Novel Methods for Rheological Characterization of Polymers and Polymeric Biofilms.

Abstract

This dissertation reports that environmental conditions significantly impact the bulk mechanical properties of Staphylococcus epidermidis bacterial biofilms. Bacterial biofilms are commonly found as infections of implanted medical devices, which experience large shear forces within the bloodstream. The biofilm’s ability to withstand these forces and host immune responses makes infections difficult to eliminate. We aim to reduce the disease burden of biofilms by understanding the mechanical properties that allow them to survive in the bloodstream. In this dissertation, we will discuss various methods of in situ characterization of these biofilms that allows them to be studied directly in their natural growth environments. Additionally, we present a technique to weaken the biofilm that may allow for easier removal of infections. First, we design an in situ parallel plate bio-rheometer to grow the biofilm while replicating the shear stress (0.1 Pa) and temperature (37°C) environment that Staphylococcus epidermidis would encounter in the bloodstream. We are then able to directly characterize the elastic modulus (G’) and determine how biofilms respond to environmental conditions, such as osmotic stresses and temperature. We notice a non-monotonic dependence of G’ on NaCl concentration and an irreversible decrease in G’ after heating up to 60°C. Additionally, we determine the yield stress (~20 Pa) and fit the linear creep behavior with viscoelastic models to find the relaxation time (~750 s). We then investigate the effects of temperature on biofilm on three different scales: the bacterial cells, the extracellular polymers, and the bulk biofilm. We follow our growth protocol with a one-hour exposure at three temperatures: 37°C, 45°C, and 60°C. We find little difference between the lower temperatures, but significant decrease in cell viability and yield stress following a 60°C treatment. Finally, we examine a technique, cavitation rheometry, to rapidly characterize the elastic modulus of a material, which we believe can be used for in vivo diagnostics of soft biological matter. Through experimentation, simulation, and theoretical analysis, we extend this technique to viscoelastic materials of ~1 microliter volumes, comparable to typical clinical biofilm infections. Collectively, these results facilitate diagnostics of biological soft matter and bacterial biofilm infections based on material elasticity.