Measuring forces in liver cutting: new equipment and experimental results.

We are interested in modeling the liver cutting process as accurately as possible by determining the mechanical properties experimentally and developing a predictive model that is self-consistent with the experimentally determined properties. In this paper, we present the newly developed hardware and software to characterize the mechanical response of pig liver during (ex vivo) cutting. We describe the custom-made cutting apparatus, the data acquisition system, and the characteristics of the cutting force versus displacement plot. The force-displacement behavior appears to reveal that the cutting process consists of a sequence of intermittent localized crack extension in the tissue on the macroscopic scale. The macroscopic cutting force-displacement curve shows repeating self-similar units of localized linear loading followed by sudden unloading. The sudden unloading coincides with observed onset of localized crack growth. This experimental data were used to determine the self-consistent local effective Young's modulus for the specimens, to be used in finite element models. Results from finite element analyses models reveal that the magnitude of the self-consistent local effective Young's modulus determined by plane-stress and plane-strain varies within close bounds. Finally, we have also observed that the local effective Young's modulus determined by plane stress and plane strain analysis decreases with increasing cutting speed.