Interactions of Cosmetic Ingredients with Human Stratum Corneum

390 JOURNAL OF COSMETIC SCIENCE
The mechanisms by which the above type of moisturizing ingredients provide benefit from
wash-off systems vary significantly depending upon the nature of the ingredients. Since these
mechanisms are likely to be similar to their function from leave-on systems, they will be
discussed in later sections. It should be noted that there are differences between leave-on
and wash-off systems. The presence of high levels of surfactants and skin hydration under
cleansing conditions often helps to deliver certain actives rapidly into deeper layers. However,
the time of contact during cleansing is limited, and therefore the overall benefits from wash-
off systems will always be much lower than what can be expected from leave-on systems.
OTHER INGREDIENTS IN CLEANSERS
In addition to surfactants, other ingredients present in cleansers include polymers,
preservatives, pH modifiers, skin benefit agents, and fragrances.
POLYMERS
Polymers are generally used for structuring formulations, conditioning skin and hair, or for
deposition emollient and occlusive materials onto skin and hair. Polymers, because of their
high molecular weight, do not penetrate through the human SC and do not pose any major
threat to the skin barrier. Role of polymers in cosmetics has been reviewed by Lochhead,57
and a detailed review of polymers is beyond the scope of this paper. Cationic polymers
help to enhance the deposition of droplets and dispersed materials on skin and hair, as
both these surfaces are negatively charged under cleansing conditions. Polymers adsorbed
on skin and hair by themselves also can modify their wet and dry sensory and lubrication
properties. Silicone polymers, especially crosslinked silicone elastomers, are well known for
their consumer desired silky feel on skin and hair.58
Minimize Damage
Compensate for
surfactant damage
Provide posig415ve
benefits
Reduce
protein
damage
Reduce
loss of
NMFs
Reduce
lipid
damage
Mild surfactants
(Lower micellar
charge density-
large head-groups,
low acg415ve levels)
e.g. Isethionates,
Glycinates
SLES-Betaine
Combos (SLS-free),
Other aminoacid based
surfactants
Pre-saturate
surfactant
micelles with
sacrificial lipids
e.g. fag425y acids &
sterols in Bars &
Liquids
Replenish
NMFs/
humectants
Replenish
lipids
Provide
masking/
occlusion
for barrier
repair
Provide
“nutrients”/
Pro-lipids
for maintaining/
rebuilding
healthy corneum
Deposit /
deliver
Glycerol, PCA
&amino acids
during
cleansing
Deposit/
deliver
fag425y acids
&sterols
during
cleansing
Deposit
PJ/triglyceride
oils during
cleansing
Deliver
acg415ves that can
penetrate into
deeper layers &
enhance healthy
barrier rebuilding,
e.g. Esseng415al
fag425y acids,
Strategies for Mild and Moisturizing Cleanser Technologies
Maintain skin’s
Natural pH
Neutral pH to
mildly acidic pH
cleansers that
have no impact
on steady state
pH of SC
Figure 8. Technology routes to mild cleansing and moisturizing cleanser technologies.

391 The Human Stratum Corneum
Polymers, because of their interaction with surfactants, can also modulate the surfactant
monomer activity and, in the process, can enhance the mildness of surfactant systems.59
Hydrophobically modified polymers have been shown to be even more effective in
modulating the monomer activity than the unmodified ones.60 Another area of application
of hydrophobically modified polymers is in emulsification.61–64 Their superior emulsification
and stabilization properties have made them the preferred materials for formulators.
MOISTURIZING AND SKIN BENEFIT INGREDIENTS
Leave-on skincare products are generally multiphase systems containing emulsions or
dispersions for such varied functional benefits as moisturization, sun protection, and other
advanced care benefits as antiaging, acne or dandruff treatment.65–68 Typical moisturization
ingredients include occlusives, humectants, and emollients.66–67,70 Commonly used
advanced skin benefit ingredients include AHAs (alpha hydroxy acids), BHAs (beta hydroxy
acids), retinol family of actives (vitamin A family), Niacinamide (Vitamin B group) and
antioxidants like Vitamin C (e.g. ascorbic acid) and Vitamin E.68 In addition to the active
ingredients, they may also contain polymers, emulsifiers, penetration enhancers, fragrances,
and preservatives. Volatile silicones and silicone elastomers are also often found in skincare
products specifically for their ability to spread on the skin surface and provide consumer
desired skin feel.69,70 As discussed earlier, polymers themselves are not of any safety concern
from a skin barrier point of view. In the sections below the interaction of other ingredients
with SC is reviewed briefly.
