Granular hydrogels improve myogenic invasion and repair after volumetric muscle loss

Skeletal muscle injuries including volumetric muscle loss (VML) are marked by excessive scarring and functional disability that inherent regenerative mechanisms are unable to reverse. Despite high prevalence in civilian and military populations, there is currently no effective treatment for VML but bioengineering interventions such as biomaterials that fill the VML defect to support tissue growth and repair are a promising strategy. However, traditional biomaterials developed for this purpose are rigid, non-porous constructs that hinder cell infiltration. In the present study, we test the effects of granular hydrogels on muscle repair - hypothesizing that their inherent porosity will support the invasion of native myogenic cells and their flowability will permit conformable filling of the defect site, leading to effective muscle repair. We used photocurable hyaluronic acid crosslinked with matched muscle stiffness to prepare small or large particle fragments via extrusion fragmentation and facile size sorting. In assembled granular hydrogels, particle size and degree of packing significantly influenced pore features including porosity, pore size, and pore density, as well as rheological behavior including storage moduli and yield strain. We tested the ability of granular hydrogels to support early-stage (satellite cell invasion) and late-stage (myofiber invasion) muscle repair compared to bulk hydrogels in a VML injury model in the tibialis anterior (TA) muscles of 12-14 week old mice. Histological evaluation revealed granular hydrogels supported these regenerative processes while control bulk hydrogels restricted them to the gel-tissue interface in line with the absence of invading cells. Together, these results highlight the promising potential of injectable and porous granular hydrogels in supporting endogenous repair after severe muscle injury.