Water-responsive (WR) materials that strongly swell and shrink in response to changes in relative humidity (RH) have shown a great potential to serve as high-energy actuators for soft robotics and new energy-harvesting systems. However, the design criteria governing the scalable and high-efficiency WR actuation remain unclear, and thus inhibit further development of WR materials for practical applications. Nature has provided excellent examples of WR materials that contain stiff nanocrystalline structures that can be crucial to understand the fundamentals of WR behavior. This work reports that regenerated Bombyx (B.) mori silk can be processed to increase β-sheet crystallinity, which dramatically increases the WR energy density to 1.6 MJ m−3, surpassing that of all known natural muscles, including mammalian muscles and insect muscles. Interestingly, the maximum water sorption decreases from 80.4% to 19.2% as the silk's β-sheet crystallinity increases from 19.7% to 57.6%, but the silk's WR energy density shows an eightfold increase with higher fractions of β-sheets. The findings of this study suggest that high crystallinity of silk reduces energy dissipation and translates the chemical potential of water-induced pressure to external loads more efficiently during the hydration/dehydration processes. Moreover, the availability of B. mori silk opens up possibilities for simple and scalable modification and production of powerful WR actuators.
Bibliographical noteY.P. and Y.J. contributed equally to this work. This work was supported by The Office of Naval Research (ONR) (N00014‐18‐1‐2492). R.T. and Y.J. thank the support of the National Science Foundation under Grant No. 1605904. The authors thank Haozhen Wang and Zhi‐lun Liu for technical assistance with AFM experiments, and Roxana Piotrowska, Scott McPhee, and Elizabeth J. Biddinger for helpful discussion about FTIR analysis.
- water-responsive materials
- β-sheet nanocrystals