Passive electrical properties of human stratum corneum in vitro depending on time after separation

The passive electrical properties of human skin after separation from the body are predominated by the stratum corneum. Skin within a bath medium (150 mM phosphate buffered saline) at constant temperature (37 degrees C) exhibits a characteristic change of the passive electrical properties with time. Independent of the time the locus in the Z-plane is a depressed semicircular are. The angle between the lines from the center of the are to the points where the locus reaches the real axis remains unchanged. The difference between the high and low frequency resistivity (R(0) - R(x)) increases over 10 h, reaches a plateau and decays after 20 h exponentially with a time constant of about 40 h. As model for the impedance we used a 5 element electrical circuit (R(0), R(1), R(2), C-1, C-2), describing 3 pathways, (0) the de path (appendages; R(0)), (1) tortuous pathways around the cell structures (R(1), C-1) and (2) direct pathways involving the corneocytes (R(2), C-2). There are characteristic changes with time in the elements of the equivalent circuit up to about 200 h after excision. Dramatic changes in C-1 and R(2) at about this time after separation strongly suggests destruction of the lipid structures. It will be suggested that the use of separated human stratum corneum as model for in vivo yields unreliable results after this time.