6th Annual International IEEE EMBS Conference on Neural Engineering
San Diego, California, 6 - 8 November, 2013

Evaluation of Post-Fabrication Thermoforming Process forIntracortical Parylene Sheath Electrode*
Brian J. Kim, Student Member, IEEE, Seth A. Hara, Student Member, IEEE, Benny Chen, Jonathan
T.W. Kuo, Curtis Lee, Student Member, IEEE, Christian A. Gutierrez, Member, IEEE, Tuan Hoang,Malancha Gupta, Victor Pikov, and Ellis Meng, Senior Member, IEEE

Abstract² The chemical, mechanical, and electrochemical
attributes of the Parylene sheath electrode (PSE) wereevaluated following a post-fabrication thermoforming process
to determine its impact on both the polymer and thin film
platinum materials. The three-dimensional conical shape of thePSE was formed via thermal molding of a surface
micromachined Parylene C microchannel using a custom
shape-forming microwire having the desired taper at 200°C for
48 hours under vacuum. Contact angle and Fourier transform
infrared spectroscopy measurements indicated that thethermoforming process resulted in no significant changes to thesurface and bulk chemistry of Parylene. The thermoformedParylene samples possessed greater <RXQJ¶V PRGXOXV EXW
retained their flexibility. Electrochemical characterization ofelectrodes before and after thermoforming revealed adecreased storage charge capacity and increased electrode
impedance, however, recording functionality was not lost as
resolvable neuronal unit activity was successfully obtained
post-implantation.
I. INTRODUCTION
Chronic implementation of intracortical prosthetic
devices has been limited by multiple failure mechanisms
encountered during long-term implantation. In addition toabiotic failure mechanisms (e.g. electrode corrosion, broken
connectors), the decline of recording performance ofelectrodes has been attributed to biotic causes that occur invivo [1]. Specifically, the inflammatory response caused byinsertion trauma and the presence of a foreign body within
the cortex [2, 3] forms glial cell encapsulation and neuronal
retraction from the recording sites, which limit the longevityof high fidelity recordings [4]. This response is further
exacerbated by the mechanical mismatch between current
electrodes, made of silicon, metal, or stiffer polymers, and*This work was sponsored by the Defense Advanced Research ProjectsAgency (DARPA) MTO under the auspices of Dr. Jack Judy through theSpace and Naval Warfare Systems Center, Pacific Grant/Contract No.
N66001-11-1-4207.
B.J. Kim, S.A. Hara, J.T.W. Kuo, C. Lee, T. Hoang, and E. Meng arewith the Department of Biomedical Engineering at the University ofSouthern California, Los Angeles, CA 90089 USA (corresponding author:213-821-3949; e-mail: ellis.meng@usc.edu).
B. Chen and M. Gupta are with the Mork Family Department ofChemical Engineering and Materials Science at the University of Southern
California, Los Angeles, CA 90089 USA.
C. A. Gutierrez is an independent engineering consultant in Los Angeles,
CA 90089 USA.
V. Pikov is with the Huntington Medical Research Institutes (HMRI),Pasadena, CA 91105 USA.

978-1-4673-1969-0/13/$31.00 ©2013 IEEEthe surrounding neural tissue, inflicting continuous damage
during brain micromotion over time [5].

Figure 1. Optical micrograph of the tip of a Parylene sheath electrode. Scalebar is 300 µm.

