Wrinkle

A wrinkle, also known as a rhytid, is a fold, ridge, or crease in an otherwise smooth surface.

While wrinkles can appear in fabric, they are most commonly discussed regarding human skin. They are often viewed as a typical sign of the aging process.

Many different factors contribute to their formation over time. These include sun damage, smoking, and poor hydration. Habitual facial expressions and sleeping positions also play significant roles. Even temporary immersion in water can change the appearance of the skin.

To understand how wrinkles form, we must look at the skin's internal structure. The misrepair-accumulation aging theory suggests that wrinkles develop from incorrect repairs of injured fibers. Within the dermis, there are elastic fibers and collagen fibers that maintain skin structure. When these fibers are repeatedly extended or compressed, they can suffer injuries. During the repair process, the body may not restore the original fibers perfectly. If an elastic fiber breaks while the skin is extended, it may be replaced by a "long" collagen fiber. This accumulation of long fibers makes the skin loose and stiff, creating a large fold. Conversely, if a fiber breaks while the skin is compressed, it may be replaced by a "short" collagen fiber. These shorter fibers restrict the extension of longer fibers, trapping the skin in a permanent fold or wrinkle.

Sleep wrinkles, or sleep lines, are a specific type of wrinkle caused by external forces. These form primarily from repeated mechanical compression and shear forces during sleep. This is especially common for individuals who sleep in side or stomach positions. Unlike dynamic wrinkles, which are caused by muscle contractions like crow's feet, sleep wrinkles result from sustained pressure against a pillow. These forces can create temporary creases that eventually become permanent due to cumulative mechanical stress. The location of these wrinkles often follows patterns related to facial anatomy, such as the nasolabial folds or the periorbital regions. To mitigate this, some people use low-friction pillowcases made of silk or satin. These materials may reduce skin drag and shear compared to rougher cotton.

Low-friction pillowcases have become a popular skincare trend since the late 2010s. These products are designed to minimize skin shear and drag during the night. Commercial options include silk-based brands like Blissy, which uses 22-momme mulberry silk, or YouthSecret. Prices for these specialized pillowcases typically range from $20 to $100. Many are designed with specific features, such as hypoallergenic certifications to resist dust mites. Some even feature specialized weaving methods to ensure a uniform, smooth surface. While independent lab testing shows these fabrics have a low coefficient of friction, direct clinical evidence for wrinkle prevention remains limited.

Another unique form of wrinkling is seen during prolonged water immersion. This is often called "pruney fingers" or water aging.

For a long time, people believed this happened because the skin absorbed water and expanded. However, a 1935 study by Lewis and Pickering suggested the nervous system is actually involved. They discovered that wrinkling did not occur in skin where the nerves were damaged. Recent research indicates that water initiates a process called vasoconstriction. This is when blood vessels constrict, or tighten, reducing the fluid underneath the skin. This decrease in fluid reduces tension, causing the skin to fold. Some studies suggest this may provide an evolutionary benefit by increasing traction. For example, a 2021 study found that wrinkles decreased the force required to grip wet objects by 20%.

Many different methods exist to manage the appearance of wrinkles. Retinoids are a class of vitamin A analogues used widely for anti-aging. Topical versions like tretinoin are approved medical agents that promote cell turnover and stimulate collagen synthesis. Another option is the use of glycosaminoglycans (GAGs), such as hyaluronic acid. These act as natural moisturizers and lubricants between cells to maintain fluid balance. For more immediate changes, dermal fillers can be injected to correct depressions in the skin. These are often composed of hyaluronic acid or calcium hydroxylapatite.

Neuromodulators offer a different approach by targeting the muscles themselves. Botulinum toxin, such as the brand Botox, is a protein produced by the bacterium Clostridium botulinum. It treats wrinkles by immobilizing the muscles that cause them, specifically for glabellar lines between the eyebrows. Other approved versions include Dysport and Xeomin. Finally, there is photorejuvenation, which uses lasers to improve skin condition. Laser ablation removes thin layers of skin, while nonablative lasers work by stimulating the production of collagen.