Dynamic mechanical analysis of porcine liver viscoelasticity: a reproducible protocol and reference values for...

Multiple tissue-mimicking biomaterials for the hepatic environment have recently been proposed for a wide range of applications, from preoperative training phantoms to the development of 3D cell cultures for in vitro modelling. Despite their different nature and scope, these platforms are linked by the need to closely mimic the mechanical properties of the liver. To this end, a thorough understanding of the mechanical behavior of this organ is needed. Many studies applied indirect testing methods or quasi-static analyses, which provided some indications regarding the mechanical characteristics of the liver. Furthermore, variations in testing conditions and sample origins affect the few studies applying dynamic mechanical analysis to this aim. We present the development of a reproducible procedure for analyzing the viscoelastic properties of porcine hepatic tissue under small dynamic shear and compressive strains, accounting for possible anisotropies and inhomogeneities in the tissue. Various frequencies and pre-compression levels were considered, and the feasibility of preserving specimens by freezing was assessed. Livers from different sources were tested to evaluate intra-species variability, and two pathological organs were examined. The analysis revealed that liver tissue exhibits solid-like behavior, with a higher storage modulus than loss modulus in the considered frequency range (G' = 107-279 Pa, G" = 18-54 Pa, E' = 31.3-33.6 kPa, E" = 4.2-4.9 kPa). Tests performed in oscillatory shear show that the physiological liver has an isotropic, homogeneous mechanical behavior, whereas the hepatitic organ shows inhomogeneous properties. Moreover, the two pathological organs were analyzed as preliminary, single-organ case studies (one steatotic, one hepatitic), and in both the diseased tissue was stiffer than the physiological one. Finally, the difference in mechanical properties between oscillatory shear and compression measurements is consistent with describing the liver's microstructure as a fluid-filled closed-cell foam, which we propose here as a working hypothesis. These results provide new insight into the mechanical properties of the liver, which can be used to develop accurate and reliable material-based models for disparate applications in the field of life sciences.