EMULSIFIERS
Traditional emulsifiers tend to be nonionic surfactants with ethylene oxide groups (EO)
or hydroxyl (OH) groups as their hydrophilic moieties and hydrocarbon chain as the
hydrophobic part.71 As mentioned earlier, polymeric emulsifiers which are essentially
hydrophobe-modified polymers have become popular lately because of their ease of use and
their ability to provide enhanced emulsion stability.61–64
The traditional nonionic surfactant-based emulsifiers are generally thought to be
nonirritating to skin. They do not induce any significant swelling of the human SC by
strong interaction with skin proteins but may impact the lipid layers. They are known to
alter the permeability of the SC by interacting with the bilayer lipid layers. In leave-on
moisturizers and advanced skincare formulations, this property is often thought to be
beneficial in helping the main active penetrate deeper layers of skin. However, this can
result in skin irritation and inflammation because of enhanced penetration of actives as
well as other potentially irritating ingredients such as fragrance molecules in a formulation.
It is therefore important that the irritancy potential of fully formulated systems be tested
rather than that of a single ingredient.
Barany et al. investigated the impact of a range of emulsifiers while keeping the
moisturization package same.72 Their in-vivo results on normal skin showed that for the same
moisturization package, certain emulsifiers (PEG-2 Stearate, PEG-9 Stearate, Steareth-10,
and Steareth-21) increased the TEWL, while some others decreased it. The enhancement
in TEWL was not accompanied by any erythema or increased blood flow, which suggests
that these emulsifiers were perturbing the lipid layer but that they themselves or other

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390 JOURNAL OF COSMETIC SCIENCE
The mechanisms by which the above type of moisturizing ingredients provide benefit from
wash-off systems vary significantly depending upon the nature of the ingredients. Since these
mechanisms are likely to be similar to their function from leave-on systems, they will be
discussed in later sections. It should be noted that there are differences between leave-on
and wash-off systems. The presence of high levels of surfactants and skin hydration under
cleansing conditions often helps to deliver certain actives rapidly into deeper layers. However,
the time of contact during cleansing is limited, and therefore the overall benefits from wash-
off systems will always be much lower than what can be expected from leave-on systems.
OTHER INGREDIENTS IN CLEANSERS
In addition to surfactants, other ingredients present in cleansers include polymers,
preservatives, pH modifiers, skin benefit agents, and fragrances.
POLYMERS
Polymers are generally used for structuring formulations, conditioning skin and hair, or for
deposition emollient and occlusive materials onto skin and hair. Polymers, because of their
high molecular weight, do not penetrate through the human SC and do not pose any major
threat to the skin barrier. Role of polymers in cosmetics has been reviewed by Lochhead,57
and a detailed review of polymers is beyond the scope of this paper. Cationic polymers
help to enhance the deposition of droplets and dispersed materials on skin and hair, as
both these surfaces are negatively charged under cleansing conditions. Polymers adsorbed
on skin and hair by themselves also can modify their wet and dry sensory and lubrication
properties. Silicone polymers, especially crosslinked silicone elastomers, are well known for
their consumer desired silky feel on skin and hair.58
Minimize Damage
Compensate for
surfactant damage
Provide posig415ve
benefits
Reduce
protein
damage
Reduce
loss of
NMFs
Reduce
lipid
damage
Mild surfactants
(Lower micellar
charge density-
large head-groups,
low acg415ve levels)
e.g. Isethionates,
Glycinates
SLES-Betaine
Combos (SLS-free),
Other aminoacid based
surfactants
Pre-saturate
surfactant
micelles with
sacrificial lipids
e.g. fag425y acids &
sterols in Bars &
Liquids
Replenish
NMFs/
humectants
Replenish
lipids
Provide
masking/
occlusion
for barrier
repair
Provide
“nutrients”/
Pro-lipids
for maintaining/
rebuilding
healthy corneum
Deposit /
deliver
Glycerol, PCA
&amino acids
during
cleansing
Deposit/
deliver
fag425y acids
&sterols
during
cleansing
Deposit
PJ/triglyceride
oils during
cleansing
Deliver
acg415ves that can
penetrate into
deeper layers &
enhance healthy
barrier rebuilding,
e.g. Esseng415al
fag425y acids,
Strategies for Mild and Moisturizing Cleanser Technologies
Maintain skin’s
Natural pH
Neutral pH to
mildly acidic pH
cleansers that
have no impact
on steady state
pH of SC
Figure 8. Technology routes to mild cleansing and moisturizing cleanser technologies.

