| In this study, analytical and Finite Element Method (FEM) are employed to determine the contact pressure on the surface of a tissue being grasped by an endoscopic grasper, in Minimally Invasive Surgery (MIS). Normally, an endoscopic grasper has corrugated teeth in order to maintain a firm grip on slippery tissues. Because it is very important to avoid damage while grasping and manipulating tissue during endoscopic surgery, it is essential to determine the exact contact pressure on the surface of the tissue. To this end, a comprehensive closed form analysis is undertaken followed by the finite element and experimental analyses of the grasping contact pressure on viscoelastic materials which have similar properties as that of biological tissues. The behavior of a grasper with wedge and semi-cylindrical teeth is examined when pressed into a linear viscoelastic material. Initially, a single tooth penetrating into a solid is studied and then is extended to the multi teeth grasper. The elastic wedge and semi-cylinder penetration is the basis of the closed form analysis. Also the effects of time are included in the equations by considering the corresponding integral operator from viscoelastic stress-stain relations. In addition, a finite element analysis is carried out. Finally, the experimental results will be presented to validate both analytical and FEM analysis. The results of this study provide a closed form expression for grasping contact pressure, force and contact area along with the variations of stress in tissue obtained through FEM analysis. The variation of contact pressure and the rate of growth of the contact area with time are also presented. In order to determine the properties of the biological tissues during MIS, we present the design, analysis, fabrication and assembly of four-tooth annular micro-fabricated tactile sensors incorporated to the upper and lower jaws of an endoscopic surgical grasper tool. Two viscoelastic models, namely, Kelvin-Voight and Kelvin are employed for tissue characterization. The relationship between the force ratio, compliance and the equivalent viscous damping of the tissue are studied. The designed sensor uses a Polyvinylidene Fluoride (PVDF) film as its sensing element. The sensor consists of arrays of rigid and compliant elements which are mounted on the tip of an endoscopic surgical grasper tool. The relative force between adjacent parts of the contact object is used to measure the viscoelastic properties. |
