Synergism of binary carbon nanofibres and graphene nanoplates in improving sensitivity and stability of stretchable strain sensors

Composites Science and Technology 172 (2019) 7–16

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Synergism of binary carbon nanofibres and graphene nanoplates in improving sensitivity and stability of stretchable strain sensors

T

Fan Zhang, Shuying Wu, Shuhua Peng, Zhao Sha, Chun H. Wang∗ School of Mechanical and Manufacturing Engineering, University of New South Wales, Sydney, New South Wales, 2052, Australia

A R T I C LE I N FO

A B S T R A C T

Keywords: Hybrid carbon fillers Synergistic effect Sensitivity Stability Flexible strain sensors

Stretchable strain sensors with high sensitivity and good stability are crucial for wearable healthcare devices and tactile sensors for robots. Herein we present a new technique to synergistically improve sensors' sensitivity and cyclic stability by hybridising carbon nanofibers (CNFs) with graphene nanoplates (GNPs) within polydimethylsiloxane (PDMS) medium. The results reveal that, compared with equivalent sensors containing only CNFs or GNPs, the hybridised sensors show significantly better performance with a greater linear range up to ∼50% of strain and much-improved stability (less drift) under repeated loading, which is quantitatively reflected by the synergy ratio of linear range and drift rate. Increasing the concentration of hybrid carbon fillers can further increase sensors sensitivity. Therefore, the hybridisation of 1D and 2D nano-carbon materials offers a new route for increasing the sensitivity and cyclic stability of flexible strain sensors.

1. Introduction Flexible strain sensors capable of high strain (e.g. greater than 50%) have received keen interests prompted by their potential for wearable electronics demanded by health and patient care [1–5], physical treatment [6], sports performance monitoring [7,8], soft robotics [9,10], and artificial skin [11,12]. This new type of sensors overcomes the limitation of conventional strain sensors made of metal foils and semiconductors that do not go beyond strain of 2–5%. Critical to the functionality of flexible strain sensors are their sensitivity, stretchability, and stability under cyclic loading [2,13]. One promising method for manufacturing flexible strain sensors with high strain capability is to embed conductive fillers in an elastomer matrix [14], with considerable efforts being devoted to enhancing the key properties of this new type of strain sensors [15–21]. Li et al. [20] reported that sensors based on carbon nanotubes (CNTs)/PDMS sponges exhibited a gauge factor (GF) of 1.1 for strain ε < 15% and 15 for ε > 15%, respectively. Jeong et al. [18] fabricated strain sensors using the composite of fragmented graphene foam and PDMS, yielding gauge factors of 15–29 and a maximum sensing strain of 70%. Li et al. demonstrated that ultrathin graphene film-based strain sensors offered extremely high sensitivity with a gauge factor of 1037 at a strain of 2% [16], although the stretchability for such thin film-based sensors is reported to be less than 5%. A high gauge factor of 248 at a small strain of 5% was achieved in the aligned multi-walled carbon nanotubes (MWCNTS) in

∗

an ethylene-α-octene block copolymer (OBC) matrix [22]. To improve sensors' stretchability, Yan et al. [17] demonstrated that sensors made by embedding crumpled graphene and nanocellulose in an elastomer matrix attained a high stretchability up to 100% with a GF of 7.1. A carbonized plain-weave silk fabric was used to enhance both the sensitivity and stretchability of strain sensors, showing a GF of 9.6 within strain range of 250% and a GF of 37.5 within strain range of 250–500%, respectively [19]. A multi-dimensional strain sensor was also fabricated through the highly aligned CNT films on an elastic substrate, showing a sensitivity of 36.2 in the strain range of 0–150% and a sensitivity of 1198 in the strain range of 150–260% along one specific direction [23]. Recently, hybrid fillers of single scale [24,25] or multiple scales [26–30] have been reported to improve the stability and sensitivity of flexible strain sensors due to their synergistic effect on properties. Cheng et al. [24] reported a wearable sensor based on percolating network of gold nanowires and silver nanowires, achieving a GF of ∼236 at a low strain (< 5%) but a lower gauge factor of ∼5.0 at a strain of 70%. Ke et al. [31] reported that melt mixing hybrid fillers, including carbon nanotubes (CNTs, 0.5–1 wt %) and carbon black (CB, 0.5–4 wt %) with poly(vinylidene fluoride) can tune the piezoresistivity of nanocomposites. The effects of the total volume fraction and the ratio of CNT to CB on the piezoresistivity of nanocomposites were reported in Ref. [32]. The results showed that CB-CNT/PDMS nanocomposite containing 2 wt% of CB-CNTs (mCNTs/mCB = 1:2) could provide a high GF of 13.1 in the strain range of 150%–300%. Fu's group reported that

