In 1876, Charles Darwin observed the secretion of yellow matter from the rootlets of ivy. Little was known about the material until 2008, when Mingjun Zhang, PhD, an associate professor of biomedical engineering at the University of Tennessee, and his research team found nanoparticles in the yellow matter.
Zhang’s described his first findings as being focused on the climbing mechanisms of English ivy (Hedera helix) and Boston ivy (Parthenocissus tricuspidata). “I was trying to understand the strong force ivy generates to attach to structures,” noted Zhang, whose team analyzed the secretions with atomic force microscopy (AFM). “These nanoparticles are secreted by the ivy to fill holes and dents in the material that it attaches to, which generates crosslinking with the surface to increase adhesion forces.”
A year and a half later, Zhang attended a nanobiotechnology conference in San Francisco, where a speaker discussed toxicity concerns over metal-based nanoparticles used as UV filters. Since Zhang had access to the English ivy nanoparticles, he and his team tested them and found they could absorb and scatter UV light.
TiO2 and ZnO vs. English Ivy
Due to their white appearance on skin, titanium dioxide (TiO2) and zinc oxide (ZnO) often are reduced to nano sizes to offer more transparent yet equal UV protection. Many nanoparticles, according to Zhang, have the ability to absorb and scatter UV radiation due to their large surface-to-volume ratio; however, he finds that English ivy nanoparticles may offer better sun protection than metal-based nanoparticle UV filters with improved optical properties, less safety concern and a more uniform particle size.
Zhang and his team compared the optical extinction spectra of English ivy nanoparticles against that of TiO2. At 4.92 μg/mL, the English ivy nanoparticles exhibited significant extinction in the UV region (280–400 nm) but decreased after that, suggesting the English ivy nanoparticles would be effective at protecting skin. The results also indicated that English ivy nanoparticles have high transmittance in the visible UV region, which makes them “invisible,” according to Zhang. At the same concentration, TiO2 exhibited a lower extinction level in the UV region (250–350 nm) and decreased slowly after the UV region.
The ability of English ivy nanoparticles to protect skin from UV also is enhanced by their size and smoothness. “We observed that the [English ivy nanoparticles] are uniform at 65.3 ± 8.04 nm, and they have a smooth surface. This smoothness will enhance absorption or scattering of light,” said Zhang.
Although it was important for Zhang to assess the UV protection of the English ivy nanoparticles, it was equally as important to assess their skin penetration and toxicity for comparison with metal-based nanoparticle UV filters. It was determined that nanoparticles with a diameter <10 nm could reach the deeper layers of the stratum corneum (SC), while nanoparticles larger than 40 nm could only reach 5–8 μm into the SC after 8 hr of application, and 8–13 μm after 20 hr of application. It was therefore concluded that English ivy nanoparticles could not penetrate the SC from cosmetic applications.
The team also tested for cytotoxicity by incubating 1 μg/mL of English ivy nanoparticles with HeLa cells for 24 hr. With propidium iodide staining, the researchers noted no toxicity of the cells treated with English ivy nanoparticles, compared with control cells.
In personal care, the adhesion properties of the ivy nanoparticles could enhance the durability of sunscreens, reducing the frequency of re-application. These adhesion properties are also viable for other industries.
“We have investigated nanoparticle enhanced adhesion for medical glue,” said Zhang, who added that his team is working with the National Science Foundation to examine the ability of the nanoparticles to interface with surfaces.
“It seems that when nanoparticles are added to a nanocomposite, they enhance a material’s bonding properties, due to crosslinking,” said Zhang, who noted that nanoparticles have been added to tires to enhance their strength. In yet another project, the team is working on applying the nanoparticles to drug delivery, due to their biocompatibility.
Zhang’s team is still refining ways to optimize the nanoparticles for specific applications, as well as an eco-friendly scale-up method. “We are working to biosynthesize [rather than] chemically synthesize the material, meaning that we are trying to get the English ivy plant to produce the nanoparticles,” explained Zhang.
Of all the projects, however, Zhang believes the simplest project is perhaps the hardest—to observe the secretion process. “Nobody knows how English ivy secretes the nanoparticles, so we have set up a high magnification camera with which we hope to catch the secretion process in real time.”