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Sensors and Materials, Vol. 32, No. 7 (2020) 2527–2538
MYU Tokyo
S & M 2280

Flexible Porous Carbon Black–Polymer
Composites with a High Gauge Factor
Yi-Te Huang,1 Naoki Inomata,1 Zhuqing Wang,1 Yu-Ching Lin,2 and Takahito Ono3*
1

Department of Mechanical Systems Engineering, Tohoku University, Sendai, Miyagi 980-8579, Japan
2
Goertek Technology Japan Co., Ltd., Tokyo 101-0021, Japan
3
Micro System Integration Center, Tohoku University, Sendai 980-8579, Japan(Received March 6, 2020; accepted April 14, 2020)

Keywords: nanoporous, carbon black, conductive polymer, high gauge factor, strain sensing

In this work, new conductive polymer films suitable for microfabrication have beendeveloped. The solutions used are prepared by dispersing highly conductive nanoporous carbon
blacks (n-CBs) in PHOTONEECE® (CBPh), polyvinyl alcohol (CBsPV), or gelatin (CBsGel).
Conductive polymer composite thin films are formed by spin-coating on a clean glass substrate.
The electrical conductivity and its response to strain are evaluated by a four-point bending
method, which shows high gauge factors (GFs) of 150 and 523 for PVA- and gelatin-based
composite films, respectively, in a low strain region. This clearly reveals the high potential
capability of these novel nanocomposite films with the reported CB composite polymer showing
a GF of ~100. It is considered that the porous structure of the n-CBs provides high sensitivity offlexible sensing elements that can be implemented into microfabrication processes easily.

1.

Introduction

Recently, research studies on microdevices for strain and pressure sensing have been
extensively performed because of various potential applications,(1–3) including reliable, flexible,
and sensitive strain and pressure sensors for upcoming robotic and IoT applications.(4–7)
For flexible device applications, flexible materials sensitive to strain are one of the key
technologies. Among strain sensors, semiconductors,(8,9) ceramics,(10) and metals(11) are often
used; nevertheless, these materials are basically not flexible. Conductive polymer composites
made of nonconductive polymer and conductive filler are potential candidates of flexible
strain-sensitive sensing materials suitable for the microfabrication of soft strain and pressure
sensors.(12,13) Carbon black (CB) with a zero-dimensional feature is one of the carbon allotropes
and is often used as a conductive filler for polymer composites.(14,15) Carbon-based materials
including carbon nanotubes and graphene have been used for improving mechanical, electrical,
and thermal properties.(16–18) CB has the advantage over graphene and carbon nanotubes interms of cost and accessibility; however, the gauge factors (GFs) of CB-based composites instrain sensing are mostly lower than 100.(19,20) Therefore, a high-GF material based on the CB*

Corresponding author: e-mail: ono@nme.mech.tohoku.ac.jp
https://doi.org/10.18494/SAM.2020.2853
ISSN 0914-4935 © MYU K.K.
https://myukk.org/

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filler is desired for low-cost, accessible, and high-performance strain sensors. In this study, ahighly conductive nanoporous CB with a smaller diameter than a conventional CB(21) is chosen
as the filler to improve GF. In this work, highly conductive nanoporous carbon black composite
specimens with three types of matrix are synthesized and their GFs are evaluated.

2.

Materials and Methods

2.1

Conductive polymer for strain sensitivity evaluation

The percolation effect determines the conductivity of a conductive polymer, which is the
crucial point in strain sensing. A schematic structure of the CB polymer composite for theevaluation of the percolation threshold(22) is shown in Fig. 1. The nanoporous CB particles(CB from Lion Specialty Chemicals; mean particle diameter, 34 nm) in solid-state powders orin a nanoparticle solution are mixed with the polymer. As the concentration of CB increases,
the resistivity of the CB/composite decreases abruptly after reaching the percolation threshold,at which the highest sensitivity is expected in strain sensing.(23,24) A schematic of the test
specimen is shown in Fig. 2. Porous CB particles are used as fillers, and three types of polymer
are examined as the base matrixes of the composites, i.e., PHOTONEECE (Toray®), polyvinylFig. 1.

(Color online) Schematic of percolation effect in conductive polymer.

(a)Fig. 2.
state.

