Controlling\nMicroparticle Aspect Ratio via Photolithography\nfor Injectable Granular Hydrogel Formation and Cell Delivery

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