The salt in the hydrogel crystallizes
when dry. Credit: Andrew Brodhead
Scientists
in recent years have sought to efficiently draw moisture from ambient air and
condense it into potable water using materials made of salt and absorbent
polymers. But these materials, known as hydrogels, until now have degraded too
quickly to be practical or cost-effective.
Researchers have now discovered a way to
harvest water from air using solar power and a hydrogel that lasts for eight
months or more. Attached to metal coated to prevent corrosion, the long-lasting
material can produce water at low cost almost anywhere.
"There
are a lot of people who don't have access to water or have to walk hundreds of
hours per year to procure water," said Carlos Diaz-Marin, an assistant
professor of energy science and engineering in the Stanford Doerr School of
Sustainability and co-lead author of the research published May 7 in Nature Communications. "There are also
very water-intensive industries like semiconductor manufacturing and data
centers that are putting even more pressure on water systems. We believe this
could potentially be a way to provide additional water resources."
"These
new hydrogels are exceptionally exciting because they give us a way to produce
potable drinking water in really extreme conditions," said co-lead author
Chad Wilson, who worked on the hydrogel as a graduate student at the
Massachusetts Institute of Technology.
Until
now, materials known as hydrogels have degraded too quickly to be practical or
cost-effective for producing clean drinking water. Credit: Andrew Brodhead
In previous work, published in 2025, the team brought a hydrogel device—a square about cookie-sheet-size with a metal
frame—to Chile to test it in the Atacama Desert, one of the world's driest
places. They used a hydrogel made of the superabsorbent salt lithium chloride
and a polymer commonly used in diapers, polyacrylamide.
Even in the Atacama Desert's
parched environment, the hydrogel filled up with water overnight. The
researchers used a sheet of aluminum painted black to absorb the sun's heat and heat
up the hydrogel. As it warmed during the day, the hydrogel released water as
vapor, which could then be condensed back into liquid water and collected for
drinking. The gel proved highly absorbent, holding between two to four times
its weight in water.
While the gel was effective at
attracting moisture even where it's scarce, the researchers soon found a
problem. It lasted only about 30 cycles of filling up and releasing water
before it degraded. This is a problem not only for producing water cheaply, but
also for safety. "Any degradation could make either the salt or the
polymer go into the condenser," said Diaz-Marin. "That would
basically destroy the potability of the water."
More than half a million U.S.
households lack access to running water. One in four people globally lack
access to safe drinking water.
Building a stable sorbent
Through lab experiments over the
past four years, the researchers have investigated how the hydrogel breaks
down. They found that problems arise from the gel's contact with a metal
surface, such as the painted foil in the Atacama Desert experiment. The metal
casing is key to powering the water-harvesting process with heat from the sun,
but it also releases ions that form radicals in the hydrogel and attack the
polymer's long chains. The gel turns to goo as bits of polymer leach into the
water.
"The radicals are very
efficient at eating the polymer away," said Diaz-Marin. "To our
knowledge, nobody had thought of durability and degradation of these materials,
despite it being a critical parameter for water production."
The researchers tested
interventions to block the metal ions. When they applied an anti-corrosion
coating to the metal, the hydrogel's lifespan dramatically extended. In one
test, the hydrogel remained stable for more than eight months while kept at
167°F, a temperature meant to stress-test the material under extreme
conditions. The researchers also found the hydrogel on coated metal remained
stable for more than 190 water-harvesting cycles.
Durable hydrogels, cheap water
This level of durability advances
the hydrogel toward producing water at a competitive cost. The improvements
"could let us get to a point where we produce water at maybe one cent per liter," said Diaz-Marin. This would be about 1% of the
cost of bottled water and about 10 times the rate U.S. households pay for tap
water. "We see a path to this technology to perhaps even being competitive
with tap water."
At the right price, a future
hydrogel-based water system could bring potable water to rural communities
facing water shortages in arid inland regions, where other technologies such as
desalination are not an option. Since it's solar-powered, it doesn't need a
grid connection and would have a minimal environmental footprint compared to
water that needs to be pumped or trucked in.
It's not ready to supply
communities just yet, but the team is optimistic. Diaz-Marin and his students
are now working to further improve efficiency and cost. Their current design
can produce up to two liters, or a little over half a gallon, of water daily
with a thin layer of material spread over a panel roughly the size of a bath
towel. That's around the amount of water generally needed per person per day to
maintain basic health during emergencies.
Diaz-Marin said his goal is to
increase the output to five liters daily. "Especially being at
Stanford," he said, "I could see us translating this into the world
either by a startup or licensing it." In the not-so-distant future, we could
be sipping sky water.
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