Fabrication of High Aspect Ratio Millimeter-Tall Free-Standing Carbon

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Fabrication of High Aspect Ratio Millimeter-Tall FreeStanding Carbon Nanotube Based Microelectrode Arrays Guohai Chen, Berg Dodson, David M. Hedges, Scott C. Steffensen, John N. Harb, Chris Puleo, Craig Galligan, Jeffrey Ashe, Richard Vanfleet, and Robert Davis ACS Biomater. Sci. Eng., Just Accepted Manuscript • DOI: 10.1021/acsbiomaterials.8b00038 • Publication Date (Web): 29 Mar 2018 Downloaded from http://pubs.acs.org on March 29, 2018

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ACS Biomaterials Science & Engineering

Fabrication of High Aspect Ratio Millimeter-Tall Free-Standing Carbon Nanotube Based Microelectrode Arrays Guohai Chen,a,† Berg Dodson,a David M. Hedges,b,‡ Scott C. Steffensen,b John N. Harb,c Chris Puleo,d Craig Galligan,d Jeffrey Ashe,d Richard R. Vanfleet,a and Robert C. Davisa,* a

Department of Physics and Astronomy, Brigham Young University, Provo, UT 84602, USA

b

Department of Psychology and Neuroscience, Brigham Young University, Provo, UT 84602, USA

c

Department of Chemical Engineering, Brigham Young University, Provo, UT 84602, USA

d

General Electric Global Research (GE-GR), 1 Research Circle, Niskayuna, NY 12309, USA

* Corresponding author. E-mail address: [email protected] †

Present address: CNT-Application Research Center, National Institute of Advanced Industrial Science and

Technology (AIST), 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan ([email protected]). ‡

Present Address: Scientific Computing and Imaging Institute, University of Utah, 72 S Central Campus Drive,

Room 3750, Salt Lake City, UT 84112

Abstract Microelectrode arrays of carbon nanotube (CNT)/carbon composite posts with high aspect ratio and millimeter-length were fabricated using carbon-nanotube-templated microfabrication with a sacrificial “hedge”. The high aspect ratio, mechanical robustness, and electrical conductivity of these electrodes make them a potential candidate for next-generation neural interfacing. Electrochemical measurements were also demonstrated using an individual CNT post microelectrode with a diameter of 25 µm and a length of 1 mm to perform cyclic voltammetry on both methyl viologen and dopamine in a phosphate-buffered saline solution. In addition to detection of the characteristic peaks, the CNT post microelectrodes show a fast electrochemical -1-

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response, which may be enabling for in-vivo and/or in-vitro measurements. The CNT post electrode fabrication process was also integrated with other microfabrication techniques resulting in individually addressable electrodes.

Keywords: carbon nanotube, neural probe, microelectrode array, cyclic voltammetry, electrical resistivity

Introduction Multielectrode neural recording is an enabling technology for dense recording of in-vivo neuronal network activity with the temporal resolution required (10 kHz) for single-spike recording. However there is still a significant gap between the spatial resolution of these electrode array technologies and the requirements of single neuron recording.1-2 Demand is ever increasing for technologies that can provide stable recordings at a higher density while keeping tissue damage/inflammation to a minimum level.1,3-4 Several kinds of multielectrode arrays (MEAs) have been developed by researchers and these can be classified into two main categories: 1) manually assembled arrays of electrodes and 2) directly fabricated multielectrodes on a single substrate. Manually assembled array elements generally consist of versatile metal wire electrodes; however, these arrays suffer from low spatial resolution, needle-to-needle variation, and poor control over spacing.5-7 More recent approaches have assembled arrays from silicon wires or carbon fibers to improve spatial resolution.8-10 The second category of MEAs are those directly fabricated on a single substrate. Some of these are fabricated using microsystem fabrication

