Strong and tough conductive silk fibroin/poly(vinyl alcohol) composite hydrogel by a salting-in and salting-out synergistic effect

Hofmeister effect has been used to prepare highly tough and conductive hydrogel. However, it usually needs the complex steps of post-treatment of a pre-formed weak hydrogel by soaking in salt solution. Herein, strong and tough silk fibroin/poly(vinyl alcohol) (SF/PVA) ionic conductive hydrogel was prepared by a simple one-pot method by direct introduction of sodium citrate (Na3Cit) into SF/PVA solution. It was found that Na3Cit could not be directly introduced into SF/PVA solution due to its salting-out effect to prepare tough hydrogel. Fortunately, the salting-out effect of Na3Cit could be weakened with the presence of calcium chloride (CaCl2) and glycerol in SF/PVA solution. Finally, the strong and tough SF/PVA hydrogel was obtained by the one-time freezing/thawing process of SF/PVA/CaCl2/Na3Cit/glycerol solution. The tough SF/PVA/CaCl2/Na3Cit/glycerol organohydrogel with the tensile strength of 0.33 MPa and elongation-at-break of 540% were obtained. The SF/PVA/CaCl2/Na3Cit/glycerol organohydrogel with the excellent mechanical properties and ionic conductivity was used as the strain sensor. The SF/PVA/CaCl2/Na3Cit/glycerol organohydrogel sensor could detect various human motions with high sensitivity (GF = 6.87). The SF/PVA/CaCl2/Na3Cit/glycerol organo hydrogel strain sensor could generate reliable and reproducible electrical signals in response to various mechanical stimuli. We intend to present a simple and effective method to prepare high performance ionic hydrogel by Hofmeister effect.