FORMULATING
in this quest, the data so far indicating that (using zinc oxide as the ‘active drug’) a particle size of 150 nm and above does not penetrate skin but remains on the surface as a white precipitate. Reducing the particle size reduces the white appearance on skin but also reduces the zinc oxide’s effectiveness as a sunscreen. Current theory is that a particle size of 80 nm-120 nm is the best balance. Regulatory authorities are currently concerned with one manufacturer’s claims to have an effective zinc oxide sunscreen with a particle size of approximately 20 nm-50 nm, particularly when you need around 20% to give an SPF of 30+. The reason for both the loss of efficacy and the FDA’s concern is probably due to increased concern regarding absorption of the zinc oxide into the dermis (and probably the bloodstream) with the smaller particle sizes.
In a paper entitled: ‘Nanotechnology and skin delivery – infinitely small or infinite possibilities’ by Dr Johann Wiechers, where he quotes world experts, including Professor Michael Roberts of Queensland University’s Therapeutic Reserarch Unit, the author presents available evidence that shows nanoparticles in the range 3 nm-10 nm tend to accumulate in the hair follicles and sweat glands, but do not penetrate further into the dermis. Professor Roberts also shows that there are polar hindrances to particles of zinc oxide and titanium dioxide penetrating through the epidermis. From this paper and others, any evidence presented by those that say nano-sized zinc oxide penetrates skin base their evidence on finding zinc ions in the dermis. This is shown to be flawed hypothesis as the zinc ions have come from zinc oxide or zinc carbonate dissolving in the skin’s fatty acids or in the formulation and therefore being more bioavailable. In any case, evidence shows that the level of zinc found in the dermis is less than background zinc levels, indicating that the amount absorbed is insignificant. Another question Professor Roberts asks is whether or not the low levels of zinc detected are harmful. At levels less than background concentrations, one would think not. In essence, this work showing that zinc oxide or titanium dioixide do not penetrate skin has given us a hypothesis that you need to have a particle size of less than 3 nm, or to have a soluble particle for any of it to be absorbed through the epidermis and into the dermis.
As is stated previously: “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
Mulberry.
layer. The next layers are a series of lipid layers, hence lipophilic ingredients have some chance whereas lipophobic (or hydrophilic) ingredients have great difficulty.“ Also: “The intercellular channels between the layers are composed of a lamellar structure of oil layers (nearest the cells) and water layers between. The water layer is lined with surface active agents that have a polar nature. It is this polar nature that restricts polar chemicals (e.g. water soluble vitamins, amino acids and almost all water soluble drugs) from penetrating.” Two factors stand out:
Smaller molecules penetrate better than large molecules.
Non-polar molecules (and lipophilic molecules) penetrate better than polar hydrophilic molecules.
Obviously, polar hydrophilic molecules will try to get down the intercellular channels via the aqueous channel and with the polar ends of ‘emulsifiers’ lining the surface of these channels the flow will be retarded, sometimes halted depending on the affinity of the drug for the emulsifier. This theory is akin to the technology on which the gas chromatograph is based.
Reducing the polarity of the drug seems to work. Research being done by a French company indicates that condensation of small peptide chains (3 to 5 amino acids) with palmitic acid (C16) will allow these polar peptides to not only be absorbed readily through the stratum corneum but also easily penetrate living cells. The purpose of doing this is to use fragments of DNA to ‘trick’ the cell into thinking it has damaged DNA hence activate the cell’s skin repair mechanism.
Whitening agents Hydroquinone
In medical literature, hydroquinone is considered the primary topical ingredient for inhibiting melanin production. Its components have potent antioxidant abilities. Topical hydroquinone comes in 2% (available in cosmetics) to 4% (or more) concentrations (available from a physician or by prescription), alone or in combination with tretinoin 0.05% to 0.1%. Research has shown hydroquinone and tretinoin to prevent sun- or hormone- induced melasma.
Hydroquinone is a strong inhibitor of melanin production, meaning that it prevents skin from making the substance responsible for skin colour. Hydroquinone does not bleach the skin but lightens it, and can only disrupt the synthesis and production of melanin hyperpigmentation. It has been banned in some countries (e.g. France) because of fears of a cancer risk and is restricted, at levels exceeding 2% in other countries such as the US and Australia, where it can only be sold as an over-the-counter drug.
Some concerns about hydroquinone’s safety on skin have been expressed, but the research when it comes to topical application indicates negative reactions are minor or a result of using extremely high concentrations or from other skin lightening agents such as glucocorticoids or mercury iodine. Any perceived risk is most likely applicable to African women. Hydroquinone has been shown to cause leukemia in mice and other animals. The European Union banned it from cosmetics in 2001, but it shows up in bootleg creams in the developing world.
Because of hydroquinone’s action on the skin, it can be irritant, particularly in higher concentrations of 4% or greater and predictably when combined with tretinoin. Some medications have been created that combine 4% hydroquinone with tretinoin and a form of cortisone. The cortisone is included as an anti-inflammatory. The negative side effect of repeated application of cortisone is countered by the positive effect of the tretinoin so that it does not cause thinning of skin and damage to collagen. Safer alternatives are more expensive but are available.
Arbutin
Arbutin is a skin de-pigmentation and whitening agent extracted from the bearberry plant via a solid/liquid extraction process. It protects the skin against damage caused by free radicals, and is proving a popular whitening agent in Japan and Asian countries for skin de-pigmentation. Arbutin inhibits the formation of melanin pigment by inhibiting tyrosinase activity.
April 2012 PERSONAL CARE 125
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