Design and Modeling of a Wide Dynamic-Range Hardness Sensor for Biological Tissue Assessment

We report a tactile sensor for characterizing the hardness of biological tissues, targeting applications in the field of minimally invasive robotic surgeries. The sensor employs a tandem spring design for which key design parameters are proposed analytically and confirmed experimentally. The sensors are realized using a direct silicon-to-PCB fabrication/packaging scheme. We experimentally demonstrated that the sensitivity of the tactile sensor exhibits a scaling factor dependent on contact conditions, proving that the precise control of contact angle and contact force is important. The sensor modeling takes account of practical, non-ideal contact angles, as well as variable magnitude of contact forces. The MEMS based hardness sensor is able to measure within a hardness range of 0.3-360 psi (2.1-2482.1 kPa), covering most biological tissue types of interest during robotic surgeries or other medical procedures. To our knowledge, it is the first time that hardness measurement with such wide dynamic range has been experimentally demonstrated.