Yeojin Jung1,2,Maheen Khan1,2,Darjan Podbevsek2,Raymond Tu1,2,Xi Chen1,2,3
City College of New York1,Advanced Science Research Center2,The Graduate Center of the City University of New York3
Yeojin Jung1,2,Maheen Khan1,2,Darjan Podbevsek2,Raymond Tu1,2,Xi Chen1,2,3
City College of New York1,Advanced Science Research Center2,The Graduate Center of the City University of New York3
Water-responsive (WR) materials that mechanically swell and shrink in response to changes in relative humidity or water gradient have shown capability to exert higher energy than conventional actuators and artificial muscles, and thus hold potential to be used as high-energy actuating components for various engineering applications, including robotics, shape morphing, and smart structures. Despite the growing interest in this emerging category of WR materials, the underlying mechanisms of their significant performance are still not fully understood, hampering any rational design. One common feature of high-performance WR materials, such as bacteria peptidoglycan, is that they consist of hierarchical and stiff structures with nanoscale pores. Here, we systematically varied the porosity of <i>Bombyx (B.)</i> mori silk and tested the effect of porosity on silk’s water adsorption and WR performance. To fabricate porous silk, blends of regenerated silk fibroin and Poly(ethylene glycol) solutions were cast on thin polyimide substrates, and subsequently treated with methanol and immersed in water to extract PEO. The porosity was varied by controlling the volume concentration of PEO from 0 to 60%. We found that <i>B. mori</i> silk’s WR actuation energy densities increased first with increasing PEO concentration, and then decreased when the PEO concentration is higher than 20%. Notably, when silk is processed with 20%PEO, the silk showed extremely high WR energy densities of ~3 MJ m<sup>-3</sup>, which is about four hundred-fold higher than that of mammalian muscles (~8 kJ m<sup>-3</sup>). Further studies on silk’s secondary structures, water adsorption, and water structures using scanning electron microscopy (SEM), dynamic water vapor sorption (DVS), and Fourier transform infrared spectroscopy (FTIR) showed that increasing silk’s porosity can lead to dramatic changes in water-silk interactions and the structure of water confined within silk’s nanopores. Our finding suggests that the pore structure-dependent water properties could play a crucial role in silk's WR performance.