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DOI: 10.1002/adma. 201601572 based on dry-spun CNT fibers which could be stretched by over 900%. Unfortunately, it exhibited a GF of only 0.54 in the strain range of 0–400%, limiting its applications in detecting subtle deformations of a human body induced by pulse, respiration, facial expression, and phonation. Very recently, a graphenebased fiber strain sensor [25] was reported for its high sensitivity and relatively wide workable strain range (up to 100%), which is still not enough for detecting motions induced by multiple joints which are usually larger than 100%.[19] Thus, the development of flexible strain sensors with both of high sensitivity and ultrahigh stretchability remains a big challenge. Compared to CNTs and graphene, carbon-based materials from natural biomaterials have received extensive research interests owing to their good electrical conductivity, large-scale, and low-cost production capability as well as environmental benignity.[32] Silk, as a kind of widely used ancient natural material, is composed of sericin and fibroin, among which fibroin is the major component and consists of repetitive crystalline heavy chains (forming discrete β-sheet crystallites) and noncrystalline light chains (forming amorphous domains).[33, 34] Silk fibroin has been reported to transform into graphitic nanocarbon by thermal treatment because of the fact that the β-sheet crystallites in the silk polypeptides could be aromatized or cyclized into sp2-hybridized carbon structure.[35] It has been reported that carbonized silkworm cocoon or regenerated silk fibroin, based on their good electrical conductivity, showed good performance for energy conversion …