(165o) Water-Responsive Actuation of Gram-Negative/-Positive Bacterial Peptidoglycan

Human-made machines rely on actuators that typically transduce electrical fields, heat, or pressurized gas/liquid into mechanical motions. In nature, plants use water-responsive (WR) materials that deform in response to humidity changes to drive their essential movements. These WR actuators could be powerful, and hold great potential to advance actuating components for robotics, shape-morphing, and energy harvesting. Here, we present that peptidoglycan (PG) extracted from Escherichia coli, Saccharomyces cerevisiae, and Staphylococcus aureus bacteria show significant water-responsiveness such that their WR actuation energy density and efficiency reach 39.6 MJ m^-3 and 38.4%, surpassing those of frequently used actuator materials. PG widely exists in gram-negative/-positive bacterial cell walls, and PG in gram-positive species, such as Saccharomyces cerevisiae (~2000 nm) and Staphylococcus aureus (~500 nm), is thicker than that in gram-negative species, such as Escherichia coli (~100 nm). Despite the differences in chemical structures, we found that both gram-positive and gram-negative PG exhibit water-responsiveness. For instance, Staphylococcus aureus PG shows a WR strain of 19.1 % and energy density of 39.6 MJ m^-3, and Saccharomyces cerevisiae PG shows a WR strain of 11.2 % and energy density of 6.0 MJ m^-3. We also found that, while gram-negative Escherichia coli PG possesses lower WR performance than Staphylococcus aureus PG, it exhibits higher WR strain (12.0 %) and energy density (22.0 MJ m^-3) than Saccharomyces cerevisiae PG. Using AFM nanoindentation, we found that the water confined in Staphylococcus aureus PG is extremely viscous, and its viscosity is higher than those of two other species. Our findings suggest that Staphylococcus aureus PG’s highly viscous confined water facilitates the energy transfer from water’s chemical potential to PG’s mechanical deformations, serving as a guideline for the rational design of high-efficiency WR structures.