Graphene brain implants.

A team of researchers from Technische Universität München (TUM) and the Jülich Research Center published results in the journal Advanced Materials that demonstrated work on a graphene-based transistor array that is capable of recording the electrical signals of the living biological cells on which it is placed. According to a statement, so-called bioelectronic applications have been proposed that would place sensors and, in some cases, actuators inside a person’s brain, eye, or ear to help compensate for neural damage.

Of the several material systems being explored as alternatives to silicon microelectronics, graphene appears suited to bioelectronic applications as it offers excellent electronic performance, is chemically stable and biologically inert, can readily be processed on flexible substrates, and should lend itself to large-scale, low-cost fabrication.

The latest results from the TUM-Jülich team confirm key performance characteristics and open the way for further advances toward determining the feasibility of graphene-based bioelectronics. The experimental set-up reported in Advanced Materials began with an array of 16 graphene solution-gated field-effect transistors (G-SGFETs) fabricated on copper foil by chemical vapour deposition and standard photolithographic and etching processes.

‘The sensing mechanism of these devices is rather simple,’ said Dr Jose Antonio Garrido, a member of the Walter Schottky Institute at TUM. ‘Variations of the electrical and chemical environment in the vicinity of the FET gate region will be converted into a variation of the transistor current.’

Directly on top of this array, the researchers grew a layer of biological cells similar to heart muscle.

Not only were the ‘action potentials’ of individual cells detectable above the intrinsic electrical noise of the transistors, but these cellular signals could be recorded with high spatial and temporal resolution. For example, a series of spikes separated by tens of milliseconds moved across the transistor array in just the way action potentials could be expected to propagate across the cell layer.

Also, when the cell layer was exposed to a higher concentration of the stress hormone norepinephrine, a corresponding increase in the frequency of spikes was recorded.

Separate experiments to determine the inherent noise level of the G-SGFETs showed it to be comparable to that of ultra-low-noise silicon devices that, as Garrido explained, are the result of decades of technological development.

‘Much of our ongoing research is focused on further improving the noise performance of graphene devices and on optimising the transfer of this technology to flexible substrates such as parylene and kapton, both of which are currently used for in vivo implants,’ said Garrido. ‘We are also working to improve the spatial resolution of our recording devices.’

Meanwhile, the team is working with scientists at the Paris-based Vision Institute to investigate the biocompatibility of graphene layers in cultures of retinal neuron cells, as well as within a broader European project called Neurocare, which aims at developing brain implants based on flexible nanocarbon devices.
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