Submicroscopic characterization of biopolymer networks in solution by Thermal Noise Imaging

Biopolymer networks display a wide range of interesting mechanical properties that are essential for living organisms. For example, a highly nonlinear elastic response to strain gives biopolymer networks the ability to comply with small stresses but to resist large ones. These macroscopic mechanical properties have their origin in the properties of the individual filaments and their connectedness, like cross-linking geometry and pore size distribution. While the macroscopic properties of biopolymer networks have been extensively studied, there has been a lack of experimental techniques that can simultaneously determine mechanical and architectural properties of networks in situ with single filament resolution. This work introduces Thermal Noise Imaging (TNI) as a novel quantitative method to address these issues. TNI is a three-dimensional scanning probe technique that utilizes the confined thermal motion of an optically trapped particle as a three-dimensional, noninvasive scanner for soft, biological material. Using a photonic force microscope (PFM) custom built for this research, the position of the probe can be detected with nanometer precision and megahertz bandwidth. Two sets of single molecule experiments are described that demonstrate the microscope's exceptional precision and stability. Micrometer scale thermal noise images inside a collagen network are shown and quantitative information about cross-linking geometry is extracted from the data. Further, by imaging microtubules grafted to a support it is shown that the acquired data yield information about the transversal fluctuations of the imaged fibers and about fiber elasticity. These results pave the way for an investigation of force distributions inside biopolymer networks on the single filament level.