384 JOURNAL OF COSMETIC SCIENCE
consuming. Alternate methods that bring systems closer to real SC are ideally suited for
surfactant and product evaluations. A corneosurfametry technique originally developed by
Pierard et al.37 and further modified to make it a high-throughput methodology is noteworthy
in this regard.38 These studies, carried out with tape stripped samples of human stratum
corneum, involve treating the SC sample with the product/surfactant followed by exposing
it to a dye solution. The degree of staining of the dye is an indicator of the potential for
irritation. Figure 4 shows rank ordering of various commonly used surfactants in cleansing
products by this method. Corneosurfametry also can be done on tape-stripped samples after
a clinical study to assess the degree of damage in deeper layers.38 Such assessments can
provide information on the quality of the corneum in the deeper layers, which could be
masked otherwise because of deposition of occlusive ingredients such as petrolatum.
It is important to note that surfactant-induced swelling itself is not the mechanism of skin
irritation. Swelling will increase the permeability of ingredients in a product, including
surfactants, into deeper layers of the skin. The surfactant itself can be an irritant on its own.
In a fully formulated product with surfactants, it is also possible that the surfactant-induced
enhancement in skin penetration can lead to other irritants reaching deeper layers, causing
a biochemical reaction resulting in inflammation and irritation. Release of inflammatory
markers such as IL1α and IL1-Ra in the deeper layers are indicative of the potential of
ingredients/products to cause skin irritation.
Repeat washing and drying cycles with high swelling surfactants can contribute to
SC barrier damage by another mechanism. During the wash cycle with high swelling
surfactants, the SC is in a hyper-hydrated state.39 During rinse, water soluble NMFs can
be removed routinely. The higher the swelling, the higher is the extent of NMF removal.
Figure 3. 5A: (left) The D2O depth profile in the SC of forearm skin after the treatment with six different
surfactant solutions and D2O measured using confocal Raman spectroscopy. Figure reproduced with permission
from Endo et al. J of Surf. &detergents, 2018, 21, 777-788 (36). EC – Polyoxyethylene 5 EO lauryl ether
carboxylate, ES – Sodium mono-oxyethylene alkyl ether sulfate, SDS-Sodium dodecyl sulfate, LK – Potassium
laurate (pH 10), AGS – monosodium salt of alkyl glutamic acid, EC/ES is a 1:1 wt %mixture of alkyl ether
carboxylate and alkyl ether sulfate surfactants and D2O is deuterated water as the control. All other pH values
adjusted to 6.2. Surfactant concentration 2 wt %.The plateau level shows the maximum level of hydration,
and the length of the plateau shows how far deep into the SC the high level exists. 5B (right) shows correlation
between protein denaturation vs plateau thickness of skin swelling upon exposure to surfactants.

385 The Human Stratum Corneum
As one steps out of the shower, the excess water must evaporate from skin as the corneum
comes to equilibrium with the surrounding environment. The steeper the water gradient,
the rate of evaporation is also expected to be higher, and this often results in a drying stress
that is felt by consumers as after wash tightness. This is typically evident in winter months
when one washes one side of the face with a harsh cleanser and the other with a milder
cleanser. The shrinkage of corneocytes upon dehydration can lead to possible debonding of
the corneocytes from the lipid matrix, leading to the beginning of flake formation. Repeat
cycles with such harsh products can in the long-term result in skin dryness. Such uplifting
of the cells can weaken the barrier further and allow increased penetration of irritants into
the deeper layers causing erythema and inflammation. Note that this process is a slower
process of weakening the barrier compared to the rapid penetration of irritants that could
occur during the swollen state. It is reasonable to speculate that the rapid penetration in a
swollen state is more likely to be through hydrophilic protein channels, whereas the slow
damage from repeat wash cycles is from progressive damage to the lipid matrix.
Skin drying stress after exposure to surfactants can be measured using in vitro or ex vivo
skin samples. Purohit et al.40 and German et al.41–42 have measured surfactant-induced
drying stress in ex vivo skin samples and have concluded that surfactant exposure increases
the drying stress. Importantly, chloroform-methanol treatment, which mainly delipidates
the SC, increases the drying stress to the maximum. This shows that both protein and
lipid damage are important in assessing the impact of cleansers and other chemicals on
SC. With anionic surfactants, since protein denaturation seems to correlate with their
irritation tendency, it may be the protein interactions that dominate overall behavior. In
Figure 4. CIM (Corneosurfametry index of mildness) values for individual surfactants. Bars having the
same letter are not significantly different from each other. CAPB – Cocoamido propyl betaine, APG – alkyl
polyglucoside, SLES 1 EO – Sodium lauryl ether sulfate with average 1 EO ethoxylation and SLES 3 EO with
average 3 EO ethoxylation, SDS – sodium dodecyl sulfate. Results showed CAPB was the mildest and SDS
was the harshest. Also showed SLES 3 EO to be milder than SLES 1EO and SLES 1 EO and 3 EO blends with
Betaine were milder than the corresponding SLESs. Tape strips from 12 subjects and for each tape strip there
were three measures of CIM values. Figure reproduced with permission from Liu M et al.38