1

ACS: Applied Materials and Interfaces. 2017 to be published.

Novel Electrically-Conductive Porous PDMS/Carbon Nanofibre Composites

for Deformable Strain-Sensors and Conductors

Shuying Wu,†, ‡ Jin Zhang,§ Raj B. Ladani,† Anil R. Ravindran, † Adrian P. Mouritz,† Anthony J.

Kinloch,∥ and Chun H. Wang‡,*

†Sir Lawrence Wackett Aerospace Research Centre, School of Engineering, RMIT University,

GPO Box 2476, Melbourne, VIC 3001, Australia

‡School of Mechanical and Manufacturing Engineering, University of New South Wales, Sydney,

NSW 2052, Australia

§Australian Future Fibres Research and Innovation Centre, Institute for Frontier Materials,

Deakin University, VIC 3220, Australia

∥Department of Mechanical Engineering, Imperial College London, London, SW7 2BX, UK

*E-mail: chun.h.wang@unsw.edu.au

KEYWORDS: Porous nanocomposites, stretchable, strain-sensor, conductor, piezoresistivity,

carbon nanofibres, polydimethylsiloxane

ABSTRACT

Highly flexible and deformable electrically-conductive materials are vital for the emerging field

of wearable electronics. To address the challenge of flexible materials with a relatively high

electrical conductivity and a high elastic limit, we report a new and facile method to prepare

porous polydimethylsiloxane/carbon nanofibre composites (denoted by p-PDMS/CNF). This

method involves using sugar particles coated with carbon nanofibres (CNFs) as the templates.

The resulting three-dimensional porous nanocomposites, with the CNFs embedded in the PDMS

pore walls, exhibit a greatly increased failure strain (up to ~ 94%) compared to that of the solid,

2

neat PDMS (~48%). The piezoresistive response observed under cyclic tension indicates that the

unique microstructure provides the new nanocomposites with excellent durability. The electrical

conductivity and the gauge factor of this new nanocomposite can be tuned by changing the

content of the CNFs. The electrical conductivity increases, while the gauge factor decreases,

upon increasing the content of CNFs. The gauge factor of the newly-developed sensors can be

adjusted from approximately 1.0 to 6.5 and the nanocomposites show stable piezoresistive

performance with fast response time and good linearity in ln(R/R0) versus ln(L/L0) up to ~ 70%

strain. This tunable sensitivity and conductivity endow these highly stretchable nanocomposites

with considerable potential for use as flexible strain-sensors for monitoring the movement of

human joints (where a relatively high gauge factor is needed) and also as flexible conductors for

wearable electronics (where a relatively low gauge factor is required).

1. INTRODUCTION

Highly flexible and stretchable electrically-conductive materials are a key to future wearable

electronics with a diverse range of applications such as electronic skins,1 human-friendly

interactive electronics,2 and wearable health monitors.3-4 Recently, elastic and conductive

polymer nanocomposites have been extensively investigated due to their excellent deformation

capability compared to the traditional rigid metallic strain-sensor gauges and conductors.2 One

common technique is to incorporate electrically conductive nanomaterials, such as nanowires,4-5

carbon nanotubes (CNTs),6-7 carbon nanofibers (CNFs),8 metal nanoparticles9 and graphene10,

into flexible polymers to achieve a relatively high elastic limit and electrical conductivity. In

particular, carbon nanomaterials with excellent mechanical and electrical properties are emerging

3

as a leading solution for fabricating flexible strain-sensors and conductors. However, due to the

strong π-π interactions in such carbon nanomaterials, it remains a challenge to disperse such

nanomaterials uniformly in polymeric matrices. This leads to the need for a relatively high

concentration of carbon nanomaterials to be added in order to reach the percolation threshold and

so achieve sufficient electrical conductivity for their use as flexible strain-sensors and

conductors.8

To avoid the dispersion problem, and to improve greatly the electrical conductivity of the

polymer nanocomposites, researchers have recently shown that arranging the carbon

nanomaterials into three-dimensional (3D) interconnected structures is a promising strategy.11-15

By selecting an appropriate polymer matrix (e.g. polydimethylsiloxane (PDMS)), stretchable

strain-sensors based on 3D-structured graphene have been developed by our group and other

researchers.11-14 Such nanocomposites strain-sensors have shown high sensitivity with the gauge

factor being as high as 99, which is significantly higher than the typical value of about 2 for

metallic foil strain-sensor gauges. Nonetheless, a relatively poor deformation capability has been

observed (i.e. the maximum failure strain being approximately 30%).11 Additionally, the

complicated fabrication process associated with their manufacture limits their applications.

Recent research has revealed that making elastomeric polymers porous improves their

deformability and that their failure strain greatly exceeds that of their solid (i.e. non-porous)

counterparts.16-18 Using a photolithography technique,16 Park et al. showed that 3D

nanostructured pores can be created in a PDMS polymer which enhances the fracture strain by

225 % compared with the solid counterpart. Following this pioneering work, porous PDMS

4

polymers have been fabricated using a 3D printing technique17 or nickel foam as the template.18

Nevertheless, the thickness of the porous PDMS using the photolithography technique is limited

to the micrometer scale, while the 3D printing technique and nickel-foam procedures require

time-consuming etching processes and involve the use of toxic solvents. Hence, it is of great

importance to explore new cost-effective approaches to fabricate porous polymer

nanocomposites.

Herein, we demonstrate a novel strategy for creating stretchable and conductive porous

PDMS/CNF nanocomposites (denoted by p-PDMS/CNF) by using CNF-coated sugar particles as

the templates. CNFs with a high-aspect-ratio are used due to their high electrical conductivity (i.e.

~ 103 S/cm) and relatively low cost (i.e. they are typically three and five hundred times cheaper

than multi-wall or single-wall carbon nanotubes, respectively).19-22 Recent work has

demonstrated that conductive nanomaterials can be incorporated into porous polymer substrates

via dip-coating.17-18, 23-25 However, detachment of the nanomaterials from the substrates may

occur under cyclic loading-unloading, leading to an ineffective stress transfer from the polymer

substrate to the nanomaterials, and thereby a reduction in their long-term durability.25 In the

present work, the CNFs, which are originally on the surfaces of sugar particles, are transferred

and embedded in the pore walls of the porous PDMS, which ensures good adhesion between the

CNFs and the polymer. In comparison with the nanocomposites prepared by dip-coating, these

new polymer nanocomposites demonstrate a significantly higher durability. Furthermore, the

sensitivity of these new p-PDMS/CNF nanocomposites is tunable by simply varying the content

of the CNFs. The potential applications of these conductive nanocomposites as a strain-sensor

5

for detecting movement of the finger joint and as a stretchable conductor in a LED circuit are

demonstrated.

