Independent and Synergistic Modulations of Viscoelasticity and Stiffness of Dynamically Cross-Linked Cell-Encapsulating ClickGels by Covalently Tethered Polymer Brushes.

We report independent and synergistic modulations of the stiffness and viscoelasticity of ClickGels, formed by a combination of the bio-orthogonal covalent and dynamic noncovalent cross-linking, by covalently incorporating nonionic, zwitterionic, or anionic polymer brushes. Tethering nonionic and zwitterionic brushes at the cost of noncovalent cross-linking increased stiffness and slowed stress relaxation, respectively, without altering the other properties. Meanwhile, tethering anionic brushes significantly increased ClickGel stiffness, while also slowing its stress relaxation. ClickGels with faster stress relaxation, not reduced stiffness, promoted short-term (24 h) viability and YAP/TAZ nuclear localization of encapsulated bone marrow-derived stromal cells (BMSCs). In contrast, ClickGel stiffness, not viscoelasticity, inversely correlated to the short-term dehydrogenase activity of encapsulated BMSCs. This work highlights the role of the ionic state of polymer brushes in modulating the hydrogel elastic modulus and viscoelasticity and establishes the brush-modified ClickGel as a facile and reproducible tool for manipulating mechanical cues of the synthetic cellular niche.