An Extremely Inexpensive, Simple, and Flexible Carbon Fiber

Mar 7, 2019 - ... artificial intelligence-based hysteresis modeling, long short-term memory recurrent neural network, to describe the hysteresis nonli...
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An Extremely Inexpensive, Simple, and Flexible Carbon Fiber Electrode for Tunable Elastomeric Piezo-Resistive Sensors and Devices Realized by LSTM RNN Min-Young Cho,† Jeong Heon Lee,‡ Seong-Hoon Kim,‡ Ji Sik Kim,§ and Suman Timilsina*,§ †

Department of Automotive Engineering, ‡Department of Software, §School of Nano & Advanced Materials Engineering, Kyungpook National University, Kyeongbuk 742-711, Republic of Korea

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ABSTRACT: Here, we describe the utility of a carbon fiber (CF) electrode that is inexpensive, simple, and flexible and can be embedded with elastomeric nanocomposite piezo-resistive sensors fabricated from silicone rubber (Ecoflex) blended with carbon nanotubes (CNTs) and various wt % of silicone thinner to tune the sensitivity and softness range. The performance of the CF electrode was evaluated on the basis of piezo-resistive responses from the sensors subjected to dynamic sinusoidal compressive strains at different levels and frequencies. The responses were positive-pressure effects with rate-dependent asymmetric nonlinear hysteresis characteristics. Developing a mathematical model to describe the rate-dependent asymmetric nonlinear hysteresis behavior is technically impossible; therefore, we employed artificial intelligence-based hysteresis modeling, long short-term memory recurrent neural network, to describe the hysteresis nonlinearity. The debonding strength of the CF electrode was determined in the pull-off testing and was found to be much higher than that of a copper wire electrode. The debonding mechanism was further elucidated via an in situ resistance profile. The importance of a robust conductive interface between a CF electrode and a nanocomposite was experimentally demonstrated. It was found that the inherent piezo-resistance of the CF was negligible compared with the piezo-resistance of the sensor; therefore, the signals from the sensor were free of interference. We believe CF-embedded tunable piezo-resistive sensors could be used in biomedical devices, artificial e-skins, robotic touch applications, and flexible keyboards where the required stretchability of the electrode can be introduced via an appropriate geometrical design. KEYWORDS: tunable piezo-resistive sensor, carbon fiber electrode, positive-pressure-effect, artificial intelligence, nonlinear hysteresis, conductive interface

1. INTRODUCTION In recent years, the number of studies related to pressure and strain sensors fabricated from flexible elastomeric polymers by incorporating functional nanofillers has risen remarkably.1−3 These types of sensors are in great demand for use in biomedical devices,4 in artificial skin,5 and in robotic touch applications,6 where an externally applied force is converted into readable electrical signals that allow them to function. On the basis of their functional mechanism, these sensors can be classified as piezo-resistive,7 piezo-electric,8 tribo-electric,9 or capacitive sensing.10 Considering the cost of production, ease of fabrication, large measurement range, and simple assessment of output data, the advantages of piezo-resistive sensors outweigh those offered by other types; hence, they are being widely investigated. Most studies have focused on increasing the sensitivity, linearity, and mass production and widen the field of applications.1−3 However, the study of electrodes that could bridge the piezo-resistive nanocomposite and data acquisition (DAQ) systems has not been given substantial priority compared with efforts allocated to the investigation of the © XXXX American Chemical Society

piezo-resistive material itself. In many reports, metallic electrodes such as copper are either connected to nanocomposites using a silver paste or they are inserted directly into a nanocomposite.1−3,11−15 Because flexible piezo-resistive sensors are built to perform under large strains and dynamic loading rates, the interfaces of functional nanocomposites and electrodes face high levels of repetitive strain. Therefore, to withstand such conditions, the electrodes must be flexible. Moreover, interfacial conductivity and bond strength must be sufficient to enhance the performance and durability of the sensor. Under such situations, the rigidity of a metallic wire coupled with low levels of interface bonding with an elastomer attributed to high levels of surface tension of the metallic wire make them unsuitable for use in pressure and strain sensors.16 We faced similar problems in past studies where we used copper electrodes to bridge from a flexible smart e-skin or a flexible keyboard to a neural network to Received: January 8, 2019 Accepted: March 7, 2019 Published: March 7, 2019 A