To address these concerns, we developed a threedimensional (3D), flexible Parylene sheath electrode (PSE)
constructed from surface-micromachined Parylene C forneuronal recording (Fig. 1) [6]. The PSE consists of ahollow sheath structure with four electrode sites within thelumen and another four electrode sites on the outer perimeterof the sheath. The aim of the PSE is to utilize a drug-coated,sheath-based design to minimize the immune response andencourage neuronal proliferation and growth towards andwithin the sheath, which houses the electrode sites andfollows the previous success of the neurotrophic cone
electrode [7]. The PSE however, also has added benefits of aflexible, thin (10 µm) substrate material that would further
reduce the mechanical mismatch (and subsequent damage)
between the PSE and surrounding cortical tissue comparedto the rigid, glass-based neurotrophic cone electrode.
To construct this 3D sheath structure from a flat, surfacemicromachined device, we harness the thermoplastic
property of Parylene C, and thermally mold, or³WKHUPRIRUP´, a conical shape from a flat microchannel [8].
Thermoforming these devices involves holding a sample inthe desired configuration within a mold, and heat treating theassembly at a temperature between the glass transition
temperature (~60-90°C) and the melting temperature(290°C) of Parylene C [9]. By annealing the assembly
between these temperatures while maintaining the desiredconfiguration, the polymer chains can rearrange and thereby
facilitate conformation to the molded orientation. As theassembly is cooled and mold removed, the final molded
configuration remains, due to the immobility of the polymer
chains at temperatures below the glass transition
temperature. It is important to note that the thermoforming
process should be carried out in a vacuum environment toinhibit thermal oxidation of Parylene C at temperatures379

>125°C [10]. The PSE was thermoformed at 200°C for 48
hours to not only achieve its 3D shape, but also to enhance
the adhesion between the Parylene C layers [11]. Here we
present the evaluation of the thermoforming process on thechemical and mechanical properties of Parylene C as well as
the electrochemical performance of the PSE, to capture any
material changes and their possible impacts on the final
thermoformed device, and on the feasibility of the PSE as animplantable intracortical neural probe. We seek to assess theadvantages and disadvantages of this simple post-fabrication
process for the development of novel 3D structures using
Parylene C for applications in neural electrode development.
II. MATERIALS AND METHODS
A detailed description of the fabrication of the PSE waspreviously described [6]; a short description is presented
here for completeness. Platinum electrodes and leads were ebeam evaporated and patterned by a liftoff method onto aParylene C substrate layer on top of a silicon carrier wafer.
A Parylene C microchannel was then constructed on top of
the four inner electrodes by depositing a Parylene C layer on
top of a sacrificial photoresist structure in the shape of araised trapezoid. Openings were etched on the tip and baseof this microchannel to allow for the insertion of themicrowire mold for thermoforming. Lastly, the flat probes
were oxygen plasma etched to their final shapes and werereleased from the wafers and soaked in acetone to remove
the sacrificial photoresist structures to reveal the hollow
microchannel to be thermoformed (Fig. 2a).

Parylene samples using a nanoindenter with a Berkovich tip.
A custom made large-scale indenter (>100 µmdisplacement) also conducted additional structural stiffness
measurements to assess the mechanical resilience andflexibility of the newly thermoformed structure.
C. Electrochemical Characterization
Thermoforming effects on electrode (and thus recording)properties of the PSE were assessed using cyclic
voltammetry (CV; 1x PBS, -0.6 to 0.8V, scan rate of 50
mV/s for 3 cycles, Ag/AgCl reference) and electrochemical
impedance spectroscopy (EIS; 1x PBS, 1-100,000 Hz,
Ag/AgCl reference), which are widely used to assess
electrode properties and investigate changes to the electrodesurfaces [12]. CV and EIS measurements were taken ofelectrodes before and after the thermoforming process toobserve possible annealing effects on the electrode material,and give insight into the recording capability of the PSE.
D. In Vivo Chronic Implantation and Measurements
Chronic implantation of the PSE was performed in theM1 motor cortex of young male Sprague Dawley rats (>340g). Electrophysiological recordings were carried out weekly,
with rats anesthetized with Ketamine/Xylazine (90/10mg/kg, IP). The electrophysiological data was acquired at 16bit and 40 kHz per channel using a 64-channel dataacquisition system (OmniPlex; Plexon Inc., Dallas, TX) andhigh-pass filtered at 300 Hz to remove the low-frequency
fluctuations from the baseline.
III. RESULTS AND DISCUSSION

Figure 2. (a) Flat, surface micromachined Parylene device following
removal of sacrificial photoresist. (b) Insertion of microwire mold into
microchannel. Scale bars are 200 µm.

