05-12-2022 | By Liam Critchey
There are many natural processes and systems that exist today and perform highly specialised and highly optimised functions that scientists could only dream of emulating on a synthetic level. Natural selection and evolution have resulted in nature being the best scientist, and many scientists and engineers take inspiration from nature when building new innovations—be it medicines or electronics.
On the electronics side, there has been a notable rise in biologically inspired devices in recent years. One example of this is ion rectification devices. Ion rectification is an electrodynamic phenomenon that gives rise to an asymmetric ion flux across a channel, and this ion flux facilitates asymmetric electrical current-voltage characteristics when building electronic devices.
Ionic transport in ion rectification processes often favours one direction over another, and this has led to an interest in harnessing the ion rectification phenomena in diodes. Because biological ion channels use ions as charge carriers and have a unidirectional ion flow to transmit signals, there has been interest in creating ion artificial but biologically inspired ion rectification diodes for human-machine communications applications.
There are several molecular structures and mechanisms that can be utilised to achieve ion rectification. Ion rectification is usually realised when ions become trapped or transported in an asymmetric structure—such as a heterojunction—and this generates non-linear current-voltage (I-V) curves. There are different synthetic materials being trialled for these ion rectification devices, but many have so far focused on conical structures such as nanopores. Another common nanomaterial system is nanochannel heterojunctions.
Ion rectification diodes are also different to electrolytic ionic diodes, as the latter produces rectification using asymmetric faradaic reactions at the electrodes. So, electrolytic ionic diodes are technically classed as electron-based rather than ion-transport-based. Polyelectrolyte ionic diodes (PID) are seen as the stand-out device for ion rectification systems, where the rectification performance is governed through an ionic double layer and behaves similarly to a conventional p-n junction that controls electron transportation—but in this case, the double layer controls the movement of ions rather than electrons. PIDs are now being looked at for mimicking biological ion channels in communications applications.
There are many instances of ionic transport in nature. Our cells have multiple protein channels and ion pumps where different ions—such as sodium, potassium, calcium, and chlorine—are passed in and out of the cell to perform essential functions, such as producing non-linear signals that can be used to transmit information between cells. One such example is where voltage-gated ion channels are responsible for the sodium ion influx in sweet, bitter, and umami taste bud cells.
Synthetic ion-transport devices are now being developed to control ionic charge carriers. They are possible thanks to scientists emulating the information transmission process found within biological systems—where the diffusion and movement of ions can be controlled by manipulating the ion concentration to generate, transmit and store signals. Ion-based transport systems have a better chance of communicating with biological systems compared to electron-based solid-state systems because they use the same communication and data transfer principles.
However, in biological systems, the ions are evenly distributed between the inside and the outside of the cell when it is in a resting state, and the ions are subsequently transported via an electrochemical gradient. The unidirectional ion transport is triggered by a stimulus, and the channel performs a diode-like rectifying behaviour using ionic charge carriers. It’s thought that this rectification process could be mimicked for artificial devices. As it stands, synthetic ion rectification systems do not directly mimic biological systems, as synthetic systems rely on utilising molecular surfaces that have a fixed charge, but there is interest in finding ways of not relying on these charged surfaces.
While the dependence on a charged surface is an undesirable factor for these devices (especially from a biomimicry perspective), there are some other issues with other PID rectification diodes. One aspect is that many of the polyelectrolytes used are hydrogels, so easily evaporate and narrow the electrochemical window of the device. Additionally, some of the elastomers used in the polyelectrolyte have poor ionic conductivities, and the dependence on nanopores/nanochannels to facilitate an ion rectification mechanism means that processing and fabrication costs of these devices would be high if they made it to larger scales, making them less commercially feasible—even for higher-tech applications.
The combination of issues regarding the performance and economic feasibility of the common polyelectrolytes, alongside their biological incompatibility, has prompted new research into ion rectification diodes and new polymeric materials/polymeric mixtures that could offer better overall results.
To better mimic the mechanisms of ion transport in biological systems, the researchers designed and fabricated an ion rectification diode using a gel polymer electrolyte using PMMA and PVDF-HFP polymers. This gel-based electrolyte enabled ion rectification to occur via ion migration/diffusion and offered a much better way of emulating natural ion channels.
To overcome the stability issues (especially around evaporation), the polymers in the gel were selected because of their high boiling point, low volatility in organic solvents, and hydrophobic properties. The polymers were then combined with an the electrochemically stable ionic liquid, salts, and organic solvent. This approach enabled a thermally stable electrolyte to be created that did not rely on electrochemical redox reactions.
In the ion rectification diode, both positive and negative ions are mobile and can freely diffuse around the channels without being influenced (attracted or repelled) by any charged surface. By utilising the different diffusion and migration rates of the ions within the gel polymer electrolyte heterojunction, a preferential ion flow could be generated within the device, leading to ion rectification phenomena manifesting.
To overcome the stability issues (especially around evaporation), the polymers in the gel were selected because of their high boiling point, low volatility in organic solvents, and hydrophobic properties. The polymers were then combined with an the electrochemically stable ionic liquid, salts, and an organic solvent. This approach enabled a thermally stable electrolyte to be created that did not rely on electrochemical redox reactions.
Electrical tests performed on the diode showed a high rectifying ratio of 23.11 and showcased the ability for the electrolyte to operate in a wide temperature window from −20 °C to 125 °C. Electrochemical impedance spectroscopy cyclic voltammetry confirmed the absence of electron-based redox reaction and showed that the device was performing purely using an ion-transport mechanism (not electron-based).
After the electronic tests showed promising results, the ionic rectification diodes were combined with resistors and used to construct logic gates for signal communication systems. The system was further enhanced using a triboelectric nanogenerator to harvest energy from the local surrounding environment. It was further postulated that rectification of the triboelectric nanogenerator could also be possible, which could potentially lead to the creation of synaptic devices with the ion rectification diode system.
Overall, the diode produced uses a new gel-based electrolyte that is cheaper to produce than more complicated channels and has a much higher thermal stability and temperature tolerance than water-based ionic diodes. There’s a lot of potential for human-machine interface applications, especially as synaptic devices are theoretically possible with these systems. Still, the key feature of these devices is that they operate on the basis of non-faradaic ion diffusion and migration—and it is this mechanism that mimics the biological processes found within our cells and is the mechanism that could make these devices much more compatible with biological systems.