(b)(Color online) Schematic of test specimens for strain sensitivity evaluation. (a) Original and (b) bendingSensors and Materials, Vol. 32, No. 7 (2020)

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alcohol (PVA), and gelatin. PHOTONEECE is mixed with a solid-state CB powder owing to its
lack of affinity with water, while PVA and gelatin are common polymers, which are misciblein water and water-based CB solutions. The deformation of the CB polymer composites
changes the distance between CB particles, which in turn changes the electrical conductivity.(25)
However, if the CB particles cannot be dispersed very well owing to aggregation, the strain
sensitivity will decrease.
2.2

Fabrication process of test specimen

The premixing processes of the conductive polymer composites are shown in Fig. 3.
N-Methyl-2-pyrrolidone (NMP) is mixed with PHOTONEECE using a planetary centrifugal
mixer for 10 min, as shown in Figs. 3(a1) and 3(a2). Then, the CB powder is mixed with the
PHOTONEECE solution [Fig. 3(a3)] using the planetary centrifugal mixer again for 10 min.
For the premixing process of CBsPV, a PVA powder is poured into 95 °C hot water withstirring until the PVA powder is completely dissolved, as shown in Figs. 3(b1) and 3(b2).
The CB water solution is then mixed with the PVA solution [Fig. 3(b3)] using the planetarycentrifugal mixer for 10 min.
For the premixing process of CBsGel, a gelatin powder is added to 65 °C warm waterwith stirring at 800 rpm for 20 min, as shown in Fig. 3(c1). Glycerol is also added to gelatin[Fig. 3(c1)] to plasticize the gelatin. Then, the above solutions [Fig. 3(c2)] are mixed by rotatingthem for 10 min [Fig. 3(c3)]. Next, the CB solution is mixed with the gelatin solution.

Fig. 3.

(Color online) Premixing processes of the CB polymer nanocomposite films.

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Then, ultrasonication (60 min), mixing (10 min), and vacuum pumping (60 min) are
performed to obtain a well-dispersed CB composite polymer. Finally, the dispersed CB
composite polymer is spin-coated onto a 0.21 mm thick glass substrate at a rate of 2000 rpm for
25 s to obtain a conductive polymer thin film.
2.3

Experimental setup for evaluation of strain sensitivity to resistanceTo evaluate the intrinsic resistivity of the samples, the sheet resistance of the CB
polymer composite thin films on the glass substrate is measured five times for each sample
using a four-point probe tester (K-705RS Kyowariken®). Through measuring the different
concentrations of CB polymer composite thin films using the four-point probe tester, we candetermine the relationship of sample sheet resistance with increasing concentration. The film
thickness is measured four times for each sample from the film height using a surface profiler(P-10 Tencor®). The average thicknesses of CBPh-, CBsPV-, and CBsGel-based composite
films are approximately 1600 ± 300, 2500 ± 600, and 800 ± 100 nm, respectively. In addition,
the resistivity ρ can be obtained asρ = Rst,(1)

where Rs is the sheet resistance and t is the thickness. The homemade experimental setup forstrain sensitivity evaluation using a four-point bending measurement setup is shown in Fig. 4.
The sample is electrically connected to an I–V measurement system. While the sample is set
on a four-point bending system with the upper load and bottom support bars, force is applied tothe sample at four contact points to produce the strain calculated from the applied force. I–V ismeasured under various force applications to evaluate the resistance variation caused by strain.
In I–V measurement, a voltage of 0.1 V is applied to the sample from −5 V to 5 V and back andforth. The I–V of each sample is measured three times at a specific strain. The resistance of thesamples is evaluated from the average reciprocal of the slope of I–V measurement results.

Fig. 4.

(Color online) Schematic of strain sensitivity evaluation setup using four-point bending setup.

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3.

Results and Discussion

3.1

Formation of CB polymer composite films

From the optical micrographs of the prepared films shown in Fig. 5, the aggregated CB
particles are recognized as black spots in the PHOTONEECE matrix. This means that CB
particles are not dispersed well. The aggregation of the CB particles in the matrix possibly
causes low sensitivity to strain. As shown in Figs. 5(b) and 5(c), the CB particles are welldispersed in the PVA and gelatin matrixes. SEM images of the CB morphology of the CB
polymer/composite are shown in Fig. 6. The CB particles on CBPh [Fig. 6(a)] aggregate, whichindicates the inadequate dispersion of the CB particles. In addition, Fig. 6(b) reveals that theextent of CB aggregation is lower than that of CBPh aggregation, whereas Fig. 6(c) indicatesthat the extent of aggregation of the CB particles in gelatin is lower than that of other materials.
Because of the superior miscibility of the CB solution in the water-based polymer solution, theCB particles are dispersed well in the polymer.

(a)

(b)

(c)

Fig. 5. (Color online) Micrographs of samples: (a) CBPh-2.28 wt% CB, (b) CBsPV-1.60 wt% CB, and (c)CBsGel-1.94 wt% CB.

(a)

(b)

(c)

Fig. 6. (Color online) SEM images of morphology of CB on composites (a) CBPh-2.28 wt% CB, (b) CBsPV-1.60
wt% CB, and (c) CBsGel-1.94 wt% CB.