A. Thermoforming of Parylene Devices
Following removal of the sacrificial photoresist, acustom-made microwire mold of the desired taper width was
manually inserted into the lumen of the microchannel tomechanically open and create the desired 3D conicalstructure (Fig. 2b). This assembly was then placed inside ofa vacuum oven, where upon reaching maximum vacuum(~10 mTorr), the temperature was ramped 1.3°C/min to
200°C. Once the ramp was complete, the temperature washeld for 48 hours, after which the oven was slowly cooled toroom temperature overnight. Following the thermoforming
process, the microwire was easily removed from theParylene devices to reveal the 3D cone shapes (Fig. 3).

A. Thermoforming
Vacuum thermoforming of the Parylene C microchannels
produced consistent 3D sheath structures without observablematerial oxidation. Different conical taper widths (sharp andmoderate) were produced from Parylene microchannels andmicrowire molds of varying tapers, and a cylindricalstructure was also formed by using a cylindrical microwire
mold (Fig. 3a-c). A design iteration of the PSE withperforations on the sheath and substrate to facilitate innerouter cell-to-cell signaling was also thermoformed using thesame procedure to produce a 3D perforated sheath (Fig. 3d),demonstrating the robustness of the process in forming 3D
sheath structures.

B. Chemical and Mechanical Characterization
Surface and bulk chemical changes to the thermoformed
Parylene material were characterized using contact angle
measurements and Fourier transform infrared spectroscopy(FTIR), respectively. These tests were conducted on
unmolded Parylene test samples (15 µm thick films) thatwere also subjected to the same thermoforming process as
microwire-molded samples. Mechanical changes wereassessed by measuring the <RXQJ¶V PRGXOXV of heat-treated380

Figure 3. SEMS of thermoformed sheath structures showing different PSE
designs: (a) sharp taper, (b) moderate taper, (c) cylindrical, and (d)perforated with sharp taper. Scale bars are 200 µm.

B. Chemical Characterization
Contact angle measurements of Parylene test samples
indicated that the surface energies and surface roughness
between untreated and thermoformed samples of Parylene
were relatively unchanged following the thermoforming
process (Table 1).
TABLE I.

RESULTS OF CONTACT ANGLE MEASUREMENTS

Condition
Untreated
Thermoformed (200°C, 48 hours)

Contact Angle
73.4 ± 0.9°; Mean ± SE, n = 3
74.7 ± 1.1°; Mean ± SE, n = 3

The bulk material also remained consistent, as seen
through absorbance measurements using FTIR (Fig. 4).
There was no observable change in the bulk chemicalfunctionality as no new significant peaks were detected
between the untreated and thermoformed samples. Theseresults suggest that the thermoformed films still retain theas-deposited Parylene C surface and bulk chemical
properties, which is important to note when relying on thebiocompatibility of as-deposited Parylene films.

Though the Parylene substrate increases in stiffness
following the thermoforming process, flexibility of the PSE
was not lost. This is supported by mechanical deformation
tests that demonstrated that the Parylene sheath structure
could return to its original open position following a
deformation of 200 µm imposed by a 25 mN load (Fig. 5).
The sheath is also sufficiently robust to resist any
deformation in vivo as seen in histological data [14].
Although slightly stiffer than the untreated material, thethermoformed PSE is still a large improvement inminimizing the mechanical mismatch between cortical tissue
and current technologies, and compares favorably to otherpopular polymers used in microfabrication, such aspolyimide.

Figure 5. Displacement (black squares) and load force (red triangles)
measurements taken during deflection studies of thermoformed PSE sheathstructure. Inset and plot indicate the (1) starting position, (2) deflection of
200 µm, and (3) return to initial position indicating following release ofload. Sheath remains flexible and is robust to mechanical perturbations.
Sheath deformation is outlined in black to facilitate visualization. Scale bar
is 200 µm.

