Stretchable, Stable, and Degradable Silk Fibroin Enabled by Mesoscopic Doping for Finger Motion Triggered Color/Transmittance Adjustment
Zijie XuWu QiuXuwei FanYating ShiHao GongJiani HuangAniruddha PatilXuyi LiShutang WangHongbin LinChen HouJizhong ZhaoXing GuoYun Jung YangHezhi LinLianfen HuangXiangyang LiuWenxi Guo
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As a kind of biocompatible material with long history, silk fibroin is one of the ideal platforms for on-skin and implantable electronic devices, especially for self-powered systems. In this work, to solve the intrinsic brittleness as well as poor chemical stability of pure silk fibroin film, mesoscopic doping of regenerated silk fibroin is introduced to promote the secondary structure transformation, resulting in huge improvement in mechanical flexibility (∼250% stretchable and 1000 bending cycles) and chemical stability (endure 100 °C and 3–11 pH). Based on such doped silk film (SF), a flexible, stretchable and fully bioabsorbable triboelectric nanogenerator (TENG) is developed to harvest biomechanical energy in vitro or in vivo for intelligent wireless communication, for example, such TENG can be attached on the fingers to intelligently control the electrochromic function of rearview mirrors, in which the transmittance can be easily adjusted by changing contact force or area. This robust TENG shows great potential application in intelligent vehicle, smart home and health care systems.Keywords:
Fibroin
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Electrospinning
Smart material
Silk has a long history of use in medicine as sutures. To address the requirements of a mechanically robust and biocompatible material, basic research to clarify the role of repeated sequences in silk fibroin in its structures and properties seems important as well as the development of a processing technique suitable for the preparation of fibers with excellent mechanical properties. In this study, three silk-like protein analogs were constructed from two regions selected from among the crystalline region of Bombyx mori silk fibroin, (GAGSGA)2, the crystalline region of Samia cynthia ricini silk fibroin, (Ala)12, the crystalline region of spider dragline silk fibroin, (Ala)6, and the Gly-rich region of spider silk fibroin, (GGA)4. The silk-like protein analog constructed from the crystalline regions of the spider dragline silk and B. mori silk fibroins, (A6SCS)8, that constructed from the crystalline regions of the S. c.ricini and B. mori silk fibroins, (A12SGS)4, that constructed from and the crystalline region of S. c.ricini silk fibroin and the glycine-rich region of spider dragline silk fibroin, (A12SGS)4,were expressed their molecular weights being about 36.0 kDa, 17.0 kDa and 17.5 kDa, respectively in E. coli by means of genetic engineering technologies. (A12SCS)4 and (A12SGS)4 undergo a structural transition from α-helix to β-sheet on a change in the solvent treatment from trifluoroacetic acid (TFA) to formic acid (FA). However, (A6SCS)8 takes on the β-sheet structure predominantly on TFA treatment and FA treatment. Structural analysis was performed on model peptides selected from spider dragline and S. c.ricini silks by means of 13C CP/MAS NMR.
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Structures of Bombyx mori silk fibroin have been studied in solution, in silkworm and in the solid state by means of solution and solid 13C and 15N NMR spectroscopies. The silk fibroin yields very sharp 13C NMR signals in aqueous solution and in silkworm, indicating the fast segmental motion of the main chain in spite of a fairly high molecular weight, 3 x 105. This makes detailed sequential and conformational analyses of the silk fibroin possible. The structure of the silk fiber in the solid state was studied with 15N CP NMR and 15N isotope-labeled silk fibroins on the basis of the chemical shift tensors in detail. The torsion angles of the glycine, alanine and tyrosine residues were determined.
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The Thaumetopoeid silkmoth Anaphe reticulata is highly abundant in equatorial and southern Africa and a potential commercial source of silk. In this paper, we report detailed structural characteristics of the silk fibroins. Comparison of the 13C solution NMR spectra of Anaphe silk fibroin and several model peptides with Ala and Gly residues indicates that (AAG)n1 and (AG)n2 are the main sequences. In addition this analysis also indicates that the sequence contains (A)m (where m > 2) such as (AAAG)n3, (AAGAG)n4, and (AAAGAG)n5. GG sequences were absent at a level that could be detected by our NMR method. The 13C CP/MAS NMR study shows that the fiber structure is heterogeneous, but predominantly β-sheet structure and the length of (AG)n2 is too short to form the Silk I structure detected in Bombyx mori silk fibroin. X-ray diffraction analyses gave information on the higher order structure and hydrogen-bonding character of Anaphe silk fiber.
