The hydrophylic monomer used to prepare the hydrophyhc polymer is preferably a hydroxyalkyl monoacrylate or methacrylate such as 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, diethylene glycol monoacrylate, diethylene glycol monomethacrylate, hydroxypropyl acrylates and methacrylates, e.g. Z-hydroxypropyl acrylate, 2-hydroxypropyl methacrylate, 3-hydroxypropyl acrylate, 3-hydroxypropyl met