A study on utilizing multi-scale indentation and compression methods to robustly characterize the mechanical properties of viscoelastic materials

Abstract

Mechanical cues imparted onto cells, such as the stiffness and stress-relaxation properties of the surrounding microenvironment, can significantly impact cell function and behavior. In the case of cancer, the increased stiffness of the tumor stroma, relative to healthy tissue, has been found promote tumor progression by enhancing cell proliferation and motility. Due to the fundamental role of biomechanical signals in disease progression, researchers often seek to elucidate the relationship between tissue characteristics and their impact on cell behavior, a task that requires robust methods to characterize their mechanical properties. While many studies have characterized materials by measuring their stiffnesses, few have attempted to describe the rate-dependent, viscoelastic behavior of soft tissues and their hydrogel models.

In this study, we aimed to robustly characterize the mechanical properties of various viscoelastic materials using indentation and compression methods across a multitude of length scales. By performing stress-relaxation tests on a set of hydrogels and in vitro tumor models, we have determined an appropriate set of material properties to describe their elastic and viscoelastic loading responses. We then use these properties to identify mechanical markers in tumor models derived from phenotypically distinct cancer cell lines, and we further elucidate on the contribution of specific molecular pathways to these mechanical properties by adding contraction-inhibiting drugs to tumor models. Finally, we establish the mechanical heterogeneity of soft tissues at different length scales by performing milli- and nano-scale indentation on hydrogel and tumor samples. Our findings support the notion that both elastic and viscoelastic properties are required to fully characterize the mechanical behavior of soft tissues. The results of this work may provide insight into the appropriate loading conditions and material descriptors that should be used for mechanically testing and characterizing hydrogels and tumor models at different length scales.