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technology (e.g. micro-machining) with various materials but in particular silicon.11-14 Microfabricated electrode arrays, like the “Michigan array,” demonstrate fabrication of multiple electrodes on a single probe shank with precise control over electrode size and placement, as well as the overall shank geometry.14-15 Microfabricated electrode arrays are made using state-of-the-art silicon MEMS technology and can be integrated with on-chip circuitry, signal processing and wireless interfaces.16 Recording sites on microfabricated probes are often made of iridium,17 platinum,18 and gold,19 and can be also modified by coating other conductive polymers and even CNTs to improve long-term performance and decrease impedance.20-22 Michigan array style electrodes provide a high density of sensors at precisely determined positions; specifically, they can have an electrode thickness of 15 or 50 µm, an electrode length of 2-15 mm, a recording channel count from 8 to 256, and a spacing between probe shanks in a 3D array of 400 µm.23 Direct fabrication of MEAs consisting of a 2D array of vertical recording posts on a single substrate has resulted in the “Utah array”.13 In this approach, a silicon wafer is diced to produce an array of square silicon columns. The dicing is followed by wet etching to achieve the final needleshaped electrode shanks. The Utah array has resulted in some of the longest lasting implanted devices to date and is the only intracortical microelectrode array that is currently FDA approved for long-term human studies.3,24 Currently, the commercial Utah array can have up to 128 electrodes in various configurations. It has a single recording site per probe and the probe pitch has remained at 400 µm. The spacing between neurons in the brain is on average about 25 µm, which is still significantly smaller than the spacing between recording sites of either of the above electrode

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arrays. However, each recording site can effectively record signals from multiple neurons up to a distance of 40-65 µm, which is still about 4 times smaller than the spacing between probe shanks in state-of-the-art silicon electrode arrays.1 Of potentially greater concern is the lifetime of implanted MEAs.3,25-26 While Utah Arrays have demonstrated functional longevity, showing control of reach and grasp five years after the implantation, they are subject to reduction over time in both channels contributing and spike amplitude.27 Improvements in tissue response and probe material stability are certainly of great interest.28-29 A chronically implanted MEA can cause a persistent reactive response resulting in a glial sheath or scar, which can be caused by mechanical factors such as agitation and mechanical impedance mismatches with surrounding tissue, or responses to electrode surface chemistry and porosity.30 The formation of a glial sheath can lead to difficulty in signal acquisition due to high electrical impedance and the local neuronal cell death or neurofilament loss (~100-200 µm radius).10 Mechanical impedance mismatch is caused by the difference in materials; the MEAs use metal or silicon material which have much higher Young’s modulus compared to tissues. Such MEAs can induce significant tissue damage/inflammation due to the mismatch of mechanical properties.31-32 Although some soft polymer or flexible electrodes have been developed, they often need reinforcement during insertion, which can result in greater tissue damage and also greatly lower their suitability for use in a high density electrode array.33 The efficacy of any of these MEAs is also determined by the quality of the neuron– electrode interface. In all types, a metal surface is still the final contact with the tissues. A fundamental impediment of microelectrodes is their relatively high specific impedance, which increases the overall electrode impedance values for MEAs and consequently increases noise

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levels, especially when signals are propagated long distances without lowering the impedance with pre-amplifier stages on the MEA backplane. As such, in order to improve the recording performances of MEAs, an extensive effort has been made to reduce the impedance of the electrodes. One efficient way to decrease impedance is to increase the effective area of the electrodes while maintaining the small overall electrode dimensions. There is still potential for array improvements, such as reducing electrode surface impedance, increasing the density of microelectrodes, improving process uniformity and enhancing manufacturability. Choosing a suitable electrode material and proposing an efficient method to fabricate MEAs with high density, small spacing, controlled electrode size, closely matched mechanical properties, high sensitivity, and long-term stability still remains a challenge. Here we explore fabrication of vertically oriented 2D arrays of recording posts, a form factor very similar to that of the UEA, using an approach that could enable smaller probes and finer probe spacings. Our goal is to fabricate a 2D array of 20 µm diameter probe shanks with a height of over 1 mm and spacing between probes ≤ 100 µm. These geometries cannot be generated with present silicon or wire-based microfabrication technologies. Deep silicon etching, the only widely used microfabrication technique for high-aspect-ratio structure generation, is limited to structures with heights of several hundred microns due to very slow etch rates (500 µm takes several hours), finite mask selectivity, control over etch verticality, and other limitations.34 Previously

we

developed

carbon-nanotube-templated

microfabrication

(CNT-M)

techniques to fabricate extremely high-aspect-ratio microstructures from a variety of materials.35-37 These techniques use patterned vertically aligned CNT structures as a mechanical framework for infiltration. After growing the framework by patterning the CNT growth catalyst, the framework is

