Photoplethysmograph fingernail sensors for measuring finger forces withouthaptic obstruction

A new type of touch sensor for detecting contact pressure at human fingerti ps is presented. Unlike traditional electronic gloves, in which sensor pads are placed between the fingers and the environment surface, this new senso r allows the fingers to directly contact the environment without obstructin g the human's natural haptic senses. The finger touch force is detected by measuring changes in the coloration of the fingernail; hence the sensor is mounted on the fingernail rather than on the fingertip. Specifically, the f ingernail is instrumented with miniature light emitting diodes (LEDs) and p hotodetectors in order to measure changes in the reflection intensity when the fingertip is pressed against a surface. The changes in intensity are th en used to determine changes in the blood volume under the fingernail, a te chnique termed "reflectance photoplethysmography." A hemodynamic model is u sed to investigate the dynamics of the blood volume at two locations under the fingernail. A miniaturized prototype nail sensor is designed, built, an d tested. The theoretical analysis is verified through experiment and simul ation.