Electrical properties of stretchable and skin–mountable PDMS/MWCNT hybrid composite films for flexible strain sensors

Abstract

Flexible strain sensors based on carbon nanofillers have great potential in the application of skin-adhesive sensors, wearable sensors, and tactile sensors, due to their superior electrical properties. Herein, the electrical properties of highly sensitive PDMS/MWCNT strain sensors made by vacuum filtration method were investigated. In order to obtain the electrical percolation curve of the flexible conductive films, first different samples were made with the same surface area but with different wt. % of CNTs. Then, depending on CNT content, the obtained conductive films exhibited initial electrical resistance in the range of 12.5 KΩ to 22.8 MΩ. The piezoresistive films with the CNT concentration of 1.4 to 2.9 $mg / m m^{2}$ had shown superior resistance drop, so this interval was determined as the percolation threshold region. According to the SEM images, the nanocomposite layer thickness of the flexible strain sensors in this region was 790 nm to 1210 nm. Afterward, the percolation curve was obtained using curve fitting to the experimental data and the exact value of the percolation threshold was defined as $φ_{c} = 1.992 mg / m m^{2}$ . Finally, in order to determine the minimum gauge factor ( $G F_{min}$ ) of the sensors in percolation region, a flexible strain sensor in the upper limit of this region was selected and the piezoresistive properties of the selected sample were investigated.

Keywords

Flexible strain sensors electrical percolation curve vacuum filtration polydimethylsiloxane

Introduction

Conductive polymer nanocomposites play an important role in stretchable electronics, health monitoring, tactile sensors, biosensors, soft actuators, and flexible sensing elements as well as strain and pressure sensors.^1,2 Nanocomposite sensing elements convert mechanical stimuli to electrical signals by three main mechanisms including piezoresistivity, capacitance, and piezoelectricity.³ These mechanisms are highly dependent on the type of nanofillers used in the nanocomposite. There are a wide variety of nanofillers with different geometrical shape, electrical,⁴ optical⁵ and magnetic⁶ properties. From the geometrical aspect, the nanofillers have at least one dimension less than 100 nm, and divided into three main groups including two- and three-dimensional (e.g. graphene and box-shaped graphene^7–9), one-dimensional (e.g. nanowires and nanotubes^7,10,11), and zero-dimensional (e.g. carbon black nanoparticles^7,12). CNTs and nanowires have an extreme surface area-to-volume ratio and are one of the best candidates for manufacturing piezoresistive sensing elements.

As one of the most popular solution-based coating method, vacuum filtration can be used in the preparation of CNT transparent conducting films on flexible substrates.¹³ Wang et al.¹⁴ fabricated highly transparent ultrathin films of SWCNT using vacuum filtration process. They peeled off SWCNT ultrathin films from the filtration membrane by the surface tension of deionized water, and then transferred that onto polyethylene terephthalate (PET) substrates to make highly sensitive flexible chemiresistive sensors. Xu et al.¹⁵ prepared the flexible conductive films of silver nanowires-PET (AgNW-PET) for high-performance flexible electronics and photovoltaic devices by applying a facile method including vacuum filtration and mold transferring. The effect of silver nanowire weight density on the electrical properties of films, as well as the electrical percolation, was investigated. The resulting typical AgNW-PET film exhibited a high figure of merit with a low sheet resistance and high transparency.

Wang et al.¹⁶ fabricated reduced graphene oxide ultrathin films using vacuum filtration by a reproducible exfoliation method at the liquid/air interface, and developed a flexible relative humidity sensing matrix with high transparency, tunable sheet resistance, uniform electric conductivity, and structural homogeneity over a large area. Hyun et al.¹⁷ used vacuum filtration method to disperse graphene suspension on the surface of a hydrophobic polytetrafluoroethylene (PTFE) membrane filter (pore size: 0.2 µm), and develop foldable paper-based electronic circuits. The shapes of circuits were drawn on the back of the membrane filter with a pen, and the filtered graphene was transferred onto the paper, while the graphene in the other parts remained on the filter. The graphene circuits had shown a small change in conductance under various folding angles and maintain an electronic path on paper substrates after repetition of folding and unfolding. Jiang et al.¹⁸ manufactured foldable and stretchable graphene-based resistors using vacuum filtration method for portable and multifunctional devices and investigated the electrical stability of the films under bending and tension. They showed that nanocomposite resistors made using this method have unique characteristics and some advantages such as mechanical and thermal stability, rapid prototyping, flexibility, and stretchability over the traditional resistors.