Figure 4. Comparison of FTIR results of (a) untreated and (b)thermoformed Parylene C samples indicating no new peaks following thethermoforming process.

C. Mechanical Characterization
Measurements of <RXQJ¶V PRGXOXV LQGLFDWHG WKDW theParylene substrate increased in stiffness following
thermoforming (Table 2). These results are also consistent
with the increase in crystallinity that is observed in literature
with the heat treatment of Parylene due to the formation ofadditional crystalline domains within the amorphous regionsof the polymer [13]. Previous studies have shown that thisstiffness can be tuned by varying the parameters ofthermoforming such as the temperature and duration of heat
treatment [8].
TABLE II.

D. Electrochemical Characterization
CV measurements of the platinum electrode surface
indicated a noticeable reduction in the area inside the CV
curve compared to the untreated electrodes, indicating adecreased charge storage capacity (Fig. 6).

RESULTS OF YOUNG¶S MODULUS MEASUREMENTS

Condition
Untreated
Thermoformed (200°C, 48 hours)<RXQJ¶V 0RGXOXV
2.70 ± 0.06; Mean ± SE, n = 16a3.97 ± 0.06; Mean ± SE, n = 28aa. n values taken at different test regions over a single large area sample

Figure 6. Representative CV curves of untreated (blue dashed) andthermoformed (black solid) electrodes.

Electrochemical impedance also supported an alteredelectrode surface as the impedance magnitude had increased
and the phase plot had shifted to the right; the 1 kHz

381

impedance magnitude increased by 375% to ~NŸfollowing thermoforming (Fig. 7). Taken together with theCV results, we hypothesize that one or more phenomena
may have contributed to the observed changes: (1)contamination of the surface of the platinum electrodes bychlorine molecules mobilized from Parylene C during
annealing that diffuse through microvoids within platinum
grain boundaries formed following platinum thin-film
annealing [15, 16] and/or (2) a combination of the relaxation
effects of the intrinsic residual stress of the deposited thinfilm platinum [17] and possible formation of microcracks
during thin-film platinum annealing [18]. Additional studies
are underway to determine the cause of the alteredelectrochemical properties of thin film electrodes supported
on thermoformed Parylene substrates.

Figure 7. Representative plots impedance magnitude (a) and phase (b) of
EIS measurements of untreated (red triangles) and thermoformed (blacksquares) electrode.

Despite the chemical or mechanical disruptions to theelectrode surface caused by thermoforming, the electrodes
were still able to detect resolvable neural signals in vivo as
seen in Fig. 8.

Figure 8. Representative 500 msec trace from a 300-Hz high-pass filteredelectrophysiological record from thermoformed PSE at 21 days postimplantation into the rat motor cortex.

IV. CONCLUSION
Evaluation of the chemical, mechanical, andelectrochemical
effects
ofthepost-fabrication
thermoforming process revealed little changes to the surface
and bulk chemistry of the material, but an increase inmaterial stiffness as well as alterations to the electrodesurface. Despite these effects, the PSE still performed in its
intended design as a sheath-based neural probe. Currently,
additional studies are underway with more specificmeasurement methods to pinpoint the cause of the aberrantelectrochemical effects of thermoforming. Although the PSE
has been able to perform in vivo despite these factors,
understanding the cause of the phenomenon may help in
improving its chronic performance. Overall, our studies
indicate that thermoforming is a simple process forachieving 3D Parylene C structures for neural interface
applications with minimal effects on performance.

ACKNOWLEDGMENT
The authors would like to thank Dr. Saiyun Hou of theHuntington Medical Research Institutes (HMRI, Pasadena,CA 91105) for performing the chronic animal implantations
and electrophysiological recordings, Mr. Yuzheng Zhang foraid in nanoindentation measurements, and the members of
the USC Biomedical Microsystems Laboratory for theirsupport.
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