a chemically crosslinked hydrogel includes an active ingredient for transdermal transport selected from the group of lidocainehydrochloride , hydrocortisone , menthol , and methyl salicylate and utilizes as a crosslinking agent an aluminum acetate , which is a polyvalent salt , which renders the resulting hydrogel electrically conductive with a resistance of less than 10 , 000 ohms per linear centimeter .