Effect of tunable stiffness on immune responses in 3D-bioprinted alginate–gelatin scaffolds

Tissue engineering is an approach used to restore damaged tissues and organs using biomaterials that support cell adhesion, growth, and proliferation. However, immune responses triggered by tissue injury and biomaterial implantation can lead to undesired reactions such as foreign body response and fibrotic capsule formation. Macrophages play a critical role in these immune responses. Therefore, comprehending and controlling the immune responses to biomaterials are crucial for successful clinical translation in tissue engineering. In this experimental study, we fabricated three-dimensional-bioprinted hydrogel scaffolds with adaptable stiffness by adjusting the alginate–gelatin ratio. We examined the physical properties of these scaffolds and assessed the immune responses they provoked both in vitro and in vivo. Our results revealed that higher-stiffness implants could drive macrophage polarization toward pro-inflammatory phenotypes in vivo. Furthermore, our animal experiments demonstrated that high-stiffness hydrogels elicited elevated immune responses through the TLR4/Myd88/NF-κB signaling pathway and IL-6/JAK-STAT signaling pathway. Collectively, our study demonstrates that increased implant stiffness correlates with stronger immune responses. These findings are expected to provide novel insights for the clinical application of alginate–gelatin composite hydrogels.