Compliance-dependent load allocation between sensing versus non-sensing portions of a sheet-array contact stress sensor.

Piezoresistive array pressure sensors are widely used in orthopaedic research to determine contact stress distributions across articular joint surfaces. Experience with such sensors has shown there can be inaccuracies in how the sensor perceives applied load, depending on the material stiffnesses between which it is compressed experimentally, versus in calibration. A study was undertaken to quantify the relationship between load perception of one such sensor design (Tekscan) and the stiffness of the materials between which it is compressed. A three-dimensional finite element model of a 3x3 sensel portion of the sensing matrix was formulated, along with a layer of compression test material on each side of the sensor. The elastic modulus of the test material was varied across the range representative of cartilage (12 MPa) to hard plastic (10 GPa). Using the computed contact pressure results between contacting surfaces of the sensor layers, the percentage of load passing through the active conductor intersections was determined. The results revealed that with increase of the elastic modulus of the material between which the sensor was compressed, the percentage of load on the active conductor intersections increased monotonically. The highest sensitivity of perceived loading to test material modulus (0.1%/MPa) was seen at the low end of the modulus range. The more compliant the test material, the more the sensor layers conformed around each other's geometric incongruities, the larger the true contact areas, and the higher the fraction of the total load that passed through the intermediate (non-sensing) regions between the conductors.