Original Research
published: 08 September 2015
doi: 10.3389/fmech.2015.00010

Mechanical properties of thin-filmParylene–metal–Parylene devices
Curtis D. Lee 1 and Ellis Meng 1,2*
1
Department of Biomedical Engineering, University of Southern California, Los Angeles, CA, USA, 2 Ming Hsieh Department
of Electrical Engineering, University of Southern California, Los Angeles, CA, USA

Edited by:
Weileun Fang,
National Tsing Hua University, Taiwan
Reviewed by:
Chingfu Tsou,
Feng Chia University, Taiwan
Christian Bergaud,
CNRS, France*Correspondence:
Ellis Meng,
Biomedical Microsystems Laboratory,
Department of Biomedical
Engineering, 1042 Downey Way,
DRB-140, Los Angeles,
CA 90089-1111, USAellis.meng@usc.edu
Specialty section:
This article was submitted to Microand Nanoelectromechanical
Systems, a section of the journal
Frontiers in Mechanical Engineering
Received: 01 July 2015
Accepted: 18 August 2015
Published: 08 September 2015
Citation:
Lee CD and Meng E (2015)
Mechanical properties of thin-filmParylene–metal–Parylene devices.
Front. Mech. Eng. 1:10.
doi: 10.3389/fmech.2015.00010

Structures and testing methods for measuring the adhesion strength, minimum bending
diameter, and bending fatigue performance of thin-film polymer electronic architectures
were developed and applied to Parylene–metal–Parylene systems with and without the
moisture barrier Al2O3 [deposited using atomic layer deposition (ALD)]. Parylene–metal–
Parylene interfaces had the strongest average peel test strength and Parylene–Paryleneinterfaces had the weakest peel test strength. Layers of ALD Al2O3 deposited within
the device increased the average peel strength for Parylene–Parylene interfaces whencombined with silane A-174, but did not increase that of the Parylene–metal–Paryleneinterface. Metal traces in the middle of 24 μm thick Parylene–metal–Parylene devices
had a minimum bending diameter of ~130 μm before breaking and being measured as
an open circuit. The addition of one layer of Al2O3 above the traces allowed them to becompletely creased when bent away from the Al2O3 layer without producing an open
circuit, but increased the minimum bending diameter to ~450 μm when bent towards
the Al2O3. Although fatigue testing produced cracks in all devices after 100k bends,
the insulation of the Parylene–metal–Parylene devices without Al2O3 performed well withelectrochemical impedance spectroscopy showing only small decreases in impedance
magnitude and small increases of impedance phase at low frequencies. However,devices with Al2O3 failed during EIS due to Al2O3 being deteriorated by water.
Keywords: Parylene, thin-film polymer, peel test, bend test, fatigue test, EIS, Al2O3Introduction
Unlike traditional silicon-based electronics, thin-film polymeric devices are inherently flexible, which
is important for devices targeting soft, moving tissues for various applications, such as monitoring
electrical activity from muscles, measuring nerve impulses from the central or peripheral nervous
system, and delivering therapeutic electric current, such as in pacemakers, deep brain stimulation,cochlear implants, and spinal cord stimulation. For such applications, flexibility is required to maintain good contact (important for sensing weak electrical signals and reducing the amount of current
required for a therapeutic effect) and avoid damaging tissue through tethering effects.
In addition, thin-film polymeric devices use photolithography and other technologies borrowed
from the semiconductor industry to pattern electrodes or other sensors or structures onto the surfacesof materials, such as Parylene (Johnson and Wise, 2012; Kuo et al., 2013; Zhao et al., 2013), polyimide(Kisban et al., 2009; Tolstosheeva et al., 2011; Viventi et al., 2010), or polydimethylsiloxane (Gray et al.,
2004; Guo et al., 2014). Because thin-film polymeric devices are based on microfabrication technology
used to make transistors or micro-sensors, these devices have micron-scale resolution or better andallow sensors, such as transistors (Nomura et al., 2004) or force sensors (Gutierrez et al., 2010), to

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Mechanical properties of thin-film Parylene–metal–Parylene devices

be integrated directly onto the flexible device. These devices arethin enough to be rolled up and delivered via catheter (Fonseca
et al., 2006) or incorporated into the catheter itself (Li et al., 2008a).
Thin-film polymeric devices for cochlear stimulation (Johnson andWise, 2010), spinal cord stimulation (Rodger et al., 2007) as wellas electrocardiography (ECG) (Viventi et al., 2010; Yu et al., 2012),
electrocorticography (ECOG) (Toda et al., 2011), and intracorticalneural recordings (Kim et al., 2013) have been demonstrated.
While thin-film polymeric devices are flexible, biocompatible,
and allow for high resolution features, they are prone to delamination (Li et al., 2008b; Seymour et al., 2009) and carry concerns
about their mechanical robustness (Schuettler et al., 2005).
Mechanical tests for individual devices for specific applications
have been performed, such as measuring the properties of a device
while wrapping it around fixed-diameter structures (Chen et al.,2010; Kisban et al., 2009; Ochoa et al., 2013), fixing it to a moving
tissue (Metallo et al., 2011; Viventi et al., 2010; Zhao et al., 2013),
or allowing it to self-curl (Yamagiwa et al., 2013). In addition, tests
have been performed to screen for methods to improve adhesion(Chang et al., 2011; Seymour et al., 2009). However, systematicstudies that investigate in detail the mechanical performance andadhesion of multi-layer film stacks in generalized polymeric film
devices are lacking in research literature.
Also, typical reports in literature present data from single devices(such as bend testing across cylinders of multiple diameters) from
non-destructive experiments. Destructive and continuous peel/
bend tests can provide specific limits to device performance.
In this work, designs for polymeric thin-film devices and tests
are presented as a way to determine the robustness of thin-film
structures. In particular devices and tests to measure peel strength,
minimum bending diameter, and bending fatigue characteristics ofParylene–metal–Parylene, structures are tested to determine theirpotential and limitations for use as thin-film devices. In addition,these devices and tests are used to measure the properties of Al2O3[deposited using atomic layer deposition (ALD)], which – in combination with Parylene – has been shown to improve water-barrier
properties of encapsulated silicon devices (Ghosh et al., 2005) andchosen in the present study to act as a water barrier and adhesion
promoter for Parylene–metal–Parylene devices.

a second layer of Parylene was deposited to insulate the Pt patterns, and O2 plasma was used to expose contact pads and cut
out devices. ALD was used to deposit Al2O3 either before andafter metal traces or only on top of metal traces (Figure S1B in
Supplementary Material, details of Al2O3 given below in Section‘‘Atomic Layer Deposition of Al2O3 Barrier’’).

