New Brain Implants Exploit 3D Cortical Architecture
by James Cavuoto, editor
March 2020 issue
Neural engineers developing new brain implants for applications in brain-computer-interfaces, cortical stimulation, and neuroprosthetics have made continued progress at improving the function and the footprint of microelectrode arrays. Several research institutions and at least two commercial firms have literally added a new dimension to neural interfacing by taking advantage of the three-dimensional architecture of the cerebral cortex.
Researchers at Stanford University recently showcased their new device connecting the brain directly to silicon-based technologies. “Nobody has taken these 2D silicon electronics and matched them to the three-dimensional architecture of the brain before,” said Abdulmalik Obaid, a graduate student in materials science and engineering at Stanford. “We had to throw out what we already know about conventional chip fabrication and design new processes to bring silicon electronics into the third dimension. And we had to do it in a way that could scale up easily.”
The device, the subject of a paper published in Science Advances, contains a bundle of microwires. These thin wires can be gently inserted into the brain and connected on the outside directly to a silicon chip that records the electrical brain signals passing by each wire. Current versions of the device include hundreds of microwires but future versions could contain thousands. The technology described in the study is also the basis for a fully integrated BCI that is being developed by Paradromics, the Austin, TX company founded by Matthew Angle, one of the authors of the paper. Investigators from the Francis Crick Institute and University College London collaborated on the paper.
“Electrical activity is one of the highest-resolution ways of looking at brain activity,” said Nick Melosh, professor of materials science and engineering at Stanford and co-senior author of the paper. “With this microwire array, we can see what’s happening on the single-neuron level.”
Another firm seeking to exploit the 3D architecture of the brain is Modular Bionics Inc. of Berkeley, CA. That company’s N-Form implant can be customized to a 3D configuration that reaches a large volume of neurons across multiple cortical layers and columns. Their microwire arrays feature shanks with a thickness of 125 microns, with up to eight contact sites per shank.
Modular Bionics CEO Ian Halpern took some issue with Obaid’s claim. “Silicon and microwire based arrays recording individual neurons in three dimensions has been demonstrated for many years,” he said. “As an example, we have demonstrated this capability with our N-Form microwire based array since 2015 after we scaled manufacturing and launched it and its insertion system as products. The N-Form is composed of many microscale fingers, each containing multiple electrode recording sites creating a volume of three-dimensional recording.
“Since 2015, the N-Form has successfully recorded individual neurons in true 3D over long-duration chronic experiments within the brain and peripheral nervous system across multiple large animal species, including humans, in labs across the world. By ‘true 3D’ in this context, I mean reliable microwire recording sites along the side of each biocompatible polymer finger of the N-Form array, instead of exclusively at the tips of wires grouped in an array.”
The N-Form is configurable in three dimensions across a wide range of parameters, including depths between one half of a millimeter and 15 mm to record different regions and circuits within the nervous system. The device currently offers up to 128 channels, but a 250 channel unit has been successfully implanted in a macaque brain.
The Stanford researchers tested their BCI on isolated retinal cells from rats and in the brains of living mice. In both cases, they successfully obtained meaningful signals across the array’s hundreds of channels. Ongoing research will further determine how long the device can remain in the brain and what these signals can reveal. The team is especially interested in what the signals can tell them about learning. The researchers are also working on applications in prosthetics, particularly speech assistance.
Following their initial tests on the retina and in mice, the researchers are now conducting longer-term animal studies to check the durability of the array and the performance of large-scale versions.