a flexible pressure sensor with sandwiched carpets of
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Abstract— This paper presents a flexible pressure sensor
composed of two face-to-face electrodes of vertically aligned
carbon nanotubes (VACNTs) carpets partially embedded in
polydimethylsiloxane (PDMS) substrates. VACNTs were grown
using atmospheric pressure chemical vapor deposition (APCVD)
on a Si/SiO₂ substrate and transferred onto PDMS for partial
embedment. This unique synthesis permitted a rapid and facile
integration of a flexible and stretchable platform for the pressure
sensor. The change of resistance occurs due to the external
pressure applied orthogonal to the surface. The substrate was
stretched to 180% and bent at bending radii of 87 mm and 105
mm, respectively. The pressure sensor exhibited the response
times of 40 ms during loading and 60 ms during unloading,
respectively. As a proof-of-concept, the sensor was attached on
human skin; the heart rate, muscle flexing, and walking signals of
an individual have been measured. In respect to durability, this
sensor also showed a stable performance for 10,000
loading/unloading cycles with a resistance retention of 82%
demonstrating its long-term use for repeated cycles toward
practical applications.
Index Terms— carbon nanotubes, flexible electrodes, pressure
mapping, pressure sensor, stretchability.
I. INTRODUCTION
ecent research and development have enabled the
manufacturing and integration of high-performance
electronic systems on flexible substrates with promising
prospect for e-skin, wearable, and bio-integrated applications
[1]–[7]. These electronic systems incorporate a wide range of
sensors, including electrophysiological, temperature, strain,
and pressure sensors [8].
Pressure sensors are used to measure applied pressure via
Manuscript received XXX; revised XXX; accepted XXX. Date of
publication XXX; date of current version XXX. This work was partially carried out in the Micro Devise Laboratory (MDL) at Stevens Institute of Technology
funded by US Army under Contract No. W15QKN-05-D-0011. The associate
editor coordinating the review of this paper and approving it for publication was XXX. (R. Zhang and A. Palumbo are co-first authors) (R. Zhang and A.
Palumbo contributed equally to this work.) (Corresponding author: Eui-Hyeok
Yang.) R. Zhang, A. Palumbo, G. Hader, K. Yan, and J. Chang are with the
Laboratory of Nanomaterial Growth and Nanofabrication, Department of
signal generation, which is often electrical. Silicon-based
pressure sensors have long been realized commercially by well-
established silicon micromachining technologies [9]–[13].
However, the rigid and brittle structure that constitutes the
sensors can limit the functionality and applications of pressure
sensors as wearable electronics [14], [15]. To this end, thin and
flexible pressure sensors have been studied for flexible
electronics and e-skin applications [16]–[19]. Efforts have been
made to develop pressure sensors on flexible substrates and to
improve their performance (i.e., sensitivity, response time,
pressure range, flexibility, and durability). Flexible pressure
sensors are usually realized by replacing silicon with flexible
substrates such as polymers, paper, or textiles [20]–[23]. In
most cases, electrode materials are sandwiched between two
flexible substrates. For the electrode materials, gold nanowires
[24], silver nanowires [21], carbon nanotubes (CNTs), and
graphene [25], [26] have been widely used, and various
structures [27], [28] have been developed toward enhancing the
performance of sensors. Several flexible pressure sensors with
CNTs as electrode materials exhibited the flexibility with up to
120% of stretching and bendability with up to 90˚ bending
angles [14], [19], [20], [22], [29]. A multi-functional stretchable
sensor composed of orthogonal CNT-polyurethane sponge
strips demonstrated the independent detection of
omnidirectional bending and pressure [22]. Another pressure
sensor comprised of hierarchical structures of aligned
CNT/graphene and microstructured PDMS indicated a
detection limit of 0.6 Pa and a response time of 16.7 ms [28].
Piezoresistive interlocked microdome arrays made of CNT-
composite elastomer films detected various mechanical
Mechanical Engineering, Stevens Institute of Technology, Hoboken, NJ 07030,
United States. H. Wang is with the Faculty of Biomedical Engineering, Stevens Institute of
Technology, Hoboken, NJ 07030, United States.
