Hydrogels with hierarchical hydrogen bonds enable tunable stress relaxation to direct macrophage-driven immunoregulation

Matrix viscoelasticity as the typical mechanical feature of the extracellular matrix (ECM) of living tissues has been recently identified to impose profound impact on cellular behavior and functionalities. However, their effect on macrophages and the related inflammatory responses have been rarely explored, in which a major hurdle is the lack of candidate ECM-mimicking biomaterials with independently fine-tunable viscoelasticity. Herein, we developed a class of photopolymerizable hydrogels based on hierarchical hydrogen bond (H-bond) design to enable controllable stress relaxation behavior. Specifically, acrylamide (AM) and N-acryloyl glycinamide (NAGA) monomers were polymerized to form hydrogel networks consisting of varying ratio of single H-bond by AM and dual H-bond by NAGA, thus hydrogels with tunable stress relaxation rate but constant matrix elasticity can be developed via mediating monomer mixing ratios and polymerization degree. We further revealed that slower stress relaxation induced macrophage towards pro-inflammatory M1 polarization, while faster stress relaxation stimulated macrophage to polarize towards anti-inflammatory M2 phenotype. In vivo subcutaneous implantation of different hydrogel matrices using a mice model showed similar impact on host immune responses, as evidenced by severer inflammation characterized by thicker fibrosis encapsulation for matrix with slower stress relaxation than the faster ones. In general, we demonstrate that matrix viscoelasticity can significantly affect macrophage-mediated inflammatory responses, which provides new insights for the design and applications of implantable biomaterials. STATEMENT OF SIGNIFICANCE: This work revealed how extracellular matrix viscoelasticity characterized by stress relaxation rate can significantly impact macrophage polarization and inflammatory responses, which has rarely been explored previously. We proposed an innovative and ease-of-preparation class of copolymer hydrogels based on tailoring the mixing ratio of acrylamide (AM) and N-acryloyl glycinamide (NAGA) monomers, wherein matrix with fine-tunable stress relaxation rates can be achieved via controlling the combination of singular or dual hydrogen bonds. We further demonstrated that slower stress relaxation induced macrophage towards pro-inflammatory M1 polarization, while faster stress relaxation stimulated macrophage to polarize towards anti-inflammatory M2 phenotype. Our study suggests matrix viscoelasticity can significantly affect macrophage-mediated inflammatory responses, and provides new insights for the design and applications of implantable biomaterials.