Cartilage tissue engineering strategies often rely on hydrogels with fixed covalent crosslinks for chondrocyte encapsulation; yet the resulting material properties are largely elastic and can impede matrix deposition. To address this limitation, hydrazone crosslinked poly(ethylene glycol) hydrogels were formulated to achieve tunable viscoelastic properties and to study how chondrocyte proliferation and matrix deposition vary with the time-dependent material properties of covalent adaptable networks. Hydrazone equilibrium differences were leveraged to produce average stress relaxation times from hours (4.01×103s) to months (2.78×106s) by varying the percentage of alkyl-hydrazone (aHz) to benzyl-hydrazone (bHz) crosslinks. Swelling behavior and degradation associated with adaptability was characterized to quantify temporal network changes that can influence the behavior of encapsulated chondrocytes. After four weeks, mass swelling ratios varied from 36±3 to 17±0.4 and polymer retention ranged from 46±4% to 92±5%, with higher aHz content leading to loss of network connectivity with time. Hydrogels were formulated near the Flory-Stockmayer bHz percolation threshold (17% bHz) to investigate chondrocyte response to distinct levels of covalent architecture adaptability. Four weeks post-encapsulation, formulations with average relaxation times of 3 days (2.6×105s) revealed increased cellularity and an interconnected articular cartilage-specific matrix. Chondrocytes embedded in this adaptable formulation (22% bHz) deposited 190±30% more collagen and 140±20% more sulfated glycosaminoglycans compared to the 100% bHz control, which constrained matrix deposition to pericellular space. Collectively, these findings indicate that incorporating highly adaptable aHz crosslinks enhanced regenerative outcomes. However, connected networks containing more stable bHz bonds were required to achieve the highest quality neocartilaginous tissue.

The inherent reactivity of lignin in conventional biomass processing commonly prevents isolation of native lignin and limits monomer yields from catalytic depolymerization strategies that target aryl-ether bonds. Here we show...

Lignin valorization approaches, which are critical to biorefining, often involve depolymerization to aromatic monomers. Alkaline oxidation has long held promise as a lignin depolymerization strategy, but requires high concentrations of...