Skin penetration
Although it is necessary to consider the function
of topically applied cosmetic actives in some detail,
a majority of those substances are not intended for
deeper skin penetration and absorption, and will
only have a superficial action. Percutaneous
absorption includes both active penetration and
absorption into the blood stream, possibly with
pharmacological action at sites far from the appli-
cation area. Cosmetic actives are intended to have
a predominantly local effect, thus it is undesired to
have an absorption into the blood. Lipid nanopar-
ticle dispersions show the ability to control the
rate of penetration of actives into the skin [34].
Penetration of actives when applying common
topical base ingredients does not occur to any
large extent, although such materials may become
enmeshed or entrained in the outer regions of the
horny layer when massage is applied. Modulation
of release and active penetration into certain lay-
ers of the skin can be achieved as a consequence
of e.g. the creation of supersaturated systems
[33]. These systems can be created by incorpora-
tion of lipid nanoparticles into topical formula-
tions (creams, ointments, emulsions, gels). The
increase in saturation solubility will lead to an
increased diffusion pressure of the active into the
skin. During shelf life, the active remains
entrapped into the lipid matrix because this latter
preserves its polymorphic form. After application
of supersaturated cream onto the skin, and
because of an increase in temperature and water
evaporation, increasing the thermoactivity, the
lipid matrix transforms from a more unstable
polymorph into a more ordered polymorph lead-
ing to the release of active into a system already
saturated with the same active, and thus creating
a supersaturation effect. Penetration studies using
several interesting cosmetic ingredients have been
performed, e.g. retinoids [26] and molecular sun-
screens [35].
In some special circumstances, a prolonged
release of the active but with little penetration
may be desired. This is the case of particulate and
molecular ultraviolet (UV) blockers, because of the
side effects they show if penetration into the skin
occurs.