VISCOELASTIC CHARACTERISATION OF PIG LIVER IN UNCONFINED COMPRESSION

Understanding and modelling liver biomechanics represents a significant challenge due the complex nature of this organ. Several methods and models based on direct measurements on the liver (e.g. rheological, compressive or indentation tests) or image-based techniques (e.g. magnetic resonance or ultrasound-based elastography) are reported in literature to characterise the liver viscoelastic behaviour in-vitro or in-vivo [Marchesseau et al, 2010]. Unfortunately, there is no consensus on liver viscoelastic properties, and results are strongly dependent on adopted testing method, sample type, status and testing conditions. We focused on in-vitro unconfined bulk compressive tests for deriving liver viscoelastic parameters in the linear viscoelastic region (i.e. small strain region). We propose the use of the ε̇M (epsilon dot method) which we developed to address the major drawbacks of standard tests (e.g. step response or dynamic mechanical tests) such as long test duration and initial contact between sample and testing apparatus, that may significantly pre-stress/strain very soft and hydrated samples and alter their status [Tirella et al, submitted]. With the ε̇M, samples are characterised using standard compressive tests at different strain rates (ε̇). Stress-time series collected at various ε̇ are then fitted using a multi curve shared parameter fitting approach. Liver viscoelastic parameters estimated with ε̇M were compared to those obtained using conventional dynamic mechanical (DMA) testing systems.