In the last decade, scientists have tried to explore new ways to interface the brain with computers, so modeling new hardware that complies with our biological wetware has become increasingly important. This has led to the constant need to innovate transistors technology, the fundamental components of electronic systems at the base of brain-computer interfaces and which can interact with biological substrates.
Transistors are the constituent elements of circuits capable of performing specific operations such as signal amplification, filtering, detection of signal characteristics and the provision of electrical or chemical stimulation. The key features required for the safe, efficient and prolonged use of transistors in biological environments include:
- biocompatibility and stability of constituent materials;
- the conformability of the substrate to prevent mechanical misalignment with the tissues;
- high speed and amplification to detect potentially low – signal -amplitude at physiologically relevant time scales;
- independent gating to allow integrated circuit calculations.
Although a wide range of transistor architectures and types are available, none incorporates all these features. Conventional silicon-based field-effect transistors (FETs) can be fabricated in multi-electrode matrices to acquire data from neural tissue in vitro and in vivo, but the great mechanical imbalance between Si and biological tissue, as well as its lack of biocompatibility, precludes chronic use in humans due to the risk of neural damage. It happens that the implanted electrodes irremediably activate the immune system of the brain, with consequent formation of scar tissue around the implant site. This problem is partially solved, incorporating the transistors into biocompatible plastics, which are tolerated by our body, but this makes the devices remarkably rigid and bulky, causing a significant reduction in performance.
New experiments in the organic electronics sector have also been conducted to create intrinsically flexible plastic transistors, such as electrolytic or electrochemical transistors that can modulate their production based on ionic currents. However, these devices are not fast enough to perform the calculations required by bioelectronic devices used in neurophysiology applications. In short, we can say so far researchers have not been able to build transistors that have all the features needed for safe, reliable and fast operations in these environments for long periods.
The internal-ion-gated organic electrochemical transistor: a surprising technological innovation
To solve this problem a group of Columbia University scientists led by Dion Khodagholy, assistant professor of electrical engineering at Columbia Engineering, and Jennifer N. Gelinas, Columbia University Medical Center, Department of Neurology and the Institute for Genomic Medicine, has developed a new transistor whose operation is based on ions rather than electrons (as happens in traditional transistors).
It is the first biocompatible, conformable, stable, high-speed, internal ionic organic electrochemical transistor (IGT) with high transconductance, which allows integrated bioelectronics and is fast enough to allow real-time signal detection and stimulation of brain signals.
“Importantly, we only used completely biocompatible material to create this device. Our secret ingredient is D-sorbitol, or sugar,” says Khodagholy. “Sugar molecules attract water molecules and not only help the transistor channel to stay hydrated, but also help the ions travel more easily and quickly within the channel.”
The internal ionic gate organic electrochemical transistor (IGT) operates through mobile ions contained within a conductive polymeric channel to allow both the volumetric capacity (ionic interactions involving the entire mass of the channel) and the reduced ionic transit time. The IGT has a great amplification speed and can be independently gated. In their study published in Science Advances, researchers demonstrate their IGT’s ability to provide a miniaturized, soft and conformable interface with human skin, using local amplification to record high-quality neural signals, suitable for advanced processing some data.
“We’ve made a transistor that can communicate using ions, the body’s charge carriers, at speeds fast enough to perform complex computations required for neurophysiology, the study of the nervous system function,” Khodagholy says. “Our transistor’s channel is made out of fully biocompatible materials and can interact with both ions and electrons, making communication with neural signals of the body more efficient. We’ll now be able to build safer, smaller, and smarter bioelectronic devices, such as brain-machine interfaces, wearable electronics, and responsive therapeutic stimulation devices, that can be implanted in humans over long periods.”
The study also offers excellent prospects for other uses in the medical field. These devices could also be used to create closed-loop implantable devices, such as those currently used to treat some forms of epilepsy or be integrated into devices for the control of muscle movement, cardiac and ocular.
Khodagholy and Gelinas are now exploring if there are physical limits to the type of mobile ions they can incorporate into the polymer. They are also studying new materials in which they can incorporate mobile ions and refine their work on the use of transistors to make integrated circuits for reactive stimulation devices.
With such speed and amplification, combined with their ease of microfabrication, these transistors could be applied to many different types of devices. There is great potential for the use of these devices to benefit patient care in the future
~ Dion Khodagholy, assistant professor of electrical engineering at Columbia Engineering speaking about the future use of the new IGT technology