Effect of residual sericin on the structural characteristics and properties of regenerated silk films

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Abstract

Regenerated silk film has been increasingly attracting the research community’s attention for biomedical applications due to its good biocompatibility and excellent cyto-compatibility. However, some limitations regarding its mechanical properties, such as brittleness, have restricted the use of silk films for industrial biomedical applications. In this study, regenerated silk films with different residual sericin content were prepared applying controlled degumming conditions to evaluate the effect of sericin content on the structure and properties of the films generated. When the residual sericin content increased to 0.6%, crystallinity index and breaking strength of silk films were increased. Above this value, these parameters then decreased. A 1.5 fold increase of silk film elongation properties was obtained when incorporating 16% sericin. Regardless of sericin content, all regenerated silk films showed excellent cyto-compatibility, comparable to the one obtained with tissue culture plates.

Introduction

Silk has been used as a textile material for a long time. Recently, researchers have explored new applications of silk polymers in order to raise their value. In this regard, studies focusing on developing regenerated silks and describing their properties have been carried out. These researches confirmed that natural silk fiber could be successfully transformed into regenerated silk film [1], [2]. Additionally, such regenerated silk films showed good blood compatibility [3], [4], excellent cyto-compatibility [5] and high biodegradability [6], which are useful features for biomedical applications such as wound and burn dressings.

Because regenerated silk films are too brittle to be used for biomedical applications, many studies have been performed to improve their mechanical properties. To do so, researchers have blended the regenerated silk with other polymers [7], [8], [9], [10]. However, these blending techniques might lead to a deterioration of other advantageous properties of regenerated silk films.

Some researchers tried to enhance the mechanical properties of regenerated silk films employing other methods than blending. For example, Chung et al. reported that the breaking strength and elongation of regenerated silk films could be improved using silk from the proper silkworm variety [11].

Silk is composed of two polymers: fibroin and sericin. Previous studies reported that residual sericin improves the wet spinnability, post drawing and mechanical properties of silk [12], [13]. Also previously described is the fact that the residual sericin can improve electro-spinnability [14] and electro-spinning rate [15] of regenerated silk. Silk is a relatively expensive polymer material and sericin is usually fully removed from it when preparing regenerated silk. As a result, an expensive silk component (sericin) is disposed of. In addition, chemicals and energy are being used during this highly polluting removal process (degumming). Therefore, remaining sericin in silk may improve mechanical properties of regenerated silk film while reducing production cost and pollution. In this study, the sericin was partially removed from raw silk, which was then regenerated to fabricate silk film. Subsequently, the effect of residual sericin content on the structure and properties of regenerated silk film was examined.

Section snippets

Preparation

The regenerated silk was prepared using a previously described method [14]. Briefly, to prepare silk with different residual sericin content, the Bombyx mori cocoons were degummed with a sodium oleate (0.024–0.6% (w/v)) and sodium carbonate (0.016–0.4% (w/v)) aqueous solution at boiling temperature for 1 h. Table 1 lists the degumming conditions, the degumming ratios, and residual sericin content in silk. The liquor ratio was 1:25. After the degumming process, the cocoons were rinsed thoroughly

Molecular conformation of regenerated silk film

The molecular conformation of regenerated silk has been studied extensively in past studies as it strongly influences the properties of regenerated silk [2], [13]. Therefore, in this study, FTIR measurements were performed on regenerated silk films with different residual sericin content and the results are shown in Fig. 1(A).

When the regenerated silk film did not contain sericin, IR absorption peaks could be observed at 1620, 1510 and 1260 cm−1 in amide I, II and III bands, respectively. These

Conclusion

In this study, the effect of sericin content on the structure and properties of regenerated silk films was examined. The crystallization behavior of silk films was affected by residual sericin. At low sericin content (0.6%), sericin helps the β-sheet crystallization of silk. However, above this threshold, β-sheet crystallizations of silk polymers (fibroin and sericin) were restricted due to the crystallization competition between both polymers. This unique modulation of crystallization behavior

Acknowledgments

This study was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2014R1A1A2056892).

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