Schematic diagram showing where the MTA device is implemented in a rat. (Image Credit: Zifang Zhao and Claudia Cea/Columbia Engineering)
Responsive neurostimulation is becoming effective at studying neural circuit function and treating epilepsy and Parkison's disease. However, designing an implantable and biocompatible device that performs those interventions comes with significant restrictions. The resolution needs to be higher, and most contain large, bulky components, making it difficult to implant with complication risks.
Engineers at Columbia University designed a new technique that could potentially improve these devices. The researchers are building on an earlier iteration of their work to create smaller, more efficient bioelectric transistors and materials. These devices were orchestrated to produce high-performance implantable circuits capable of reading and manipulating brain circuits. Compared to existing methods requiring the same amount of amplifiers as the number of channels, this multiplex-then-amplify (MTA) system only needs one amplifier per multiplexer.
"It is critical to be able to detect and intervene to treat brain-disorder-related symptoms, such as epileptic seizures, in real-time," said Dion Khodagholy, a leader in bio- and neuroelectronics design. "Not only is our system much smaller and more flexible than current devices, but it also enables simultaneous stimulation of arbitrary waveforms on multiple independent channels, so it is much more versatile."
Recording, detecting, and localizing epileptic discharges is achieved by logging brain activity in various locations with high temporal resolution. A high-sampling-rate multi-channel acquisition and stimulation device and circuit are needed to perform this task. Traditional circuits need the same amount of amplifying circuits as the number of channels before linking the signals into a data stream via multiplexing. As a result, the circuits' size increases linearly with the number of channels.
Image showing the micro-fabricated conformable conducting polymer-based electrode. (Image Credit: Zifang Zhao and Claudia Cea/Columbia Engineering)
The need for an implantable system with capabilities to record, process, and stimulate brain activities has become more evident. Such a system allows researchers to develop personalized therapies. To record brain activity, the team required multi-channel amplifiers, but these were too large and unwieldy. While improving the efficiency of the electrodes by using a conducting polymer to lower the impedance, the team thought of placing the multiplexer in front of the amplifier to see what happens.
They developed the MTA device and verified its functionality. This was achieved by creating an implantable, responsive embedded system that obtains single neural action potentials using conformable conducting polymer-based electrodes. It accomplishes this using low-latency arbitrary waveform stimulation and local data storage. All of which occurs within a quarter-sized physical footprint.
"The key challenge was to create an electric-charge drainage path during the multiplexing operation to eliminate any unwanted charge accumulation," said Zifang Zhao, a postdoctoral fellow in the department of electrical engineering and the first author of the study.
The MTA device allowed the team to create a closed-loop protocol that suppresses pathological coupling between the hippocampus and cortex in real-time within an epileptic network. Epilepsy sufferers dealing with memory issues could benefit from this approach.
"These devices will allow application of targeted high-spatiotemporal resolution responsive neurostimulation approaches to a variety of brain functions, greatly broadening our ability to chronically modify neural networks and treat neuropsychiatric disease," Jennifer Gelinas, a specialist in pediatric epilepsy, said.
Now, the team is incorporating this system with experimental platforms. Ultimately, their goal is to help improve neural network functionality and cognitive skills.
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