Viscoelasticity and plasticity of living tissue revealed by a microfluidic rheometer

Tissue stiffness has long been correlated with the homeostatic or pathogenic state of the tissue. Here, we postulate that, in addition to stiffness, tissue viscoelasticity and plasticity are important biomarkers for evaluating the biological function of the tissue. In this work, we characterized the viscoelasticity and plasticity of non-tumorigenic versus malignant breast tumor spheroids using a recently developed microfluidic rheometer and a theoretical power law model. Our results revealed that the measured strain relaxation curves can be successfully described by a modified power law model. Using computed effective viscosity we found that malignant spheroids were more fluid like in longer time scales compared to non-malignant spheroids and this was consistent with our observation of increased invasion by malignant spheroids. Furthermore, non-tumorigenic tumor spheroids were more elastic, and less plasticity than those of tumorigenic spheroids. Together, these results highlight that viscoelastic properties can serve as complementary mechanical biomarkers of tumor malignancy and demonstrate the validity of a modified power law model for the mechanical characterization of a living tissue.