Microelectromechanical force and tactile sensors for minimally invasive surgery

Abstract

Sense of touch plays an important role in surgery. Minimally invasive surgical (MIS) procedures provide great benefits to patients; however, the surgeon’s ability of perceiving force and tactile sensation from tissues is severely impaired. Performing surgery without such sensory information could lead to increase of tissue trauma and vital organic tissue damage. This dissertation presents the development of miniaturized force and tactile sensors that are able to measure the contact force and relative hardness of contact objects, with the purpose of enhancing the surgeon’s touch sensibility during minimally invasive surgery. The force and tactile sensors developed in this dissertation are based on microelectromechanical systems (MEMS) technology utilizing the transduction principles of silicon nanowire (SiNW)-based piezoresistance and bulk acoustic resonance. A miniaturized force sensor that can be integrated on the distal tip of a commercial guidewire is firstly developed. The sensor has a sensory area of 200 µm × 200 µm, and utilizes SiNWs as piezoresistive sensing element by taking advantage of its ultra-small dimension and high sensitivity. Together with the movable core wire, the sensor can detect contact forces while maintaining the original welded tip which is an essential part of the standard guidewire assembly. A finite element (FE) model is built in COMSOL Multiphiphysics® v4.2, in the aim of finding the maximum stress location in order to locate the SiNWs at the high stress regions and simulating the mechanical behaviour of the sensor-guidewire interaction as well. The results from simulation have been verified by the experimental measurement results. By taking advantages of the high sensitivity of SiNWs, the fabricated sensor is capable of detecting small displacement in nano-meter scale with a sensitivity of 13.4 x 10-3 µm-1 in the z-direction. It has been shown from the characterization results that the sensor has high linearity (> 99.9%) to the applied load without obvious hysteresis. To enhance the force sensitivity of the ring-shaped sensor that has a flat surface, a biomimetic tactile sensor is then designed and characterized. The Meissner corpuscle inspired sensor is designed with a rigid spherical ball attached on top of the suspended ring structure mimicking the biological structure of the Meissner corpuscles. The sensor is developed with a simplified fabrication and packaging process by dropping spherical balls on the SiNW-based ring structure. Finite element analysis (FEA) has been used to predict the mechanical behaviours of the tactile sensor with rigid solder ball and with elastic bump covering of rigid solder ball underneath. The performance of the SiNWs on the bio-inspired sensor and on the bare ring-shaped sensing element is comparatively studied by repeatability and hysteresis tests. The effectiveness of sensitivity enhancement of the bio-inspired tactile sensor with solder ball attached is verified from the experimental results, in which the fractional change in resistance in normal direction is improved from 1.34 percent to 2.8 percent per micro meter. Following, a miniaturized resonant tactile sensor is developed for material elasticity measurement. The sensor is based on a MEMS silicon bulk acoustic wave (BAW) resonator, incorporated with a rounded sensing tip and mechanical stoppers to limit the maximum loading force and contact area. Upon contact with the material, the resonant frequency shifts corresponding to the amount of mass and stiffness loading from the contact object. The BAW resonator is excited in the square-extensional (SE) mode and is fabricated using a silicon-on-insulator (SOI) multi-user MEMS process through MEMSCAP. The resonant frequency of the resonator is 2.2 MHz and has a Q-factor of 10430 in air. The ability of the sensor to measure the relative hardness of the contact object is tested with experiments by contacting the sensor with materials of different degrees of Young’s modulus. Results show that the BAW resonant tactile sensor can provide information not only about the stiffness of the materials in contact with the sensor, but also the extent of force/stress applied on the sensing tip. The sensor is demonstrated to be able to differentiate materials of Young’s modulus in the GPa range.