Iontronic Implants May Supplant Metal Electrodes
by James Cavuoto, editor
May 2020 issue
Most of the neural implants in use today rely on metal electrodes that either stimulate or record from neural tissue using the flow of electrons at the neural interface. But at least one commercial firm and several research institutions in Europe and North America are looking at implants that exploit the flow of ions, as opposed to electrons. The capacitive current that flows at an ionic interface is much safer and effective than the faradic current produced by metal electrodes.
Dion Khodagholy and colleagues from Columbia University have made considerable progress on ion-driven soft and organic transistors designed to record individual neurons and perform real-time computation. Writing in Science Advances, they demonstrated a soft, biocompatible smart composite—an organic mixed-conducting particulate material—that enables the creation of complex electronic components which traditionally require several layers and materials. It also enables easy and effective electronic bonding between soft materials, biological tissue, and rigid electronics. “Instead of having large implants encapsulated in thick metal boxes to protect the body and electronics from each other, we could do so much more if our devices were smaller, flexible, and inherently compatible with our body environment,” said Khodagholy.
Khodagholy’s team took advantage of both the electronic and the ionic conduction of organic materials to create ion-driven transistors they call e-IGTs, or enhancement-mode, internal ion-gated organic electrochemical transistors, that have embedded mobile ions inside their channels. Because the ions do not need to travel long distances to participate in the channel-switching process, they can be switched on and off quickly and efficiently. The researchers used their e-IGTs to acquire a wide range of electrophysiological signals, such as in vivo recording of neural action impulses, and to create soft, biocompatible, long-term implantable neural processing units for the real-time detection of epileptic discharges.
Panaxium Inc., a Canadian startup firm with facilities in France, is seeking to commercialize closed-loop, bidirectional brain implants based on ionic conduction. Panaxium CEO Brad Schmidt described the benefits of his firm’s iontronic devices last month during the 2020 Bioelectronic Medicine Forum.
The ions that are transported in Panaxium’s device can range from simple molecules like potassium ions to more complex molecules like neurotransmitters. They also offer higher resolution and less tissue damage.
Iontronic implants also require a much lower power envelope, which enable smaller implants that can be placed with a single burr hole. And they have better biomechanical capabilities, since they can flex and stretch around a sulcus. Schmidt said his devices can deliver a 100- to 1000X improvement in signal to noise ratio. “That means we can measure single action potentials in vivo with high reliability and we have proprietary fabrication techniques to do this,” he said.
Schmidt said that metal electrodes produce peroxide and other chemical byproducts that can harm tissue. “We recognize that the body’s electrical system is not the electrical system of a microelectronic computer chip” he said. “ Neurons rely on this cascading change in ion concentrations. You can consider it a combination of electrical stimulation and drug delivery.”
Panaxium scientific advisors include George Malliaras, head of the bioelectronic lab at the University of Cambridge, and Roisin Owens, who heads the bioelectronic systems technologies group at Cambridge. Khodagholy is also an advisor. The company’s first product will be an intraoperative probe that would serve as a regulatory predicate.
