Conditioning Agents for Hair and Skin

Introduction 3
subsequent damage. With continued physical abrasion these cuticular defects
can become cracks or fissures which penetrate into the cortex. Sufficient degradation
of the cortex will ultimately lead to breakage and loss of the hair shaft.
Therefore, maintaining a smooth cuticular surface is essential to the condition
of hair.
b. Skin. Skin is also composed of protein but, unlike hair, it is a living
organ. Chapters 2 and 3 discuss the biology of skin structures in detail. The
skin's outer layer, the stratum corneum, is analogous to the hair's cuticle in
that it consists of flat, hardened, keratinized cells. As with hair, severe perturbations
in the outer protective layer of skin can result in increasing degrees of
damage. This damage is especially prevalent at low temperature/humidity conditions,
which can cause skin to become relatively inflexible and inelastic. The
resulting cracks and fissures in the stratum corneum can cleave the epidermis
and lead to bleeding, inflammation, and infection (2). For both skin and hair,
a contiguous surface structure is key to ensuring the good condition of the
substrate.
2. Compositional Elements
a. Hair. In addition to the proteinaceous structure of hair, there are additional
elements of composition to consider. Lipids, which are essential to
maintaining the pliability of hair, are present both in the free form and bound
in cell structures. The free lipids are primarily the oily secretions of the sebaceous
glands which provide surface gloss and lubrication to the hair; however,
they may also detract from its condition by making the hair appear greasy or
weighed down. The structural lipids are part of the intercellular complex and
are important because they help cement together the protein structures, particularly
in keeping the cuticle cells. It is worth noting that the intercellular
spaces, where these lipids reside, may be important pathways to deliver conditioning
agents inside the hair (1, p. 44). When these lipids are stripped from
hair, the result is that the hair becomes more brittle and prone to damage.
Water is another important component in hair. Water also helps maintain
pliability, which keeps the hair shafts from fracturing. Generally, the moisture
content in hair will equilibrate to the external humidity. However, when hair
is dried by heating, the water level does not equilibrate to preheating levels
without submersion or rewetting. This phenomenon, known as histeresis
(1, p. 77) can occur as a result of the heating processes commonly used in hair
care (e.g., blow dryers and curling irons).
b. Skin. Skin, being a living organ, has a more complex structure than
hair, but it also depends on a mixture of lipids and water to maintain its optimum
condition. In fact, epidermal lipids account for about 10-12% of skin's
dry tissue weight. These lipids consist of phospholipids, sphingolipids, free and