Polyethylene glycol (PEG) is a hydrophilic polymer. It can be easily synthesized by the anionic ring opening polymerization of ethylene oxide, into a range molecular weights and variety of end groups. When crosslinked into networks PEG can have high water content, forming "hydrogels". Hydrogel formation can be initiated by either crosslinking PEG by ionizing radiation or by covalent crosslinking of PEG macromers with reactive chain ends. PEG is a suitable material for biological applications because it does not trigger an immune response.