Atomic Layer Deposition of Al2O3 Barrier

Various processing techniques, such as thermoforming, cleaning,and chemical treatments, have been investigated to improve thinfilm adhesion (Chang et al., 2011; Kim and Najafi, 2005). Also,barrier layers and adhesion promoters have been used to prevent
failure of thin-film insulation (Xie et al., 2012; Xie et al., 2013).
However, only static conditions were used to test for improved
bond strength and insulation integrity. In the current study, ALD
Al2O3 combined with the silane adhesion promoter A-174 was
deposited between layers of Parylene to act as both a barrier layer
and adhesion promoter. The effect of this layer, not only on adhesion promotion but also on trace performance in bending failure
and bending fatigue, was measured.
Al2O3 has been shown to be an excellent barrier to water vapor(Ghosh et al., 2005) and for this reason was deposited with Parylene
to improve the performance of implanted neural electrodes (Xieet al., 2013). Parylene serves as a barrier for bulk water, ions, andchemicals, while the Al2O3 serves as a barrier for water moleculesthat are able to penetrate the Parylene. Plasma-enhanced ALD isa process that sequentially deposits monolayers of molecules onto
a substrate using self-limiting chemical reactions. ALD deposited
Al2O3 is preferred over sputtered or plasma enhanced chemical
vapor deposition (PECVD) because of argon sputtered Al2O3 cantrap argon into the coating making it porous, and PECVD canlead to less dense films (Ghosh et al., 2005). ALD coatings arealso highly conformal and pinhole free, thus avoiding gaps which
can be major sources of corrosion in encapsulated electronics incontact with water (Vanhoestenberghe and Donaldson, 2013).
To incorporate Al2O3 into PMP devices, ALD was used todeposit 17–24 nm of [250 cycles on a Fiji 2 System (Ultratech,
MA, USA)] either before and after metal deposition or only after
metal deposition (Figure S1B in Supplementary Material). When
Al2O3 was deposited before metal deposition, an adhesion layer of
20 nm layer of Ti was deposited over the Al2O3 to promote adhesion of Pt to Al2O3 and an adhesion layer of A-174 (Momentive,
Waterford, NY, USA) was deposited over the top layer of Al2O3 topromote the adhesion of the top layer of Parylene C. In addition,
because Al2O3 is unaffected by O2 plasma, wafers were soaked forextra time (~6 min) in developer [AZ 340 (Microchemicals, Ulm,
Germany):DI H2O, 1:4 dilution]. These processes made devices
with a variety of different combinations of Parylene C, Al2O3, andmetal architectures (Table 2).

Materials and Methods
Table 1 summarizes the distinct test structure designs for measuring the peel strength, minimum bending diameter, and bending
fatigue. Subsequent sections describe details of the fabrication,
design, and testing of these devices.

Device Fabrication

Peel test, minimum bending diameter test, and bending fatigue
test devices were all fabricated using similar fabrication methods(Figures S1A,B in Supplementary Material). Details of Parylene
C-based thin-film devices can be found in previous publications(Kuo et al., 2013), therefore, only a brief description is given here.
First, ~13 μm of Parylene C was deposited onto a prime siliconwafer using CVD (Specialty Coating Systems Inc., IN, USA). Next,
standard photolithographic liftoff was used to pattern 200 nm of
e-beam deposited Pt on top of the underlying Parylene C. Finally,

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Peel Tests
Peel Test Device Design

Delamination is a common concern for thin-film polymeric devices,
especially in aqueous solutions (Li et al., 2008b). Any interface in athin-film polymer device that delaminates could potentially create
paths to water intrusion and so determining the adhesion strength
between layers is important. Thin films can bond to underlying

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TABLE 1 | Summary of devices and tests used to measure peel strength, minimum bending diameter, and bending fatigue.
Test name and purpose

Description of device

Peel test strength
Measures the strength ofadhesion between two thinfilm layers

Peel test devices contain a bonded region
connected to two flaps that are clamped
to a load cell, which measures the force
required to separate the joined interfaces.

Minimum bending diameter
Determines the minimum outer
diameter a thin film device canbe bent to before encapsulated
traces break and result in open
circuit

Minimum bending diameter devices
consist of a rectangular cable with a single
serpentine trace which transverses thelength of the cable several times. The
resistance of the trace is measured at the
exposed ends while the device is crushed
between two plates until an open circuit isdetected.

Bending fatigue failure
Measures the performance ofthin film trace insulation after
cyclic bending of thin film
cables

Bending fatigue failure devices consist ofa rectangular cable with multiple insulated
traces with only one end exposed to makean electrical connection. The device isbent ±90° for a given number of cycles
and then the electrochemical impedance
between each trace and an external
electrode as well as between adjacentelectrodes is measured. This process isthen repeated.

