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Nanotechnology and Skin Delivery: Infinitely Small or Infinite Possibilities?
By: Johann W. Wiechers, PhD, JW Solutions
Posted: December 19, 2008, from the January 2009 issue of Cosmetics & Toiletries.
- Figure 1. Theoretical predictions of particle penetration
- Figure 2. Histological sections demonstrating the penetration depth
- Figure 3. Kinetics of the storage of nanoparticles
- Figure 4.The effect of particle size on the UV attenuating properties of titanium dioxide.
- Figure 5. Schematic representation of the size-dependent occlusive effect of lipid nanoparticles
- Figure 6: Cumulative amount of ketorolac
page 5 of 13
From this, it can be concluded that—theoretically&mdashthere will be no significant penetration of particulate matter into the viable epidermis, and hence the exposure should be very, very small. However, the Friends of the Earth would argue these are nothing but theoretical predictions and that real experimental data is necessary. And they would be fully right. Therefore, the next topic to be discussed is the "growing body of (experimental) evidence."
This body of evidence is indeed growing and it is growing rapidly. Most articles describing skin penetration of nanoparticles deal with particulate sunscreen agents such as titanium dioxide and zinc oxide.
Experimental Skin Penetration Data on Nanoparticles in Cosmetics
The first thing that becomes apparent when examining articles describing skin penetration of nanomaterials is that the transfollicular route of skin penetration plays an important role. This is not amazing considering that in 1993, Rolland et al.13 had already discussed how fluorescent particles with a diameter between 3 µm and 10 µm penetrated almost exclusively via the sebaceous glands and the transfollicular orifice, whereas particles smaller than 3 µm penetrated generally into the skin and those greater than 10 µm did not penetrate the stratum corneum at all.
Subsequent investigations conducted mainly by Jürgen Lademann and his team at the Center of Experimental and Applied Cutaneous Physiology, Department of Dermatology, Charité-Universitätsmedizin Berlin, have shown that fluorescein that was covalently bound to microparticles penetrated more efficiently into the hair follicle than free fluorescein (see Figure 2).14 In the stratum corneum, the topically applied substances were stored only in the upper layers of the corneocytes. These layers become a short-term reservoir because, every day, one layer is sloughed off. In contrast, the orifices around the follicles represent a long-term reservoir because their depletion only takes place by hair growth or sebum production, which are slow processes.14
This long-term reservoir function of the infundibulum, the orifice in the skin where a hair follicle emerges from the skin surface, was beautifully visualized in an article by the same group (see Figure 3).15 It was shown that the smaller microparticles penetrate deeper into the stratum corneum, especially in combination with cyanoacrylate skin stripping.16