In the present study, stretchable and skin-mountable PDMS/MWCNT hybrid composite films for flexible strain sensors were made using the vacuum filtration process. Afterward, the electrical properties of the flexible strain sensors containing different %wt of MWCNTs were evaluated and the electrical percolation region of the stretchable sensors made by this manufacturing method was obtained. Additionally, in order to determine the exact value of the percolation threshold, the percolation curve was obtained through curve fitting using the Gompertz growth numerical model.

Experimental study

Probably, compared with the other methods, the vacuum filtration technique is one of the best methods for the fabrication of ultrathin, electrically conducting films. In this process, surfactant-based suspension of MWCNTs was poured into filtration funnel and due to the differential pressure across the nanofiltration membrane, a homogeneous wafer-scale film of randomly oriented MWCNTs was formed on the filtration membrane.¹⁹ Then, the surfactant was washed away with purified water and the ultrathin layer of MWCNTs was used to prepare flexible strain sensor (see Figure 5).

In this method, the electrical conductivity and mechanical integrity of the resulted film are maximized, because under vacuum, the nanotubes tend to lie straight, gaining maximum overlap and interpenetration with each other. According to Figure 1(a), homogeneity of the MWCNT films is guaranteed since by accumulation of the nanotubes the local permeation rate decreases and the deposition rate will be tuned automatically. Moreover, the CNT nanoparticles in the MWCNT film have a random distribution which results in nanocomposite film with uniform piezoresistive behavior in different directions. Additionally, it is possible to control the MWCNT film thickness with nanoscale precision by adjusting the suspension volume and nanotube concentration.²⁰

Figure 1.

Schematic representation of vacuum filtration process: (a) filtration of the MWCNT suspension, (b) ultrathin film of randomly oriented MWCNTs on a nanofiltration membrane.

Materials

To fabricate nanocomposite sensing elements, Sylgard 184 from Dow Corning was used as a matrix and MWCNT as a filler of nanocomposite. Sylgard 184 is the commercial PDMS with desirable properties such as facile polymerization, simple molding and prototyping, long gel time, biocompatibility, transparency, and desirable mechanical and dielectric properties. This elastomer mainly used in stretchable sensors, soft actuators and microfluidics. Schneider et al.²¹ measured the rheological, mechanical and optical properties of Sylgard 184 for optical MEMS and showed that Sylgard 184 can endure strains up to 115%. This elastomer can be cured at different temperatures. In this paper, PDMS/MWCNT nanocomposites were cured at 70 ℃ for 2 h.

In addition, a suspension of MWCNTs was used as a nanocomposite filler because of its appropriate properties.²² MWCNTs have desirable geometrical characteristics and physical properties.²³ The electrical conductivity of MWCNTs and their aspect ratio are the most important factors to create piezoresistive behavior in PDMS matrix. For instance, carbon nanotubes (CNT) possess high aspect ratios (length-to-diameter ratio) of up to 10,000²⁴ and also electrical conductivity of 103 to 104 S/cm.²⁵

Preparation of CNT suspension

In order to disperse MWCNTs in deionized water, the sodium dodecyl sulfate (SDS) was used as a surfactant material. Each SDS molecule has a polar and non-polar head which helps to disperse MWCNTs (non-polar) in deionized water (polar) and creates a stable suspension. At the beginning, CNT was mixed with SDS by the weight ratio of 1:23 in 1 L of deionized water. This value is an optimum quantity for the weight ratio of nanoparticle to surfactant. For this ratio, nanoparticle's surface is completely occupied by surfactant's molecules. Higher amount of surfactant affected the conductivity of CNTs. In lower ratios, the nanoparticles will not be stable.

