Schematics of the all diamond MEA fabrication process.Better understanding of brain function is driving the use of microelectronic devices that interface with neural tissue. New research indicates that boron-doped diamond could be an ideal material for these devices because it is highly stable and biocompatible [Hébert, C., et al., Materials Science & Engineering C 46 (2015) 25-31, DOI: 10.1016/j.msec.2014.10.018].
Currently, microelectrode arrays (MEAs) based on platinum are most commonly used for interfacing with neural tissue both in human patients, where they provide a link to external prosthetic devices such as cochlear implants, and in basic research. But devices tend to become overgrown by cells or degrade during long-term use, affecting the reliability and stability of performance. Moreover, as magnetic resonance imaging (MRI) is becoming increasingly common in the analysis of brain function, implanted devices should not, ideally, disturb magnetic fields.
Diamond is a promising candidate for MEAs because it is known to be highly biocompatible and has low magnetic susceptibility, but has not been widely explored for neural-interfacing devices. Now researchers from Institut Néel, INSERM, CEA/INAC/SPrAM/CREAB, Université Grenoble Alpes, CNRS, and CEA-LETI-DTBS Minatec in France have fabricated prototype 121-microelectrode arrays with boron-doped polycrystalline diamond (B-PCD) as the conductive layer sandwiched between insulating diamond layers on a silicon substrate.
“There is a lack of data concerning the electrochemical properties of diamond in biological-like media,” explains Clément Hébert, formerly of Institut Néel now at the Diamond Sensor Lab of CEA LIST.
The researchers’ B-PCD electrodes show electronic characteristics comparable to typical platinum-based electrodes in terms of double layer capacitance, charge storage capacitance, and interfacial electrochemical impedance, which determine overall performance.
“[Our] values are as good as the classical ones obtained on flat (non-structured) platinum electrodes, which has long been the reference material in academic studies and industrial applications,” says Hébert. “This is a promising result for the application of diamond in neural interfacing.”
In vivo testing of boron-doped diamond MEAs in rats showed promising results too: six months after implantation, the diamond MEAs did not appear to be producing any inflammatory reaction and nor did they induce any MRI artifacts when scanned. The findings indicate, say the researchers, that diamond can be used to fabricate functional and addressable arrays that perform just as well as standard platinum electrodes.
“Boron-doped diamond is a fantastic material for neural interfacing since it meets all the requirements,” says Hébert. “It is one of the best options [for] long-term, stable implants for neural prostheses. It still requires a few years of development but all the recent progress makes me hope that it could be integrated into commercial medical systems.”
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