2. EXPERIMENTAL SECTION

2.1 Materials

The carbon nanofibres (Pyrograf®-III, grade PR-24-XT-HHT) used were supplied by Applied

Sciences Inc. The CNFs had a diameter of about 70–200 nm and a length of 50–200 μm. The

Sylgard 184 silicone elastomer base and curing agent were supplied by Dow Corning Co. Ltd

(Australia). Brown sugar with an average particle size of about 225 μm was used. Isopropanol

(IPA) was sourced from Sigma-Aldrich. Graphene oxide (GO) was synthesized using an

improved Hummers' method.14, 26 The high-purity silver-paste (Silver Paste Plus™) was from

SPI Supplies (USA).

2.2 Preparation of Porous PDMS/CNF (p-PDMS/CNF) Nanocomposites Using CNF-coated

Sugar-Particle Templates

The p-PDMS/CNF nanocomposites were prepared by the following procedure (illustrated in

Figure 1): firstly, the CNFs were dispersed in IPA with GO as the dispersant by using a Sonics

VCX-750 Vibra Cell Ultra Sonic Processor (40% amplitude, pulse duration: 5 second on, 5

second off). GO is used as dispersant since it has been previously utilized as a two-dimensional

surfactant to disperse carbon nanotubes in water.27 The mass ratio of CNF:GO was 30:1 and

there were no visible aggregates after sonication for 30 minutes. Sugar particles were added to

the dispersion (3 mg/mL) to get a slurry-like mixture, which was then placed in a vacuum oven

to remove the IPA. Subsequently, the CNF-coated sugar was compacted into a thin sheet

6

template with dimensions of 40 mm (length) × 9 mm (width) × 2 mm (thickness) using a

hydraulic press operated at a pressure of 2 MPa. Sugar particles were selected as the pore-

creating agent since they are eco-friendly and easily removed at a later stage, i.e. simply by

dissolving them in water.

The PDMS prepolymer was mixed with the curing agent at a weight ratio of 10:1. This precursor

mixture was then magnetically stirred for 10 minutes and degassed for 30 minutes, after which

the mixture was poured onto the CNF-coated sugar template. Owing to its low viscosity and low

surface energy, the PDMS prepolymer infiltrated gradually into the 3D interconnected pores

between the CNF-coated sugar particles. To facilitate the infiltration, the template infiltrated with

the PDMS prepolymer was placed in a vacuum oven for 1 h, which was subsequently heated at

65 oC for 2 h for curing. Afterwards, the excess PDMS polymer around the edges was trimmed

away to expose the sugar particles, which were removed by warm water under sonication. It was

found that a negligible concentration of CNFs was removed during the sugar dissolving and

sonication processes. Finally, porous p-PDMS/CNF nanocomposites, with interconnected CNFs

in the pore walls, were obtained by drying in a fume hood for 24 h. Different p-PDMS/CNF

nanocomposites were prepared by changing the weight content of the CNFs (i.e. 0.1, 0.3, 0.5, 0.7,

1.4, and 2.8 wt% of CNFs). Porous, neat PDMS polymer sheets were also prepared following the

same procedure but using a sugar-particle only template which was made by pressing the sugar

particles into a 2 mm thin sheet using the hydraulic press at the same pressure as above (i.e. 2

MPa). Also, 2 mm thin sheets of the solid, neat PDMS polymer were prepared.

7

2.3 Preparation of Porous PDMS/CNF (dc-PDMS/CNF) Nanocomposites Using Dip-coating

For comparison, another type of porous PDMS/CNF nanocomposite, denoted by dc-PDMS/CNF,

was prepared using a typical dip-coating method.17-18, 20-22 Firstly, thin sheets of porous, neat

PDMS were prepared as described above. The CNFs were then coated onto the porous PDMS by

dipping the porous PDMS into a dispersion of CNFs in IPA, with GO as the dispersant, and then

drying the material in a vacuum oven. Before dip-coating, the porous PDMS was treated in an

oxygen plasma chamber (Zepto, Diener Electronics with 60W power under 0.5 mbar air pressure)

for 3 minutes to increase the polar nature of the exposed surfaces. This plasma treatment is

essential for the deposition method to be successful due to the superhydrophobic nature of the

PDMS. To be consistent, the dc-PDMS/CNF nanocomposite was prepared by repeating the dip-

coating and drying process several times, so that the dc-PDMS/CNF nanocomposite contains

0.3wt% of CNFs.

Figure 1. Schematic illustration of the preparation methods for the two different types of porous

PDMS/CNF nanocomposites.

8

2.4 Characterization.

The morphologies of the p-PDMS/CNF nanocomposites were examined using a scanning

electron microscope (FEI Nova NanoSEM). The nanocomposite specimens were cryogenically

fractured in liquid nitrogen and then coated with a thin layer of gold prior to observation. The

degree of porosity of the porous PDMS prepared by using the sugar particles was estimated to be

~ 79% based on the ratio between the density of porous (0.23 g/cm3) and solid PDMS (1.1

g/cm3). This value is similar to that reported by Zhang et al.28

A tensile testing machine (Instron Model 4466) was employed to characterize the mechanical

properties. The PDMS/CNF nanocomposites were tested under quasi-static tension and also

under cyclic tensile loading-unloading cycles under displacement control. The test specimens

had dimensions of 40 mm (length) × 6 mm (width) × 2 mm (thickness). For the quasi-static

tension test, a strain rate of 8%/min was used. The cyclic tests were conducted by applying a

sinusoidal cyclic load at a frequency of 0.08 Hz. The electrical resistance was measured using a

digital multimeter (34465A, Keysight Technologies). Thin copper wires were attached to the two

ends of the specimens using a conductive silver-paste adhesive. The response time was measured

by rapidly (i.e. at 40 mm/s) stretching the sensor to 40% strain and holding for 30 s. The

specimen was then unloaded at the same rate back to 0% strain and held for 30 s before starting

the next cycle.

3. RESULTS AND DISCUSSION

3.1 Microstructure and Mechanical Properties of the Porous PDMS/CNF Nanocomposites

9

Figure 2a shows a SEM image of the sugar particles, from which it is estimated that their

average size is approximately 225 µm. Figure 2b is a SEM image of the sugar particles coated

with CNFs. From the insert taken at a relatively high magnification, it is seen that the CNFs are

uniformly coated onto the surfaces of the sugar particles and form a network. When the liquid

PDMS prepolymer is cast onto the CNF-coated sugar templates, the PDMS infiltrates into the

interconnected pores of the CNF-coated sugar particles, owing to the relatively low viscosity and

low surface energy of the PDMS prepolymer.29 After curing the PDMS, the CNFs are entrapped

within the PDMS, near the surfaces of pores, forming a nanocomposite layer of CNFs and

PDMS. The SEM image (Figure 2c) shows the microstructure of the p-PDMS/CNF

nanocomposites and clearly reveals that the sugar particles are removed by the water and that

interconnected pores are formed. The high-magnification SEM image (Figure 2d) shows that the

majority of the CNFs are embedded just below the surface of the PDMS, with some CNFs being

visible on the surface. The contact and close proximity between the CNFs in the p-PDMS/CNF

nanocomposites give rise to a relatively high electrical conductivity, as discussed below. By

contrast, for the nanocomposites prepared by the dip-coating method (i.e. the dc-PDMS/CNF

nanocomposites), although the porous structure is similar, the CNFs are only loosely attached to

the surface of the pore walls, as indicated by the SEM image in Figure 2e.

