Bakajin says this has been tried with more conventional membrane materials, but adding compounds for attracting carbon dioxide decreases the permeability of these membranes to the point that they are no longer practical. The extraordinarily high permeability of carbon nanotubes could help with this problem. "We have a lot of permeability to lose," she says. "If the permeability goes down as much as with other membrane materials, we're still fine."

She says the company has identified several promising candidates for modifying the nanotubes, but says the details are proprietary. In addition to selecting one of these, she says, the company is also working out how best to manufacture the carbon nanotube membranes, which includes deciding what material to use to bind the nanotubes together and serve as a support material. "Some have advantages in fabrication, some are better structurally, some are more resistant to harsh environments," she says. "The more we do it, the more we think of new things to try."

Bruce Hinds, a professor of chemistry at the University of Kentucky who has also demonstrated the high permeability of nanotube membranes, isn't convinced that carbon capture is the best use for these membranes, in part because of the challenge of making carbon nanotube membranes selective for carbon dioxide. He's starting with pharmaceutical applications--such as using the membranes to deliver drugs or to separate chemicals during drug manufacturing. These don't require large-scale manufacturing, which is good, since large-scale manufacturing of the membranes hasn't been demonstrated yet. The drug applications also command higher prices, allowing for more expensive materials.

Porifera is also pursuing other potential applications. It recently announced funding from DARPA, the research and development office for the U.S. Department of Defense, for producing portable desalination systems for soldiers. Carbon nanotubes can transport fluids 1,000 times as fast as conventional membranes. In addition to saving energy, such fast transport makes it possible to use much smaller membranes, which are better suited for portable devices.