By measuring how the electric current of an ionic fluid is generated by the flow of water through ångstrom-sized channels, researchers in France and the UK have discovered that this current is sensitive to an electric field when pressure is applied. This transistor-like electrohydrodynamic effect, as they have explained it, is very similar to that recently observed in biological ion channels, such as PIEZO, and could help advance the emerging field of “iontronics”.
“Modern-day computing relies on electrons to perform calculations, but the circuitry in living organisms is different in that it exploits the transport of ions, such as sodium, chlorine and calcium, through molecular-scale channels,” explains Lydéric Bocquet of the École Normale Supérieure in Paris, who led this research effort together with Radha Boya and Nobel laureate Andre Geim of the University of Manchester. “Achieving this – often exotic – behaviour of ion transport at the nanoscale in artificial channels remains a considerable challenge, however.”
Ångström-scale channels
The researchers obtained their result by studying ion transport though ångstrom-scale channels made from two (roughly 10-nm and 150-nm) thin crystals of graphite or boron nitride separated by bilayer graphene strips on a silicon/silicon nitride substrate. The channels are assembled atop a micron-sized slit etched in the substrate, which serves as the opening of the fluidic channel, with its exit being on the other side of the wafer. These channels were developed by Boya and Geim.
Bocquet and colleagues then connect the channels to two macroscopic reservoirs filled with solutions of potassium chloride containing chlorinated silver/silver chloride electrodes. “We measure the change in ionic current using these electrodes as it flows through the channel while applying pressure drops and an applied electric field along the channel,” says Bocquet. “This so-called patch-clamp technique is similar to that employed in physiology experiments since it can measure minute electric currents.”
Indeed, the researchers say that the set up allows them to measure the pressure-driven component of the ionic current, known as the streaming current, which is an indirect measure of how water flows when confined in extremely narrow channels.
Pressure-driven flow is a supplementary parameter
“It is not easy to explain the effect we have observed using a simple physical picture,” Bocquet tells Physics World, “but it does have similarities with how electrons flow in Shockley diodes and field-effect transistors. The difference in our system is that there is pressure-driven flow as a supplementary parameter, which does not exist in electronics. This flow is probably key to iontronics.”
Frictionless flow in 2D channels
The finding was made possible thanks to the “ultimately thin” channels that Boya and Geim developed because it is only at these extremely small scales, typical of biological channels, that fluid and ionic transport does not fit the classical framework of hydrodynamics, says study lead author, Timothée Mouterde. “This exotic behaviour opens up a whole new world for fluid transport.”
Such devices are ideal platforms in which to mimic the behaviour of biological channels in which ions are driven though natural nanoscale channels under osmotic pressure and bioelectric potentials, add the researchers, reporting their work in Nature. “Studying such systems could help us better understand biological ionic channels such as TRAAK, TREK and PIEZO, which were recently discovered to be sensitive to pressure,” explains Mouterde. “The effect we have discovered could be the first step to assembling more advanced functions for iontronics inspired by these natural structures.”
