Electrospun Fibers from Biopolymers

Keywords

Silk fibroin • Electrospinning • Solvent • Recombinant protein • Lithium thiocyanate

59.1 Electrospun Fibers from Silk Fibroin

Due to the distinct advantage of silk for various applications, considerable attempts have been made to reproduce silk in the laboratory with specific properties for targeted applications. For instance, to exploit the advantages of protein-based biomaterials and nanostructures for medical applications, silk fibroin was electrospun into fibers [10Zha]. To form the fibers, silk (Bombyx mori) was first degummed to remove sericin. Later, the silk fibers were dissolved in 9.3 M lithium bromide solution at 60 °C, and the dissolved solution was dialyzed against a 2,000 molecular weight membrane to obtain a 3-7.2 % protein solution [10Zha]. In addition, lyophilized silk fibroin was also dissolved using HFIP at room tempera­ture. Silk solutions were blended with polyethylene oxide (PEO) to improve spinnability and enable fiber formation. Fibers with relatively larger diameters, between 700 and 880 nm, were obtained. Electrospun mats obtained were treated with methanol to induce crystallization in silk and transform the silk into p-sheet configuration. Methanol treatment removed PEO and increased the surface rough­ness of the fibers [02Jin]. In another study, silk fibroin has been electrospun and the potential of using the silk nanofibers for various applications has been studied [08Kaw, 05Kim]. Silk nanofibers with diameters from 8 to 2,500 nm have been produced and used for tissue engineering [09Zha].

Fibroin obtained from B. mori and Samia cynthia ricini and a recombinant protein containing sequences from both the silks were electrospun into fibers. To produce the fibers, B. mori silk fibroin was dissolved in 9 M lithium bromide at 40 °C and made into films. The silk films and ricini silk fibers were dissolved using hexafluoroacetone (HFA) solution in 2-10 % concentrations and electrospun into

fibers with diameters ranging from 100 to 1,000 nm. Proteins in the fibers assumed the p-sheet configuration for B. mori silk but not for S. ricini silk. Electrospun mats produced from B. mori had strength of 15 MPa and elongation of 40 % compared to strength of 20 MPa and elongation of 20 % for the S. ricini silk. Fibers with average diameters of 100 nm were obtained from the recombinant proteins, but no tensile properties were reported [03Ohg].

The biocompatibility and possibility of using electrospun fibroin membranes for bone regeneration were studied [05Kim]. Silk fibroin was dissolved in CaCl2/ CH3CH2OH/H2O (1:2:8 molar ratio) for 6 h at 70 °C to obtain sponges and the sponges were later dissolved in acetic acid to form the electrospun fibers. Cells grown on the scaffolds had ALPase activity and calcification similar to cells cultured on petri dishes. When implanted into a rabbit, the scaffolds supported cell attachment and growth and showed complete bone regeneration in 12 weeks as seen in Fig. 59.1 [05Kim].

Using the same approach of dissolving fibroin, Zhang et al. obtained fibroin fibers with widths between 234 and 1,016 nm. Fibers with grooves that facilitated cell attachment, growth, and spreading were obtained by coagulating the fibers in methanol [14Zha]. Treating with methanol promoted crystallization through con­formational transition of random coils to p-sheet structure [08Ale]. Similar improvement in degree of crystallinity was also observed. Murine fibroblasts (L929) showed good attachment and growth on the scaffold. Blends of silk fibroin and hydroxybutyl chitosan were dissolved in 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) and trifluoroacetic acid (TFA) and electrospun into fibers with average diameters of 215 ± 84 nm for the pure fibroin and with a diameter of 315 ± 150 nm for the 50/50 fibroin-chitosan blend fibers.

Silk fibroin with and without PEO was electrospun into relatively coarse fibers with average diameter of 700 nm for potential use as tissue engineering scaffold. Fibroin was first degummed, later dissolved in 9.3 m lithium bromide (LiBr), and used for electrospinning [04Jin]. Inclusion of PEO improved the strength of the matrices obtained but inhibited cell growth. After removing the PEO, the fiber matrices were conducive to cell growth and extensive growth and proliferation of bone marrow stromal cells could be seen [04Jin]. Instead of using fibroin from B. mori, He et al. developed electrospun fibers from the fibroin from Antheraea mylitta. Proteins were dissolved in lithium thiocyanate and extruded into fibers with an average diameter of 422 nm. Structural analysis showed that the as-spun fibers had a complete a-helix/random coil configuration with no p-sheet content [13He]. However, after insolubilization, fibers were found to have both the a-helix and p-sheet configurations [13He]. Since in vivo biodegradation of the fibroin matrices is important for tissue engineering, it has been demonstrated that controlling recrystallization during fiber formation can provide matrices with desired degradation rates [12Kim]. By varying the ratio of ethanol/propanol, matrices that degraded between 14 weeks to one year were obtained.