DOI: 10.1021/acsami.9b00464 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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CF electrodes were inserted before solidification. To observe the CF performance in terms of piezo-resistive responses, sensors with different ranges of tunable sensitivity and tunable softness were attained through the incorporation of silicone thinner (ST) at various wt % and subjected to dynamic sinusoidal compressive strain at different levels and frequencies. An artificial intelligence (AI)-based hysteresis model was employed to describe the nonlinear hysteresis behavior of the response. A pull-off test was undertaken to elucidate the debonding mechanism at the interface of the CF and the nanocomposite via an in situ resistance profile. Eventually, to reveal the importance of a robust conductive interface, the origins of both positive and negative piezo-resistances were experimentally demonstrated. Bulky sensors were devised to produce a large displacement to determine the compatibility of the CF, and only compressive strains were applied as strain and pressure sensors mostly face strain of a compressive nature. Ecoflex was selected in this work rather than polydimethylsiloxane (PDMS) because Ecoflex is ultrasoft with a Young’s modulus of 125 kPa and exhibits mechanical compliance as high as that of the human skin (25− 220 kPa). Therefore, Ecoflex is a better matrix for pressure and strain sensors where human/machine interaction is required. Furthermore, Ecoflex shows water resistivity that is higher than that of PDMS, which makes Ecoflex environmentally stable and allows it to deliver a lifelong performance.

identify the intended position or a letter based on a piezoresistive signal.14,15 The use of wire limited the flexibility, durability, and conductivity. Because of poor conductivity, electrical interface signals were affected by noise that caused problems with the neural networks identifying a specific position or a letter. To overcome the drawbacks of metallic electrodes, only a few studies have focused on the possible development of flexible and stretchable electrodes composed of elastomers blended with homogeneous mixtures of conductive fillers [carbon nanotubes (CNTs), graphene, metal particles, and metal oxide particles]17 as well as elastomers coated with conductive nanofilms.18,19 However, problems such as nonlinear conductivity under strain, effects of temperature, a complicated fabrication process, high susceptibility to corrosion, and production cost are the challenges that must be solved.20−26 To address such issues, an intrinsically flexible and stretchable conductive polymer can be an important substitutional electrode.27,28 One of the best conductive polymers which has attracted the attention of the researchers is poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS). By incorporating ionic additives and a plasticizer, the stretchability and conductivity have been remarkably enhanced, and hence, PEDOT:PSS could be the next generation of flexible, stretchable, and transparent electrodes.29 However, different types of electrodes are preferred for different applications. Considering the basic constituents and the highly deformable physical nature of elastomeric piezo-resistive pressure and strain sensors, the best electrode could be the one which would provide a large total surface area to make contact with the nanoconductive fillers and at the same time offer sufficient flexibility. Therefore, in the present study, we proposed an extremely inexpensive, simple, and flexible carbon fiber (CF) as an electrode, which is considered to have a large total surface area. CF has excellent properties that include high stiffness, chemical stability, high resistance to pressure and humidity, long-term durability, negligible temperature sensitivity, and low density.30 Consequently, CF is widely used in various structural applications of automotive, aerospace, sports, and military equipment, medical instruments, and in power plants to enhance the mechanical strength and durability while reducing the structural weight.31,32 In addition to the previously mentioned properties, there is another important property that CF possesseselectrical conductivity.33 A limited number of applications considering the electrical conductivity of CF have been reported for structural health monitoring: structural damage detection,34 protection against lightning strikes,35 detection of fracture initiation,36 and self-diagnosis to prevent catastrophic disaster.37 In addition to the application for structural health monitoring, CF has been used as a base electrode for electrophysiological, electrochemical, biosensor, and biofuel cell applications.38−40 The utility of CF as an electrode for supercapacitors and rechargeable batteries has also been reported.41,42 However, the complementary nature of the relationship between the CF and tunable elastomeric nanocomposite piezo-resistive sensors has never been reported without technical difficulties. In this work, we describe the compatibility of an inexpensive, simple, and flexible CF electrode with tunable elastomeric nanocomposite piezo-resistive sensors. An elastomeric nanocomposite piezo-resistive sensor was fabricated from a highly flexible, superstretchable, and ultrasoft silicone rubber that is known as Ecoflex, which was then blended with CNTs, and two

