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:

[email protected])

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: [email protected]

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

Copyright (c) 2020 IEEE. Personal use is permitted. For any other purposes, permission must be obtained from the IEEE by emailing [email protected].


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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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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.



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