A KIRIGAMI-BASED PARYLENE C STRETCH SENSOR
Alex Baldwin and Ellis Meng
University of Southern California, Los Angeles, USA
and selectively increasing trace resistance in regions which
tend toward either compressive or tensile strain, a trace’s
DC electrical resistance can correlate with displacement, asseen in previously reported polyimide devices [8]. We
developed a stretch device consisting of a thin-filmParylene C substrate with an etched kirigami slit array (Fig.
1). Gold traces were patterned around slits, and both DC
and high-frequency properties of these traces wereevaluated during stretching with the aim of transducing
stretch distance. Previously reported polymer-based stretch
sensors exhibited relatively low dynamic ranges and stretch
distances that maxed out at around 20% [8, 9]. The
combination of flexible, thin-film Parylene with kirigami
slit patterns could enable stretch sensors with much higher
dynamic ranges.

ABSTRACT
We present the first slotted Parylene C kirigami device
for transducing stretch-induced displacement undermultiple modes of operation. Devices consist of thin-film
gold traces wound among multiple rows of offset slits
etched through a free-film Parylene substrate. Slits wereetched via a switched-chemistry DRIE in oxygen plasma,
resulting in densely packed slit designs. Multiple slit
designs were evaluated and devices with more tightly
packed slits were found endure greater stretch distance
before failure, with devices containing more tightly packed
slits able to stretch up to 9 mm (180%) without plastic
deformation or compromise of electrical integrity. Multiple
modes of stretch transduction were evaluated, including
DC resistance changes due to strain at trace inflection
points and high-frequency changes in self-impedance andinter-trace capacitance. DC resistance was found toincrease linearly with stretch distance with sensitivities
between 0.04 and 0.1 Ω/mm which are comparable topreviously reported polyimide sensors but with a wider
dynamic measurement range. Capacitance was also foundto increase linearly with stretch distance after 2 mm, andhigh-frequency impedance increased nonlinearly, withdifferent trace routing designs resulting in different
patterns of impedance change with stretch.

INTRODUCTION

Figure 1: Parylene stretch sensors consist of an actuationsection 5 mm long and 4 mm wide containing gold traces
wound through an etched kirigami slit array, with two
contact pad regions at either end.

Kirigami (a Japanese term translating to ‘cut’ and‘paper’) is a technique often used to complement origami
and enables creation of three-dimensional shapes from a
two-dimensional sheet of paper. By strategically placing
cuts in a planar paper substrate and then subjecting thesubstrate to tension or compression, a huge variety ofcomplex three-dimensional patterns can be produced.
Kirigami techniques have been employed in industrial andresearch applications and proven valuable for producing
3D shapes and actuators in fields ranging from architecture[1] to batteries [2] to biomimetic robots [3]. A common
kirigami technique is the use of offset rows of slits toenable a material to stretch far beyond what its normal
tensile properties would allow; by converting tensile stress
to torsion at specific points between slits, a kirigami slit
pattern allows linear movement over a high dynamic range
without compromising the structural integrity of thesubstrate [4]. This effect has been utilized in graphenekirigami devices to achieve stretch distances of up to 267%
[5, 6], and on a more flexible Parylene C substrate, tightly
packed slits have enabled stretch distances of up to 1100%
before tearing occurred [7].
Slotted kirigami devices exhibit minimal material
stress in columns between slits, and tensile andcompressive stress average to zero along each sliver ofmaterial adjacent to a slit, implying stability of electricalconnectivity in metal traces wound among these features.
However, by introducing non-uniform conduction paths978-1-5090-5078-9/17/$31.00 ©2017 IEEE

DESIGN & FABRICATION
Stretch sensors consisting of gold traces wound among
etched slits in a free-standing Parylene substrate weredesigned and fabricated. Parylene C was chosen as thesubstrate material due to its high elongation to break (up to200% [10]), which can lead to higher stretch distances
compared to more rigid substrates, and its etchability whichallows for placement of precisely aligned, high aspect ratio
microscopic slits. Devices contained a slit array over a 5mm long and 4 mm wide area with two contact pad regions
at either end to enable electrical connection and aid intesting.
Two variations of the kirigami slit array weredesigned. Type A devices contained slits spaced 400 µm
apart with inter-row spacing of 200 µm, while type B
devices contained slits spaced 150 µm apart in offset rows
every 100 µm (Fig. 2). 100 µm was chosen as the smallest
separation distance to enable two 20 µm wide gold traces
between slits with 20 µm clearance. Both device variations
contained slits 600 µm long and 20 µm wide with circular
ends 50 µm in diameter to reduce stress. Traces between 5and 20 µm wide with nominal DC resistance values
between 50 and 150 Ω were wound throughout the slit

227

MEMS 2017, Las Vegas, NV, USA, January 22-26, 2017

array. Traces either wound to one side of the slit
consistently, to enhance proximity between traces andincrease inter-trace capacitance, or alternated sides toisolate self-impedance (Fig. 2B).

