Control of Matrix Stiffness Using Methacrylate–Gelatin Hydrogels for a Macrophage-Mediated Inflammatory Response

The successful tissue integration of a biomedical material is mainly determined by the inflammatory response after implantation. Macrophage behavior toward implanted materials is pivotal to determine the extent of the inflammatory response. Hydrogels with different properties have been developed for various biomedical applications such as wound dressings or cell-loaded scaffolds. However, there is limited investigation available on the effects of hydrogel mechanical properties on macrophage behavior and the further host inflammatory response. To this end, methacrylate-gelatin (GelMA) hydrogels were selected as a model material to study the effect of hydrogel stiffness (2, 10, and 29 kPa) on macrophage phenotype in vitro and the further host inflammatory response in vivo. Our data showed that macrophages seeded on stiffer surfaces tended to induce macrophages toward a proinflammatory (M1) phenotype with increased macrophage spreading, more defined F-actin and focal adhesion staining, and more proinflammatory cytokine secretion and cluster of differentiation (CD) marker expression compared to those on surfaces with a lower stiffness. When these hydrogels were further subcutaneously implanted in mice to assess their inflammatory response, GelMA hydrogels with a lower stiffness showed more macrophage infiltration but thinner fibrotic capsule formation. The more severe inflammatory response can be attributed to the higher percentage of M1 macrophages induced by GelMA hydrogels with a higher stiffness. Collectively, our data demonstrated that macrophage behavior and the further inflammatory response are mechanically regulated by hydrogel stiffness. The macrophage phenotype rather than the macrophage number predominately determined the inflammatory response after the implantation, which can provide new insights into the future design and application of novel hydrogel-based biomaterials.