Physical regulation of cell behaviors: multifrequency AFM measurement of mechanoelectric, mechanochemical, and hydration shell effects on ECM and cell substrate viscoelasticity

The extracellular matrix (ECM) is a scaffold of molecules, constructed and maintained by cells, that determines tissue structure. Cell-ECM interactions are crucial for maintaining healthy tissues. Understanding these interactions could provide insight into regenerative medicine and help develop new disease treatments. The mechanical properties of cells and the ECM are crucial in this interaction. Biological systems are viscoelastic (viscous and elastic), meaning they respond to applied forces with characteristic time delays known as time responses. For models of viscoelasticity that describe biological structures, time responses are the ratio of viscosity to elasticity. Furthermore, viscoelastic, electrical, and chemical properties of biological systems are coupled. In this thesis, I applied advanced dynamic atomic force microscopy (AFM) to quantify effects of electrical and chemical stimuli on ECM viscoelasticity at cellular length scales. Mechanoelectric coupling in the ECM was studied by measuring how ions altered collagen (the most prevalent component of the ECM) mechanics. Increased ion concentration increased collagen elasticity and viscosity, maintaining constant time responses. Ion flow decreased collagen time responses. Additionally, these experiments demonstrated that collagen hydration shells are viscoelastic, suggesting fluid within tissues contributes directly to tissue viscoelasticity. Next, mechanochemical coupling in the ECM was studied by measuring viscoelasticity of ECMs produced by cells in response to growth factors and mutation. Cells deposited more ECM in the presence of growth factors, increasing ECM viscosity and elasticity, yet maintaining constant time responses. Mutants exhibited reduced ability to produce ECM, modify viscoelasticity, and less control over time responses. The final set of experiments measured viscoelasticity of substrates known to alter cell behaviors, demonstrating a potential relationship between substrate time responses and cell behaviors. Together, my findings suggest that ECM time responses remain constant in many environmental conditions. Alteration of time responses may be a physiological mechanism of regulating cell activities and tissue health.