Production of Submicron Diameter Silk Fibers under Benign Processing Conditions by Two-Fluid Electrospinning

An environmentally benign, entirely aqueous process is described to produce solid, crystallized silk fibers with diameters that are much smaller than natural silk fibers. The process entails three steps. First, submicron diameter fibers with a coaxial internal structure consisting of a silk core and poly(ethylene oxide) (PEO) shell are produced by a two-fluid electrospinning technique at ambient conditions using water as the common solvent. The two-fluid electrospinning process is the key to produce a continuous core filament of unblended silk. No significant mixing between the two fluids was observed to occur during electrospinning. Second, the silk/PEO fibers are annealed under high humidity to induce the conformation transformation from the random coil and/or silk I to predominantly β sheet (silk II) structure. The large surface area of the submicron diameter fibers and the molecular orientation of silk fibroins obtained through electrospinning are believed to facilitate the conformational transformation. Finally, the PEO shell is extracted with water from the now insoluble silk core filament, resulting in a solid, crystallized silk fiber with diameter as small as 170 nm.