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Chemicals in Arizona: nanoparticles in sunscreen creams

Posted on March 31st, 2017 by in New Materials & Applications

Arizona sun screen

Most of us do not think about sunscreen (sunblock) all of the year round except for when we go to the beach.  Aptly, questions about sunscreen occurred to me while vacationing in central Arizona in March 2017.  Although the spring had barely begun the sunlight was intense and hot.As an intermediate hiker I set out to hike the Sonoran Mountains after being lathered with copious layers of sunscreen in addition to a hat.  While the hike was grand and invigorating the sunscreen’s effectiveness left a great deal to be desired.  I came back from the hike with a significant tan, despite the sunscreen, after all of 4-5 hours of sun exposure.

Some questions occurred: how “screening” is the sunscreen on the market?  Is there any research underway for safer and more effective sun blocking creams?  Even more important, are the existing sunscreens safe?

A 2016 paper, based on research at Yale University, discusses new studies on more effective sun blockers (Source: Deng, Ediriwickrema, Saltzman et al, Nature Materials, 14, 1278–85, 2015).  According to the authors a majority of commercial sunblock preparations use organic or inorganic ultraviolet (UV) filters.  Despite protecting against skin phototoxicity, direct cellular exposure to UV filters has raised a variety of health concerns.  The authors encapsulated Padimate O (2-ethylhexyl 4-dimethylaminobenzoate), a model UV filter, in bioadhesive nanoparticles. Encapsulates were tested as a substance to prevent epidermal cellular exposure to UV filters while enhancing UV protection.

Deng eta al have developed a new generation of sun blockers that wrap a UV light filter—the active ingredient of most chemical sunscreens—inside a sticky polymer nanoparticle.  The new encapsulate has proven effective in addition to preventing potentially harmful chemicals from direct contact with human skin. Professor Saltzman at the Biomedical Engineering Department of Yale University and his team have demonstrated the large skin-adhesive nanoparticles they have developed stayed on the surface skin of mice.  In contrast commercial sunscreens penetrated into deeper skin layers. The new sunblock formulation also remained on the skin of mice for days, even after water exposure.  In contrast the existing sunscreens wash off easily, requiring repeated application even as often as once per hour.

“This is a new type of material,” said Professor Robert Langer, at the Chemical Engineering Dept of  Massachusetts Institute of Technology, who was not involved in the study. “The novelty is the high adherence to the skin that works very well in the models used.” (Source: A. Azvolinsky, Next Generation: Nano Sunblock, www.the-scientist.com, Sep 28, 2015)  The significance is most sunscreens on the market contain either inorganic filters—such as zinc dioxide—or organic ones—like Padimate O and Avobenzone. Both micronized inorganic filters and organic ones are chemical sunscreens that effectively absorb UVA and UVB light before it penetrates the skin, but, there are concerns “regarding their potential to act as allergens, to release oxidative species after UV exposure, and to partially bind to hormone receptors,” according to the 2016 study coauthor Michael Girardi, a Yale dermatologist and researcher.  When these UV filters absorb potentially harmful light, reactive oxygen species form that have the potential to mutate DNA and cause other types of cellular damage.  “Almost all of the organic sunblock molecules are not chemicals we want penetrating into the skin, or into the bloodstream,” Girardi wrote.

While the benefits of using sunscreens that absorb harmful UV rays and help prevent DNA damage is thought to outweigh the potential risks, the researchers have sought to make a safer form of sun protection. “Our idea was to encapsulate the UV filter inside a polymer nanoparticle in a stable way so that the UV filter stays inside the nanoparticle core,” Professor Saltzman explained (Source: A. Azvolinsky, Next Generation: Nano Sunblock, www.the-scientist.com, Sep 28, 2015).

So far the nanoparticle-based sunblock has been found to stay on the surface of mouse skin, and has not been detected within the crevices of hair follicles. The consequence of applying the product to broken skin has not yet been tested.  One would expect the adhesiveness of the particles might prevent them from penetrating the skin.  The ultimate concern is the entry of nanoparticles into the blood stream.

Next, the nano sunscreen will be tested in a small human study and the nanoparticles will be combined with other widely used UV filters. “This concept can also be extended to inorganic filters such as zinc oxide,” said Professor Saltzman, noting that this might require reformulating the nanoparticles themselves.

Desert Spiny Lizards (in header photo) are much luckier than human beings!  Their colors become darker during the cool season, which allows them to absorb more heat from the sun.  These Lizards become lighter during warm times so they reflect more solar radiation.  No sunscreen required…


 

All opinions shared in this post are the author’s own.

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