Fabrication of Amyloid Curli Fibers-Alginate Nanocomposite Hydrogels with Enhanced Stiffness.

Alginate hydrogels are biocompatible, biodegradable, low-cost, and widely used as bioinks, cell encapsulates, three-dimensional culture matrices, drug delivery systems, and scaffolds for tissue engineering. Nevertheless, their limited stiffness hinders their use for certain biomedical applications. Many research groups have tried to address this problem by reinforcing alginate hydrogels with graphene, carbon nanotubes, or silver nanoparticles. However, these materials present nanotoxicity issues, limiting their use for biomedical applications. Other studies show that electrospinning or wet spinning can be used to fabricate biocompatible, micro- and nanofibers to reinforce hydrogels. As a relatively simple and cheap alternative, in this study we used bioengineered bacteria to fabricate amyloid curli fibers to enhance the stiffness of alginate hydrogels. We have fabricated for the first time bioengineered amyloid curli fibers-hydrogel composites and characterized them by a combination of (i) atomic force microscopy (AFM) to measure the Young's modulus of the bioengineered amyloid curli fibers and study their topography, (ii) nanoindentation to measure the Young's modulus of the amyloid curli fibers-alginate nanocomposite hydrogels, and (iii) Fourier-transform infrared spectroscopy (FTIR) to analyze their composition. The fabricated nanocomposites resulted in a highly improved Young's modulus (up to 4-fold) and showed very similar physical and chemical properties, opening the window for their use in applications where the properties alginate hydrogels are convenient but do not match the stiffness needed.