Branched polyethylene glycol (four arms, MW = 15 000) having a cinnamylidene acetyl moiety as a pendant group was synthesized by an esterification reaction between polyethylene glycol and cinnamylidene acetyl chloride. The photosensitive polymer was irradiated with a 450 W medium pressure Hg lamp (lambda > 300 nm) from 5 min to 3 h to produce polyethylene glycol hydrogels. These gels were swollen in water and showed characteristic properties of a hydrogel. The degree of swelling was controlled by the content of cinnamylidene acetyl moiety in the polymer and the time of ultraviolet irradiation. A reduced degree of substitution resulted in increased swellability of the synthesized hydrogel. The photoscission of the gel, which was monitored by its UV spectrum, was performed by irradiating the hydrogel with a 150 W Xenon lamp at 254 nm using a bandpass filter. The biocompatibility of the synthesized gel was also determined. The antithrombogenic behavior (99.6% reduction in platelet deposition) of the synthesized b-PEG-CA hydrogel was demonstrated by measuring platelet adhesion onto coverslips which had been coated with PMMA with a second coating film of b-PEG-CA hydrogel.