2. EXPERIMENTAL PROCEDURE The sensor was developed using an acrylic molder. To form the molder, a 3D printer produced two vertical symmetrical halves of a cylindrical container that had a hollow 20 mm inner diameter with four 1.5 mm × 1 mm (height × width) rectangular notches at the edges, as shown in Figure S1. Then, the symmetrical parts were joined using a soft glue. After the glue dried, a CF (Toray Carbon Fibers America, Inc.; filament diameter = 7 μm and tow size = 3 K) as an electrode was inserted through each hole. Finally, the bottom of the container was attached to an acrylic plate using a soft glue. The acrylic was chosen owing to its nonsticky nature with Ecoflex. To construct an Ecoflex and CNT nanocomposite, 0.2 wt % of CNTs (Carbon Nano-Material Technology Co. Korea; multilayer wall, diameter = 5−20 nm, length = ∼10 μm, and purity = 90 wt %) was added to part A of Ecoflex (Ecoflex 00-30, Smooth-On, Inc.) in a plastic container. To homogeneously mix the CNT and avoid agglomeration, a few ZrO2 balls, 10 mm in diameter, were added to the container, which was then transferred to a planetary mixer for 2 h. Subsequently, an equal amount of part B of Ecoflex was added, and again, the container was transferred to the planetary mixer for 10 min. The homogeneously mixed composite was then degassed for 5 min to remove trapped air bubbles. The composite was then poured into the container and allowed to solidify at room temperature. Further solidification was accomplished using a vacuum oven at 60 °C for 2 h. Eventually, the solidified sample was extracted from the mold. The entire process is illustrated in Figure S1. To construct a nanocomposite comprising 5 and 10 wt % of ST (Smooth-On, Inc.), the respective amounts of ST were added along with the CNT and part A and transferred to the planetary mixer for 2 h; all other steps were the same as previously described. Thus, three different samples were constructed: 0 wt % ST (ST-0), 5 wt % ST (ST5), and 10 wt % ST (ST-10). It should be noted that the amount of CNT was 0.2 wt % of the amounts of parts A and B (the ST weight was not considered). Various mechanical tests were undertaken using an Instron E3000 testing machine. Meanwhile, the load and displacement data from the Instron machine and the output voltage from the voltage-divider circuit were synchronized using a NI-Max DAQ system. The details of the experimental setup are highlighted in Figure 1. B

DOI: 10.1021/acsami.9b00464 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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of a thinner wt %. Figure S3 shows that there is no shift in the characteristic peaks of the nanocomposite, and, rather, there is a variation in the intensity depending on the wt % of the thinner. The variation in the intensity might be due to the degree of covalent bonding of the thinner with the relaxed polymer chains and the degree of disorder in the structure caused by swollen polymer chains, depending on the amount of the thinner.45 Furthermore, via the incorporation of ST, softness can be tuned as depicted in the insets of Figure 2. High sensitivity under low pressure is a desirable characteristic of piezo-resistive sensors when used in a human/machine interface, and ST could be useful for tuning both sensitivity and softness. The cause of softness can be attributed to the migration of the thinner toward the polymer chain, to the swelling effect on the packing density, and to the effect of lowered viscosity.45 In Figure 2a,b, a clear delay response can be observed. In Figure 2a, for ST-0 and ST-5, a shoulder peak appeared during relaxation after a strain at 8%, but ST-10 showed no such shoulder. With strain at 23%, the shoulder that appeared at 8% strain disappeared, but there was a delay response for all samples. The delay is caused by the incomplete recovery of the effective conductive path, which was destructed while loading, and the degree that depends on the level of strain, as illustrated in Figure 2. The incomplete recovery of the effective conductive path is mainly due to the visco-elastic nature of the elastomer. The effective conductive path in a nanocomposite is mainly comprised of conductive networks formed by CNTs themselves and tunneling effects among neighboring CNTs. The change in the resistance of the nanocomposite under cyclic compression is due to the deformation and formation of conductive networks of CNTs and the change in the distance between the neighboring CNTs (tunneling effects). As reported in previous works,44,46,47 the piezo-resistivity originated mainly from the tunneling mechanism if the wt % of CNT is lowered significantly. Because a low wt % of CNT (0.2 wt %) was used in the current work, the variation of resistance should have originated mainly from the tunneling effects. This is supported by the signal features such as the positivepressure effect, nonlinearity, sensitivity dependency on the strain level, and delay responses that are characteristic of piezo-resistive signals originating from the tunneling effects.44,46,47 The working mechanism can be more clearly explained from the schematic diagram illustrated in Figure S4. The figure shows the random distribution of CNTs in the insulating matrix (Ecoflex) at the steady state. When the compressive strain is applied, the CNTs in Ecoflex are dragged along with the polymer chains and pulled apart in the direction perpendicular to the applied pressure. Thus, the orientation of CNTs changes in the direction perpendicular to the applied pressure, and the degree of orientation depends on the strain

Figure 1. Illustration of the experimental setup and DAQ.

