Artificial Mechanoreceptors from Hydrogels—Capacitive, Triboelectric and Piezoionic Sensors for Ionic Skin and as Interfaces with the Nervous System

Millivolt potentials and nanoampere ionic currents are the signals at the heart of our sensory system, and our nervous system in general. Can we make use of ionic signals in artificial sensors and neural interfaces? Here we discuss three versions of artificial ionic sensors.<br/><br/>In the first, the hydrogels are used as ionic conductors in capacitive or piezoresistive sensors. These are low power, soft sensors that build on the ionic skin concept developed almost a decade ago - combining mechanical toughness, transparency and compliance. Here there is no need for hard electronics or wires, except to deliver and record electronic signals. The interface between the ionic and electronic, and the wet nature of the hydrogels, are challenges - but there are solutions. The scalability of these approaches to create skins containing arrays of sensors is discussed, and compared with implementations using conductive elastomers.<br/><br/>In the second approach, change in surface contact between a hydrogel and a conductor or other charged interface, can lead to the generation of significant voltages. Hydrogel toughness and deformability, combined with mobile salt content, provide advantages for creating compliant sensors or even generators. Potentials at the level of volts or higher can be generated, making detection simple. Charge density and electromechanical coupling are relatively small, so generators will need to operate at high frequencies to run electronics. They rely on an electronically conductive interface, but can be self-powered (i.e. no input electrical energy is needed - the energy from the mechanical work done is sufficient).<br/><br/>A third sensing approach uses motion of ions induced by pressure gradients. This can include streaming, in which ions of one polarity are carried by moving solvent, producing currents and charging. In general, what can be called a piezoionic effect is involved - where there is an effect of pressure on the motion of ions. This is akin to piezoelectrics, except that it relies on a gradient, and charges are mobile rather than bound into a crystal. In hydrogels with both mobile anions and cations, such as Na+ and Cl-, the net current produced may be simply a result of anions being effectively larger than cations, experiencing more drag as they move through the hydrogel, and leading to a net current as the cations flow more freely. Voltages are small - typically millivolts. But charge can be large. And responses can be surprisingly fast - in the same range as our mechanoreceptors. The net effect can stimulate a nerve and lead to muscle contraction, fully self-powered. This transduction and transmission should be possible without the use of any metallic conductors, though this has yet to be shown, and our demonstration uses fine wires. Wires allow the signals to be transferred over larger distances, due to the much lower resistivity of metals than ionic conductors. As with any interface between electronics and tissue, there are electrical and mechanical impedance mismatches. These can be managed using conducting polymer coatings that absorb enormous concentrations of ions, or, perhaps, by avoiding the use of electronics completely. To expand the possibilities of ion only sensors, an active ionic system like that provided by our sodium/potassium ion pumps, is likely needed.