As the brain is essentially an electrical organ, miniaturized electronics are able to decipher a good deal of its secrets. This is how imec’s silicon neural probes set the standard in neuroscientific research.
To uncover the workings of the brain, and how neuronal disorders arise, neuro-electrophysiologists study the electrical activity of the brain. The goal is to ascertain how the brain processes data and controls behavior, by picking up signals from single neurons and across wider areas.
The primary tools to unravel these operational details of the brain are neural probes. These are small brain implants that contain electrodes for recording the electrical signals from neurons. In the future, they will also be essential components in brain-machine interfaces that, for instance, enable paralyzed persons to communicate more easily.
By leveraging its expertise in the extreme miniaturization of electronic circuits, imec is one of the key drivers behind the next generation of neural probes. More particularly, imec plays a key role in Neuropixels, the probes that are widely considered to be the ‘gold standard’ in neuroscience.
Not surprisingly, the usefulness of a neural probe is largely determined by its size. Damage to the brain must be avoided at all costs – both direct damage to the neurons and indirect damage due to inflammatory reactions, that results in scar tissue blocking electrical signals.
The shank of the neural probe – the part entering the brain – must therefore be as narrow as possible. On the other hand, you want to incorporate the highest possible number of electrodes, to maximize the recording resolution.
The solution to this challenge is to integrate electronics on the probe using advanced CMOS technology. This enables the multiplexing of electrodes, and the simultaneous transport of several signals over the same cable. As the number of wires in the shank is a major contributor to its dimension, this means the resolution can go up without impacting the size.
There’s also a material challenge associated with neural probes. The shanks must be rigid enough to stay straight when entering, yet flexible enough to remain intact within a jiggling brain – especially during long-term research.
To avoid loss of the signal when it leaves the probe and travels through wires where it picks up noise, it can be pre-processed and digitized on the base of the probe, instead of on external equipment.
In 2022, imec presented a scalable neural readout microchip that features one of the smallest recording channels for simultaneous acquisition of local field and action potentials. It holds the promise for even higher-density neural recording tools.
The most advanced neural probe on the market today is the Neuropixels probe. It’s developed through a collaboration funded by Howard Hughes Medical Institute (HHMI), Wellcome Trust, Gatsby Charitable Foundation and Allen Institute for Brain Science. The probes are designed, developed and fabricated at imec, in collaboration with HHMI Janelia Research Campus, Allen Institute for Brain Science and University College London.
The first generation of the Neuropixels probe is currently used in research labs around the world. It consists of a single shank that’s monolithically integrated with a non-implantable base for amplification, digitization and power management.
The Neuropixels 2.0 probe was announced in 2021 and will be only a third of the size of its predecessor. Each probe will consist of four shanks with a total of more than 5,000 electrodes – an unprecedented resolution.
The Neuropixels 2.0 system is especially well-suited for tracking the electrical activity of individual neurons over extended time periods – days or even weeks. This is needed to study the neural basis of phenomena such as learning and memory. And it’s challenging because neurons tend to ‘drift’ due to small movements of the brain. The second generation of the Neuropixels probe solves this through a more even and dense spacing of the electrodes, and stabilization algorithms.
For more information about Neuropixels, and to order Neuropixels probe, visit www.neuropixels.org.
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