3. RESULTS 3.1. Compatibility of a Flexible CF Electrode for Use with a Tunable Elastomeric Piezo-Resistive Sensor via the Use of ST. At first, to determine the possibility of using a CF electrode in elastomeric piezo-resistive sensors, the nature of the piezo-resistive responses of sensors with various values of ST wt % were characterized by applying sinusoidal strain cycles with different strain levels at 1 Hz. The results are illustrated in Figure 2, which show the positive-pressure effect (i.e., the resistance increases with increasing pressure). The dependency of piezoresistance on strain is due to the destruction and formation of the effective conducting path.43,44 Under compression, destruction of the conduction networks is dominant and results in an increment of resistance; when the strain is released, however, the re-formation of the effective conducting paths leads to a decrease in the resistance.44 Furthermore, the sensitivity dependency on the proportion of the ST content was evident. It should be noted here that the resistance increased by ∼3279% for ST-10 at a 23% strain, whereas ST-5 and ST-0 recorded resistance increases of 969 and 584%, respectively, which indicates that the sensitivity is tunable by varying the ST content. The effect of ST can also be seen in Figure S2, which shows the decreasing conductivity of the sensor in response to increases in the amount of ST content, ascribed to the contributions of two different effects. The first effect is that the less-viscous form of the liquid elastomer that results from the addition of ST can easily pass through the CNT networks as well as increase the distance between CNTs, resulting in a reduction in the number of effective conductive paths. The second effect could be the breakdown of the continuous filler networks by the swollen polymer chains.45 The Fourier transform infrared spectroscopy (FTIR) technique was applied to detect the effect

Figure 2. Piezo-resistance responses of ST-0, ST-5, and ST-10 (a) at 8% strain and (b) at 23% strain. C

DOI: 10.1021/acsami.9b00464 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces level as shown in the figure. The variation in the orientation of CNTs changes the distance between the neighboring CNTs drastically, thereby affecting the tunneling mechanism. Furthermore, the tunneling resistance depends exponentially on the distance between adjacent CNTs as reported in the literature,46,47 thus leading to nonlinear responses. Upon the release of strain, the effective conductive path is not fully recovered because of the visco-elastic nature of Ecoflex, thus causing a delay in the responses. The addition of the thinner increased the sensitivity of the sensor significantly. The addition of the thinner reduced the conductivity of the sensor for the reasons explained above and consequently enhanced the tunneling resistance, which further increased the sensitivity of the sensor. Furthermore, the repeatability of the responses is good even for high levels of compressive strain. This shows that there is strong binding between the CNTs and the Ecoflex matrix. In the past work,48 a transmission electron microscopy image of the fractured surface showed very strong binding between the CNTs and Ecoflex, which was attributed to the mechanical interlocking of CNTs and polymer molecules because of the very low viscosity of the liquid Ecoflex, covalent chemical bonding, and noncovalent bonding such as van der Waals and electrostatic forces. However, the FTIR spectra shown in Figure S3 show no significant shift in peaks, revealing that there might not be any chemical bonding between the CNTs and Ecoflex. Nevertheless, the repeatability of the responses from the sensors illustrates good interfacial adhesion between the CNTs and Ecoflex, which might be due to mechanical interlocking and noncovalent bonding such as van der Waals and electrostatic forces. For intuitively comparing the sensitivity of the present work, the gauge factor [(R − R0)/R0 divided by strain] has been calculated and compared with other works and listed in Table 1.