391 The Human Stratum Corneum
Polymers, because of their interaction with surfactants, can also modulate the surfactant
monomer activity and, in the process, can enhance the mildness of surfactant systems.59
Hydrophobically modified polymers have been shown to be even more effective in
modulating the monomer activity than the unmodified ones.60 Another area of application
of hydrophobically modified polymers is in emulsification.61–64 Their superior emulsification
and stabilization properties have made them the preferred materials for formulators.
MOISTURIZING AND SKIN BENEFIT INGREDIENTS
Leave-on skincare products are generally multiphase systems containing emulsions or
dispersions for such varied functional benefits as moisturization, sun protection, and other
advanced care benefits as antiaging, acne or dandruff treatment.65–68 Typical moisturization
ingredients include occlusives, humectants, and emollients.66–67,70 Commonly used
advanced skin benefit ingredients include AHAs (alpha hydroxy acids), BHAs (beta hydroxy
acids), retinol family of actives (vitamin A family), Niacinamide (Vitamin B group) and
antioxidants like Vitamin C (e.g. ascorbic acid) and Vitamin E.68 In addition to the active
ingredients, they may also contain polymers, emulsifiers, penetration enhancers, fragrances,
and preservatives. Volatile silicones and silicone elastomers are also often found in skincare
products specifically for their ability to spread on the skin surface and provide consumer
desired skin feel.69,70 As discussed earlier, polymers themselves are not of any safety concern
from a skin barrier point of view. In the sections below the interaction of other ingredients
with SC is reviewed briefly.
EMULSIFIERS
Traditional emulsifiers tend to be nonionic surfactants with ethylene oxide groups (EO)
or hydroxyl (OH) groups as their hydrophilic moieties and hydrocarbon chain as the
hydrophobic part.71 As mentioned earlier, polymeric emulsifiers which are essentially
hydrophobe-modified polymers have become popular lately because of their ease of use and
their ability to provide enhanced emulsion stability.61–64
The traditional nonionic surfactant-based emulsifiers are generally thought to be
nonirritating to skin. They do not induce any significant swelling of the human SC by
strong interaction with skin proteins but may impact the lipid layers. They are known to
alter the permeability of the SC by interacting with the bilayer lipid layers. In leave-on
moisturizers and advanced skincare formulations, this property is often thought to be
beneficial in helping the main active penetrate deeper layers of skin. However, this can
result in skin irritation and inflammation because of enhanced penetration of actives as
well as other potentially irritating ingredients such as fragrance molecules in a formulation.
It is therefore important that the irritancy potential of fully formulated systems be tested
rather than that of a single ingredient.
Barany et al. investigated the impact of a range of emulsifiers while keeping the
moisturization package same.72 Their in-vivo results on normal skin showed that for the same
moisturization package, certain emulsifiers (PEG-2 Stearate, PEG-9 Stearate, Steareth-10,
and Steareth-21) increased the TEWL, while some others decreased it. The enhancement
in TEWL was not accompanied by any erythema or increased blood flow, which suggests
that these emulsifiers were perturbing the lipid layer but that they themselves or other

Help

388 JOURNAL OF COSMETIC SCIENCE
charge density measured from dynamic light scattering measurements.46 Morris et al. also
showed that the micelle size did not correlate with the penetration into the skin, suggesting
that size is not the main factor, but rather that charge density is the primary factor driving
the penetration of charged surfactants into skin. Figure 7 shows the correlation of surfactant
micelle charge density with skin penetration and protein denaturation.