Corresponding author. E-mail address: [email protected] (C.H. Wang).

https://doi.org/10.1016/j.compscitech.2018.12.031 Received 4 November 2018; Received in revised form 27 December 2018; Accepted 31 December 2018 Available online 04 January 2019 0266-3538/ © 2019 Elsevier Ltd. All rights reserved.

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Fig. 1. Schematic illustration of the fabrication procedures for sandwich strain sensors.

used in the study were purchased from XG Science. The average thickness and diameter are approximately 6–8 nm and 25 μm, respectively. The Sylgard 184 silicone kit including PDMS base and curing agent was supplied by Dow Corning Incorporation.

[33] combining multi-walled carbon nanotubes (MWCNTs) with CB could reduce the entanglement of conductive network, which improves the piezoresistive sensitivity. Despite these recent research work, it remains unclear how the hybridisation of conductive fillers affects the strain sensing behaviour of flexible strain sensors, particularly sensitivity and cyclic stability. Recent research has highlighted a major issue with the stability of flexible sensors under cyclic or repeated loading [32,34]. Zheng et al. [32] reported that a highly stretchable sensor based on CB-CNT/PDMS composites exhibited a substantial drift under long-term cyclic loading. Jin et al. [34] reported that the electrical resistance of CNT thin films exhibited a hysteretic dependence on strain under cyclic loading. Hence, the drift under cyclic loading is presently a major problem for stretchable strain sensors that calls for new solutions. In our recent work [35] we reported the use of dual-scale network of carbon nanofibres (CNFs) and short carbon fibres within PDMS to achieve greater piezoresistive sensitivity than the CNF-reinforced composite sensor while retaining similar stretchability. However, this type of sensors also displayed a substantial drift in the electrical resistance under long-term cyclic loading. To simultaneously improve the sensitivity and stability of flexible sensors, we propose herein a hybridisation technique to form a synergistic conductive network of one-dimensional (1D) and two-dimensional (2D) nanocarbon materials, i.e., CNFs and GNPs. These two nanomaterials have been chosen for their lower cost than other carbon materials of similar scale and dimension. For example, CNFs are much cheaper (up to 500 times) to manufacture than CNTs [36]. A facile fabrication combing probe sonication and spin-coating were used to prepare flexible strain sensors by sandwiching a sensing layer between two layers of PDMS. Three different concentrations of conductive fillers were investigated, with the weight ratios of CNFs to GNPs being varied to examine their effects on the sensitivity and stability under monotonic and cyclic tension. Finally, demonstrations of these flexibles strain sensors to monitor human joints motion were made by attaching them directly on human skins.

