Ilya Beskin Final Defense

Abstract: Filament networks play a vital role for processes at the cellular level. Inside the cell, microtubules provide the cell with structure, are responsible for internal organization, facilitate for active transport, and help with cell division. Animal cells exist in a 3D matrix structure called the extracellular matrix. Different organizations of collagen fibrils, the main load bearing component of the extracellular matrix, are responsible for tissue mechanical properties. Cells actively interact with individual collagen fibrils through mechanotransduction. An understanding of the mechanical properties of individual filaments and of filament networks is crucial to studying these processes. To achieve this goal, a technique for visualizing the filaments and for extracting structural information is required. In this work, we develop a quantitative optical tweezer based microscopy technique capable of not only label-free visualization of individual filaments, but also accurate measurements of filament diameters. By combining this method with activity microscopy, a technique developed in our lab for measuring filament fluctuation amplitudes, we are able to extract the Young's modulus of single collagen fibrils at small deformations. A small number of studies have published values for the Young's modulus of collagen, which vary by over an order of magnitude. Varying sources of collagen and collagen preparation techniques, assumptions about fibril radius, and strain stiffening behavior inside collagen fibrils contribute to this wide spread of published values. By accounting for each fibril's radius profile and measuring the Young's modulus at very small displacements, activity microscopy becomes a sensitive tool for measuring the effects of collagen preparation on its mechanical properties. Furthermore, careful analysis of the fibril dynamics reveals hints of multiple oscillating modes. By studying different oscillating modes, we can probe mechanisms of internal friction and strain stiffening within filaments.