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JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 27, NO. 6, DECEMBER 2018

Kirigami Strain Sensors MicrofabricatedFrom Thin-Film Parylene C
Alex Baldwin

and Ellis Meng , Fellow, IEEE

Abstract— A kirigami-based strategy was investigated forstrain transduction, using serpentine gold traces embedded inthin-film Parylene C. Multiple kirigami slit designs were evaluated via COMSOL simulation and mechanical force testing;sensors having tightly packed slits stretched up to 17.5 mm(350% strain) before mechanical failure and 9 mm (180% strain)
before electrical failure. Strain transduction was achieved bymonitoring DC resistance changes during stretching. DC resistance linearly increased with strain, with sensitivities up to
0.16 /mm (gauge factor = 0.007) and minimal hysteresis. Highfrequency trace impedance and inter-trace capacitance were also
investigated during strain cycling. Capacitance increased withstrain and high-frequency impedance show a nonlinear strain
relationship. The biocompatible construction and extremely low
profiles (20 μm thick) of these sensors are attractive for minimallyinvasive in vivo strain sensing applications.
[2018-0130]
Index Terms— Electromechanical sensors, kirigami, microsensors, microfabrication, Parylene C, strain sensors, thin film
sensors.

I. I NTRODUCTION
IRIGAMI is a paper cutting technique often used tocomplement origami. By strategically placing cuts in aplanar substrate and then subjecting the substrate to tension
or compression, complex three-dimensional patterns can beproduced [1]–[3]. Research groups in several fields implemented kirigami techniques for manufacturing, packaging,
actuation, or sensing [4]–[7]. When applied to thin film devices
such as microelectromechanical systems (MEMS), kirigami
enables complex three-dimensional structures using only twodimensional planar microfabrication techniques [8]. Several
groups reported lithium-ion batteries or supercapacitors fabricated using kirigami patterns [7], [9], [10]. These devices
were first fabricated on a planar substrate, then released andfolded into their final, packaged shape. Kirigami devices canalso generate out-of-plane movement from linear displacement or applied force; applications include sun-tracking solar

K

Manuscript received June 10, 2018; revised August 8, 2018; accepted
September 3, 2018. Date of publication September 19, 2018; date of current
version November 29, 2018. This work was supported in part by the U.S.
National Science Foundation under Grant EFRI-1332394, in part by the USC
Provost’s Fellowship, and in part by the Alfred E. Mann Institute Fellowship.
Subject Editor H. Jiang. (Corresponding author: Ellis Meng.)
A. Baldwin is with the Department of Biomedical Engineering, Universityof Southern California, Los Angeles, CA 90089 USA (e-mail: axbaldwin@
gmail.com).
E. Meng is with the Department of Biomedical Engineering, Universityof Southern California, Los Angeles, CA 90089 USA, and also with theMing Hsieh Department of Electrical Engineering, University of Southern
California, Los Angeles, CA 90089 USA (e-mail: ellis.meng@usc.edu).
This paper has supplementary downloadable material available at
http://ieeexplore.ieee.org provided by the authors.
Color versions of one or more of the figures in this paper are available
online at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/JMEMS.2018.2869090

