In Situ Forming, Dual-Crosslink Network, Self-Healing Hydrogel Enabled by a Bioorthogonal Nopoldiol-Benzoxaborolate Click Reaction with a Wide pH Range

The use of click chemistry as a hydrogel cross-linking reaction is often limited by slow reaction rates and harsh conditions, such as exposure to UV light and/or use of nonspecific or toxic reagents. On the other hand, the process of boronic ester formation between arylboronic acids and diols suffers from its intrinsic reversibility and low binding affinity at low pH, which impede its potential in many biomedical applications where a fast and stable click reaction is needed. Herein, we report a new concept of click hydrogel fabrication that combines a traditional sugar-based boronic ester and a novel nopoldiol-based benzoxaborolate as a dual-crosslink network (DCN) system. The cooperation of dynamic and rigid networks and the unique sensitivity of benzoxaborolate cross-links toward stimulus provide an intelligent hydrogel with a set of interesting features: (i) catalyst/light-free nopoldiol-benzoxaborolate bioorthogonal click cross-linking, (ii) rapid in situ formation within 26 s, (iii) wide self-healing pH range from 8.5 to 1.5, (iv) exceptional stability under acidic condition and polyol solutios, (v) reactive oxygen species/pH-responsive degradation, (vi) pH-responsive drug release, and (vii) capability for viable cell encapsulation. The complementary click partners, a rigid diol monomer [1R)-(-)-nopol-methacrylamido-diol (nopoldiol)] and a benzoxaborole-based monomer [5-methacrylamido-1,2-benzoxaborole (MAAmBO)], can be easily incorporated into a variety of synthetic polymers through free-radical polymerization with poly(ethylene glycol) methyl ether methacrylate (PEGMA) as the backbone component. The shortened gelation time, improved mechanical properties, and excellent self-healing properties of the resulting DCN hydrogel PBNG were evaluated through rheological measurements. The stability/degradation of PBNG under low pH buffer and H2O2 were monitored via hydrogel weight changes, and the potential of PBNG as a drug-releasing carrier was assessed by the pH-responsive release of doxorubicin. Finally, HeLa cells were successfully encapsulated and cultured in the 3D network to confirm the hydrogel's biocompatibility as a cell culture scaffold. The nontoxic components and their fast click reaction under mild conditions make the nopoldiol-benzoxaborolate click hydrogels promising candidates for future biomedical applications such as gene delivery, cell therapy, and tissue engineering.