Piezoionic Currents: Mechanical Deformation of Hydrogels Induces Ionic Currents that Stimulate Nerves

The nervous system operates at tens of millivolts and employs ionic currents. Similar voltages and ionic currents are generated when hydrogels are compressed. This opens the possibility of stimulating the nervous system directly using the electrical energy produced when ionically conductive materials are deformed. To demonstrate this effect, a finger was firmly tapped on a slab of polyacrylamide hydrogel containing 1.5 M of sodium chloride solution. The resulting current was fed into PEDOT-coated Pt-Ir electrodes wrapped around a rodent peripheral nerve, The mechanical input produced currents of 10 μA at voltages of less than 100 mV. Hhindlimb twitches were produced. This 'self-powered' sensing interfaces with nerves without any signal conditioning or amplification. We dub the underlying mechanims "piezionic", in loose analogy to piezoelectrics. A pressure gradient applied to the hydrogel produces a flow of electrolyte and ions within it. Some ions displace faster than others as they translate through the nanopores of the hydrogel, likely due to differences in size between anions and cations. Similar effects are seen in other ionically conducting polymers including conjugated polymers and ionically conductive membranes. Response times can be quite fast (~30 ms) when thin gels are used, which create large pressure gradients around the indentation area, and fast transport. The piezonionic effect creates the possibility of a fully ionic artificial mechanoreceptor that can interface with the nervous system - though long distance, high density signal conduction may be best achieved using thin metal wires.