cells [11], dynamic mechanical filters with tunable properties [12], and tunable diffraction gratings [13]. Combinedwith piezoelectric or pneumatic actuation, kirigami can beused to produce complex motion from simple one-dimensional
forces. Mechanical hands [14], biomimetic robots [15], [16],piezoelectric energy harvesters [17], and artificial muscles [18]that combine kirigami techniques with actuators were reported.
A common kirigami technique is the use of offset rowsof slits to enable a material to stretch far beyond what its
normal tensile properties would allow. By converting tensilestress to torsion, a kirigami slit pattern allows linear movement over a high dynamic range without compromising thesubstrate’s structural integrity [13], [19]. Kirigami slits were
utilized in graphene to achieve up to 267% strain before
mechanical failure [20]–[22], while on Parylene C substrates,
strain up to 1100% was reported [23], [24]. Slotted kirigami
devices exhibit minimal stress between slits, suggesting that
conduction paths placed in these regions will exhibit electronicstability. By introducing non-uniform conduction paths andselectively increasing trace resistance in regions which tend
toward either compressive or tensile strain, a trace’s DC
electrical resistance can correlate with strain [25]. Flexible
strain sensors typically exhibit relatively low strain operation[25], [26]. The potential combination of flexible, thin-filmParylene C with kirigami slit patterns could enable strain
sensors with much higher dynamic ranges.
II. S ENSOR D ESIGN
To test kirigami patterns for stretchable Parylene C electronics, devices with serpentine traces wound among an etchedkirigami slit array were designed and fabricated (Fig. 1). Eachfilm contained eight gold traces sandwiched between 10 μm
thick Parylene C layers with each trace forming a single
device. Parylene C is highly flexible but has low elongation atbreak (between 20 and 25% [27], [28]). However, Parylene C’s
compatibility with oxygen plasma etching allows fabricationof precisely aligned, high aspect ratio kirigami slits [29], [30]which permit operation exceeding the limits of the contiguous
film.
Each device consisted of a kirigami slit array 5 mm long by
4 mm wide with contact pads at either end. Slit dimensions
were held constant at 600 μm long and 20 μm wide. Slit
ends were capped with 50 μm diameter circles to avoid stressconcentration. However, lateral etching of Parylene during
deep reactive ion etching caused feature wash-out and resulted
in rounded slits 600 μm long and 50 μm wide. Two variationsof the kirigami slit array were designed: A-type devices
contained slits spaced 400 μm apart with inter-row spacing

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BALDWIN AND MENG: KIRIGAMI STRAIN SENSORS MICROFABRICATED FROM THIN-FILM PARYLENE C

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Fig. 2.
The von Mises stress in Pascals within (A) A-type and(B) B-type kirigami devices. B-type devices exhibited much lower stress undercomparable strain than A-type devices. Strain is shown at a 1:1 scale, withan outline showing the geometry of the unstretched device. Inset: stress in
B-type devices is concentrated near the ends of each slit.

Fig. 1. Top: Parylene C kirigami strain sensors consist of a 5 mm by 4 mmregion containing gold traces within a kirigami slit array, with contact pads ateither end. Bottom left: A-type devices contained slits spaced 400 μm apartin offset rows, with 200 μm separation between rows. Bottom right: B-typedevices contained slits spaced 150 μm apart in offset rows with 100 μm
separation.

of 200 μm, while slits in B-type devices were spaced 150 μm
apart in offset rows every 100 μm. Serpentine gold traces
between 10 and 20 μm wide with nominal DC resistancevalues between 50 and 150  were wound throughout theslit array.
The design of the kirigami devices was selected to achieve
high strain operation based on the guidelines that the length Lof each slit is greater than the distance d between slits, bothin the vertical and lateral directions (L > d) [19]; the L : dratio of B-type devices was 4:1, indicating that devices will becapable of high-strain operation, while A-type devices had an
L : d ratio of 3:2, indicating that elastic operation will only
occur over a limited strain range. In addition, slit width andminimum trace width were constrained based on lithography
resolution for etch and liftoff resists, and the spacing between
slits in B-type devices was selected as the minimum spacing
necessary for two 20 μm wide traces to pass between slits
while reserving enough space for fabrication variability andoveretching. A-type devices were designed with twice thevertical separation between slits as B-type devices. Slit length(600 μm) was chosen such that eight strain sensors (eachsensor consisting of a single gold trace with contact pads ateach end) could be placed on each fabricated die. Additional
devices with slits 1200 μm long and slit spacing identical toB-type devices were also designed and fabricated but were toodelicate for successful testing.
III. S IMULATION
Stress concentration and elongation to break in bothA- and B-type kirigami devices were investigated using asimple finite element model. Simulations were performed
using COMSOL Multiphysics 4.3 (COMSOL Inc., Stockholm,
Sweden). Mechanical models were constructed for the active

