Zinc oxide (nano form)

5. Do ZnO nanoparticles pass through the skin?

Separate tests were made related to the important question of whether ZnO nanoparticles are more likely to penetrate the skin than larger particles.

This is a complex question, as the skin has a multi-layered structure. Passage of these very small particles through the layers, if it occurs, has to be followed with great care – using spectroscopic tests and electron microscope observation. There is also normally zinc in the body at low concentrations, as an essential trace element. To test whether externally applied zinc finds its way through the skin in real situations requires a method to distinguish zinc that has been applied to the skin from what is already inside the body. This can be done using special preparations enriched with one of the rarer isotopes (chemically identical but physically distinguishable forms) of zinc.

Tests have been performed to identify whether ZnO nanoparticles penetrate the skin barrier. Some experiments made use of skin removed during plastic surgery, usually from the abdomen. These indicate that a very small proportion of zinc (0.03 of the applied dose in one series of experiments) may be absorbed when ZnO is applied to the skin. These tests did not identify nanoparticles penetrating the skin, so the passage of zinc was probably due to zinc ions released from nanoparticles into solution. This is the form in which zinc is normally already found in the body.

Animal experiments extended these tests to investigate if skin penetration might be increased when skin is damaged by sunburn. UVB damaged skin did not prove any more likely to allow penetration of nanoparticles.

Further tests examined whether penetration is enhanced by additional chemicals, such as solvents. These might affect the outer layer of the skin, the stratum corneum, which is normally a toughened assembly of dead cells (corneocytes). Solvents such as ethanol may affect the material between these cells. Animal experiments showed that this can enhance penetration of this outer layer, but did not investigate whether this extended to deeper skin layers.

More extensive tests used human volunteers and were designed to mimic how sunscreens are actually used. For example, after a 2 hour exposure to a nano-sized ZnO-containing sunscreen, nanoparticles were only found in the outermost layers of the volunteers’ skin (the stratum corneum). Even in the deeper portions of that layer, the concentration of zinc was no higher than normally found in the body. Increasing exposure, by using the sunscreen for 48 hours, or stripping some corneocytes from the skin before application, still yielded a negative result.

The most sensitive tests are probably those in which penetration of zinc was tested using the variant isotope method. One such study used possibly the most authentic test site – volunteers using sunscreen twice daily for five days while at the beach. The product they used also contained a chemical, isopropyl nitrate, which is known to enhance skin penetration. After using nano-particle ZnO sunscreen, the volunteers gave blood and urine samples, which were analysed to see if the isotopic composition of zinc in their bodies was altered. There was some evidence of absorption of soluble zinc in the beach trial, but the amounts were very small. The recommended daily dietary intake of zinc is 8 mg for women and 11 mg for men. The amounts absorbed from sunscreen, which amounted to 0.001 per cent of the applied dose, were between 100 and 1,000 times smaller than this. Women in this study absorbed slightly more zinc than men, and the change in blood zinc did not appear for several days.