The effects of fibre bundling on the mechanics of stochastic fibre networks

Fibrous networks form the structural basis of many biological and synthetic materials such as collagen and self-assembling peptide hydrogels (SAPHs). While the macroscopic mechanical response of these materials is known to emerge from their mesoscale structure, the specific impact of fibre bundling (fibres associating into bundles) remains under explored. Using computational and analytical modelling, we analyze the effects of bundling on the bulk mechanical behaviour of fibrous networks. We develop a framework that generates 2D and 3D stochastic Mikado-type networks with a controllable bundling strength and demonstrate that bundling influences network mechanics by reshaping the network geometry and through internal mechanical coupling within bundles. We define two distinct modes of bundling: merged bundles, which behave as singular, mechanically coupled, thick fibres, and proximal bundles, where the fibres remain distinct. We find that bundling generally leads to a reduction in small strain stiffness and modifies large deformation strain stiffening by postponing the onset of stiffening and changing its rate. Notably, networks with identical topologies can exhibit different stiffness levels and stiffening behaviour depending on the degree of internal coupling within their bundles. These results identify bundling strength and bundle coupling as two independent design parameters, offering new strategies for tuning the mechanical properties of engineered hydrogels and fibrous materials.