Flexible Brain Electrodes Cause Less Damage to Brain Tissue, can be Customized
How do individual neurons behave in a circuit over time, as the brain performs its daily routine of complex cognitive processes? This is a fundamental question for neuroscientists.
To decipher the activity of individual neurons requires greater resolution than that afforded by non-invasive approaches to recording brain activity like magnetic resonance imaging (MRI), electroencephalography (EEG) and magneto encephalography (MEG).
For years neuroscientists have been fine-tuning the procedure of implanting electrodes under the skulls of research animals to measure the activity of neurons.1
Increasing channel number improves recording resolution
From single electrodes to tetrodes, to multi-channel systems, neuroscientists and engineers have been pushing the boundary of what’s possible in terms of measuring neuronal activity as animals perform behavioural tasks.
In November 2017, neuropixels probes were released.2 These silicon probes incorporate CMOS technology to enable researchers to record from over 900 channels on an electrode a fraction of the thickness of a human hair. The closely packed channels facilitate unprecedented resolution of the electrical activity of the neural processes that intersect the probe shank.
Rigid probes damage tissue
However, rigid shank electrodes have in the past caused issues for researchers as they damaged the tissue they are trying to record from. The brain is gelatinous and any transverse movement of a stiff electrode can have a shearing effect on the brain tissue, rupturing the tiny blood vessels that serve the brain.
Single shank electrodes also have the drawback of all the recording sites being aligned in a single plane. Multiple probes can be used to record from an array of sites over a wider area, but placement is awkward, and this makes freely moving behavioural tasks difficult to perform.
Flexible, fine probes are the future of in vivo electrophysiology
Loren Frank’s group from the University of California San Francisco have developed a polymer array of electrodes that enable recording from ~1000 different sites. The flexible polymer prevents disruption of blood vessels and brain tissue, and the group are able to record from the probes for a very prolonged amount of time, they recorded single-neuron activity for up to 11 months. The polymer probes are arranged into 16 individual fork-shaped recording modules. The forks have four tines that contain 16 electrodes each, totalling 64 electrodes per fork, 1024 electrodes in total. The fork arrangement enables simultaneous recording from multiple sites in the brain.3
At Neuroscience 2018, the Society for Neuroscience’s main meeting in San Diego, London-based neuroscientist Dr Romeo Racz of the Crick Institute unveiled his novel electrode design. Named jULIEs, which stands for juxta-neuronal ultra-low impedance electrodes, his probes are unlike all other probes on the market.
Using a special microfabrication process, Romeo and his team can engineer the probes to be atomically flat. Their smoothness and small size enables them to slide past blood vessels and sit next to neurons, whilst their ultra-low impedance facilitates excellent signal to noise. What’s more, the arrangement of the electrodes can be customised to facilitate measuring from brain areas with special geometries, or to measure from different layers in a cortical column, for example.
jULIEs probes are flexible, customizable electrodes revolutionising in vivo recording. Cedit: MCI-Neuroscience, YouTube.
The new flexible electrodes from both the UCSF team and the Crick team can be customized to the scientists’ needs. This means the scientist can use the probes to answer their research question, not be restricted to only answering the research questions possible with the current electrodes on the market.
Whatever your choice of electrode it is important to consider the damage being caused to brain tissue by the implanted fibres as this damage will affect your experiment.
1. Adrian, E. D., & Moruzzi, G. (1939). Impulses in the pyramidal tract. The Journal of physiology, 97(2), 153-199.
2. Jun, J. J., Steinmetz, N. A., Siegle, J. H., Denman, D. J., Bauza, M., Barbarits, B., … & Barbic, M. (2017). Fully integrated silicon probes for high-density recording of neural activity. Nature, 551(7679), 232.
3. Chung, J., Joo, H., Fan, J., Liu, D., Barnett, A., Chen, S., Geaghan-Breiner, C., Karlsson, M., Karlsson, M., Lee, K., Liang, H., Magland, J., Pebbles, J., Tooker, A., Greengard, L., Tolosa, V. and Frank, L. (2018). High-Density, Long-Lasting, and Multi-region Electrophysiological Recordings Using Polymer Electrode Arrays. Neuron.