under tension. Therefore, the destruction of the tunneling paths should be more dominant under compression than under tension and hence contribute to large gauge factors. It has been reported that for hard polymer nanocomposites, such as epoxy nanocomposites, the sensitivity of the sensor is lower under compressive strain than under tensional strain.46,47 Under compressive strain in a hard polymer nanocomposite, the orientation of the CNTs is not altered much, and, rather, there is a reduction in the distance between adjacent CNTs, which results in the negative-pressure effect signals (i.e., resistance decreases with increasing pressure) and the reduction of sensitivity. However, in elastomeric nanocomposites, the positive-pressure effect occurs because of a drastic change in the orientation of CNTs both in compressive strain and tensional strain. Furthermore, the sensitivity under compressive strain is much higher for the aforementioned reasons. Under the given loading conditions, the nanocomposite filler type, the polymer type, the curing temperature, and the mixing speed, the sensitivity is related to the number of effective conductive paths: a lower number of effective conductive paths equaled a higher level of sensitivity.44,46−48 Generally, the wt % of nanoconductive fillers is lowered to enhance the sensitivity. However, in the current work, the amount of CNT was kept fixed, and the sensitivity was tuned by incorporating the ST. Tuning the sensitivity by employing ST is much easier to handle than from the wt % of the nanoconductive filler. In the real world, sensors are subjected to dynamic straining conditions, and these were simulated to test the performance of the CF electrode. To simulate dynamic loading, the sensors were first compressed at a 16% strain, and then sinusoidal ± 8% strain cycles (+8% is compressive and −8% is relaxation cyclic strain) were applied with various frequencies ranging from 1 to 21 Hz. The overall response of piezo-resistance and the stress of ST-0 can be seen in Figure S5. It can be seen that stable responses occur for each straining rate after a few loading cycles. This is common in piezo-resistive materials as a result of the restabilization of the effective conductive path. Responses of 1, 9, and 21 Hz taken from the stable part are compared in Figure 3a after converting into change in percentage, considering the prestrained state as the reference point. It can be seen that the change in peak oscillation from the reference point shows a consistent response despite different straining rates. Under the same experimental conditions, the frequency responses of ST-5 and ST-10 were determined and have been compared with those of ST-0 in Figure 3b−d via normalized hysteresis curves. The delay response seen in Figure 2 can be observed in the hysteresis curves as well. It is apparent that the shapes of the curves differ for different strain rates, which illustrate the effect of frequency on the tunneling mechanism; however, the general piezo-resistance response was the same at a given frequency despite the variation in ST wt %. It is noteworthy that the illustrated rate-dependent asymmetric nonlinear hysteresis characteristic of piezo-resistive responses are typical piezo-resistive responses of elastomeric piezoresistive materials, and the causes of sensitivity, nonlinearity, positive-pressure effect, delay response, and softness all originate from the nature of the elastomeric nanocomposite, as described above. Thus, we believe CF simply acts in a neutral fashion to bridge the nanocomposite and DAQ systems and performed well over a range of sensitivities.

Table 1. Comparison of the Sensitivity of the Elastomeric Piezo-Resistive Sensor reference 49

this study

materials

gauge factor

Ecoflex-carbon black Ecoflex-carbon grease Ecoflex-CNT Ecoflex-ionic liquid Ecoflex-CNT-thinner (10%) Ecoflex-CNT-thinner (5%) Ecoflex-CNT-thinner (0%)

1.62−3.37 3.8 1.75 1.75−3.75 142 42 25

The gauge factors of the current work have been calculated considering the results of Figure 2b. The gauge factors for ST-10, ST-5, and ST-0 were 142, 42, and 25, respectively. Therefore, the ST content played a significant role in tuning the sensitivity of the elastomeric piezo-resistive sensor. As highlighted in Table 1, the gauge factors of the current work are much higher even for the sample without ST. In addition to the effect of filler wt %, thinner, and mixing technology, the loading conditions might have also influenced the gauge factors. In the current work, the samples were tested under compression, whereas the works highlighted in the table considered the tensional test to study the performance of their sensors. As shown in Figure S4, CNTs tend to orient toward the direction perpendicular to the applied pressure; however, under tension, the CNTs should tend to orient toward the direction parallel to the applied pressure. Furthermore, the physical properties, such as stiffness of the elastomeric nanocomposite, are higher under compression than D

DOI: 10.1021/acsami.9b00464 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 3. (a) Piezo-resistance response of ST-0 under conditions of dynamic strain, and comparison of hysteresis effect on ST-0, ST-5, and ST-10 at (b) 1, (c) 9, and (d) 21 Hz.