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Sensors and Materials, Vol. 32, No. 7 (2020)

Resistivity of CB composites

The resistivity change of the CBPh composite film shows a logarithmic decrement withincreasing CB concentration, as shown in Fig. 7. This reveals that the percolation thresholds of
CBPh, CBsPV, and CBsGel are observed at approximately 2.3, 1.6, and 1.9 wt%, respectively.
In the next subsection, the samples with a CB concentration at the percolation threshold arechosen to evaluate the strain sensitivity.

3.3

Temperature dependence of resistivity of CB polymer composites

The temperature dependence of the samples is investigated using a temperature-controlled
oven with the experimental setup shown in Fig. 8. The variation in resistance is defined as
Stemp =

∆Rt
,
Rt 0

(2)

where ΔRt = Rt − Rt0, with Rt and Rt0 being the resistance at a specific temperature and theresistance at room temperature (20 °C), respectively. The results indicate that CBsGel is theFig. 7.

(Color online) CB concentration dependence of resistivity of composite films.

Fig. 8. (Color online) Relationship between sensitivity and temperature of (a) CBPh-2.28 wt% CB, (b)CBsPV-1.60 wt% CB, and (c) CBsGel-1.94 wt% CB.

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most sensitive to the variation in temperature (−0.58 %/K) before reaching the environmental
temperature of 60 °C. On the other hand, the variations in the resistances of CBsPV and
CBPh are only −0.15 %/K and −0.09 %/K approximately before reaching the environmental
temperature of 60 °C, which reveals that CBsPV and CBPh are less sensitive to temperaturethan CBsGel. In addition, the changes in the resistances of CBsPV (−0.15 %/K) and CBPh(−0.09 %/K) are less than those in other work on a CB composite polymer strain sensor(25)(−0.2 %/K). This clearly demonstrates the potential use of CBsPV and CBPh as strain sensing
materials.
3.4

Strain sensitivity of CB composites

To estimate the strain sensitivity to the resistance of the thin films on the glass substrate, an
Au electrode pattern is formed on the films by electron beam evaporation and photolithography,
as shown in Fig. 9. Titanium (20 nm) is deposited on the CB polymer composite thin film as
an adhesion layer, and a 180 nm gold layer is deposited on the titanium layer as the electrodes.
In addition, the surface roughness of each sample is measured three times using a surface
profiler (ET-200 Kosaka Laboratory®), and the arithmetical mean roughness (Ra) is obtained
as the parameter of the surface roughness of the samples. The Ra values of CBPh, CBsPV, andCBsGel are 0.006 ± 0.001, 0.011 ± 0.004, and 0.007 ± 0.004 μm, respectively. This reveals that
the CB polymer composite thin films have a roughness with a dimension of 0.001 μm, which isbeneficial for contact with the Ti adhesion layer.
Force sensitivity is measured using the four-point bending setup, as described previously.
Tensile stress is applied to the nanocomposite film and the resistance change ΔR is evaluated.
The resistance of the samples subjected to different forces is obtained by I–V measurement
and typical results are shown in Fig. 10. Because of the good contact between the electrodes
and the CB polymer composite thin film, the I–V curves are straight in a cycle test, which also
indicates a good ohmic contact between the electrodes and the CB composite polymer thin film.
Similarly, a good ohmic contact is observed at high strains, which shows that the electrodes are
not peeled-off from the CB polymer composite thin film by the applied strain.

Fig. 9.

(Color online) Photograph of test specimen.

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Fig. 10. (Color online) I–V curve variation of CBsGel composite with various loads evaluated using four-point
bending setup.

Fig. 11. (Color online) Strain dependence of sensitivity (ΔR/R0) in composite films, CBPh-2.28 wt% CB (inset),
CBsPV-1.60 wt % CB, and CBsGel-1.94 wt% CB.

The strain sensitivity of the samples to the resistance is defined as ΔR/R0. The results ofthe strain sensitivities of the CBPh films with various CB concentrations are shown in Fig. 11.
It is observed that the sensitivity does not show a linear relationship in the measured strain
region. However, a high sensitivity is observed in a limited region, Here, we discuss the highest
sensitivity performance. The CBPh composite film exhibits the lowest sensitivity performance,
and the strain sensitivities of CBsPV and CBsGel composite films are much higher than that of
the CBPh composite. The reason is that the aggregation of the CB particles in PHOTONEECE
reduces the strain sensitivity. On the other hand, the strain sensitivity performance measured
using the two-probe sensing setup may degrade owing to the contact resistance between theelectrodes and the CB polymer composite thin film.
Additionally, the CB particles in an aqueous solution are miscible in other water-based
solutions. Thus, the CB particles can be well dispersed in the polymer, which enhances
the strain sensitivity. On the other hand, the slight aggregation of the CB particles in
PHOTONEECE causes the low sensitivity to strain. Because of the limited dispersion, theresistance of the CBPh film decreases with increasing applied force. Figure 12 shows thevariation in the resistance of the CB polymer composite film under cyclic strain application.

