Why does our skin form wrinkles as we age?

This post will outline the answer in a few steps:

Under what conditions do materials form wrinkles, in general?
How does the general theory of wrinkles apply to aging human skin?
What underlying factors drive the physiological changes which result in wrinkles?

In the process, we’ll draw on sources from three different fields: mechanical engineering, animation, and physiology.

Why do Materials Wrinkle?

Imagine we have a material with two layers:

A thin, stiff top layer
A thick, elastic bottom layer

We squeeze this material from the sides, so the whole thing compresses.

The two layers want to do different things under compression:

The thin top layer maintains its length but wants to minimize bending, so it wants to bow outward and form an arc
The elastic bottom layer wants to minimize vertical displacement, so it wants to just compress horizontally without any vertical change at all.

Because the two layers are attached, these two objectives trade off, and the end result is waves - aka wrinkles. Longer waves allow the top layer to bend less, so a stiffer top layer yields longer waves. Shorter waves allow the bottom layer to expand/compress less vertically, so a stiffer bottom layer yields shorter waves. The “objectives” can be quantified via the energy associated with bending the top layer or displacing the bottom layer, leading to quantitative predictions of the wavelength - see this great review paper for the math.

Engineers do this with a thin metal coating on soft plastic. The two are bound together at high temperature, and then the whole system compresses as it cools. The end result is cool wrinkle patterns:

Other interesting applications include predicting mountain spacing (with crust and mantle as the two layers) and surface texture of dried fruit - see the review paper for more info and cool pictures.

The same thing happens in skin.

Skin Layers

For our purposes, skin has three main layers:

The epidermis is a thin, relatively stiff top layer
The SENEB (subepidermal non-echogenic band, also sometimes called subepidermal low-echogenic band, SLEB) is a mysterious age-related layer, mostly absent in youth and growing with age, between the epidermis and dermis - more on this later
The dermis is the thick base layer, containing all the support structure - blood vessels, connective tissue, etc

Both the SENEB and the dermis are re...

Gears of Aging