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then infiltrated (using vapor or liquid based infiltration) with matrix materials such as carbon, silicon, silicon nitride, or metals like Ni or Cu, etc. The final filled structure consists mostly of the matrix material due to the very low density of unfilled CNT forests. The CNT-M technique has several other potential advantages for fabricating MEAs. First, the mechanical properties of the structures can be modulated by the choice of infiltration materials and by varying the degree of infiltration. Consequently, it should be possible to lower the mechanical compliance mismatch by providing electrode materials with a Young’s modulus closer to that of the tissues. More closely matching the mechanical impedance of the neural probe to the impedance

of

the

surrounding

neural

tissue

can

significantly

reduce

tissue

damage/inflammation.31-32,38-39 Second, the CNTs used as the framework materials can have intrinsically large surface areas and high electrical conductance, valuable properties for the fabrication of low impedance electrodes.40-41 Additionally a recent study shows that nanoporous CNT based electrodes demonstrate long-term stability due to their high resistance to biofouling when compared to carbon fiber electrodes.22 As a result, MEAs may benefit from the use of CNTs directly as the electrode material. Despite its promise, significant limitations of the CNT-M technique are observed when attempting to fabricate isolated needle-like neural probes. Attempts to directly grow high-aspectratio free-standing CNT posts result in significant bending during CNT growth.37 For moderateaspect-ratio structures, borders of unconnected but nearby nanotubes can assist in the straight growth of free-standing CNT posts; however, even that is not adequate for the very high aspect ratios needed here. In this work, we have modified the CNT-M process to allow for direct growth of straight,

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high aspect ratio, mm-tall CNT post arrays. The modified process uses sacrificial CNT “hedges” to support the posts during growth, followed by a controlled etching process to remove the hedges resulting in a densely packed vertical electrode array. As a result, arrays of isolated, free-standing CNT posts with aspect ratios of up to 60:1 were achieved. The CNT posts have a diameter of ~20 µm and are spaced at 100-400 µm. The etching process consists of non-directional plasma etching (to remove the top portion of the hedges), carbon infiltration (to strengthen the CNT posts), and directional plasma etching (to remove the remaining hedge structure). Electrical characterization measurements confirm the high electrical conductance of the posts and the ability to address them individually. Additionally, electrochemical measurements show the compatibility of the probe with chemical detection including the use of fast-scan rate cyclic voltammetry for detection of dopamine. We also demonstrate a process for electrical integration to individually address each of the CNT probes within the array.

Methods Growth of CNT arrays. The CNT arrays were prepared on a single crystal Si substrate using the following procedure. First, a ~30 nm Al2O3 buffer layer was deposited onto the Si substrate by electron beam evaporation. Next, a 4 nm Fe catalyst film was thermally evaporated onto the substrate and then lithographically patterned using a liftoff process. Vertically aligned CNTs were grown by chemical vapor deposition (CVD) in a 1-in tube furnace. To accomplish this, the substrate was heated to the growth temperature of 750 °C in H2 at a flow rate of 230 sccm (standard cubic centimeters per minute). When the growth temperature was reached, H2 flow was shut off and C2H4 was flowed at 250 sccm for CNT growth. Growth was continued for 35 min until the CNT tube height exceeded 1 mm (about 1.3 mm, average growth rate of ~40 µm/min).

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Finally, the furnace was cooled down in Ar. Etching of CNT support hedges. A multi-step process was used to remove the CNT support hedges (Figure 1a). First, a non-directional oxygen plasma etch was performed in a 6-in barrel cleaner (Harrick Plasma) on the as-grown CNT arrays with RF power in the range of 6-18 W and an air flow rate of 0-20 sccm. Next, a thermal carbon infiltration was performed at 900 °C for 1.5 min to strengthen the CNT structures; thermal carbon growth was performed in the same tube furnace with the same flow rates of H2 and C2H4 used in CNT growth. Lastly, a directional oxygen plasma etch was performed in a parallel plate etcher (Technics Planar Etch II), to remove the remaining CNT hedges (power: 150 W, oxygen flow rate: 10 sccm, and pressure: 500 mTorr). Microscopic Imaging and Electrical Characterization. The microstructures of the CNT microelectrode arrays were characterized by transmission electron microscopy (TEM, FEI Tecnai F20) and scanning electron microscopy (SEM, S-Feg XL30 FEI). The electrical resistivity of the CNT electrodes was investigated parallel (out-of-plane) and perpendicular (in-plane) to the CNT growth direction using 2-point and 4-point measurements. Electrochemical Measurements. For electrochemical measurements, an individual CNT post was attached to a tungsten needle using silver paste. Two chemical analytes were used for the tests: methyl viologen (MV) and dopamine. Various concentrations of MV (0.2, 1, and 5 millimolar) or dopamine (0.01, 0.1, and 1 millimolar) were dissolved in phosphate buffered saline (PBS) buffer at a pH of 7.4 as the electrolyte test solutions. All solutions were de-oxygenated for at least 5 min by sparging with nitrogen gas prior to the electrochemical measurements. For MV electrochemical tests, a three-electrode configuration with an Ag/AgCl reference electrode and a platinum counter electrode was used. Cyclic voltammetry (CV) characterizations

