.6 pF to 42 pF. The capacitance changes 31.4–65.1% for a strain range of 0–1000 μɛ depending on the design parameters. An analytical model is developed for the sensor mounted to a cantilever test bar and compared to experimental results of actual devices. The model and experimental results show an average difference of 5% for all nine designs investigated. The final sensor design achieved a linear gauge factor of 252 and was fabricated for the spinal fusion application.