Experiment And Theory Research On Cutting Mechanism Of Biological Soft Tissue

Robotic minimally invasive surgery has been paid more and more attention due to minimal damage caused in surgical operations. Similar to flight simulators, a surgical simulator is expectedly used for training surgeons before performing surgical procedures on human patients, which has become a research hotspot in recent years. With the development of robot minimally invasive surgery and a surgical simulation system, it is necessary to understand the deformation characteristics of soft tissue and know the responses of soft biological tissues during a surgical process."Cutting" is the core task of "surgery". It is very important for the developmet of robotic minimally invasive surgery and surgical simulation system to investigate the biolocical deformation characteristics and model the the responses of soft tissue during a surgical process. In this study, cutting experiments were carried out on porcine ascending aorta tissue. Cutting force was measured by a loadcell. The characteristics of soft tissue cutting process were analyzed. The impacts of the tissue cutting parameters such as initial holding force, tissue holding distance, cutting speed, inclination angle, blade radius and tissue cutting direction were investigated by cutting experiments. The soft tissue cutting process was analyzed by an energy approach from the view of fracture mechanics. Based on energy balance, a physical model was built to describe biological soft tissue cutting process. A finite element model was established to simulate the interaction between tool and tissue during biological soft tissue cutting process. The research work mainly includes:(1) Aorta tissue constituents and structure, especilly the medial aortic microstructure were decribed in detail. The relationship between tissue structures and their mechanical response were analyzed. Uni-axial tensile experiments were carried out to measure the mechanical properties of porcine aorta tissue including stress-strain response, stress relaxion and tensile strength.(2) A tissue cutting apparatus was designed and constructed. The cutting experiments were carried out ex vivo on porcine ascending aorta tissue which was a representive soft tissue. The cutting force was measured and the cutting process was characterized. We explored the impact of the tissue cutting variables such as initial holding force, tissue holding distance, cutting speed, tool inclination angle, blade radius, anatomic orientation and tissue thickness. The effects of tissue cutting variables on cutting force and local stiffness were investigated. Initial holding force and tissue holding distance were very important for soft tissue cutting since they had important effects on the cutting force and local stiffness. A large tool inclination angle and a small blade radius could reduce the cutting force, but had no effect on tissue local stiffness. Cutting speed had an effect on cutting force, but not on tissue local stiffness in the range of0-150mm/min. The effect of tissue bulging was study during the soft tissue cutting process. The impacts of cutting parameters on tissue bulging were analyzed.(3) The soft tissue cutting process was analyzed from the perspective of fracture mechanics. The rupture mechanism during soft tissue cutting was exposed. The effect of cutting parameters on "break-in" energy was investigated.The fracture toughness of porcine ascending aorta was calculated from cutting experiments under different tissue holding conditions based on an energy balance equation and the value is in a range from800-1800J/m2.(4) Based on the energy balance in the cutting process, the energy conversions were analyzed for each phase. The cutting force was modeled based on the energy balance. The cutting force in the deformation phase showed a nonlinear J-shaped curve. An exponential function was used to fit the cutting force in the deformation phase. The cutting force in rupture phase shows a linear decrease in a short time. The cutting force obtained with the model fits the experimental force curve well, which justifies the cutting force model.(5) A finite element model was built to simulate the interaction between tool and tissue during cutting. Ogden model was used as a material model of porcine aorta tissue. Different cutting conditions such as various initial holding force, tissue holding distance, wedge angle and blade radius have been simulated using FEM by changing the boundary conditions and tool geometric dimension. The experimental cutting force was fit by the cutting force from FEM in order to testify the validity of the model. The cutting process was simulated by FEM, which provided the maximun stress and stress distribution of blade surrounding the tissue at the "break-in" point. The rupture stresses of aorta tissue were used in FEM under different cutting conditions. The effects of tissue holding conditions and tool geometric dimension were investigated.The research results reported in this study provide a basis for understanding the characteristic responses of soft tissue to scalpel cutting and soft tissue cutting mechanism, which is helpful for surgeons to create new surgical tools and improve surgical operations. The results of this study are useful to the development of minimally invasive robotic surgery and surgical simulation systems.