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for MV were done using a potentiostat (Gamry Reference 600) under ambient conditions with two scan rates (5 and 10 V/s), sweeping the voltage from -0.1 to -1.5 V, for each constant MV concentration. A large portion (greater than 700 µm in length) of the CNT post was dipped into the electrolyte and microampere level current was obtained. For dopamine detection, a fast-scan technique was used. Briefly, a continuous flow of dopamine solution was introduced into a chamber filled with PBS solution and a voltage sweep from -0.6 to 1.3 V was applied to the CNT probe electrode with a 400 V/s scan rate (ChemClamp voltage clamp amplifier made by Dagan Corporation, Demon Voltammetry and Analysis Software42). In this way, a continuous change of measured current was obtained, associated with the continuous change of dopamine concentration in the chamber (see the details in the next section). To lower the current level to the range of the fast-scan amplifier, most of the CNT probe was covered with paraffin wax, exposing only a small portion of CNT probe (about 50-60 µm in length) to the electrolyte. These wax coated probes were prepared by dipping into a molten paraffin wax so that the silver paste and the CNT post were insulated, and xylene was used to remove the wax from only the front portion of the CNT post.

Results and Discussion As mentioned above, a sacrificial CNT “hedge” structure (i.e. Sacrificial CNTs that are used to support vertical CNT electrode growth) was used to assist in the straightness of mm-tall highaspect-ratio CNT post arrays during growth (Figure 1a). The hedge structure is a thin wall of CNTs with a width of 5 μm. Without assistance of the CNT hedges, the CNT posts bend in random directions when grown to high aspect ratios. Figure 1b shows top and side view SEM images of the CNT posts, 20-40 μm in diameter, without hedges. As seen in the image, many of -9-



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them already show significant bending at a post height of around 400 μm. It is suggested that the bending is caused by small variations in the growth rate.37,43 Any difference in catalyst nanoparticle densities or in carbon feedstock concentrations between the edge and the center of the post area can cause differences in the CNT growth rate. Bending may not be appreciable when the aspect ratio is low, however, as the aspect ratio increases bending becomes very apparent. This severe bending renders CNT-M posts unusable for MEAs. However, by applying very thin CNT hedges (5 μm width), CNT posts that are highly vertical can be grown. Figure 1c shows top and side view SEM images of the CNT post arrays grown with the help of support hedges. The diameter of the posts is ~40 μm and the height is ~1.3 mm, corresponding to a high aspect ratio of ~30:1. Clearly, the posts remain very straight without noticeable bending, even at high aspect ratios. The well-ordered post arrays (compared to the randomly bent posts) highlight the effectiveness of the CNT hedges. The as-synthesized CNTs were also investigated by TEM and showed crystalline multi-shell cylindrical structures with average diameter of 9 nm (Supporting Information Figure S1).

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Figure 1. (a) Schematic of the combination fabrication process. (b) SEM images of the bending CNT post arrays directly-grown without support hedges. (c) SEM images of the straight CNT post arrays directlygrown with the help of support hedges. (d) Top view and (e) Side view SEM images of the isolated freestanding CNT post arrays after the combination fabrication process. (f) SEM images of an individual freestanding CNT post showing that the support hedges have been removed.

