Research On Heart Contact Meachics In The Minimally Invasive Surgery For Mitral Valve

In minimally invasive surgery, the physical properties of biological tissue can be measured by tactile sensor, and be fed back to surgeon. These properties play an important role in the whole planning and the specific implementation of the surgery. Recently, the research on this kind of medical tactile sensor is still in its initial phase. The thesis, taking a percutaneous catheter MIS for mitral valve regurgitation as practical background, is devoted to the following research i.e., the design of tactile sensor,the mechanics of heart tissue, and the contact problem between sensor and heart tissue.Taking account of the tiny implementation space and the complex working environment of the catheter, this paper designed a piezoresistive tactile sensor, which is compatible with the configuration of the catheter and highly robust. This tactile sensor is composed of two cylindrical force sensors with different radii, and a filler plate. In the prototype experiment, when the tactile sensor makes contact with several different polymers, its outputs can correctly reflect the hardness levels of these polymers.As a result, these polymers can be accurately distinguished.Due to small deformations experienced by atrium during heart beating, the mechanics of the atrial tissue is described by two linear viscoelastic models i.e., the generalized Kelvin model and the fractional order viscoelasticity model. These two models have their own advantages and disadvantages,the generalized Kelvin model shows explicit physical meaning, and the fractional order viscoelasticity model can be used in a broader applications. To obtain the experimental data about the mechanics of the atrial tissue, the viscoelastic properties of the atrial tissue are measured via dynamical mechanical analysis test. Based on the experimental data, the material parameters in the generalized Kelvin model and the fractional order viscoelasticity model are identified through the genetic algorithms. Comparing the experimental value with the theoretical value, it is found that both the two models can be in good agreement with the experimental data.In the research on the mechanics of the ventricular tissue, an orthotropic, hyper-viscoelastic model,based on the invariant theory in continuum mechanics and the ventricular microstructure, is proposed.In this model, the thermomechanical principles are satisfied by introducing internal state variables. According to the experimental data on the simple shear of the ventricular tissue, the material parameters in this viscoelastic model are identified via the genetic algorithm. Then, the cylindrical and the axisymemtrical ventricle models are separately analyzed. The analyses illustrate that the residual stresses and the transmural Cauchy stresses predicted by the hyper-viscoelastic model are matchable with the results given in the literature, meanwhile, in the analysis of shear deformation, the results obtained from this model are better agreeable with the experimental dataWhen dealing with the contact of the sensor and the atrial tissue, the smooth contact problem and the frictional contact problem are treated, separately. Especially, the traditional contact mechanics is used to solve the smooth contact problem. First, based on the general Papkovich–Neuber solution in the boundary-value problem, the linear elastic solution of the contact problem is obtained through Combination–of–Harmonics method. Then, use of the correspondence principle between elasticity and viscoelasticity is made to get the viscoelastic contact solution. Under various boundary conditions, the contact process between the sensor and the atrial tissue can be directly analyzed by the viscoelastic solution. The final results show, although the sensor can measure the hardness of the atrial tissue, its output is also closely related to the normal force enforced on the sensor. When analyzing the frictional contact problem, finite element method is adopted. In this analysis, a split strategy for the constitutive relation is proposed. In this strategy, the generalized Kelvin model is split to be several traditional Kelvin model,hence, the finite element analysis can be conveniently implemented. Frictional model is an important factor influencing the convergence of the finite element contact analysis, an elastoplastic friction model based on the interfacial microstructure is utilized in the thesis. The results of this frictional contact analysis demonstrate the friction delays the contact time between the atrial tissue and the small force sensor, but plays a little role in the whole contact analysis.Due to the large deformation experienced by ventricle in heart beating, hence, the contact analysis between the sensor and ventricular tissue is performed via the finite-deformation contact algorithm. To obtain good robustness and convergence, the mortar method based on dual Lagrange multiplier is used to derive the weak formulation of the contact problem. Then, the weak formulation are implemented efficiently through semi-smooth Newton computational strategy. After the analysis, it is found that the muscle fiber pattern has a little effect on the contact between the tactile sensor and the ventricular tissue,however, the constraint imposed by the pericardium sac influences the contact process in an apparent way. If the motion of the outer boundary of the ventricle is fixed, the tactile sensor can measure the hardness of the ventricular tissue, and if the outer boundary of the ventricle can move freely, the tactile sensor fails to measure.