Infrared kinetic/structural studies of barrier reformation in intact stratum corneum following thermal perturbation

Stratum corneum, the outermost layer of the epidermis, constitutes the main barrier to permeability in skin. As such, it has been the target of many approaches for transdermal drug delivery based on methods involving transient modifications of the barrier. An infrared (IR) spectroscopic method has been developed to monitor the kinetics of barrier restoration following an external perturbation. In the current case, temperature perturbation was selected as a convenient means to induce structural changes in the barrier. The method is based on the observation that the ordered lipid phases of the barrier in isolated human stratum corneum exist in part in orthorhombically packed subcells. Such phases display a characteristic splitting of the CH₂ rocking vibrations with component frequencies at 720 and 729 cm^-1. The latter is reliably diagnostic for orthorhombic phases and is markedly reduced in intensity following a thermal perturbation to 55 °C. The kinetics of barrier recovery following quenching to either 25 °C or 30 °C were monitored by tracking the restoration of the 729 cm^-1 band intensity. The kinetics were dominated by exponential growth in the initial stages, followed by linear increases at longer times. The half lives for exponential growth regimes were 52.4 h for the 25 °C quench and 13.8 h for the 30 °C quench. These values are in reasonable accord with those determined with more phenomenological approaches, typically based on restoration of some barrier function. This novel method for monitoring structural reorganization kinetics in intact stratum corneum can readily be extended to evaluate barrier recovery following a variety of treatments used to enhance drug delivery.