Fingernail touch sensors: spatially distributed measurement and hemodynamic modeling

Advancements on a new type of touch sensor for detecting contact pressure at human fingertips are presented. A fingernail is instrumented with miniature LEDs and photodetectors in order to measure changes in the nail color when the fingertip is pressed against a surface. The fingernail sensor allows the fingers to directly contact the environment without obstructing the human´s natural haptic senses. Reflectance photoplethysmography is used to measure the pattern of nail color, i.e., the blood content under the fingernail. Hemodynamic modeling, based on actual fingertip anatomy and physiology, is used to investigate the dynamics of the change in blood volume at multiple locations under the fingernail. An optical model is created to relate the blood volume to the light intensity measured by the photodetectors. The theoretical analysis is verified through model simulation and experimentation using a prototype fingernail sensor