Abstract
Silk fibroin (SF) produced by the domesticated wild silkworm, Samia cynthia ricini (S. c. ricini) is attracting increasing interest owing to its unique mechanical properties, biocompatibility, and abundance in nature. However, its utilization is limited, largely due to lack of appropriate processing strategies. Various strategies have been assessed to regenerate cocoon SF, as well as the use of aqueous liquid fibroin (LFaq) prepared by dissolution of silk dope obtained from the silk glands of mature silkworm larvae in water. However, films cast from these fibroin solutions in water or organic solvents are often water-soluble and require post-treatment to render them water-stable. Here, we present a strategy for fabrication of water-stable films from S. c. ricini silk gland fibroin (SGF) without post-treatment. Aqueous ethanol induced gelation of fibroin in the posterior silk glands (PSG), enabling its separation from the rest of the silk gland. When dissolved in 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP), the SGF-gel gave a solution from which a transparent, flexible, and water-insoluble film (SGFHFIP) was cast. Detailed structural characterization of the SGFHFIP as-cast film was carried out and compared to a conventional, water-soluble film cast from LFaq. FTIR and 13C solid-state NMR analyses revealed both cast films to be α-helix-rich. However, gelation of SGF induced by the 40%-EtOH-treatment resulted in an imperfect β-sheet structure. As a result, the SGF-gel was soluble in HFIP, but some β-sheet structural memory remains, and the SGFHFIP as-cast film obtained has some β-sheet content which renders it water-resistant. These results reveal a structure water-solubility relationship in S. c. ricini SF films that may offer useful insights towards tunable fabrication of novel biomaterials. A plausible model of the mechanism that leads to the difference in water resistance of the two kinds of α-helix-rich films is proposed.
Highlights
Fiber spun by the domesticated wild silkworm, Samia cynthia ricini (S. c. ricini), is essentially different from that spun by the mulberry silkworm, Bombyx mori (B. mori)
To account for the peak shift and broadening detected in the profile of the film treated with HFIP gas, the gradual increase in temperature during the differential scanning calorimetry (DSC) scan was considered to increase the interaction between fibroin and HFIP molecules, eventually resulting in a structure similar to that of the SGFHFIP as-cast film
The silk gland fibroin (SGF)-gel dissolved in HFIP at room temperature within several hours, implying that it has an imperfect β-sheet structure
Summary
Fiber spun by the domesticated wild silkworm, Samia cynthia ricini (S. c. ricini), is essentially different from that spun by the mulberry silkworm, Bombyx mori (B. mori). This has limited effective utilization of this unique bioresource [9] To overcome this shortcoming, fabrication of biomaterials using liquid fibroin (LF) obtained directly from the posterior silk gland (PSG) of mature S. c. Β-sheet formation, often induced by treatment with aqueous alcohol solutions, and thermal- and water-vapor annealing (refer to Yoshioka et al [13] and references therein), is the most widely used strategy to render the biomaterials water-resistant Such post-treatments often modify inherent desirable properties of the LF-based materials, limiting their applicability [9]. We developed a mild approach to fabricate high-molecular-weight, water-resistant cast films directly, that is, without the need of the time-consuming steps as well as post-product treatment, from B. mori silk gland fibroin (SGF) [13]. On the basis of these experimental results, a plausible model of the mechanism leading to water resistance of the SGFHFIP as-cast film is proposed
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