Enhanced Release of Molecules upon Ultraviolet (UV) Light Irradiation from Photoresponsive Hydrogels Prepared from Bifunctional Azobenzene and Four-Arm Poly(ethylene glycol)

Advances in biosensors and drug delivery are dependent on hydrogels that respond to external stimuli. In this work, we describe the preparation and characterization of photoresponsive hydrogels prepared by cross-linking of di-NHS ester of azobenzoic acid and four-armed, amine-terminated poly(ethylene glycol). The porous structure and composition of the hydrogels were confirmed by scanning electron microscopy (SEM) and Fourier transform infrared (FTIR) spectroscopy. The reversible photoisomerization of the azobenzene-containing hydrogel cross-linkers in the gels was confirmed by absorption spectroscopy. Specifically, the photoisomerization of the cross-linkers between their trans and cis configurations was observed by monitoring the absorbance of the hydrogels at the two characteristic peaks of azobenzene (π-π* at 330 nm and n-π* at 435 nm). The effect of photoisomerization on the hydrogel structure was investigated by microscopy. Ultraviolet (UV) irradiation-induced reduction in hydrogel size was observed, which may be a result of the inherently smaller footprint of the cis azobenzene conformation, as well as dipole-dipole interactions between the polar cis azobenzene and the polymer network. The UV-triggered reduction in hydrogel size was accompanied by enhanced release of the near-infrared fluorescent dye Alexa Fluor 750 (AF₇₅₀). Enhanced release of AF₇₅₀ was observed in samples irradiated with UV versus dark control. Together, these data demonstrate the potential of these systems as reversible photoresponsive biomaterials.