Device layout

TABLE 2 | Different material combinations investigated for flexible
Parylene devices.
Abbreviation

Interfacial layers (from bottom to top)

PP
PMP
PAP
PAMAP

Parylene/Parylene
Parylene/platinum/Parylene
Parylene/Al2O3/Parylene
Parylene/Al2O3/titanium/platinum/Al2O3/Parylene
Parylene/Al2O3/Parylene
Parylene/titanium/platinum/Al2O3/Parylene

PAAP
PMAP

linkage with the underlying substrate (Charmet et al., 2013). Also,
a combination of heat and compression can bond Parylene layers
together by causing the polymer chains on the surface of the touching films to become entangled (Noha et al., 2004). Computationalmodels suggest that metals, such as platinum and titanium can form
stable and covalent bonds when deposited on Parylene (Vasenkov,
2011). Water intrusion (both from the edges of thin-film devices
and through the polymer layers themselves) can weaken adhesion
by penetrating between physical bonds or dissolving intermediate
layers. Delamination occurs when upward forces perpendicular tothe substrate are stronger than adhesion forces the top film.
Several methods have been used to measure the adhesion forceof thin films. The “scotch test tape” qualitative tests the adhesionof a film deposited on a substrate by applying a piece of pressure
sensitive tape to the film and pulling the tape off (Lim et al., 2006;
Sridhar et al., 2009). If the top layer of the deposited film comesoff (either in parts or in pieces), it is said to have “failed” the test.
The adhesion force of the film can also be determined using aload cell connected to the free end of a film which is then pulled

Thickness of layers: Parylene C (top and bottom layers) 12–13 μm; Al2O3 (top andbottom layers) 17–25 nm, Ti 20 nm; Pt 200 nm.

substrates physically by filling in pores or surface topography,
through covalent bonds, or by van der Waals forces. In addition,interfaces that typically have poor adhesion, such as Parylene
deposited on silicone, can be improved with adhesion promoters,such as the silane A-174, which bonds covalently both with Parylene
free radicals during the deposition process and provide a strong

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Mechanical properties of thin-film Parylene–metal–Parylene devices

at a 90° angle to a fixed substrate (Chang et al., 2011) or 180°
to a second attached flexible film, to measure force required toseparate the film from a substrate. The peel strength is defined as
the average load per unit width of bondline required to progressively separate the two materials.
When the film is pulled at a 180° angle, the test is called a“T-peel test” because of the shape formed by the two free ends
pulled and the bonded perpendicular region forming base ofthe ‘‘T’’ (Table 1). T-peel tests are used for the simplicity of thetest setup and ability to compare different forms of bonding or
adhesives (Song and Yu, 2002); however, these tests are sensitive
to the thickness of the substrates, which require energy to deform
during the test and must be accounted for if direct measurementof cohesion strength is desired (Kim, 1988).
In the current experiment, T-peel tests were performed withspecimens based on ASTM standard D1876-08, but modified to
be used on thin-film devices. These tests allowed the peel strengthof multiple interfaces to be measured, including PP, PMP, PMAP,
PAP, PAMAP, and PAAP (Table 2).

Peel Test Device Fabrication

In addition to the fabrication steps described in Section ‘‘Device
Fabrication’’ (Figure S1 in Supplementary Material), two flaps(4.3 mm × 14 mm) with 3 mm holes (for gripping) werecreated using a sacrificial photoresist that was later removed
to enable them to be separated after the device was released(Figure S2 in Supplementary Material). The flaps were used togrip while pulling apart the bonded region (4.3 mm × 3 mm)of the device. Each Parylene layer of the peel test device was
13 μm thick.

FIGURE 1 | (A) Flaps of peel test sample separated with tweezers after
removing sacrificial PR. Bonded metal region is circumscribed by dotted line.
(B) Peel test sample placed over mounting posts on motorized stage withload cell before top clamp is secured. White arrow points to bonded region ofpeel test device [scale bars for (A) and (B) are 5 mm].

Peel Test Setup and Characterization

where M is the bending moment, y is the vertical distance fromthe neutral axis, and I is the moment of inertia around theneutral axis. The neutral axis within the sample is where thetransition from compressive to tensile forces occurs (Figure S3B
in Supplementary Material). Because materials are generally faildue to cracks propagating perpendicular to tension forces, cracks
in bent materials tend to propagate from the outside surface tothe inside surface.
The effect of bending stress on thin-film devices can beassessed by wrapping films around cylinders of specific diameters ranging from hundreds of microns to several millimeters
and then either measuring the resistance across the tracesor the testing the performance (Ai et al., 2008; Chen et al.,2010; Kim et al., 2010; Lacour et al., 2010; Ochoa et al., 2013).
Alternatively, films can be wrapped around spheres (Ochoa
et al., 2013; Rodger et al., 2007) and inflatable balloons (Kimet al., 2011). Some devices are self-curling and were shown tomaintain function while being bent 90° (Takeuchi et al., 2004)or down to a radius of 2 mm or less (Johnson and Wise, 2012;
Yamagiwa et al., 2013).
A convenient and automated setup determining the minimum
bend radius of a thin-film device called the “bending plate” method(Roll, 1976) involves bending a flexible film between two plates
until failure (Sekitani et al., 2010). The bending plate method is
a versatile technique that was developed to measure the tensile
strength for optical fibers based on the distance between the platesPeel test devices were tested by attaching one flap to a stationary
clamp and the other to a 50 g load cell (Omega, Stamford, CT,
USA), which was attached to a motorized stage (Z812, Thorlabs,
Newton, NJ, USA) that pulled the two flaps apart at a rate of
0.02 mm/s (Figure 1). A custom Labview program was used tocontrol the stage and measure the voltage output from the loadcell (Figure 1B). PMP devices were tested both dry and after a
24-h soak in 1× PBS solution at 37°C for 24 h. Scanning electron
microscope (SEM) images were taken of peel test surfaces to
examine morphology of adhesion failure.

