Controlling Microparticle Aspect Ratio via Photolithography for Injectable Granular Hydrogel Formation and Cell Delivery

Granular hydrogels are injectable and inherently porous biomaterials assembled through the packing of microparticles. These particles typically have a symmetric and spherical shape. However, recent studies have shown that asymmetric particles with high aspect ratios, such as fibers and rods, can significantly improve the mechanics, structure, and cell-guidance ability of granular hydrogels. Despite this, it remains unknown how controlled changes in the particle aspect ratio influence the injectability, porosity, and cell-instructive capabilities of granular hydrogels. Part of the challenge lies in obtaining microparticles with precisely tailored dimensions using fabrication methods such as flow-focusing microfluidics or extrusion fragmentation. In this work, we leveraged facile photolithography and photocurable hyaluronic acid to fabricate rod-shaped microparticles with widths and heights of 130 μm and lengths that varied from 260 to 1300 μm to obtain aspect ratios (ARs) of 2, 4, 6, 8, and 10. All AR microparticles formed porous and injectable granular hydrogels after centrifugation jamming. Interestingly, the longest microparticles neither clogged the needle nor fractured after extrusion from a syringe. This was attributed to a relatively low elastic modulus that permitted microparticle pliability and reversible deformation under shear. Cells (NIH/3T3 fibroblasts) mixed with the jammed microparticles and injected into molds remained viable, adhered to the particles' surface, and showed a significant and rapid rate of proliferation over a period of 7 days compared to bulk hydrogels. The proliferation rate and morphology of the cells were significantly influenced by the particle AR, with higher cell numbers observed with intermediate ARs, likely attributable to the surface area available for cell adhesion. These findings showcase the utility of injectable granular hydrogels made with high-aspect-ratio microparticles for biomedical applications.