No abstract

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-[PEG-Dopa] hydrogels are formed through Fe-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]:[Dopa] molar ratio in precursor solutions. Networks formed from precursors with high [Fe]:[Dopa] ratios create mechanically robust networks (G' = 6880 ± 240 Pa) that are largely impervious to US-mediated disintegration at intensities of ≤43 W/cm. Conversely, lightly cross-linked networks formed through [Fe]:[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-[PEG-Dopa] 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.

Interfacial adhesion‐mediated transfer printing processes can integrate functional electronic microstructures with polymeric substrates that are bendable and stretchable. Transfer printing has also been extended to catechol‐bearing adhesive hydrogels. This study presents indentation adhesion tests between catechol‐bearing hydrogel substrates with catechol concentrations varying from 0 to 10% (mol/mol) and thin‐film materials commonly used in microelectronic fabrication including polymers, noble metals and oxides. The results indicate that the interfacial adhesion of catechol‐bearing hydrogels is positively correlated with the concentration of catechol‐bearing monomers as well as the retraction velocity during transfer printing. This study can inform transfer printing processes for microfabricated structures to compliant hydrated substrates such as hygroscopic monomers, mesoporous polymer networks and hydrogels. © 2016 Society of Chemical Industry