Human stratum corneum penetration by copper: in vivo study after occlusive and semi-occlusive application of the metal as powder.

Aim of the study was to shed light on the long-standing controversy whether wearing copper bangles benefits patients suffering from inflammatory conditions such as arthritis. Sequential tape stripping was implemented on healthy volunteers to examine the diffusion of copper through human stratum corneum in vivo following application of the metal as powder on the volar forearm for periods of up to 72 h. Exposure sites were stripped 20 times and the strips analyzed for metal content by inductively coupled plasma-mass spectroscopy with a detection limit for copper of 0.5 ppb. Untreated skin was stripped in the same fashion, to determine baseline copper levels for comparison with exposure values resulting from exposure in respective volunteers. Under occlusion with exclusion of air, up to 72 h copper values decreased from the superficial to the deeper layers of the stratum corneum with gradients increasing commensurately with occlusion time, characteristic of passive diffusion processes. From the tenth strip on, however, levels reverted to background values. Under semi-occlusion allowing access of air by covering the skin with "breathable" tape, initial copper values lay significantly above baseline values and concentration gradients increased proportionally with occlusion time. At 72 h, from the tenth to the twentieth strip reaching the glistening epidermal layer, copper values continued at constant levels, significantly above baseline values. The results indicate that, in contact with skin, copper will oxidize and may penetrate the stratum corneum after forming an ion pair with skin exudates. The rate of reaction seems to depend on contact time and availability of oxygen. A marked inter-individual difference was observed in baseline values and amounts copper absorbed.