occurred in late 1949, when Dr. William Wellman, et al of
the (then) Colgate Palmolive Peet Co. developed “Colgate
Instant Barber Shave.” The level of glycerol was estimated as
about 4.5%.
Much later, formulators used such similar humectants as
1,2-dihydroxypropane, the d.sorbitol and d.mannitol hexanhexols
and simple polyethylene glycols (PEGs). Sometimes
there is a small benefit in using combinations, like glycerol
and d.sorbitol. The TEWL reduction due to glycerol can last
up to about a week, but slightly longer if d.sorbitol is added.
The mechanisms by which glycerol increases the moisture
inventory of the SC is complex and not fully comprehended.
One factor involves cellular shrinkage. Glycerol works best
in moderate to high humidity—below about 10% relative
humidity there is little or no effect. This suggests that the
water-glycerol solutions can offset TEWL by their hygroscopic
action. A supporting factor is the finding that a film
of glycerol will absorb its weight of water in an hour or two
at 50% relative humidity.
Other Moisturizing Approaches
Alpha-hydroxy acids (lactic, glucolic, citric, mandelic, tartaric,
etc.) have been used as peels to smooth and plump the
skin while reducing fine lines and wrinkles. Over time they
can cause skin thickening. Under slightly alkaline conditions
they can stimulate natural hyaluronic acid (HA) deposition.
HA is an important and major component of the skin
and body. About half of the inventory resides in skin cells.
It is a complex disaccharide polymer produced in vivo by a
single enzyme: hyaluronidase. The molecular weight varies
from about 2,500,000 to 25,000,000. The structure consists
of alternating units of gluconic acid and N-acetylglucosamine
in loose coils. The structure was established in 1951
by M.M. Rapport.
In the skin, HA polymers have a half-life of 10 to 20 hours due to enzyme actions. The decomposition
is inhibited by Vitamin C and other chemicals. HA is constantly regenerated by other
enzymes. Synthetic versions (enzyme free) last almost indefinitely and are available commercially
as both acids and salts. The larger
molecules are more biologically
active.
HA regulates TEWL by influencing
cell behavior, especially its
metabolism. It also affects osmotic
pressure, acts as a lubricant, promotes
cell motility and assists in
wound repair by regeneration
processes. Even a brief exposure
to UV causes an increase in HA.
Antioxidants present in the skin
slow HA degradation. Examples
include Vitamin E and ubiquinone,
which are also depleted by
sunlight. Vitamin C stimulates
HA but is also depleted. In summary,
the HA polydisaccaride is
a major metabolically active SC
component, often seen in skin care
cosmetics.
Urea is also widely used for
the treatment of dry skin. While
St. Ives Fresh Hydration Lotion comes in Citrus & Vitamin C, Coconut Milk
& Orchid Extract and Oatmeal & Shea Butter.
32 Spray November 2017