Researchers at the University of Tennessee report that English ivy nanoparticles may protect skin from UV radiation better than other metal-based nanoparticles such as titanium dioxide (TiO2). Their work, published in the Journal of Nanobiotechnology, responds to research claiming that metal-based nanoparticles could be linked to environmental and animal toxicity, and that TiO2, maghemite and iron nanoparticles less than 15 nm are capable of penetrating the stratum corneum, potentially leading to increased aging, pathological effects in the liver and particle accumulation in the brain.
The team isolated nanoparticles from Hedera helix (English ivy) and evaluated them for potential use in sunscreens based on their ability to absorb and scatter UV light, safety toward mammalian cells, biodegradability and potential for diffusion through skin. The team observed that the isolated ivy nanoparticles had a diameter of 65.3 ± 8.04 nm, based on measurements of 30 randomly counted nanoparticles that were not dominated by other particles.
To test the ability of the nanoparticles to protect skin from UV radiation, a UV and visible wavelength spectrophotometer was used to measure the optical extinction spectra of the nanoparticles. The team observed that at a concentration of 4.92 μg/mL, the nanoparticles exhibited significant extinction in the UV region, while having little extinction at the visible and near infrared regions. This indicated that the ivy nanoparticles could effectively block UV radiation without the opacity observed in other metal-based nanoparticles.
Comparing the UV blockage with TiO2 nanoparticles at the same concentration indicated that the total extinction of the ivy nanoparticles from 280 nm to 400 nm was better than the TiO2 nanoparticles. In addition, extinction of the ivy nanoparticles decreased sharply after the UV region, which makes ivy nanoparticles more effective in the UVA/UVB region and gives them high transmittance in the visible region, making them virtually "invisible."
To assess the toxicity of the nanoparticles, the team incubated 1 μg/mL ivy nanoparticles with HeLa cells for 24 hr. Using propidium iodide staining, researchers examined the cells upon incubation with the ivy nanoparticles by flow cytometry, noting no toxicity in comparison with the control cells.
Biodegradation of the nanoparticles was determined through a number of digestion techniques. The researchers incubated the ivy nanoparticles in RPMI at 37°C for up to 24 hr and found the nanoparticles were not digested as assessed by AFM. To test the ability of the particles to be broken down by enzymatic digestion, proteinase K was used. After incubation for 30 min, the nanoparticles were no longer detectable by AFM. Therefore, after enzymatic digestion, the ivy nanoparticles were degraded and lost their normal structure.
Finally, a mathematical model was developed to determine the potential for ivy nanoparticles to penetrate through human skin. Based on the model and obtained parameters, the dynamics of nanoparticle diffusion into the SC layer of the skin were simulated. While nanoparticles with a diameter of less than 10 nm had a chance to reach deeper into the SC layers, nanoparticles over 40 nm could only reach 5–8 μm into the SC layer after 8 hr of application, and 8–13 μm after 20 hr. The researchers therefore determined that with the standard 8 hr of sun exposure, the ivy nanoparticles, at 65 nm, could not penetrate the stratum corneum.
In addition to demonstrating that the ivy nanoparticles can be used as a UV filter in sunscreens, the researchers also emphasized that these nanoparticles demonstrated an adhesive effect, which reportedly enhances the UV protective ability of the nanoparticles.
Since question and debate remains over the safety of metallic nanoparticles in sunscreens, interesting alternatives such as these pose new opportunities for formulators in this highly debated field.