The age-related changes in the electrical and physiological properties of the skin were examined in rats at the ages of 5, 10, 21, 90, and 180 d. The resistance of the stratum corneum, the resistance of the viable skin (epidermis and dermis), and the capacitance of the stratum corneum were analyzed from skin impedance data using an equivalent circuit. With development and aging, the resistance of the stratum corneum and the viable skin increased, whereas the capacitance of the stratum corneum decreased. Physiological characteristics such as the thickness of skin strata and the content of lipid and water in the stratum corneum were also measured. The lipid content in the stratum corneum was constant at all ages. The water content in the stratum corneum decreased, and the thickness of skin strata increased with age. Comparison between electrical data and physiological properties suggested that the increase in the resistance of the stratum corneum with aging is primarily caused by the decrease in the water content and that the capacitance of the stratum corneum and the resistance of the viable skin depend on age-related increases in the thickness of skin strata. In conclusion, the age dependency of cutaneous electrical properties may affect the permeation profile of drugs through the skin, and impedance analysis can be used to estimate age-related changes in transdermal drug delivery.
2002 The Pharmaceutical Society of Japan