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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 11 of 13
After having established that human exposure to nanomaterials used in cosmetics is infinitely small, and that the hazards are over-estimated and more caused by the experimental conditions than reflecting what might occur in real life, it can be concluded that the risk of nanotechnology in cosmetics via topical application is also over-estimated. But what are the benefits of using nano-sized materials in general and in cosmetics in particular?
The unique size-dependent properties of nanomaterials mean that in some ways they behave like new chemical substances. For example, nanoparticles can scatter and absorb short-wavelength UV radiation but leave longer-wavelength visible light virtually unaffected (see Figure 4). Quantum dots can be used in medical imaging techniques because when they absorb UV radiation, they emit visible light, and the color of the emitted light differs for nanoparticles of different diameters.7 But there are also some very specific purely cosmetic benefits, other than translucent sun care products, that can be obtained from the use of nanoparticles in cosmetics.
Souto and Müller describe the cosmetic features of two nanostructured delivery systems: solid lipid nanoparticles (SLNs) and nanostructured lipid carriers (NLCs).33, 34 They mention both the skin protective and lubrication/emolliency properties of these nanoparticles. When applying lipid particles onto the skin, a film layer will be formed, having a surface area that depends on particle size. The air-filled space within a layer of optimal packing density is independent of the particle size, which is considered to be 24% if assuming a three-dimensional hexagonal packing of ideal spherical-like particles. However, the air channels in a layer of nanoparticles (see Figure 5; top) will be much smaller than the air channels in a layer of microparticles (see Figure 5; bottom). The transepidermal water loss will therefore be more reduced in a formulation containing nanoparticles than in a formulation containing microparticles.34 Experimentally, this was also shown to be the case. The occlusion factor of lipid micro-particles with a diameter greater than1 m was only 10%, relative to a factor of 50% when using lipid nanoparticles of approximately 200 nm.35
Souto and Müller also claim that this occlusion may help to enhance the skin elasticity and skin penetration of active ingredients, but this depends first of all on the polarity of the active ingredient and there are many alternatives to nanoparticles available to increase the skin elasticity or moisture content of the skin. The same applies for their reputed lubricating effects that are due to their spherical-like shape.34 But these cosmetic benefits of nanoparticles are indeed only secondary because there are many other ingredients offering the same effect.
In this article titled "Nanotechnology and Skin Delivery: Infinitely Small or Infinite Possibilities?" it was concluded that the penetration of the particulate matter is infinitely small indeed. Nano-sized structures that are flexible will go into the skin, as shown in Figure 6 and exemplified by liposomes, which were designed to penetrate the skin. The interested reader is directed to References 28 and 36-38 for more on the skin penetration, hazards and biocompatibility of liposomes.