10

0 40 80 120 1600.0

0.2

0.4

0.6

0.8 Solid PDMS Porous PDMS p-PDMS/CNF dc-PDMS/CNF

Strain (%)

Stre

ss (M

Pa)

(f)

Figure 2. SEM images of (a) raw sugar particles; (b) CNF-coated sugar particles (the insert is a

magnified image from the region marked by the cycle); (c) p-PDMS/CNF nanocomposite (0.3 wt%

of CNFs); (d) is an image taken from the pore wall marked by the circle in (c); (e) a dc-

11

PDMS/CNF nanocomposite (0.3 wt% of CNFs); (f) quasi-static tensile stress-strain curves of the

porous and solid, neat PDMS polymers and the two PDMS/CNF nanocomposites.

The porous, neat PDMS polymer exhibits a significantly higher tensile breaking strain (of

~150%) than the solid, neat PDMS polymer (of ~48%) (Figure 2f). The underlying mechanism

for the increased fracture strain has been attributed to local rotations of narrow structural

elements (i.e. the pore walls) in the 3D network relative to the stretching direction under the

tensile deformation, which leads to a lower strain acting in the pore walls.16-17 This observation

was based on finite element modelling studies, which indicate that the applied strain is

distributed in the 3D structure of the porous material, resulting in less actual strain on the

interconnecting material.16-17 Now, the introduction of the CNFs into the porous PDMS increases

the stiffness of the resulting nanocomposite but lowers the fracture strain. Interestingly, the p-

PDMS/CNF nanocomposite shows a higher modulus but a lower fracture strain than the dc-

PDMS/CNF nanocomposite. Specifically, the modulus and fracture strain for the p-PDMS/CNF

nanocomposite are approximately 314 kPa and 87%, respectively, while the dc-PDMS/CNF

nanocomposite exhibits a modulus of ~ 115 kPa and a fracture strain of ~ 120%. The electrical

conductivity of the dc-PDMS/CNF nanocomposite was measured to be ~ 0.007 S/m, whilst the

p-PDMS/CNF nanocomposite shows conductivity nearly five times higher of ~ 0.033 S/m. These

different properties may be attributed to the different microstructures, i.e. the CNFs are loosely

coated on the surface of the PDMS pore walls in the dc-PDMS/CNF nanocomposite, whilst the

CNFs are embedded and uniformly dispersed within the PDMS pore walls in the p-PDMS/CNF

nanocomposite.

12

3.2 Comparison of the Piezoresistivity of the Two Types of Porous PDMS/CNF

Nanocomposites

3.2.1 Quasi-static Tensile Loading

To evaluate the strain-sensing capability of the two types of PDMS/CNF nanocomposites, they

were firstly subjected to quasi-static tensile loading (see Figure 3). For both nanocomposites,

which contained 0.3 wt% of CNFs, the relative change in the resistance, ΔR/R0, increases

continuously with the applied strain, ε. The resistance of the dc-PDMS/CNF nanocomposites

increases rapidly after being stretched to ~60% strain and reaches values beyond the

measurement capacity of the digital multimeter used. As shown in Figure 3a, the values of

ΔR/R0 for both types of PDMS/CNF nanocomposites show a nearly linear dependence on the

applied strain up to around 20%-30% strain. However, under larger deformations, nonlinear

responses are observed. It is interesting to note that better linearity is obtained by expressing the

experimental data using natural logarithms (Figure 3b). Therefore, instead of the commonly

used linear ratio of (ΔR/R0)/(ΔL/L0) to define the gauge factor, GF,30 the sensitivity of the

strain-sensor under large deformations may now be expressed quantitatively by the gauge factor,

GFld, defined as:

GFld = ln(R/R0)ln(L/L0)

(1)

where R0 and L0 are the initial resistance and length while R and L are the resistance and length

of the strain-sensor under an applied strain, ε. The value of GFld obtained by linear regression

analysis of the data according to eq.(1) is 6.4 (with a R2 = 0.997) and 4.1 (with a R2 = 0.999) for

the dc-PDMS/CNF and p-PDMS/CNF nanocomposites, respectively. Thus, the dc-PDMS/CNF

13

nanocomposite exhibits a higher sensitivity than the p-PDMS/CNF nanocomposite, which is

likely to be due to the different microstructures of the strain-sensors and piezoresistivity

mechanisms, as discussed below.

0.0 0.2 0.4 0.6 0.80

10

20

30

40

50

0.0 0.1 0.2 0.30

2

4

6

∆R

/R0

Strain (ε)

∆R/R

0

Strain (ε)

p-PDMS/CNF dc-PDMS/CNF

(a)

0.0 0.1 0.2 0.3 0.4 0.5 0.6

0

1

2

3

4

5

R-square=0.999

p-PDMS/CNF dc-PDMS/CNF

ln (R

/R0)

ln (L/L0)

(b)

R-square=0.997

Figure 3. Comparison of the variation in resistance under quasi-static loading for the two

different types of PDMS nanocomposites containing 0.3 wt% of CNFs: (a) ΔR/R0 versus strain

and (b) ln(R/R0) versus ln(L/L0). (The insert in (a) is the enlarged view of the rectangular region.)

3.2.2 Cyclic Loading and Unloading

To investigate the physical durability of these two types of PDMS/CNF nanocomposites as

strain-sensors they were subjected to cyclic loading and unloading (i.e. stretching and release

cycles). Figure 4 shows the resistance changes of the nanocomposites being stretched to 30% of

strain and then gradually unloaded to 0%. From Figures 4a and 4c it can be seen that the value

of the relative resistance change, ΔR/R0, steadily increases upon stretching and then gradually

decreases during unloading. After the first cycle, the resistance (unloaded to 0% strain) increases

irreversibly by ~35% for the p-PDMS/CNF and ~113% for the dc-PDMS/CNF nanocomposite,

14

respectively (Figure 4c). However, in the subsequent stretching-releasing cycles, both of these

two nanocomposites essentially recover their resistance after releasing (Figure 4a), indicating

excellent durability.

Figure 4b shows the value of ΔR/R0 at the maximum strain of 30% as a function of the number

of cycles of loading and unloading. For the dc-PDMS/CNF nanocomposite, each consecutive

cycle generates a regular, continuous downward drift of the relative resistance change, ΔR/R0.

This downward drift is more noticeable in the first few cycles and slows down as the number of

cycles increases. Indeed, for the dc-PDMS/CNF nanocomposite, this downward drift in the value

of ΔR/R0 is never arrested. In contrast, for the p-PDMS/CNF nanocomposite, the value of ΔR/R0

decreases somewhat during the initial few cycles but then rapidly stabilizes all the way up to

10,000 cycles (the maximum number of cycles employed in the present tests). These

experimental measurements demonstrate that strain-sensors fabricated from the dc-PDMS/CNF

nanocomposite, where the CNFs are deposited on the surface of the pores, are not as durable and

hence as stable as those fabricated from the p-PDMS/CNF nanocomposite, where the CNFs are

embedded in the pore walls of the PDMS.