E. H. Yang is with the Faculty of Mechanical Engineering, Stevens Institute of Technology, Hoboken, NJ 07030, United States (e-mail:
eyang@stevens.edu)
A Flexible Pressure Sensor with Sandwiched
Carpets of Vertically Aligned Carbon Nanotubes
Partially Embedded in Polydimethylsiloxane
Substrates
Runzhi Zhang#, Anthony Palumbo#, Grzegorz Hader, Kang Yan, Jason Chang,
Hongjun Wang, and Eui-Hyeok Yang*
#These authors are co-first authors and contributed equally to this work *Correspondence: eyang@stevens.edu
R
This is the author's version of an article that has been published in this journal. Changes were made to this version by the publisher prior to publication.The final version of record is available at http://dx.doi.org/10.1109/JSEN.2020.2999261
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stimuli, including normal, shear, stretching, bending, and
twisting forces [29]. Pattern transfer techniques have been
commonly employed to fabricate electrodes towards flexible
pressure sensors [30]–[35]. For example, graphite transferred
onto PDMS using a two-step process by direct-write laser
printing and patterned transfer resulted in flexible sensing
resistors for fingertip pressure detection up to a range of 12 kPa
[33]. Inkjet-printed CNTs of up to 35 layers transferred onto a
PDMS substrate yielded a pressure sensor exhibiting the change
in resistance with force up to 0.5 N [31]. CNT thin films
fabricated by transferring mechanically drawn CNT sheets via
metal stick directly onto a polyethylene naphthalate (PEN)
substrate resulted in a flexible pressure sensor with a pressure
detection limit less than 2 Pa [30]. Although aforementioned
reports on flexible pressure sensors have demonstrated good
mechanical flexibility and stable performance essential for
realizing e-skin or wearable applications, there is room for
improvement on both the complicated fabrication process and
their performance.
Here we demonstrate a flexible pressure sensor using
VACNT as the electrode materials and PDMS as a flexible
platform. VACNTs are grown via atmospheric pressure
chemical vapor deposition (APCVD) and then transferred onto
partially cured PDMS. As a result, VACNTs are partially
embedded in PDMS, ensuring that individual CNTs are
strongly held by the fully cured PDMS. This enables the
structure to sustain various strains. We measure the change in
resistance due to the application of pressure orthogonal to the
surface, while demonstrating the flexibility with stable
performance. We determine that the fabricated
VACNTs/PDMS structure achieves a high level of integrity
under various strains. Detailed characterization on the
durability and flexibility of the sensor is performed under
various strains and for 10000 loading/unloading cycles.
II. MATERIALS AND METHODS
A. Fabrication of Flexible Pressure Sensors
The fabrication of CNT/PDMS structure is similar to the
process reported in our previous work [36]. VACNTs were
grown via APCVD, and Raman spectroscopy confirmed
VACNT quality such that individual nanotubes are not affected
by the lateral stretching of PDMS, as shown in our previous
work [36]. Physical vapor deposition (PVD) was used to
deposit 5 nm Al and 3 nm Fe as catalyst on the substrate of
Si/SiO2. Following addition of catalyst, the substrate was
placed in a quartz tube furnace for CNT growth via APCVD
with a growth temperature increased to 750 °C while flowing
500 sccm Ar flow, and the furnace temperature was maintained
at 750 °C for 15 min with 60 sccm H2 and 100 sccm C2H4. The
CVD tube was then rapidly cooled to room temperature by
opening the furnace cover, while maintaining Ar flow. The
grown CNTs displayed a vertically aligned structure, composed
of individual wavy nanotubes mechanically cross-linked with
one another. The grown CNTs on Si/SiO2 were transferred onto
partially cured PDMS, as detailed in our previous work [36].
First, a liquid mixture of PDMS base and curing agent (Sylgard
184 Silicone Elastomer, Dow Corning) was mixed with a ratio
of 10:1 and degassed under reduced pressure in a vacuum
environment to remove gas bubbles. The liquid PDMS was
heated on a hot plate at 65 °C for approximately 30 min to a
state of partially cured PDMS. The curing condition of PDMS
was optimized to wet fully the roots of individual CNT carpets
upon the full curing of PDMS. To transfer the CNT carpet onto
partially cured PDMS, as grown CNT on Si/SiO2 was placed
face-to-face onto the partially cured PDMS, and PDMS
infiltrated between each individual carbon nanotube, owing to
the capillary effect and the viscoelastic property of PDMS. As
a result, tips of interwoven CNTs were embedded into PDMS,
and the CNT/PDMS was left in an ambient condition for 12 h
to fully cure PDMS. After PDMS was fully cured, VACNTs
were partially embedded in PDMS and the Si/SiO2 substrate
was easily peeled away, resulting in a single CNT/PDMS
electrode. Then two CNT/PDMS electrodes were stacked face-
to-face to fabricate the resistive-based flexible pressure sensor.
