The skin has a lot of protective barriers but it’s always possible to make something better so a sunblock that doesn’t penetrate the* skin at all seems like it might be a good thing. Interestingly, this new sunblock or sunscreen is enabled by nanoparticles but not the metallic nanoparticles found in what are sometimes called nanosunscreens. From a Sept. 29, 2015 news item on Nanowerk,

Researchers at Yale have developed a sunscreen that doesn’t penetrate the skin, eliminating serious health concerns associated with commercial sunscreens.

Most commercial sunblocks are good at preventing sunburn, but they can go below the skin’s surface and enter the bloodstream. As a result, they pose possible hormonal side effects and could even be promoting the kind of skin cancers they’re designed to prevent.

But researchers at Yale have developed a new sunblock, made with bioadhesive nanoparticles, that stays on the surface of the skin.

A Sept. 28, 2015 Yale University news release by William Weir, whch originated the news item, describes the research in more detail,

“We found that when we apply the sunblock to the skin, it doesn’t come off, and more importantly, it doesn’t penetrate any further into the skin,” said the paper’s senior author, Mark Saltzman, the Goizueta Foundation Professor of Biomedical Engineering. “Nanoparticles are large enough to keep from going through the skin’s surface, and our nanoparticles are so adhesive that they don’t even go into hair follicles, which are relatively open.”

Using mouse models, the researchers tested their sunblock against direct ultraviolet rays and their ability to cause sunburn. In this regard, even though it used a significantly smaller amount of the active ingredient than commercial sunscreens, the researchers’ formulation protected equally well against sunburn.

They also looked at an indirect — and much less studied — effect of UV light. When the active ingredients of sunscreen absorb UV light, a chemical change triggers the generation of oxygen-carrying molecules known as reactive oxygen species (ROS). If a sunscreen’s agents penetrate the skin, this chemical change could cause cellular damage, and potentially facilitate skin cancer.

“Commercial chemical sunblock is protective against the direct hazards of ultraviolet damage of DNA, but might not be against the indirect ones,” said co-author Michael Girardi, a professor of dermatology at Yale Medical School. “In fact, the indirect damage was worse when we used the commercial sunblock.”

Girardi, who specializes in skin cancer development and progression, said little research has been done on the ultimate effects of sunblock usage and the generation of ROS, “but obviously, there’s concern there.”

Previous studies have found traces of commercial sunscreen chemicals in users’ bloodstreams, urine, and breast milk. There is evidence that these chemicals cause disruptions with the endocrine system, such as blocking sex hormone receptors.

To test penetration levels, the researchers applied strips of adhesive tape to skin previously treated with sunscreen. The tape was then removed rapidly, along with a thin layer of skin. Repeating this procedure allowed the researchers to remove the majority of the outer skin layer, and measure how deep the chemicals had penetrated into the skin. Traces of the sunscreen chemical administered in a conventional way were found to have soaked deep within the skin. The Yale team’s sunblock came off entirely with the initial tape strips.

Tests also showed that a substantial amount of the Yale team’s sunscreen remained on the skin’s surface for days, even after exposure to water. When wiped repeatedly with a towel, the new sunblock was entirely removed. [emphasis mine]

To make the sunblock, the researchers developed a nanoparticle with a surface coating rich in aldehyde groups, which stick tenaciously to the outer skin layer. The nanoparticle’s hydrophilic layer essentially locks in the active ingredient, a hydrophobic chemical called padimate O.

Some sunscreen solutions that use larger particles of inorganic compounds, such as titanium dioxide or zinc oxide, also don’t penetrate the skin. For aesthetic reasons, though, these opaque sunscreen products aren’t very popular. By using a nanoparticle to encase padimate O, an organic chemical used in many commercial sunscreens, the Yale team’s sunblock is both transparent and stays out of the skin cells and bloodstream.

This seems a little confusing to me and I think clarification may be helpful. My understanding is that the metallic nanoparticles (nano titanium dioxide and nano zinc oxide) engineered for use in commercial sunscreens are also (in addition to the macroscale titanium dioxide and zinc oxide referred to in the Yale news release) too large to pass through the skin. At least that was the understanding in 2010 and I haven’t stumbled across any information that is contradictory. Here’s an excerpt from a July 20, 2010 posting where I featured portions of a debate between Georgia Miller (at that time representing Friends of the Earth) and Dr. Andrew Maynard (at that time director of the University of Michigan Risk Science Center and a longtime participant in the nanotechnology risk discussions),

Three of the scientists whose work was cited by FoE as proof that nanosunscreens are dangerous either posted directly or asked Andrew to post comments which clarified the situation with exquisite care,

Despite FoE’s implications that nanoparticles in sunscreens might cause cancer because they are photoactive, Peter Dobson points out that there are nanomaterials used in sunscreens that are designed not to be photoactive. Brian Gulson, who’s work on zinc skin penetration was cited by FoE, points out that his studies only show conclusively that zinc atoms or ions can pass through the skin, not that nanoparticles can pass through. He also notes that the amount of zinc penetration from zinc-based sunscreens is very much lower than the level of zinc people have in their body in the first place. Tilman Butz, who led one of the largest projects on nanoparticle penetration through skin to date, points out that – based on current understanding – the nanoparticles used in sunscreens are too large to penetrate through the skin.

However, there may be other ingredients which do pass through into the bloodstream and are concerning.

One other thing I’d like to note. Not being able to remove the sunscreen easily ( “When wiped repeatedly with a towel, the new sunblock was entirely removed.”) may prove to be a problem as we need Vitamin D, which is for the most part obtainable by sun exposure.

In any event, here’s a link to and a citation for the paper,

A sunblock based on bioadhesive nanoparticles by Yang Deng, Asiri Ediriwickrema, Fan Yang, Julia Lewis, Michael Girardi, & W. Mark Saltzman. Nature Materials (2015) doi:10.1038/nmat4422 Published online 28 September 2015

This paper is behind a paywall.

*’teh’ changed to ‘the’ on June 6, 2016.