The Photochemistry of Sunscreens
The correlation between excessive exposure to sunlight – specifically, ultraviolet (UV) radiation – and skin cancer is now well established. Nevertheless, and despite the extensive availability of sunscreen products, skin cancer cases have increased in recent years. Therefore, developing sunscreen formulations which maximise photoprotection is of the utmost importance, as is educating the public on the correct usage of sunscreen. I propose to improve sunscreen efficacy by pursuing a comprehensive understanding of the mechanisms by which sunscreen molecules dissipate the excess energy resulting from absorption of UV – referred to as photodynamics.
Ideally, sunscreen molecules’ photodynamics should occur without any detriment to itself, the mixture which surrounds it (the sunscreen lotion) or the human skin. In addition, photodynamics processes should occur within an ultrafast timescale; that is on the order of femto- to picoseconds (one quadrillionth to one trillionth of a second, respectively). In order to follow such fast processes, I use ultrafast laser pulses in combination with time-resolved pump-probe techniques. These techniques follow a “stop-motion style”, with several “pictures” being taken at several time intervals to create a continuous film that describes the ultrafast process under study. To achieve this, I use two laser beams: one, called the pump, to photoexcite the molecule (i.e. mimic the action of sunlight) and another, called the probe, to monitor the ensuing processes at several points in time.
Using this methodology I have found drastic differences in the photodynamics of different sunscreen molecules. For example, our group found, in accordance with previous studies, that one of the most widely used sunscreen ingredients, 2-ethylhexyl-E-4-methoxycinnamate (EHMC), dissipates energy very quickly, i.e. after a couple of picoseconds, after UV radiation absorption without any molecular damage occurring. This is ideal for a sunscreen molecule, because not only it absorbs the energy that could damage the skin and lead to skin cancer, but it also dissipates it without causing any harm. However I have observed a different sunscreen molecule, menthyl anthranilate (MenA, trade name Meradimate), to behave very differently to this. MenA actually does not effectively dissipate this excess energy and remains in what is called an excited state much longer than EHMC. This would suggest that MenA is, in fact, a bad sunscreen ingredient, despite the fact that it is still being used in sunscreen lotions. These results highlight the relevance of the research I am conducting, and serve as an example of the important issues that it can address. In addition, these conclusions raise further questions: what exactly does change a molecule’s photodynamics, so that it behaves as an ideal sunscreen? Can we tweak the currently available sunscreens so that their photodynamics translate to optimum photoprotection? Despite sunscreens having been on the market for 80 years, there is still no comprehensive answer to these questions. However, targeting research towards a comprehensive understanding of photoprotection could be more efficient than the current trial-and-error methods for sunscreen development, and eventually give rise to a ground-breaking generation of sunscreens which are tailor made for optimum photoprotection.
Professor Vasilios G. Stavros