Moo Kon Kim

Refining hot-water extracted silk sericin by ethanol-induced precipitation.

In order to improve some inherent disadvantages in the mechanical properties of silk sericin (SS), we prepared ethanol-precipitated sericin (EpSS) by adding ethanol into hot-water extracted sericin (HS) solution. EpSS had higher viscosity compared to HS and it was due to the differences in their molecular weight distribution (MWD). The different MWD of EpSS was due to the different solubility of sericin molecules at various concentrations of ethanol, and the amino acid composition of EpSS showed that the more hydrophobic sericin is precipitated more readily in the presence of ethanol. The secondary structure of sericin is also changed from a random coil to β-sheet structure when the amount of ethanol added is high enough. The DSC analysis also revealed that EpSSs has a more compact structure. Finally, when beads were prepared from EpSS, they had enhanced compressive strength compared to those from HS.

Effects of different Bombyx mori silkworm varieties on the structural characteristics and properties of silk

Silk has attracted the attention of biomedical researchers because of its good biocompatibility. Although various characteristics of silk are needed for its successful application in biomedical fields, the performance of silk material is limited. Although there are many varieties of Bombyx mori silkworm, the effect of different silkworm varieties on regenerated silk has not been considered in detail. That is, the use of a diverse variety of silkworms has not been considered in non-textile applications resulting in limited performance of silk materials. In this study, the effects of different silkworm varieties on the structural characteristics and properties of silk cocoon and regenerated silk fibroin (SF) were examined. Structural characteristics of silk cocoon including color, fiber diameter, and porosity, differed depending on the silkworm variety. Furthermore, molecular weight, solution viscosity, and mechanical properties of regenerated SF were influenced by the variety of silkworm, while the amino acid composition, β-sheet crystallization by formic acid, and cyto-compatibility of regenerated SF did not differ between the samples from different varieties of silkworm. These results imply that diverse performance of silk can be obtained by controlling the silkworm variety, and that the use of different varieties of silkworm might be a good way to strengthen the performance of silk in biomedical fields.

Extraction conditions of Antheraea mylitta sericin with high yields and minimum molecular weight degradation

Although the technique for extracting the Bombyx mori sericin has been extensively known, the extraction of sericin from wild-silkworm cocoons is not yet standardized. The aim of this study was to find the optimal conditions for the extraction of sericin from Antheraea mylitta cocoons, with high yields and minimum degradation. We attempted to apply various protocols for the extraction of the A. mylitta sericin (AmS). Among these, we found that the extraction of AmS with a sodium carbonate solution exhibited the highest yield except the conventional soap-alkali extraction. To find the optimal conditions for the AmS extraction with the sodium carbonate, we changed the concentration of sodium carbonate and the treatment time. With an increase in the sodium carbonate concentration and the extraction time, the yield of AmS increased, but the molecular weight (MW) of AmS decreased. Considering the yield, molecular weight distribution (MWD) and amino acid composition of AmS, we suggest that the optimal conditions for the AmS extraction require treatment with 0.02 M sodium carbonate and boiling for 60 min.

Preparation and characterization of silk sericin/glycerol/graphene oxide nanocomposite film

Sericin (SS) is a protein that is secreted by silkworms, but it is usually discarded during the degumming process. To obtain and make use of the sericin, we prepared sericin/glycerol/graphene oxide nanocomposite film. The inherent brittleness of pure sericin film was improved by the addition of glycerol (Glc) as a plasticizer. To compensate for the reduced stiffness, we added graphene oxide (GO) into the SS/Glc film. At concentrations of up to 0.8 wt% relative to SS, GO dispersed evenly in the SS matrix without any agglomeration. The maximum tensile strength (9.5±0.7 MPa) and Young’s modulus (414.4±23.2 MPa) were obtained when the GO content was 0.8 wt% relative to SS. The elongation of SS/Glc/GO nanocomposite film also increased by approximately 40 % compared to SS/Glc film. The strong interfacial interaction between the SS and the GO was responsible for the increased stiffness. The increased elongation was due to the reduced crystallinity of the sericin matrix in the presence of GO.

The role of glycerol and water in flexible silk sericin film.

Silk sericin (SS) can be obtained as a byproduct during the silk fiber process, but its application has been limited due to the brittleness of the SS film. To enhance the flexibility of the SS film, glycerol (Glc) has been added as a plasticizer. The addition of Glc enhanced the elongation property of the SS film when the Glc content was 50-70 wt% of SS. Glc also induced the structural transition of SS from a random coil structure to a β-sheet structure. The inconsistent increase of elongation and β-sheet structure of the SS/Glc film were explained by the content of moisture in the SS/Glc film. The moisture content of the SS/Glc film increased proportionally when the Glc content was higher than 50 wt% of SS, which was the same Glc content range that exhibited the plasticizing effect. Therefore, the plasticizing effect on the SS film may occur not only because of Glc but also because of water. Furthermore, water also contributed to the increase in the β-sheet structure development. Our results suggest that the moisture content in the plasticized protein film may play an important role when the plasticizer has hygroscopic properties.

