Clean water for drinking, for agriculture and for sanitation has become an increasingly precious resource, primarily as a consequence of climatic changes and of overpopulation. The challenge has to be tackled from different fronts, including improved infrastructures for efficient distribution as well as adequate behaviour to avoid wastes. Technology also plays an essential role, for example clean water can be obtained by removing salt from seawater or brackish water.

Artistic impression of water flowing through the artificial water channels studied by Song et al. Credit: Erik Zumalt, The University of Texas at Austin

Reverse osmosis is perhaps the most popular desalination process. Salty water gets pushed through a porous membrane at high pressure and the membrane captures salt while letting water permeate through. In simple terms, the efficiency of a membrane can be measured by comparing how much water can permeate and how much salt can be rejected. Unfortunately, permeability and selectivity tend to compete against each other. The best results can be obtained with thin-film composite (TFC) polymer membranes, which are in fact used commercially, and despite efforts to look at other materials, so far no definitive improvement has been demonstrated.

In an Article published in this issue, Woochul Song et al. report promising results using membranes based on artificial water channels. Membranes mimicking aquaporins have been investigated before. The new aspect of this work is that the water does not flow through static channels created in the membranes. Rather, the nanoarchitecture that forms the membranes results in the formation of a network of water wires that allows a permeability similar to that of natural aquaporins and a very high salt selectivity, enough to predict better performance than TFCs, once scaled up.

Of course, scaling up may end up being a big challenge. But the essential aspect of the work is that it provides a clear example of a case in which — when a technology reaches its limits — the best way forward is to go back to the drawing board and explore issues from a fundamental perspective. We have reason to think that this type of approach is becoming more urgent in more branches of nanotechnology, including, for example, medical and energy applications. In this specific case, whether the membranes investigated will themselves end up in commercial applications is secondary to the potential inspiration for new ways to design membranes that the results may inspire.