Ultra-Stretchable and Skin-Mountable Strain Sensors Using Carbon Nanotubes-Ecoflex Nanocomposite

Morteza Amjadi, Yong Jin Yoon, and Inkyu Park*

M. Amjadi Department of Mechanical Engineering and KI for the NanoCentury (KINC) Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Republic of Korea

Prof. Y. J. Yoon School of Mechanical & Aerospace Engineering Nanyang Technological University (NTU) 50 Nanyang Ave, Singapore 639798 [*] Prof. I. Park Department of Mechanical Engineering and KI for the NanoCentury (KINC) Korea Advanced Institute of Science and Technology (KAIST) 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Republic of Korea inkyu@kaist.ac.kr

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Fabrication of the Rosette Type Strain Sensors:

Figure S1: a-e) Fabrication process of rosette strain sensor. a) Patterning of polyimide film by

plotter. b) Spraying the high density CNT solution in the electrode areas. c) Very low density

CNT coating in the sensing area. d) Transferring the whole deposited CNT thin film to the

Ecoflex matrix. e) Coverage of the peeled-off CNT thin film with another layer of Ecoflex. f)

Photograph of the fabricated rosette type strain sensor.

TEM observations:

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Figure S2: Microstructural observation of the CNTs-Ecoflex nanocomposite-based strain

sensor: a) TEM image of the transferred CNT thin film to the polymer matrix; CNTs are

randomly dispersed into the Ecoflex matrix in the form of percolation network. b) TEM image of

a CNT cross-linked with Ecoflex; the whole structure of CNT is covered by Ecoflex molecules

providing good interfacial adhesion between CNT and polymer.

Transfer of the CNT thin film from donor substrates to the target polymer matrix:

Different types of substrates and elastomeric polymers were employed for the successful transfer

of the CNT thin film to the elastomer matrix. Polyimide film as donor substrate enabled

successful transfer of the CNT thin film to the elastomer due to its very low surface energy and

high contact angle ensuring the weak adhesion with CNTs-Ecoflex nanocomposite film. The

CNT thin films were fully transferred to different elastomer matrices such as Ecoflex® and

PDMS (see Figure S3a and S3b). However, in case glass was used as donor substrate, strong

binding between CNTs-Ecoflex nanocomposite film and glass prohibited successful transfer to

the elastomer matrices (see Figure S3c).

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Figure S3: a) Complete transfer of the CNT thin film to the Ecoflex matrix by using polyimide

as a donor substrate. b) Complete transfer of the CNT thin film to the PDMS matrix by using

polyethylene terephthalate (PET) substrate. c) Incomplete transfer of the CNT thin film to

Ecoflex matrix due to strong binding between CNTs-Ecoflex nanocomposite and glass substrate.

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Wrinkling and plastic deformation of the CNTs-Ecoflex nanocomposite: We propose

sandwich structured (Ecoflex layer/CNTs-Ecoflex nanocomposite thin film/Ecoflex layer) strain

sensors to remove the wrinkling and out-of-plane deformation of the CNTs-Ecoflex

nanocomposite thin film in simple embedded samples (CNTs-Ecoflex nanocomposite thin film

on Ecoflex layer with no covering Ecoflex layer on top). Figure S4a and c illustrate the

fabricated sandwich and embed structured samples, respectively. As the figures depict, there are

no wrinkling and out-of-plane deformation of the nanocomposite thin film before applying strain

to both samples. Figure S4b shows the cross-sectional image of the sandwich structured sample

after 2,000 cyclic stretching/releasing from 0% to 300% without any emerged wrinkling and out-

of-plane deformation of the nanocomposite thin film because of the full coverage of the thin film

from both sides and uniform stress distribution. However, we observed large wrinkle patterns

and out-of-plane deformation of the nanocomposite thin in case of the simple embedded sample

only after a few cycles of loading/unloading, which critically limits their application as

stretchable strain sensors (Figure S4d). Moreover, in a bilayer system such as the simple

embedded sample with the stiff CNTs-Ecoflex nanocomposite layer on the compliant Ecoflex

substrate, spontaneous wrinkle patterns emerge to release the compressive strain caused by

mechanical instability [1].

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Figure S4: a) Cross-sectional image of the sandwich structured sample before stretching. b)

Cross-sectional image of the sandwich structured sample after more than 2,000 cycles of

loading/unloading from 0% to 300%. c) Cross-sectional image of the simple embedded sample

before applying the strain. d) Cross-sectional image of the simple embedded sample after three

cycles of few cyclic loading/unloading from 0 % to 50 %. e) Top-view optical image at the

boundary between the Ecoflex layer and wrinkled CNTs-Ecoflex nanocomposite.

