Abstract

Dynamic covalent chemistry at the polymer-nanoparticle interface enables the engineering of soft materials that simultaneously exhibit high modulus and adaptability. In this study, we grafted Wulff-type boronic acid methacrylate chains onto SiO2 nanoparticles using surface-initiated RAFT polymerization and embedded these fillers in alginate modified with phenylboronic acid. Increasing the pH to 7.4-8.0 promoted the formation of stable boronate esters between the nanoparticle brushes and alginate diols, resulting in nanocomposite hydrogels with 0.10-0.50 wt% inorganic content. Rheometry revealed that nanoparticles increase the storage modulus of the network by an order of magnitude compared to the control. Frequency sweeps showed pH-responsive stiffening, and cycle tests demonstrated full recovery after five large-amplitude cycles. Remarkably, the nanocomposite hydrogels displayed strain stiffening. Analysis using the stiffness index and critical stress indicated that while the nanoparticles enhance the overall strength of the network, they have little effect on the microscopic or cooperative mechanical response. All formulations passed a 21-G needle injectability test, requiring 1-3 N, within the clinically acceptable range of injection forces, and exhibited hemolysis levels below 1.3%, suggesting good hemocompatibility. These findings imply that multiple dynamic boronate contacts provide energy-dissipating junctions that reinforce alginate hydrogels without relying on permanent bond cross-linkers.