regions of both A- and B-type kirigami devices; models
measured 5 mm long, 4 mm tall, and 20 μm thick, with slit
dimensions and spacings identical to the fabrication masks forthe experimental devices. Material properties of Parylene C
were based on literature from Specialty Coating Systems,
Inc., which manufactures Parylene dimer and deposition systems [31]. The material was assumed to be isotropic. To determine elongation to break, the left end of the device was held inplace while the right end was displaced at intervals of 0.1 mm.
For each displacement, the von Mises stress was calculated
throughout the device; when the maximum von Mises stresswithin a device exceeded Parylene C’s reported yield strengthof 3200 MPa [31], mechanical failure was said to occur.
The stress in A-type devices exceeded the yield strengthof Parylene C after only 1.7 mm displacement (34% strain),while the stress in simulated B-type devices did not exceed
Parylene C’s yield strength until 11.8 mm displacement(236% strain). Simulated stress inside A-type devices
concentrated in four lateral bands between offset rows ofslits, while stress in B-type devices was more uniform,
concentrating near the ends of each slit (Fig. 2).
IV. FABRICATION AND T ESTING M ETHODSBoth A- and B-type kirigami devices consisted of
Parylene C layers sandwiching gold traces. Devices were fabricated using previously reported micromachining techniques[29], [30]. First, 10 μm of Parylene C was deposited onto a4” prime silicon wafer using an SCS LabCoter 2 deposition
system (Specialty Coating Systems, Indianapolis, IN, USA). A
200 Å titanium adhesion layer and 2000 Å of gold were thendeposited using a Temescal electron-beam metal deposition
system and patterned via liftoff using a 2 μm thick layer of AZ
5214 image reversal photoresist (Integrated Micro Materials,
Argyle, TX, USA). A second 10 μm layer of Parylene wasdeposited as insulation, then a 15 μm thick etch mask of AZ
P4620 photoresist (Integrated Micro Materials, Argyle, TX,
USA) was spin coated and patterned via UV lithography. The
slit array was etched in an Oxford PlasmaLab 100 inductively
coupled plasma etcher (Oxford Instruments, Abington, UK)
using a switched-chemistry deep reactive ion etch, withoxygen plasma as the etchant and C4 F8 for passivation. Thisetching process can generate high aspect ratios in Parylene C,

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JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 27, NO. 6, DECEMBER 2018with sidewall angles exceeding 70° [32]. Contact pads anddevice cutouts were exposed with a second, lower power deep
reactive ion etch using a 15 μm thick AZ4620 etch mask.
To prevent gold sputtering, RF power during the second
etch was lowered so that voltage did not exceed 70V. Afterfabrication, wafers were cleaned by immersion in acetone,
isopropyl alcohol, and deionized water for 10 minutes
each. Then devices were released from the silicon carrier
wafer by gently peeling while immersed in deionized water.
Overetching of slits and the device cutout were necessary
since peeling devices off the carrier wafer could result in thedestruction of the delicate kirigami slit array.
Electrical connection was achieved by attaching a PEEK
(polyether-ether-ketone) backing to contact pads using
cyanoacrylate glue and inserting both ends into zero insertion
force (ZIF) connectors (8 channel, 0.5 mm pitch; Hirose
Electric Co. Ltd., Tokyo, Japan) [33]. Stretch testing of devices
was accomplished by immobilizing one end and displacing theother end at 0.2 mm/s using a ThorLabs Z812 motorized stage.
For mechanical testing, force was measured using a calibrated
force gauge. For electrical testing, DC resistance was measured using a Keithley 2400 SourceMeter, and high-frequency
electrical testing was performed using an Agilent E4980A
precision LCR meter. All data was collected using a custom
LabVIEW program. Testing first involved stretching devices
to failure while measuring the mechanical forces involved
and the DC trace resistance. Then, strain transduction was
evaluated by measuring the DC resistance of traces overmultiple stretch cycles within the device’s elastic strain regime.
Finally, the high-frequency electrical properties of each trace
were investigated during repeated strain cycling.
V. R ESULTS
A. Mechanical Testing
Devices with A-type and B-type slit arrays were stretcheduntil mechanical failure while measuring the applied force.
Identical devices without etched kirigami slits were tested as acontrol. Three devices of each type were stretched until tearing
occurred while measuring the resultant force generated byeach device. Displacement was defined as the linear distancefrom the unstretched sensor along the axis between contact
pad regions, and strain was defined relative to the lengthof the slit array (5 mm displacement = 100% strain). BothA- and B-type kirigami slits drastically increased the distancethat devices could stretch before breaking (Fig. 3).
Kirigami devices have been shown to possess nonlinear
stretching dynamics, with three key regions appearing on
force-displacement curves [19], [34]. Region I corresponds toan initial in-plane deformation of the material between slits,
and is characterized by a linear force-displacement relationship. Region II is characterized by a flat force-displacement
relationship, due to the transition from in-plane deformation toout of plane deformation in the form of rotation between slits.
In region III, plastic deformation begins at the tips of slits,
resulting in a transition back to a linear force-displacement
relationship and eventually to device tearing and failure.
All three of these regions were visible in the force-

