Characterization, processing and modeling of silk and silk-like polymers.

Abstract

Genetically engineered protein polymers have been produced with the crystallizable <italic> Bombyx mori</italic> silk fiber peptide repeats (GAGAGS) alternated with bioactive (fibronectin or laminin) sequences or flexible elastin sequences. These protein polymers have potential use as coatings for neural prosthetic devices. These materials have precise amino acid sequences, but their morphological characterization and processing are largely unexplored. The structure of native silk and silk-like polymers and the electrospinning deposition process were examined in this study. Studies of native silk fiber using X-ray diffraction (XRD), molecular modeling and infrared spectroscopy (IR) suggested that the extended-chain beta-sheet crystal structure had predominantly parallel chain packing. Comparisons between XRD simulations of packing combinations and experimental data were performed by creating a program for full diffraction pattern comparison. The IR spectrum for native fibroin also supported parallel molecular packing; the amide I peak was not split as in the re-crystallized silk. Finite element modeling of the banded structure showed a stress redistribution from the curved geometry of the bands. The silk-like protein polymers showed two crystalline polymorphs, Silk I (crankshaft) and Silk II (beta-sheet). The SLP4 polymer, containing long, continuous repeats of GAGAGS, was received in the Silk I form but converted to Silk II after re-crystallization, allowing for conformation-dependent studies. XRD patterns of the silk-like polymers were similar to native silk, indicating that the silk-like domains aggregated into crystallites. The tendency to form a certain polymorph was a function of the amount and domain length of the silk sequences. Electrospinning uses an electric field to deform and draw a fluid material toward an opposite electrode. When the material is a viscous solution of polymer in a volatile solvent, the emitted stream remains filamentous, and is deposited as solid. A theoretical model was developed to elucidate the influence of the important parameters for electrospinning. An oscillating waveform was superimposed on the direct potential in order to influence the morphology of the resulting filaments. The oscillating waveform can drive the formation of a beaded morphology in which the number of beads increases with signal frequency.