One last point to be addressed regarding penetration of anionic surfactants into skin is
whether micelles penetrate through intact SC. The observation that micelle charge density
correlates with penetration of surfactants into skin does not imply that micelles penetrate
through healthy SC. In fact, Morris et al. have shown that the penetration remains essentially
plateaued above CMC, indicating that micelles do not penetrate through SC.47 If the skin
is exposed to surfactant solutions for a longer period such as several hours, it can damage
the SC, possibly resulting in penetration of the micelles.47 The reason micelle charge density
correlates with penetration is because the surfactant aggregates forming on the protein
backbone are similar to that of micelles, and have similar charge densities as the micelles.
The focus of discussion so far has centered around surfactant interactions with proteins
which correlate with the skin irritation tendencies of surfactants. This may be because the
rapid penetration of surfactants may be through hydrophilic pathways linked to protein
channels rather than the slower lipid pathway. Progressive alterations of the lipid matrix by
harsh surfactants also can contribute to SC damage, which may impact the desquamation
process, resulting in the formation of dry and flaky skin. In this context, the interaction of
surfactants with the lipid matrix is examined below.
SURFACTANT INTERACTIONS WITH SC LIPIDS
Surfactants are designed to remove oily lipids, specifically sebaceous lipids, during cleansing.
However, the cleansing system does not know the difference between sebaceous lipids and
the integral bilayer lipids that imparts SC its barrier properties. Surfactants can damage
Figure 7. Surfactant penetration into human cadaver skin in vitro from a variety of anionic-based surfactant
solutions versus zeta potential of the surfactant solutions (graph on the left) (b) Zein solubility in surfactant
solutions versus (absolute) zeta potential of the surfactant solutions (graph on the right). Figure reproduced
from Morris et al.47 Micelle zeta potential correlates with both surfactant penetration into skin and protein
denaturation.

389 The Human Stratum Corneum
the lipids mainly in two ways: by intercalating into the bilayer lipids and introducing a
charge repulsion in the bilayer, and by solubilizing some of the more surfactant-soluble lipid
components such as cholesterol and fatty acids. Both these effects will increase the permeability
of the bilayer lipids, allowing surfactants and other foreign matter to penetrate deeper layers.
Repeat washing with such products can lead to progressive damage to the bilayer lipids
in deeper layers. These alterations will eventually increase the TEWL. Dehydration of the
surface layers also can affect the desquamation process, resulting in dry flaky skin.
Several in vitro and ex vivo tests have been developed to determine the lipid damage
potential of surfactants. These include simple solubilization of SC lipid components by
cleanser surfactants, disruption of liposomes made up of model lipids or SC identical lipids,
extraction of lipids from a model lipid film on a substrate such as glass, and lipid dissolution
from isolated SC or from in vivo wash measurements.28,32,33,48,49 Spectroscopic studies have
also been conducted to determine the changes in SC lipid organization upon exposure to
surfactants and cleansing products.50–52 All studies suggest that damage to SC lipids and
model lipid systems occurs upon exposure to harsh surfactants. Details of the molecular
mechanisms involved are not fully established so far. Based on the model system studies
of de la Maza et al., it is reasonable to hypothesize that the first stage of lipid damage is
intercalation of surfactants into the bilayer, which leads to fluidization of the lipid bilayer
followed by extraction of the more soluble components into the surfactant micelles.48
Frobe et al., by soaking an isolated piece of SC with SDS, showed that fatty acids and
cholesterol are extractable by surfactants and that ceramides are not.49 This is reasonable since
ceramides are two tailed highly hydrophobic molecules and are difficult to be solubilized by
conventional micelles. Imokawa and coworkers, on the other hand, showed in their in vivo
experiments that all lipids get removed during a surfactant wash.32 Note that with in vivo
experiments there will be some exfoliation of cells from the surface, and therefore removal
of all lipids associated with the corneocytes can be expected in this respect the Imokawa
experiments differed from that of Frobe’s. Importantly, Imokawa et al. found that fatty acids
are extracted at a higher rate than other lipids which were in similar ratios as expected in
the bilayer lipids.32 These results suggest that fatty acids are the more extractable lipids
among the various SC lipids, and therefore preventing extraction of FAs by presaturating
the micelles with FA could be a strategy to prevent lipid damage to SC. This has been
demonstrated in controlled arm wash studies with an anionic surfactant base and increasing
levels of FAs of chain length C16–C18, which showed that the addition of FAs reduced both
the TEWL and skin dryness significantly.53 However, it is important to ensure that such
presaturation does not have any impact on the cleansing or sensory properties of the system.
