Design and microfabrication of a strain - gauge array on polymer substrate for tactile neuroprosthesis in rats

Abstract

In this work design, microfabrication and characterization of a strain gauge array which can be used for sensory neuroprostheses in rats is presented. The array is composed of an array of 2×7 cells, each of which has a series combination of 4 strain gauges. Each group of four strain gauges is placed around a square membrane with the size of 2.5×2.5 mm2. Unlike most common tactile sensors based on silicon substrate, we used 3D-printed polylactic acid (PLA) as a substrate. Strain gauges were fabricated by depositing and patterning a thin aluminum (Al) film on a polyimide sheet with a thickness of 0.125 mm. Polydimethylsiloxane (PDMS) elastomer was bonded on the top surface of the polyimide membrane. PDMS layer is prepared in two different thicknesses for the sake of a comparative investigation into influence of the thickness of the elastomer membrane in static response of the sensor. The maximum allowable force corresponding to a maximum deformation of 0.9 mm of the center of each cell in two sensors differs according to the thickness of PDMS layer. The average force that each sensor cell operates linearly is 3N with an average resistance variation of 200 mΩ/N (for the sensor with 1.2-mm thick PDMS) and 4 N with an average resistance variation between 70 mΩ/N (for the sensor with 1.5-mm thick PDMS), and a nonlinearity of less than 3%. The cells have comparatively low average cross-talk around 5 mΩ/N. The static response of the cells was calibrated by using a micromanipulator and a digital balance and dynamic response of the individual cells of the sensor was characterized at several frequencies by using a vibrotactile stimulation system and a high gain amplifier. Then the sensor was tested inside the conditioning chamber to demonstrate the reliability of the sensor response for both static and dynamic stimulations.