Fig. 1. (a) Schematic showing the mechanisms of pressure sensing. (b–c) SEM
images depicting the cross-sectional view of an area interfacing the two
VACNTs/PDMS layers, with scale bars of 5 μm and 500 nm, respectively. (d–
e) Digital photograph images of the pressure sensor at (d) initial length, and
(e) after stretching with 100% stretching strain, both with scales bars of 1 cm.
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B. Characterization and Measurements
The scanning electron microscope (SEM) (Auriga Small
Dual-Beam FIB-SEM, Carl Zeiss, Jena, Germany) was used to
characterize the samples. The resistances of the sensors were
measured using potentiostat. The sensors were connected to the
potentiostat and a voltage was applied to the sensor while the
current was measured. A force testing system (Chatillon
TCD110 Series) was used to apply various pressures to the
sensor. A constant displacement rate of 0.5 mm/minute was
applied, compressing the pressure sensor vertically.
III. RESULTS AND DISCUSSION
Fig. 1a shows a schematic of the mechanisms of the flexible
pressure sensor. Fig. 1b–c show scanning electron microscope
(SEM) images depicting the cross-sectional view of an area
interfacing the two VACNTs/PDMS layers, where CNT carpets
of upper and bottom layers make contact. Fig. 1d–e present
digital photographs of a pressure sensor sample, showing the
stretchability for wearable application. When an external
pressure is applied orthogonal to the surface of VACNT
carpets, the contact areas between individual CNTs embedded
on both top and bottom PDMS substrates increase; the
resistance of the contacting nanotubes then decreases because
of the decrease of the electrical conducting length.
The fabricated devices showed a broad range of pressure
sensing from 26 Pa to 56 kPa, as shown in Fig. 2a, with
capabilities in the subtle-pressure regime such as screen touch
(<1 kPa), the low-pressure regime such as daily activity (1–10
kPa), and also the medium-pressure regime such as object
manipulation (10–100 kPa) [17], [27]. As shown in Fig. 2a, the
recovery curve and response curve result in hysteresis due to
interlocking of CNTs on opposing electrodes with increased
pressure. As pressure is alleviated, individual CNTs maintain
contact with CNTs of opposing electrodes during the recovery
curve that were not present during the initial response curve.
The long saturation period shown in Fig. 2a is attributed to the
loading becoming stabilized. Fig. 2b shows the response times
of the sensor, measured by loading/unloading a weight on top
of the sensor manually. The response times were 40 ms under
loading and 60 ms under unloading, respectively. As shown in
Fig. 2b, the initial baseline is 1 mA corresponding to 0 Pa, and
the sudden jump and drop of the current are upon the loading
and unloading of the pressure, as shown in the respective insets.
Following the first cycle, the baseline decreases from 1 mA to
approximately 0.98 mA. The decrease in baseline current is
attributed to an initial deformation of individual VACNTs upon
first application of pressure, resulting in slightly less ordered
CNTs with a consequently lower overall current at 0 Pa (i.e.,
baseline).
The mechanical flexibility was tested by applying strains to
the substrate and measuring the resistance change
simultaneously. The experimental setup is shown in Fig. 3a
with initial state (no stretching) (top image) and with 50%
stretching (bottom image). The amount of stretching is
controlled by clamping both ends of PDMS into the system and
physically extending one clamped end while the other end
remains fixed to introduce strain. As shown in Fig. 3b, the
sample was stretched up to 180% until it reached the breaking
point of the PDMS substrates. With the increase of tensile
strains, some individual nanotubes may be separated from each
other, increasing the overall resistance, especially at high
stretching strains. Previously, a direct correlation has been
observed with spacing of CNT arrays and the macroscopic
electric field [37]. As strain is increased, a greater quantity and
degree of stretching between individual CNTs likewise occurs,
resulting in the observed trend herewith. Fig. 3c shows the
measured resistances of a sensor at various stretching strains
from 0% to 13% at different pressures of 0 Pa, 500 Pa, 509 Pa,
975 Pa, and 1783 Pa. As shown in Fig. 3c, there is no significant
change in resistance with respective pressure values within this
range of relatively lower stretching strains. We attribute this to
intermingling contact maintained among neighboring CNTs at
relatively low strain values, maintaining resistance values at
respective applied pressures. As shown in Fig. 4a, various
bending radii of 85 mm, 105 mm, and 200 mm were tested, with
bending angles of 6.8º, 5.5º, and 2.9º, respectively, calculated
using a 1 cm x 1 cm sample. Different pressures were applied
Fig. 2. (a) Change of resistance in response to increasing and decreasing
pressure loadings. (b) Response times under the pressure loading and
unloading by instantaneously putting a weight on top of the pressure sensor
manually.
