Examples of polymer mixtures that may be used include 95-99 wt % polyethylene glycol and 1-5 wt % of another water-soluble polymer such as polyvinyl alcohol, polyvinylpyrrolidone, and polyethylene glycol derivatives such as polyethylene glycol acrylate, polyethylene glycol diacrylate, polyethylene glycol methyl ether acrylate, polyethylene glycol methacrylate, polyethylene glycol dimethacrylate, p