Cell membrane biology inspires design of new saltwater filters

Researchers at the Francis Crick Institute, King’s College London and the University of Fribourg have developed polymer water channels, similar to commonly used plastics, that can draw salt out of water, inspired by the body’s own water filtering system.

If their innovation could be scaled up and produced industrially, this could help to filter seawater to create drinking water.

Aquaporins are proteins that rapidly transport water across cell membranes while excluding salt. They are critical for maintaining the right balance of water inside and outside cells and for concentrating or diluting urine in the kidneys.

In research published recently in Angewandte Chemie International Edition, an international team of researchers took inspiration from aquaporins to design artificial water channels that can be used to filter salt out of water.

These could be used for a process called desalination, where salts and minerals are removed from seawater to produce water safe for drinking.

This process, which usually involves heating the water, condensing the vapor and leaving the salt behind, is regularly used in areas such as the Middle East, where rainfall is low. But it is energy intensive and very expensive, and so cheaper alternatives are needed.

To address this challenge, the team developed artificial channels using long molecules of plastic, organized into a helix structure called polymers, or into cyclic structures called macrocycles.

The pores inside the two types of channels were filled with a chemical mixture of fluorine and molecules called hydrocarbons, which together create a greasy layer.

The channels were then tested inside vesicles, sacs of water contained in a greasy layer, similar to cells, which were then placed in a sugary solution.

Water moves out of vesicles naturally by osmosis, even without water channels, but this process is slow. But when the artificial channels were added to the vesicles, water movement increased in speed and volume.

The team also tested different lengths of polymers, finding that the most efficient water channel was the same length as the vesicle’s greasy layer, and nearly matched the performance of biological aquaporins.

Finally, the team confirmed that salts couldn’t cross the channels by testing the most efficient polymer in a salty solution. They saw that salts didn’t cross into the vesicle.

Charlie McTernan, Group Leader of the Artificial Molecular Machinery Laboratory at the Crick and Senior Lecturer at the Department of Chemistry at King’s College London, said, “Many systems in nature, like the body’s ability to filter water using aquaporins, have evolved to be very efficient at their job, so are hard to mimic artificially. But with some clever chemistry, we’ve achieved efficiencies close to those of natural aquaporins with our artificial water channels, which can effectively transport water and remove salts.

“We’re experiencing climate warming and potentially a lack of available drinking water, so cheaper and more scalable solutions to filter seawater are crucial. Our work could be scaled up for industrial use, but there will be challenges in manufacturing at scale to overcome, including testing with more stable membranes than those used in this research. We’d also need to see how the channels interact with other substances in seawater.”

Javid Ahmad Malla, postdoctoral researcher in the Artificial Molecular Machinery Laboratory at the Crick, said, “This collaborative project came about through a chance meeting at a conference—our team at the Crick and the lab in the University of Fribourg realized we could bring together our joint expertise to approach this real-life problem. We’re now trying to maximize our channels’ efficiency so we can even beat natural aquaporins.”