ARRAYED 3D PARYLENE SHEATH PROBES FOR NEURAL RECORDINGSJ.T.W. Kuo1, B.J. Kim1, S.A. Hara1, 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
ABSTRACT
Parylene C arrayed neural probes possessing 3D sheath structures that can be decorated with neurotrophic factors forattracting and promoting neural tissue ingrowth are presented for long-term intracortical neural recording. 3D sheathstructures were constructed through thermoforming of surface micromachined Parylene microchannels around a solid
microwire mold. Multiple Pt electrodes were patterned along the sheath lumen and exterior. Electrochemical characterizationof the electrodes confirmed impedance values (25-150 kΩ at 1 kHz) suitable for neural recordings. Sheath probes were
temporarily stiffened for a successful implantation demonstrated in agarose brain tissue model.

KEYWORDS:

Parylene C, Thermoforming, Neural Probe, Neuroprosthetic, Brain-machine-interface

INTRODUCTION
Achieving long-term reliable neural recordings for brain-machine interfaces is an ongoing challenge. Commercial probe
technologies in the forms of microwire arrays or micromachined Si probes (Utah and Michigan) experience loss of neuralsignal recording over time; this loss may be attributed to aggressive foreign body response, mechanical mismatch aggravatedby brain micromotion, and the resulting gliosis shielding recording sites from neuronal activity [1].
The neurotrophic electrode, however, has achieved long-term neural recordings in humans on the order of years [2]. This
success is attributed to the release of neurotrophic factors from the probe into the surrounding tissue that attracts neural tissue
ingrowth into the conical device towards recording electrodes. These probes are manually assembled which suffers from poor
reproducibility and thus has limited its adoption.
Microfabrication technologies overcome manufacturing challenges and achieve batch fabricated devices with consistent
repeatability and fine features. Recording sites constructed with thin film metal electrodes increases the number and densityof recording sites per probe for greater spatiotemporal signal. Probes constructed fromflexible polymer materials can partially alleviate the mechanical mismatch. Finally,
promotion of tissue integration post-implantation can be achieved with slow release ofneurotrophic coatings. Therefore, to achieve long-term neural recordings, we previously
introduced flexible Parylene neural probes possessing multiple Pt recording sites withnovel 3D sheath structures that can be decorated with neurotrophic factors for neuraltissue ingrowth (Fig. 1) [3]. Here, improved sheath probe designs in the forms of ×and Figure 1: SEM (left) and profile
1× arrays are described. Sheath probe arrays maximize neural signal inputs and view (right) of Parylene sheathprobe. The 3D sheath structureaccessible brain volume per device.
allows for ingrowth of neuralprocesses.
METHODS
Materials
Parylene C, a USP Class VI biocompatible polymer that can be processed with standard micromachining technologies,
was used to construct the sheath probes. Parylene C (E~3 GPa) is significantly more compliant than traditional probe
materials such as Si or glass (E~100-200 GPa). Thin-film Pt, a non-corrosive and inert metal widely used in neural interfaces
was selected for recording site construction, simplifying sheath probe fabrication since no adhesion layer between Pt andParylene was required [4].
Device design
Probes consist of sheath structures with four Pt electrodes (45 µm diameter) on each of the inner and outer surfaces(eight total electrodes). Sheath dimensions and electrode placement were selected to match the anatomy of rat barrel motor
cortex to target recordings of vibrissal (whisker)-related neuronal activity. Probes were arrayed in a × or 1× arrangement
for maximal access to neural signals; allowing simultaneous recordings from multiple vibrissal-related sites per device.
Device fabrication
A bare Si wafer with native oxide was used as a carrier substrate during the microfabrication process and aided in thesubsequent release of Parylene probes from the wafer. The Si wafer was first coated with 5 m Parylene. Electrodes, traces,and contact pads were liftoff patterned using AZ 5214 E-IR and e-beam deposited Pt (2000 Å). Parylene insulation (1 m)
was then deposited, patterned, and plasma etched, exposing recording sites and contact pads. Next, AZ 4620 (9.6 m) waspatterned and covered with 5 m Parylene to create sheath structures. Selective plasma etching of Parylene exposedperipheral electrodes, contact pads, and sheath openings while simultaneously defining each individual probe. Finally, theprobe was lifted from the wafer surface and an acetone soak released the sheath structure.
3D Parylene sheath thermoforming
A microwire mold was aligned and inserted into the Parylene microchannel under a microscope. The assembly was thenplaced into a vacuum oven and the Parylene was thermoformed at 200 °C and held for 48 hours followed by a controlled cooldown. Nitrogen purging prevented Parylene oxidative degradation by minimizing oven oxygen content. The microwire wasthen removed and the sheath retained its 3D structure. The thermal annealing process also served to improve adhesion
between Parylene layers.

RESULTS
Arrayed devices were successfully fabricated and thermoformed (Fig. 2). Thermoformed
Parylene sheaths were mechanically robust and could withstand repeated indentation
demonstrating that probe structures will not collapse upon implant. Electrical connections weremade with use of a ZIF connector (Fig. 3a). Electrochemical characterization of Pt recordingsites through electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV)demonstrated good neural recording properties with acceptable electrode impedances andelectrochemical surface properties matching that of bulk Pt wires, respectively.
An insertion technique that temporarily stiffened the sheath probes for implantation wasdeveloped since the Parylene probes were too compliant for direct insertion into neural tissue(Fig. 3b and 3c). Each sheath probe was individually affixed to a microwire with polyethylene
glycol (PEG). The assembly was then attached to a stereotactic tool and inserted into 0.5% agar
solution, brain tissue model. A saline flush released the sheath probes by dissolving the PEG
and the microwires were removed; leaving the sheath probes implanted (Fig. 3d).

Figure 2: Released
2×2 sheath probearray.

Figure 3: (a) Electrical packaging of 2×2 arrayed sheath probes with a ZIF connector to a PCB. (b) 1×2 array attached tomicrowires on an introducer tool with PEG. (c)2×2 array inserted into brain tissue mimic while affixed to microwires. (d)Array remained implanted after probe release and microwire removal.

CONCLUSION

3D Parylene sheath probes were arrayed in ×and 1×2 architectures for long-term neural recordings over multiple sites
of interest and large areas. Biocompatible materials were selected for maximizing long-term recording success. Probes
featured multiple Pt recording sites on both interior and exterior sheath surfaces. Electrochemical characterization of the thinfilm Pt confirmed acceptable properties for neural recordings. An implantation method was developed and successfully
demonstrated in brain tissue mimic; thus this technique is easily translated for in vivo implantations.
Future work entails development of deposition processes for decorating neurotrophic factors onto probe surfaces andlong-term cortical recordings from rat. This probe architecture will also be investigated for peripheral and central nervous
system recording applications.

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