Figure 4. Comparison of the actual hysteresis loops with their predicted hysteresis loops by the LSTM network-based hysteresis modeling: (a) actual hysteresis loops of 1 Hz, (b) predicted hysteresis loops of 1 Hz, (c) comparison of actual hysteresis loops of 3, 6, 9, 12, 15, and 21 Hz, and (d) comparison of predicted hysteresis loops of 3, 6, 9, 12, 15, and 21 Hz.

4. DISCUSSION 4.1. Short-Term Memory Recurrent Neural NetworkBased Hysteresis Modeling. The strain-resistance relationship, as depicted in Figure 3b−d, has a rate-dependent asymmetric nonlinear hysteresis effect arising from the viscoelastic nature of the elastomer. To apply the elastomeric piezoresistive sensor for engineering applications, the hysteresis effect has to be minimized by using the elastic nature elastomer;

however, there is no such percent of a linearly behaving elastomer existing today. The other option to realize an elastomeric piezo-resistive sensor could be from the proper modeling of the nonlinear hysteresis behavior either by mathematical modeling or by AI-based hysteresis modeling. Unfortunately, it is almost impossible to develop a mathematical model that would describe the rate-dependent asymmetric nonlinear hysteresis behavior. Therefore, hysteresis models E

DOI: 10.1021/acsami.9b00464 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 5. Comparison of the debonding process for the (a) CF and nanocomposite and (b) CW and nanocomposite based on load−displacement and resistance−displacement relationships.

the CW and the nanocomposite. In addition, as illustrated in Figure S7, the sensor resistance (without any strain) accompanied by the CF was 40 times smaller than the resistance of a sensor accompanied by the CW, which proves the higher interfacial conductivity of the CF and the nanocomposite. The higher interfacial strength can be attributed to the mechanical interlocking of the nanocomposite between the CF filaments, which is caused by a very low level of viscosity of the liquid Ecoflex. To further support the mechanical interlocking of CF filaments, scanning electron microscopy (SEM) images were taken from the surface (Figure S8a) and from the cross-section (Figure S8b) of the CF electrode after it was pulled out from the sensor. It can be seen that nanocomposite passed through the filaments and subsequently enhanced the mechanical interlocking. The atomic force microscopy (AFM) image of the CF filament’s surface (Figure S8c) shows a grooved surface, which could have also played a role in enhancing the mechanical interlocking. Because the nanocomposite, as shown by the SEM images, has moved into the filaments, the CNTs in the nanocomposite would contact the filament, as illustrated in Figure S8d, and thereby increase the interfacial conductivity substantially in addition to the interfacial strength. The FTIR spectra illustrated in Figure S9 do not show any shift in peaks, indicating that there is no chemical bonding between the CF electrode and the nanocomposite, and hence the higher interfacial robustness should be attributed to mechanical interlocking. The mechanical interlocking could have been further enhanced by van der Waals and electrostatic forces between CNTs, CF, and Ecoflex.51 It is also important to mention here that the strong conductive interface between the nanocomposite and the CF electrode also allowed us to lower the wt % of CNT to 0.2 wt % and tune the sensitivity by incorporating ST. Tuning the sensitivity by ST and using the CW electrode gave no signals because of significantly infinite resistance at the interface. Thus, the complementary nature of the relationship between CF and elastomeric nano-composite piezo-resistive materials has been revealed. The interfacial bonding strength was further clarified by an in situ resistance profile. A sudden drop in the conductivity at an approximate peak load verified the complete debonding. The exponential rise in resistance after the onset of loading was the result of a combination of interfacial shear strain and the formation of cracks at the interface.52−54 The extension after the peak load was derived from the frictional force between the debonded surface of the electrode and the nanocomposite.52−54 A short extension refers to a higher frictional force, whereas a large extension refers to a lower frictional force. As illustrated in the figure, the extension for the CW was large and indicated a