Effect of shear viscosity on the preparation of sphere-like silk fibroin microparticles by electrospraying.

Silk fibroin (SF) is known to be a biocompatible material, and different forms of SF are used for various applications. However, the application of SF in particle form is rarely reported, compared to other forms. In this study, SF microparticles with a diameter of approximately 250 μm were prepared by the electrospray method, using 1 M LiCl/DMSO as a solvent. The dissolution time of SF in the CaCl2/CH3CH2OH/H2O solution and the concentration of the SF dope solution affected the final morphology of the microparticles. A long dissolution time and a low SF concentration led to the formation of irregular microparticles, but a short dissolution time and a high concentration produced sphere-like microparticles. The shear viscosity of the SF dope solution was the main parameter that affected the morphology of the SF microparticles. Regardless of the dissolution time in the CaCl2/CH3CH2OH/H2O solution and the concentration of the SF dope solution, the shear viscosity of the dope solution must be higher than 0.33 Pa s to produce sphere-like microparticles. Finally, cell adhesion experiments demonstrated that these SF microparticles show potential for use as cell carriers.

Effect of degumming methods on structural characteristics and properties of regenerated silk

In the present study, the effects of different degumming methods on the structural characteristics and properties of regenerated silk fibroin (SF) were examined. The crystallinity index of the degummed silk increased with the degumming ratio. The crystallinity index at any given degumming ratio differed depending on the degumming method. The soda method and the soap/soda method using sodium carbonate resulted in a higher crystallinity index than the other methods The degumming method strongly affects the molecular weight (MW) and solution viscosity of the regenerated SF. The MW and viscosity of the regenerated SF, according to the degumming method, was in the order of urea method>HTHP method≈acid method>soap/soda method≈soda method. The turbidity of a silk formic acid solution decreased as a result of increasing the degumming ratio and was a minimum at a degumming ratio of around 26%. However, it was not affected by the degumming method. The mechanical properties of a regenerated SF film were strongly affected by the degumming method and the trend in the strength and elongation with the various degumming methods was the same as that of the MW and viscosity of the regenerated SF.

Surface modification of silk fibroin nanofibrous mat with dextran for wound dressing

In this study, we examined the effects of a dextran-modified silk fibroin nanofibrous mat (D-SFNM) on wound healing. To increase the hydrophilicity of silk fibroin (SF), the SF nanofibrous mat (SFNM) was modified with oxidized dextran. The D-SFNM absorbed water faster than the SFNM, and the swelling ratio was increased by approximately 80 % compared with the SFNM. An in vitro cell (NIH3T3) test revealed that fewer cells attached to the D-SFNM than the SFNM, but the proliferation of cells was not significantly affected by the presence of dextran. An in vivo wound healing test with mice indicated that the D-SFNM resulted in a good wound recovery effect similar to a commercial wound dressing material. The increased hydrophilicity of the D-SFNM might balance the moist environment at the wound site, which improves the wound healing compared with the SFNM.

Wound-Healing Potential of Cultured Epidermal Sheets Is Unaltered after Lyophilization: A Preclinical Study in Comparison to Cryopreserved CES

Lyophilized Cultured Epidermal Sheets (L-CES) have been reported to be as effective as the cryopreserved CES (F-CES) in treating skin ulcers. However, unlike F-CES, no preclinical study assessing wound-healing effects has been conducted for L-CES. The present study was set out to investigate the microstructure, cytokine profile, and wound-healing effects of L-CES in comparison to those of F-CES. Keratinocytes were cultured to prepare CES, followed by cryopreservation at −70°C and lyophilization. Under microscopic observation, intact cells with apparent intracellular junctions were observed in L-CES. The L-CES, like fresh CES, consisted of three to four well-maintained epidermal layers, as shown by the expression of keratins, involucrin, and p63. There were no differences in the epidermal layer or protein expression between L-CES and F-CES, and both CES were comparable to fresh CES. TGF-α, EGF, VEGF, IL-1α, and MMPs were detected in L-CES at levels similar to those in F-CES. In a mouse study, wounds treated with L-CES or F-CES completely healed at least 4 days faster than untreated wounds. CES-treated wounds completely healed by day 10, while the untreated wounds did not heal by day 14. Massons trichrome staining showed that collagen deposition in the CES-treated wounds was highly increased in the dermis of the wound center compared to that in the control wounds. Thus, this study demonstrates that L-CES is as clinically effective as F-CES for wound treatment.