114 JOURNAL OF COSMETIC SCIENCE and the receiver compartments were filled with PBS, and the p-FTS sample was left to hydrate for one hour before the beginning of the experiment to allow the skin's initial barrier property to reach steady state. At this point, the skin electrical current across the p-FTS sample was measured (see below), and only p-FTS samples with an initial skin current 3 µA were used in the permeation studies (a well-accepted criterion for se­ lecting suitable in vitro skin samples (7 ,13)). The PBS in the donor compartment was then replaced with either 1.5 ml of an SDS aqueous solution or 1.5 ml of an SDS + 10 wt% glycerol aqueous solution. The solution in the donor compartment, referred to hereafter as the contacting solution, contacted the p-FTS sample for five hours. Note that a five-hour exposure of the skin was chosen because this is a sufficiently long time to allow significant SDS skin penetration, yet a short enough time to prevent the saturation of the skin with SDS. Subsequently, the contacting solution was removed and the donor compartment and the p-FTS sample were rinsed four times with 2 ml of PBS to remove any trace chemical left on the skin surface and in the donor compartment. The receiver compartment was stirred with a magnetic stirrer at a speed of 400 rpm throughout the experiment to eliminate permeant bulk concentration gradients. Following the SDS aqueous solution and the SDS + 10 wt% glycerol aqueous contacting solution treatments of the skin, the p-FTS samples in the diffusion cells were exposed to a contacting solution of 3 H-radiolabeled mannitol in PBS (1-10 µCi/ml) for 24 hours. Mannitol is: (i) a low-molecular-weight monosaccharide (MW = 182 Da) (6,7) and (ii) a highly hydrophilic (log K O /w = -3.1) chemical (7), which is not metabolized by the body, and hence, if desired, can also be used for in vivo skin permeation studies (6,7). Being small in size and highly hydrophilic, mannitol can access similar aqueous pores as do ions in order to transport across the skin barrier. This, in turn, makes mannitol a suitable permeant to study in the context of the hindered-transport porous pathway model of the SC (6-9). Pretreatment of p-FTS with (a) SDS or (b) SDS + 10 wt% glycerol aqueous contacting solutions in this manner, followed by passive mannitol-skin permeation, allowed for a controlled comparison of the skin barrier perturbation poten­ tial of solutions (a) and (b) at fixed exposure times of five hours. Throughout these experiments, solution samples were withdrawn from both the receiver (r) and the donor (d) compartments every two hours, and the concentrations of the radiolabeled permeant (mannitol) in the two compartments (Cr and Cd, respectively) were measured using a liquid scintillation counter (Packard, Sheldon, CT). When the transport of mannitol attained steady state, the mannitol skin permeability, P, was calculated as follows (6,7): (1) where V r is the volume of the receiver compartment, A = (1. 77 cm 2 ) is the area of the SC exposed to the mannitol solution in the donor compartment, and t is the exposure time. Equation 1 is based on the following two assumptions: (i) the concentration of the permeant in the donor compartment is high, and does not deplete with time, and (ii) the concentration of the permeant in the donor compartment is always much higher than that in the receiver compartment. In the experiments reported here, assumptions (i) and (ii) were both satisfied because less than 2% of mannitol in the contacting solution permeated across the p-FTS samples.

SDS MICELLES IN SKIN BARRIER PERTURBATION 115 IN VITRO SKIN ELECTRICAL CURRENT AND SKIN ELECTRICAL RESISTIVITY MEASUREMENTS During each skin permeation experiment, two Ag/ AgCl electrodes (E242, In Vivo Metrics, Healdsburg, CA) were placed in the donor and in the receiver compartments to measure the electrical current and the electrical resistivity across the p-FTS sample (see Figure 1). A 100 m V AC voltage (RMS) at 10 Hz was generated by a signal generator (Hewlett-Packard, Atlanta, GA) and was applied across the two electrodes for 5 s. The electrical current across the skin was measured using an ammeter (Hewlett-Packard, Atlanta, GA). This ammeter was used to measure low AC currents and was accurate in the 0.1 µA range. The electrical resistance of the p-FTS sample was then calculated from Ohm's law (7). Because the measured skin electrical resistance is the sum of the actual skin electrical resistance anl the background PBS electrical resistance, the latter was subtracted from the measured skin electrical resistance to obtain the actual skin electrical resistance. The skin electrical resistivity was then obtained by multiplying the actual skin electrical resistance by the skin area (A = 1.77 cm2). The skin electrical resistivity, being an intrinsic electrical property of the skin membrane, is a preferred measure in this analysis over the skin electrical resistance, which is an extensive electrical property of the skin membrane (3 3 ). Therefore, by using the skin electrical resistivity, it will be easier to compare differences in the electrical properties of the skin barrier upon exposure of the skin to the SDS and to the SDS + 10 wt% glycerol aqueous contacting solutions. Skin electrical current and resistivity measurements were carried out before and during the permeation experiments at each predetermined sampling point. For each p-FTS sample, an average skin electrical resistivity was determined over the same time period for which the steady-state skin permeability, P! was calculated using equation 1. This average skin electrical resistivity, R! was then analyzed along with the corresponding skin perme­ ability, P1 in the context of the theoretical framework presented below in the Theoretical section. IN VITRO SKIN RADIOACTIVITY MEASUREMENTS The p-FTS samples were mounted in vertical Franz diffusion cells, as was done in the case of the skin transdermal permeability measurements described above. Following a similar protocol, p-FTS samples were now exposed to aqueous contacting solutions containing 1.5 ml of SDS or 1.5 ml of SDS + 10 wt% glycerol. Each of these contacting solutions also contained about 1 µCi/ml of 14 C-SDS. Diffusion of SDS into the skin took place for five hours, as before, and subsequently, the aqueous contacting solutions were removed and the donor compartment and the p-FTS sample were rinsed four times with 2 ml of PBS to remove any trace chemical left on the skin surface and in the donor compartment. The p-FTS samples were then heat-stripped following a well-known procedure (11). Briefly, a p-FTS sample was placed in a water bath at 60°C for two minutes, and subsequently, the epidermis (the SC and the viable epidermis) that was exposed to the contacting solution was peeled off from the dermis. The exposed epidermis was then dried for two days in a fume hood and weighed. The dried epidermis was dissolved overnight in 1.5 ml of Soluene-350 (Packard, Meriden, CT). After the epidermis dissolved, 10 ml of Hionic Fluor scintillation cocktail (Pack­ ard) was added to the Soluene-350, and the concentration of radiolabeled SDS was determined using the Packard scintillation counter. Note that we did verify that the