A close-up of the two-layer material for passive cooling; the upper layer consists of aerogel and the bottom layer of hydrogel. Photo courtesy of Jeffrey Grossman, et. al.
A close-up of the two-layer material for passive cooling; the upper layer consists of aerogel and the bottom layer of hydrogel. Photo courtesy of Jeffrey Grossman, et. al.

Camels have evolved a seemingly counterintuitive approach to keeping cool while conserving water in a scorching desert environment: they have a thick coat of insulating fur. Applying essentially the same approach, researchers at Massachusetts Institute of Technology (MIT) have now developed a system that could help keep things like pharmaceuticals or fresh produce cool in hot environments, without the need for a power supply.

Most people wouldn't think of wearing a camel-hair coat on a hot summer's day, but in fact many desert-dwelling people do tend to wear heavy outer garments, for essentially the same reason. It turns out that a camel's coat, or a person's clothing, can help to reduce loss of moisture while at the same time allowing enough sweat evaporation to provide a cooling effect. According to the researchers, tests have shown that a shaved camel loses 50% more moisture than an unshaved one, under identical conditions.

The new system developed by MIT engineers 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 composed 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.

Hydrogels are already used for some cooling applications, but field tests and detailed analysis have shown that this new 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 findings are reported today in a paper in Joule by MIT postdoc Zhengmao Lu, graduate students Elise Strobach and Ningxin Chen, research scientist Nicola Ferralis, and Jeffrey Grossman, head of the Department of Materials Science and Engineering.

The researchers say this 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 goods experience, eliminating spikes that can accelerate spoilage.

Ferralis explains that such packaging materials could provide constant protection of perishable foods or drugs all the way from the farm or factory, through the distribution chain, to the consumer's home. In contrast, existing systems that rely on refrigerated trucks or storage facilities may leave gaps where temperature spikes can occur during loading and unloading. "What happens in just a couple of hours can be very detrimental to some perishable foods," he says.

The basic raw materials involved in the two-layer system are inexpensive – the aerogel is made of silica, which is essentially cheap and abundant beach sand. 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. But at least one start-up company is already working on developing such large-scale processing to use the material to make thermally insulating windows.

The basic principle of using the evaporation of water to provide a cooling effect has been used for centuries in one form or another, including in double-pot systems for food preservation. These use two clay pots, one inside the other, with a layer of wet sand in between. Water evaporates from the sand out through the outer pot, leaving the inner pot cooler. But the idea of combining such evaporative cooling with an insulating layer, as camels and some other desert animals do, has not really been applied to human-designed cooling systems before.

For applications such as food packaging, the transparency of the hydrogel and aerogel materials is important, allowing the condition of the food to be clearly seen through the package. But for other applications, such as pharmaceuticals or space cooling, an opaque insulating layer could be used instead. This would provide even more options for the design of materials for specific uses, says Lu, who was the paper's lead author.

The hydrogel material is composed of 97% water, which gradually evaporates away. In the experimental setup, it took 200 hours for a 5mm-layer of hydrogel, covered with 5mm of aerogel, to lose all its moisture, compared to 40 hours for the bare hydrogel. The two-layered material's cooling level was slightly less than the bare hydrogel – a reduction of 7°C (about 12.6°F) versus 8°C (14.4°F) – but the effect was much longer-lasting. Once the moisture is gone from the hydrogel, the material can be recharged with water so the cycle can begin again.

Especially in developing countries, where access to electricity is often limited, such materials could be of great benefit. "Because this passive cooling approach does not rely on electricity at all, this gives you a good pathway for storage and distribution of those perishable products in general," Lu says.

This story is adapted from material from MIT, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.