The Effects of Nanoconfined Liquid Properties on the Water-Responsive Behavior of Bacterial Cell Walls
Abstract: Water-responsive (WR) materials have the ability to mechanically swell and shrink in response to changes in relative humidity (RH). These WR materials are used by many biological systems to perform essential tasks; for example, pinecones use WR materials to release their seeds in dry environments, and wheat awns open and close to propel seeds into the soil, driven by daily RH changes. The WR actuation of some biomaterials is extremely powerful, for example Bacillus subtilis cell walls display record-high actuation energy and power densities of 72 MJ m-3 and 9.1 MW m-3, surpassing those of all existing muscles and actuator materials. They hold great potential to be used as high-performance actuators for various applications, including energy harvesting, robotics, and morphing structures. However, the fundamental mechanisms of WR actuation are still poorly understood. Despite the unclear WR mechanism, recent studies have provided compelling evidence of the critical role that the properties of nanoconfined water play in these observed high-power WR actuation, and thus, adjusting the properties of nanoconfined water should substantially affect WR behavior and performance.
This thesis investigates the role of nanoconfined liquids in the WR actuation of bacterial cell walls, focusing on how modifying their behavior can improve WR performance. In this research, cell walls of E. coli, S. aureus, S. cerevisiae and B. subtilis were extracted and used to investigate the properties of their nanoconfined water. Based on these findings, we further explored the effects of kosmotropic and chaotropic solutes, known to stabilize or disrupt hydrogen bonding networks, on the WR performance of B. subtilis cell walls. We discovered that cell walls treated with low-concentration kosmotropic solutes exhibited a significant increase in WR actuation energy density, reaching 103.3 MJ m-3. However, higher concentrations of kosmotropic or chaotropic solutes led to decreased WR performance. Our observations suggest the presence of an optimal range for kosmotropic and chaotropic treatments to enhance WR energy density. These findings could be explained by the impact of the solutes on hydration forces and intermolecular interactions, which affect the ultimate WR pressure. This, in turn, provides a pathway towards achieving superior WR actuation performance and advancing the development of high-work-density actuator materials for diverse industrial applications.
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