based on AI could be the best option to describe the complex hysteresis phenomenon.50 Consequently, we are employing long short-term memory recurrent neural network (LSTM RNN) to model the rate-dependent asymmetric nonlinear hysteresis behavior. Figure S6 shows the structure diagram of the LSTM RNN designed to model the hysteresis behavior. Here, we consider stress, strain, stress rate, and strain rate as input variables while resistance is considered as an output variable. The hysteresis modeling results based on LSTM RNN are shown in Figure 4. Figure 4a illustrates the actual data, whereas Figure 4b shows the predicted data by the hysteresis model for 1 Hz. From the result, it can be seen that the hysteresis model describes the relationship between the input strain and the output resistance with sufficiently high precision. Furthermore, the actual data and predicted data for 3−21 Hz are also compared in Figure 4c,d, respectively. Despite the different straining rates, the resemblance between the experimental data and the predicted data is sufficiently high. The training accuracy and test accuracy for all frequencies can be seen in Table S1. The result is encouraging, and the incorporation of AI-based hysteresis modeling can solve the issues of the rate-dependent, nonlinear, and delay response of elastomeric nanocomposite sensors. Therefore, AI can play a complementary role to overcome the intrinsic demerits of elastomeric nanocomposite sensors and to realize sensors for real applications. 4.2. Mechanical and Electrical Properties at the Interface of CF Electrodes and Nanocomposites. To reveal the interfacial robustness between the CF and a nanocomposite, a pull-off test was performed (the test was performed on ST-0). While one of the electrodes was being pulled off at a cross-head speed of 0.1 mm/s, the resistance variation between the two electrodes was measured. This allowed indirect observation of the debonding process via an in situ resistance profile. For the sake of comparison, a pull-off test was also performed in a sensor embedded with a copper wire (CW) cable electrode (cross-sectional area = 0.75 mm2). Figure 5a,b shows the results of the pull-off test for CF and CW, respectively, which displays the distinctive debonding mechanism particularly related to the maximum load, the displacement to reach the maximum load, the length of the extension after the peak load, and the onset of infinite resistance. The maximum force that is normally considered as the interfacial bond strength was much higher for CF (19 N) than that for the CW (12 N). Furthermore, the displacement to reach the maximum force for CF was much higher (∼19 mm) than that for CW (∼7.5 mm). These two pieces of evidence validate a superior level of interfacial bonding for the CF and the nanocomposite than for F

DOI: 10.1021/acsami.9b00464 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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geometrically with the strain levels while plate electrodes attained a constant level of minimum resistance at higher strain levels. Another discrepancy was observed at the onset of loading where the Al electrode immediately showed good sensitivity while the embedded CF showed a very low level of sensitivity. Poor sensitivity at the onset of loading and exponential increases in resistance under compression are typical elastomeric piezoresistive material phenomena, as previously explained, whereas extraordinarily excellent sensitivity from the onset of loading and the attainment of a constant level of minimum resistance for a given strain level are typical piezo-resistive responses that originate from the contact mechanism between an electrode and a nanocomposite.56 Variations in the resistance signal from contact are directly related to the number of contacting points between the electrode and the nanocomposite. Therefore, with a given strain increment, the contact between the CNT on the surface and the electrode will increase, which results in an increased number of contact points, which consequently decreases the resistance of the sample. At a given level of strain, the number of contact points is saturated, which leads to a constant level of minimum resistance. If the interface between a nanocomposite and an electrode is not strong, piezo-resistive signals are always affected by the signals that originate from the contact mechanism. Thus, a robust conductive interface is indispensable to avoid dominance of the negative-pressure effect over the positive-pressure effect. Conventionally, the piezoresistive response is measured by attaching a metallic electrode to the surface of the sample using a conductive glue. However, it is very difficult to attach an electrode by a glue on the surface of an elastomeric polymer like Ecoflex for a long time because of its low surface energy, and moreover, the surface resistance is very high. The surface resistance measured in our sample was several times higher than the inner nanocomposite resistance, which was probably due to lower CNT dispersion on the surface. This can be attributed to the cohesive nature of liquid Ecoflex because the majority of CNTs are embedded just below the thin membrane of Ecoflex, and this results in lower conductivity on the surface. Thus, we believe that embedding the CF electrodes into the nanocomposite matrix helps to avoid lower conductivity on the surface, which aids electrode attachment and avoids foreign material at the interface of the electrode and the nanocomposite (e.g., conductive glue made from materials with mechanical properties that are incompatible with those of the nanocomposite). These steps help avoid the dominance of the negative-pressure effect, which enhances the durability and ensures clearer responses. It is important to mention here that many researchers have established the negative-pressure effect in elastomeric nanocomposites, and several working mechanisms have been proposed to deal with it.57,58 We believe that such a negative-pressure effect might has resulted from the contact mechanism but not from the nanocomposite itself, based on the manner in which they used electrodes and on the signals obtained. CF electrodes have one disadvantage, which is an inherent piezo-resistance that originates from the degree of filament contact in response to applied stress, as shown in Figure S10.37 As shown in Figure S8b, filaments are well-dispersed in a nanocomposite. However, there are several points where the filaments are in contact with each other, as illustrated in Figure S10 by the schematic diagram. When the applied strain is increased, the degree of filament contact increases in response to the applied strain and thereby decreases the effective resistance of the electrode. This inherent piezo-resistance of the CF

