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
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
