21-04-2020 | | By Liam Critchey
There is a saying that “nature is the world’s greatest scientist”. This statement is true, and millions of years of evolution have created highly complex systems that can perform a number of functions that humans only wished that they could replicate. After all, nature has been evolving for millions of years and over this time has found the most optimal way of functioning in many ways. So, it should be of no surprise that many scientists and engineers take inspiration from nature and try to create man-made objects which try to replicate nature but are adapted so that they are suitable for human use. Some notable examples over the years include water repellent sprays and self-cleaning surfaces, but even electronics can take inspiration from the biological environment that is all around us.
Modern-day electronics wouldn’t be able to function without the integrated circuit and it has become a ubiquitous component of any device nowadays. However, where our electronics rely on a number of diodes and transistors to manipulate and amplify a signal around a circuit board, living organisms within our natural environment rely on a completely different way of amplifying a signal. This biological ‘circuit’ is ionic in nature and relies on the transport of different chemical ions in a completely aqueous (water-based) environment.
These ionic processes, much like our integrated circuits, are a ubiquitous mechanism in many different species and are used to help cells function effectively. The backbone of the biological circuits are the ion channels which are found with the cell membrane. These channels are used to move ions in and out of the cell and is a very important process that ensures that the cell regulates itself.
Where this is of interest is that this process is the first step in a ‘biological amplification’ process, as these ionic processes enable different external stimuli to be observed by the body. For example, sound waves are converted – by hairs on the cochlea inside the ear – into ionic currents. These ionic currents are formed because the ionic channels open upon sensing the sound wave, which sends millions of ions through the channel, generating an amplified signal that can be detected by the brain. Other examples include when the nose detects a smell or the eyes detect light, and these amplification principles can be applied to man-made electronic circuits.
There have already been a number of man-made ionic circuits created which have utilised aspects of bioelectronics. However, it has been hard to create a circuit which is optimal on many fronts. The first circuits made in this way were very basic in nature and used chemically modified proteins that acted as ionic diodes.
One of the more promising approaches has been to use organic electrochemical transistors (OECTs), as they can amplify small signals into a large current using either conductive polymers or organic semiconductor materials. However, while OECTs have enabled efficient communication to take place between ionic and electronic devices, they do not provide an amplification that is ionic in nature. So, one of the next steps in man-made ionic circuits has been to create circuits that can amplify the ionic signals in the circuit so that they become ionic outputs, rather than relying on the amplification and transport of electrons and holes found in traditional circuitry. Like electrons and holes, these ions could then be sent to specific parts of the circuit, so that the circuit performed a specific function.
In recent years, some devices have been created that can amplify an ionic signal, such as individual microfluidic ionic transistors, however, high voltages had to be applied across these devices to get it to function effectively, so researchers have been looking at ways of creating similar devices that work at much lower voltages, and Zuzanna Siwy and colleagues in the U.S. have managed to do so by creating ionic transistors that are based on nanopores embedded within films rather than microfluidic channels.
This ionic amplifier circuit created by the researchers was found to be both tunable and capable of amplifying a small ionic signal into an ionic output. The tunability in the device was found to be due to the modular nature of the circuit and uses both inputs and outputs that are ionic in nature. The circuits themselves were created by etching the nanopore transistor channels into polymer-coated silicon nitride chips and were inspired by both conventional amplifier technology and biological ionic pathways to only channel selective ions through the pore.
Each of the pores was connected to three terminals, including a gate that could enhance or deplete the movement of ions within the pore. All the pores were connected to a multicomponent circuit and the overall approach used is similar to that of a Darlington Amplifier composed of transistors – where one of the transistors was paired with another, so the output from the first transistor becomes the input for the second transistor, amplifying the signal in the process.
One of the remarkable features of the device over the status quo is that it functioned at voltages less than 1 V – which in turn makes it compatible with biological signals – as well as being able to respond to sub-nanoampere gate currents, all while offering a large amplification gain of up to 300.
While there have been a number of ionic circuits created to date which use a number of fabrication methods, component architectures and materials, the ability of this device to amplify and transport an ionic output at a low voltage represents a significant improvement over similar devices to date. While ionic circuits are not at a place where they will replace conventional electron/hole integrated circuits, their continued advancement could contribute to future chemical and biochemical sensing devices, as well as in amplification devices. Their development is something that could complement conventional electronics in the future, not replace them.