(521b) Nanoconfined Water Endows Peptidoglycan Extreme Water-Responsive Actuation

Water-responsive (WR) materials that deform in response to changes in relative humidity (RH) show great potential in applications, including soft robotics and novel energy harvesting systems. The best-performance WR material up-to-date is Bacillus (B.) subtilis spore, which shows an actuation energy density of ~10 MJ m^-3, surpassing that of existing artificial muscles and actuators. However, the underlying reason of spores’ forceful actuation remains unclear. Here, we present that peptidoglycan (PG) within spores’ cortex layer dominates spores’ WR behaviors, and its WR energy density reaches 59.9 MJ m^-3, which exceeds spores’ record-high energy density by three-fold. We found that, when RH is increased from 10% to 90%, spore PG exhibits a WR strain of ~50.1%, which is ~5 times larger than that of spores. We also found that the PG from B. subtilis bacterial cell shows comparable water-responsiveness. For example, cell wall PG shows a WR strain of 27.2% and an energy density of 72.9 MJ m^-3. To elucidate water’s effect on PGs, we analyzed the effect of water’s diffusion on the PG solid network and found that the viscosity of water in PG is extremely high (~7 orders higher than bulk water), which enables the water flow to drag and deform PG’s solid network. Our finding of PG’s extreme water-responsiveness not only provides a new record energy density for artificial muscles, but also provides mechanistic insight in developing and exploring more advanced WR materials.