Modulating Viscoelasticity, Stiffness, and Degradation of Synthetic Cellular Niches via Stoichiometric Tuning of Covalent versus Dynamic Noncovalent Cross-Linking

Viscoelasticity, stiffness, and degradation of tissue matrices regulate cell behavior, yet predictive synergistic tuning of these properties in synthetic cellular niches remains elusive. We hypothesize that reversible physical cross-linking can be quantitatively introduced to synthetic hydrogels to accelerate stress relaxation and enhance network stiffness, while strategic placement of isolated labile linkages near cross-linking sites can predict hydrogel degradation, both of which are essential for creating adaptive cellular niches. To test these hypotheses, chondrocytes were encapsulated in hydrogels formed by biorthogonal covalent and noncovalent physical cross-linking of a pair of hydrophilic building blocks. The stiffer and more viscoelastic hydrogels with DBCO–DBCO physical cross-links facilitated proliferation and chondrogenic ECM deposition of encapsulated cells by dissipating stress imposed by expanding cell mass/ECM via dynamic disruption/reformation of physical cross-links. Degradation of labile linkages near covalent cross-linkers further facilitated cell proliferation and timed cell release while maintaining chondrogenic phenotype. This work presents new chemical tools for engineering permissive synthetic niches for cell encapsulation, 3D expansion, and release.