Currently, van der Waals forces or π–π* stacking is used for the non-covalent wrapping of lignosulfonate on CNTs, 37,38 during the preparation of solutions for feeding silkworms. 15 In order to enhance the mechanical properties of silk fibers, it is highly desirable to increase the CNT content in the silk fibers, which can be realized by removing excess LGS coating. ![]() 34–36 The CNT content in silk fibers can lead to protein reorganization and subsequently afford tunable mechanical properties. 17 The relationship between silk protein sequences and mechanical properties has been reported before. Lignosulfonate (LGS) is used as the surfactant to coat the CNTs because it is a natural product with stable hydrophilic functionality and biocompatibility. 15 It is especially desirable to enhance the CNT/surfactant ratio in any artificial additives, since excess surfactant will block CNTs from entering into the silk fibers. Alternatively, by feeding silkworms with mulberry leaves sprayed with single-wall carbon nanotubes or graphene dispersions, silk fibers with in situ developed mechanical properties can be obtained. 28–30 The carbon nanomaterials can be assembled on silk fibroin ex situ through dry/wet-spinning 31,32 or electrospinning 33 a mixed solution. By filling CNTs into silk fibers, the nanotubes with structural translational symmetry and structural flexibility endow the fibers with improved electrical and mechanical properties. Single- or multiple-walled carbon nanotubes (SWCNTs/MWCNTs) have been widely used in transistors, 23,24 batteries, 25 and reinforcing agents, 26,27 due to their excellent electrical, mechanical and catalytic properties. In addition, inherently reinforced fibers have been obtained through directly feeding artificial additives to silkworms or spiders. Carbon nanodots 22 can be supposed to be a promising substitute for those fluorescent dyes, due to their intense photoluminescence, low toxicity, and high aqueous solubility. 18,19 used fluorescent dyes as nanofillers to obtain colorful functionalized silk fibers. Various methods have been developed to improve the mechanical, electrical, and optical properties of silk fibers, such as filling them with carbon-based nanomaterials 15–17 functionalization with fluorescent dyes 18,19 and transgenic engineering. 1–4 In particular, silk fibers produced by the mulberry silkworm have been widely used in tissue engineering, 5,6 controlled drug release, 7,8 wound dressing, 9 silk hydrogels, 10 bio-photonics, 11,12 wearable human-motion sensors and electrochemical diagnosis, 13,14 due to their lustrous texture, biodegradability, outstanding toughness, and excellent biocompatibility. Introduction Natural silk fibers have drawn much attention in biology, physical chemistry, materials science, and other areas, because of their ecofriendly sustainability and promising mechanical properties ( e.g., strength and stretchable nature). Our coating and purification strategies provide a potential facile way to obtain natural silk fibers with high mechanical performance. The increased CNT content not only contributed to the self-assembly into buffering knots of silk fibers, but it also enhanced the conductivity of graphitized silk. The CNT-embedded silk fibers were characterized via Raman spectrometry and thermogravimetric analysis (TGA), demonstrating that the CNT content in the silk fibers increased 1.5-fold in comparison to the unpurified group. The toughness modulus is 1.69 times than that of the unpurified group. Reinforced silk fibers were produced with a mechanical strength as high as 1.07 GPa and a strain of 16.8%. Then the purified biocompatible single and multiple-walled CNTs were fed to silkworms, leading to a large CNT content in the resulting silk fibers. ![]() The resulting CNT/LGS nano-composite was further processed through an additional purification method to remove excess surfactant and enhance the CNT/LGS ratio. In order to increase the CNT content, lignosulfonate (LGS) was used as a surfactant to ameliorate the CNT solubility, dispersibility, and biocompatibility. The mechanical properties can be reinforced with carbon nanofillers, particularly carbon nanotubes (CNTs), depending on the CNT content in the silk fibers. Silkworm fibers have attracted widespread attention for their superb glossy texture and promising mechanical performance.
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