2.2. Sample fabrication CNFs, GNPs, and hybrid fillers (CNFs and GNPs) were dispersed in ethanol with the assistance of a surfactant (polyvinylpyrrolidone) to form dispersions with various concentrations of 5 mg/ml, 10 mg/mL and 15 mg/ml using a Hielscher UP200S ultrasonic homogenizer. The mass ratio between fillers and surfactant is 1:1. The ultrasonication process was carried out for 3 h. For the case of hybrid fillers, the ratios of CNFs to GNPs were 1:1, 1:3, and 1:5, respectively. Flexible PDMS substrates were fabricated by mixing Sylgard 184 silicone base and curing agent at a weight ratio of 10:1. The thickness of the PDMS substrates was 1.0 mm. The PDMS substrates were then surface treated using O2 plasma, followed by spin-coating of the prepared ethanol dispersions containing CNFs, GNPs, or hybrid fillers to create film sensors. After annealed at 90 °C for 1 h, copper wires were attached on the surfaces of the film sensors (on carbon nanomaterials side) using a conductive silver paste. Then, a liquid PDMS prepolymer mixture was poured onto the top of the carbon nanomaterials to get a PDMS layer of the same thickness to form a sandwich structure. The schematic illustrations for the detailed fabrication procedures for the sandwich strain sensors are shown in Fig. 1. Bi-layer sensors without the additional layer of PDMS were also prepared with all other parameters being kept the same for the purposes of comparing sensor performance and mechanisms. 2.3. Characterisation The microstructures of the film sensors were characterised by a scanning electron microscope (FEI Nova SEM450). In situ tension tests were conducted with another scanning electron microscope (FEI Nova SEM230) equipped with an in situ mechanical loading stage. In both cases, the samples were first sputter-coated with platinum for 60 s. To characterize their pizeoresistivity, the sandwich strain sensors were then subjected to both quasi-static tension and repeated stretching/releasing cycles using a tensile testing machine (Instron Model 3369) under displacement control. The strain rates for all tests, except the long-term stability tests (1000 cycles), were kept at 0.02 s−1. For the

2. Materials and experimental procedures 2.1. Materials Carbon nanofibers (Pyrograf-III, grade PR-24-XT-HHT) were obtained from Applied Science Incorporation. They have a diameter of 70–200 nm and a length of 50–200 μm. The graphene nanoplatelets 8

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Fig. 2. Microstructure of the sandwich sensors. (a) a schematic diagram of the top view orientation; SEM images of (b) CNFs, (c) GNPs, and (d) hybrid carbon fillers of CNFs and GNPs (ratio of 1:1) deposited on a PDMS substrate; (e) cross-section of a sandwich strain sensor; (f) SEM image of a sensor containing 10 mg/ml CNF +GNP); (g) Zoom-in image of (f).

long-term repeated stretching stability tests, the strain rate was kept at a higher rate of 0.4 s−1. The electrical resistances signals were measured by a digital multimeter (34465A, Keysight Technologies).

impregnated the pores, as shown in Fig. 2 (f) and (g). The sensor layer is now well sandwiched between two PDMS layers, forming a structurally robust sandwich structure.

3. Results and discussion

3.2. Comparison of the pizeoresistivity for three types of strain sensors

3.1. Microstructure

To evaluate the strain-sensing capability of the three types of sandwich strain sensors, the samples were subjected to a quasi-static tensile loading. The relationships between the relative change in the resistance, ΔR/R0, and the applied strain, are shown in Fig. 3 (a) and (b). The relative resistance change is approximately proportional to the applied strain up to a strain of 10% for CNFs and 15% for GNPs, respectively, at the same filler concentration. However, at higher strains the response becomes nonlinear. Interestingly, sensors with CNFs and GNPs hybrid showed much improved linearity range up to a strain of ∼50%, as shown in Fig. 3 (c), compared with the equivalent sensors

Fig. 2 shows the surface and cross-sectional views of the film sensors. It can be seen from Fig. 2 (b)-(d) that the conductive fillers of CNFs, GNPs, and their hybrids were uniformly distributed with no obvious agglomeration. A slight hoop preferential orientation was observed for the CNFs or GNPs conductive fillers spin-coated on the top of PDMS substrate, which is attributed to the shear flow of the liquid during spin-coating. When another PDMS layer was casted on the top of sensor layer, the liquid PDMS infused into the filler network and 9

