Analysis of the flexible PDMS capacitive microsensor for the 3-axis force measurement

Analysis of a flexible capacitive microsensor is described to measure both normal and shear forces simultaneously and instantaneously for the potential application of gait analysis. The microsensor consists of two pairs of sensing electrodes and an in-between dielectric layer. When normal force is applied to the microsensor, the capacitances of two sensing electrodes increase because of the uniform compression of the dielectric layer. When shear force is applied, deformation of the dielectric layer induces the capacitance difference between two sensing electrodes. Therefore, by measuring the capacitance values and capacitance difference, the normal and shear force can be determined directly and simultaneously. Since the microsensor is designed for measuring large forces, polydimethylsiloxane (PDMS) is selected as the material of the dielectric layer because it has the advantages of high dielectric constant and tunable elasticity. But PDMS is an elastic material that has highly non-linearity of stress-strain relationship. In this work, PDMS characterization was conducted to investigate the stiffness under different mixing ratios of PDMS pre-polymer and the curing agent. PDMS in 16:1 mixing ratio was selected since it has the most linear stress-strain relationship with the highest elasticity based on our results. Hence, analysis of mechanical and electrical properties of the microsensor was performed and the capacitance changes under normal and shear stresses were calculated. The microsensor was design to support the maximum normal stress of 1.39MPa and shear stress of 85KPa based on the requirement of foot pressure measurement. The microsensor is designed using flexible materials and it is easy to access to the curved surface to measure its interfacial force with minimum disturbance. It has the potential to develop portable real time microsensor for many biomechanical applications.