The critical micelle concentration (CMC) in pure water at 25 ℃ is 8.2 mM. According to the molar mass of SDS (288.372 g/mol), the CMC in 1 L pure water is formed in the presence of 23 g SDS. We used 1 g CNT to make 1 L CNT suspension, and therefore the weight ratio of CNT:SDS was set to 1:23 in the 1 L of deionized water.²⁶

In order to create CNT suspension, the materials were prepared and stirred for 15 min, then uniformly dispersed in deionized water with sonication process. According to Figure 2, the sonication process was done with BANDELIN-HD 3200 with the probe model VS-70T. The pulse duration and power were set to 1 s on/off and 70 W, respectively.

Figure 2.

Ultrasonic probe, BANDELIN HD 3200 with the probe type VS-70T.

To ensure that the suspension does not contain any agglomerated CNTs, the final step should be done. In this step, all the suspension was centrifuged at 4500 r/min for 15 min in Universal-320 centrifuge machine. After this process, any agglomerated CNTs were deposited in the centrifuge tubes and a well-dispersed CNT suspension was obtained (Figure 3(a)).

Figure 3.

(a) Final CNT suspension after centrifuging. Agglomerated CNTs deposited in centrifuge tube and the final suspension is free from any agglomeration. (b) Aluminum foil container after evaporation of deionized water.

Although the centrifuging process separates agglomerated CNTs from suspension (Figure 4(c)), however, this changes wt.% of CNTs in suspension. In order to obtain an exact wt.% of CNTs in the suspension, it is necessary to measure wt.% of CNTs again.

Figure 4.

Schematic illustration of (a) uniformly distributed, (b) aligned and (c) agglomerated CNTs of different lengths.

For this, some containers were made from an aluminum foil, then the specific amount of CNT suspension poured inside each container (Figure 3(b)) and all of the containers were weighted. To evaporate deionized water, containers were kept in the oven for 24 h at 65 ℃. After 24 h, containers were weighted again.

In Table 1, the difference weight shows the weight of both CNT and SDS. As the CNT/SDS weight ratio was fixed at 1:23 in the primary suspension (before centrifuging), the concentration of MWCNTs in the final CNT suspension can be calculated. According to Table 1, the average concentration of MWCNTs is about 0.1 mg/ml.

Table 1.

Properties of final CNT suspension after centrifuging process (see containers in Figure 3(b)).

Container	Container weight (g)	Suspension volume (mL)	Container weight after evaporation (g)	Difference weight (g)	CNT weight (g)	CNT Concentration (mg/mL)	wt.% of CNTs
A	0.5175	20	0.5636	0.0461	0.00192	0.0960	0.00972
B	0.5160	20	0.5618	0.0458	0.00191	0.0954	0.00962
C	0.2859	10	0.3083	0.0224	0.00093	0.0933	0.00952
Average						0.0949	0.0096

Nanocomposite fabrication

The first step in the fabrication of piezoresistive nanocomposite is applying CNT film on CA membrane through vacuum filtration method. In order to implement this technique, preparation of CNT suspension and membrane with porosity smaller than the size of nanoparticles is necessary. These requisites are two important inputs of this process.

To achieve the best piezoresistive properties, it is necessary to have a suspension without any agglomeration of CNTs and use a membrane type (hydrophilic or hydrophobic) that has a proper conformity with suspension. In this study, a colloidal water-based suspension was used so the hydrophilic membranes like CA, Ny, MCE, PE, NC H-PTFE, PES, and H-PVDF were proper for using in vacuum filtration. In this study, CA membrane with pore size of 220 nm and Millipore setup was used in vacuum filtration process.

