With the growing human population and accompanying contamination affecting freshwater sources, access to clean water is becoming a problem. Devices that use sunlight to clean dirty water can only produce a few gallons a day. But now, researchers have found a solution inspired by loofah sponges that could purify enough water to meet a person’s daily needs, even on cloudy days.

Sunlight-driven evaporation has been suggested as a low-energy way to purify water, but it doesn’t work well on cloudy days. One solution is temperature-responsive hydrogels made of poly(N-isopropyl acrylamide) (PNIPAm) that switch from absorbing water at cool temperatures to repelling it when heated. However, because of their closed-off pores, conventional PNIPAm gels can’t purify water fast enough. Loofah sponges, which many people use for exfoliating in the shower, have large open pores. Researchers wanted to replicate the loofah’s structure in a PNIPAm-based hydrogel to rapidly absorb and release purified water when heated by the sun’s rays, even under cloudy conditions.

Researchers made a PNIPAm hydrogel with an open pore structure like a loofah and coated its inner pores with polydopamine and poly(sulfobetaine methacrylate). When tested with artificial light equivalent to the sun’s power, it absorbed water at room temperature and released 70% of its stored water in 10 minutes, four times faster than a previously reported absorber gel. Under lower light conditions, replicating partly cloudy skies, it took 15 to 20 minutes to release a similar amount of stored water.

Finally, the new loofah-like material was tested on samples polluted with organic dyes, heavy metals, oil, and microplastics. In all tests, the gel made the water substantially cleaner. For example, in two treatment cycles, water samples with around 40 parts per million chromium were absorbed and released with less than 0.07 parts per million chromium, the allowable limit for drinking water. The researchers say the unique hydrogel structure could also be useful in other applications like drug delivery, smart sensors, and chemical separations.