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(a)

(b)

(c)
Fig. 12. (Color online) Demonstration of force sensing in composite films (a) CBPh-2.28 wt% CB, (b) CBsPV-1.60
wt% CB, and (c) CBsGel-1.94 wt% CB.

Table 1
Summarized results of the strain sensitivity.
SampleA
GF−3.3
CB particles
PowderPolymer
PHOTONEECE
Percolation threshold~2.3 wt% CB

B
150
Solution
PVA~1.6 wt% CB

C
523
Solution
Gelatin~1.9 wt% CB

Force is applied to the upper load perpendicularly (On-state) and removed (Off-state) alternately
to estimate the repeatability of measurements of the CB polymer/composite. The GFs of thesamples are summarized in Table 1, clearly indicating that the CBsPV and CBsGel composite
films show much higher GFs of 150 and 523, respectively, which are higher than those in otherworks regarding CB composites (GF < 100).(19,20) This reveals the high potential of applying
CBs to low-cost, simple-process, and flexible strain sensors based on a flexible conductive
polymer. Currently, the reason for the saturation in the sensitivity at the high-strain region
is not clear; but basically, the percolation effect on the resistivity is a nonlinear phenomenon,
which possibly affects the nonlinearity of sensitivity.

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Conclusions

In this work, three types of CB polymer composite are synthesized by dispersing highly
conductive porous nanostructured CB in PHOTONEECE, PVA, and gelatin. The CB particles
can be dispersed well in PVA and gelatin. The CB concentration dependence on the resistivity
shows a logarithmic linear relationship, also indicating the percolation effect of the CB particles
in these polymers. Among the specimens, the CBsPV and CBsGel composite films at thepercolation threshold exhibit the highest GFs of 150 and 523, respectively. These results show
the potentially high capability of the CBsPV composite film as a strain and force sensing
element with a simple process, low cost, and flexibility to detect a small strain or force.

Acknowledgments
Part of this research was performed at Tohoku University Micro System Integration Centerand Micro/Nano-Machining Research and Education Center.

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About the Authors
Yi-Te Huang received his B.S. degree from National Chung Hsing University,
Taiwan, in 2018. Since 2018, he has pursued his M.S. degree in Tohoku
University, Japan. His research interests are in MEMS, novel materials, gassensors, strain gauge, and pressure sensing. (yite@nme.mech.tohoku.ac.jp)

Naoki Inomata received his M.S. and D.E. degrees from Tohoku University,
Sendai, Japan, in 2010 and 2013, respectively. He was an assistant professor
at Micro System Integration Center, Tohoku University from 2013 to 2016.
He has been an assistant professor at the Department of Mechanical System
Engineering, Graduate School of Engineering at Tohoku University since2016. (inomata@nme.mech.tohoku.ac.jp)
Zhuqing Wang received his Dr.Eng. degree from Yamanashi University,
Kofu, Japan in 2011. He was an assistant professor at the Graduate Schoolof Engineering, Tohoku University. His research interests cover wide areas,
including micro-electromechanical systems (MEMS), nanoelectromechanicalsystems, and ultrasensitive sensing based on thermal biodevices for practicalapplications. (wzhuqing@nme.mech.tohoku.ac.jp)

		Yu-Ching Lin received her M.S. degree from National Taiwan University
and then her Ph.D. degree from Tohoku University in 2006. She has
worked as an assistant professor and then an associate professor at Tohoku
University, Japan; a scientific staff member of Fraunhofer ENAS, Germany;a visiting scholar at UC Berkeley, USA; a technical adviser of MEMSCORE
Corporation, Japan. Currently, she is Director of Advanced Microsystem
Research Institute at Goertek Technology Japan.
(yclin@nme.mech.tohoku.ac.jp)

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Takahito Ono received his D.E. degree in mechatronics and precisionengineering from Tohoku University in 1996. From 1996 to 1999, he wasa research associate in the Department of Mechatronics and Precision
Engineering, Tohoku University. From 1999 to 2009, he was an associate
professor. He is currently a professor at the Department of Mechanical
Systems Engineering of Tohoku University and the Director of Microsystem
Integration Center, Tohoku University. He was a professor at the Departmentof Mechanical Engineering, Graduate School of Engineering at The Universityof Tokyo from 2013 to 2016. He is Director of the Micro/Nano Machining
Education Center, Tohoku University from 2018. His expertise is in the areasof microelectromechanical systems (MEMS), nanoelectromechanical systems(NEMSs), silicon-based nanofabrication, ultrasensitive sensing based onresonating devices, scanning probe technologies, and nanoprobe sensing fornanoscale science and engineering. His recent interests cover nanomaterials
and their integration into microsystems for applications in IoT sensors,
environment monitoring, biomedical sensors, nanoenergy, and scientificinstrumentation. (ono@nme.mech.tohoku.ac.jp)