RANKING OF SURFACTANT-HUMECTANT SYSTEMS 607 here, which corresponds to the enhancement in the skin electrical current, can also be expressed as an enhancement in the mannitol skin permeability, within the context of the hindered-transport aqueous porous pathway model (5-10,41). Specifically, (p H(AJ ) ( � H(A )) IE H(A p E 'T t E p RM - - ----) - - ----- - ------EJ)A(Heor2r(p le -( p H(AJ ) - ( � H(AJ) - (prJ0reH(AJ)c H(A J ,) C T C where we have used equation 3 and the fact that (D /L) E = cv IL) C' 6 (5) It is instructive to consider the following characteristics of the in vitro ranking metric, RM: (a) RM is numerically equal to unity for the in vitro PBS control (iii), and (b) increasing values of RM indicate an increase in the extent of perturbation to the skin aqueous pores, and hence, an increase in the extent of skin barrier perturbation. In addition, it is also worth noting the following implications regarding the ranking metric when analyzed in the context of the hindered-transport aqueous porous pathway model: 7 (a) it scales linearly with the skin permeability (P) of a hydrophilic permeant,8 (b) it is a linear function of p and a nonlinear function of r p oreJ and (c) because it depends on both p and r p oreJ it can shed light on the mechanism of skin barrier perturbation specifically, one can determine if an aqueous contacting solution induces a high ranking metric value by increasing r p or e J p, or both. The results of the in vitro ranking metric study are compared with the in vivo skin barrier measurements in the Results and Discussion section. RESULTS AND DISCUSSION RESULTS OF THE IN VITRO SKIN BARRIER MEASUREMENTS IN THE CONTEXT OF A RANKING METRIC The average skin electrical currents induced by aqueous contacting solutions i-v are reported in Figure 1. It is important to note that the measurement of skin electrical currents in vivo is different from the measurement of skin electrical currents in vitro. The in vivo skin electrical current (or conductance) measurement is carried out on dry skin, with a low value indicating less hydrated skin that displays a greater extent of skin barrier perturbation (20). On the other hand, the in vitro skin electrical current mea­ surement is performed on skin in contact with a PBS solution (5-9,41). Consequently, a high skin electrical current in vitro indicates that the skin barrier has been compro­ mised because ions can traverse the skin barrier more freely (5-9,41). 6 Because v 1L does not depend on the nature of the aqueous contacting solution, and depends solely on the choice of the permeant (mannitol in the present case) and the skin model used (p-FTS in the present case), it follows that (D /L) E = (D /L) c 7 It is important to note that RM, which is defined as the enhancement in the skin electrical current induced by E relative to C (see equation 1), is independent of the hindered-transport aqueous porous pathway model. However, the hindered-transport aqueous porous pathway model can be used to further analyze the RM to determine r pore and p. 8 Because the skin electrical resistivity (R) scales linearly with P (5-9,41), and P scales linearly with the ranking metric, the ranking metric also scales linearly with R.

608 120 100 i so .... = 60 ·c:y.... y = 40 20 0 (I) 1 wt% sos JOURNAL OF COSMETIC SCIENCE (iii) In Vitro Control (PBS) (iv) 10 wt% PG (v) 10 wt% G Figure 1. Skin electrical currents induced by aqueous solutions i-v upon contacting p-FTS in vitro in diffusion cells. The error bars represent standard errors based on six p-FTS samples. The skin electrical currents induced by aqueous surfactant solutions i and ii ( 1 wt% SDS and 1 wt% C 12 E6 , respectively) are significantly higher than those induced by the in vitro) PBS control solution (iii) and by the aqueous humectant solutions iv and v (10 wt% PG and 10 wt% G, respectively). Clearly, and perhaps as expected, these results indicate that surfactants induce the greatest extent of perturbation to the skin aqueous pores through which ions can cross the skin barrier, thereby resulting in the largest observed skin electrical current values. 9 In addition, aqueous contacting solution v (10 wt% G) induces a lower skin electrical current than aqueous contacting solution iv (10 wt% PG). 10 This is consistent with the observation that G is able to diffuse into the SC, increasing skin hydration and relieving clinical signs of erythema and skin dryness more readily than PG and the in vitro PBS control (iii) (30-35). More specifically, the following effects of glycerol on the skin barrier have been reported in the literature: (a) Glycerol affects the crystalline arrangement of the intercellular lipid bilayers, thereby enhancing SC barrier function and decreasing SC water permeability 9 Note that 1 wt% SDS induces a larger skin electrical current in vitro, and consequently, a greater extent of perturbation to the skin aqueous pores in vitro than that induced by 1 wt% C 12 E 6 in vitro. This finding is consistent with SDS inducing a greater extent of erythema than C 12 E6, although C 12 E 6 does induce skin dryness (21-24). 10 A Student t-test with a significance (p 0.05) indicates that each of the bars in Figure 1 is statistically different from the other bars.