Disposable diapers may be full of it, but their designs are anything but.
Bill Hammack, an engineer at the University of Illinois at Urbana-Champaign, says the ordinary-seeming products actually boast an intriguing and deceptive level of complexity.
Hammack posts videos on his YouTube channel, engineerguy, celebrating the small engineering feats found in everyday products, such as coffee makers and fibre optic cables.
He says diapers aren’t the cheap blend of cotton and plastic most people assume they’d be. Rather, they’re well-organised systems of layers designed to trap human waste and move it away from a baby’s skin.
Hammack calls that a “triumph of polymer science.” Here’s why.
The 3 magical layers
Most commercially available diapers are made of three key layers. The first layer is known as the topsheet; it makes contact with the baby’s skin. The next layer is the surge layer, which is followed by the third and final layer, the absorbent core. Each one is thicker than the last.
Hammack explains that each layer performs a crucial function.
The topsheet acts as a one-way street for urine. A child pees into the diaper with enough force to send the liquid through that top layer, but a repellent material stops it from running back onto the child’s skin.
Instead, the urine sinks deeper into the surge layer, which is designed to take a localised stream and distribute it throughout the absorbent core. If the design didn’t have this feature, the liquid would saturate one spot and the rest of the diaper would be useless.
How the diaper keeps the baby dry for hours
It’s not enough that the diaper wicks moisture away from the body. It also has to store it somewhere without causing too much discomfort.
To do that, Hammack says the diaper’s three layers are all built with the same one-way design at the topsheet. As you go deeper into the layers, each one contains a denser bundle of fibres than the last, making it easy for liquid to enter but hard to escape.
The absorbent core is made of a super-absorbent polymer than can hold up to 400 milliliters of urine, Hammack explains.
That material is so good at its job that manufacturers purposely use about 15% cotton in that layer so urine can actually flow to unused spots. If the absorbent core were made entirely of the polymer, the urine would absorb too thoroughly and pool in certain spots instead of distributing evenly.
Instead, the cotton acts as a channel through which the urine can access dry parts of the core, where it can remain for approximately nine hours while the baby sleeps.
Of course, babies don’t just pee into their diapers, but Hammack seems less impressed by the system for faeces than the one used for urine.
The primary mechanisms for trapping poop inside a diaper are the two elastic cuffs that hug the baby’s legs. According to one of the original patents for the diaper, these were designed to contain “explosive liquefied bowel movements.”
It’s not the sexiest engineering marvel, but keeping solid waste trapped inside the diaper is about all you can hope for. The alternative is a messy one.