Packed hydrogel microfibers as a granular medium

Particle-based (granular) hydrogels are an attractive class of biomaterials due to their unique properties and array of applications in the biomedical space - serving as platforms for extrusion printing and injecting as well as permissive materials for 3D cell culture. Physical properties of particle-based hydrogels are governed in part by contact forces between particles, which are limited to interactions with neighboring particles. Secondary annealing mechanisms are often used to increase mechanical properties and serve to link particles across the granular material volume. Here, we present a novel particle-based hydrogel where each "particle" is a discrete electrospun hydrogel microfiber that has been segmented to a length of 93 {+/-} 51 m, with a diameter of 1.6 {+/-} 0.3 m. The fibers are flexible and have aspect ratios that are greater than one order of magnitude larger than most traditional hydrogel microparticles. This enables long-range entanglements of discrete fibers following packing into a bulk material, yielding unique properties. Without crosslinking, these packed hydrogel microfiber materials are mechanically robust, they can stretch without breaking when strained, and they exhibit stress relaxation under constant strain. As a cell culture scaffold, shear-induced alignment of the individual fibers within 3D printed filaments confers contact guidance cues to cells and promotes anisotropic cellular morphologies. Packed hydrogel microfibers can also be used as 3D cell culture environments, with cells able to spread due to the permissive nature of the scaffold. Overall, this work introduces a particle-based material system comprised of individual hydrogel microfibers that allows unique properties to be engineered into biomaterials that might be used in extrusion processes and cell cultures and, ultimately, in tissue engineering and regenerative medicine applications.