MOISTURIZING CLEANSERS
Cleansing systems these days have gone beyond simple, mild cleansing to providing
moisturization benefits by depositing and delivering benefit ingredients.53–55 Moisturizing
actives typically found in cleansing systems include lipids such as fatty acids53 and ceramides,54
occlusives such as petrolatum,55 triglyceride oils such as vegetable oils,53 and humectants such
as glycerol.53 In general, deposition efficiencies on skin from rinse-off systems are poor—
often less than 5%. Clinical studies have shown that even with such low levels of deposition,
consumer perceivable and clinically relevant results can be delivered from wash-off systems.
Recent advances in moisturizing cleansers have been reviewed recently.54,56 Typical strategies
that can be used to create moisturizing cleansers are given in Figure 8.

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392 JOURNAL OF COSMETIC SCIENCE
components in their model formulations were not causing any irritation. Interestingly, the
emulsifiers that increased TEWL in normal skin in fact reduced TEWL in SLS treated
irritated skin. Note that in SLS irritated skin, the baseline TEWL was significantly higher
than that in normal skin possibly because of both lipid and protein damage, and the
emulsifier that perturbed skin lipid structure in normal skin resulted in an improvement
in this case. It is not unreasonable to expect added lipids to strengthen an already weakened
barrier or fluidize a healthy barrier both by incorporating into the bilayer.
Unlike that of anionic and cationic surfactants, the inherent skin irritation potential of
nonionic surfactants has not been investigated in detail. Recently, Lemery et al.73 investigated
the skin toxicity potential of 4 ionic and 14 nonionic surfactants using four in vitro assays.
These include MTT assay, cell viability, and release of biomarkers such as IL1 α (primary
inflammation marker) and IL-8 (delayed inflammation marker) using Reconstructed Human
Epidermis (RHE) tissues. The tissue was exposed to 14 surfactants as emulsifiers at 3% level
along with caprylic/capric triglycerides as the oil phase. Their results for the two irritation
markers are reproduced in Table II. The authors chose a threshold value of three, with less
than three as relatively mild and the higher ones indicating increasing levels of potential
for irritation. These results show that the soluble ionic surfactants (SLS and cetyl trimethyl
ammonium bromide), irrespective of their nature of the charge (i.e. cationic or anionic),
caused more damage than the relatively insoluble longer chain ionic surfactants (sodium
stearoyl lactylate and distearyl diammonium chloride). The low value of IL8 release from
SLS was attributed to cell death occurring rapidly preventing any further release of delayed
markers. The differences observed between the soluble and the insoluble ionic surfactants
are not surprising as insoluble surfactants will not have enough monomers or micelles in the
aqueous phase to bind to proteins or intercalate with lipids from aqueous systems.