Fig. 3.
A B-type kirigami devices shown stretched to about 10 mm
displacement (200% strain), shown compared to an American 10 cent coin.

displacement curves generated for both A- and B-type kirigami
devices (Fig. 4). The control device without etched kirigami
slits experienced plastic deformation immediately and tore at
14% strain.
A-type devices appeared to plastically deform after only
4-6 mm displacement (80-120% strain), while B-type devices
remained in the elastic regime until 10 to 12 mm displacement(200-240% strain). On average, A-type devices failed via
tearing at 101% strain, corresponding to 0.7 N force, while
B-type devices failed at 350% strain and 0.3 N of force.
Parylene devices without slits broke almost immediately(14% strain) with approximately 2 N of force.
B. Electrical TestingElectrical integrity of traces during stretching was evaluatedby measuring the DC resistance of traces on an A-type anda B-type kirigami device while stretching to failure. DC
resistance on A-type device traces was stable through 3.2 mm
displacement (64% strain), then increased exponentially until
open circuit failure occurred at 3.8 mm displacement (78%strain). B-type trace resistance stayed stable through 6 mm
displacement (120% strain) and failed at 9 mm displacement(180% strain) (Fig. 5). Microcracks in the Parylene C wereobserved at inflection points near the ends of slits.
C. Strain Transduction
Kirigami devices were evaluated for strain sensitivity, withthe goal of developing a thin-film Parylene C strain sensor.
Six gold traces across two B-type kirigami devices were
tested; because of their greater mechanical and electricalstability, only B-type devices were evaluated for straintransduction. DC trace resistance was measured during
repeated cycling from 0-4 mm displacement, revealing a
positive linear correlation between resistance and strain(sensitivity 0.10-0.16 /mm) (Fig. 6). Gauge factor (GF),
which is defined as the ratio of relative change in resistanceto mechanical strain [35], varied between 0.004 and 0.007 for
DC resistance-based strain transduction. Measurement noiseBALDWIN AND MENG: KIRIGAMI STRAIN SENSORS MICROFABRICATED FROM THIN-FILM PARYLENE C

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Fig. 4. Left: Representative force-displacement curves for A- and B-type devices, compared with identical Parylene devices without kirigami slits (control).
When stretched, kirigami devices first experience brief in-plane displacement resulting in a linear force-displacement relationship (Region I), then a sharptransition to out of plane displacement and a flat force-displacement relationship (Region II), which lasts until plastic deformation begins and devices fail
(Region III). Right: A-type devices failed at around 101% strain, while B-type devices failed at around 349% strain. Devices without slits failed almost
immediately (14% strain). Break force was inversely proportional to break distance.

