Accelerated wound-healing capabilities of a dressing fabricated from silkworm cocoon

https://doi.org/10.1016/j.ijbiomac.2017.04.069Get rights and content

Highlights

  • A simple and environmentally friendly implemented technique of directly preparing a silkworm cocoon sol-gel film (SCSF).

  • The excellent optical transparency of these dressings facilitated wound observation.

  • The excellent elasticity of the SCSFs is expected to enhance wound contact.

  • SCSF exhibited marked bacteriostatic capacity against S. aureus and E. coli, and splendid biocompatibility and efficacy.

  • The application of SCSF expedited the recovery of the full cutaneous thickness of the wound.

Abstract

Silk fibroin materials have shown some success in wound dressing applications; however, their use for this purpose remains limited by a complex production process and wasted sericin. In the present study, Bombyx mori cocoon materials are used because the protective function of the silkworm cocoon resembles the manner in which the skin protects the human body. A series of silkworm cocoon sol-gel film (SCSF) wound dressings are prepared by immersion in a CaCl2-ethanol-H2O solution for different treatment times. The accelerated wound-healing capabilities of SCSFs are systematically evaluated. Among them, the SCSF sample immersed for 90 min exhibits stronger biocompatibility and antibacterial performance compared to other SCSFs. SCSF-90 also exhibits excellent transparency, a high swelling ratio, and good extensibility. Furthermore, in vivo experiments indicate that SCSF-90 can significantly accelerate the healing rate of wounds in New Zealand white rabbits, compared to the standard Mepitel® dressing, and histological examinations reveal that SCSF-90 aided in the successful reconstruction of intact and thickened epidermis. These results demonstrate that the proposed approach may be utilized in the design of antibacterial materials with promising applications in wound dressing.

Introduction

Regenerated silk fibroin materials have shown much potential for applications in various technological fields because of the material’s properties and production capacity [1], [2]. Silk fibroin has also been recognized as an important biological material owing to its excellent biocompatibility and regenerative performance [3], [4], [5]. Accordingly, silk fibroin has been investigated for use in wound dressings [6], [7], [8], [9]. However, producing these silk fibroin materials (electrospun silk scaffolds or silk fibroin films) requires a series of complex processing steps because silk fibroin cannot be acquired directly but must be extracted by dissolving degummed silk [10].

The dissolution of silk is a critical step in producing higher purity silk fibroin [11], [12]. Native silk has been dissolved in H3PO4-formic acid [13], LiBr (self-dialysis) [10], H3PO4, or ionic liquids [14] for the direct spinning of fibers and forming of film structures [15]; however, although these processes can produce silk fibroin materials, the processes are complicated, and the resulting fibers and film structures perform poorly [16]. A widely used alternative method of dissolving silk utilizes a CaCl2-ethanol-H2O solution; nonetheless, this approach requires self-dialysis, and the process is also complex and time-consuming [17], [18], [19]. These steps include the removal of sericin, the dissolving of silk proteins, and self-dialysis process, as well as the removal of the bioactive functional proteins that remain after the folding process. Following these methods, sericin is wasted in the silk degumming process, which is unfortunate because sericin has long been acknowledged to promote adhesion and the propagation of cells [20], [21], [22], [23].

To address these issues, researchers have given special attention to Bombyx mori cocoons themselves. Cocoons, which consist of twining silk fibers with continuous lengths of approximately 1000 m that are bonded together by sericin glue, are a typical natural fiber composite with hierarchical structures [24], [25], [26]. The compact structure of the silkworm cocoon (SC) provides high mechanical resistance against disturbance, allowing the cocoon to withstand threats from parasites and predators and providing security for pupal growth and development [27], [28]. As such, the protective function of the SC resembles the protection that the skin provides to the human body. This resemblance suggests that the full cocoon structure, including both sericin and fibroin might be beneficial for wound repair. However, there have been few relevant studies on the potential offered by SCs as a defense and survival tool for chrysalis [29], and there is almost no research on natural SCs that have been deployed in the field as a biological material [30], particularly for the purpose of wound healing.

Inspired by the protective functionality of the SC, we have designed a novel and easily implemented technique of directly preparing a SC sol-gel film (SCSF) wound dressing by partially dissolving a large piece of SC in a CaCl2-ethanol-H2O solution instead of indirectly preparing the material from regenerative fibroin films. This paper reports the synthesis of SCSF composites with different SC treatment times in the CaCl2-ethanol-H2O solution, as well as the systematic investigation of their structures, mechanical performances, biocompatibilities, and anti-bacterial activities. The rate of tissue regeneration and wound closure were also measured to evaluate the material’s efficacy for enhancing wound healing in comparison with Mepitel®, a commonly used silicon-coated polyamide net [31].

Section snippets

Materials and animals

The Bombyx mori cocoons used in this study were provided by the State Key Laboratory of Silkworm Genome Biology (Southwest University, Chongqing, China). All chemicals were purchased from Taixin Chemical Reagent Company (Chongqing, China) and used without further purification. All animal experiments and care were in compliance with institutional ethical use protocols and were approved by the National Center of Animal Science Experimental Teaching (ASET) at the College of Animal Science and

Transmittance characterization

Transparent wound dressings offer the advantage of allowing daily visual inspections without dressing removal, and therefore the dressings can typically be changed less frequently [37]. For this reason, Fig. 2 presents the transparency characteristics of the different SCSFs. Fig. 2A shows photographs of the SC and SCSFs. The background is entirely obscured by the SC. However, the SCSFs exhibit different degrees of transparency depending on the treatment time of the SCSF in the CaCl2-ethanol-H2O

Conclusions

SCSFs were successfully constructed by partly dissolving SCs in a solution of CaCl2-ethanol-H2O, and a series of SCSFs were prepared with different treatment times (30, 60, 90, and 120 min). Although the prolonged treatment times reduced the breaking strength of SCSFs, their extensibility was increased. SCSF-90 exhibited marked bacteriostatic capacity against S. aureus and E. coli. Following the demonstration of biocompatibility and efficacy, the results of in vivo experiments confirmed that

Acknowledgments

This work was funded by Hi-Tech Research and Development 863 Program of China Grant (No. 2013AA102507). This work was also supported by The Science and Technology Basic Condition Construction Project of Guangdong province (No. 2015A030303010).

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