Skin penetration by metal compounds with special reference to copper

The process of diffusion into and across the different structures of the skin by chemicals is reviewed, with particular attention given to copper compounds. The scarce data available from the literature indicate that, in contact with the skin, metallic copper will oxidize, and the compounds resulting will penetrate it. Results from our lab confirm that copper compounds formed with skin exudates penetrate the human stratum corneum in a time-dependent fashion. The only quantitative diffusion rates for copper compounds given in the literature so far refer to experiments performed on the cat in vitro and in vivo. Transformation of data from that study, based on certain assumptions, lead to estimated Kp values of 10(-6) to 10(-5) cm/h for the copper salts tested, values that lie at the lower end of skin diffusivity rates measured for transition metal salts. Permeability coefficients for aqueous copper sulfate and acetate through human epidermis in vitro measured in our laboratory are of the order of 10(-6) cm/h. For copper compounds formulated in combination with zinc compounds for therapeutic purposes, applied on dermatomed human skin in vitro in various vehicles, the apparent penetration coefficients Kp were in the range of 3.2 x 10(-6) and 1.6 x 10(-5) cm/h.