Ultrasound-Mediated Self-Healing Hydrogels Based on Tunable Metal–Organic Bonding

Stimulus-responsive hydrogels make up an important class of programmable materials for a wide range of biomedical applications. Ultrasound (US) is a stimulus that offers utility because of its ability to permeate tissue and rapidly induce chemical alterations in aqueous media. Here we report on the synthesis and US-mediated disintegration of stimulus-responsive telechelic Dopa-modified polyethylene glycol-based hydrogels. Fe 3+ -[PEG-Dopa] 4 hydrogels are formed through Fe 3+ -induced cross-linking of four-arm polyethylene glycol-dopamine precursors to produce networks. The relative amounts of H-bonds, coordination bonds, and covalent bonds can be controlled by the [Fe 3+ ]:[Dopa] molar ratio in precursor solutions. Networks formed from precursors with high [Fe 3+ ]:[Dopa] ratios create mechanically robust networks ( G ′ = 6880 ± 240 Pa) that are largely impervious to US-mediated disintegration at intensities of ≤43 W/cm 2 . Conversely, lightly cross-linked networks formed through [Fe 3+ ]:[Dopa] molar ratios of <0.73 are susceptible to rapid disintegration upon exposure to US. Pulsatile US exposure allows temporal control over hydrogel disintegration and programmable self-healing. Sustained US energy can also stabilize hydrogels through the formation of additional cross-links via free radical-mediated coupling of pendant catechols. Taken together, the diverse ranges of mechanical behavior, self-healing capability, and differential susceptibility to ultrasonic disintegration suggest that Fe 3+ -[PEG-Dopa] 4 hydrogels yield a class of application-specific stimulus-responsive polymers as smart materials for applications ranging from transient medical implants to matrices for smart drug delivery.