Preparation of non-woven mats from all-aqueous silk fibroin solution with electrospinning method
Introduction
In recent years, the electrospinning process has gained much attention because it is an effective method to manufacture ultrafine fibers or fibrous structures of many polymers with diameter in the range from several micrometers down to tens of nanometers [1], [2], [3], [4]. In the electrospinning process, a high voltage is used to create an electrically charged jet of a polymer solution or a molten polymer. This jet is collected on a target as a non-woven fabric. Because these nanofibers have some useful properties such as high specific surface area and high porosity [1], they can be used as filters [5], wound dressings [6], tissue engineering scaffolds [7], etc.
Silk fibroin (SF) is the protein that forms filaments of silkworm silk and gives silk high mechanical strength, elasticity, and softness. In addition to the outstanding mechanical properties, silk fibroin displays good biological compatibility [8]. It has been demonstrated that silk fibroin-derived scaffolds may have wide-range applications in the fabrication of replacement tissue [9]. Electrospinning is a unique method capable of producing fibers from both synthetic and natural polymers for biomedical application [10], because electrospun non-woven fibers have high specific surface area and highly porous 3-D structure that are desirable for high-density cell and tissue cultures; electrospun non-woven fibers are among the most promising material forms used in various tissue engineering applications [1] and can be considered as ideal candidates [11], for example, Min et al. found that the SF nanofiber matrix can promote cell adhesion [12]. SF has been electrospun with the spinning solvents such as hexafluoro-2-propanol (HFIP) [13], hexafluoroacetone (HFA) [14] and formic acid [1], [15] or in combination with PEO [16], [17].
Organic solvents can pose problems when the processed materials are exposed to cells in vitro or in vivo; avoiding the use of organic solvents can enhance the potential biocompatibility of the electrospun fibers. To address this goal, Jin et al. reported that they conducted a process for silk electrospinning in combination with PEO [17]. In this study, we tried to develop the SF electrospinning with two major purposes. First, we wanted to avoid the problem that derived from residual organic solvents when the SF fibers were exposed to cell in vitro or in vivo and improve the potential biocompatibility of the SF non-woven mats. To overcome this problem, water was used as electrospinning solvent. Second, to simplify the process of electrospinning, instead of blending with other materials, the concentrated solution that was appropriate for electrospinning was prepared by concentrating the SF solution. Thus, an all-aqueous process for SF electrospinning was developed and SF non-woven mats were fabricated.
Section snippets
Preparation of regenerated Bombyx mori SF solution [18]
Raw silk was degummed with 0.5% (w/w) Na2CO3 solution at 100 °C for 60 min and then washed with distilled water. Degummed silk was dissolved in a ternay solvent system of CaCl2/H2O/EtOH solution (1/8/2 in mole ratio) for 40 min at 80 °C and dialyzed to remove salts in a cellulose tube against distilled water for 3 days at room temperature, then the SF solution was filtered.
Preparation of the spinning solution
The obtained solution was directly concentrated to generate about 28%, 30%, 32%, 34% and 37% SF solutions by weight with slow
Stability of the SF solution
Viscosity plays an important role in the electrospinning process. The jet from low viscosity liquids breaks up into droplets more readily and few fibers are formed, while at high viscosity, electrospinning is prohibited because of the instability of flow caused by the high cohesiveness of the solution. To the SF aqueous solution electrospinning, the former is the primary problem. Increasing the concentration of the SF solutions to acquire SF solutions which are of enough viscosity is a direct
Conclusion
Electrospinning process of a stable aqueous solution at a high concentration was conducted successfully and the products were studied. The fibers had a belt-like morphology. The structure of as-spun fibers was predominantly random coil because there was not enough time for molecular arrangement and crystallization. As-spun mats had a break stress of 0.82 MPa and strain of 0.76%. After the traditional SF conformational transitions were induced with methanol, the stress and the strain at break
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