Minimum Bending Diameter Tests
Minimum Bending Diameter Device Design

Implanted thin-film polymeric devices are exposed to mechanical forces, such as muscle contractions, acceleration, gravity,hydrostatic pressure, etc., that can damage the device. Muscles
can contract to 50% of their resting length (Jenkins, 2001), which
can cause devices near or on their surface to be flexed, bent, or
pinched. When a material is subjected to bending, it experiencescompression forces on the inside surface and tensile forces on
the outside surface (Figure S3 in Supplementary Material). These
stresses follow a linear relationship according to the following
equation for bending stress (σb):
σb =

My
I

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Mechanical properties of thin-film Parylene–metal–Parylene devices(D) the overall fiber thickness (d), the uncoated fiber radius (r),and the Young’s modulus of the fiber (Eq. 2) (Matthewson et al.,1986), but has also been used to determine the smallest diameterthat thin conductive films on flexible substrates can be bent tobefore registering an open circuit (Alzoubi et al., 2011; Leterrier
et al., 2003) (i.e., the minimum bending diameter). The boundary condition of the bending plate method is that the radius ofcurvature is 0 where the plate is in contact with the cable, andthe radius of curvature is maximum on the outside edge of thecable half way between the plates. The radius of curvature of thedθ 1bend (R) is described by the equation:= where θ is theds Rangle formed by a line tangent to the curve of the cable and a line
at the midpoint at the bend parallel with the plates, and s is thelength of the line from the intersection of the line tangent to thecable with the cable to the intersection of that same line to themidline. The moments about this point are given by the bendingdθ= − Fx − M (Matthewson et al., 1986).
beam equation: EI
ds
There are several advantages in using this type of bend test
rather than a traditional tensile test, including eliminating theinfluence of the grips on the test and simple setup; however, thetest is limited by difficulty in determining the exact position of 0
separation and only being able to test the portion of the sample
between the plates.
σ max = 1.198 E

2r
D −d

Minimum Bending Diameter Test Setup

To measure the minimum bending diameter, a thin-film cable
was between two acrylic blocks while resistance was monitored(Figure S4 in Supplementary Material). One of the acrylic
blocks was attached to a 50-g load cell (Omega, Stamford, CT,
USA) which, in turn, was connected to a motorized stage (Z812,
Thorlabs, Newton, NJ, USA) for measuring axial force for the
purpose of determining complete contact between the blocks. The
devices were supported by a holder and connected to a precisionmultimeter (Keithley 2700, Keithley, Cleveland, OH, USA). Usinga custom Labview program (National Instruments, Austin, TX,
USA), the motorized stage was advanced at a rate of 0.02 mm/s,
bringing the two acrylic blocks together and reducing the bend
diameter. Failure was defined as an open circuit measurement.
The minimum diameter was measured both with the devices
folded away from the contact pads (‘‘out’’) and toward the contact
pads (‘‘in’’) (Figure S4 in Supplementary Material).
The acrylic blocks were visually confirmed to make full contact when the load cell read 4.5 N. To determine the minimum
diameter, therefore, the position of the stage where the load cellreached 4.5 N was defined as 0, and the difference of the stage
position at that point and the position of the stage where theresistance of the minimum bend diameter test device directly
before the trace was measured as an open circuit was the device’s
minimum diameter. The minimum bending curvature (radius)is half of the difference between the minimum bending diameterand the thickness of the thin-film device (26 μm).
To determine the failure diameter, the electrical resistance ofa Pt trace across cable was monitored as a loop of cable was bent
between two plates (Figure S4 in Supplementary Material).

(2)

To measure the minimum bending diameter of Parylene thinfilm cables using this method, a thin-film flexible cable (withdimensions 48 mm × 4.3 mm × 0.026 mm) with a single 360 μm
wide serpentine trace (longest straight dimension is 40 mm) was
designed, whose resistance could be measured while compressing the cable between two acrylic plates (Figure 2). Failure wasmarked by the cable reading an open circuit. These devices weremade using standard photolithographic processes described in
Figure S1 in Supplementary Material.

Bending Fatigue
Bending Fatigue Device Design

When thin films are exposed to forces less than their tensile
strength, even though they will not break on the first cycle, they
can still fail due to fatigue caused by repeated motions present
near parts of the body such as the heart, which contracts over
100k times/day, or the neck, which moves an average of 10
million times in each direction each year (Sterling et al., 2008).
Fatigue occurs because dislocations and voids move and coalesce
under bending stress. When a sufficient number of dislocations
aggregate, crack propagation sites form that weaken the material.
Fatigue in thin-film devices has been measured in test in vitro
by continuously measuring resistance across polyimide (Kisban
et al., 2009) or Parylene (Johnson, 2011) test cables bent to 90°
angles for thousands or millions of cycles, and in vivo by measuring ECG signals for ~10,000 heart beats (Viventi et al., 2010).
To measure the effect of repeated deformation on
thin-film Parylene devices, bend test samples (measuring
4.3 mm × 48 mm × 0.026 mm) with 40 mm long and 360 μm wide
fully insulated traces (Figure 3) were designed and fabricatedusing standard lithographic techniques described in Figure S1 in
Supplementary Material.
Electrochemical impedance spectroscopy (EIS) was used
identify signs of failure in the device (e.g., delamination, water
infiltration) by measuring the impedance between traces [theFIGURE 2 | (A) Cartoon and (B) photograph of minimum bending diameter
test devices (scale bar is 5 mm). In cartoon (A), the thickness of the traces
has been increased and number of turns decreased for illustrative purposes.
The metal and hole on the left side of device are for clamping the device tothe testing fixture. The contact pads on the right side allow electricalresistance measurements.

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Mechanical properties of thin-film Parylene–metal–Parylene devices

FIGURE 4 | Representative peel test raw data from (A) PP and (B) PMP
test devices. The edge effect observed for the PP device was not observed
for the PMP device, most likely due to the force magnitude of being
dominated by the larger PMP average peel strength (~50 times larger). Notethat the two plots are on different scales.

FIGURE 3 | (A) Cartoon and (B) photograph of bend test devices (scale bar
is 5 mm) [in cartoon (A), the thickness of the traces has been increased andnumber of traces decreased for illustrative purposes]. The metal and hole on
the left side of device are for clamping the device to the testing fixture. The
contact pads on the right side allow electrochemical impedance
measurements with a potentiostat.

lateral impedance (Figure S5A in Supplementary Material)] andthe impedance between a trace and an external counter electrode [the transverse impedance (Figure S5B in Supplementary
Material)] while immersed in phosphate buffered saline (PBS).

