Skinny nanotubes break aquaporin’s record for moving water

The membrane protein known as aquaporin, which shuttles water in and out of cells, has long held the record as the most efficient water-trafficking system. Now, scientists have discovered one that’s six times as fast: a membrane full of skinny carbon nanotubes. The finding could lead to desalination membranes that resist biofouling and operate more efficiently.

The nanotubes in the new system are just 0.8 nm in diameter, so water molecules must slide through them single file—a process that speeds the molecules’ transport (Science 2017, DOI: 10.1126/science.aan2438). Aquaporin also makes water travel single file, but amino acids on the inside of its channel hydrogen bond to the water molecules and slow their transit compared with those sliding along the molecularly smooth nanotubes’ interior.

“There is always interesting physics to observe by just looking at how things are done in nature,” says Aleksandr Noy, the Lawrence Livermore National Laboratory scientist who led the research. “Then you can go back to the lab and use that information to design new systems and materials.” Noy says that aquaporin was his inspiration to create the skinny-nanotube system and that it “was always the molecule to beat.”

“What is most exciting,” says Zuzanna Siwy, an expert in nanopore physics at the University of California, Irvine, is that Noy and coworkers “have identified a man-made system that is even more efficient than the biological system of aquaporin.”

Karl Johnson, a chemical engineering professor at the University of Pittsburgh who studies carbon nanotubes for desalination, says that “while this work is of tremendous scientific interest, I do not see how it provides a pathway for constructing membranes that can actually be used for large-scale water desalination and other applications.”

Siwy is more optimistic. There are obviously challenges, she says, such as making desalination membranes of sufficient area and with enough nanotube pores, but she points out that carbon nanotubes are amenable to different types of processing, unlike delicate aquaporin, and should therefore be strong contenders for desalination membrane components.