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Human brain disorders, such as epilepsy, depression, migraine, schizophrenia, autism, and dementia, emerge when large-scale interactions within the brain are disrupted. Severe damage to the central nervous system of the body occurs as a result of extensive traumatic and degenerative lesions. This leaves millions of lives (particularly young adults) world over devastatingly become victims. Available therapeutic options at present are few; this calls for extensive basic and clinical research towards restoration of voluntary functions or relief from the ailment. Investigations of the brain functioning are carried out in different subjects such as rodents, primates and patients; these studies pose numerous challenges. New experimental approaches involve repair of the damaged axons, re-establish their original connections between the neurons, understand the neurophysiology, and use prosthetic devices to restore basic motor functions.
One of these approaches requires the study of the electrical signaling fired across the neurons. When neurons communicate, they do so by producing electricity across. A brain probe measures the highly localized electric fields over the neurons, nerves or brain tissues. Brain probes, also known as neural probes, record these signals during sleep, pre-epileptic activity, or in response to various stimuli. To record electrical signals from one neuron is one thing; simultaneous recording of signals from as many as possible neurons is required ‘to make sense of the frenzy of signals coursing through the brain.’ Deconvolving neural circuitry is not without experimental limitations.
The human brain produces in 30 seconds as much data as the Hubble Space Telescope has produced in its lifetime.
Konrad Kording, Northwestern University
Real-time neurophysiology – using an array of probes, for simultaneous recording of several neural circuit dynamics – is an ambitious goal to develop neural interface systems that can reveal the interactions of individual cells and entire neural circuits in both time and space. Brain probes are electrodes made of different materials such as silicon, platinum (Pt) and its alloys with iridium, titanium nitride, conducting polymers, nanomaterials composite, semiconductor FET nanowires or graphene. Modern electrical neural interface systems are built on developing advanced electrode technologies. Single ended electrodes, silicon probes and similar devices suffer from impedance limits as the sizes of these probes shrinks. Brain probes should have the following characteristics: must be a low modulus material, have increased stability without causing irreversible electrochemical reactions, enable minimally invasive operations, have good signal amplification, have multiplexed readout possibility, and be multifunctional depth-penetrating probes.
In 2011, Jose Garrido, a nanotechnologist at the Walter Schottky Institute in Munich, Germany, showed that arrays of transistors made of graphene can detect action potentials in heart cells. Graphene has been explored to record neural activity. Graphene is a stable material, with favorable electronic properties and flexibility (enabling it a suitable bionic material). Salient features of graphene have made it a suitable material for brain probes: “graphene is impervious to the harsh ionic solutions found in the human body, and graphene's ability to conduct electrical signals means it can interface with neurons and other cells that communicate by nerve impulse, or action potential.” However, graphene electrodes also suffer from high impedance, which implies that the communication between the brain and the recording devices is hindered. Methods to improve this, has lead to loss of transparency in the material. Transparency is required for optical imaging. In a Nature Communications paper (2014), Duygu Kuzum and team used transparent and flexible graphene for simultaneous electrophysiology and neuroimaging with a strengthened signal to noise ratio.
This year a team of engineers and neuroscientists at the University of California San Diego achieved higher quality of brain imaging with an array of graphene electrodes, by lowering their impedance 100 times using Pt nanoparticles and retaining about 70% of their transparency. The Pt nanoparticles are electrodeposited on a monolayer of graphene; this provides a parallel conduction path to overcome the quantum capacitance and the lack of Faradaic reaction for the graphene electrodes. The low-impedance graphene electrodes recorded and imaged neuronal activity, such as calcium ion spikes, at both the macroscale and single cell levels in transgenic mice. Upon shrinking the electrode dimensions to single cell size, they recorded neural activity with single cell resolution without sacrificing transparency. Based on their experimental evidence, the researchers “envision that their technique is applicable to fabricate transparent microelectrode arrays with geometries specifically tailored toward probing different neural circuits and mechanism in multimodal experiments providing unprecedented spatiotemporal resolution”.
References
- The Brain Activity Map
- Brain–machine interfaces to restore motor function and probe neural circuits
- Bioelectronics: The bionic material
- Neuroscience: Solving the brain
- Soft Materials in Neuroengineering for Hard Problems in Neuroscience
- Beyond the Patch Clamp: Nanotechnologies for Intracellular Recording
- A Sprinkle of Platinum Nanoparticles Onto Graphene Makes Brain Probes More Sensitive
- Transparent and flexible low noise graphene electrodes for simultaneous electrophysiology and neuroimaging
- Ultralow Impedance Graphene Microelectrodes with High Optical Transparency for Simultaneous Deep Two‐Photon Imaging in Transgenic Mice
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