Cryogelation of human hair keratins

• Main Author: Chua, Huei Min
• Other Authors: Ng Kee Woei
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2022
• Subjects: Engineering::Materials::Biomaterials
• Online Access: https://hdl.handle.net/10356/154987
• Institution: Nanyang Technological University

Human hair keratins (HHK) are natural structural proteins that can be extracted from abundant hair waste and valorised into biomaterials for tissue regeneration, wound healing, and drug delivery etc. The emergence of HHK in biomedical applications is attributed to their proven biocompatibility as well as their potential to promote cell adhesion and proliferation due to the presence of LDV (leucine-aspartic-valine) cell adhesion motifs and residual regulatory molecules. In a clinical setting, HHK-based templates could also serve as autologous alternatives which minimise the risk of xenogeneic infections or chemically induced cyto-toxicity. Nonetheless, conventional methods to translate HHK proteins into meaningful three-dimensional (3D) constructs often require chemical modifications or cross-linking additives. Such processing routes can be complicated and may compromise the resultant biocompatibility of the scaffolds. To circumvent the use of chemical additives, cryogelation was adopted as a clean and facile technique to fabricate 3D HHK scaffolds with tuneable physical properties. This novel concept of amalgamating HHK and cryogelation was premised on the ability of cryogelation to facilitate natural crosslinking between the HHK molecules through disulphide bonds due to the abundance of cysteine residues. However, the actual assembly mechanism of HHK during cryogelation remained speculative and prompted further investigation on the contribution of various intermolecular associations including hydrophobic interactions, hydrogen, and ionic bonding. In this research, inhibition studies using bond disrupting agents and gel solubilisation studies revealed that both disulphide bonds and hydrophobic interactions are cooperatively vital for gel network formation and stabilization. Inhibition of either interaction resulted in incomplete or delayed gelation. Given that these intermolecular linkages are formed during the freeze-thaw (FT) process, the control of cryogelation parameters is therefore crucial in modulating the degree of HHK crosslinking and physical properties of the fabricated cryogel scaffolds. In this dissertation, one of the main cryogelation parameter explored is the number of FT cycles. Under optimal freezing and thawing conditions, FT cycling was found to improve the storage modulus of HHK cryogels from 116.4 Pa at FT3 to 1908.7 Pa at FT10. Similarly, uniaxial compression tests demonstrated that FT cycling, from 0 to 10, enhanced the compression modulus of the HHK sponges by 12-folds. Microarchitecture analysis revealed that FT cycling rendered a change from sheet-like lamellae in FT0 sponges to interconnected polygonal pores in freeze-thawed sponges. Moreover, differences in physical properties engendered by FT cycling were found to influence material performance such as the degradation rate and cellular response. Therefore, these findings establish the correlation between cryogelation parameters and tuneable scaffold properties. Finally, in vitro assessment using human dermal fibroblasts (HDF) and human dermal microvascular endothelial cells (HMVEC) demonstrated that HHK cryogel sponges support cell adhesion and proliferation. In addition, stiffer HHK sponges of higher FT cycles were found to enhance cell infiltration through the porous microarchitecture. In a nutshell, the major findings of this dissertation serve to elucidate the mechanism of HHK cryogelation while demonstrating the possibility of developing a cell compliant 3D HHK scaffold with tuneable properties through a facile and clean process.