Gelation of Covalently Cross-Linked PEG-Heparin Hydrogels

We study PEG-heparin hydrogels to identify compositions that lead to gel formation and measure the corresponding gelation kinetics. The material consists of a maleimide-functionalized high molecular weight heparin (HMWH) backbone covalently cross-linked with bis-thiol poly(ethylene glycol) (PEG). Using multiple particle tracking microrheology, we investigate a broad composition space, defined by the number of maleimide functional sites per HMWH (f = 3.9-11.8), the molecular weight of the PEG cross-linker (M(n) = 2000, 5000, and 10 000), and the concentrations of the heparin and PEG polymers. Gelation kinetics are characterized by time-cure superposition, yielding the gel time, t(c), and the critical relaxation exponent, n. Gelation times range from 5 < t(c) ≤ 45 min, with the fastest kinetics occurring for the highest HMWH maleimide functionalities. t(c) depends nonmonotonically on the PEG cross-linker molecular weight, suggesting that gelation is affected by the length of the cross-linker relative to intermolecular interactions between heparin molecules. The critical relaxation exponent decreases from n = 0.52 for PEG 2000 to n = 0.39 for PEG 10 000. Finally, 219 equilibrated samples taken over the entire composition space are identified as liquid or solid, defining the "gelation envelope". The boundaries of this empirical gelation envelope are in good agreement with Flory-Stockmayer theory. In all, microrheological measurements enable characterization over a large parameter space and provide crucial insight into the gelation of complex, multifunctional hydrogelators used in therapeutic applications.