Camels keep cool while conserving water in a scorching desert environment via a thick coat of insulating fur. Applying essentially the same approach, researchers have developed a system that could help keep things like pharmaceuticals or fresh produce cool in hot environments without the need for a power supply.
The camel’s coat, or a person’s clothing, can help reduce loss of moisture while at the same time allowing enough sweat evaporation to provide a cooling effect. Tests have showed that a shaved camel loses 50 percent more moisture than an unshaved one under identical conditions. The new system uses a two-layer material to achieve a similar effect. The material’s bottom layer, substituting for sweat glands, consists of hydrogel — a gelatin-like substance that consists mostly of water — contained in a sponge-like matrix from which the water can easily evaporate. This is then covered with an upper layer of aerogel, playing the part of fur by keeping out the external heat while allowing the vapor to pass through.
The two-layer material, less than a half-inch thick, can provide cooling of more than 7 °C for five times longer than the hydrogel alone — more than eight days versus less than two. The system could be used for food packaging to preserve freshness and open up greater distribution options for farmers to sell their perishable crops. It could also allow medicines, such as vaccines, to be kept safely as they are delivered to remote locations. In addition to providing cooling, the passive system — powered purely by heat — can reduce the variations in temperature that the goods experience, eliminating spikes that can accelerate spoilage.
The basic raw materials involved in the two-layer system are inexpensive — the aerogel is made of silica (essentially beach sand), which is cheap and abundant. But the processing equipment for making the aerogel is large and expensive, so that aspect will require further development in order to scale up the system for useful applications.
The hydrogel material is composed of 97 percent water that gradually evaporates away. In the experimental setup, it took 200 hours for a 5-millimeter layer of hydrogel — covered with 5 millimeters of aerogel — to lose all its moisture compared to 40 hours for the bare hydrogel.