THREE DIMENSIONAL TRANSFORMATION OF PARYLENE THIN FILM
STRUCTURES VIA THERMOFORMING
B. J. Kim1, B. Chen2, M. Gupta2, and E. Meng1
Department of Biomedical Engineering, University of Southern California, Los Angeles, CA, USA
2
Mork Family Department of Chemical Engineering and Materials Science, University of Southern
California, Los Angeles, CA, USA

1

ABSTRACT
Non-planar, three dimensional structures, not possible
with conventional microfabrication processes, wereachieved using post-fabrication thermal annealing of thinfilm Parylene-C structures facilitated by a mold(“thermoforming”). We demonstrate thermoforming ofParylene-Parylene and Parylene-metal-Parylene (PMP)
structures for increased structural and mechanicalfunctionality such as strain relief, formation of open-lumen
sheath structures, and conformation-matching of curved
surfaces that broaden applications for Parylene MEMS.
Characterization of the material and mechanical properties
as a function of thermoforming temperature is alsopresented. Thermoformed Parylene consistently retained
bulk/surface chemical material properties following thetreatment regardless of temperature, and thermoforming at
higher temperatures increased structural stiffness, which is
attributed to increased crystallinity of the polymer. By
varying the thermoforming process parameters, the final
shaped structure can be mechanically and structurally
tuned for broad range of applications, most notably,
structured implantable neural interfaces with integrated
channels for tissue ingrowth and improved integration.

INTRODUCTION
Thermoforming of polymers for biomedical
applications was previously reported to form microfluidic
and lab-on-a-chip devices [1]. Common thick-film
thermoplastics, such as polystyrene, were used to achieve
substrate material shaping for a variety of applications.
Typically, thermoforming processes involve the use ofmolds and pressure to hold a sample in a specific shapeduring heating (above its glass transition temperature but
below the melting point) which softens the material andfacilitates shaping. Subsequently, the cooling process locks
in the new shape [1]. Within the thermoforming
temperature range, polymeric chains experience increased
mobility and are free to move and undergo thermallyinduced reorganization [2]. Although this process was wellcharacterized for common polymers (e.g. PET, PLA), thepotential of this technique for shaping Parylene C, abiocompatible thermoplastic polymer [3], was introduced
recently.
Parylene C has garnered much attention as a structuraland encapsulation material for biomedical devices
attributed to its many favorable features: including
biocompatibility, chemical inertness, dielectric properties,
moisture barrier properties, and low Young’s modulus.
Thermal treatment of Parylene-based devices was explored
for the micromolding of microchannels [4], hot-embossingof patterns [5], and simple thermoforming of Parylene into
curved shapes [6-8]. In addition to the formation ofstructures, thermal annealing of Parylene C was exploredto improve encapsulation and adhesion properties [9].
Here, formation of complex three dimensional (3D) shapes
via thermoforming, together with analysis of the resulting
surface and mechanical properties of thermoformed
Parylene are reported for the first time.
Thermoforming of Parylene C involves heat treatmentof the film between the glass transition and melting
temperatures, which are ~60-90°C and 290°C, respectively[4]. By annealing Parylene between these temperatures
while simultaneously holding the final desired shape,complex, functional structures can be realized. It is
important to note that Parylene C undergoes thermaloxidation at elevated temperatures >125°C in anoxygen-rich atmosphere [10]; thus, all thermal treatment
must be performed under vacuum with nitrogen backfill toensure the absence of oxygen.
The utility of thermoforming was realized for shaping
an as-fabricated, flat surface-micromachined Parylene
channel into a 3D sheath lined with electrodes as animplantable cortical recording probe [11]. In addition tostructural shaping, the added benefits of annealing
Parylene-Parylene layers, such as enhanced adhesion [12],supports thermoforming as a means to attain 3D structuraland mechanical functionality to Parylene-based devices toexpand Parylene MEMS applications.

EXPERIMENTAL METHODS
Thermoforming of two multi-layered thin film
structures was performed: Parylene-Parylene (dual layer)
and Parylene-Metal-Parylene (PMP) sandwiches (Figure1). These thin film combinations are commonly
encountered in practical device structures and wereselected for this reason.

Figure 1: Top and cross-sectional drawings illustrating
test samples used in thermoforming experiments: (left)
Paryleneon
Parylenesamplesand(right)
Parylene-metal-Parylene (PMP) sandwiches.
To obtain the final desired shape, films were wrappedaround or placed on molds (i.e. stainless steel, aluminum,or tungsten; Figure 2), or held in place with aluminum foil.
Fixtured samples were then thermoformed in a vacuum
oven (Lindberg/Blue V0914A, 10 mTorr) with nitrogen
backfill (10 SCFH). The oven was ramped at a rate of~1.3°C/min to the desired final thermoforming
temperature. Fixtured Parylene samples were annealed for
1 hour at different temperatures (100, 120, 140, 160, 180,and 200°C) to elucidate temperature related effects.
Samples were slowly cooled overnight also under vacuum
and nitrogen backflow to ensure that thermal degradation
does not occur during the cooling process. After cooling,
the molds and fixtures were easily removed to reveal thefinal structure without the use of dedicated release layers.