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Fig. 3. Comparisons of the performance of sandwich sensors under (monotonic) quasi-static loading: (a) CNFs based sensors; (b) GNPs based sensors; (c) hybrid CNFs + GNPs based sensors, at concentration of 10 mg/ml. (d) Variation of the gauge factors with strain for CNFs based sensors, GNPs based sensors, and hybrid CNFs and GNPs sensors.

further discussed in Section 3.5 and indicated by the dashed lines in Fig. 9(c) and (d). This hybrid network is, thereforeable to maintain a near constant sensitivity than sensors made of either nanomaterial separately. As the CNF/PDMS strain sensors were stretched beyond 30% strain, the CNF conductive network starts to experience damage or destruction, increasing the resistance. Therefore, hybridising CNFs and GNPs yielded a synergistic effect and improved performance. Fig. 4 shows the variation of resistance under (monotonic) quasistatic loading for strain sensors containing different concentrations of CNFs and GNPs (the ratio of CNFs to GNPs ranges from 1:0 to 0:1). The respective gauge factors can be calculated by fitting the data within the linear range. The linearity of the sensor response to strain depends also on whether the engineering strain or the true strain is used. When the experimental results are plotted in terms of the true strain, ln(L/L0), and the true resistance change, ln(R/R0), better linearity can be observed (results pertinent to engineering strain are presented in Fig. S1). As shown in Fig. 4 (a)-(d), the gauge factor increases with the concentration of GNPs, as the ratio of GNPs to CNFs varying from 0:1 to 1:0. These results suggest that GNPs play a key role in improving the gauge factor of CNF-based sensors. It is noted that the gauge factor depends on the concentration of the conductive fillers in the composites due to changes in the density of conductive pathways [18,37]. However, the hybrid sensor with CNFs:GNPs of 1:3 and a total concentration of 15 mg/ml shows a significantly higher gauge factor of ∼5.1, compared to the other concentrations investigated in this study. Fig. 4 (e) shows the linearity of strain sensors as a function of the ratio of CNFs to GNPs at total carbon filler concentrations of 5 mg/ml, 10 mg/ml or 15 mg/ ml. The significantly enhanced linearity is achieved for hybrid CNF +GNP sensor compared with equivalent sensors containing CNFs or GNPs only, giving a range of linearity reaching approximately 50% strain for sensors containing a total carbon filler concentration of 10 mg/ml or 15 mg/ml. To further investigate the synergistic effect of hybridisation on the

containing either CNFs or GNPs alone. Therefore, the hybridisation of CNFs (1D) and GNPs (2D) nano-carbon fillers produces a broaden linear sensing range up to a large strain (∼50%), which might be attributed to the enhanced conductive network formed by the dual-scale fillers of CNFs (good stretchability) and GNPs compared with that of either CNFs or GNPs. The sensitivity of the sensor is determined by the gauge factor defined by the following expression,

GF =

ΔR εR 0

(1)

where ΔR/R0 is the relative resistance change, and ε is the applied strain. The sandwich strain sensors with CNFs only show a gauge factor value of ∼1.0 at 10% strain, while a higher sensitivity of ∼13.5 is achieved by the GNPs based strain sensors. By adding GNPs to CNFs, a sensitivity of 3.5, higher than that pertinent to equivalent sensors containing CNFs only. This can be attributed to the synergistic effect of CNFs and GNPs owing to the improved electrical network by the GNPs. The results in Fig. 3 show that the gauge factor for the hybrid CNFs +GNPs sensors remained approximately constant up to a strain of 50%, indicating greater linearity. However, sensors containing only CNFs or GNPs show linearity range less than ∼10% and ∼15%, respectively. Here the linearity range of sensors is defined as the strain at which the gauge factor, ki = (ΔR/ R 0)i / εi , would deviate from its initial value by 10%, as shown in Fig. 3 (d). At higher strains, CNFs based sensors showed a rapidly decreasing sensitivity whereas GNPs based sensors showed an increased sensitivity. This reason for this opposing behaviour is that the GNPs can undergo relative shear or sliding deformation, with the nanoplatelets moving away from each other and forming microcracks as shown in Fig. 9. These microcracks cause contact resistance to increase rapidly. Combining the two nano-fillers creates a hybrid network, with nanofibers bridging graphene nanoplatelets, as 10

