The skin is the main organ of interest to the cosmetic scientist. You might well suppose that the skin, being something that we all possess and interact with on a daily basis, was something that we would know a lot about. In fact there are still a lot of quite important questions to which we don’t know the answer. The biggest challenge is the topmost layer, the one we can actually see.
The uppermost layer of the skin is called the stratum corneum and is really really thin. It is about half the width of a sheet of photocopy paper. Amazingly, it is the stratum corneum that provides our main defence against stuff getting into the skin. If you have ever wondered where a cream goes when you apply it to your skin, the bulk of it is absorbed into the stratum corneum. The stratum corneum is constantly shedding dead skin cells which are replaced from below. This turnover of cells is a process called exfoliation. So most of the stuff you apply to your skin gets no further than the stratum corneum, and doesn’t even stay there that long.
This protective function for most of human history was probably essential to stop the toxins produced by plants, animals and germs from killing us. I wouldn’t choose to live without it even today. But useful as this stratum corneum is, it does make it very hard work to get drugs and cosmetic actives through the skin. There are some drugs it would be really brilliant if they could be delivered across the skin. Diabetics for example would love to be able to get their insulin without the need for needles. I suppose one really shouldn’t encourage them, but it would make heroin addiction slightly less dangerous.
I have made the point before that not many cosmetic ingredients that claim to reduce wrinkles are especially effective. The primary reason for this is simply that it is almost impossible to get a reasonable dose delivered across the skin. Basically the skin doesn’t allow itself to be messed with.
But despite the difficulties scientists continue to work to try and understand better the way the skin works as a barrier to stop molecules getting through. If we can get a better idea of that, we have a chance to work out ways of getting them through a bit more efficiently. One of these scientists is Majella Lane of the London School of Pharmacy, who presented some of her findings at the Making Cosmetics event in Coventry last March.
One of the ideas that has been around for a while is identifying what are known as penetration enhancers – materials you can add to a formulation to help the molecules you are interested in get through the skin. The name is probably a bit optimistic. The effects of of most of these penetration enhancers are modest in the extreme. When you take the jar of penetration enhancer off the shelf the feeling is more ‘well it can’t do any harm I suppose’ than ‘let’s see how much better this will make it’.
Majella described how she has managed to track the way different molecules move through the stratum corneum. The structure of the stratum corneum has been compared to that of bricks and mortar, with dense impenetrable dead skin cells forming the bricks and a lipid rich fluid forming the mortar. It is a good description but it is a bit misleading in one respect. The bricks are huge and the mortar is really thin compared to actual bricks and mortar. This is the key to understanding just why it is so difficult to penetrate such an apparently thin barrier. In reality the route into the body is a long thin and winding road where the route is so narrow it is hard for any large molecules to get through at all. Penetration enhancers and the ingredients they are supposed to be helping through rapidly get separated from one another.
Technology has achieved some wonderful things and maybe one day we will be able to open and close the skin barrier like a gate. A lot of things that I believed to be impossible have now actually been done, and have even become almost routine. But as things stand at the moment, the skin looks like being a barrier that we won’t know how to overcome for some time to come.