RANKING OF SURFACTANT-HUMECTANT SYSTEMS 609 (34), (b) glycerol increases the rate of corneocyte loss from the upper layers of the SC through a keratolytical effect on desmosome degradation, thereby reducing the scaliness of dry skin and maintaining the SC barrier (33) and (c) glycerol can penetrate into the SC and, due to its high hygroscopic property, is able to bind water and thus reduce water evaporation (30). The mannitol skin permeability values induced by aqueous contacting solutions i-v are reported in Figure 2. As can be seen, aqueous contacting solution i (1 wt% SDS) induces the largest mannitol skin permeability in vitro. Because the 1 wt% aqueous SDS con­ tacting solution i induces the largest perturbation to the skin aqueous pores relative to the other aqueous contacting solutions (ii-v), the mannitol skin permeability values are the largest for SDS. This result is consistent with those of the skin electrical current measurements (see above). In addition, the 1 wt% C12E6 aqueous contacting solution (ii) induces a significantly larger mannitol skin permeability relative to the in vitro control (iii) and also relative to the aqueous humectant solutions (iv and v). This result is consistent with those of the in vivo skin barrier measurements, which show that a 1 wt% C12E6 aqueous contacting solution induces a larger extent of erythema and skin dryness than a 10 wt% PG and a 10 wt% G aqueous contacting solution (see below). It is important to note that aqueous contacting solution v (10 wt% G) induces the smallest extent of perturbation to the skin aqueous pores relative to the other aqueous contacting solutions (i-iv), as reflected in both the in vitro skin electrical current and the mannitol skin permeability measurements (see Figures 1 and 2). This finding is consistent with 0 e OS � e � = ·= = OS 8 7 6 5 4 3 2 1 0 (i) 1 wt% sos (iii) In Vitro Control (PBS) (iv) 10 wt% PG (v) 10 wt% G Figure 2. Mannitol skin permeability 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.

610 JOURNAL OF COSMETIC SCIENCE the in vivo skin barrier measurements, which show that glycerol is indeed the mildest to the skin barrier (see below). 11 The results of the in vitro ranking metric, RM! corresponding to the enhancement in the skin electrical current values relative to the in vitro control (iii), are reported in Table II. The skin electrical current values were converted to skin electrical resistivity values according to the procedure described in references 5 and 6. The mannitol skin perme­ ability values, P! and the skin electrical resistivity values, R! were then analyzed in the context of the theoretical model presented in the Theoretical section to obtain the average pore radius ( r pore and the pore number density (p), which are reported in Ta­ ble II. Recall that equation 1 indicates that if the RM value corresponding to an aqueous contacting solution is less than one, then that aqueous contacting solution induces a lower extent of skin barrier perturbation relative to the in vitro PBS control (iii). Conversely, if the RM value corresponding to an aqueous contacting solution is greater than one, then that aqueous contacting solution induces a larger extent of skin barrier perturbation relative to the in vitro PBS control (iii). An examination of Table II reveals that aqueous humectant-contacting solutions (iv and v) induce a lower extent of skin barrier perturbation because their RM values are both less than one, while aqueous surfactant contacting solutions i and ii induce a larger extent of skin barrier perturbation because their RM values are both greater than one. In addition, Table II reveals the following order of ranking for aqueous contacting solutions i-v (from the harshest to the mildest): (i) (1 wt% SDS) (ii) (1 wt% C 12 E6) (iii) (in vitro PBS control) (iv) (10 wt% PG) (v) (10 wt% G). It is important to note that the ranking of the permeability enhancement, P EIPc (see the third column in Table II), follows the order of ranking obtained using the in vitro ranking metric (RM), which corresponds to the skin electrical current enhancement, ! E lle (see the second column in Table II). This result indicates a strong correlation between skin electrical current and mannitol skin permeability measurements in vitro. 11 A Student t-test with a significance (p 0.05) indicates that each of the bars in Figure 2 is statistically different from the other bars. Table II Ranking Metric (RM) Values, Permeability Enhancements, Average Pore Radii, and Pore Number Density Enhancements Induced by Aqueous Solutions i-v Upon Contacting p-FTS In Vitro in Diffusion Cells Type of aqueous contacting solutions (i) 1 wt% SDS (ii) 1 wt% C 12 £6 (iii) PBS control (iv) 10 wt% PG (v) 10 wt% G Ranking metric RM (!Elle) 7.6 ± 1.0 3.5 ± 1.0 1.0 0.8 ± 0.2 0.6 ± 0.1 Permeability enhancement (pEIPc) 9.8 ± 2.0 4.7 ± 1.0 1.0 0.7 ± 0.2 0.3 ± 0.1 Average pore Pore number density radius, r p ore (A) enhancement (pEIPc) 33 ± 5 2.6 ± 1.0 38 ± 3 0.7 ± 0.5 20 ± 3 1.0 18 ± 2 0.9 ± 0.3 11 ± 4 1.7 ± 0.8 Note that the error bars represent the standard errors based on six p-FTS samples. In addition, note that E indicates enhancer (aqueous contacting solutions i, ii, iv, and v) and C indicates the in vitro PBS control (iii).