The above results for nonionic surfactants showed that several nonionic surfactants caused
significant IL1-α release and IL8 release. A detailed examination of the results showed that
Table II
IL-1a, IL-8, and IL-8/IL-1a Ratios for Different Surfactants
Surfactant IL-1α IL-8 Ratio IL-8/IL-1α
PEG-20 stearate 0.985 0.787 0.799
PEG-100 stearate 0.735 0.985 1.341
PEG-25 hydrogenated castor oil 6.860 5.512 0.804
Laureth-23 3.105 0.484 0.156
Ceteth-10 90.400 4.250 0.047
Steareth-100 6.363 5.453 0.857
Oleth-20 34.359 3.111 0.091
Beheneth-25 24.592 11.502 0.468
Polyoxyethylene sorbitan laurate 0.801 0.441 0.55
Polyoxyethylene sorbitan stearate 0.865 0.612 0.707
Polyoxyethylene sorbitan oleate 0.784 0.866 1.105
Sucrose laurate 7.113 9.109 1.281
Sucrose stearate 1.404 0.791 0.563
Sucrose oleate 1.073 0.948 0.883
Sodium lauryl sulfate 22.121 0.810 0.037
Sodium stearoyl lactylate 0.582 3.407 5.85
Distearyldimonium chloride 1.242 0.648 0.522
Cetyl trimethylammonium chloride 6.326 0.172 0.027
*IL-1a and IL-8 above the threshold of 3 given from the frequency distribution are given in
bold characters. Table reproduced from Lemery et al.73

393 The Human Stratum Corneum
PEG ethers showed significant release of inflammatory markers compared to PEG esters.
Two exceptions among their tested esters were a PEG ester (PEG-25 hydrogenated castor
oil), and a sucrose ester (sucrose laurate). The large number of nonionic surfactants allowed
the authors to examine their results for structure-function relationships. Most parameters
such as CMC and HLB did not show any correlation with their impact on cytotoxicity.
However, packing parameter or phase behavior seem to show an interesting correlation with
the higher packing parameter, or the more crystalline or lamellar structure the structure is,
the lower its tendency is to release inflammatory markers. This observation is also consistent
with the previously reported correlation between micelle charge density and skin irritation
potential for ionic surfactants as higher charge density results in lower packing parameters.34
The results presented by Lemery et al. clearly show that certain nonionic surfactants can be
harsher than others.73 Further research to understand the molecular mechanism governing
the effect of nonionic surfactants on skin penetration and their potential for skin irritation
is warranted. With increasing interest in enhancing skin penetration for advanced skin care
benefits, the importance of the delicate balance among enhancing penetration, stabilizing
emulsions, and preventing skin irritation may become important in the coming years.
EFFECT OF PENETRATION ENHANCERS ON SC STRUCTURE
Delivery of drugs and cosmetic actives through skin have been an active area of research
for decades, and it has been reviewed extensively.74–78 It is generally believed that a
molecular weight of 500 Daltons is a cut off for penetration of molecules into skin.78 The
extent of penetration of molecules with molecular weights is very much dependent upon
factors such as charge, polarity, and hydrophobicity. In this context, an approach often
employed to enhance delivery of actives is to use chemical penetration enhancers. Typical
penetration enhancers include surfactants, solvents, terpenes, azone and osmolytes.79
Among the surfactants-class, ionic, amphoteric, and nonionic surfactants are used as
penetration enhancers. Solvents (such as water, ethanol, and propylene glycol), terpenes
(such as menthol and camphor), and osmolytes, (such as glycerol and urea) are also used for
enhancing penetration through the SC. Penetration enhancers may function by multiple
mechanisms including enhancing solubility of actives, modifying the lipid structure, and
enhancing polar pathway by swelling the SC. Mechanisms by which surfactants impact the
penetration has been discussed in earlier sections in the context of skin cleansing. Since the
focus of this paper is on the interaction of ingredients with SC, the effect different types of
penetration enhancers on the structure of SC will be briefly reviewed here.
Moghadam et al. recently reviewed the effect of various types of penetration enhancers
on the SC lipid barrier.80 In their study, they used both SAXS and WAXS to investigate
structural changes to SC upon exposure to various penetration enhancers. Depending upon
the degree of alteration to the lipid structure observed from their SAXS patterns, they
classified the penetration enhancers into five categories. These include minimal changes
to lipid structure, slight disordering, significant disordering, disruption of the lipid layer,
and formation of new lipid structure with incorporation of the enhancers. Some of their
observations are summarized below:
• Solvents such as ethanol and PG had the least impact on the SC lipid structure, even
though both have been shown to enhance skin penetration of actives. Moghadam et al.
attributed this to effect of solvent on solubility and partitioning of actives into the SC.80

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