Electrical connection was achieved by attaching aPEEK (polyetheretherketone) backing to contact pads
using cyanoacrylate glue and inserting both ends in zero
insertion force (ZIF) connectors [12] connected to flat
flexible cables (Fig. 4).

Figure 2: (A) Type A devices contained slits spaced 400 µm
apart in offset rows, with 200 µm separation between rows.
(B) Type B devices contained slits spaced 150 µm apart inoffset rows with 100 µm separation. Traces either windaround slits consistently (right two traces) or alternatedsides (right trace).

Figure 4: Devices were attached to PEEK backing andinserted into ZIF connectors during electrical testing. A
stretched device is shown between the ZIF connectors.
Testing Methods
Stretch testing of devices was accomplished byholding one end of the device fixed and stretching theopposing end at a rate of 0.2 mm/s using a ThorLabs
moveable stage. DC resistance was measured using aKeithley 2400 SourceMeter and high-frequency impedanceand capacitance were measured with an Agilent E4980A
LCR meter. Data was collected with a custom LabView
program at a rate of 5 Hz.
Three methods of stretch transduction were evaluated.
Changes in the DC resistance of a trace monitored changes
in strain seen along the conduction path. The highfrequency impedance of a single trace measured changes inself-inductance or self-capacitance due to increasing
separation of trace sections. The inter-trace capacitance
was taken as a measure of net changes in the distance
between traces as well as changes in dielectric properties
due to the imposed strain on the Parylene substrate.

Fabrication
Fabrication first involved the deposition of 10 µm ofParylene C onto a silicon carrier wafer (Fig. 3). A 200 Å
titanium adhesion layer and 2000 Å of gold were thendeposited using electron-beam metal deposition andpatterned by a liftoff process. A second 10 µm layer ofParylene was deposited as insulation, and the slit array wasetched using a switched-chemistry deep reactive ion etch
which consisted of alternating etch steps of inductively
coupled oxygen plasma and passivation steps of C4F8,
resulting in an anisotropic etch and slits with high aspectratios [11]. Contact pads at either end of the device wereexposed using a second, lower power deep reactive ion etch
to prevent gold sputtering. Devices were released from thesilicon carrier wafer by gently peeling while immersed indeionized water.

EXPERIMENTAL RESULTS
Both type A and type B devices were stretched to
failure to evaluate the effects of different slit patterns on
trace’s electrical stability. Stretch distance was defined as
the linear displacement from rest of the sensor along theaxis between contact pad regions, and percent stretch wasdefined relative to the length of the slit array (such that 5mm stretch distance = 100% stretch). DC resistances ontype A device traces were stable up to 3.2 mm (64%) ofdisplacement and increased exponentially thereafter until
open circuit failure occurred at 3.8 mm (76%), likely due
to microcracks in the gold traces at inflection points (Fig.
5). Type B device resistances stayed stable until 6 mm

Figure 3: Sensor fabrication involved (a) deposition ofParylene C on a silicon carrier wafer, (b) electron beam
deposition and liftoff patterning of 2000 Å Au with 200 Å
Ti adhesion layer, (c) etching of slits via a switchedchemistry deep reactive ion etch in oxygen plasma and (d)release from the carrier wafer by peeling while immersedin DI water.

228

(120%) of displacement and failed at 9 mm (180%),
confirming that increasing slit density enabled greater
stretch distances but increased inter-trace width did not
improve electrical stability of traces (Fig. 6).

Figure 7: The DC resistance of alternating traces on a B
device over multiple stretch cycles, showing sensitivities of
0.043 and 0.088 /mm. Both traces are of the same design;differences in baseline resistance are likely due to process
variations.

Figure 5: Microscope image of a B type device stretched to
9 mm (180%). Strain is concentrated at two inflection
points near the end of the slit.

Capacitance between adjacent consistently-woundtraces was evaluated at 100 kHz. Capacitance was found to
be relatively stable up to 2 mm displacement and toincrease linearly at a rate of 4.7 fF/mm between 2 and 5
mm displacement (Fig. 8). Such changes in capacitance
may result from net changes in separation distance between
traces or changes in the dielectric constant of Parylene
induced by stress.

Figure 6: The DC resistance of a trace on A and B typedevices stretched at 0.2 mm/s until failure. Tighter slit
spacing increased the stretch distance to failure from 76%
to 180% and showed that wider spacing between slits did
not improve the stability of embedded traces.
Due to the higher achievable stretch distances andstability of electrical traces, B type devices were evaluatedfor stretch transduction. DC trace resistance was measured
during cycling from 0-4 mm displacement, revealing a
positive linear correlation between resistance anddisplacement (sensitivity 0.04-0.1 Ω/mm) (Fig. 7). This
stretch sensitivity likely results from increases in strain in
the thin-film gold traces, which is concentrated at inflection
points near the end of slits. Even without optimization, thesensitivity of resistance-based stretch transduction wascompetitive with previously reported polyimide devices,with Parylene devices possessing a higher dynamic range
even within a conservative testing protocol (0.09 Ω/mm
over 80% displacement vs -0.18 Ω/mm over 20%
displacement in polyamide devices [6]).

