How Cross-Linking Mechanisms of Methacrylated Gellan Gum Hydrogels Alter Macrophage Phenotype

In tissue engineering scaffolds, macrophages play a critical role in determining the host response to implanted biomaterials. Macrophage phenotype is dynamic throughout the host response, and a balance of phenotypes is essential for timely progression from injury to proper wound healing. Therefore, it is important to predict how materials will modulate the response of macrophages. In this study, we investigated the effect of methacrylated gellan gum (GG) hydrogels on macrophage phenotype and proliferation with the ultimate goal of improving rational design of biomedical implants. Naïve, along with classically and alternatively activated RAW 264.7 macrophages were seeded on methacrylated gellan gum hydrogels that were fabricated with different thiol-ene ratios and cross-linking mechanisms. Live/dead assays showed that all hydrogels supported cell attachment and proliferation. Stiffer substrates enhanced anti-inflammatory production of nitrites from both naïve and classically activated macrophages compared to the softer substrates. Moreover, arginine and CD206 expression-markers for alternatively activated macrophages-were inhibited by higher thiol content. Introducing ionic cross-links using calcium did not influence the proliferation or polarization for any of the three macrophage phenotypes. Our results suggest that the macrophage phenotype shift from M1 to M2 is controlled by the different cross-linking mechanisms, physical properties, and the chemistry of methacrylated gellan gum hydrogels.