Size and shape optimization of a 1.0 mm multifunctional forceps-scissors instrument for minimally invasive surgery

A size and shape optimization routine is developed and implemented on a 1 mm multifunctional instrument for minimally invasive surgery. The instrument is a compliant mechanism, without hinges, capable of both grasping and cutting. Multifunctional instruments have proven to be beneficial in the operating room because of their ability to perform multiple tasks, thereby decreasing the total number of instrument exchanges in a single procedure. In addition, with fewer exchanges the risk of inadvertent tissue trauma as well as overall surgical time and costs are reduced. The focus of the paper is to investigate the performance effects of allowing the cross-sectional area along the length of the device to vary. This is accomplished by defining various cross-sectional segments along the device in terms of parametric variables (W_i) and optimizing the dimensions to provide a sufficient forceps jaw opening while maintaining adequate cutting and grasping forces. Two optimization problems are considered. First, all parametric segments are set equal to one another permitting all cross-sections to vary uniformly and achieving size optimization. Second, each segment is defined as a separate design variable to allow segments to vary independently and thereby achieving shape optimization. Due to the device's symmetry, one-half of the mechanism is modeled as a cantilever beam undergoing large deformation. ANSYS' optimization module is employed using the first order method because it is capable of performing optimization considering non-linear deformation and multiple loading conditions. Finally, prototypes are fabricated using wire EDM and prototype evaluations are conducted to compare size versus shape optimization, and to validate ANSYS as the solution method.