Stratum Corneum Reservoir

The stratum corneum is usually described as a barrier to be crossed, which makes it sound like a sheet. It is also a compartment with volume: twenty-odd layers of flattened dead cells set in a lipid mortar, and that mortar dissolves things. A dose applied to the surface does not simply pass through or wash off. A share of it lodges there.

Fat-loving molecules do this most readily — sunscreen filters and several actives among them. The consequence is that skin goes on being exposed after the film has dried, and after a wash that took away everything sitting on top. It is part of why once a day is a sensible instruction for a molecule that would not last an hour in a beaker.

This store also empties in a way no other compartment does: by shedding. The loaded cells are already on their way off the surface, so the reservoir drains over the same days the skin spends renewing itself.

It changes how a claim reads. “Works for X hours” may be describing a reservoir rather than any release technology, and the two are not the same achievement. It cuts both ways, too — something irritating accumulates by the same route, which is part of why trouble with a new product tends to arrive on the fourth day rather than the first. It is not a licence to apply more, though. A compartment that is full stops taking any.