Photoswitchable PEG-CA hydrogels and factors that affect their photosensitivity

The photoswitching behavior of polyethylene glycol-based hydrogels was dete rmined by monitoring the changes in their swellability and absorption spect ra upon exposure to alternating wavelengths of irradiation. Highly hydrophi lic PEG-based hydrogels were prepared by the irradiation of a cinnamylidene acetate-terminated PEG solution (PEG-CA). The degree of swelling of the PE G-CA hydrogels was predictably modulated by alternating the wavelength of e xposing irradiation. Exposing the PEG-CA gels to >300 nm irradiation result ed in the decrease of their swellability, while short exposure to 254 nm le d to more hydrophilic gels. That is, the physical properties of the PEG-CA gels can be controlled by the selection of wavelength of irradiation. Inter estingly, it was found that 254 nm irradiation could not only lead to the p hotoscission of the PEG-CA gels, but also initiate a crosslinking reaction between PEG-CA monomers. On the contrary, only photocrosslinking via a cycl obutane ring formation was observed under >300-nm irradiation. Finally, the factors that could affect an efficient photoswitching behavior of the PEG- CA gels were investigated. (C) 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 1466-1476, 2000.