CHARACTERIZATION OF VASCULAR SMOOTH MUSCLE CELL MECHANICAL AND FRICTIONAL PROPERTIES USING ATOMIC FORCE MICROSCOPY

A working hypothesis within the Laboratory of Vascular Research is that mechanical loading on vascular smooth muscle cells (VSMCs), especially due to solid contact from endovascular devices, contributes to the development of restenosis. In order to better understand the role of mechanical loading on VSMCs in vascular disease development, it is imperative to understand the mechanical properties of VSMCs themselves. To measure the viscoelastic and frictional properties of living VSMCs in an in vitro setting, an atomic force microscope (AFM) was utilized, thereby allowing for mechanical testing of living cells in a fluid environment. In the first phase of research, it was found that proliferative VSMCs, similar to those commonly found in atherosclerotic lesions, had lower stiffness and higher hysteresis values than quiescent VSMCs. Furthermore, measured stiffness values did not appear to deviate greatly within the central region of adherent cells. As VSMCs are viscoelastic, rather than purely elastic in their mechanical behavior, phase two involved the development of an AFM-based stress relaxation technique, in order to quantify VSMC viscoelastic behavior. Suitable mechanical models, including the QLV reduced relaxation function and a simple power-law model, were identified and applied to accurately describe VSMC stress relaxation. In addition, the roles of two major cytoskeletal components, actin and microtubules, in governing stress relaxation behavior, were quantified via the aforementioned mechanical models. In phase three, the surface frictional properties of VSMCs were focused upon, and a novel method to quantify surface shear forces on VSMCs using lateral force microscopy was developed. It was determined that VSMC frictional properties are greatly influenced by cell stiffness, and elastohydrodynamic lubrication was proposed as a possible cellular lubricating mechanism. During research phase four, each of the techniques developed during the preceding phases was employed to test the effects of a clinically relevant biomolecule, oxidized low-density lipoprotein (oxLDL) on VSMC mechanical properties. It was concluded that oxLDL is associated with decreased cell stiffness, and decreased viscosity, as measured by stress relaxation and indentation tests. Furthermore, frictional coefficients were found to correlate positively with more fluid-like cells. This research project has led to a better understanding of VSMC mechanical behavior, as well as the development of AFM-based techniques and models that will be useful in determining cellular mechanical and frictional effects of various stimuli in an in vitro environment