Preparation And Applications Of Silk Fibroin Microspheres Under Electric Fields

Silk fibroin has been used to form a variety of biomaterials, such as gels, scaffolds, films and microspheres for medical applications with remarkable mechanical properties, structural controllability and excellent biocompatibility. As the drug delivery carrier and the mineralization template, silk spheres have a broad prospect in drug delivery, optical materials and renewable materials of biological hard tissue replacement applications. In the present study, silk fibroin electric gel system (e-gel) composed of nano/microspheres was formed under low electric field by controlling the nanostructure of silk fibroin in aqueous solution which was in a special container with electrodes. The structures and properties of microspheres were characterized after freeze-drying. This paper provided a new method to prepare spheres that was expected to be applied as drug delivery carriers and biological mineralization templates.Firstly, silk fibroin solution was pre-treated by heat and slowly concentrating, and then 1 wt% ~ 8 wt% silk fibroin solution was prepared. In order to explore the mechanism for microspheres formation, the process parameters of low-voltage electric field was changed. Nano-structure in the fresh and pre-treated silk solution was observed by scanning electron microscope (SEM) and atomic force microscope (AFM). The results showed that nano- structure in the silk fibroin solution played a key role of regulating silk microspheres conformation. Based on preparing and regulating microspheres formation under electric field, we changed nano structure of silk fibroin solution by altering temperature and time in the heating process. 25V low-voltage electric field was used to treat silk fibroin solution, and regulating diameters in the range of 200 nm up to 3000 nm was achieved. Results also showed that structure of silk fibroin microspheres was intermediateα-helical structure.Secondly, fluorescein isothiocyanate labeled bovine serum albumin (FITC-BSA) was used as model drug. Different ratios of bovine serum albumin and silk fibroin solution were mixed. Under low-voltage electric field, the silk fibroin near the positive electrode was self assemble into microspheres which negatively charged model drugs concentrated near the positive electrode and finally encapsulated into microspheres. Results showed that the initial loading rate reached 81.7% and released slowly within 120 hours when 1.33μg/mL drugs were added. Considering the mild conditions (low-voltage electric field, room or low temperature, water), biodegradability of silk fibroin and impressive biocompatibility of drugs, preparation of silk fibroin microspheres through electric field controlling is expected to become excellent carriers for negatively charged proteins and gene medicines.Furthermore, the silk fibroin microspheres were used as templates to simulate calcium carbonate mineralization in the liquid environment and then mechanism of induction of mineralization with silk fibroin was studied. Results showed that silk fibroin microspheres could induce mineralization as templates and control the crystal formation and structure of calcium carbonate. CaCO3 microspheres were mainly composed of 63% stable vaterites, diameters ranged from 1μm to 1.5μm, which could be used for drug delivery, coatings and other areas.In summary, on the basis of formation of silk fibroin microspheres under electric field, this paper further explored application possibility of silk fibroin microspheres in drug delivery, biomineralization and other fields, prividing a new essay to expand the scope of silk fibroin materials technology and applications.