Once the post growth was complete, it was necessary to remove the CNT support hedges to produce isolated free-standing posts. To remove the hedges, we used a combination of nondirectional and directional oxygen plasma etching (Figure 1a). Initial results showed that use of a

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1-step non-directional plasma etch was only able to remove the top portion of the CNT hedges. Conversely, in a 1-step directional plasma etch, the bottom portion of the CNT hedges (and the posts) were preferentially attacked resulting in significant narrowing or even complete removal of the bottom of the post. Therefore, we proposed a combination process consisting of a nondirectional oxygen plasma etch step followed by a directional plasma etch. The non-directional etch was applied to the as-grown CNT arrays and incorporated a very low oxygen flow rate to avoid strong etching. After ~60 min, the top part of hedges, about 300-400 μm, was etched away without destroying the alignment of posts. Second, prior to applying the strong directional etching, a carbon infiltration of ~1.5 min was performed to strengthen the CNT structures. During this step, the fragile CNT posts were reinforced by the infiltrated amorphous or partially graphitized carbon to stabilize them to endure strong etching in the following step.44 Lastly, a directional oxygen plasma etch step was used to remove the remaining lower portion of the CNT hedges. This multistep process resulted in well-ordered, isolated, free-standing, (and critically) vertical highaspect-ratio CNT posts (Figure 1d). Side view SEM images clearly show that the multistep process completely removed the hedges from top to bottom (Figure 1e). A focus on the top and the bottom of an individual free-standing CNT post confirmed that the hedges were removed and only some disconnected residue remained on the substrate surface (Figure 1f). The free-standing CNT posts have diameters around 20 μm and heights over 1.2 mm, corresponding to a high aspect ratio of ~60:1. It is worth noting that longer carbon infiltration times can be used to increase the strength of the posts, but would require a longer time in the directional etcher for hedge removal. A balance between these two steps is required to successfully remove the hedges without excessive reduction of the post diameter. Overall, our combination process has resulted in efficient

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CNT hedge removal and has been used to fabricate over 50 prototype MEAs. To test the capability of our carbon infiltrated CNT posts (probes) to serve as electrochemical sensing electrodes, an individual CNT post was attached to a tungsten needle using silver paste. One end of the post was completely buried in the silver paste (~800 μm long exposed) to assure good electrical connection between the tungsten needle and the CNT post (Figure 2a). Cyclic voltammetry (CV) was performed for various concentrations of methyl viologen (MV) at two scan rates using a three-electrode configuration (Figure 2b and 2c). Several observations can be made from the curves. First, CV curves clearly show the characteristic peaks of MV.45-46 For instance, in the cathodic process, the first reduction peak (pc-1) corresponds to the reaction of MV2+ + e- → MV+• at a potential of about -0.8 V vs. Ag/AgCl. The reference Ag/AgCl electrode has a potential of +0.235 V vs. the Standard Hydrogen Electrode. The second reduction peak (pc-2) corresponds to the reaction of MV+• + e- → MV0 at a potential of about -1.3~-1.4 V. In the anodic process, the oxidation peak (pa-1) corresponds to the reaction of MV+• → MV2+ + e- at a potential of about -0.4 V and the oxidation peak (pa-2) corresponds to the reaction of MV0 → MV+• + e- at a potential of about -0.9 V. Second, as expected, the peak current increased with increasing MV concentration. Third, the peak current also increased with increasing scan rate due to the rapid depletion of MV near the electrode. These CV results indicate that the CNT probes function well as carbon electrochemical probes.

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Figure 2. (a) An individual CNT post (probe) attached to a tungsten needle using silver paste. CV curves of MV at scan rates of (b) 5 and (c) 10 V/s. (d) The CNT probe was insulated by paraffin wax with the tip exposed to act as the active electrode. (e) Current vs. time curves and (f) fast-scan rate CV curves of dopamine at various concentrations. The arrows in (b), (c), and (f) indicate the direction of the voltage sweep and the label of pc and pa is the cathodic and anodic peak, respectively.

