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ELECTROCHEMICAL CHARACTERIZATION OF A 3D PARYLENE SHEATHCORTICAL PROBE
S.A. Hara1, J.T.W. Kuo1, B.J. Kim1, C.D. Lee1, L.Yu1, C.A. Gutierrez2, T.Q. Hoang1, and E.Meng1
1
University of Southern California, USA2
Independent Consultant, Los Angeles, CA, USA

ABSTRACTWe developed a 3D Parylene sheath cortical probe for the purpose of advancing neuronal recording reliability andlongevity. Our approach is to improve probe-tissue integration through a unique 3D open-lumen probe structure and theapplication of biofunctional coatings to mitigate adverse immune reactions and promote neuronal growth. We present the useof electrochemical (EC) testing to confirm probe integrity and interrogate changes to electrode performance that may arise asa result of the post-fabrication 3D sheath structure modification and the application of biofunctional coatings.

KEYWORDS:

Neuroprosthetics, microelectrodes, electrochemical impedance spectroscopy

INTRODUCTION
Silicon-substrate and microwire intracortical recording probes suffer from loss of signal over time as a result of tissue
inflammation and the body’s immune response [1-2]. By fabricating our probe on a Parylene C substrate, we aim to mitigate
the damage to cortical tissue associated with hard, rigid probes [3]. This biocompatible material
also allows for the creation of a 3D open-lumen structure, which is shaped by thermoforming aParylene microchannel around a microwire mold. Thermoforming is conducted at 200°C for 48
hours in a vacuum environment after the standard microfabrication process. Eight Pt electrodes
are arranged on the probe, four inside and four outside of the sheath [4] (Fig. 1).
Electrochemical impedance spectroscopy (EIS) was used to measure changes to the electrodeproperties due to the modified probe geometry as well as the application of heat.
The design of the sheath structure works in conjunction with biofunctional coatings toattract neuronal growth into the open-lumen of the probe, thereby bringing neurons in close
Figure 1: Fabricatedproximity with recording sites, isolating signaling, and anchoring the probe to the corticalprobe
tissue. The role of these coatings in altering electrode function was also studied with EIS.

METHODS AND RESULTS
Following microfabrication but prior to thermoforming, electrodes were EC cleaned by cycling the potential from -0.2 to1.2V vs Ag/AgCl (3M NaCl) at 250 mV/s for 30 cycles in 0.5M H 2SO4 with a Pt counter electrode. The effect of thiscleaning process is demonstrated in
EIS measurements taken before andafter EC cleaning (Fig. 2). EIS wasperformed in 1×PBS with a 10 mV
rms perturbation signal and frequency
range of 1-100,000 Hz. A Pt counterand Ag/AgCl (3M NaCl) reference
wereused.
Theremovalofcontaminants from the electrodesurface correlates to the slightreduction of the impedance magnitude
as well as the correction of the phasecurve from multiple to the expected
Figure 2: Impedance magnitude and phase both before (black squares) andsingle time constant curve. Following
after (white circles) EC cleaning. Mean ± SE, n=8 different electrodes.
EC cleaning, the resulting EIS curvesconfirm probe integrity and indicate impedances capable of recording neuronal signaling.
To investigate the effects of the thermoforming process, EIS curves were taken at two points during the process: (1) after
mechanically opening the Parylene microchannel with the microwire mold, but prior to heat treatment and (2) after heattreatment (Fig. 3). Mechanically opening the sheath caused only a slight shift in the impedance curves, which may be
explained by the broader conductive path afforded to the inner electrodes upon widening of the sheath. After heat treatment,
impedance magnitude increases, suggesting changes to the electrode surface following thermal processing. Electrodes appear
to possess increased roughness by scanning electron microscopy and further analysis is underway to identify the cause.

Figure 3: Impedance magnitude and phase before thermoforming process (black square), after mechanical opening ofsheath (white circle) and after complete thermoforming process (gray triangle). Mean ± SE, n=8 different electrodes.
Probes were coated with biofunctional coatings consisting of an extracellular matrix, Matrigel, loaded with either theimmunosuppressant dexamethasone complexed with cyclodextrin (Dex), or neurotrophic factors nerve growth factor (NGF)
and neurotrophin-3 (NT-3), and then immersed in 1×PBS at 37°C. The normalized change in impedance from pre-coat values
was monitored over the course of three days (Fig. 4).The control and neurotrophic factor samples exhibited a gradual
impedance drop over time as the coatings eluted off of the electrode surface. The sample coated with dexamethasone,
however, showed a steady increase in impedance. This may be attributed to the tendency of the immunosuppressant coating
to swell in the presence of water, resulting in a thicker coating covering the electrode and impeding charge transfer [5].

Figure 4: Normalized impedance with respect to pre-coat impedances, monitored over three days: Day 0 (black square),Day 1 (white circle), Day 2 (gray up triangle), and Day 3 (white down triangle).

CONCLUSION
Electrochemical techniques were used to validate that our cortical probe possesses the desired electrochemical properties
for neuronal recording. Our results indicate that, in addition to standard cleanroom cleaning procedures, EC cleaning
provides a useful extra measure to remove electrode contaminants following microfabrication. Additionally, while thethermoforming and coating processes can lead to increased electrode impedance, values remain below 1MΩ and are expected
to perform well in vivo [6].

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