D. High-Frequency Impedance Characterization
The high-frequency impedance magnitude of a single trace
and the capacitance between adjacent traces were tested on
two B-type kirigami devices. The goal of this preliminary
impedance characterization was to identify future development paths for wireless strain transduction; devices were not
optimized for impedance-based strain sensing, and the effectsof strain on trace self-impedance or inter-trace capacitance
was not considered when designing the devices. Four traces(sensors) on a single B-type kirigami device were tested for
self-impedance, and a second kirigami device was evaluatedfor capacitance between traces.
Complex impedance at 2 MHz was recorded for single
traces during multiple displacement cycles from 0-5 mm. The
complex impedance of a trace includes the trace resistance R,
self-inductance L, and self-capacitance C S , and has the relationshipZ=
Fig. 5. The DC resistance of a trace on A- and B-type devices stretched until
failure. Tighter slit spacing increased distance to failure from 78% to 180%
strain. A-type traces have a lower DC resistance than B-type traces due toshorter path length; the baseline resistance of A-type traces is approximately
50  versus 110  for B-type traces. Any relationship between strain andresistance within the elastic deformation range is not visible in this figure due
to its small magnitude compared to the large resistance changes seen during
failure. Inset: Microscope image of a B-type device stretched to 9 mm
(180% strain). Microcracks are visible near slit inflection points.

was significant compared to the sensor’s sensitivity, resulting
in 3σ resolution between 0.2 and 1.6 mm (4 to 32% strain).
A small amount of hysteresis was observed over multiple
stretch cycles. Though noisy, the sensitivity of resistance tostrain was observed to be stable over multiple strain cycles;
there was no difference in sensitivity between the first timea device was stretched and subsequent stretching (Fig. 7).

R+ j ωL
1 + j ω RCs(1)√
where ω is measurement frequency and j is −1. Impedance
magnitude showed a nonlinear dependence on strain, which
varied depending on trace geometry (Fig. 8). The total increasein impedance magnitude between 0 and 5 mm displacement
varied between 0.4% and 1%. The capacitance between adjacent traces (C P ) at 100 kHz was also evaluated. Capacitance
was relatively stable up to 2 mm displacement and increased
linearly at a rate of 4.7 fF/mm between 2 and 5 mm displacement (Fig. 9).
VI. D ISCUSSION
Experimental results showed that kirigami devices fabricated out of Parylene C and gold stretched to 4.5× their
original size before mechanical failure (see supplemental
video). The strain until mechanical failure in experimental

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JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 27, NO. 6, DECEMBER 2018

Fig. 6. Left: The DC resistance of two different traces on a single B-type kirigami device during strain cycling. The top trace showed a strain sensitivityof 0.131 /mm (GF = 0.005) with 3σ resolution of 1.07 mm (21.4% strain), while the bottom traces had a strain sensitivity 0.100 /mm (GF = 0.004) with
3σ resolution of 0.81 mm (16.2% strain). Mean and standard deviation were calculated using resistance values measured at 0.1 mm intervals while increasing
and decreasing strain over three strain cycles. Both traces are the same design; differences in baseline resistance were likely due to process variations. Right:
DC resistance change versus strain for a different gold trace over three displacement cycles between 0 and 4 mm. This trace showed a sensitivity of 0.156
/mm (GF = 0.007) with a resolution of 0.21 mm (4.2% strain). A small amount of hysteresis was observed. The baseline DC resistance was 118.7 . Meanand standard deviation were calculated using resistance measurements at 0.05 mm intervals.

Fig. 7. The response of a strain sensor on a B-type kirigami device over its
first three strain cycles. Despite high noise, there was no noticeable difference
between the first strain cycle and subsequent strain measurements.

devices exceeded that of the simulation results, with B-typedevices failing at 350% strain experimentally versus 236% in
simulation and A-type devices failing at 101% strain experimentally versus 34% in simulation. Improved performanceover model results may be due to fabrication variability,
namely low lithography resolution and overetching, both ofwhich would increase slit dimensions and result in higher
elongation to break. The model also possessed inherent limitations, including the assumption that the Parylene substrate wasisotropic instead of layered and the use of material properties
from literature without adjusting for changes that might occur
during fabrication. A more accurate mechanical model could
be constructed using two Parylene C layers with gold traces,
and thin-film Parylene C could be measured after undergoing
lithography, reactive ion etching, and metal deposition to determine if thermal cycling during these processes significantly
affected material properties.
The electrical integrity of traces failed much earlier, at apoint which appeared to coincide with the transition between