For neural sensing applications, we investigated the detection of the presence of dopamine using our CNT probe with fast-scan CV. As shown in Figure 2d, after wax and xylene treatment, only the small front portion of the CNT post served as the active electrode. The SEM image indicates that the post tip (cleared of wax) is composed of aligned porous CNTs. The porosity is determined by the carbon infiltration time; here the infiltration time is ~1.5 minutes resulting in a -14-


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~85% void fraction by weight. In the first step of the measurement, PBS buffer solution was flowed into a measurement chamber containing the CNT probe and a background CV trace was recorded. A dopamine solution was then introduced into the flow while the voltage sweeps continued. With the introduction of dopamine, a current change (relative to the background scan) was observed, indicating detection of the presence of dopamine due to the electrochemical reaction of dopamine on the electrode surface (Figure 2e). The current increased, consistent with an increasing dopamine concentration, and eventually plateaued when the initial PBS solution was completely replaced by the dopamine-containing solution. To confirm that the detected signal was caused by the dopamine concentration, three concentrations of dopamine solution were used, and the signal was observed to increase with increasing concentration. After the fast-scan signal reached a plateau, CV curves were taken for each dopamine concentration (Figure 2f). The characteristic peaks of dopamine were clearly detected with an oxidation peak (pa) at ~0.8 V. The peak current was also influenced by the dopamine concentration. These successful fast-scan CV measurements indicate the capability of these CNT electrodes for neural chemical detection although these were shown at higher concentrations than seen in physiological conditions. It may be noted that the peak current for dopamine at 1 mM concentration is much lower magnitude than that for MV. We attribute this difference in current magnitude to the different effective electrode surface areas used (see the Methods section for details) and the differences in electron transfer kinetics between MV and dopamine on these surfaces.47-48 To apply our CNT probes to MEA devices, it is necessary to evaluate the possibility of using microfabrication to create the electrical connections that address individual probes. Here, we show integration of the CNT probes with conductive tungsten pads and lines using

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conventional photolithography and thin film techniques. As shown in Figure 3a, tungsten pads and lines were first patterned on a Si substrate by sputtering and lift-off, followed by Al2O3 deposition (30 nm), Fe catalyst deposition (4 nm), and CNT growth. Here, the diameter and height of the CNT probes are ~40 μm and ~250 μm, respectively. The SEM images in Figures 3b and 3c show very straight and well-ordered CNT probes grown in the presence of the underlying tungsten metal layer. Please note that for simplicity these short CNT probes were directly synthesized without the assistance of hedges for this demonstration of integration capability. Taller probes would require the hedge-assisted formation process demonstrated above. Next, we used 2-point and 4-point measurements to determine the electrical resistivity of the probe material and to measure the resistance of the electrical connections to the tungsten contacts. The 4-point measurements were performed by putting individual CNT probes onto a custom-designed micro-4-probe setup (inset in Figure 3d). The resistance of three CNT probes was measured and the electrical resistivity was calculated and found to be between 0.2 and 0.6 Ω·cm (Figure 3d). A 2-point resistance measurement was made between the top of a CNT post (by touching the post tip in a probe station) and the tungsten pad. A total of 12 posts were measured this way, yielding resistances of 580 ± 65 Ω. The calculated resistivity of these posts was 0.31 ± 0.04 Ω·cm, which is very similar to the result from 4-point measurement indicating that the presence of the Al2O3 layer between the CNT post and the tungsten does not contribute a significant resistance to the device. There is significant evidence in the literature that the Fe catalyst readily diffuses into the Al2O3 layer (subsurface diffusion).49-51 There is also a prior report demonstrating electrical contact between CNTs and metal growth substrates through a 30 nm thick Al2O3 layer, though no quantitative resistance results were presented.52 In our work, there is more than sufficient Fe in the catalyst layer to

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significantly dope the porous Al2O3 layer through sub-surface diffusion, which may be the cause of the low observed resistance between the CNTs and the tungsten. However, the mechanism details will require further investigation. The electrical resistivity of our CNT probes is at the lower end of reported resistivity values for vertically aligned CNT forests (0.2-31 Ω·cm for outof-plane),53-54 and the overall probe resistance is much lower than the expected neural probe electrode surface impedance.13 Our CNT/carbon composite electrodes have a similar form factor to the Utah Array electrodes, whose reported impedance was 100–300 kΩ at 1 kHz.24 The measured DC resistance of our samples, 0.5 kΩ, is negligible by comparison.

Figure 3. (a) Schematic of process integration to fabricate individually addressable CNT probes. (b) Low and (c) high magnification SEM images showing isolated free-standing CNT probes (posts) directly grown on tungsten lines to form electrical contact. (d) Electrical resistivity of CNT probes measured using 2-point and 4-point configurations.

