Massood Tabib-Azar, Steven Fulop, Michael Moore, David Hart: Case Western Reserve University

Abstract
We performed a feasibility study to determine if piezoresistive
pressure sensor (PPS) technology could be adapted to measure the hydrostatic pressures in human intervertebral discs. This novel application of a PPS transducer was chosen for its numerous advantages in performance, packaging, and cost over current competing technologies. Commercially available PPS chips were modified producing sensor chips 8x8x3 mm3 with a sensing element that was 1.5x1.5x1 mm3. The PPSs were calibrated for hydrostatic pressure using a dedicated pressure chamber. The sensors were found to have an average sensitivity of 1mV/PSI. Methods were developed to implant the sensor and measure the intervertebral disc pressure in response to axial compressive loads. Experiments were run on 3 human lumbar functional spinal units (FSU) using a custom built axial loading device. In these experiments the hydrostatic disc pressure varied linearly with applied axial load. The PPSs sensitivity to compressive load ranged from 1 mV/PSI down to 0.5 mV/PSI at hydrostatic pressures above 20 PSI. A needle based pressure sensor functionally identical to those used in discography procedures was used to obtain a baseline measurement of the disc pressure response to axial loading in each FSU for comparison. It was found that the modified commercially available PPS element could be functionally adapted to measure the disc pressures, and was able to do so more reliably, accurately and precisely than needle based designs at pressures below 20 PSI. This upper limit of pressure is not inherent to the sensor structure and will be extended by using a slightly different sensor geometry. We will discuss design, fabrication and characterization of a minimally invasive wireless pressure/force sensor to monitor hydrostatic pressures in human intervertebral discs.