394 JOURNAL OF COSMETIC SCIENCE
• In general, surfactants had most impact on disordering the lipid layers and they also
enhanced the penetration of actives to a higher degree than did terpenes and solvents.
Impact was seen in either SPPs or LPPS or in some cases, both.
• Use of dodecyl trimethyl ammonium bromide and benzalkonium chloride, resulted
in the disappearance of the orthorhombic lipid fraction of the SC bilayer, indicating
their strong lipid disruption potential. Similarly, SDS also caused disruption of the lipid
phase, which resulted in the formation of a new lipid phase that incorporated SDS
into the structure. The impact of nonionic surfactants was less than that of their ionic
counterparts.
• Among terpenes and solvents, those with ring structures had less impact on the lipid
structure than those with alkyl chains.
• Authors also concluded that the degree of damage did not always correlate with the extent
of enhancement in penetration suggesting that it may be possible to find penetration
enhancers that are less damaging to the lipid barrier.
Phan et al. also investigated the effect of chemical penetration enhancers on SC structure.81
They used natural abundance 13 C polarization transfer solid-state nuclear magnetic resonance
(NMR) on intact porcine SC to investigate the changes in the mobility of lipid and protein
components upon exposure to chemical penetration enhancers such as monoterpenes, fatty
acids, osmolytes, surfactant, and azone. They also included the effect SC hydration level on
the performance of these penetration enhancers. They used dry SC powder obtained from
porcine skin in their NMR experiments and hydrated them to 20% or 40% by weight of
the SC sample. Their main conclusion was that hydrophobic and amphiphilic compounds
mainly affected the lipid regions, whereas osmolytes and hydrophilic compounds affected
both lipid and protein regions. They also concluded that the effect of penetration enhancers
depended on the hydration level of the SC with higher hydration impacting the structure
more than the lower one.
It is clear from the literature that penetration enhancers influence SC barrier properties.
Importantly, the degree of damage did not necessarily correlate with the magnitude of
penetration enhancement, indicating that it is possible to enhance the penetration without
causing major damage to the corneum. It is also important to ensure that the barrier is
restored to its normal state within a reasonable time. Not much work has been done on
understanding the restoration of barrier properties after exposure to penetration enhancers.
MOISTURIZER ACTIVES AND THEIR IMPACT ON THE SC BARRIER
Typical moisturizer actives include occlusives, humectants, emollients, and lipidic materials
such as fatty acids.65–67 Lately, skin identical lipids such as fatty acids, ceramides, and
sterols in ratios that are likely to form bilayer structures are being used to improve the SC
barrier.66,82,83 The role of moisturizer is to alleviate symptoms of dry skin and to help the SC
rebuild a healthy natural barrier. Moisturizer performance can vary significantly depending
upon the actives in them.
OCCLUSIVES
Petrolatum, a hydrocarbon based occlusive, is still the gold standard when it comes to
preventing water loss from damaged skin.84–86 White purified petrolatum has been shown

395 The Human Stratum Corneum
to be safe for personal care use.86 Typical petrolatum consists of a mixture of an amorphous
and a crystalline fraction with a wide range of chain lengths exhibiting a broad melting
peak starting from around the body temperature. The low molecular weight fractions
of petrolatum are essentially similar to mineral oil. The lack of high melting and more
hydrophobic fractions in mineral oil makes it less efficient as an occlusive compared to
petrolatum. Petrolatum, or some of its fractions, have been shown to penetrate into crevices
and cracks in dry skin, resulting in its ability to mask the dry skin appearance and to
prevent water loss through them. Ghadially et al. have shown that barrier recovery of tape-
stripped skin is faster under an occlusive layer of petrolatum compared to its control.87
Petrolatum, though an excellent exclusive, will not increase the inherent water holding
capacity of skin, but it will prevent excessive water loss. In this regard, petrolatum, in
combination with a humectant such as glycerol, is more effective in moisturizing skin. In
contrast to petrolatum, triglycerides and lipidic materials will increase the water holding
capacity while providing a moisture barrier because of their polar head groups.
Because of the recent consumer desire towards moving away from petroleum-based materials
to naturals, several petrolatum mimics have also begun to appear in the marketplace.
These include plant-based waxes such as candelilla wax, carnauba wax, and shea butter.88,89
Vegelatum and Oleogels are essentially technologies in which natural waxes are mixed
with liquid oils to create a rheology and feel like that of petrolatum.88–90 In most cases, in
vivo data comparing the performance of alternatives with petrolatum is still lacking.
HUMECTANTS
Humectants will bind to water molecules and help retain more water in the corneum.
Glycerol, sorbitol, polyols, pyrrolidone carboxylic acid (NMF component), lactate salts,
and urea are among the humectants used in cosmetic formulations.91 Among small
molecules, ionic materials have a higher water holding capacity and a higher tendency
to remain in the corneum than their nonionic counterparts. Polymeric materials such as
hyaluronic acid and its hydrolyzed forms are beginning to find applications as humectants
in cosmetic products.92 Urea is often used as a humectant in pharmaceutical formulations.
At concentrations higher than 10%, urea has been also shown to have keratolytic activity
and hence it is used in treatments for psoriatic skin.93
Among the various humectants, glycerol has been studied extensively in the literature and
has been shown to have multiple benefits for skin. Glycerol has been shown to reduce the
visible signs of skin dryness, to enhance exfoliation, and to increase corneum extensibility.91
The combination of humectants with occlusives such as petrolatum has been shown to be
more effective in vivo than either of them alone.94
EMOLLIENTS
The term emollient is used rather loosely in literature, sometimes referring to a fully
formulated skin care product or an ingredient in a moisturizer. In this paper, the term
emollient refers to an ingredient rather than a fully formulated system. Emollients provide
surface lubrication, increased hydration, and enhance skin elasticity and skin softening.95
Emollients are generally not very effective as occlusive moisturizers. Emollients can
penetrate the surface layers of dry skin and provide increased hydration and softening of