Mechanical model of flex sensors used to sense finger movements

Flex sensors change in their electrical resistance value depending on the amount of bend they are subjected. These sensors can be realized with different techniques, but the mostly adopted are based on a resistive element film, which can be made of a PEDOT:PSS polymer or of a carbon-based element, printed on a plastic substrate. We investigated from a theoretical point of view the mechanical aspect of these sensors related to their utilization to sense finger movements, to determine the relationship with the electrical behavior. This was to provide enhanced degrees of understanding and predictability in performances. Experiments were conducted on custom-made flex sensors based on PEDOT:PSS polymer, to measure the resistance variations vs. the amount of bend radius induced in the sensors, in either of two opposing directions (outward and inward). Results demonstrated that the mechanical behavior influences the electrical counterpart but linearly, the more the bent the more the increase in the resistance, in a direct proportion. When a nonlinear behavior is measured, as in the case of commercially available Flexpoint sensors, it was found to depend only to causes strictly related to physical changes in the resistive element (cracks under stress).