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It is noteworthy that probe arrays with different materials surfaces such as Ni or W can be fabricated through our CNT-M technique.55 As such, the CNT probes could potentially be used as electrodes to perform differential chemical detection, or used to create devices containing multisensing modalities (i.e. neural electrical recording and chemical neurotransmitter detection) within the same probe array. Beside the presented promising results, we would also like to point out that there are still some mechanical questions remaining that need to be explored in anticipation of the insertion and use of our CNT/carbon composite probes in tissue, such as the compliance and strength of the probes at these lower carbon infiltrations, and the adhesion strength between the probes and the substrate.

Conclusion To summarize, we fabricated high-aspect-ratio (up to 60:1), mm-tall, free-standing, straight microelectrode arrays of carbon infiltrated CNT posts. In the fabrication process, sacrificial CNT hedges were used to enable direct growth of CNT posts with high aspect ratios. A process that combines non-directional plasma etching, carbon infiltration, and directional plasma etching was used to remove the sacrificial support hedges to achieve the isolated CNT posts. Electrochemical sensing with the CNT probes was demonstrated using an individual post to measure the behavior of MV in conventional CV and that of dopamine through fast-scan rate CV, indicated the suitability of the CNT probes (posts) for electrical and electrochemical recording in neural probe MEAs. We also demonstrated a process to fabricate individually addressable CNT probes through tungsten pads and lines. In addition to being a promising method for fabrication of high density MEAs for neural recording and stimulation, these CNT microelectrode arrays have the potential

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for use in a variety of other electrochemical sensing applications.

Acknowledgments The authors gratefully acknowledge funding for the work from the General Electric Company.

Supporting Information TEM images, showing the structure of as-synthesized CNTs grown from 4 nm thick Fe catalyst film.

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For Table of Contents Use Only

Fabrication of High Aspect Ratio Millimeter-Tall Free-Standing Carbon Nanotube Based Microelectrode Arrays Guohai Chen,a,† Berg Dodson,a David M. Hedges,b,‡ Scott C. Steffensen,b John N. Harb,c Chris Puleo,d Craig Galligan,d Jeffrey Ashe,d Richard R. Vanfleet,a and Robert C. Davisa,* a

Department of Physics and Astronomy, Brigham Young University, Provo, UT 84602, USA

b

Department of Psychology and Neuroscience, Brigham Young University, Provo, UT 84602, USA

c

Department of Chemical Engineering, Brigham Young University, Provo, UT 84602, USA

d

General Electric Global Research (GE-GR), 1 Research Circle, Niskayuna, NY 12309, USA

* Corresponding author. E-mail address: [email protected] †

Present address: CNT-Application Research Center, National Institute of Advanced Industrial Science and

Technology (AIST), 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan ([email protected]). ‡

Present Address: Scientific Computing and Imaging Institute, University of Utah, 72 S Central Campus Drive,

Room 3750, Salt Lake City, UT 84112

Table of Contents Graphic

-27-



Figure 1. (a) Schematic of the combination fabrication process. (b) SEM images of the bending CNT post arrays directly-grown without support hedges. (c) SEM images of the straight CNT post arrays directlygrown with the help of support hedges. (d) Top view and (e) Side view SEM images of the isolated freestanding CNT post arrays after the combination fabrication process. (f) SEM images of an individual freestanding CNT post showing that the support hedges have been removed. 129x99mm (300 x 300 DPI)


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Figure 2. (a) An individual CNT post (probe) attached to a tungsten needle using silver paste. CV curves of MV at scan rates of (b) 5 and (c) 10 V/s. (d) The CNT probe was insulated by paraffin wax with the tip exposed to act as the active electrode. (e) Current vs. time curves and (f) fast-scan rate CV curves of dopamine at various concentrations. The arrows in (b), (c), and (f) indicate the direction of the voltage sweep and the label of pc and pa is the cathodic and anodic peak, respectively. 119x81mm (300 x 300 DPI)



Figure 3. (a) Schematic of process integration to fabricate individually addressable CNT probes. (b) Low and (c) high magnification SEM images showing isolated free-standing CNT probes (posts) directly grown on tungsten lines to form electrical contact. (d) Electrical resistivity of CNT probes measured using 2-point and 4-point configurations. 80x37mm (300 x 300 DPI)


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Fabrication of High Aspect Ratio Millimeter-Tall Free-Standing Carbon

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