such as increasing the mechanical flexibility, [3,4] sensitivity [5,6] and accuracy, [7] lowering down the operating voltage, [8,9] response speed, [1,10,11] etc. To realize the close-fitting to human bodies and the long-term monitoring, mechanical flexibility has become one of the most critical properties for wearable devices [12] and on-skin sensors. [13][14][15][16] Previously, conventional flexible devices usually applied thin polymer films as the substrates, such as poly(ethylene terephthalate), [17][18][19][20] poly(d imethylsiloxane), [21] and polyimide. [9] It is noted that some problems exist for such polymer-based substrates. For instance, the aforementioned polymers are nondegradable so that they would give rise to large amount of electronic waste. Besides, compared with the natural skin which serves as the transportation routine for nutrients and body fluids, most of the polymer-based substrates are neither biocompatible nor air/water permeable, suggesting they would damage the skin when continuously paste such polymer-based substrates on skin.In this regard, it is in great demand for developing advanced substrate materials which maintain good biocompatibility, tight adherence to biological tissues, and ideal air/water permeability. Recently, owing to the superior biocompatibility and relatively low cost, silk fibroin (SF) has become one of the most promising candidates for the future flexible substrate materials which display both high dielectric property and excellent mechanical compliance. [5,22,23] Moreover, the excellent air permeability of SF films (in the wet state), which is even comparable to that of natural human skin, also triggers their potential for application in artificial skin systems. [13,23] Despite the superior natural biodegradability and biocompatibility, the development of SF-based on-skin electronics is still at a preliminary stage due to the following reasons: First, the intrinsic brittleness and poor chemical stability of SF films prevent the fabrication of SF-based electronics through traditional techniques. Although the mechanical/chemical stability can be improved to a certain extent by doping polyurethane, [24] polyvinylalcohol (PVA), [25,26] glycerol, [27] or metal ions, [28] it is still far from the demand of real applications. Second, except for mechanical/chemical stability, the SF film served as the substrate is also supposed to be stretchable, so that they can be comfortably cling to skin. [29][30][31][32][33] Similarly, the conducting film on SF substrate should also be Due to the natural biodegradability and biocompatibility, silk fibroin (SF) is one of the ideal platforms for on-skin and implantable electronic devices. However, the development of SF-based electronics is still at a preliminary stage due to the SF film intrinsic brittleness as well as the solubility in water, which prevent the fabrication of SF-based electronics through traditional techniques. In this article, a flexible and stretchable silver nanofibers (Ag NFs)/SF based electrode is synthesized th...
