Fast and flexible ionic transistors for bioelectronic devices

Many major advances in medicine, particularly in neurology, have been brought about by recent advances in electronic systems capable of acquiring, processing and interacting with biological substrates. These bioelectronic systems, which are increasingly used to understand dynamic living organisms and to treat human diseases, require devices that can record body signals, process them, detect patterns, and provide electrical stimulation. or chemical to deal with the problems.

Transistors, devices that amplify or switch electronic signals on circuits, constitute the backbone of these systems. However, they must meet many criteria to function effectively and safely in biological environments such as the human body. Until now, researchers have not been able to build transistors with all the features needed for safe, reliable and fast operation in these environments over long periods.

A team led by Dion Khodagholy, an assistant professor of electrical engineering at Columbia Engineering, and Jennifer N. Gelinas, of the Department of Neurology of the Columbia University Medical Center and the Institute for Genomic Medicine, set out point the first biocompatible transistor controlled by fast ions. to enable real-time signal detection and stimulation of brain signals.

The internal ion organic electrochemical transistor (IGT) operates via mobile ions contained in a conductive polymer channel to allow both a volumetric capacitance (ionic interactions involving the entire channel) and a reduced ion transit time. The IGT has a high transconductance (amplification rate), a high speed and can be independently controlled as well as microfabricated to create adaptable and scalable integrated circuits. In their study published today in Progress of science, the researchers demonstrate the ability of their IGT to provide a miniaturized, flexible and adaptable interface to human skin, using local amplification to record high quality neural signals, suitable for advanced data processing.

"We have manufactured a transistor capable of communicating with the help of ions, the charge carriers of the body, at a speed fast enough to perform complex calculations necessary for neurophysiology, at the study of the function of the nervous system, "explains Khodagholy. "The channel of our transistor is composed of fully biocompatible materials and can interact with ions and electrons, making communication with the neuronal signals of the body more efficient.We will now be able to build safer, smaller, and smaller bioelectronic devices. more intelligent, such as brain-machine interfaces, portable electronics and reactive therapeutic stimulation devices that can be implanted in humans over long periods. "

In the past, traditional silicon-based transistors were used in bioelectronic devices, but they had to be carefully encapsulated to avoid any contact with body fluids, both for the safety of the patient and for the proper functioning of the device. This requirement makes implants based on these voluminous and rigid transistors. At the same time, numerous works have been carried out in the field of organic electronics to create inherently flexible plastic transistors, in particular electrolyte or electrochemical transistors capable of modulating their output as a function of the ionic currents. However, these devices can not operate fast enough to perform the necessary calculations for bioelectronic devices used in neurophysiology applications.

Khodagholy and his postdoctoral researcher, George Spyropoulos, the first author of this work, constructed a transistor channel based on conductive polymers for ion modulation. In order to make the device fast, they modified the material in order to have its own mobile ions. By reducing the distance that the ions had to travel in the polymer structure, they improved the speed of the transistor by an order of magnitude compared to other ion devices of the same size.

"It's important to note that we only used completely biocompatible materials to create this device, and our secret ingredient is D-sorbitol, or sugar," says Khodagholy. "The sugar molecules attract the water molecules and not only help the transistor channel stay hydrated, but also help the ions to travel more easily and faster in the channel."

The IGT can significantly improve the ease and tolerance of electroencephalography (EEG) procedures for patients, the researchers chose this platform to demonstrate the translational ability of their device. Using their transistor to record human brain waves at the surface of the scalp, they showed that local amplification of IGT directly at the device-scalp interface allowed to reduce the size of the contact five orders of magnitude: the entire device easily inserted between the hair follicles. , greatly simplifying placement. The device could also be easily manipulated by hand, thereby improving mechanical and electrical stability. In addition, since the micro-EEG IGT device conforms to the scalp, no chemical adhesive was required, so that the patient did not show any skin irritation due to the adhesives and was generally more comfortable.

These devices could also be used to fabricate implantable closed-loop devices, such as those currently used to treat some forms of medically refractory epilepsy. The devices could be smaller and easier to implement, and also provide more information.

"Our initial inspiration was to make an adaptable transistor for neural implants," notes Gelinas. "Even though we have specifically tested it for the brain, IGTs can also be used to record cardiac, muscular and ocular moments."

Khodagholy and Gelinas are currently studying the possibility of defining physical limits as to the type of mobile ions they can integrate into the polymer. They are also studying new materials in which they can integrate moving ions and refine their work on the use of transistors to realize integrated circuits for reactive stimulation devices.

"We are very excited to dramatically improve ion transistors by adding simple ingredients," says Khodagholy. "With such a speed and such amplification, coupled with their ease of microfabrication, these transistors could be applied to many types of devices.Their use could be very beneficial in the future."

Provided by
School of Engineering and Applied Sciences of Columbia University