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Abstract This study focuses on the conformational characterization of differently processed Bombyx mori silk fibroin samples by Raman spectroscopy. The Raman spectra of silk fibroin film and liquid silk are discussed in comparison with those of the crystalline fractions of Bombyx mori silk fibroin (Cp, chymotryptic precipitate) with Silk I (Silk I‐Cp) and Silk II (Silk II‐Cp) structures. The complete 1800–200 cm −1 Raman spectrum of Silk I‐Cp is reported for the first time. The amide I and amide III modes were found to be scarcely suitable for the spectroscopic characterization of silk fibroin in the Silk I form in the presence of a random coil conformation. Raman marker bands for the Silk I form were identified in other spectral ranges at about 1415, 950, 930, 865, 260 and 230 cm −1 . On the basis of the above findings, the comparison of the Raman spectra of film, liquid silk and Silk I‐Cp in the range 1000–800 cm −1 clearly indicates that in addition to random coil, both film and liquid silk contain local domains of Silk I structure; their amount is higher in liquid silk, as indicated by the relative intensity of the bands at about 950, 930 and 865 cm −1 and by the I 1415 / I 1455 intensity ratio. The assignments of the bands at about 1275 and 1107 cm −1 are also discussed. These bands were previously assigned to the presence of α‐helical conformation in Bombyx mori silk but, from the results reported, they should rather be attributed to the Silk I form. Copyright © 2001 John Wiley & Sons, Ltd.
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The fiber formation mechanism of Bombyx mori silk fibroin by silkworm is essentially the structural change from silk I (the silk fibroin structure before spinning in the solid state) to silk II (the silk fibroin structure after spinning) under external forces in both silk gland and spinneret of B. mori silkworm. Recently, we proposed structural models for silk I and silk II forms of the model peptide (Ala-Gly)15 of B. mori silk fibroin using mainly solid-state NMR methods. In this paper, molecular dynamics (MD) calculation was performed to simulate the structural change of poly(Ala-Gly) from silk I to silk II and to clarify the detailed mechanism of the silk fiber formation. The silk I structure (repeated β-turn type II) changes to silk II structure (heterogeneous structure, but mainly antiparallel β-sheet) by stretching of the chain with MD simulation, but the change occurs only under very high temperature such as 1000 K and large tensile stress (1.0 GPa). However, the structural change during the MD simulation occurs more easily by taking into account several external forces (the presence of water molecules around the silk chains, and application of both shear and tensile stresses to the silk fibroin) applied to the silk fibroin simultaneously. The heterogeneous structure of the silk fiber determined previously with solid-state NMR could be reproduced well with the MD calculation and then molecular mechanics calculation after removal of water molecules.
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The past 15 years have seen a major increase in our understanding of the structure and dynamics of Bombyx mori silk fibroin, largely as a result of NMR studies. We now have a reasonably good idea of the structure before spinning (Silk I) and a good model for the crystalline regions after spinning (Silk II), though there are still some big outstanding questions. The details of the structures of Silk I and Silk II are the starting point for discussion of production of man-made silk fibroin, the origin of the strong silk fiber, and the mechanism of fiber formation in vivo.
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The study of silk - its review and prospects structural basis of silk fibre. Part 1 Biological aspects of silk yarn: wild silkworms in Japan pjysiology and heredity of silkgland of the domestic silkworm variations in the silk proteins in the silkworm reflection properties of light in the silkworm cocoon ultrastructure of silkgland of silkworm biosynthesis of silk fibroin molecular weight and sub-unit structure of fibroin molecular genetics of silk protein synthesis. Part 2 Physical aspects of silk yarn: mechanical properties of silk thread scanning electron microscopic observation of fine structure of fibroin fibres fine structure of silk fibres and lousiness fibres fine texture and physical properties of silk fibres structural formation of silk fibroin molecular motion in silk fibres structure of fibroin the crystal structure of silks conformation of silk protein in solution mechanism of silk spinning.
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Silk fibroin demonstrates great biocompatibility and is suitable for many biomedical applications, including tissue engineering and regenerative medicine. Current research focuses on manipulating the physico-chemical properties of fibroin, and examining the effect of this manipulation on firobin's biocompatibility. Regenerated silk fibroin was modified by in vitro enzymatic phosphorylation and cast into films. Films were produced by blending, at several ratios, the phosphorylated and un-phosphorylated fibroin solutions. Fourier transform infra-red spectroscopy was used to determine the specific P-OH vibration peak, confirming the phosphorylation of the regenerated silk fibroin solution. Differential scanning calorimetry showed that phosphorylation altered the intra- and inter-molecular interactions. Further experiments demonstrated that phosphorylation can be used to tailor the hydrophylicity/hydrophobicity ratio as well as the crystalinity of silk fibroin films. Release profiling of a model drug was highly dependent on silk modification level. Cytotoxicity assays showed that exposure to lixiviates of phosphorylated films only slightly affected cellular metabolism and proliferation, although direct contact resulted in a strong direct correlation between phosphorylation level and cell proliferation. This new method for tuning silk biomaterials to obtain specific structural and biochemical features can be adapted for a wide range of applications. Phosphorylation of silk fibroins may be applied to improve the cytocompatibility of any silk-based device that is considered to be in contact with live animals or human tissues.
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