What’s the best sunscreen? It’s a question that troubles beachgoers, athletes, and scientists alike. Mark Saltzman, who falls into the last category, was so concerned by the time his third child was born that he wanted to engineer a better sunblock. “The initial goal … was to make a sunblock that lasted longer,” says Saltzman, a biomedical engineer at Yale University. “But as I read more about sunscreen, I became aware of people’s concerns about safety.” Now, he and his colleagues have unveiled the results of their research: a nanoparticle-based sunblock, which they say is longer lasting and less likely to leak into the body than traditional sunscreen.
The sunscreen we lather on works in one of two ways: by blocking out sunlight entirely with a zinc oxide paste (think white-nosed lifeguard) or by chemically filtering out harmful UV rays before they contact skin. And although the more popular UV-filtering version doesn’t leave you looking like a ghost, it has some drawbacks. Its chemicals have been found in urine and breast milk, showing it can penetrate the outer layer of the skin and leach deeper into the body. Some studies have suggested that the same chemicals shielding us from UV damage can also produce DNA-damaging molecules after absorbing UV radiation.
To keep the invisible appeal of the chemical filters but prevent them from penetrating the skin, Saltzman and his colleagues turned to nanoparticles, tiny particles that are between 1 and 100 nanometers in size. They found a variety that tightly adheres to protein-rich surfaces, like skin, and engineered them to encapsulate a chemical UV filter. Each nanoparticle is—in essence—a sunscreen-filled bubble that forms an ultrathin cloak by sticking to the skin. Researchers found that it not only stayed outside the body, but it was more efficient than traditional sunscreen, needing only 5% of the UV-protective chemicals, they report today in Nature Materials. What’s more, any DNA-damaging molecules produced from the chemical-UV reaction stayed sequestered in the nanoparticle layer, safely outside the body.
To test their sunblock, the team shaved the fur off four mice, sprayed the clear nanoparticle solution onto the rodents’ bare skin, and exposed them to UV light. They compared this group of mice with another one covered in commercially available sunscreen. Mice covered in both types of sunscreen were protected from key signs of UV damage called cyclobutane pyrimidine dimers, a “molecular signature” of sunburn. This suggests that the nanoparticle sunscreen is just as good as the traditional store-bought sunscreen, says Yale dermatologist and co-author Michael Girardi.
The nanoparticle sunblock—which is water resistant and does not penetrate mouse skin—can stay on for up to 5 days, but comes off with the wipe of a towel. What happens to it after it enters the environment is still the subject of future studies, but Girardi speculates that the lower dose of chemicals will lead to less leaching into the environment. The new study suggests a way to “[make] sunscreen filters a bit more effective and a bit less toxic,” says Anthony Oro, a dermatologist at Stanford University in Palo Alto, California, who was not involved with the research.
Meanwhile, Saltzman and his team are also preparing to test how the nanoparticle sunblock fares against the multitude of sunscreen options that line drugstore shelves. So far, they have compared their sunblock with only one commercially available type. But ultimately their goal is to get this sunblock onto people. Saltzman says they are not yet allowed to try out the product on humans, but they are in the process of applying for permission. “I’ll certainly be the first volunteer if it’s allowed.”