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Fig. 4. Plots of ln(R/R0) versus ln(L/L0) for sensors made of CNFs, GNPs, or hybrid of CNFs and GNPs at various concentrations of (a) 5 mg/ml; (b) 10 mg/ml; (c) 15 mg/ml. (d) Gauge factors of strain sensors as a function of ratio of CNFs to (CNFs+GNPs) at the total carbon filler concentrations of 5 mg/ml, 10 mg/ml or 15 mg/ ml (The resistance value of 5 mg/ml GNPs/PDMS is beyond the measurement capacity of the digital multimeter.) (e) Range of linearity as a function of the CNFs to (CNFS+GNPs) ratio for a total filler concentration of 5 mg/ml, 10 mg/ml, 15 mg/ml, respectively.

the equivalent sensors containing either CNFs or GNPs only, the hybrid sensor with the ratio CNF:(CNF+GNP) of 1:4 and a total concentration of 15 mg/ml shows a significantly greater range of linearity up to a strain of 47%. Moreover, the gauge factor of this hybrid strain sensor is approximately 5.1, which is slightly lower than the predicted value of 5.83 using an additive model. To quantify the level of synergy between CNFs and GNPs in improving the linearity range, a synergy ratio is defined below

Table 1 Synergistic effect behaviour of 15 mg/ml sandwich strain sensor. Types of strain sensors

Linearity

Gauge factor

15 mg/ml CNFs/PDMS 15 mg/ml GNPs/PDMS 15 mg/ml CNFs+3GNPs/PDMS

6.2% 24.3% 47.0% Predicted linearity by additive model 19.8%

2.20 7.04 5.13 Predicted gauge factor by additive model 5.83

15 mg/ml CNFs+3GNPs/PDMS

LRCNFs + GNPs Synergy ratio = ⎛ − 1⎞ × 100% ⎝ γ∗LRCNFs + (1 − γ) ∗ LRGNPs ⎠ ⎜

range of linearity and gauge factor, detailed characterizations were carried out on sensors made of 15 mg/ml CNFs+3GNPs (CNFs:GNPs = 1:3), which has been found to give the highest gauge factor among the sensors examined in this work. To illustrate the significance of the results presented in this paper, a comparison is shown in Fig. S2 of the sensitivity and stretchability of the new sensors with those reported in the literature. As indicated in Table 1, compared to

⎟

(2)

where γ stands for the ratio of CNFs to GNPs at the total carbon filler concentrations. LR denotes linearity. According to this definition, the synergy ratios for the linearity range and gauge factor are 137.4% and −12%, respectively. It is clear that hybridising CNFs and GNPs has achieved a significant synergistic effect on the linearity range but no synergy can be observed in the effect on the gauge factor. 11

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Fig. 5. Cyclic strain senor behaviour of 15 mg/ml GNPs, 15 mg/ml CNFs+GNPs, 15 mg/ml CNFs+3GNPs, 15 mg/ml CNFs+5GNPs and 15 mg/ml GNPs sandwich sensors. The maximum strain increases from 10%, 20%, 30%, 40% and 50% for (a)–(e). The normalised kN / k1 versus cycle number N to show the drift behaviour of sensors for (d) 15 mg/ml CNFs based sensors for (f) 15 mg/ml GNPs based sensors; (g) 15 mg/ml CNFs+GNPs sandwich sensors; (h) 15 mg/ml CNFs+3GNPs sandwich sensors; (i) 15 mg/ml CNFs+5GNPs sandwich sensors; (j) 15 mg/ml CNFs based sensors. The bold fitting lines based on the f (N ) formula are also shown in (f)–(j). The fitting parameters for both (k) a and (l) b as a function of γ (ratio of CNFs to (CNFs+GNPs)) at the total carbon filler concentrations of 15 mg/ml.