Different volume fractions of MWCNT suspension were used from 1 to 10 mL to create ultrathin films of MWCNT on CA membrane with Millipore setup. As surfactant material (SDS) reduces the electrical conductivity of CNT nanoparticles, to achieve the maximum electrical conductivity in MWCNTs, it is better to deterge surfactant from nanoparticles. For this purpose, after the filtration process, the CNT layer on CA membrane was washed with deionized water.

Next step is applying PDMS on a CNT-coated CA membrane and curing the PDMS to obtain piezoresistive PDMS/CNT nanocomposite. In this step, a PDMS film with a thickness of 500 µm was applied on CA membrane (Figure 5(b)), and then a liquid PDMS was cured at 70 $\circ C$ for 2 h. After curing PDMS, CA membrane was peeled off from PDMS/MWCNT piezoresistive nanocomposite (Figure 5(c)).

Results and discussion

Electrical property

According to the traditional percolation theory,²⁷ the electrical conductivity of conductive composites can be predicted by

σ = σ_{0} (Φ - Φ_{c}) t; Φ > Φ_{c}

(1)

where

σ

is the electrical conductivity of FCFs,

Φ

is the volume fraction of the filler,

Φ_{c}

is the percolation threshold,

σ_{0}

is a parameter basically depending on the electrical conductivity of the filler and t is the critical exponent universally related to the dimensionality of the conductive network. According to the S-shaped conductance curve of nanocomposites (Figure 6), the transition from nonconductive to conductive state occurs at some critical volume fraction, known as percolation threshold.

Figure 5.

Schematic representation of experimental procedure.

Figure 6.

Schematic illustrations of the percolation curve of fillers in conductive FCFs.

After passing through the percolation threshold ( $Φ = Φ_{c}$ ), the dependence of the electrical conductivity of the nanocomposite on the filler reduces and eventually reaches a saturation point and adding more conductive nanoparticles will no longer improve the electrical conductivity of the nanocomposite.²⁸ The highest piezoresistive sensitivity is achieved when the volume fraction of nanotubes is near to the percolation threshold. Given the above description, in order to achieve the maximum sensitivity, it is necessary to find the percolation threshold and manufacture the sensing element with optimum weight percentage of conductive nanofillers.

According to Figure 7, sensing elements with the same size but different CNT contents are made using the manufacturing process illustrated in Figure 5. The sensing elements are hybrid films containing 0.7 to 7.2 mg of CNT nanofillers per square millimeter. Table 2 shows the values of initial resistance (R₀) and the PDMS/MWCNT nanocomposite layer thickness on pure PDMS substrate for different strain sensors made by 1 to 10 mL of CNT suspension. Figure 8 demonstrates the variation of nanocomposite layer thickness with CNT concentration for different strain sensors.

Figure 7.

(a–j) Piezoresistive PDMS/ MWCNT nanocomposites with different CNT content made by 1 to 10 mL of CNT suspension, respectively. (k) SEM image of cellulose acetate membrane with pore size of 220 nm. (l) Uniform distribution of MWCNTs on the CA membrane surface. (m) Nanocomposite surface. (n–p) Cross-section of (c), (e) and (i) piezoresistive films with the nanocomposite thickness of $850 nm$ , $1400 nm$ , and $2160 nm$ , respectively.

Figure 8.

Variation of nanocomposite layer thickness with CNT concentration for different strain sensors.

Table 2.

Initial resistance versus nanocomposite layer thickness of strain sensors made of 1 to 10 ml CNT suspension.

Suspension value (mL)	Initial resistance (kΩ)	Nanocomposite thickness (nm)	CNT concentration $(\frac{mg}{(m m^{2})})$
1	22,800	480	0.7
2	12,700	790	1.4
3	781	850	2.2
4	74.9	1210	2.9
5	39.6	1400	3.6
6	30.2	1450	4.3
7	21.8	1790	5
8	14.3	2075	5.8
9	13.4	2160	6.5
10	12.55	2540	7.2

Table 3.

Constant parameters of the electrical conductivity curve of strain sensors.

