The skin is a relatively thick and complex organ designed, amongst other features, to protect tissues and organs within the body from externally applied substances. It varies in thickness from between 1.5mm (as found on the eyelids) to 4mm (as found on the soles of the feet) and is composed of the outer (non-living) epidermis, living epidermis, dermis and hypodermis.

In order to be their most effective, cosmetic ingredients need to penetrate the epidermal layers sufficiently to perform their specific function, and this will vary depending on the type of product you are creating. For example, a moisture protective product only needs to penetrate (and preferably provide some sort of film forming) at the stratum corneum level; while a product designed to boost hydration over a prolonged period will need its ingredients to penetrate to the mid-layers of the epidermis whilst also preferably providing some moisture protective properties at the stratum corneum level. Cosmeceutical ingredients, particularly advanced cosmeceutical ingredients that activate dermal-epidermal communications, need to penetrate to the stratum basale level.

The key thing to remember when formulating is: where do you want the ingredients to go, and how are you going to get them there?

Stratum corneum hydrophilic pathways require water soluble substances to be less than 0.4nm in diameter to penetrate past this layer; while the stratum corneum lipid bilayer will allow passage of much larger lipophilic substances (on a nano scale!) of less than 13nm. The stratum corneum intercorneocyte space will allow passage of lipophilic substances with a diameter of 20-75nm, but then you face the challenge of the substance being able to pass through the protective intercellular lipids which have a stratum corneum thickness of 10,000 – 40,000nm!
Cosmetic substances applied topically may:

• sit on the surface of the skin and/or penetrate the very outer layers of the epidermis (stratum corneum):
  – this provides a moisture protective barrier to reduce transepidermal water loss (TEWL).
  – the net results will be an improvement to the suppleness and the appearance of the skin.
  – many common lipids will achieve this function.
  – film forming and barrier agents will be used in products to specifically provide a lightweight coating to the skin to prevent TEWL for a short period of time and improve the moisture content and suppleness of the skin.
  – DHA and some colourants (some D&Cs, FD&Cs) are used purposely to alter the colour of keratinocytes; this will provide temporary ‘staining’ effects of the skin at this level.

OR

• penetrate to the mid-layers of the epidermis (stratum granulosum):
  – this includes osmolytic substances such as humectants (glycerin, propylene glycol, sodium hyaluronate etc); these will typically penetrate up to the stratum granulosum and attract water from the environment and from within the deeper layers of the skin to provide a more supple, dewy appearance to the skin.
  – substances used to reduce the polarity index of the skin, such as dimethyl isosorbide (DMI) can enhance delivery of actives, particularly water soluble actives, to mid-layers of the epidermis.

OR

• penetrate into the deeper layers of the epidermis (stratum basale):
  – skin whitening agents will penetrate to this level and alter the normal skin pigmentation process; by reducing deposition of melanin into keratinocytes, the skin will appear lighter.
  – cosmeceutical substances such as peptides and other actives may induce a recuperative action on the skin by stimulating communicative responses to create physiological changes to the dermal matrix (i.e. stimulate collagen and elastin synthesis or have an impact on other dermal functions such as an anti-inflammatory response).
  – claims about physiological changes cannot be made about a cosmetic product; any claims about the activity at this level cannot be made besides the visible or appearance based changes they induce e.g. ‘reduces the appearance of redness’ or ‘visibly improves elasticity of the skin’.
  – changes induced by these substances will be transient – once the substance is no longer used, the effect will quickly dissipate.
  – to provide this type of action, the active material must be in a suitable carrier base to be able to penetrate through the outer layers of the epidermis and reach the stratum basale target site.

We have created a summary of different delivery methods that can be used to help enhance delivery of actives to target sites – but take careful note of the typical size of each of these delivery options and in particular, whether the substances are likely to travel via hydrophilic or lypophilic pathways.

Very few cosmetic substances applied topically will penetrate into the dermis; and even when they do, they will commonly be acted upon by macrophages within this layer to be removed or deactivated before they can reach the bloodstream. Pharmaceutical (e.g. steroids) or injected cosmetic substances (e.g. sodium hyaluronate), on the other hand, will be designed specifically to reach the dermis and/or bloodstream for a greater physiological action. Pharmaceutical and injected substances have greater safety and regulatory requirements for this reason, and are beyond the scope of a cosmetic substance or our discussion on cosmetic formulations.

Belinda Carli is the Director of the Institute of Personal Care Science (IPCS). IPCS provides distance and on-site training in Cosmetic Formulation, Brand Management and Regulatory Affairs. Contact Belinda and the team for assistance with your training, formulation, brand management or regulatory requirements: belinda@personalcarescience.com.au or visit www.personalcarescience.com.au for more information.