attributed to a gradual degradation of the conductive network. The normalised gauge factor, kN / k1 versus the number of cycles N (from 1 to 10 for each loading block), as shown in Fig. 5 (f)-(j), can be correlated using the following relationship,

3.3. Cyclic loading To evaluate its strain-sensing performance and durability, the CNFs, GNPs and hybrid of CNFs and GNPs strain sensors, with the ratio CNFs to (CNFs+GNPs) ranging from 0 to 1, were subjected to a sequence of block loadings of increasing amplitude. Each block consisted of ten cycles, and the maximum strains ramped up in 10% increment to a peak strain of 50%. The results are presented in Fig. 5 (a)-(e). It is seen that the CNF-based sensors showed a significant downward drift under each sequence of block loading. The maximum resistance change ΔR/R0 shows a slight downward drift, with the irreversible resistance change also gradually increasing with the number of cycles, which can be

f (N ) =

kN = a + (1−a) e−b (N − 1) k1

(3)

where k1 denotes the gauge factor at the first cycle of a loading block. The fitting parameter b in the above-mentioned formula indicates the rate of drift of the strain sensors under repeated stretching. The values of b for the various hybrid ratios and maximum strain levels are presented in Fig. 5 (l). A significant drift in terms of large kN / k1 value is 12

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To quantitatively illustrate the sensor drift behaviour, the synergy ratio of drift rate based on the fitting parameter b at the maximum strain of 20% is calculated using the following formula

bCNFs + GNPs Synergy ratio = ⎛ − 1⎞ × 100% γ b + (1 − γ) bGNPs CNFs ⎝ ⎠ ⎜

⎟

(4)

where b stands for the fitting parameter in Fig. 5 (l), and γ stands for the ratio of CNFs to (CNFs+GNPs) at the total carbon filler concentrations of 15 mg/ml. Fig. 6 shows the significantly reduced b value of the hybrid strain sensors for a range of hybridisation ratio γ ranging from 1/6 to 1/2 , confirming that a greatly reduced drift has been achieved for the hybrid strain sensors when compared equivalent sensors containing only CNFs or GNPs, especially for the hybrid strain sensors of CNFs+3GNPs (γ= 1/4 ). Furthermore, the response and relaxation time of this optimal hybrid of CNFs and GNPs strain sensors (15 mg/ml CNFs+3GNPs) were also measured and are shown in Fig. S3. From these results, the response time and relaxation time were estimated to be about 0.132 s and 2.489 s, respectively.

Fig. 6. Cyclic performance as a function of 1000 loading-unloading cycles for 15 mg/ml CNFs+3GNPs strain sensor (with a cyclic maximum applied strain of 50% and a minimum applied strain of 0%).

3.4. Mechanics of composite sensors To further illustrate the synergistic effect on the mechanical properties, such as stiffness and stretchability, a detailed analysis was carried out on the change of modulus and strain in composite sensors by comparing the hybrid strain sensors with equivalent sensors containing only CNFs or GNPs. When both CNFs and GNPs were used in hybrid strain sensors, the synergy ratio can be calculated based on the following equation:

ECNFs + GNPs Synergy ratio = ⎛ − 1⎞ × 100% E γ + (1 − γ) EGNPs CNFs ⎝ ⎠ ⎜

⎟

(5)

Therefore, the synergy ratio of modulus can be used to assess the synergistic effect of stiffness. For the ductility demonstrated in strain sensors, the capable strain (ductility) displayed in the hybrid strain sensors was also evaluated and compared with equivalent sensors containing either CNFs or GNPs. The synergy ratio for stretchability can be assessed by

Fig. 7. Synergy ratio as a function ratio of CNFs to (CNFs+GNPs) at the total carbon filler concentrations of 15 mg/ml for linear range LR, fitting parameter b based on the maximum strain of 20% (to illustrate the drift rate), modulus E and ductility εf.

