Abstract

The commercially available endoscopic graspers are tooth like in order to grasp slippery tissues during a Minimally Invasive Surgery (MIS). However, they don't provide the tactile feedback to the surgeon during the operation. One of the most important requirements of a tactile feedback in MIS is softness sensing. This thesis presents the design, analysis, fabrication and testing of an annular microfabricated piezoelectric tactile sensor to determine the softness of tissues during MIS. This tooth-like sensor consists of an array of four sensors on each jaw of the endoscopic grasper. Each sensor consists of a rigid inner cylinder surrounded by a soft hollow cylinder. The sensor works on the principle of relative deformation between the inner rigid cylinder and the outer compliant cylinder. In this design Polyvinyledene Fluoride (PVDF) film is used as the sensing element. The fabrication sequence of the sensor involves three steps, namely, fabrication of cylinders, PVDF film patterning and sensor assembly. The compliant cylinders are realized by micromolding of liquid silicone rubber. The PVDF film was patterned using photolithography techniques and the patterned portions are etched using commercially available aluminium etchant. Different parts of the sensor are assembled under a stereomicroscope. Different materials are tested under dynamic applied load and their modulus of elasticity has been found using the force ratio output from the PVDF sensing element. A 3D analysis in ANSYS has been done to compare the force ratios with the experimental values. It has been shown that good agreement exists between experimental and finite element results. A viscoelastic Kelvin model is employed for tissue characterization. A closed form expression is derived to express the relationship between the force ratio and compliance. Using the force ratios output from the experiments performed, the compliance of various soft materials in contact with the tactile sensor are obtained. These results are shown to be in close agreement with the manufacturer's value