Tough silk fibroin hydrogel via polypropylene glycol (PPG) blending for wearable sensors

Abstract Regenerated silk fibroin is a promising bio‐material because of its sustainability, biocompatibility, degradability, and eco‐friendliness. However, silk fibroin hydrogel's intrinsic hard and brittle mechanical properties hinder its application on human skin as a wearable sensor. We used polypropylene glycol (PPG) as a plasticizer to physically blend with a silk fibroin solution, and designed a set of green and environmentally friendly preparation methods without organic solvents to obtain the hydrogel for wearable sensors. PPG molecules will aggregate into clusters to act as a plasticizer. Meanwhile, a large number of hydrogen bonds are formed between the hydroxyl groups in PPG and silk fibroin molecular chains. When the hydrogel is stretched, the hydrogen bond in the silk fibroin chain will release heat in an irreversible form to increase the toughness. The PPG/silk fibroin hydrogel exhibits mechanical strength of 16.6 MPa, breaking elongation of 160%, and toughness of 17.7 MJ/m 3 . The Ca 2+ introduced during the sample preparation process gives the hydrogel ionic conductivity, which provides the possibility of using the hydrogel as a human skin sensing material.