Creating a
Graphene-Based
Neural Interface
Researchers are investigating the potential of graphene to function
at the electrode-tissue interface as a recoding and stimulating electrode for application
in neural prostheses—devices that restore functions lost due to neural damage.
Graphene—a novel 2D material comprised of a single layer of carbon atoms—is rapidly making
its way through research and development
phases. It is electrically and thermally
conductive, chemically stable, transparent,
flexible and strong. Leveraging these
attributes to improve and create new
technologies is a common theme across
many fields.
But how could one material be so
ubiquitous? The answer is versatility.
Its structure can be modified and
functionalized to serve a multitude of
different purposes. Furthermore, the
production and modification of graphene-based materials can range from processes
involving chemical vapor deposition (CVD)
or bulk chemical synthesis in solution using
graphitic raw materials. As the production
technology improves, the material properties
advance closer to their theoretical upper
limits and the utility of the material expands
to a wider range of applications.
Neural interface technology
One of the many potential uses of graphene
is in neural interface technology. As part
of the Graphene Flagship, the lab at the
Catalan Institute of Nanoscience and
Nanotechnology (ICN2) in Barcelona,
Spain, in collaboration with the Institute of
Microelectronics of Barcelona (IMB-CNM),
is investigating the ability of graphene to
function at the electrode-tissue interface
as a recoding and stimulating electrode for
application in neural prostheses— devices
that restore functions lost due to neural
damage.
The goal is to produce a graphene-based
microelectrode technology that outperforms
traditional electrodes derived from metals,
such as gold, platinum, and titanium. A
high-performing electrode is characterized
as having a large signal-to-noise ratio
(SNR) during electrical recording, and a
high charge injection capacity (CIC) during
electrical stimulation. Electrode materials
with a low electrochemical impedance
generally provide a better SNR, whereas
materials with a large effective surface
area, and thereby a large capacitance
through which charge transfer can occur,
provide a higher CIC. Materials based on
conductive polymers and other forms of
carbon (e.g., carbon nanotubes, diamond
and carbon black) have been proposed as
means to improve upon the traditional
metallic technology. So far they have yet
to replace metal electrodes because their
high performance comes at the cost of
impaired electrochemical stability and
biocompatibility, and vice versa. This
research aims to leverage the versatility of
graphene to tailor-make electrodes with
competitive electrochemical properties.
Graphene form follows function
Graphene as a neural interface material can
take many forms, such as single layer two-
dimensional graphene, multilayer graphene,
vertically-aligned graphene nanowalls,
and graphene flakes. Each of these has
different properties. Single layer graphene
is conductive and transparent, but it has a
relatively low interfacial capacitance. It is
suitable for recording electrodes and can be
used in optoelectronic studies that induce
neural activation through light stimulation
while electrically recording the neural
response using the electrodes. Graphene’s
transparency enables interrogation of neurons
directly beneath the electrode that would
otherwise be occluded by an opaque material.
Multilayer graphene can offer higher
conductivity and interfacial capacitance
than single layer graphene, but with reduced
optical transparency. Vertically-aligned
graphene nanowalls consist of few-layer
graphene layers oriented perpendicular
to the substrate surface. They can be
produced using plasma-enhanced CVD.
This improves the interfacial capacitance
of the material by accessing the surface
area in the vertical dimension, similar to
what happens for carbon nanotubes. This
increased capacitance is attractive for neural
stimulation electrodes. Graphene flakes can
be derived from liquid phase exfoliation of
bulk graphite.
The random alignment of the flakes can
result in a porous architecture once these are
deposited to form a film, with a surface area
that scales with the thickness of material. The
resulting capacitance makes the printed film
promising as electrode for neural recording
and stimulation, although it has drawbacks,
such as the reduced transparency caused by
the film thickness, and the high resistance
caused by the fragmented nature of the
material. Each of these graphene forms can
By Steven Walston, PhD and Jose A. Garrido, PhD, Catalan Institute of Nanoscience and Nanotechnology
A representation
of a retina implant.
Credit: ICN2