εf (CNFs + GNPs) Synergy ratio = ⎜⎛ − 1⎟⎞ × 100% ε γ ⎝ f (CNFs) + (1 − γ) εf (GNPs) ⎠

(6)

where εf stands for the ductility of strain sensors with CNFs or GNPs or hybrid; γ stands for the ratio of CNFs to (CNFs+GNPs) at the total carbon filler concentrations. Fig. 7 shows the synergy ratio as a function of the ratio of CNFs to (CNFs+GNPs) at the total carbon filler concentrations of 15 mg/ml for linear range LR, drift rate parameter b, modulus E, and ductility εf. The hybrid sensors demonstrated the significant synergistic effects in both increasing the linearity range (LR) and reducing cyclic drift rate (fitting parameter b), achieving substantially enhanced linearity range and significantly lower drift rate when compared with equivalent sensors containing only CNFs or GNPs. However, little synergy has been observed in both modulus and ductility.

observed in both 15 mg/ml CNFs and 15 mg/ml GNPs sensors, as shown in Fig. 5 (f) and (j). By comparison, the hybrid CNFs and GNPs strain sensors (15 mg/ml CNFs+5GNPs, 15 mg/ml CNFs+3GNPs, 15 mg/ml CNFs+GNPs) exhibit a largely improved stability with smaller kN / k1 value, as can be seen in Fig. 5 (g)-(i). In particular, Fig. 5 (k) and (l) show that hybrid sensors displayed significantly reduced drift when compared with equivalent sensors containing only CNFs or GNPs, especially for the 15 mg/ml CNFs+3GNPs (γ= 1/4 ) hybrid strain sensors. Therefore, hybridisation has produced a significant synergistic effect in reducing the drift of strain sensors under cyclic loading. To demonstrate the long-term durability of the hybrid sensors, the sensors were subjected to 1000 cycles at a maximum strain of 50% and a minimum strain of 0%, as shown in Fig. 6. A slight drop in gauge factor is observed during the initial several cycles, followed by a stable performance during the ensuring long-term cycles. These results confirm that the hybrid carbon fillers of CNFs and GNPs form a very stable, robust conductive network after the initial loading. The higher stability of hybrid strain sensors was quantitatively analysed by the synergy ratio of drift rate compared with equivalent sensors containing only CNFs or GNPs. Therefore, the strain sensors exhibit a remarkably stable performance and durability over many repeated cycling of peak strain of 50%, making it a very promising sensor for practical applications as wearable electronics.

3.5. Piezoresistivity mechanism To reveal the dominant strain-sensing mechanism of the present hybrid sensors, in situ tension tests were carried out on the sandwich sensor made of 15 mg/ml (CNFs+3GNPs)/PDMS. Fig. 8 shows the in situ SEM images of the strain sensor under different strains of 0%, 10%, 30% and 50%. As shown in the selected areas in Fig. 8 (a)-(d), no obvious change can be observed in the sandwich strain sensor as the strain was increased from 0% to 10%, while some microcracks and the initial debonding started to appear between the PDMS and the filler when the strain was increased from 10% to 50%, as indicated by the red arrows in Fig. 8 (b)-(d). When the strain was removed, crack opening decreased 13

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Fig. 8. In situ tension test of the cross-sectional views of the sandwich strain sensor based on 15 mg/ml CNFs+3GNPs. (a)–(e) SEM images of the strain sensors stretched from 0% to 50%; (f) a schematic diagram to illustrate the working mechanism of strain sensors.