Bending Fatigue Testing

To apply repeated bend cycles to the bending fatigue test devices,a custom cable flex tester (based on ASTM standard D4565-10 for
testing cable insulation) was constructed consisting of a Sparkfun
stepper motor (ROB-09238) (Sparkfun, Boulder, CO, USA) withan Arduino UNO (Arduino, Torino, Italy) controller and mounting fixtures (Figure S6 in Supplementary Material). In this setup,
the top of the cable was attached to a rotating wheel and passed
through two mandrels at the center of the wheel and two stationary rollers below the wheel. A weight [consisting of a section offlat flexible cable (FFC)] was attached to the bottom of the cable(below the stationary rollers) to keep the cable taut. The wheel wasthen rotated ±90°. As the wheel turned the cable was bent aroundthe center mandrels, while the cable below the mandrels remained
free to move up and down through the stationary rollers. For acable bent around a mandrel, the total stress in the cable (σtotal) canbe approximated as the sum of the stress due to the tensile forces(σt) caused by the weight hanging from the cable) and the bending
forces (σb). Because the radius of curvature can be assumed to be
the cylindrical radius of mandrel σb can be approximated as:
σb =

Et
D +t

FIGURE 5 | (A) Combined average peel test data (mean ± SE, n = 3–8) fromvarious trials. Parylene/Parylene interfaces had the lowest peel strength andwere regraphed separately (B) using a different scale. The slash indicates theinterface at which delamination occurred.

impedance magnitude was normalized to absolute impedance ofthe magnitude at 0 bends and plotted.
In addition to EIS, devices were examined using optical andNomarski differential interference coherence (DIC) microscopy aswell as scanning electron microscopy (SEM), before and after bending.
The impedance was expected to behave according to thesimplified Randle’s model, with the values for the capacitive andresistive components changing as the insulation fails [Figure S7 in
Supplementary Material (Loveday et al., 2004)]. Delamination of
the Parylene film is expected to cause a drop in measured impedance starting at lower frequencies, which has been shown to be
more sensitive to electrode failure (Takmakov et al., 2015).

(3)

Results

where E is the elastic modulus of Parylene, t is the thickness of thecable, and D is the diameter of the mandrel. In a fatigue test, σtotal
is much less than the tensile stress of the cable so that multiple
cycles are required before signs of fatigue become apparent. In thecurrent test setup, devices were subject to 10–100k bending cycles
in which one cycle is defined by bending the between −90° and+90° around a mandrel (1.25 mm OD).
EIS was performed in 1× PBS at 37°C with an amplitude of
10 mVrms and a frequency range of 1–100,000 Hz. A platinum
wire was used as a counter electrode and an Ag/AgCl (3 M NaCl)electrode was used as a reference (transverse impedance). The

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Peel Testing

Peel testing resulted in characteristic T-peel curves with anincrease in force once the samples become taut, and a relatively
continuous force as the bonded area was separated by peeling(Figure 4). Overall, the average peel test strength of the PP
devices was low with relatively large spikes observed in the force
plots at the beginning and inconsistent spikes at the end of thepeel tests (representative plot in Figure 4A).
Mean values of average peel strength are presented in
Figures 5A,B indicated PMP devices, exhibited the highest
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Mechanical properties of thin-film Parylene–metal–Parylene devices(Figures 6C–F, right images), contained clusters of thin stringsof Parylene.

adhesion (21.9 ± 1.3 N/mm, mean ± SE), followed by the PMAP,
PAP, PAMAP, and PAAP devices (Figure 5A). The force required
to pull apart PP and PP wet devices was the lowest (0.43 ± 0.06
and 0.21 ± 0.04 N/mm, respectively) (Figure 5B). PP samples
soaked overnight at 37°C in 1 M PBS solution (PP wet) had significantly lower peel strength than devices that were not exposed
to soaking (Figure 5B). In all cases, the top layer of Parylene wasseparated from the underlying layers.
Significant differences were found between PAP and PMAP
and the PMAP and PAMAP devices. However, there is no
significant difference between the PAMAP and PAAP devices.
The cumulative mean average peel test bar graphs also show thatsamples with either one or two layers of Al2O3 showed muchsmaller variation in peel strength than samples without Al2O3(Figure 5A).
SEM images were acquired for all conditions (Figure 6). The
PMP surfaces (Figure 6A) contained cracked and delaminatedPt on the bottom layer of Parylene (left image) and plastically
deformed Parylene on the top layer (right image). In contrast,
the PP devices (Figure 6B) were relatively smooth and clean
except for thin strands of Parylene on the bottom layer (leftimage). The devices with Al2O3 with or without a layer of Pt
were similar (Figures 6C–F). The bottom layer of these interfaces (left images), which included the bottom layer of Parylene
and one or two layers of Al2O3, contained parallel ridges topped
with plastically deformed flaps of Parylene. The top layers

Minimum Bending Diameter

A diameter of 0 mm was defined as the point in which the force
measurement on the acrylic clamping blocks was 4.5 N and alldata was normalized to this setting. Full contact of acrylic blocks
at 4.5 N was confirmed using video images. The resistance ofthe traces as the bending diameter was reduced was graphed(Figure 7) and the minimum bending diameter just prior to theformation of an open circuit was recorded (Figure 8).
The minimum bending diameter of PMP devices was measured
to be 138 ± 16 μm with no significant difference between bending
the device toward the top layer of Parylene (in) or toward the bottom layer of Parylene (out). Contrarily, the PMAP device with one
layer of Al2O3 underneath the top layer of Parylene failed muchearlier (476 ± 60 μm) than the PMP devices when bent toward theAl2O3 layer, but did not break at all when bent away from the Al2O3
layer. The low yield of PAMAP devices was attributed to difficultyin establishing electrical contact and therefore, PAMAP devices
were only tested under bending toward the top layer of Parylene.
SEM images were taken of both the top and bottom surfacesof devices that had been bent away from (Figure 9) and toward(Figure 10) the contact pads. Cracks that formed due to tension
on the outside of the bend (Figures 9A,C and 10A,C) appeared
to be deeper with cleaner edges compared to cracks formed due to

FIGURE 6 | SEM images of peel test surfaces. For each individual panel,
the left image is the bottom (closest to the silicon wafer during processing)layer (consisting of Parylene, metal, and Al2O3) and the right image is the top
layer of Parylene. The PMP surface (A) contained cracked and delaminated Pt
on the bottom layer of Parylene (left image) and plastically deformed Paryleneon the top layer (right image). PP devices (B) were relatively smooth and clean

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except for thin strands of Parylene on the bottom layer (left image). Devices
with Al2O3 with or without a layer of Pt (C–F) were similar in that the bottom
layers (left images) contained parallel ridges topped with plastically deformed
flaps of Parylene and the top layers (right images) contained clusters of thinstrings of Parylene. Note that images are at different scales (surfaces weresputtered coated with Pt before imaging).

