Novel 3D printed capacitive shear stress sensor

This paper describes the research into 3D printed capacitive shear stress sensors. The context is the development of a technology that can potentially be used in shoe insoles. This is interesting in medical applications, such as prevention of diabetic foot ulcers. The base geometry consist of a capacitive layout with parallel wire structures and has two flexible elements on two sides. Experimental devices are optimized using electrostatic and mechanical models and printed using material extrusion additive manufacturing (AM) with a flexible thermoplastic polymer (TPU). Two geometries with different sizes are designed and tested. The smallest design is 8.75×8.75×11mm and its responsivity is measured to be 0.9 pF/MPa for a measurement range of ±140kPa. The design can easily be altered to meet specifications for a variety of other applications. The realised sensors show some hysteresis, which we attribute primarily to the mechanical behavior. The capacitive measurement of the displacement is quite linear. The mechanical hysteresis can partially be explained by the properties of the printed material. The results are promising for application in shoe insoles and can relatively easily be adapted to meet requirements for other applications.