Spherical Indentation Behavior of Soft Electronics

This chapter is dedicated to the mechanical modeling of spherical indentation behavior and its applications in soft electronics. In the mechanical modeling part, some assumptions are made. First, the spherical indenter is a rigid body with infinite Young's modulus compared to the measured objects. Second, the feature size of the contact surface between the indenter and the specimen is much smaller than the feature size of the specimen, so the specimen is assumed to be a semi-infinite isotropic elastic solid with a finite modulus. Third, the interface between the indenter and the semi-infinite solid is frictionless. Theoretically, the relationship between the indentation displacement, the applied force, and Young's moduli of the specimens tested could be obtained explicitly. Two typical indentation-based sensors are presented here as application cases. Experimentally, the indentation displacement and the applied force could be measured directly or indirectly, for example, using a strain sensor. But for a single strain sensor, we can only get one type of data, strain or stress, derived from the resistance or capacitance signal. If we want to obtain both deformation and force information, some improvements are essential. Here, the advanced and latest methods adopted to simultaneously obtain both deformation and force information from only one kind of electrical data are also introduced. By solving this equation, the spherical indentation behavior could be applied in detecting the softness of materials, which is essential in scenarios requiring in situ softness measurement, including restoring human-like haptic behavior in elasticity sensing and medical palpation.