Figure 3: Thermoformed Parylene and PMP samples: (a)cylindrical sheath for promoting neural integration, (b)SEM of cylindrical sheath showing 3D structure, (c) PMP
cone for cone shaped sheath electrodes, and (d) curved
PMP cables bonded at the central crossing point.
Figure 2: Thermoforming fixture consisting of a microwire
inserted into a Parylene channel to form cone shaped
sheath electrodes for neural recordings. The microwire
was positioned in the lumen of the cone to maintain thefinal desired shape during the thermal treatment.
Chemical and mechanical properties werecharacterized as a function of varying temperatures. To
assess effects on bulk chemical properties, Fouriertransform infrared spectroscopy (FTIR) (Nicolet iS 10FT-IR Spectrometer) was used to identify any molecularchanges or additions to the bulk material following
thermoforming. Changes to surface properties were also
assessed by measuring contact angle on the heat-treatedParylene surfaces (10 L droplet, Ramé-Hart Model
290-F1). Mechanical properties, notably molded structure
stiffness, were also of interest to see if thermoforming
could not only affect structural but also mechanicalfunctionality. Structural stiffness measurements wereobtained on thermoformed Parylene cone structures using ananoindenter (Berkovich tip, MTS Nano Indenter XP).

RESULTS AND DISCUSSION
Thermoformed Structures
Vacuum thermoformed structures intended for avariety of applications were achieved. To form 3D neuralinterfaces, sheath-shaped structures consisting ofParylene-Parylene and PMP samples were formed by theinsertion of a microwire mold into an as-fabricatedParylene microchannel (Figure 3a-c). The 3D shapeformed by thermoforming the microchannel is clearlyobserved in the SEM image (Figure 3b), forming a hollow,
tubular structure that would be difficult to fabricate using
conventional microfabrication techniques. Thermaltreatment of Parylene also yielded bonded PMP structures
without the need for applied pressures (Figure 3d).
However, the bonded structures contained unboundedvoids and did not uniformly survive the scotch tape test
indicating the need for additional research to optimize thebonding parameters.

In addition to forming primary structural elements for
Parylene-based devices, secondary structures such as astrain-relief coil in Parylene-based flexible cables weredemonstrated (Figure 4a). Thermoforming could also be
repeated in sequential processes; secondary elements wereformed in a second sequential step without affecting thepreviously molded primary structures (Figure 4b).

Figure 4: (a) Thermoformed PMP neural probe formed bysequential thermoforming processes where the cone wasformed first followed by the strain-relief coil. (b) SEM
image of thermoformed cone.
Parylene-based electrode arrays were thermallyshaped to allow for conformation matching ofphysiological surfaces. An electrode array was curled 360°
which allows wrapping around nerve fiber bundles (Figure5a) and a sensor array was curved to match the shape of theretina (Figure 5b).

Figure 5: Shaped PMP samples: (a) coiled and (b) curved
electrode arrays.

It is important to note that thermoforming is associatedwith bulk dimensional changes in Parylene. This is ofparticular concern when the precise pitch of structures afterthermoforming must fall within tight tolerances. Directelectrical connections can be established with flexible
Parylene ribbon cables using commercially available
zero-insertion force (ZIF) connectors [13], but requires a
specific contact pad spacing for proper alignment between
connector pins and contact pads. However, it was observedthat thermoformed PMP samples exhibited altered contact
pad pitch that prevented proper pin-pad alignment (Figure6). Shrinkage was largely limited to inter-lead Parylene
spaces and resulted in an overall ~5% reduction of the total
lead spacing in thermal-treated samples. This shrinkage
error propagates with the increasing number of channels
and therefore must be compensated in larger lead countconnections. For 32-count linear contact pad arrays inParylene cables (0.5 mm pitch; Figure 6a), a significant
change in dimension was encountered whereas in 8-countarrays (0.5mm pitch; Figure 6b), the dimensional change
was tolerated. This shrinkage was accounted for in futureversions of Parylene ribbon cables.