Figure 8: The capacitance between two adjacent traces ina B device, measured at 100 kHz. At around 2 mm
displacement, the capacitance begins to increase linearlywith stretch distance.
Complex impedance at 2 MHz was recorded for asingle trace during stretching from 0-5 mm. Impedance
magnitude showed a nonlinear dependence with stretch
distance which varied depending on whether traces wound
to one side of slits consistently or alternated winding side.
Consistent traces showed a sigmoidal increase in
impedance with an inflection point at approximately 2.5mm displacement and a total resistance change between 0and 5 mm of displacement of about 1%, while alternating229

traces showed a simpler curved trend and a total resistancechange of around 0.4% (Fig. 9).

geometries which may improve stretch sensitivity.
Capacitive and impedance-based stretch transduction will
also be further explored with the goal of developing sensors
which transduce stretch via changes in resonant frequency
instead of resistance. The attachment of a Parylene-based
coil would render such systems wireless and fully
implantable.

(a)ACKNOWLEDGEMENTS
We would like to thank Dr. Lawrence Yu for his advice on
device design and fabrication, Jessica Ortigoza for her help
in designing the testing setup, and Dr. Donghai Zhu forcleanroom and fabrication assistance. This work wasfunded under NSF award number EFRI-1332394.

REFERENCES

(b)

1.
2.
3.

4.
Figure 9: The high-frequency impedance magnitude showsa non-linear relationship with stretch distance, with (A) asigmoidal relationship for consistent traces and (B) acurved relationship for alternating traces.

5.
6.

DISCUSSION & FUTURE WORK

7.

Kirigami devices constructed of Parylene and gold
were fabricated and found to have multiple electricalparameters which vary predictably with stretch distance.
DC resistance’s sensitivity to stretch distance was found to
be of comparable sensitivity to previously reported
polymer sensors but with a higher dynamic range andsimpler trace geometry. Inter-trace capacitance and highfrequency inductance also varied with stretch distance,
which could lead to additional sensing modalities based off
of frequency transduction as opposed to resistivetransduction.
Construction out of only Parylene and gold renders
these sensors not only resistant to corrosion but fully
biocompatible and suitable for chronic implantation in thehuman body. This contrasts with other common strain
sensors which utilize semiconductor-based piezoresistive
materials or carbon nanotubes for stretch sensing, neitherof which is able to be chronically implanted. A highdynamic range, low strain stretch sensor could potentiallybe used in numerous biomedical applications, such as
monitoring bladder size for the treatment of urinary
incontinence, sensing muscle stretch for improved
proprioception, or integrating with fabric-based wearables
for non-invasive movement transduction.
Future work will involve optimizing slit size andorientation in order to maximize stretch distance andminimize internal stress, as well as exploring different trace

8.

9.

10.
11.

12.

230

Weston,M.,
AnisotropicOperations.
International Journal of Architectural Computing,2012. 10(1): p. 105-120.
Song, Z., et al., Kirigami-based stretchable
lithium-ion batteries. Scientific reports, 2015. 5.
Sareh, S. and J. Rossiter, Kirigami artificial
muscles with complex biologically inspiredmorphologies. Smart Materials and Structures,2012. 22(1): p. 014004.
Kotov, N.A., et al., Kirigami Nanocomposites as
Wide-Angle Diffraction Gratings. ACS nano,
2016.
Blees, M.K., et al., Graphene kirigami. Nature,
2015.
Shyu, T.C., et al., A kirigami approach toengineering elasticity in nanocomposites throughpatterned defects. Nature materials, 2015.
Morikawa, Y., et al. An origami-inspired
ultrastretchable bioprobe film device. in 2016
IEEE 29th International Conference on Micro
Electro Mechanical Systems (MEMS). 2016.
IEEE.
Firouzeh, A. and J. Paik, The design and modelingof a novel resistive stretch sensor with tunablesensitivity. IEEE Sensors Journal, 2015. 15(11): p.
6390-6398.
Lee, C., L. Jug, and E. Meng, High strain
biocompatiblepolydimethylsiloxane-based
conductive graphene and multiwalled carbon
nanotube nanocomposite strain sensors. Applied
Physics Letters, 2013. 102(18): p. 183511.
Kim, B.J. and E. Meng, Micromachining ofParylene C for bioMEMS. Polymers for
Advanced Technologies, 2015.
Meng, E., P.-Y. Li, and Y.-C. Tai, Plasma
removal of Parylene C. Journal of
Micromechanics and Microengineering, 2008.
18(4): p. 045004.
Gutierrez, C.A., et al. Epoxy-less packaging
methods for electrical contact to parylene-based
flat flexible cables. in 2011 16th International
Solid-State Sensors, Actuators and Microsystems
Conference. 2011. IEEE.