Double-Sided Conformable Piezoelectric Force Sensor with Enhanced Performance and Bending Correction

Abstract

Flexible piezoelectric devices have gained considerable interest due to their potential for new applications, particularly in wearable technology. However, a significant challenge remains in measuring low forces on nonplanar and deformable surfaces. Indeed, conformability on complex surfaces induces bending stresses in the piezoelectric sensors, interfering with the measurement of compressive force. Yet such measurements can be valuable, especially in medical applications that involve assessing forces on soft tissues. This study presents an innovative highly sensitive conformable sensor based on a thin film of P(VDF-TrFE) copolymer. The selection of the substrate is essential for ensuring the device's conformability, but it is also demonstrated that it can provide a substantial improvement in performance if its Young's modulus is lower than that of the active polymer. The effective piezoelectric charge coefficient $d_{{33}_{eff}}$ of a sensor on TPU substrate is measured equal to −340 pC.N⁻¹, representing a tenfold increase in the theoretical compression sensitivity of P(VDF-TrFE). Additionally, a double-sided structure to eliminate the contribution of bending in the piezoelectric signal and tackle the challenge of conformability on complex surfaces is developed. Overall, the proposed device shows promising results for measuring low forces applied to soft biological tissues such as skin or heart valve leaflets.