Contrary to popular belief, the skin is a relatively thick and complex organ designed, amongst other features, to limit the entry of externally applied substances. There is a lot of misinformation on the internet suggesting most cosmetic chemicals penetrate the skin quite easily, when in fact, one of the biggest challenges we have as formulators is to make sure the active ingredients get to the intended target site so they can work as proven and expected!
Why is delivery important?
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; whilst advanced cosmeceutical ingredients typically need to penetrate to the stratum basale level.
For effective skin penetration, substances need to be less than 500 daltons and be in a suitable carrier system. This means even a very small substance may not penetrate very far through the epidermis if it is not in an effective delivery system.
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 scal
e!) 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!
So, where do you want the ingredients to go, and how are you going to get them there?
Cosmeceuticals vs pharmaceuticals
Very few cosmetic substances applied topically will penetrate to the dermal-epidermal junction; because of their size and/or the distance they need to traverse through the epidermis and the pathways to do so.
Pharmaceutical (e.g. steroids) or injected cosmetic substances (e.g. sodium hyaluronate), on the other hand, are 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 a discussion on cosmetic formulations or cosmeceuticals.
What are the key delivery sites?
Where you want the active to go will impact your delivery agents. For example:
- osmolytic substances such as humectants (glycerin, propylene glycol, pentylene glycol, low molecular weight sodium hyaluronate etc.) 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.
- peptides and other small cosmeceutical substances intended to penetrate to the deeper layers of the epidermis (str
- atum basale) must not only be below 500 daltons but also in a suitable carrier base to traverse the outer layers of the epidermis and reach the stratum basale target site.
What are the latest advances in delivery?
When you want an active to reach the mid or deeper layers of the epidermis, what are the latest ‘formulation buzz’ options to do this?
- Liposomal delivery can assist delivery of hydrophilic substances where this is enclosed in the core. The liposome will generally be in a biomimetic phospholipid form that can utilise liposoluble pathways for deeper penetration and release of the active substance at a deeper site than it could reach without the liposomal outer shell.
- Nanoencapsulation can be used to enable controlled release of internal actives while also stabilizing the internal core materials – but care must be taken to conform with nano-particle rules, which may impact how far these substances can travel based on their size.
- Micro and nano-emulsions can enable deeper skin penetration of actives, especially where osmolytic substances are used in the water phase or the polarity index is reduced (for water soluble actives), or lipid-soluble actives are used.
- 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.
The final word…
Remember to check safety data carefully in line with efficacy and delivery sites when determining the best delivery system/s to use, the actives you are using, and where they need to reach.
The fantastic in-vivo data that accompanies cosmeceutical active materials shows how well these substances can work when they reach their intended delivery sites, but if they can’t reach the site they simply won’t perform as the data suggests. If you’re investing in the use of specialized materials then it makes sense to ensure the substance comes packaged in, or the base contains, a suitable delivery vehicle to get it where it needs to be for best efficacy.