Preparation of a Stable Highly Concentrated Keratin Solution by PEGylation in a Deep Eutectic Solvent

Keratin molecules contain many hydrophobic amino acids and disulfide bonds that render them poorly soluble in water, thus greatly limiting their properties and applications as biomaterials. Improving the solubility of keratin and the stability of highly concentrated solutions is the key to solving this. In this paper, wool keratin was first dissolved and then PEGylated in a novel reduced deep eutectic solvent (DES) to obtain a stable keratin solution with a concentration of 300 g/L. The hydrophobicity, particle size distribution, conformation, and rheological properties of this high-concentration keratin solution were investigated. The results showed that a thiol–ene click reaction reduced the thiol content by up to 60% and inhibited thiol–thiol cross-linking. PEGylation also reduced the hydrophobic areas on the keratin surface, thus improving its stability in an aqueous solution. In the highly concentrated PEGylated keratin solution, the secondary conformation changed from α-helix to β-sheet. Polarized light microscopy indicated the formation of liquid crystals after applying friction. Rheological tests showed that the solution had a shear-thinning behavior at high shear rates, indicating that molecular chain reorientation was induced under the action of shear forces, which may help to strengthen the materials.