Fig. 9. In situ tension test of the top views of the bi-layer-structured 15 mg/ml (CNFs+3GNPs)/PDMS. (a)–(e) SEM images of the strain sensors stretched from 0% to 50% and then returned to 0%.

mechanism of microcracking of the strain sensors under small and large strain. To further verify the observed phenomena responsible for the cyclic response of the sandwich strain sensors, repeated tests and measurements were carried out, revealing the formation and growth of

(Fig. 8 (e)) with only a very slight gap remaining visible between the crack surfaces after returning to 0% strain, in comparison with the initial state of zero strain (Fig. 8 (a)). However, the unrecoverable debonding between the fillers and PDMS matrix remained, as shown in Fig. 8 (e). The schematic diagrams in Fig. 8 (f) illustrate the dominant 14

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Fig. 10. Applications of the sandwich strain sensors (15 mg/ml CNFs+3GNPs) to detect human motions: (a) response of the strain sensors from finger bending from 0° to 90°. (Inset: photograph images of fingers motion); (b) response to motion of wrist bending from 0° to 90° (Inset: photograph images of wrist motion); (c) response to motion of frowning. (Inset: photograph images of frowning motion).

strain. The sensitivity or gauge factor of the sensors has been found to increase with the concentration of hybrid carbon fillers and the relative ratio of GNPs to CNFs, with the highest sensitivity occurring at 15 mg/ ml concentration and a CNFs to (CNFs+GNPs) ratio of 1:4 (the hybrid being denoted as CNFs+3GNPs). This improved performance of hybrid carbon filler sensors is attributed to the synergistic effect of CNFs and GNPs in creating a stable conductive network. The flexible strain sensors based on hybrid carbon fillers demonstrate much better stability (less drift) under long-term cyclic stretching. The synergy ratios of linearity range and drift have been quantitatively analysed to confirm the synergism of hybrid CNFs and GNPs in increasing the linear range and reducing the drift rate of strain sensors. A demonstration of the application of the strain sensors developed in this work has been presented to illustrate the stable response of the sensors in characterising the large strain deformation of human joints. The newly created sandwich strain sensor based on hybrid nanocarbon materials provides significant potential as wearable sensors and electronic skins.

microcracks with an increasing strain from 10% to 50% (see Fig. S4). In situ tension tests were also conducted on bi-layer sensors (15 mg/ml (CNFs+3GNPs)/PDMS without the top capping PDMS layer) to examine the consequence of leaving the sensor layer exposed. The in-situ SEM images are presented in Fig. 9 (a)- (d). As shown by the red arrows, the introduction of CNF fillers bridged the gap between GNPs to enhance the conductive network. Upon stretching under small strain, the conductive network tends to undergo some disruption and reconstruction. However, when the strain was increased from 10% to 50%, a significant opening of the cracks was observed in Fig. 9 (d) and (e). These microstructural changes are responsible for the observed changes in the electrical conductivity of the sensors with the applied strain. 3.6. Potential applications The sandwich strain sensors offer great potential as a skin-mountable sensor for detecting human motion characterized by very large strains (the large strain can reach over 50%). The newly developed strain sensors in this work were attached to different human joints as shown in Fig. 10. Finger bending, wrist bending and even the eyebrow frowning (Fig. 10) can be detected by measuring the electrical resistance. The stable results confirm that the sandwich strain sensors made of hybrid nanofillers are a very promising wearable electronic device.

Acknowledgements F. Zhang would like to appreciate the PhD scholarship support from the New South Wales University. S Wu would like to acknowledge the financial support from Australian Research Council (ARC) via Discovery Early Career Researcher Award (DE170100284). The authors also appreciate the assistance from Dr. Eldad Ben-Ishay and Mr. Seetharam Mahadevan for equipment training.

4. Conclusion A highly flexible sandwich strain sensor with improved sensitivity and stability has been fabricated by a novel method of hybridising carbon nanofibers (CNFs) and graphene nanoplates (GNPs) in polydimethylsiloxane (PDMS). This hybridisation of 1D and 2D nano-carbon fillers demonstrates a largely improved linearity range up to ∼50% of

Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.compscitech.2018.12.031. 15

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