If skin is a watertight barrier, how do medicinal creams work?

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If skin is a watertight barrier, how do medicinal creams work?

In: Biology
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It’s watertight, but some substances that are not water can penetrate the skin. A famous example is dimethylmercury, which can penetrate not just the skin but a rubber glove on top of it, with [fatal results](https://en.wikipedia.org/wiki/Karen_Wetterhahn).

Its not watertight, its semipermiable.

You have cells that act as a sort of mesh. They allow some things in and some things out. Medications can be transpported into those cells when applied to the skin and they can transport that to other cells and into the blood.

Medicinal cream isn’t water?

You’re skin cells are selectively permeable which means they may not let water into the body but they can still absorb topical medicines into themselves.

Short answer – skin isn’t actually watertight.

Slightly longer answer – skin is quite porous; has to be to let sweat out, for example. What skin blocks is not so much water as (significantly larger) particles. As an example, silver miners can develop argyria, recognizable by a bluish skin tone from a high concentration of silver dust that’s gotten stuck in their skin. Not going through, just stuck in the skin; water particles are smaller, and get absorbed by the skin, as it is allowed to seep ever so slightly through it.

Watertight does not necessarily mean chemical tight. Many substances can be carried through the skin by interacting with the cell walls. Because they largely consist of lipid (fat-like substances) layers some chemicals can interact and pass through. It is particularly true of many organic solvents, also things like organophosphorus pesticides and related nerve agents. It will also absorb hydrophilic (water-loving) chemicals, just more slowly. Some organic carrier solvents also aid the passage of those compounds where they would not pass ordinarily.

This is actually a really complicated question. For the most part, they need some kind of solvent in order to get the cream (which is typically water-based) past the skin (which relies on oils to be waterproof). There are multiple chemicals that can do this, for example surfactants will temporarily dissolve the oils, or emulsifiers or alcohols can allow the water-based cream to bind with the oils in your skin. If you read the inactive ingredients list on medicinal creams, you’ll find a lot of these chemicals

Source: https://biomeddermatol.biomedcentral.com/articles/10.1186/s41702-020-0058-7