15

0 1000 2000 8000 9000 10000

0246

Stretching cycles

∆R/R

0 p-PDMS/CNF

0246

0 2 4 6 8 10024

∆R

/R0

dc-PDMS/CNF

(a)

0 2000 4000 6000 8000 100000.0

1.5

3.0

4.5

6.0

∆R

/R0

Stretching cycles

dc-PDMS/CNF p-PDMS/CNF

(b)

0.0 0.1 0.2 0.3

0

1

2

3

4

5

6

Stretch Release

Strain (ε)

∆R/R

0

dc-PDMS/CNF p-PDMS/CNF

(c)

Figure 4. Comparison of the piezoresistivity of the two types of PDMS/CNF nanocomposites

containing 0.3 wt% of CNFs: (a) ΔR/R0 under cyclic tension with a maximum strain of 30% and

a minimum strain of 0% (the insert in (a) is ΔR/R0 for the first ten cycles); (b) ΔR/R0 at an

applied strain of 30% versus cyclic numbers; (c) ΔR/R0 versus ε curves corresponding to the first

cycle in (a).

3.2.3. Mechanisms of the Piezoresistive Behaviour

The observations noted above, and hence the differences in the piezoresistive behaviour, of these

two PDMS/CNF nanocomposites may be attributed to their different microstructures. The

conductive layer in the p-PDMS/CNF nanocomposite shows a similar microstructure to PDMS

16

nanocomposites containing uniformly dispersed conductive nanomaterials.8, 31 By contrast, the

structure of the dc-PDMS/CNF nanocomposite is similar to those prepared by depositing a

conductive nanomaterial layer onto a PDMS substrate.32-33 Electrons can tunnel through a

polymer matrix if the distance between the adjacent CNFs is less than the tunneling distance (of

the order 5–50 nm).34 During stretching, the PDMS experiences a much larger deformation in the

direction of stretching and the CNFs may slide past each other and move apart, leading to

enlarged gaps. The resistance, R, is thus increased in the stretching direction due to an increase

in the tunneling resistance. A further increase in the applied strain may result in total loss of

contact, leading to a further increase in the resistance. During the initial stretching, the buckling

of the CNFs may occur due to the Poisson’s effect in the transverse direction of stretching. Some

of the CNFs may therefore buckle out of the stretching plane and fracture (Figures 5a-b).5, 29, 35

Thus, the resistance may not be completely recovered after releasing the strain back to 0%. After

the initial few cycles of loading-unloading, the rate of buckling and breaking of CNFs would be

expected to diminish and the piezoresistivity would now arise from the reversible movements of

the inter-CNFs gaps.

By contrast, microcracks are observed in the CNF layer in the dc-PDMS/CNF nanocomposites

after multiple cyclic loading and unloading (Figures 5c-d), which is believed to play an

important role in the piezoresistive behaviour of these nanocomposites. Before stretching, as

shown in Figure 2e, a densely packed layer of CNFs, containing no microcracks, is lying on the

surface of the PDMS pore walls. After stretching, microcracks appear, leading to an increase in

the resistance. Upon unloading, the PDMS substrate relaxes, causing the shrinkage, or even

complete closure, of these microcracks. During subsequent cycling, the reversible

17

opening/closing of these microcracks causes the resistance to increase and decrease. The plasma

treatment was employed to introduce some functional group (oxygen-containing groups) to

enhance the adhesion between the CNFs and the PDMS substrate. However, the adhesion is

likely to remain relatively weak and some CNFs are detached from the surface of the PDMS in

the dc-PDMS/CNF nanocomposite after repeated stretching (as indicated by the dashed oval in

Figure 5d), which limits the stress transfer from the PDMS to the CNFs. Therefore, the relative

resistance change in such nanocomposites shows a continuous decline up to 10,000 cycles. The

changes in the microstructure of these two PDMS/CNF nanocomposites after cyclic tension are

compared and illustrated schematically in Figure 5e.

18

Figure 5. Typical SEM images for (a) and (b) the p-PDMS/CNF and (c) and (d) the dc-

PDMS/CNF nanocomposites after being stretched between 0% and 30% for 200 cycles; (e)

schematic illustration of the morphology changes after cyclic loading. (Arrows in (a) and (b)

indicate the buckling, fracture, and detachment of the CNFs in the p-PDMS/CNF while those in

(c) and (d) indicate the microcracks in the CNFs layer on the pore wall surface of the dc-

19

PDMS/CNF. The dashed oval in (d) shows CNFs which are detached from the surface of the dc-

PDMS/CNF.)

3.3 Piezoresistivity of the p-PDMS/CNF Nanocomposites and its dependence on the CNF

Content

As the above results show, the p-PDMS/CNF nanocomposites exhibit a superior durability, and

hence stability, compared to the dc-PDMS/CNF nanocomposites. These properties are key

characteristics and requirements of effective and useful flexible strain-sensors and conductors. In

the present section, the piezoresistivity of the p-PDMS/CNF nanocomposites will be discussed in

more detail. Figure 6a shows the voltage, V, versus current, I, characteristics of a p-PDMS/CNF

nanocomposite, containing 0.3 wt% of CNFs, at different applied strains. It can be seen that the

slope of the V versus I curves increases with the applied strain, indicating that the electrical

resistance depends on the applied strain. Figure 6b presents the relative resistance change,

∆R/R0, over ten cycles, as the maximum applied strain is increased from 10% to 50%, with the

minimum applied strain always being 0%. The relative resistance change, ∆R/R0, versus strain

with a maximum strain of 10%, 30%, and 50% pertinent to the first and tenth cycle of Figure 6b,

are presented in Figure 6c. It may be seen that the value of ΔR/R0 increases by ~ 60% to 445%

when the maximum applied strain is increased from 10% to 50%. Moreover, there is no

significant difference in the relationships between the relative resistance change, ∆R/R0, versus

strain for the first and tenth cycle, indicating excellent durability of the p-PDMS/CNF

nanocomposite in this respect.

