Schematic of a highly MRI compatible G-Cu neural electrode and the artifact comparison with Pt electrodes of the same size under 7.0 T MRI.Coating electrodes used to probe brain activity with graphene could be safer for patients and enable more accurate measurements, according to researchers [Zhao et al., Nano Letters (2016), doi: 10.1021/acs.nanolett.6b03829].
Magnetic resonance imaging (MRI) provides an unequalled means of mapping brain activity and, when combined with direct probes into neural tissue, can enable detailed measurements and stimulation of electro-physiological activity. But neural probes or electrodes must be compatible with MRI, as well as biocompatible and nontoxic. Inert metals such as Pt-Ir, W, Au, Ni-Cr, and stainless steel are commonly used for implantable neural electrodes but can cause blind spots in MRI. These image distortions arise because the magnetic susceptibility of these metals is very different to that of neural tissue.
There is one metal that has a potentially compatible magnetic susceptibility is Cu − but it is extremely toxic to brain tissue. So Xiaojie Duan and coworkers at Peking University in China got around this problem by coating Cu microwires in a very thin layer of graphene.
“We achieved a new type of neural microelectrode made from graphene-encapsulated copper (G-Cu) microwires, [which] has high MRI compatibility compared to other neural electrodes,” says Duan.
The researchers grew a single atomic layer of high-quality graphene directly onto Cu microwires using low-pressure chemical vapor deposition (CVD). The conducting G-Cu microwires are then covered with a thin insulating layer of parylene-C, leaving a bare conductive tip (bottom left). The G-Cu microwires are highly compatible with MRI, according to the study, and show no toxicity towards brain tissue (bottom right).
“The main function of graphene is to act as a barrier between brain tissue and the Cu surface to prevent corrosion and eliminate the cytotoxicity of Cu to cells and brain tissue,” explains Duan.
Graphene has the advantage of being highly electrically conductive, so that it does not compromise the recording capability of the metal electrodes. Moreover, the thinness of the encapsulating graphene layer ensures that it does not interfere with the magnetic susceptibility of Cu either.
The combination of magnetic compatibility and low toxicity make G-Cu ideal for implantable electrodes for combining electrophysiological measurements with anatomical and functional MRI imaging. This new electrode could make neurophysiological studies of the brain safer and easier, as well as enabling clinical evaluation, monitoring, and even stimulation to treat diseases such as Parkinson’s.
“This approach is quite practical,” suggests Duan. “We don’t see major obstacles for this new G-Cu electrode in applications for now.”
Charles M. Lieber of Harvard University agrees that the advance could solve the critical problem facing the use of Cu as an in vivo neural electrode.
“Cu is, perhaps,the ‘best’ materialfrom a magnetic susceptibility perspective for MRI because it is most closely matched to neural tissue, which reduces any artifacts in the MRI signal, but it is very cyto/neurotoxic,” he says. “The unique aspect of this work is using graphene-encapsulated Cu wires as MRI-compatible neural probes – and demonstrating that these could be used for both MRI and neural recording, two important and complimentary techniques for understanding brain circuitry.”
This article was originally published in Nano Today (2016), doi: 10.1016/j.nantod.2016.12.004