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. 2018 Feb;15(1):016007.
doi: 10.1088/1741-2552/aa8f8b.

Amorphous silicon carbide ultramicroelectrode arrays for neural stimulation and recording

Felix Deku  1 , Yarden Cohen, Alexandra Joshi-Imre, Aswini Kanneganti, Timothy J Gardner, Stuart F Cogan
Affiliations

Amorphous silicon carbide ultramicroelectrode arrays for neural stimulation and recording

Felix Deku et al. J Neural Eng. 2018 Feb.

Abstract

Objective: Foreign body response to indwelling cortical microelectrodes limits the reliability of neural stimulation and recording, particularly for extended chronic applications in behaving animals. The extent to which this response compromises the chronic stability of neural devices depends on many factors including the materials used in the electrode construction, the size, and geometry of the indwelling structure. Here, we report on the development of microelectrode arrays (MEAs) based on amorphous silicon carbide (a-SiC).

Approach: This technology utilizes a-SiC for its chronic stability and employs semiconductor manufacturing processes to create MEAs with small shank dimensions. The a-SiC films were deposited by plasma enhanced chemical vapor deposition and patterned by thin-film photolithographic techniques. To improve stimulation and recording capabilities with small contact areas, we investigated low impedance coatings on the electrode sites. The assembled devices were characterized in phosphate buffered saline for their electrochemical properties.

Main results: MEAs utilizing a-SiC as both the primary structural element and encapsulation were fabricated successfully. These a-SiC MEAs had 16 penetrating shanks. Each shank has a cross-sectional area less than 60 µm2 and electrode sites with a geometric surface area varying from 20 to 200 µm2. Electrode coatings of TiN and SIROF reduced 1 kHz electrode impedance to less than 100 kΩ from ~2.8 MΩ for 100 µm2 Au electrode sites and increased the charge injection capacities to values greater than 3 mC cm-2. Finally, we demonstrated functionality by recording neural activity from basal ganglia nucleus of Zebra Finches and motor cortex of rat.

Significance: The a-SiC MEAs provide a significant advancement in the development of microelectrodes that over the years has relied on silicon platforms for device manufacture. These flexible a-SiC MEAs have the potential for decreased tissue damage and reduced foreign body response. The technique is promising and has potential for clinical translation and large scale manufacturing.

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Conflict of interest statement

Conflict of interest: none declared Abstract

Figures

Figure 1
Figure 1
Microfabrication of amorphous silicon carbide microelectrode arrays (a-SiC MEAs). The process flow features at least three photolithography steps: one for defining the metal traces and electrodes, a second for patterning the top a-SiC layer for electrode site and bond pad openings, and a third photolithography step to singulate the a-SiC device geometries. A fourth lift-off lithography step is used (not shown) to restrict deposition of SIROF or porous TiN low impedance coatings to the electrode sites.
Figure 2
Figure 2
Surface morphology and topography of a-SiC films. (a) AFM image (2μm × 2μm) and (b) SEM images show the surface morphology of 2 μm thick PECVD amorphous-SiC deposited on a silicon wafer. The surface roughness estimate from the AFM is below 4 nm rms.
Figure 3
Figure 3
SEM images show released bundles of a-SiC MEAs (a, c, and e) taken at 5kV, and shanks still attached to the carrier silicon wafer (b, d, f, and g) taken at 2kV acceleration voltage. (a) The shanks of a released a-SiC MEA form a bundle when drawn out of water. The tip of the bundle is shaped by the layout design of the shank arrays, as shown in (b–c and d–e), (f) shows the exposed electrode tip at the distal end of the shank and (g) shows the side wall profile of the exposed electrode site at a 25 degree viewing angle.
Figure 4
Figure 4
Optical micrographs show that the 16 shanks naturally bundle when the as-fabricated device is pulled out of the deionized water. Omnetics connectors were mounted on the arrays using a solder reflow process and medical grade epoxy. The figure shows (a) the as-fabricated a-SiC MEA after release from deionized water, (b) after an Omnetics connector is soldered onto the bond pads and (c) a packaged device for implantation or in vitro electrochemical characterization.
Figure 5
Figure 5
Electrochemical properties of a-SiC UMEs coated with SIROF, TiN and Pt compared with Au. (a) Cyclic voltammogram measured at 50mV/s between −0.6V and 0.8V limits. (b) Electrochemical impedance spectroscopy measured using a 10 mV rms AC sinusoid.
Figure 6
Figure 6
Electrical stimulation capabilities of ultramicroelectrodes (a) Voltage transient response to current waveforms for TiN and SIROF electrodes biased at 0.6V vs Ag|AgCl (solid lines) and without anodic bias (dash lines). The electrodes were polarized to a cathodal potential limit of −0.6 V. The average current passed across the interface within the ‘safe’ electrochemical limit is 86.4 μA for SIROF (0.6 V bias) and 40.2 μA for TiN (0.6 V bias). (b) Maximum charge injection capacity and charge per phase as a function of interpulse bias. (Frequency= 50 pps, Pulse width = 200 μs).
Figure 7
Figure 7
Acute neural recording immediately following implantation in basal ganglia of Zebra Finch brain. The 16 recorded channels showed no strong coupling between contacts (a) single channel acute voltage trace with a subcutaneous reference on the head and (b) an overlay of a neuronal spike waveforms, detected by setting the threshold of the trace in (a) at − 50 μV.
Figure 8
Figure 8
Spontaneous neural activity in rat motor cortex: (a) simultaneous spike activity recorded across three channels using the a-SiC MEA; (b) sorted single units on CH1 with average peak-to-peak amplitudes of 45 μV (Unit A), 87 μV (Unit B) and 114 μV (Unit C); and (c) the corresponding autocorrelograms processed with a bin size of 2 ms.
Figure 9
Figure 9
A SEM image of the distal tip of an a-SiC UME shank with two electrode sites located on the same shank. The GSA of the exposed Au electrode sites is 100 μm2 but with unequal perimeter. The perimeter of the square electrode site is 40 μm versus 104 μm for the rectangular site.
Figure 10
Figure 10
A SEM image of a-SiC MEA with a built-in shank curvature. The intrinsic curvature is expected to improve splaying capabilities of the a-SiC MEAs which form bundles when drawn out of water.
Figure 11
Figure 11
Raw traces showing simultaneously recorded spontaneous spike activity on channel 10 and 11 (CH10 and CH11) and their corresponding local field potentials (FP10 and FP11). (spike and local field potential amplitudes in mV).
See this image and copyright information in PMC

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