Polymer crystals are one of those special items that are almost magical. Polymer crystals gel very similar to a gelatin dessert. One of their major uses is by the disposable diaper manufacturers, where they are referred to as super absorbent polymers. The polymers allow for thinner and lighter disposable diapers.
Add water to the white crystalline flakes and they absorb many times their size in the polymeric gel that forms. One pound of the crystals holds about 50 gallons of water! The gel that forms swells by absorbing 30 grams of water for each gram of the crystals. The odorless crystals are shown below.
Construction workers, bicyclists and motorcyclists may be seen wearing cloth headbands or wearing a cloth collar around their neck. The crystals are sewn into a pouch in the cloth. The cloth item is then soaked in water for several hours. A tiny handful will swell up to many times it size and provide moist evaporative cooling for the wearer.
Florists also use polymer crystals as a dirt-free way to store water and to keep cut flowers fresh for a long time.
It takes about 45 minutes to hydrate the typical headband or collar. Refrigeration ahead of time is not necessary, but it does help extend the cooling time. The same polymer crystals can be rehydrated many times. The following picture shows how the same crystals appear (several hours later) after about an ounce of water was poured on the crystals: