After cracking the Enigma code to turn the tide of World War 2 and laying the groundwork for the development of computers, Alan Turing published a paper on biochemistry. Sixty-six years later, the ideas he proposed are being harnessed to fight a modern war against the lack of clean drinking water putting millions of lives at risk.
Turing's paper proposed how two chemicals, one of which triggers a reaction while the other inhibits it, could create complex structures. This process, called morphogenesis, explained the way zebras develop stripes and the way leaves wrap around a stem. Four years ago, the theory was confirmed experimentally for the first time. Although the role of morphogenesis in the development of body shape is still debated, there is evidence it is essential to our formation of fingers and toes.
A team at Zhejiang University, China, applied Turing's work to the production of polymer membranes. In Science, they announce that through this approach they were able to produce a membrane that removes salt from seawater better than existing alternatives.
Morphogenesis occurs when the inhibitor diffuses more easily than the activator, leading to a process described as “local activation and lateral inhibition”. We see this effect on leopards' coats, where the activator produces dark hairs around an initiation site but the inhibitor suppresses this further away, leaving other parts yellow.
Dr Lin Zhang and colleagues produced a thin polymer from the molecules piperazine and trimesoyl chloride, but added molecules that limit piperazine's diffusion. The result was porous polymer membranes, some of which had spotty patterns a few billionths of a meter wide, while others were striped rather than the even product seen without the anti-diffusion molecules. When seen in three dimensions, the authors describe the activated areas as “bubble or tube-shaped”. Membranes produced in this way have about half the average thickness of existing alternatives.
Importantly, these Turing-structured membranes let water through much more easily (while blocking the majority of salt) than those currently used for reverse osmosis, with the tubes easily beating the bubbles. Consequently, they out-performed existing membranes in turning saltwater fresh. Investigations revealed this was because of high permeability sites within the membranes.
Whether it will prove practical to mass produce membranes in this manner and scale production up to what is needed remains to be seen. Potentially, however, this work could provide a lifeline for cities threatened by shortages of drinking water and for drought-stricken farmers.
As climate change shifts rainfall patterns and leaves some regions high and dry, ideas for better desalination are emerging to fill the need. Whether membranes based on Turing's work prove the best option remains to be seen, but clearly we haven't finished mining his astoundingly broad contributions.