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Mechanical properties of thin-film Parylene–metal–Parylene devices

FIGURE 7 | Resistance data from (A) PMP, (B) PMAP, and (C) PAMAP minimum bend diameter tests. Minimum bending diameter was defined as theresistance measurement closest to 0 mm not measured as an open circuit.

metal (Figure 12) showed a small drop in magnitude at lower frequencies, but remained relatively constant at higher frequencies(Figure 12A). However, as the number of bend cycles increased,
the impedance at lower frequencies became lower. The phaseof the PMP devices was initially −90°, but increased at lower
frequencies after the first set of bends. At higher frequencies, thephase remained capacitive (around 90°) even when the devices
were bent through 100k cycles (Figure 12B). In contrast to the
PMP devices, the PMAP and PAMAP exhibited larger fluctuations in magnitude over most frequencies (Figures 12C,E) andcompromised insulation prior to bending based on the phase data
(Figures 12D,F).
The phase for PMP devices under lateral impedance testing
started greater than −90° at lower frequencies (Figure 13B).
However, similar to the transverse impedance, there was no
broad spectrum shift of magnitude, but only a decrease at lower
frequencies (Figure 13A). Similar to the transverse impedance,
the lateral impedance of the devices with Al2O3 had large shifts
in magnitude with large SEs (Figures 13C,E). In addition, theinitial phase was greater than −90° even before the devices werebent (Figures 13D,F).
Cracks in the Parylene that started at edges (not shown) werevisible by optical microscopy after 2k bend cycles (Figure 14A)
and became pronounced after 5k bend cycles (Figure 14B).
The cracks were either chevron-like, radiating from a central
point such (top left traces in Figure 14B) or perpendicular to
the traces (right trace in Figure 14B). Cracks were observed
to cross over the metal traces and the Parylene region between
traces after 100k bend cycles (Figure S8 in Supplementary
Material).
Optical micrographs of PMAP and PAMAP devices showed
noticeable water intrusion after 100 bend cycles (images were
not taken after the initial pre-bend EIS and before first bend
testing cycles) and continued to be present after subsequent
bend tests (Figure S9 in Supplementary Material). In Figure
S9 in Supplementary Material, Newton rings were observed in
the Parylene next to the edge of the device, but there was also
evidence of Newton rings near the bottom of the right trace.
Evidence of Al2O3 etching was evident in pock marks on the
traces (Figure S9 in Supplementary Material) as well as discoloration and branched patterns that initiated from the outside traces(Figure 15A). The impedance of the traces measured at 1 kHz
before bend testing commenced was much lower toward the edgesof the device, but as the device was subjected to further bending

FIGURE 8 | Summary of minimum bend test data. PMAP (in) had asignificantly larger minimum bending diameter than either PMP (in) or PMP(out) (mean ± SE, n = 3–4).

compression on the inside of the bent device (Figures 9B,D and
10B,D), which are characterized by rope-like ridges of Parylene
accompanied by plastically deformed shark-tooth shaped fringes.

Bending Fatigue

EIS of a PMP device released from a wafer that showed inconsistent adhesion during peel testing (data not shown) demonstrated
the ability to reveal insulation damage without destructive
testing (Figure 11). The magnitude of the transverse impedance gradually decreased at lower frequencies after even only
100 bend cycles, followed by a broad spectrum decrease after
5k bend cycles (Figures 11A,B). Similarly, the phase decreased
at lower frequencies during the first 4k bend cycles, and thenat higher frequencies after 5k bend cycles (Figure 11C). The
lateral impedance proved to be more sensitive, showing a broad
spectrum decrease in the magnitude of the impedance at 4k as
well as 5k (Figures 11D,E). Before bend testing (0 cycles), thelateral impedance had a −90° phase at all frequencies, which isindicative of a fully insulated trace. However, the phase dropped
at lower frequencies as soon as bend testing began, and there
was a broad spectrum drop after 4k cycles (Figure 11F). Broad
spectrum decreases in magnitude and phase suggested that theinsulation has been damaged and there was a path for ions tomove between traces.
The average transverse impedance from PMP devices (whichpeel testing showed good adhesion between the Parylene andFrontiers in Mechanical Engineering | www.frontiersin.org

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Mechanical properties of thin-film Parylene–metal–Parylene devices

FIGURE 9 | SEM of cracks resulting from bending (A,B) PMP
devices and (C,D) PMAP devices away from the contact pads (“out”
orientation). The top layers of Parylene (A,C) were the outside of thebent device, and the bottom layer (C,D) were from the inside of the bent

device (images on the right side of each panel are zoomed in views ofimages on the left with scale bars of the left and right images being 50
and 10 μm, respectively. Parylene sputtered coated with Pt before
imaging).

