Partially insoluble regenerated silk fibroin film induced by UV irradiation for electronic skins

The regenerated silk fibroin (RSF) film has been regarded as an ideal substrate for biocompatible, flexible, and biodegradable electronic skin (e-skin) devices. However, it is still a great challenge to balance the flexibility and solubility of the RSF film by adjusting its secondary structure. Herein, a film prepared with the hydrolyzed RSF was exposed to the 254 nm ultraviolet (UV) light to prepare a crosslinked and partially water-insoluble substrate for a strain-sensing e-skin. The hydrolyzed low-molecular-weight RSF was produced by heating the LiBr-silk fibroin solution at 85 degrees C for a certain duration. The film cast with the hydrolyzed RSF solution could be thoroughly dissolved in water rapidly. The UV irradiation could induce the crosslinking of the low-molecular-weight RSF to form insoluble substances, thus producing a partially insoluble RSF film. After silver nanowires painting, an e-skin strain sensor was successfully constructed based on the UV-irradiated film. The sensor shows a fast response time (2.01 s), high sensitivity (GF = 1.03 within 0%-40% strain range), and good stability. The device could be tightly attached to human skin with a drop of water. The finger, wrist, elbow, and knee bending could be sensitively detected in real-time. The head nodding and mouth opening could also be sensed by sticking the e-skin at the neck and cheek, respectively. This work may provide a facile way to prepare a stretchable and stickable RSF film, which could serve as an ideal substrate of low-cost, biodegradable, direct-to-skin sensors for wearable applications.