Injectable hydrogels are biomaterials that can be administered minimally invasively in liquid form and are considered promising artificial extracellular matrix (ECM) materials. However, ordinary injectable hydrogels are synthesized from water-soluble molecules to ensure injectability, resulting in non-phase-separated structures, making them structurally different from natural ECMs with phase-separated insoluble structural proteins, such as collagen and elastin. Here, we propose a simple material design approach to impart phase-separated structures to injectable hydrogels by adding inorganic salts. Injecting a gelling solution of mutually cross-linkable tetra-arm poly(ethylene glycol)s with potassium sulfate at optimal concentrations results in the formation of a hydrogel with phase-separated structures in situ. These phase-separated structures provide up to an 8-fold increase in fracture toughness while allowing the encapsulation of live mouse chondrogenic cells without compromising their proliferative activity. Our findings highlight that the concentration of inorganic salts is an important design parameter in injectable hydrogels for artificial ECMs.
Keywords: biomaterial; extracellular matrix (ECM); injectable hydrogel; phase separation; poly(ethylene glycol) (PEG); porous structure.