FIGURE 10 | SEM of cracks resulting from bending (A,B) PMP devices
and (C,D) PMAP devices toward the contact pads (“in” orientation). The
bottom layers of Parylene (A,C) were the outside of the bent device, and the
top layer (C,D) were from the inside of the bent device (images on the rightside of each panel are zoomed in views of images on the left with scale bars of
the left and right images being 50 and 10 μm, respectively. Parylene sputteredcoated with Pt before imaging). In the PMAP sample, delamination at the bend
was observed.

and EIS scans (each time an EIS was performed on all traces, thedevices were immersed in PBS for ~45 min), the impedance was
reduced across all traces and became more uniform (Figure 15B).

than the PP interfaces and likely dominated any edge effects(Figure 4B).
Peel tests revealed that PMP interfaces had the strongest adhesion measuring nearly 20 times more than for PP interfaces. PP
interfaces were further weakened by soaking for 24 h in 1 M PBS at
37°C (Figure 5). The reduced adhesion in soaked samples is likely
due to water intrusion between the layers. The Al2O3–Paryleneinterface was weaker than Pt–Parylene but stronger than PP.
However, the adhesion between Al2O3–Parylene interfaces fordifferent processing conditions was inconsistent. The PAMAP
and PAAP devices had lower strength than the devices with only
one layer of Al2O3. This is attributed to extra processing steps
required for the extra layer of Al2O3. These results imply thatexcellent adhesion is possible between platinum and Parylene,
which has been suggested through the results of modeling(Vasenkov, 2011), and that platinum could potentially be used

Discussion
Peel TestingSEM images (not shown) revealed trace amounts of sacrificial
photoresist that had not been fully removed from the Parylene
flap regions, which may have caused the large spikes at the
beginning of the PP peel tests (Figure 4A). The inconsistencies
at the end of the test were mostly likely due to edge effects as the
sample twisted when only a portion of the adhered regions wereseparated. This caused the samples to reorient so that the pulling
force acted preferentially on the bonded surface area over thebondline. In contrast, the PMP interfaces were much stronger

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Mechanical properties of thin-film Parylene–metal–Parylene devices

FIGURE 11 | Representative transverse (A–C) and lateral (D–F)
impedance from a PMP device released from a wafer that peel
testing revealed had uneven adhesion of the top layer of Parylene.
A clear change in impedance over 5k bend cycles is evident andrepresented by downward shifts in magnitude (A,D) emphasized bynormalizing the data to the impedance magnitude at 0 bends(B,E), and shifts in phase (C,F) at high and low frequencies (mean ±SE, n = 8).

FIGURE 12 | Transverse impedance from (A,B) PMP, (C,D) PMAP, and(E,F) PAMAP Parylene cables (mean ± SE, n = 16). Magnitude wasnormalized with respect to the impedance at 0 cycles (PMAP and PAMAP
devices were only bent for 10k cycles).

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FIGURE 13 | Lateral impedance from (A,B) PMP, (C,D) PMAP, and (E,F)
PAMAP Parylene cables (mean ± SE, n = 16). Magnitude of impedance
was normalized to the impedance of the device at 0 cycles (PMAP and
PAMAP were only bent for 10k cycles).

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Mechanical properties of thin-film Parylene–metal–Parylene devices

Minimum Bending Diameter

The variation in minimum bending diameter between the PMP
and the PMAP devices was likely due to the adhesion strength
between the two layers. Because materials are stronger in compression than compression, cracks generally initiated on theoutside edge of a bent cable and then propagated toward theinner edge. However, if there was poor adhesion between the
Al2O3 and top layer of Parylene, then the crack could not easily
propagate between the two layers of Parylene separated by Al2O3.
Conversely, it was observed that for PMAP devices oriented “in”,
the metal trace is bonded to the outside layer of Parylene andcracked when the outside layer of Parylene cracks. When there
was good adhesion on both sides, such as with the PMP devices,
the traces lay along a neutral plane (Figure S3 in Supplementary
Material), which reduced forces on the traces and resulted inbreaking below the same minimum bending diameter, whether
being bent “in” or “out”.
These results suggest that if it is desired to make devices withvery tight diameters in one direction, it should be possible todeposit metal on only one side of a material, or make one layer of
insulation thinner than the other and then fold the device toward
the thinner layer of insulation. Depositing metal on only one
side of the Parylene would also make the device prone to curling
due to residual stresses in the various layers, which have beenmeasured as 22 MPa (Dabral et al., 1992; Harder et al., 2002),
230 MPa (Branger et al., 1996), and 383 MPa (Tripp et al., 2006),
for Parylene, platinum, and Al2O3, respectively. Curling was notobserved on our devices because the sandwich structure caused
the residual stress in the two layers to cancel out (Branger
et al., 1996). Also, the Pt and Al2O3 layers are thin comparedto Parylene and the radius of curvature is proportional to thesquare of the thickness of the substrate over the thickness of thefilm. Accordingly, after the top layer of Parylene was removed
from the PMP peel test samples, the side with the metal formed
tight curls as would be predicted by the Stoney equation (Eq. 4)(Ohring, 2001) where σf and hf are the internal stress and thickness of the film, Es, hs, and υs are the Young’s modulus, thickness,and the Poisson’s ratio of the substrate, and κ is the radius ofcurvature.

FIGURE 14 | Nomarski differential interference contrast images offatigue cracks in the top Parylene layer covering three traces of a
PAMAP devices after (A) 2k bend cycles and (B) 5k bend cycles. Metal
regions are wide, light gray strips and Parylene appears as thin, dark graystrips. The majority of cracks appeared after 5k bend cycles; however, thedashed circle marks cracks present after 2k cycles (circle is 200 μm in
diameter).

FIGURE 15 | (A) Micrograph of all eight traces of a PMAP device showing
discoloration and branched patterns occurring on outside traces (scale bar is
500 μm). (B) The transverse impedance at 1 kHz for the traces shown in (A)versus their proximity to the edge. Before bend testing, traces closest to theedge had the lowest impedance, but this difference decreased as the device
was exposed to more bend cycles.

σf =

(4)

The results of the minimum bending diameter tests also suggest that improving adhesion when the metal traces lie along theneutral plane should improve bidirectional bending of devices.

as an adhesion promoter between Parylene layers. The higher
PMAP and PAMAP peel strength compared to PP also suggested
that Al2O3 and A-174 could promote adhesion between two layersof Parylene.
In all cases, the top layer of Parylene deposited on Parylene,
metal, or Al2O3, always separated from the underlying layer after
peel testing. This indicated that the interface between materials(metal and Al2O3) deposited onto Parylene possessed greater
adhesion than the interface formed when Parylene was deposited
onto the same materials. Therefore, strategies to promote adhesion should focus on the substrate-Parylene interface.

