Personal Care April 2012

FORMULATING a

Stratum corneum

Stratum lucidium

Stratum granulosum

Lamellar granules

Keratinocyte

Dead keratinocytes

Stratum corneum

Stratum lucidium

Stratum

Stratum spinosum

Langerhans cell

Merkel cell Tacltile disc Sensory neuron

granulosum

Stratum spinosum

Stratum basale

Dermal papilla

Stratum basale

Melanocyte Dermis

Superficial b

Dermis Deep 480x Figure 1: The epidermis: a) Four principal cell types in the epidermis, b) A photomicrograph of a portion of the skin.

and sexual areas). This is the layer that contains the hair roots, sweat glands, sebaceous glands (sebum production), nerve endings (pain and touch), Meissner’s corpuscles, Paccinian corpuscles and other corpuscles (pressure and touch), Krause’s end bulbs (cold) and Ruffini’s endings (warmth). Smaller blood vessels also proliferate in the dermis. The dermis consists of cellular, fibrous and ground substance components in two layers: papillary dermis and reticular dermis.

Conclusion

It is apparent that any active ingredient has a treacherous path to the site of action particularly if this involves blood transport. The chances of materials passing through the skin are slim and many do not pass through the skin at all. The outermost layer is a layer of dead skin and it must pass through the intercellular channels between cells in this layer. The next layers are a series of lipid layers, hence lipophilic ingredients have some chance, whereas lipophobic (or hydrophilic) ingredients have great difficulty.

Factors affecting the rate and extent of absorption are:  Structure of skin – Hair density, skin thickness and physical health of skin have an effect. Increased hair density will assist absorption as the drug may lodge itself in hair follicles (and outlets from sweat glands) and be ‘available’

122 PERSONAL CARE April 2012

longer, particularly at a moist surface, which is an ideal condition for absorption. Thicker or tougher skin will have a reduced rate of absorption due to the longer pathway the drug has to navigate.

 Biochemistry of skin – Lipid composition (from diet), moisture content (from external humidity) and the general energy levels (fitness) of the subject have an effect. One must also consider the rate of exfoliation, of skin, as any drug that remains on the surface of the skin would be lost when the outer layer (stratum corneum) is lost through abrasion (clothes, towelling, etc.) or normal desquamation.

 Sex – Male skin tends to be a greater barrier than female skin.

 Age – The older you get the less penetration occurs due to the loss of general structure of skin.

 Heredity – Some may have enhanced ability because of heredity factors such as thin skin.

 Disease – Dermatological (e.g. exposure of lower layers of skin increasing absorption) and systemic (hardening of the blood vessels slowing down transfer to the circulatory system).

 Physical damage to the skin – Scarring will prevent absorption while abrasion or cuts will increase absorption.

 Environment – Increasing temperature and increasing humidity will increase absorption.

 Diet – Nutrition will affect absorption, as healthy skin will behave normally.

 Drugs – Drugs will affect absorption, as some drugs will alter skin physiology and either aid or decrease skin absorption. Both diet and drugs can affect blood flow and glandular function which in turn affect absorption.

Modes of transfer are:  Diffusion – The passage or movement of an active ingredient across a membrane (usually skin or cell membrane in this field). It usually involves diffusion against a concentration gradient where the active ingredient diffuses across the membrane from an area of high concentration to one of low concentration, ie. passive diffusion along a concentration gradient. The result is an equal concentration on both sides of the membrane, unless there are other factors which may affect the result. Water transfer into and out of skin cells or in the kidneys to excrete excess water are classic cases of diffusion.

 Adsorption – Where an active ingredient is adsorbed or attached to the surface of a carrier which is then transported across the skin membrane where the active ingredient can work. It is a means of protecting the active ingredient from chemical change during transport. It can also mean the technique of attaching the active ingredient onto

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