Figure 6: Optical micrographs comparing PMP contact
pad arrays shown as-fabricated (left) and afterthermoforming (right). (a) Pitch shrinkage observed inthermoformed PMP samples for large lead count (32channels). (b) Reduced pitch shrinkage observed for 8channel cable.
In cases where oxygen was inadvertently introduced
into the vacuum chamber, oxidative degradation ofParylene was observed. The normally transparent
appearance of Parylene would instead be marked by abrownish hue. These samples were noticeably brittle anddid not retain the flexibility of samples thermoformed inoxygen-free conditions. Such mechanical changes are
undesirable and highlight the importance of the vacuum
and nitrogen backfill conditions.
Chemical and Mechanical Characterization
FTIR spectra were acquired on samples treated at low(100°C) and high (200°C) thermoforming temperatures.
Compared to the control, no new peaks were observed,
indicating that thermoforming does not significantly
change bulk chemical functionality (Figure 7). Thisobservation is further supported by contact angle
measurements. Samples treated across a range ofthermoforming temperatures exhibited similar measured
contact angles differing only by a few degrees andindicated that the surface energies (hydrophobicity) wererelatively unchanged following thermal treatment (Table1). These results suggest that the thermoformed films
remain largely unchanged in their bulk and surface

chemical properties.
Mechanically, the treated films exhibited changes instiffness, which were noted qualitatively andquantitatively. The stiffness of shaped sheath structures
processed at different temperatures was measured andcompared to that of a mechanically opened sheath that was
not thermally treated. Increased stiffness of the formed
structure was observed as a function of increasing
thermoforming temperature (Figure 8). This is consistent
with reports in literature in which the stiffness increase
arises from an increase in film crystallinity following
thermal treatment due to the development of additional
crystalline domains within the polymer’s amorphous
regions [14] and the introduction of a new second
crystalline phase [2]. It was also reported that the film
crystallinity increases with increasing treatment
temperature; this is confirmed by structural stiffness
results, where a sample thermoformed at 200°C exhibited ameasured stiffness increase up to sixty times that of an
untreated sample. Thus, mechanically tuning athermoformed structure to the required stiffness is possible
by selecting the appropriate annealing temperature.

Figure 7: FTIR spectra of (a) untreated Parylene film
(control) and films after thermoforming at (b) 100°C and(c) 200°C for 1 hour indicating no bulk chemical changes
in the material following thermal treatment.
Table 1: Measured contact angles of thermoformed
samples indicating insignificant surface energy changes.
Treatment Temperature (°C)
Contact Angle (°)
Control
80.310075.812080.2
140
77.8
160
79.1
18085.8
20082.1
The effect of increased crystallinity due tothermoforming also contributes additional benefits specificto long term reliability for chronic in vivo applications;more crystalline films were reported to possess greater
tensile strength, improved thermal resistance [14], andimproved water barrier properties [2].

CONCLUSIONA
post-fabrication,
thermalmolding(“thermoforming”) process was developed to form
non-planar, 3D structures with high aspect ratio from
Parylene thin films that would otherwise be difficult toattain with standard planar microfabrication processes.
Thermoforming was conducted under a vacuum
environment with temperatures above the glass transition
and below of the melting temperatures of Parylene C, andresulted in 3D structures with wide applicability, including
bonded and shaped structures, and those formed insequential steps. The effects of thermoforming on thechemical and material properties were characterized.
Thermoformed results retained bulk/surface chemical
properties and exhibited increased mechanical stiffness
due to increased crystallinity at higher temperatures. These
results indicate the potential to achieve structures with abroad range of mechanical stiffnesses simply bycontrolling thermoforming parameters.

[2]

[3]

[4]

[5]

[6]

[7]

[8]

[9]
Figure 8: Stiffness measurements of thermoformed
Parylene cone structures obtained from nanoindentationmeasurements. As the treatment temperature is increased,
the cone stiffness also increases, demonstrating theincreased rigidity of the final thermoformed structure.
Additional experiments are being performed to study
the relationship between treatment temperature andduration to further tune the thermoforming process toachieve specific mechanical qualities and investigate
improvement of moisture barrier capabilities, especiallyfor in vivo applications.

[10]

[11]

[12]

ACKNOWLEDGEMENTS
The authors would like to thank Mr. Yuzheng Zhang
for aid in nanoindentation measurements and members ofthe USC Biomedical Microsystems Laboratory for theirassistance. An OAI model 30 light source was used forprocessing of Parylene samples.
This work was sponsored by the Defense Advanced
Research Projects Agency (DARPA) under the auspices ofDr. Jack Judy through the Space and Naval Warfare
Systems
Center,Pacific
Grant/Contract
No.
N66001-11-1-14207 and the University of Southern
California Provost Ph.D. Fellowship (BK).

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CONTACT*E. Meng, tel: +1-213-8213949; ellis.meng@usc.edu