20

-60 -30 0 30 60

-10

-5

0

5

10

Volta

ge (V

)

Current (µA)

0% 10% 30% 50%

(a)

0 2 4 6 8 100.0

1.6

3.2

4.8

6.4 10% 20% 30% 40% 50%

∆R/R

0

Stretching cycles

(b)

0.0 0.1 0.2 0.3 0.4 0.5

0

1

2

3

4

5

6

50%

30%

∆R/R

0

Strain (ε)

1st Cycle 10th Cycle

(c)

10%

Figure 6. (a) The voltage, V, versus current, I, plot of a p-PDMS/CNF nanocomposite (0.3 wt%

CNFs) under different levels of applied tensile strain; (b) the relative resistance change, ∆R/R0,

during 10 cycles of loading-unloading and (c) the relative resistance change, ∆R/R0, versus strain

(for both loading and unloading) for the first and tenth cycle in (b). (The maximum applied strain

is indicated with the minimum applied strain always being 0%)

It is widely recognized that the gauge factor of nanocomposite strain-sensors is affected by the

number of conductive pathways,36 which can be altered by changing the concentration of CNFs

in the nanocomposite. Figure 7a presents the electrical conductivity of the p-PDMS/CNF

nanocomposites containing different concentrations of CNFs. Upon increasing the concentration

21

of CNFs from 0.1 to 2.8 wt%, the electrical conductivity increases continuously from 0.01 to 4.2

S/m. At a low concentration of CNFs, a sparse conductive network is formed, leading to a low

electrical conductivity. Upon increasing the CNF content, a much denser network of CNFs is

formed, resulting in an increased electrical conductivity. The tensile strain-stress curves of the p-

PDMS/CNF nanocomposites containing various contents of CNFs are given in Figure S1. It is

seen that the failure strain varies from 72% to 94%, with an initial increase upon increasing the

CNF concentration up to 0.7 wt% but a decrease is observed thereafter (Figure 7b). In contrast,

the modulus and tensile strength increase continuously upon increasing the concentration of

CNFs. Similar results have been recently reported for porous PDMS/CNT nanocomposites.37

The enhanced modulus and tensile strength properties may be attributed to the reinforcing effects

of the CNFs or the CNTs which possess relatively high values of modulus and strength. However,

an excessive concentration of CNFs or CNTs reduces the failure strain, which may result from an

effective increase in the degree of crosslinking of the PDMS due to the increased concentration

of CNFs or CNTs.

Figure 7c presents the representative relative resistance change, ∆R/R0, under quasi-static

loading for the p-PDMS/CNF nanocomposites containing various concentrations of CNFs. It

should be noted that, regardless of the content of CNFs, the piezoresistive response of the

nanocomposites is characterized by a linear increase up to a certain applied strain and an

exponential increase thereafter. Figure 7d shows a plot of ln(R/R0) versus ln(L/L0), see eq. (1),

and by fitting the data in the linear range the value of GFld can be calculated. The average value

of GFld increases from 1.0 to 6.5, upon decreasing the CNF content from 2.8 to 0.1 wt%, as

shown in Figure 7a. The scatter in the measured values of the gauge factor, indicated as the error

22

bars in Figure 7a, was obtained by testing at least three specimens for each type of sensor. As an

example, Figure S2 presents the variation of the performance of three specimens with a

concentration of 0.1 wt% CNFs. The variation in the gauge factor may be mainly ascribed to

slight variations in the microstructures of the nanocomposites, resulted from the inhomogeneities

in the size and shape of the sugar particles (i.e. the templates) and the coating of the CNFs on the

sugar-particle surfaces.

23

0.0 0.6 1.2 1.8 2.4 3.00.0

0.4

0.8

3.5

4.24.9

Gauge factor

El

ectri

cal c

ondu

ctivi

ty (S

/m)

CNFs content (wt%)

(a)

0

2

4

6

8

0.0 0.6 1.2 1.8 2.4 3.0

60

75

90

105

120

Failu

re s

train

(%)

CNFs content (wt%)

(b)

0.0 0.2 0.4 0.6 0.8

0

10

20

30

40 0.1 wt% 0.3 wt% 0.5 wt% 0.7 wt% 1.4 wt% 2.8 wt%

Strain (ε)

∆R/R

0

(c)

ln(L/L0)

0.1 wt% 0.3 wt%0.5 wt% 0.7 wt%1.4 wt% 2.8 wt%

0.0 0.2 0.4 0.60

1

2

3

4

5

Linear fitting

ln(R

/R0)

(d)

0 2 4 6 8 10

0

2

4

6

8 0.1 wt% 0.3 wt% 0.5 wt% 0.7 wt% 1.4 wt% 2.8 wt%

∆R/R

0

Stretching cycles

(e)

Figure 7. (a) The electrical conductivity and gauge factor, GFld, (b) failure strain, (c) and (d) the

resistance change versus strain under quasi-static loading, and (e) relative resistance change,

∆R/R0, over 10 cycles of loading-unloading (with a cyclic maximum applied strain of 40% and a

minimum applied strain of 0%) of the p-PDMS/CNF nanocomposites containing different

concentrations of CNFs.

24

The nanocomposites with different concentrations of CNFs were also subjected to cyclic

loading-unloading cycles between 0% to 40% strain. Figure 7e shows the relative resistance

change during the first ten cycles. For the p-PDMS/CNF nanocomposites containing 0.1, 0.3, 0.5,

0.7, 1.4 and 2.8 wt% of CNFs, the relative resistance changes, ΔR/R0, were 620%, 320%, 229%,

182%, 98%, and 46%, respectively, at the maximum applied strain of 40%. The relative

resistance change exhibited a slight reduction during the first few cycles but then stabilized

quickly, indicating an excellent stability.

The nanocomposite with the highest sensitivity, which contained 0.1 wt% of CNFs, was further

subjected to cyclic loading under various cyclic maximum applied strains from 10% to 50%

(Figure 8). Figure 8a shows the relative resistance change in the first ten cycles while Figure

8b shows the relative resistance change, ΔR/R, for the loading and unloading curves pertinent to

the first cycle. The value of ΔR/R0 increases from 62% to 1115% upon increasing the applied

strain from 10% to 50%. Also, a maximum strain of 40% was applied to the sensor at a ramping

rate of 40 mm/s (i.e. 260 %/s) and then maintained for 30 s to investigate the response and

relaxation properties. (It should be noted that 40 mm/s is the highest ramping rate that can be

achieved by the equipment employed.). Figure 8c shows the relative resistance changes and

strain in three cycles and Figure 8d indicates the response and relaxation time of the sensors.

The response time was estimated to be around 0.160 s, representing a latency of 0.006 s when

considering the time required for the sensor to ramp to 40% strain (0.154 s). This indicates a very

rapid response time for the p-PDMS/CNF nanocomposite. However, inherent creep behavior was

observed with a sharp overshoot and a relatively long relaxation time (i.e. 3.982 s with a latency

of 3.828 s) to reach a value within 10% of the baseline value. This may be attributed to the

25

viscoelasticity of the PDMS.38 However, this relaxation behavior is markedly faster than the

100s recovery time observed for thermoplastic elastomer/carbon black composites39 and the 50s

recovery time for sensors based on single-walled carbon nanotube films in a PDMS matrix.38

26

0 2 4 6 8 10

0

3

6

9

12

15 10% 20% 30% 40% 50%

∆R/R

0

Stretching cycles

(a)

0.0 0.1 0.2 0.3 0.4 0.50

3

6

9

12

15 10% 20% 30% 40% 50%

∆R/R

0

Strain (ε)

(b)

0.00.20.40.6

Stra

in (ε

)

(c)

0 40 80 120 1600369

∆R/R

0

Time (s) 120.8 121.0

1

2

3

4

5

6

7 Relaxationtime

(d)∆R

/R0

Responsetime

152 154 156Time (s)

Figure 8. Response of the p-PDMS/CNF nanocomposite (0.1 wt% CNFs) being cycled to

various levels of maximum applied strain, with the minimum applied strain always being 0%: (a)

the relative resistance change under 10 cycles of loading-unloading and (b) the relative resistance

change versus strain curves pertinent to the first cycle in (a); (c) and (d) the response and

relaxation properties of the p-PDMS/CNF nanocomposite (0.1 wt% CNFs). (Note: (d) is the

detail of the response and relaxation times taken from the third cycle in (c).)