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corresponding to weights of 0 g, 2 g, 5 g, and 20 g. Fig. 4b
shows the schematic of the bending test of the sensor. The
resistance of the sensor decreased with the increase of the
applied loading but remained stable at various bending radii at
equal loading, showing that the sensor was relatively robust
against bending. Fig. 4c shows the relative resistance of a
number of samples at different applied pressures of 0 Pa, 500
Pa, 1 kPa, 1.5 kPa, and 2 kPa. The variation in relative
resistance for the pressure sensor is at 19.3%, 19.4%, 18.2%,
and 17.0% at applied pressures of 500 Pa, 1 kPa, 1.5 kPa, and 2
kPa, respectively. We attribute this variation in relative
resistance between devices to variations in the degree of curing
Fig. 4. (a) Resistances at various bending radii of 85 mm, 105 mm, and 200 mm with different applied pressures corresponding to weights of 0 g (shaded
gray zone), 2 g (red), 5 g (blue), and 20 g (green), respectively. (b) Schematic
of the bending test of the pressure sensor. (c) Relative resistances of 5 different
samples at different pressures of 0 Pa, 500 Pa, 1 kPa, 1.5 kPa, and 2 kPa.
Fig. 3. (a) Photo image of tensile testing setup, showing the sensor with a
stretching strain of 0% and 50%. (b) Changes in relative resistance (i.e.,
ΔR/R0) at various stretching strains from 0% to 180%. (c) Resistances of the flexible pressure sensor at various stretching strains from 0% to 13% at
different pressures of 0 Pa, 500 Pa, 509 Pa, 975 Pa, and 1783 Pa.
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of PDMS when transferring CNT in the fabrication process.
To test the long-term cyclic stability, the sample was tested
using a force testing system (Chatillon TCD110 Series) for
10,000 cycles and applied load at a maximum of 20 N, as shown
in Fig. 5a. Photographs of the experimental setup and
equipment are shown in Fig. 5c–d. The pressure sensor sample
had an electrode area of 1 cm x 1 cm, and a cross-sectional area
of 0.5 cm2. For clarity, Fig. 5a, a small snapshot of the 10,000
cycles at different representative intervals, is shown. Each line
on the graph (i.e., differing in color, as shown in the legend)
represents the first 100 seconds at that initial cycle. For
example, the first spike at “0 s” for the “7,000 Cycles” black
line represents the 7,000th cycle. Each subsequent cycle of that
black set represents the next cycle in that sequence (i.e., 7,001st,
7,002nd, ect.). After many cycles and accumulated pressure, the
upper current values decreased, whereas lower current values
increased, as shown in Fig. 5b. We attribute this change to the
altering overall contact area between opposing electrodes
following repeated cycles. As previously reported, compressive
strain introduced parallel to VACNT electrodes increases
electrical conductivity [38], and the application of lateral
stretching via PDMS may not induce large strain to the CNT
material itself. As pressure is applied, individual CNT buckle
and the cumulative height of the electrode is lowered, resulting
in local changes in contact surface area [39]. When contacting
opposing electrodes of VACNT herewith, individual CNTs
similarly undergo bending with a change of overall
conductivity dominated by the change of contact areas.
To test the suitability of the fabricated sensor for an e-skin
application, we attached the sensor on the skin of a person to
obtain the pulse, muscle flexion, and step count (on sole of
shoe), as shown in Fig. 6. We used the same sample that was
used to obtain the pulse, muscle flexion, and step count for this
experiment, demonstrating its potential ability to be used for
wearable electronics. The experimental setup is detailed in the
Materials & Methods section, similar to other experiments
reported herewith. However, we used long wires connecting to
the sample from the potentiostat to allow respective motion.
The sensor was placed on the forearm of the person. As the
person flexed the forearm muscle, the force caused by flexion
produced higher pressure, increasing the measured current.
Then the sensor was placed on the bottom of a heel of the
person, and the sensor detected the walking signals. These
capabilities demonstrate that this sensor has potential to be a
promising wearable pressure sensor.
Fig. 5. (a) Cyclic testing results up to 10,000 cycles with a constant applied
voltage of 5 V. A small snapshot of the 10,000 cycles at different representative
intervals is shown; each line on the graph (i.e., differing in color, as shown in the legend) represents the first 100 seconds at that initial cycle starting at “0 s”.