Parameter	Value
A	0.039769427
B	3.176950293
C	6.477276124
D	0.589179378

According to Figure 9, with increasing the CNT amount, the initial resistance of the strain sensors decreases exponentially. The decrease of the initial resistance is impressive in the strain sensors with the CNT concentration values lower than 2.9 $mg / m m^{2}$ .

Figure 9.

Initial resistance versus CNT concentration for strain sensors.

There is a transition from nonconductive to conductive state in the strain sensors made of 2 to 4 ml of CNT suspension. The suspension used in this study had a concentration of 0.1 mg/ml, and the surface area of all sensors was about 1385 $m m^{2}$ . It means that the nonconductive to conductive transition is achieved with 1.4 to 2.9 $mg / m m^{2}$ of MWCNTs concentration (see Table 2). According to Figure 10, this optimum range is the percolation range of the strain sensors made by vacuum filtration method and can be used for manufacturing flexible strain sensors with maximum sensitivity.

Figure 10.

Electrical conductivity versus CNT concentration for strain sensors.

Numerical fitting

In order to find the exact value of percolation threshold ( $φ = φ_{c}$ ) and predict the electrical conductivity of strain sensors in desired CNT concentration value, an equation was defined using Gompertz growth curve in the form of equation (2). This equation was used in curve fitting and was fitted to the experimental data

Conductivity = a \times 10^{(b \times 10^{(- c \times 10^{(- d \times x)})})}

(2)

According to equation (2), the electrical conductivity curve of the strain sensors was defined by determining the constant parameters a, b, c and d through nonlinear least-squares best fit.

The electrical conductivity curve of flexible strain sensors is achieved by replacing the constant parameters a, b, c and d in equation (2). The exact value of percolation threshold ( $φ = φ_{c}$ ) was determined by calculating the inflection point of electrical conductivity curve. For this purpose, the second derivation of the conductivity curve was calculated using numerical derivation method and the percolation value was determined as illustrated in Figure 11.

Figure 11.

Exact value of percolation threshold for PDMS/MWCNT flexible strain sensors.

Figure 12.

(a) SEM image of possible tunneling effect among neighboring CNTs (b) Variation of tunneling resistance among neighboring CNTs by applying tensile strain to nanocomposite.

According to Figure 11, in order to manufacture the flexible strain sensors with maximum sensitivity, it is suggested to adjust the CNT concentration of the strain sensor to the value of 1.99 $mg / m m^{2}$ . By determining the concentration of CNT suspension ( $mg / ml$ ) and also the surface area of CNT deposition on filtration membrane ( $m m^{2}$ ), it is possible to find the exact CNT suspension volume to use in the vacuum filtration method and manufacture a flexible strain sensor which is in the percolation threshold ( $φ = φ_{c}$ ) and has the maximum sensitivity.

Piezoresistive property

As for the working mechanisms in the piezoresistive nanocomposite strain sensors, from the accumulated knowledge until now, the piezoresistivity observed in this kind of strain sensors can be mainly attributed to the following three aspects; (a) significant variation of conductive networks formed by CNTs, e.g. loss of contact among CNTs. (b) Tunneling resistance change in neighboring CNTs due to the distance change. (c) piezoresistivity of CNTs themselves due to their deformation.²⁹ In the 3D resistor network of polymer nanocomposites, tunneling resistance has a major role in piezoresistivity. For instance, Fang et al.³⁰ enhanced conductivity of the Ag-NW/PI-PANI composite films by reducing the tunneling resistance between Ag-NWs. The tunneling resistance between two neighboring CNTs as shown in Figure 12, can be approximately estimated as follows³¹

R_{tunneling} = \frac{V}{AJ} = \frac{h^{2} d}{A e^{2} \sqrt{2 m λ}} exp (\frac{4 π d}{h} \sqrt{2 m λ})

