Beauty Forum Feb23 - Flipbook - Page 40
The skin is an organ where one of the main tasks is
protection - as a rule, it should not allow "foreign"
substances to penetrate the body. Of course,
however, there are many substances capable of
overcoming the barrier of the epidermis, as well as
reaching the dermis. The issue of penetration is
also important in the context of cosmetic products thanks to this ability, some substances can act not
only on the surface of the skin, the issue of skin
absorption is also considered when assessing the
safety of cosmetic ingredients.
The 500 Dalton (Da) rule
How can substances penetrate the skin?
What exactly should be understood as "penetration
through the skin"? This term covers both the
penetration into one of the layers of the skin
(primarily the epidermis) and the entry of
substances into the bloodstream. Dermal
absorption can be divided into stages:
penetration one – i.e. penetration into a specific
layer of the skin, most often we think of the
epidermis (the stratum corneum, where some
substances may be retained) and the dermis
(which few compounds are able to reach),
penetration two– i.e. passing from one layer of
the skin to another,
resorption – i.e. absorption into the lymphatic or
circulatory system, which leads to systemic
absorption (this is not desirable and should not
even take place in the case of a cosmetic –
such action is reserved for medicinal
substances).
There are three main ways of transporting
substances through the skin:
intercellular – penetration through the so-called
intercellular cement,
transcellular – moving from cell to cell,
transappendageal – through hair follicles and
sebaceous glands.
Pathways through the skin
Whether, at all, and how quickly and in what
amount substances are able to penetrate the skin
depends on a number of factors. Some of them are
related to the properties of the substances
themselves (size, lipo/hydrophilicity, polarity,
concentration), the properties of the entire cosmetic
formula (pH, other ingredients, viscosity), the
presence of penetration promoters and the
condition of the skin (age, sex, damage to the
barrier function, disease).
All skin penetration processes are based on
diffusion, active transport is not relevant here, and
substances migrate along a concentration gradient
(i.e. from a place of higher concentration to a place
of lower concentration). One of the basic factors
determining penetration is the size of the molecule,
or more precisely - the molecular weight, and this is
what we will focus on today.
What is the molecular weight?
Molecular weight is the sum of the atomic weights
that make up a given molecule, and a commonly
used unit of molecular weight is the Dalton. There
is no simple way to relate molecular weight to
molecular size (e.g. diameter), although there are
methods for large molecules, e.g. proteins and
polymers (e.g. 5 kDa is approximately 1.1 nm and
500 kDa is 5, 2nm). For small molecules (below
800-900 Da), the size can be described in Å (1 Å =
10-10 m), but this is of no practical use.
Mass is one of the basic physicochemical
properties describing a molecule. When describing
permeation through the skin, we use molecular
