Amyloid fibril-nanocellulose interactions and self-assembly

Amyloid fibrils from inexpensive food proteins and nanocellulose are renewable and biodegradable materials with broad ranging applications, such as water purification, bioplastics and biomaterials. To improve the mechanical properties of hybrid amyloid-nanocellulose materials, their colloidal interactions need to be understood and tuned. A combination of turbidity and zeta potential measurements, rheology and atomic force microscopy point to the importance of electrostatic interactions. These interactions lead to entropy-driven polyelectrolyte complexation for positively charged hen egg white lysozyme (HEWL) amyloids with negatively charged nanocellulose. The complexation increased the elasticity of the amyloid network by cross-linking individual fibrils. Scaling laws suggest different contributions to elasticity depending on nanocellulose morphology: cellulose nanocrystals induce amyloid bundling and network formation, while cellulose nanofibrils contribute to a second network. The contribution of the amyloids to the elasticity of the entire network structure is independent of nanocellulose morphology and agrees with theoretical scaling laws. Finally, strong and almost transparent hybrid amyloid-nanocellulose gels were prepared in a slow self-assembly started from repulsive co-dispersions above the isoelectric point of the amyloids, followed by dialysis to decrease the pH and induce amyloid-nanocellulose attraction and cross-linking. In summary, the gained knowledge on colloidal interactions provides an important basis for the design of functional biohybrid materials based on these two biopolymers.