(3)

where J is the tunneling current density (Figure 12), V is the electrical potential difference, e is the quantum of electricity, m is the mass of the electron, h is Planck's constant, d is the distance between CNTs, λ is the height of the barrier (for epoxy, 0.5 to 2.5 eV), and A is the cross-sectional area of the tunnel.³²

The strain sensor shown in Figure 7(d) was selected and its piezoresistive properties were evaluated. This specimen has total thickness of about $540 μ m$ in which $1.2 μ m$ of its thickness is an ultrathin layer of PDMS/MWCNT nanocomposite. The nanocomposite layer (see Figure 7(n) to (p)) is made of 4 ml CNT suspension and has a CNT concentration of 2.9 $mg / m m^{2}$ . According to Figure 10, this strain sensor is the upper limit of the suggested region for design strain sensors and has the minimum gauge factor ( $G F_{min}$ ) of the sensors in this region. By evaluation of the piezoresistive properties of this sensor, $G F_{min}$ was determined.

The strain versus normalized resistance changes of this specimen is shown in Figure 13. The most important parameter of the strain sensor to determine its sensitivity is the gauge factor (GF). This parameter can be calculated as follows

GF = \frac{Δ R / R_{0}}{Δ L / L_{0}} = \frac{Δ R}{R_{0} \times ɛ}

(4)

Figure 13.

Strain versus normalized resistance change of the strain sensor.

According to Figure 13, a fitted line has an average slope of 5 and so the average gauge factor of this sensing element is equal to five. We can conclude that the minimum gauge factor ( $G F_{min}$ ) of the strain sensors in the design region (illustrated in Figure 10) is about 5. Figure 14 shows the gauge factor variation of this sensor in different strains.

Figure 14.

Variation of gauge factor with strain for strain sensor containing 2.9 $mg / m m^{2}$ of CNT nanofillers.

Interaction between components

In addition, in order to determine the contribution of the main elements, two different specimens were selected and tested using energy-dispersive X-ray spectroscopy (EDS) and SEM scanning. EDS is an analytical technique used for the elemental analysis or chemical characterization of a sample which can show both the percentage and distribution of the main elements if accompanied by SEM scanning. EDS spectrum of the flexible strain sensors containing 2.2 and 6.5 mg/mm² MWCNT concentration is illustrated in Figure 15(a) and (b), respectively.

Figure 15.

EDS spectrum of the flexible strain sensors with (a) 2.2 and (b) 6.5 mg/mm² MWCNT concentration.

Moreover, the distribution of the main elements in the aforementioned specimens extracted by SEM scanning is presented in Figure 16(a) and 16(b), respectively.

Figure 16.

Distribution of the main elements in the flexible strain sensor containing (a) 2.2 mg/mm² and (b) 6.5 mg/mm² MWCNTs made by SEM scanning.

Moreover, the interaction between components was analyzed with FTIR method and the final results are shown in Figure 17.

Figure 17.

Results of FTIR analysis.

Conclusion

In this study, piezoresistive hybrid films contain an ultrathin layer of PDMS/MWCNT nanocomposite with different wt.% of MWCNTs which were made by vacuum filtration method and their electrical and piezoresistive properties were discussed. First, the initial resistance of each strain sensor was measured, and then the percolation curve and best region for design flexible strain sensors were defined. It was shown that the percolation threshold of the strain sensors is achieved with the CNT concentration of 1.4 to 2.9 $mg / m m^{2}$ . Afterward, the piezoresistive specimen with the CNT concentration of 2.9 $mg / m m^{2}$ and nanocomposite thickness of $1.2 μ m$ was selected and its piezoresistive properties were evaluated. Finally, the distribution of the main elements and the interaction between components for two strain sensors with different CNT concentration were discussed.

Footnotes

Acknowledgments

The authors would like to thank the Iran Polymer and Petrochemical Institute (IPPI) and the members of the Institute of Applied Hydrodynamics and marine technology of Iran University of Science and Technology (IUST) for their help in the work.

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was partially supported by the Iran Nanotechnology Initiative Council (INIC).

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