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Eshs2κ
6hf (1 − υs )Bending Fatigue

The average transverse impedance from a PMP devices (whichpeel testing showed good adhesion between the Parylene andmetal (Figure 5) dropped slightly in magnitude at lower frequencies which are more sensitive to device failure (Fernández et al.,2014), but remained relatively constant at higher frequencies(Figure 12A). The −90° phase of the PMP devices suggested
that traces were fully insulated before bending, however, theinsulation was in part compromised after the first set of bend

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Mechanical properties of thin-film Parylene–metal–Parylene devices

tests as indicated by the drop in phase at lower frequencies.
Notably, however, the phase remained capacitive at higher frequencies even when the devices were bent through 100k cycles(Figure 12B) indicating that the insulation was at least partiallyintact.
In contrast to the PMP devices, the PMAP and PAMAP devices
showed large variations in normalized magnitude, indicating largechanges in the insulation. Increases in the magnitude (Figure 12E)
could be due to cracks that broke the traces which would increase
the impedance. The decrease in impedance back to the baseline
after 10k bends could be due to re-established contact between
broken traces, or further deterioration of the insulation throughdelamination. The small phase even at 0 bends (Figures 12D,F)
indicates that there was an almost immediate intrusion of ions
across the insulation to the traces in those devices. The large SE in
the magnitude and phase of the PAMAP devices (Figures 12C,E)
pointed to a wide variation in trace performance, which could becaused by flaws introduced during processing Al2O3 (addressedin the next section) or water intrusion.
The phase of the lateral impedance of the PMP devices was
greater than −90° at lower frequencies before the devices werebent indicating that there were ionic paths between traces that
could not be detected when measuring transverse impedance(Figure 13B). However, similar to the transverse impedance, theabsence of a broad spectrum decrease in magnitude and the presence of an enduring capacitive phase (−90°) at higher frequencies
implied that at least part of the Parylene insulation remained
intact up to 100k bend cycles. Also similar to the transverse
impedance, the phase of the lateral impedance of Al2O3 devices
had small phase shifts, which suggested open ion paths between
traces (Figures 13C–F).
Fatigue cracks in the Parylene were visible in the cables after 2kbends (Figure 14A). The number and size of the cracks increased
with more bends (Figure 14B). The cracks appeared to originate
in the Parylene layer and not the metal as the first appearance wason the device edges (in which there was no metal) and the cracks
spanned across areas with and without metal (Figures S8A,B in
Supplementary Material). In addition, while all devices showed
extensive cracking, PMP devices did not exhibit impedancechanges. Cracking in Parylene can also occur due to excessive
heat (Hsu et al., 2007; Li et al., 2010), when flexed during accelerated lifetime testing (Minnikanti et al., 2013), and in long-termin vivo testing (Schmidt et al., 1988).

Parylene thin-film devices [Figure S1 in Supplementary Material(steps 5 and v)] allowed Al2O3 to be etched by the surrounding
water. Assuming that Parylene films can sufficiently prevent
water condensation, Al2O3 should be completely encapsulated in
Parylene for devices intended for use in wet applications including implantation.

Conclusion
Thin-film flexible electronics have great potential as implantabledevices. Many different thin-film devices have been demonstrated to be used to stimulate and record from many electrically
active tissues such as the spinal cord (Rodger et al., 2007), heart(Viventi et al., 2010; Zhao et al., 2013), muscle (Spence et al.,2007), and cortex (Kim et al., 2013; Tolstosheeva et al., 2011).
Although these studies have demonstrated the performance ofthe devices for short periods of time for their given application,
there have not been comprehensive methods to test the mechanical properties of such thin-film architectures until failure. In
this paper, simple devices and testing methods were presented
to test the mechanical properties of thin-film architectures. In
particular methods for measuring the peel strength, minimum
bending diameter and bending fatigue failure were presented.
These methods were then applied to a Parylene–metal–Parylene
system with and without an Al2O3 layer, which others have used
as a water barrier.
Peel tests showed that Al2O3 and A-174 can be used to increase
the adhesion between two Parylene films, but that adhesion force
is less than for Parylene–platinum interfaces. Parylene films
deposited onto surfaces did not adhere as well as platinum andAl2O3 deposited onto Parylene. Minimum bend diameter tests
showed that adding Al2O3 underneath the top Parylene allows
the devices to be bent into tighter diameters when bent awayfrom the Al2O3, but increased the minimum bending diameter
when bent toward the Al2O3, likely due to poor adhesion. EIS
performed on devices undergoing fatigue tests, showed that PMP
can withstand up to 100k bends without significant evidence of
insulation failure. When exposed to water, devices having Al2O3
failed earlier due to the etching of the film exposed at device
boundaries.

Acknowledgments
This work was funded by the NSF under award number EFRI1332394. The authors thank Dr. Donghai Zhu of the Keck
Photonics laboratory for help with fabrication, Jon Curulli of theCenter for Electron Microscopy and Microanalysis for help with
SEM images, Dr. J. Provine of Stanford University for applying
the Al2O3 coatings and members of the Biomedical Microsystems
laboratory of USC for their assistance.

Failure of Al2O3 Devices

Optical micrographs of PMAP and PAMAP devices showed
noticeable water intrusion after 100 bend cycles (images were
not taken before 100 bend cycles) (Figure S9 in Supplementary
Material). Water intrusion was more pronounced on Al2O3
devices and could be attributed to the reduced hydrophobicity of
Al2O3 compared to Parylene and the removal of Al2O3 upon exposure to water by etching (Abdulagatov et al., 2011). Based on the
impedance measurements, the water intrusion related etching of
Al2O3 started on the device edges and propagated inward toward
the center. These results indicate the open edges on the sides of

Frontiers in Mechanical Engineering | www.frontiersin.org

Supplementary Material
The Supplementary Material for this article can be found online at
http://journal.frontiersin.org/article/10.3389/fmech.2015.00010

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Mechanical properties of thin-film Parylene–metal–Parylene devices

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Conflict of Interest Statement: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could beconstrued as a potential conflict of interest.
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