Preparation Of Regenerated Silk Fibroin Materials

In recent years, the human society has been developed rapidly, but it is followed by the over-consuming of natural resources, such as oil and coal, the increasing of carbon dioxide emissions and global climate warming. To reduce the negative impact during the development process, the materials scientists have begun to focus on natural polymer materials such as silk protein, cellulose, soy protein and so on. Natural polymer material is a valuable gift of nature with low prices and environment-friendly. It is quite valuable under such a sustainable development and low-carbon economic environment.China is not only the home of Bombyx mori silkworm but also the world’s largest silk producer with annual production of 100,000 tons of cocoon silk. With the development of science and technology, silks and their proteins (silk fibroin) have aroused the attention of material scientists as biomedical materials, biomimetic materials, nano materials and photovoltaic materials, besides as textile materials.A standardized preparation process of regenerated silk fibroin (RSF) from silkworm cocoon silk has been well developed in the past decades. Silk fibroin, as a structural protein without biological activity, shows unique advantages including low cost, renewability, biocompatibility, biodegradability and outstanding mechanical properties. We investigated the moisture effects on the mechanical properties of RSF film, and then we enhanced the toughness of the RSF film through uniaxial extension. The superhydrophilic RSF film was obtained through UV irradiation, and patterned superhydrophobic-superhydrophilic RSF sponge was fabricated.At first, the moisture effects on the mechanical properties of the silk fibroin film (RSF) have been considered. It could be found that the strength, modulus of RSF film would decrease while strain at break would increase with the relative humidity increasing from 40% to 97%. The water content of the RSF films balanced under different relative humidity was obtained by TGA. Then it has been found that there is an obvious transition for the mechanical properties of the silk fibroin films when the relative humidity is at around 80%, but there is not freezing bound water and free water as shown in DSC curves. So it is said that the non-freezing water induced the change of the mechanical properties of RSF film as water molecules play a role of plasticizer in the RSF films.RSF films are usually brittle and weak, which prevents its wide application as a structural material. To improve the mechanical properties of RSF film, uniaxial extension under swollen conditions was employed to introduce preferred orientation of molecular chains of silk fibroin. Such a pre-stretching treatment resulted in the strain at break, ultimate stress, Young’s modulus, and energy to break along the predrawn direction of the RSF film increasing from approximate 5%,90 MPa,2.7 GPa, and 2.1 kJ/kg to 35%,169 MPa,3.5 GPa, and 38.9 kJ/kg, respectively, which is an attractive combination of strength and toughness. The mechanism of these property enhancements was investigated by various techniques such as small-angle X-ray scattering, wide-angle X-ray diffraction, atomic force microscopy, and dynamic mechanical analysis.We found that the water contact angle on the surface of RSF films decreased as the increase of the UV irradiation duration, and RSF films have changed to be superhydropphilic eventually. X-ray photoelectron spectroscopy showed that the elements and chemical components of the surface of RSF film changed after UV irradiation. There were more hydrophilic groups such as carboxyl, which induced the decrease of the water contact angle on the surface of RSF film. At the same time, the superhydrophilicity of the surface of RSF films disappeared slowly, when it was stored under room condition. Yet when the water contact angle was about 25° (which needs 30 days), it would keep around there even after 90 days.The silica nanoparticle was introduced in situ onto the RSF sponge, leading to a rougher surface structure. After being modified with a perfluoroalkyl silane, the water contact angle of the samples was more then 150°, which meant it was superhydrophobic. And then the sample was irradiated by UV light, which induced the superhydrophilicity of the surface. Using Cu mesh cover mold, the superhydrophobic RSF-SiO2 changed to be patterned superhydrophibic-superhydrophilic surface after UV irradiation. We also prepared superhydrophobic silk fabric through this method, and the L929 cell culture was carried out.