Fig. 8. High-frequency impedance magnitude versus displacement for asingle trace on a B-type kirigami device. The impedance magnitude shows anon-linear relationship with displacement. Mean and standard deviation arecalculated using impedance magnitudes measured at 0.1 mm intervals during
strain cycling.

regions II and III as seen in force/displacement curves. Microcracks in Parylene were visible near the ends of each slit after
stretching, and represent the likely cause of electrical failure.
Electrical and mechanical tests confirm that increasing slit
density enables high strain operation, but increasing inter-trace
width does not improve the electrical stability of traces.
The DC resistance of traces on B-type devices varied
quasilinearly with strain. This strain sensitivity likely resulted
from stress within the thin-film gold traces, which concentrated
at inflection points near the end of slits. DC resistance began
to increase linearly as soon as strain was applied, but on somedevices, the strain-resistance slope decreased after a sharptransition point, occurring around 60% strain. This drop could
reduce overall sensitivity if a linear model is used and could
cause measurement errors at the high end of the sensor’s operating range. However, the largest contributor to sensor error isnoise during individual measurements, and inaccuracy due toBALDWIN AND MENG: KIRIGAMI STRAIN SENSORS MICROFABRICATED FROM THIN-FILM PARYLENE C

Fig. 9. The capacitance between two adjacent traces on a B-type kirigami
device during two strain cycles to 5 mm, measured at 100 kHz. At around
2 mm displacement, capacitance begins to increase linearly with strain.
TABLE IT HIS W ORK C OMPARED TO O THER P ROPOSED F LEXIBLE S TRAINS ENSORS [25], [26], [38]. B REAK S TRAIN R EFERS TO THE
S TRAIN AT W HICH M ATERIALS FAILURE WAS R EPORTED ,WHILE A CTIVE S TRAIN R EFERS TO THE R ANGE OVER
W HICH S TRAIN T RANSDUCTION WAS S UCCESSFULLYT ESTED . CNT S = C ARBON N ANOTUBES ,EGaIn = E UTECTIC G ALLIUM -I NDIUM(L IQUID M ETAL )the bilinear response in some traces will not significantly affectmeasurement accuracy until noise is reduced. Strategies for
improving reproducibility between traces include using thicker
metal layers and wider traces for increased trace stability
and improving the placement of traces around kirigami slits
using guidance from more advanced COMSOL simulations
that directly model electrical traces during device stretching. Strategies for noise reduction include the use of more
thermally stable conductive materials, improvements to thetesting setup to further reduce vibration and provide a stabletemperature during testing, and the implementation of signal
filtering to eliminate high-frequency resistance variations.
Even without optimization, the sensitivity of resistancebased strain transduction is competitive with previously
reported polyimide devices. Parylene devices possessed ahigher dynamic range despite a conservative testing protocol(0.16 /mm over 80% strain vs −0.18 /mm over 20% strain
for polyimide sensors [25]). Construction from Parylene C
and gold renders sensors not only resistant to corrosion
but biocompatible and suitable for use in the human body.
This contrasts with other reported strain sensors that utilize
semiconductor-based piezoresistive materials [36], carbon nanotubes [26], [37], or liquid metal [38].
The scope of this study was limited to investigating thestrain sensitivity of electrical traces embedded in thin-film