27

3.4 Potential Applications

It is noteworthy that the strain sensors developed in the present work demonstrate tunable

piezoresistive sensitivity and stable piezoresistive performance (as indicated in Figure S3) with

good linearity in the relationship between ln(R/R0) and ln(L/L0) up to ~ 70% strain. Thus, from

recently reported results for stretchable strain sensors, as listed in Table S1 and given in the

review paper by Amjadi et al.,40 it is seen that the newly-developed nanocomposites are very

promising as stretchable electronics. More importantly, the use of CNFs-coated sugar particles as

templates is a novel, simple, and cost-effective strategy, which can be easily scaled up. The high

sensitivity and stretchability of porous PDMS/CNF sensors offer great potential for a wide range

of novel applications. Two such examples are presented below.

3.4.1 Strain-sensors for Detecting Movements in Human Joints

To demonstrate the potential applications in wearable devices for detecting and monitoring the

movement of human joints (e.g. the bending of fingers), the p-PDMS/CNF nanocomposite

containing the lowest content of CNFs (i.e. 0.1 wt%), which gives the highest piezoresistive

sensitivity (with GFld of 6.5), was employed to characterize human joint movements. The

flexible strain-sensor was attached onto the index finger of a glove by bonding the two ends with

an adhesive. Five repeated bending/relaxation cycles (with the bending angle from 0 to 45° and

then 0 to 90°, followed by 0 to 45° again) were applied to the index finger. Figure 9a shows the

detected relative resistance change during the bending motion of the finger. It is clearly seen that

the bending and relaxing of the finger can be detected by the increase and decrease of the relative

resistance change and the degree of bending can be quantitatively monitored by the change in the

resistance of the strain-sensor. By comparing the response of the strain-sensor to different

28

applied strains (Figures 8a-b) with the data from bending of the finger, the strain from bending

the finger can be estimated (Figure 9b). For instance, when bent to 45° and 90°, strains of

approximately 12.8% and 42.5% are generated. These values are in very good agreement with

the estimates from a photographic analysis and the values reported in the literature.2, 34 Thus, this

p-PDMS/CNF nanocomposite is a very promising wearable stretchable strain-sensor.

Figure 9. (a) Response of a p-PDMS/CNF nanocomposite strain-sensor (0.1 wt% CNFs) under

five repeated bending/relaxation cycles when attached to an index finger (the insert illustrates the

bending of the index finger to different angles); (b) the estimated strain based on the relative

resistance change.

3.4.2 Stretchable Conductive Materials

Maintaining high electrical conductivity under large deformations is essential in the development

of wearable electronics. This requires that electrical conductors show a low sensitivity to an

applied strain, contrary to the requirements for strain-sensors. Herein, the p-PDMS/CNF

nanocomposite containing 2.8 wt% of CNFs was selected for further study as a stretchable

29

conductor, since it possesses the lowest gauge factor amongst the nanocomposites investigated

with a GFld of ~1.04. A simple circuit was constructed to connect an LED with the p-PDMS/CNF

nanocomposite and a Keithley source meter with a stabilized output of 10 V. Figure 10a shows a

photograph of the illuminated LED before applying any strain. It is readily observed from

Figure 10b that the LED did not show any noticeable brightness degradation, even when the

nanocomposite was stretched to ~50% strain. For this p-PDMS/CNF nanocomposite containing

2.8 wt% of CNFs, its initial electrical resistance was 500 Ω, which was increased by 53.2% to

766 Ω at 50% strain. Thus, when this conductor undergoes a relatively large deformation of 50%,

it is capable of maintaining the required potential drop to activate the LED light. It should be

noted that the conductivity of the p-PDMS/CNF nanocomposites is relatively low compared to

metallic and some previously reported polymer nanocomposites-based stretchable conductors.18,

29, 41 However, the operating voltage needed for this simple demonstration was not unduly high

and future work will study further increasing the electrical conductivity by increasing the

concentration of the CNFs employed and/or by introducing more conductive materials such as

silver (or gold) nanoparticles or nanowires.

Figure 10. Photograph of an illuminated LED in a circuit connected using a p-PDMS/CNF (2.8

wt% CNFs) conductor at (a) 0% and (b) 50% strain.

30

4. CONCLUSIONS

Stretchable porous polydimethylsiloxane/carbon nanofibre (denoted by p-PDMS/CNF)

composites have been developed via a new, cost-effective method using CNF-coated sugar

particles as the templates. The successful transfer of the CNFs from the sugar surface into the

PDMS, and subsequent removal of the sugar particles, yielded porous structures with the CNFs

embedded within the pore walls of the PDMS. The interconnected CNF network provided

electrically conductive paths that, when subjected to mechanical deformation, gave rise to a

change in the electrical resistance. These new p-PDMS/CNF nanocomposites exhibit more stable

piezoresistive behaviour than those prepared by a conventional dip-coating method (termed dc-

PDMS/CNF nanocomposites). These observations have been explained by demonstrating that

the piezoresistivity of the p-PDMS/CNF nanocomposites arises from the variation of the

tunneling resistance and contact resistance, whilst that of the dc-PDMS/CNF nanocomposites is

largely due to the formation of microcracks in the CNF layer.

The electrical conductivity and piezoresistive sensitivity of these p-PDMS/CNF nanocomposites

can be tuned by simply varying the content of the CNFs. Upon decreasing the concentration of

CNFs from 2.8 to 0.1 wt%, the electrical conductivity of the nanocomposite decreased

continuously from 4.2 to 0.01 S/m, while the gauge factor, GFld, increased from 1.0 to 6.5. It was

also found that the p-PDMS/CNF nanocomposites exhibited stable piezoresistive performance

with a fast response time and a good linearity in terms of ln(R/R0) versus ln(L/L0) up to ~ 70%

strain. The p-PDMS/CNF nanocomposites have been demonstrated to be effective as strain-

sensors for the quantitative monitoring of the bending of a human finger and as stretchable

conductors. It is believed that such a simple and cost-effective fabrication protocol provides new

31

insights into the design of 3D, highly-elastic conductive polymer nanocomposites for use as

flexible and stretchable strain-sensors and conductors for future wearable electronics.

