Water-Processable, Stretchable, and Ion-Conducting Coacervate Fibers from Keratin Associations with Polyelectrolytes

Abstract

Keratin is one of the most abundant biopolymers, produced on a scale of millions of tons per year but often simply discarded as waste. Due to its abundance, biocompatibility, and excellent mechanical properties, there is an extremely high interest in developing protocols for the recycling of keratin and its conversion into protein-based materials. In this work, we describe a novel protocol for the conversion of keratin from wool into hybrid fibers. Our protocol uses a synthetic polyanion, which undergoes complex coacervation with keratin, leading to a viscous liquid phase that can be used directly as a dope for dry-spinning. The use of polyelectrolyte complexation allows us to use all of the extracted keratin, unlike previous works that were limited to the fraction with the highest molecular weight. The fibers prepared by this protocol show excellent mechanical properties, humidity responsiveness, and ion conductivity, which makes them promising candidates for applications as a strain sensor.

Bioinspired Processing of Keratin into Upcycled Fibers through pH-Induced Coacervation

“…Keratin is a protein with a strong tendency to self-interact and aggregate through different intermolecular forces, such as hydrophobic interactions and hydrogen bonds. Coacervate formation in keratin has already been observed upon complexation with a polyanion, and in the formation of KB itself. However, since in this case phase separation was clearly linked to a decrease in pH, we hypothesize that this process was related to the protonation of negatively charged groups in keratin, such as glutamate and aspartate residues, and the −COOH terminal ends of keratin (with p K a ’s close to 2).…”

“…Here, we will refer to the upper phase as “keratin supernatant” (KS), and the dense, bottom phase as “keratin bottom” KB (Figure d). This spontaneous phase separation is driven by the aggregation of different-sized peptides: keratins in KB have a larger molecular weight than the ones in KS, while both of them are present in KH . To quantify the protein concentration in the different phases, we dialyzed (3.5 kDa cutoff) them against water for 1 week, and freeze-dried them to obtain the pure keratin as a solid.…”

“…35 We also noted a small peak in the DSC profile of KB at pH 7.0, assigned to a more ordered selfassembled structure due to the higher content in large peptides, as we have described previously. 19 To probe the relative changes in the secondary structure of keratin molecules due to coacervation, we used Fourier transform infrared spectroscopy to study the amide(I) band (1600−1700 cm −1 ). The deconvolution of the amide(I) band is given in Figure S3, and the relative spectral intensities of the various components are summarized in Table S1.…”

“…28 We also found that KB has a higher content of hydrogen-bonded nitrogen than KS and KH, indicating a more ordered structure, in agreement with the FTIR analysis discussed above and previous results. 19 To investigate the influence of coacervation on the morphology of regenerated keratin, KS, KB, and KH prepared at different pH values were analyzed with SEM. As shown in Figure 3, interconnected porous morphologies were obtained for the pH 1.0 and 1.7 samples, which agrees well with other reported studies on complex coacervates, where porous structures appear upon desalting.…”

“…However, their method only used the peptides of higher molecular weight, losing around one-third of the extracted keratin. As an alternative, we have recently reported a method where all of the extracted keratin could be processed through complexation with a synthetic polyanion, but the resulting materials had completely lost their hierarchical structure, in detriment of their mechanical properties . Furthermore, the fibers produced by these methods rely on the presence of LiBr in the final fibers, which would result in an additional environmental impact if produced on a large scale.…”