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kirigami devices. Further work is needed to develop the
kirigami devices presented in this work into practicalstrain sensors for biomedical applications. For example,
it is important to investigate high-frequency operation ofstrain sensors for application involving rapid movement.
Unfortunately, the testing setup available for straintransduction and evaluation was limited to a displacement rateof 0.2 mm/s. Future tests require the construction of a new
testing setup for rapid strain cycling, and future studies will
investigate the mechanical frequency response and bandwidthof optimized kirigami devices. Measurement noise was higher
than desirable for precise strain measurements. High noiselevels may have been due to thermal noise in the conductive
trace or environmental noise in the testing setup. A small
amount of hysteresis was observed, possibly due to polymer
reorganization during device displacement. There was also alarge difference in sensitivity between traces, even within thesame device; the trace’s position on the device did not appear
to be the cause of these sensitivity differences. Further work
is needed to determine the causes of sensitivity differences
and to optimize trace designs for DC strain sensing.
Preliminary experiments indicate that both self-impedance
and inter-trace capacitance increased with strain. Strain transduction via complex impedance or capacitance would be
advantageous in the development of passive, wireless strain
sensors. Increases in trace impedance were likely a resultof variations in self-inductance within a trace. Changes ininter-trace capacitance may have resulted from varying either
the separation between traces or the dielectric properties ofParylene C under stress. Future work will continue to explore
high-frequency strain sensing with optimized devices, and will
pair kirigami strain sensors with thin-film inductive coils [39]to develop passive, wireless strain sensors.
VII. C ONCLUSION
Using kirigami slit designs, devices were fabricated out of
Parylene C and gold which could stretch to multiple times
their length while maintaining electrical integrity of traces.
The devices were found to have multiple electrical parameterswhich vary predictably with strain. DC resistance’s sensitivity
to strain was found to be of comparable sensitivity to previously reported polymer sensors but with a higher dynamic
range and simpler trace geometry. Inter-trace capacitance andhigh-frequency inductance also varied with strain, which could
lead to additional sensing modalities based off frequency
transduction as opposed to resistive transduction. Construction
out of only Parylene and gold renders these sensors not only
resistant to corrosion but fully biocompatible and suitable forchronic implantation in the human body. This contrasts withother common strain sensors which utilize semiconductorbased piezoresistive materials or carbon nanotubes for strain
sensing, neither of which is amenable to chronic implantation.
A high dynamic range, low stress strain sensor could potentially be used in numerous biomedical applications.
ACKNOWLEDGMENT
The authors would like to thank Jessica Ortigoza-Diaz forher help with testing and characterization, Dr. Donghai Zhu of1088

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the Keck Photonics Laboratory for his assistance with fabrication, and all the members of the Biomedical Microsystems
Lab of USC for their help and support.
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Alex Baldwin received the B.S. degree in electrical engineering from the University of Arkansas,
Fayetteville, in 2013, and the M.S. and Ph.D.
degrees in biomedical engineering from the University of Southern California (USC) in 2015 and 2018,
respectively. He is currently involved in improving
the treatment of hydrocephalus using chronicallyimplanted wireless sensors. His research interests
include polymer micromachining, implantable biomedical sensors, and novel microelectromechanical devices, which operate using electrochemical
impedance. He was a recipient of the USC Viterbi Fellowship and the Alfred
E. Mann Institute Fellowship.
Ellis Meng (M’02–SM’09–F’16) received the B.S.
degree in engineering and applied science and the
M.S. and Ph.D. degrees in electrical engineeringfrom the California Institute of Technology (Caltech), Pasadena, in 1997, 1998, and 2003, respectively. She also holds a joint appointment with theMing Hsieh Department of Electrical Engineering.
She has been a Professor of biomedical engineeringwith the University of Southern California, Los
Angeles, since 2004. Her research interests include
bioMEMS, implantable biomedical microdevices,microfluidics, multimodality integrated microsystems, and packaging. She isa fellow of ASME, BMES, and AIMBE. She is a member of the Tau Beta
Pi, the Biomedical Engineering Society, the Society of Women Engineers,
and the American Society for Engineering Education. She was a recipientof the Intel Women in Science and Engineering Scholarship, the Caltech
Alumni Association Donald S. Clark Award, and the Caltech Special Institute
Fellowship. She has also received the NSF CAREER Award and the Wallace
H. Coulter Foundation Early Career Translational Research Award. In 2009,
she was recognized as one of the TR35 Technology Review Young Innovators
under 35. She was the Viterbi Early Career Chair of the Viterbi School ofEngineering and the Department Chair from 2015 to 2018.