ASSOCIATED CONTENT

Supporting Information.

Additional figures and table include: 1) The tensile stress-versus strain properties of p-

PDMS/CNF nanocomposites containing different concentrations of CNFs; 2) The relative

resistance change from replicate specimens of the same p-PDMS/CNF nanocomposite (0.1 wt%

of CNFs); 3) The relative resistance change of p-PDMS/CNF nanocomposites (0.3 wt% of CNFs)

over 10 cycles of loading-unloading (with a cyclic maximum applied strain of 70% and a

minimum applied strain of 0%); 4) A table summarizing the sensing performance of recently

reported strain sensors.

AUTHOR INFORMATION

Corresponding Author

*E-mail: chun.h.wang@unsw.edu.au

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval

to the final version of the manuscript.

Notes

The authors declare no competing financial interest.

32

ACKNOWLEDGMENTS

The authors are grateful for the financial support received from the Australian Research

Council’s Discovery Grant (DP140100778).

REFERENCES

(1) Schwartz, G.; Tee, B. C. K.; Mei, J.; Appleton, A. L.; Kim, D. H.; Wang, H.; Bao, Z.

Flexible Polymer Transistors with High Pressure Sensitivity for Application in Electronic

Skin and Health Monitoring. Nat. Commun. 2013, 4, 1859.

(2) Yan, C. Y.; Wang, J. X.; Kang, W. B.; Cui, M. Q.; Wang, X.; Foo, C. Y.; Chee, K. J.; Lee, P.

S. Highly Stretchable Piezoresistive Graphene-Nanocellulose Nanopaper for Strain Sensors.

Adv. Mater. 2014, 26, 2022-2027.

(3) Liu, Q.; Chen, J.; Li, Y.; Shi, G. High-Performance Strain Sensors with Fish-Scale-Like

Graphene-Sensing Layers for Full-Range Detection of Human Motions. ACS Nano 2016, 10,

7901-7906.

(4) Gong, S.; Lai, D. T. H.; Su, B.; Si, K. J.; Ma, Z.; Yap, L. W.; Guo, P.; Cheng, W. Highly

Stretchy Black Gold E-Skin Nanopatches as Highly Sensitive Wearable Biomedical Sensors.

Adv. Electron. Mater. 2015, 1, 1400063.

(5) Amjadi, M.; Pichitpajongkit, A.; Lee, S.; Ryu, S.; Park, I. Highly Stretchable and Sensitive

Strain Sensor Based on Silver Nanowire-Elastomer Nanocomposite. ACS Nano 2014, 8,

5154-5163.

(6) Zhang, R.; Baxendale, M.; Peijs, T. Universal Resistivity–Strain Dependence of Carbon

Nanotube/Polymer Composites. Phys. Rev. B 2007, 76, 195433.

33

(7) Hu, C. H.; Liu, C. H.; Chen, L. Z.; Peng, Y. C.; Fan, S. S. Resistance-Pressure Sensitivity

and a Mechanism Study of Multiwall Carbon Nanotube Networks/Poly(dimethylsiloxane)

Composites. Appl. Phys. Lett. 2008, 93, 033108.

(8) Toprakci, H. A. K.; Kalanadhabhatla, S. K.; Spontak, R. J.; Ghosh, T. K. Polymer

Nanocomposites Containing Carbon Nanofibers as Soft Printable Sensors Exhibiting Strain-

Reversible Piezoresistivity. Adv. Funct. Mater. 2013, 23, 5536-5542.

(9) Borghetti, M.; Serpelloni, M.; Sardini, E.; Pandini, S. Mechanical Behavior of Strain Sensors

Based on PEDOT:PSS and Silver Nanoparticles Inks Deposited on Polymer Substrate by

Inkjet Printing. Sens. Actuators, A 2016, 243, 71-80.

(10) Boland, C. S.; Khan, U.; Ryan, G.; Barwich, S.; Charifou, R.; Harvey, A.; Backes, C.; Li, Z.;

Ferreira, M. S.; Möbius, M. E.; Young, R. J.; Coleman, J. N. Sensitive Electromechanical

Sensors Using Viscoelastic Graphene-Polymer Nanocomposites. Science 2016, 354, 1257-

1260.

(11) Li, J.; Zhao, S.; Zeng, X.; Huang, W.; Gong, Z.; Zhang, G.; Sun, R.; Wong, C.-P. Highly

Stretchable and Sensitive Strain Sensor Based on Facilely Prepared Three-Dimensional

Graphene Foam Composite. ACS Appl. Mater. Interfaces 2016, 8, 18954–18961.

(12) Samad, Y. A.; Li, Y.; Schiffer, A.; Alhassan, S. M.; Liao, K. Graphene Foam Developed

with a Novel Two-Step Technique for Low and High Strains and Pressure-Sensing

Applications. Small 2015, 11, 2380-2385.

(13) Samad, Y. A.; Li, Y. Q.; Alhassan, S. M.; Liao, K. Novel Graphene Foam Composite with

Adjustable Sensitivity for Sensor Applications. ACS Appl. Mater. Interfaces 2015, 7, 9195-

9202.

34

(14) Wu, S.; Ladani, R. B.; Zhang, J.; Ghorbani, K.; Zhang, X.; Mouritz, A. P.; Kinloch, A. J.;

Wang, C. H. Strain Sensors with Adjustable Sensitivity by Tailoring the Microstructure of

Graphene Aerogel/PDMS Nanocomposites. ACS Appl. Mater. Interfaces 2016, 8, 24853-

24861.

(15) Qin, Y.; Peng, Q.; Ding, Y.; Lin, Z.; Wang, C.; Li, Y.; Xu, F.; Li, J.; Yuan, Y.; He, X.; Li, Y.

Lightweight, Superelastic, and Mechanically Flexible Graphene/Polyimide Nanocomposite

Foam for Strain Sensor Application. ACS Nano 2015, 9, 8933-8941.

(16) Park, J.; Wang, S.; Li, M.; Ahn, C.; Hyun, J. K.; Kim, D. S.; Kim, D. K.; Rogers, J. A.;

Huang, Y.; Jeon, S. Three-Dimensional Nanonetworks for Giant Stretchability in Dielectrics

and Conductors. Nat. Commun. 2012, 3, 916.

(17) Duan, S.; Yang, K.; Wang, Z.; Chen, M.; Zhang, L.; Zhang, H.; Li, C. Fabrication of Highly

Stretchable Conductors Based on 3D Printed Porous Poly(dimethylsiloxane) and

Conductive Carbon Nanotubes/Graphene Network. ACS Appl. Mater. Interfaces 2016, 8,

2187-2192.

(18) Chen, M. T.; Zhang, L.; Duan, S. S.; Jing, S. L.; Jiang, H.; Li, C. Z. Highly Stretchable

Conductors Integrated with a Conductive Carbon Nanotube/Graphene Network and 3D

Porous Poly(dimethylsiloxane). Adv. Funct. Mater. 2014, 24, 7548-7556.

