Mechanical Characterization of Bio-Tissues with Optical Techniques and Global Optimization

Availability of detailed information on mechanical behavior of biological materials may optimize design of biomedical devices maximizing their bio-functionality and durability. Nowadays, researchers are spending considerable effort to properly describe biological systems and to correctly define different stages of possible clinical interventions. The Finite Element Method is often utilized to predict mechanical behavior of complicated structures but numerical predictions are strongly affected by mechanical properties given in input to the FEM model. Another possible approach relies on the use of experimental techniques that have the inherent advantage to measure displacements without making any assumption on material properties. However, it may be difficult to reproduce effective conditions experienced in-vivo by biological districts. Furthermore, there may be limitations on the effective capacity of reproducing loads and dealing with very small sized specimens. Generally speaking, structural behavior is univocally defined once displacements are known for each point of the specimen. That is, displacements are known once material properties, loads and boundary conditions are assigned. Material characterization is a reverse engineering problem that can be seen as an inverse solution of the general elastic problem. The inverse problem can be solved by minimizing the difference between the measured displacements and their counterpart predicted numerically. Non-contact optical techniques such as Moiré and Electronic Speckle Pattern Interferometry (ESPI) fit very well in the material characterization process since they are superior over other experimental techniques in view of their capability to accurately measure displacements in real time and to gather full field information without altering specimen conditions. Fringes will appear on the surface of a specimen subjected to deformations and represent the loci of iso-displacement regions. Strain distribution can hence be recovered from fringe frequency. Versatility of optical techniques also permits to choose the experimental set-up most suited to the particular case under investigation. Global optimization algorithms such as Simulated Annealing will perform better than gradient based optimizers because of their inherent ability to deal with a highly nonlinear and non-smooth problem like the identification process. Another important aspect that must be considered when analyzing bio-structures is how accurately geometry is reproduced and modeled. In fact, mechanical behavior may be very sensitive even to little changes in geometry. Therefore, accurate reconstructions of geometry will certainly improve the reliability and the overall efficiency of numerical models. The first part of the present chapter will discuss the implementation and application of a hybrid procedure for material characterization. The procedure combines optical techniques and Simulated Annealing. Feasibility of the procedure has been checked for materials such composite laminates and hyperelastic membranes for further applications on bones and soft bio-tissues. The second part of this chapter will discuss issues involved in the accurate geometric reconstruction of biomechanical districts. In particular, a specifically designed device used for reconstructing the shape of a porcine vascular segment is described in detail.