Sericin Film Research Articles

Molecular Orientation Behavior of Silk Sericin Film as Revealed by ATR Infrared Spectroscopy

This paper reports the structure-dependent molecular orientation behavior of sericin, an adhesive silk protein secreted by silkworm, Bombyx mori. Although application of sericin as a biomaterial is anticipated because of its unique characteristics, sericin's physicochemical properties remain unclear, mainly because of its vulnerability to heat or alkaline treatment during separation from fibroin threads. This study employed intact sericin obtained from fibroin-deficient mutant silkworm to investigate the relationship between molecular orientation and the secondary structure of sericin. Sericin films were artificially stretched after moistening with aqueous ethanol of various concentrations. The resulting molecular orientation was analyzed using polarized infrared spectroscopy. These analyses indicated that formation of aggregated strands among extended sericin chains induced by ethanol treatment is the key to generating molecular orientation. Strong intermolecular hydrogen bonds are inferred to allow aggregated strands' stretching-force transmission, thereby causing molecular orientation.

Moisture Characteristic and Structure of High Molecular Weight Sericin Film

Sericin, a silk protein, shows a high moistureretaining property and an ultraviolet ray absorbability, and has been applied for a textile processing of medicines and a cosmetic additives.1 It is clarified by recent studies that sericin has many physiological effects, such as an anti-oxidative effect, a cancer prevention and a dermatitis improvement, etc.2–6 Furthermore, the practical physical properties of the sericin make it a promising material for the field of medical supplies. Sericin adheres firmly with fibroin protein. Since it has high molecular weight, most of sericin is insoluble in water at room temperature.7 Industrially, sericin can be extracted by the scouring of silk fabric and silk yarn in an alkaline solution under high temperature and pressure. The molecular weight of the sericin obtained is greatly lowered by the hydrolysis, and then the water solubility increases. Therefore, the material, such as a film or membrane, composed only the low molecular weight sericin is lacking in the practicability, because of the material simply dissolves in the water. One of the authors examined the chemical cross-linking using dimethylolurea (DMU), and membranes obtained showed water resistance.8 However, to utilize for medical supplies, it is desirable to unused the other substance, such as cross-linking agent because of toxicity. In addition, Katoh suggested that the physiological effects of sericin depend on the molecular weight.9 Recently, we developed a new process to efficiently obtain the sericin, which maintain high molecular weight.10, 11 Furthermore, we found that the film prepared by this sericin showed water resistance and large mechanical properties. In order to apply this material to the medical field, it is necessary to evaluate the moisture characteristic or the structure. In this paper, it is reported that the high molecular weight sericin film compared with the cross-linking sericin film on moisture characteristic and structure. EXPERIMENTAL

Deposition of bone-like apatite on silk fiber in a solution that mimics extracellular fluid.

The fabrication of apatite-organic polymer hybrids is one of several attractive methods for the development of biomaterials as a substitute for bone. Such materials have both bone-bonding ability and mechanical properties analogous to natural bone. The biomimetic process has focused attention on fabricating such hybrids, where bone-like apatite is deposited on an organic polymer surface in solutions that mimic physiological conditions. In this process, a bone-like apatite layer can be coated onto organic substrates either by using a simulated body fluid (SBF) with ion concentrations nearly equal to those of human extracellular fluid, or by using fluids that are supersaturated with respect to apatite at ambient conditions. In this study, we investigated the ability of natural silk and its related materials to facilitate apatite deposition under biomimetic conditions. Cloths made of raw silk or normal silk fibers were soaked in 1.5SBF, which has 1.5 times the ion concentration of SBF. Sericin film, which is made from an extract of degummed raw silk, was soaked in 1.5SBF. The cloth and the film soaked in 1.5SBF then were characterized by scanning electron microscopic (SEM) observation, energy dispersive X-ray microanalysis (EDX), and thin-film X-ray diffraction (TF-XRD). Apatite deposition was observed on the surface of cloth made from raw silk fiber after it was soaked in 1.5SBF, but it was not observed on cloth made from normal silk fibers. The apatite deposition on the raw silk fiber cloth was accelerated when the fibers were subjected to treatment with CaCl(2) solution at a concentration of at least 1 kmol/m(3) before immersion in 1.5SBF. Apatite deposition also was observed on the sericin film after the film was soaked in 1.5SBF for 7 days. These results indicate that apatite deposition on raw silk cloth is attributable to the catalytic effect of sericin because the surface of raw silk consists of sericin whereas that of normal silk contains fibroin. The deposition of the apatite and its crystal growth are accelerated by the presence of calcium ions on the sericin after treatment with CaCl(2) solution. Thus, sericin on natural silk fiber has the potential to facilitate apatite deposition and can be useful as a polymer material in the fabrication of hybrid materials analogous to bone through biomimetic processes.