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Spotted: Water is a necessity, but it takes a good deal of energy to filter and purify it. This can be a problem, especially in areas where both clean water and energy is either hard to come by or expensive. One possible solution is an invention that uses sunlight to drive water purification. This was inspired by the pufferfish, which takes in water to swell its body when threatened, and then releases it when danger passes.

Developed by researchers at Princeton University, the device resembles a large sponge made from gel, which soaks up water but leaves behind contaminants. The purified water is then collected by placing the sponge on top of a container and leaving it in the sunlight. As the sun warms the gel, pure water trickles into the container.

At the heart of the new device is a gel that changes depending on temperature. At room temperature, the gel can act as a sponge, soaking up water. When heated to 33 degrees Celsius (91 degrees Fahrenheit), the gel does the opposite – it pushes the water out of its pores.

The gel contains a honeycomb-like structure of long chains of repeating molecules that are cross-linked to form a highly porous mesh. Some regions of the mesh are hydrophilic (‘water-loving’), while other regions are hydrophobic, or water-repelling. At room temperature, the chains are long and flexible, and water can easily flow into the water-loving regions. When the sun warms the material, the hydrophobic chains clump together and force the water out of the gel.

This gel is surrounded by two layers that filter out contaminants but allow sunlight to heat up the inner material. Xiaohui Xu, postdoctoral researcher and lead author of the paper on the gel, explains the benefits of the gel, saying “Sunlight is free, and the materials to make this device are low-cost and non-toxic, so this is a cost-effective and environmentally friendly way to generate pure water.”

As both water and energy shortages loom, sustainable and affordable water purification is becoming an important concern in a wide number of regions around the world. We have seen this echoed in a number of recent innovations, including the use of algae tiles to purify water in India and a device that uses solar power to desalinate seawater.

Written By: Lisa Magloff

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