Brain Function: Novel Technologies Driving Novel Understanding - Bioinspired Approaches for Human-Centric Technologies - page 317

usually insufficient to resolve subthreshold potentials arising from synaptic activity.
Miniaturization of the electrode size helps to avoid signal overlap from multiple
neurons, although the consequent increase of the electrode impedance negatively
affects the SNR. In general, the SNR of extracellular recordings could be increased
by (1) achieving a closer apposition between the electrode and the cell membrane
and (2) increasing the surface area of the cell/electrode junction. Close apposition
increases the resistance to ionic currents in the cleft between the electrode and the
cell, thus increasing the extracellular potential, while a more extended junction
leads to a higher total current in the cleft together with a higher resistance of the
cleft itself. Classical approaches for enhancing the cell-to-substrate interaction rely
on protein coatings (Cai et al. 2008 ; Wrobel et al. 2008 ) or topographical patterning
of the electrode surface (Sniadecki et al. 2006 ; Spatz and Geiger 2007 ), the latter
also increasing the effective surface area of the junction.
A recent method to achieve both a closer apposition to the membrane and a
larger contact surface was based on fabricating mushroom-shaped electrodes that
are effectively engulfed by cells (REFs), leading to SNR orders of magnitude
higher than with standard extracellular recording. The method exploits the tendency
of cells to engulf three-dimensional microprotrusions in a phagocytosis-like fashion
(Hai et al. 2010 , 2009 , 2010 ; Hai and Spira 2012 ; Spira et al. 2007a , b ; Fendyur and
Spira 2012 ; Van Meerbergen et al. 2008 ). Analysis of the interface revealed a lower
average distance of the cell membrane from the structured electrode surface than
for flat surfaces. Thus, 3D nanostructures offer a promising approach to extend the
capabilities of microelectrode recording.
An emerging fabrication technique for realizing 3D nanostructures is the focused
ion beam (FIB), a top-down nanofabrication technique. As the name implies, the
method consists of a focused beam of accelerated ions directed toward the substrate
of interest. Due to the large mass of the ion beams, a physical sputtering of the
substrate material occurs on the spot hit by the beam. This technique can shape
materials in three dimensions with great precision. The FIB can be scanned on the
surface with a predefined pattern to generate the desired planar structure, akin to
electron beam lithography. The dwell time of the beam on the same spot determines
the depth of the etching in the area, so that three-dimensional structures can be
created.
While FIB has low throughput, for neuroscience applications, it offers the
advantage of rapid and versatile prototyping as well as the ability to combine
nanostructuring with other technologies, such as novel microelectrode arrays. For
example, we have examined the use of different nanostructured protrusions to
enhance microelectrode-neuron interactions for electrophysiological recording.
FIB allows the fabrication of nanostructures of different shapes and sizes on the
same substrate, allowing direct comparisons in neurophysiological experiments. At
IIT, Martiradonna et al. ( 2012 ) fabricated straight nanopillars and nail-headed and
sphere-headed pillars on top of the recording electrodes of a multielectrode-array
substrate (Fig. 10.11 ). Hippocampal neurons were cultured on the modified chip
after functionalization of the chip surface with poly- L -lysine (PLL). After 7 days of
culturing, cells were found to adhere to the nanostructured electrodes. Pillars were