Examples of a cross-linking agent that can be used include: compounds, each having at least two polymerizable double bonds in its molecule, such as N,N???-methylenebis(meth)acrylamide, (poly)ethyleneglycol di(meth)acrylate, (poly)propyleneglycol di(meth)acrylate, (polyoxiethylene)trimethylolpropanetri(meth)acrylate, trimethylolpropanedi(meth)acrylate, (poly)ethyleneglycol di(??-acryloyloxipropiona