Why does skin wrinkle when it is exposed to lots of water?
The skin’s outermost layer, the epidermis, is responsible for this wrinkly reaction, which usually occurs after you’ve spent about half an hour in the water. The epidermis contains the protein keratin, which strengthens your skin and helps keep it moist. Dead keratin cells make up the epidermis’ own surface layer known as the stratum corneum, which is Latin for "horned layer." These dead keratin cells in the stratum corneum absorb water easily and start to swell after extended periods in the water, but the living keratin cells deeper in the skin do not. As the dead cell layer expands, it begins to take up more surface area, but it’s still connected to the living cells beneath and doesn’t have anywhere to go. As a result, the stratum corneum wrinkles to give the temporary new surface area someplace to go.
Your skin is made up of two layers. The outer layer is called the epidermis and the lower level is the dermis. The outer layer (epidermis) produces an oily substance called sebum. You can see this substance when you touch a window or mirror and your “oily” fingerprint is left there. One job sebum does is it keeps water out of your skin—a job that it does very well. However, after long periods of time in a swimming pool, shower, or bathtub, much of the sebum is washed off and your outer layer of skin starts to absorb water. As it absorbs water it swells, but since the epidermis is “tied” down to your dermis in certain spots, it expands more where it is not “tied” down and this causes your skin to wrinkle. The difference between your skin and a raisin is that raisins wrinkle because they shrink, but your skin wrinkles because it is getting bigger.
