Optimization of stretchable fingernail sensor fabrication based on finite element modeling

This paper describes the fabrication of a novel stretchable fingernail sensor prototype, mounted on the fingernail for detecting fingertip touch force direction. Fingernail sensors are optical reflectance photoplethysmograph sensors that detect the change in blood perfusion in the nail bed, as the finger pad touches a planar surface with various forces. The original fingernail sensor was rigidly attached to the human fingernail and accordingly needed to be customized to the geometry of the fingernail. Hence, to eliminate the need for customized fabrication for each individual user, it is desired to fabricate a stretchable fingernail sensor that is independent of the fingertip geometry. In this paper, we develop novel fabrication techniques that overcome the challenges of patterning conductive lines on a stretchable substrate, and of embedding rigid optical components in a stretchable platform while maintaining electrical conductivity. We conduct finite element analysis to optimize the sensor's design parameters. We also demonstrate the successful fabrication of a stretchable sensor prototype and test its functionality in relation to the design parameters. The outcomes of this work will also be applicable to a broad spectrum of other stretchable sensors that have an analogous concept of operation. Stretchable circuits are generally useful for any application where a circuit must conform to a surface with arbitrary two-dimensional curvature, such as the human body. Ultimately, similar sensors can also be placed on other parts of the body to monitor blood flow and perfusion.