Why should cosmetics ingredients be water or fat soluble?

The stratum corneum is packed tight with skin cells (bricks) and fatty lipids (mortar), so to move through this tier an ingredient must be compatible with one of them. Theoretically, lipophilic, or fatty, molecules can penetrate fat easily - taking an intercellular approach meaning that they move in between the cells through the fatty lipids (mortar). Aquaphilic, or water-soluble, ingredients may be able to pass through watery cells, taking a transcellular path.

But each cell has its own individual barrier, the fatty cell membrane, which makes infiltration tricky. An ingredient has to be watery but moderately lipophilic too.

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