(b) Upper and lower current values of the sensor at different testing cycles. (c–
d) Experimental setup.
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IV. CONCLUSIONS
We have demonstrated highly flexible pressure sensor
enabled by opposing two VACNT layers, in which the tips of
CNTs are partially embedded into PDMS, warranting a robust
sensing performance under various mechanical strains. The
device fabrication is extremely facile (i.e., two sandwiched
electrodes each achieved by transferring directly grown CNTs
onto partially cured PDMS), and the fabrication cost is also very
low. The pressure sensor showed a broad sensing range from 26
Pa to 56 kPa, with capabilities in subtle-pressure regime such
as screen touch (<1 kPa), the low-pressure regime such as daily
activity (1–10 kPa), and also medium-pressure regime such as
object manipulation (10–100 kPa). The response time of 40 ms
enabled the pressure sensor to detect any biological ‘pressure’
signals real-time. This sensor also demonstrated stretching up
to 180% and bending at different bending radii. The sensor
showed a stable performance for 10,000 loading/unloading
cycles with the resistance retention of 82%. This sensor
detected the pulse signatures, muscle flexing, and step
count/intensity which is very promising for e-skin or wearable
applications.
ACKNOWLEDGEMENT
The authors thank Lichen Wang and Deep Parikh for their
help with the force testing system of Chatillon TCD110 Series.
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This is the author's version of an article that has been published in this journal. Changes were made to this version by the publisher prior to publication.The final version of record is available at http://dx.doi.org/10.1109/JSEN.2020.2999261
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This is the author's version of an article that has been published in this journal. Changes were made to this version by the publisher prior to publication.The final version of record is available at http://dx.doi.org/10.1109/JSEN.2020.2999261
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8
Runzhi Zhang received her B.S. in
engineering mechanics in 2013, and her
M.S. in solid mechanics from Xi’an
Jiaotong University; she is currently
pursuing her Ph.D. in mechanical
engineering from Stevens Institute of
Technology. Her research focuses on
flexible electronics, including
pressure/strain sensors and
supercapacitors.
Anthony Palumbo received his B.S. in
physics in 2012 from New York
University; he received his B.E. in
mechanical engineering in 2014, his M.E.
in mechanical engineering in 2016, and is
currently pursuing his Ph.D. in mechanical
engineering at Stevens Institute of
Technology. His current research focus
involves flexible electronics, including
sensors and flexible energy-storage devices.
Dr. E. H. Yang is currently a full professor
of the Mechanical Engineering Department
at Stevens Institute of Technology. Dr.
Yang received his Ph.D. from Ajou
University in Korea in 1996. He served as a
postdoctoral scholar first at University of
Tokyo in Japan (1996–1998) and then as at
Caltech/Jet Propulsion Laboratory (1999),
and joined NASA’s JPL one year later first as a member of the
engineering staff and then a senior member of the engineering
staff at JPL. Dr. Yang joined Stevens Institute of Technology
as an Associate Professor in the Department of Mechanical
Engineering in 2006 and became a tenured member of faculty
in 2012 and full Professor in Mechanical Engineering in 2014.
Currently, his group’s research covers the growth and
nanofabrication of graphene, carbon nanotubes and 2D
materials, as well as the implementation of tunable wetting and
surface interaction. Dr. Yang's professional service credits
include editorial or editorial board positions for several
journals, including Nature’s Scientific Reports and multiple
track chair positions for ASME International Mechanical
Engineering Congress and Exposition (IMECE). He has
produced more than 300 journal papers, conference
proceedings, and presentations and has delivered 86 keynote or
invited talks. He holds 17 issued or pending patents in the fields
of micro- and nanotechnology. Dr. Yang was a featured Micro-
and Nano- Systems Engineering and Packaging track plenary
speaker at IMECE in 2018. He received the Award for Research
Excellence at Stevens in 2019. Dr. Yang has been elected a
Fellow of the National Academy of Inventors, the highest
professional distinction for academic inventors. He has also
been elected a Fellow of the American Society of Mechanical
Engineers (ASME) for his extensive contributions to the fields
of micro- and nanotechnology.
This is the author's version of an article that has been published in this journal. Changes were made to this version by the publisher prior to publication.The final version of record is available at http://dx.doi.org/10.1109/JSEN.2020.2999261
Copyright (c) 2020 IEEE. Personal use is permitted. For any other purposes, permission must be obtained from the IEEE by emailing pubs-permissions@ieee.org.
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