Stratum corneum as a barrier – does size matter? (by Clotilde Cheignon)

The stratum corneum as a physical barrier has been of increased interest over recent years, as scientists have studied the effect of the nanoparticles ingredients on the skin, in medical and cosmetic applications. Earlier findings suggested that they could enhance penetration into the skin delivering active ingredients.
In 2000, V. Jenning, A. Gysler, M. Schäfer-Korting and S. H. Gohla, Eur J Pharm Biopharm. 2000 May; 49 (3): 211-8. studied the Vitamin A use in topical [Vitamin A loaded solid lipid nanoparticles for topical use: occlusive properties and drug targeting to the upper skin]. The use of solid lipid nanoparticles helps for a penetration of retinyl palmitate and retinol format onto the skin.
With “Nanoparticles and microparticles for skin drug delivery“ Adv Drug Deliv Rev. 2011 May 30;63(6):470-91, T. W. Prow and Cie, in 2011, demonstrated that depending on the size of the particles of the carrier (smaller or bigger than 10 nanometers), the drugs will be delivered on the top of the skin or via a deposition on the follicule.
In 2007, S. E. Cross, B. Innes, M. S. Roberts, T. Tsuzuki, T. A. Robertson and P. McCormick in their publication “Human Skin Penetration of Sunscreen Nanoparticles: In-vitro Assessment of a Novel Micronized Zinc Oxide Formulation” Skin Pharmacol Physiol. 2007;20(3):148-54 could not demonstrate a deep penetration of Zinc Oxide in the skin, but concluded that further studies need to be performed regarding other format of ZnO as well as other ingredients, as safety concerns have been raised.
For other scientists, such as S. K. Murthy, declared in 2007, that nanoparticles have a future in medicine applications [Nanoparticles in modern medicine: State of the art and future challenges].
In cosmetic application, the issue is that regarding the regulation you do not want ingredients penetrating deeply into the skin – indeed for sunscreens and some barrier enhancing ingredients it is preferable for them to stay close to the surface, However in medical applications, greater penetration could be a benefit.
The fear in cosmetic industry is so real that in October 2011, the EC adopted the recommendation 2011/696/EU on a common definition of the term “nanomaterial” to facilitate the regulation of products containing such materials.
From July 2013, cosmetic products placed on the market in the European Economic Area are obliged to comply with the European cosmetics regulation (EC) 1223/20092. Therefore the nanoparticles must be specified on the label with such an ingredient using the suffix “nano”.
Following the European commission, a series of methods have been proposed to measure the particle size of nanoparticles.
In 2012, Professor Guy’s group, challenged the view that nanoparticle ingredients could penetrate deeply the skin [Objective assessment of nanoparticle disposition in mammalian skin after topical exposure, C. S. J. Campbell, L. R. Contreras-Rojas, M. B. Delgado-Charro, R. H. Guy,  J Control Release. 2012 Aug 20;162(1):201-7. ].
His team used a laser scanning confocal microscopy to examine fluorescently-tagged polystyrene beads (ranging in size from 20 to 200 nanometers) absorbed into the skin.
Their conclusion is that even when the skin was damaged by stripping outlayers with adhesive tape, the nanoparticles did not penetrate further down the Stratum corneum over time. Despite the conclusion, it should be emphasised that this research has clearly not been able to make a systematic evaluation of nanoparticle disposition on the skin for the entire spectrum of particle properties, including shape and charge. The impact of such other important variables remains to be determined.
The findings could allow the safe use of insoluble nanoparticles into cosmetic applications, such as sunscreens.
It reinforces the fact that the Stratum corneum, which first role is to act as a barrier and to reduce the water loss, works well.
It will be interesting to see how this field of stratum corneum research develops.  There maybe people working on this right now; if so it would be good to hear your views.

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