Transmission electron microscopy images of BNPs. The scale bar represents 200 nm. (Photo credit: Yang Deng.)
Transmission electron microscopy images of BNPs. The scale bar represents 200 nm. (Photo credit: Yang Deng.)

Encasing the active ingredient of sunscreens inside sticky polymer nanoparticles stops penetration into the skin, preventing exposure to potentially harmful chemicals, researchers at Yale University have found.

Commercial sunscreens contain organic or inorganic ultraviolet (UV) filters that block radiation from the sun, which can cause sunburn, skin aging, and skin cancers. But over the past few decades, studies have indicated that organic UV filters in particular can penetrate the skin or hair follicles and find their way into the bloodstream and other bodily fluids, where they could act as allergens or hormone disruptors. In combination with sunlight, there are also concerns that some of these active agents can generate reactive oxygen species (ROS) that cause damage at the cellular and DNA level.

To get around this problem, W. Mark Saltzman and his team encapsulated one such organic UV filter, padimate-O, with polymeric bioadhesive nanoparticles (or BNPs) [Deng et al., Nature Materials (2015), 10.1038/nmat4422]. The nanoparticles, which are typically just under 100 nm in diameter, comprise a polylactic acid (PLA) core and a surface of hyperbranched polyglycerol (HPG). The UV filter is held in the core of the nanoparticle and the hydroxyl surface is converted to one rich in aldehyde groups, which stick to biological molecules like proteins (Fig. 1).

‘‘The bioadhesive nanoparticles adhere strongly to the skin and retain UV filter molecules within the core,’’ explains Saltzman.

In mouse models, the team found that the UV filter—containing BNPs remain on the surface and do not penetrate into the skin, in marked contrast to nonbioadhesive nanoparticles or padimate-O alone. Moreover, a much lower concentration of active sunscreen has a comparable anti-UV effect when applied in combination with BNPs compared with commercial formulations. In fact, the researchers found that just 5% of the amount of UV filter used in commercial sunscreens is needed to achieve the same level of protection in their nanoparticle—based formulation.

The BNP—based sunscreen has an additional practical advantage—while it is water resistant, it can be readily wiped off with a towel or left to slough off with dead skin cells without any harmful effect.

Encapsulating padimate-O in nanoparticles appears to have another benefit, as well. In the researchers’ study, the BNP—based formulation significantly reduces a type of DNA damage called double-strand breaks.

‘‘We have taken UV agents that are known to be effective, combined them with materials that are known to be safe, and produced a better sunblock,’’ says Saltzman. ‘‘[Our] sunblock is more effective, longer-lasting, and less toxic than any previously described.’’

Saltzman believes the tactic should work with other UV filters too, and is now planning the first tests of the BNP—based formulation on human volunteers.

‘‘This is a promising approach that could protect against the collateral damage potentially inflicted by the penetration of active sunscreen agents into cells,’’ says Peter J. McHugh of the University of Oxford.

This paper was originally published in Nano Today (2015), doi:10.1016/j.nantod.2015.10.003