(19) Allaoui, A.; Hoa, S. V.; Pugh, M. D. The Electronic Transport Properties and

Microstructure of Carbon Nanofiber/Epoxy Composites. Compos. Sci. Technol. 2008, 68,

410-416.

(20) Ladani, R. B.; Wu, S.; Kinloch, A. J.; Ghorbani, K.; Zhang, J.; Mouritz, A. P.; Wang, C. H.

Improving the Toughness and Electrical Conductivity of Epoxy Nanocomposites by Using

Aligned Carbon Nanofibres. Compos. Sci. Technol. 2015, 117, 146-158.

35

(21) Wu, S.; Ladani, R. B.; Zhang, J.; Kinloch, A. J.; Zhao, Z.; Ma, J.; Zhang, X.; Mouritz, A. P.;

Ghorbani, K.; Wang, C. H. Epoxy Nanocomposites Containing Magnetite-Carbon

Nanofibers Aligned Using a Weak Magnetic Field. Polymer 2015, 68, 25-34.

(22) Ladani, R. B.; Wu, S.; Kinloch, A. J.; Ghorbani, K.; Zhang, J.; Mouritz, A. P.; Wang, C. H.

Multifunctional Properties of Epoxy Nanocomposites Reinforced by Aligned Nanoscale

Carbon. Mater. Design 2016, 94, 554-564.

(23) Liu, W.; Chen, Z.; Zhou, G.; Sun, Y.; Lee, H. R.; Liu, C.; Yao, H.; Bao, Z.; Cui, Y. 3D

Porous Sponge-Inspired Electrode for Stretchable Lithium-Ion Batteries. Adv. Mater. 2016,

3578–3583.

(24) Zhang, H.; Liu, N.; Shi, Y.; Liu, W.; Yue, Y.; Wang, S.; Ma, Y.; Wen, L.; Li, L.; Long, F.;

Zou, Z.; Gao, Y. Piezoresistive Sensor with High Elasticity Based on 3D Hybrid Network of

Sponge@CNTs@Ag NPs. ACS Appl. Mater. Interfaces 2016, 22374–22381.

(25) Amjadi, M.; Min Seong, K.; Park, I. In Flexible and sensitive foot pad for sole distributed

force detection, Proceedings of the 14th IEEE International Conference on Nanotechnology,

Toronto, Canada, August 18-21, 2014.

(26) Marcano, D. C.; Kosynkin, D. V.; Berlin, J. M.; Sinitskii, A.; Sun, Z.; Slesarev, A.;

Alemany, L. B.; Lu, W.; Tour, J. M. Improved Synthesis of Graphene Oxide. ACS Nano

2010, 4, 4806-4814.

(27) He, L. X.; Tjong, S. C. Aqueous Graphene Oxide-Dispersed Carbon Nanotubes as Inks for

the Scalable Production of All-Carbon Transparent Conductive Films. J. Mater. Chem. C

2016, 4, 7043-7051.

36

(28) Zhang, A.; Chen, M.; Du, C.; Guo, H.; Bai, H.; Li, L. Poly(dimethylsiloxane) Oil Absorbent

with a Three-Dimensionally Interconnected Porous Structure and Swellable Skeleton. ACS

Appl. Mater. Interfaces 2013, 5, 10201-10206.

(29) Xu, F.; Zhu, Y. Highly Conductive and Stretchable Silver Nanowire Conductors. Adv.

Mater. 2012, 24, 5117-5122.

(30) Rubaiya, R.; Peyman, S. Effects of Inter-Tube Distance and Alignment on Tunnelling

Resistance and Strain Sensitivity of Nanotube/Polymer Composite Films. Nanotechnology

2012, 23, 055703.

(31) Duan, L. Y.; Fu, S. R.; Deng, H.; Zhang, Q.; Wang, K.; Chen, F.; Fu, Q. The Resistivity-

Strain Behavior of Conductive Polymer Composites: Stability and Sensitivity. J. Mater.

Chem. A 2014, 2, 17085-17098.

(32) Lee, J.; Kim, S.; Lee, J.; Yang, D.; Park, B. C.; Ryu, S.; Park, I. A Stretchable Strain Sensor

Based on a Metal Nanoparticle Thin Film for Human Motion Detection. Nanoscale 2014, 6,

11932-11939.

(33) Liu, Y.; Zhang, D.; Wang, K.; Liu, Y. Y.; Shang, Y. A Novel Strain Sensor Based on

Graphene Composite Films with Layered Structure. Composites, Part A 2016, 80, 95-103.

(34) Morteza, A.; Yong Jin, Y.; Inkyu, P. Ultra-Stretchable and Skin-Mountable Strain Sensors

Using Carbon Nanotubes-Ecoflex Nanocomposites. Nanotechnology 2015, 26, 375501.

(35) Lipomi, D. J.; Vosgueritchian, M.; Tee, B. C. K.; Hellstrom, S. L.; Lee, J. A.; Fox, C. H.;

Bao, Z. N. Skin-Like Pressure and Strain Sensors Based on Transparent Elastic Films of

Carbon Nanotubes. Nat. Nanotechnol. 2011, 6, 788-792.

37

(36) Jeong, Y. R.; Park, H.; Jin, S. W.; Hong, S. Y.; Lee, S.-S.; Ha, J. S. Highly Stretchable and

Sensitive Strain Sensors Using Fragmentized Graphene Foam. Adv. Funct. Mater. 2015, 25,

4228-4236.

(37) Fan, Y. J.; Meng, X. S.; Li, H. Y.; Kuang, S. Y.; Zhang, L.; Wu, Y.; Wang, Z. L.; Zhu, G.

Stretchable Porous Carbon Nanotube-Elastomer Hybrid Nanocomposite for Harvesting

Mechanical Energy. Adv. Mater. 2017, 29, 1603115.

(38) Cai, L.; Li, J.; Luan, P.; Dong, H.; Zhao, D.; Zhang, Q.; Zhang, X.; Tu, M.; Zeng, Q.; Zhou,

W.; Xie, S. Highly Transparent and Conductive Stretchable Conductors Based on

Hierarchical Reticulate Single-Walled Carbon Nanotube Architecture. Adv. Funct. Mater.

2012, 22, 5238-5244.

(39) Mattmann, C.; Clemens, F.; Troster, G. Sensor for Measuring Strain in Textile. Sensors

2008, 8, 3719-3732.

(40) Amjadi, M.; Kyung, K.-U.; Park, I.; Sitti, M. Stretchable, Skin-Mountable, and Wearable

Strain Sensors and Their Potential Applications: A Review. Adv. Funct. Mater. 2016, 26,

1678-1698.

(41) Kim, Y.; Zhu, J.; Yeom, B.; Di Prima, M.; Su, X. L.; Kim, J. G.; Yoo, S. J.; Uher, C.; Kotov,

N. A. Stretchable Nanoparticle Conductors with Self-Organized Conductive Pathways.

Nature 2013, 500, 59-U77.