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Hysteresis performance: Stretching/releasing cycles have been applied to a strain sensor while

the electrical resistance was measured. To investigate the hysteresis performance, different strain

levels (i.e. 50% and 100%) and rates (i.e. 10 %/s, 20 %/s, 30 %/s, and 40 %/s) were applied to

the sensors. Figure S5 illustrate the hysteresis performance of the strain sensors under constant

stretching of 100% with different strain rates. As shown in Figure S5, the hysteresis performance

is almost independent from strain rates.

Figure S5: Hysteresis performance of a strain sensor under different strain rates.

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Overshooting behavior of the CNTs-Ecoflex nanocomposite: Magnified plot for overshooting

behavior of a strain sensor at the strain of 58% is illustrated in Figure S4. Figure S6a shows the

current variation in the response of the strain sensor with 120 s of holding time. Relative change

of the resistance to the based resistance of the strain sensor under 120 s of holding time is

depicted in Figure S6b.

Figure S6: Overshooting of the CNTs-Ecoflex nanocomposite to the applied strain.

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Measurement of the failure strain: We tried to measure the failure strain of the CNTs-Ecoflex

nanocomposite films, as shown in Figure S7. A 10 mm length sample was continuously stretched

with the rate of 30%/s while the relative change of the resistance was measured. The film was

electromechanically robust for over ~ 1380% of strain and respond to the applied strain with

good sensitivity and linearity. Moreover, we could not apply higher strain level due to the

limitation of our experimental setup to measure the failure strain.

Figure S7: Response of the CNTs-Ecoflex nanocomposite the applied strain.

Table S1: A comparison between our strain sensors and previously reported strain sensors.

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Reference Device type Stretchability Gauge factor (GF)

Linearity Hysteresis Response time

Our strain sensor

Resistive type strain

sensor 500% 1-2.5 >95% Negligible 332 ms

Ref [1], ACS Nano

Resistive based strain

sensor 70% 2-14 Linear up

to 50%

Negligible for up to

40% 200 ms

Ref [2], Advanced Materials

Resistive type strain

sensor 50% 116 Linear Unavailable 140 ms

Ref [3], Nature

Nanotechnology

Resistive type strain

sensor 200%

0.82 for 40% of

stretchability

Two linear regions Large Unavailable

Ref [4], Nano Letters

Resistive type strain

sensor Unavailable ~ 14 Linear Unavailable Unavailable

Ref [5], Nanoscale

Resistive type strain

sensor 2.3% 9.49 Linear Unavailable Unavailable

Ref [6], Nano Letters, ACS Appl. Mater.

Interfaces

Resistive type strain

sensor 10%

~0.46 for 10% of

stretchability

Unavailable Unavailable Unavailable

Ref [7], CARBON

Resistive type strain

sensor 2% 2.4 Linear Unavailable Unavailable

Ref [8], Sensors

Resistive based strain

sensor 80% 20 Nonlinear Unavailable Unavailable

Ref [9], Nanosclae

Capacitance based strain

sensor 50% 0.7 Linear Negligible 40 ms

Ref [10], Scientific

report

Capacitance based strain

sensor 300% 1 Linear Negligible 100 ms

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Maximum strain measurements: We conducted the motion detection of the joints using our

stretchable strain sensors. To validate our measurements, we simply conducted the bending strain

calculation by a flexible ruler, as shown in Figure S8. The flexible ruler was placed on the wrist

and elbow joints and length of our strain sensors was marked by a pen. Then, the distance

between pointed made by pen was measured under stretching, see Figure S8. The maximum

strain values for the wrist and elbow full bending was measured to be 46.7% and 62.5%,

respectively. Our data measured by our strain sensors for bending of the wrist and elbow is 45%

and 63%, respectively.

Figure S8: Actual bending strain calculation for the wrist and elbow joints.

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Rosette type strain sensors: The rosette configuration of our strain sensor is demonstrated in

Figure S7. All three strain sensors are fixed with the angle of 120° to each other. For such a

configuration, all strains , , and can be demonstrated as following equations using the

‘‘strain transformation method’’ known as Mohr’s circle:

strain components of , , and can be measured experimentally from strain sensors.

Therefore, unknown variables, maximum and minimum principal strains and and their

orientation ( ) can be express in terms of known quantities , , and as follows:

Figure S9: Configuration of the rosette type strain sensor in Cartesian coordinate.

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