Nano-materials in cosmetic products are defined as insoluble or biopersistant substances with one or more external dimensions or internal structure between 1 and 100nm. Their use has come under scrutiny from groups prone to scare-mongering that these substances can actively penetrate the epidermis and dermis due to their relatively small dimensions, entering the bloodstream and possibly causing safety concerns. This scare-mongering is particularly popular on the internet, despite the strong body of scientific evidence to show that the nano-materials approved for use in cosmetic products are safe when used within current regulatory limits. It is also a requirement of all products marketed in the EU to indicate all nano-materials in the ingredient list.
The truth is, it is actually very difficult for topically applied cosmetic substances to penetrate far into the epidermis, and even harder for them to penetrate into the dermis and bloodstream. It is more of a challenge for the cosmetic chemist to enhance delivery of active substances to deeper layers of the epidermis to achieve efficacious visible results than many people think!
To help you understand the way some substances can selectively travel through the outer layers of the skin, take note of the following pathways and sizes:
|Particular||Size nm (= x10-9m)|
|Stratum corneum hydrophilic pathways(hydrophilic substances need to be smaller than this to traverse the stratum corneum)||0.4|
|Stratum corneum lipid bilayer(lipophilic substances need to be smaller than this to traverse the stratum corneum)||13|
|Stratum corneum intercorneocyte space||20-75|
|Stratum corneum thickness||10,000 – 40,000|
To pass through the stratum corneum (and deeper layers) substances either need to pass through the cells or around them (the intercorneocyte space). As you can see, water soluble substances need to be smaller than nano-materials to traverse the stratum corneum. Lipid soluble substances can travel through lipid bilayers when measuring small on the nano-material scale, but even if they can enter a pathway, they then have an incredibly large distance to travel (compared to their size) to penetrate beyond the stratum corneum!
Due to the complexity of the skin and the difference in the layers, substances that have a molecular weight below 1000 daltons may be able to penetrate the skin if in an amphiphilic base. Standard emulsions are amphiphilic bases, containing both hydrophilic (water) and hydrophobic (lipid) phases as well as amphiphilic substances (the emulsifiers), so act as excellent delivery vehicles for small substances to the stratum corneum level. Even if the molecular weight of a substance is below 1000 daltons and it is in an emulsion base, the distance it must traverse through is still incredibly large on a molecular scale. In addition to this, the substance would still be challenged to travel through deeper layers of the skin, which can be affected by its pH, polarity, solubility and amphiphilic nature (out of the base). Formulators can manipulate the delivery of agents to certain layers of skin through careful formulation techniques; but in general, substances with a molecular weight of greater than 1000 daltons in a hydrophilic base will generally have very limited penetration and sit primarily on the surface of the epidermis.
Cosmetic substances applied topically may:
- sit on the surface of the skin and/or penetrate the very outer layers of the epidermis (stratum corneum):
- many common lipids will achieve this function – applying an oil will by its very virtue help reduce TEWL and improve the suppleness and appearance of the skin. Applying an emulsion that has a high lipid content will also help reduce TEWL to some extent and provide a more supple appearance and smoother skin feel.
- DHA and some colourants (some D&Cs and FD&Cs) are used purposely to alter the colour of keratinocytes; this will provide temporary ‘staining’ effects of the skin at this level.
- penetrate to the mid-layers of the epidermis (stratum granulosum):
- this includes osmolytic substances such as humectants which 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.
- actives are often paired with a humectant or in a humectant base (including butylene and pentylene glycol) to enable them to be pulled to the stratum granulosum level.
- 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.
- cosmeceutical substances such as peptides and other actives may stimulate communicative responses to create transient physiological changes to the dermal matrix (i.e. stimulate collagen and elastin synthesis) – the substance itself does not reach the dermal layer but instead reside in the stratum basale and induce communications to direct activity deeper in the skin. This has nothing to do with nano-materials entering the blood stream but instead the action of the substance on cell communicative processes while still located in the epidermis.
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.
Research to definitively prove the safety of nano-materials when used in topical cosmetic applications is ongoing and a requirement of the EU Regulations on Cosmetic Products. In the meantime, it is up to industry to provide education to consumers to remove the fear that surrounds what is otherwise, and where used in compliance with regulatory requirements, an innovative and advanced application of cosmetic science for better acting personal care products.