Experimental Observation and Modelling of Preconditioning in Soft Biological Tissues

Constitutive models for soft biological tissues and in particular for human organs are required for medical applications such as surgery simulation, surgery planning, diagnosis. In the literature the mechanical properties of biosolids are generally presented in “preconditioned” state, i.e. the stabilized conditions reached after several loading-unloading cycles. We hereby present experiments on soft tissues showing the evolution of the mechanical response in a series of loading and unloading cycles. The experimental procedure applied in this study is based on the so called “aspiration experiment” and is suitable for in-vivo applications under sterile conditions during open surgery. In the present study this technique is applied ex-vivo on bovine liver. A small tube is contacted to the target organ and a weak vacuum is generated inside the tube according to a predefined pressure history. Several identical loading and unloading cycles are applied in order to characterize the evolutive behaviour of the tissue. The experimental data are used to inform the fitting of uniaxial and threedimensional continuum mechanics models. This analysis demonstrates that a quasi-linear viscoelastic model fails in describing the observed evolution from the “virgin” to the preconditioned state. Good agreement between simulation and measurement are obtained by introducing an internal variable changing according to an evolution equation.