Facile construction of mechanically tough collagen fibers reinforced by chitin nanofibers as cell alignment templates.

Fiber-shaped cellular architectures have drawn attention due to their structural similarity to the extracellular matrix. For such an application, fibers that are both mechanically robust and biocompatible in the targeted biological environment are required. In this work, a facile method has been developed to prepare mechanically tough reconstituted collagen fibers by extrusion of collagen acetic acid solution into a coagulation bath containing sodium alginate. The structure and morphology of the collagen fiber were studied by XPS, FT-IR, SEM, TEM and DSC. The results indicated that the instant electrostatic interaction and hydrogen bonding between sodium alginate and collagen contributed to the formation of the mechanically strong microfiber, which exhibited a tensile stress and Young's modulus of 313.4 MPa and 5848.5 MPa. During fiber formation, the triple-helix structure of collagen remained undamaged, and self-assembled into closely stacked nanofibrous morphology. In addition, positively charged chitin nanofibers (CNFs) were incorporated into the collagen fiber as reinforcing filler. With increasing CNF concentration, the tensile strength and modulus of the air-dried collagen fiber increased and reached the highest values of 506.6 MPa and 12109.5 MPa at 15 wt% CNFs. The wet strength of the composite fibers was also significantly improved with addition of CNFs, showing mechanical properties (stress and modulus of 47.3 MPa and 988.9 Mpa) comparable to the reported post-crosslinked collagen fibers. Yet, this has been achieved through a physical approach under mild conditions without using toxic chemical crosslinking reagents or causing collagen denaturation, allowing their direct application as promising biomaterials. In vitro tests showed that the addition of CNFs did not induce obvious cytotoxicity of the fibers towards fibroblast cells and the whole composite fibers exhibited a cell viability of >80% for a culture time of up to 72 h. Moreover, the fibroblast alignment on the surface of the fibers was observed by confocal and scanning electron microscope imaging, indicating potential application of the fibers as a cell alignment template in tissue engineering fields for vascular, muscle and neural engineering applications.