lower frictional force. This type of monotonic gradual decrease in the frictional force would have arisen from the friction between the debonded surface of the CW and the fractured nanocomposite surface. On the other hand, a sudden drop after the peak load for CF suggests that the friction happens not only between the debonded CF surface and the fractured nanocomposite but also between the mechanically interlocked nanocomposites in the CF and a fractured nano-composite. The effect of contact between a mechanically interlocked nanocomposite in the CF and a nanocomposite can be seen in the resistance profile as well, where a frequent reappearance of conductivity can be observed. However, the lack of a mechanically interlocked nanocomposite in the CW resulted in a total disappearance of conductivity after debonding. Because copper has an incredibly high surface tension, adhesion between the copper surface and elastomers is very weak, and this results in low levels of mechanical interlocking in the nanocomposite. It should be noted here that the CF bought in the commercial market is usually coated with an epoxy resin, which was a part of our electrode as well. Surface modification of the CF by different methods55 and applying it as an electrode could result in higher bond strength and an enhanced electrical connection. 4.3. Importance of Interfacial Conductivity between the Electrode and the Nanocomposite. A robust conductive interface between an electrode and a nanocomposite is very important to receive signals without the superposition of the contact resistance. To demonstrate the origin of the contact resistance and compare it with the piezo-resistance of the sensor, we performed an experiment on ST-0. At first, a CF electrode was used to receive the signal while straining at 0.5 mm/s. As previously mentioned, a positive-pressure effect was obtained, and the result is illustrated in Figure 6a. Then, an aluminum

Figure 6. Illustration of (a) positive-pressure and (b) negative-pressure effects of piezo-resistance in response to discrepancies at the interface between the nanocomposite and the electrode.

electrode was used to receive the signal under the same strained conditions; however, the Al electrode was neither embedded nor attached by any conductive glue; the electrode simply made contact with the surface of the sensor, as illustrated in the inset of Figure 6b. The obtained signal is illustrated in Figure 6b and shows the negative-pressure effect (decrease in the resistance while increasing the pressure). Looking carefully at the signals, the piezo-resistance obtained from the embedded CF increased G

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ACS Applied Materials & Interfaces

of the CF generated from the filament contact has a negligible effect on sensor signals. We believe CF-embedded, tunable piezo-resistive sensors can be used in biomedical devices, artificial e-skins, robotic touch applications, and flexible keyboards where the required stretchability of the electrode can be introduced via an appropriate geometrical design.

electrode would affect the sensor signal. We found, however, that when the high conductivity of CF is compared with that of the nanocomposite, the impact of CF piezo-resistance is rendered negligible, as shown in Figure S10. On the basis of the findings of the present study, CF should be considered a candidate for use as an electrode for tunable piezoresistive strain and tunable pressure sensors, where the sensor must function under an environment of high strain because of its extremely inexpensive, simple, and flexible merits. Such sensors are usually much in demand in biomedical devices, artificial eskins, robotic touch applications, and flexible keyboards, where electrodes endowed with stretchability and flexibility are a requisite. As a matter of fact, CF is not stretchable, but stretchability can be incorporated via appropriate geometrical designs of electrodes such as serpentine and helical conformations (as illustrated in Figure S11).59,60 The utility of CF electrodes could play a role in reducing the weight of sensors, reducing the costs of production, enhancing the robustness of sensors, providing noise-free signals, and providing flexibility along with geometrical design-aided stretchability. The excellent properties of CF such as chemical stability, negligible temperature sensitivity, and high resistance to humidity and pressure are important features that will enhance the sensor performance.30 Moreover, depending on the size of the elastomeric piezo-resistive sensor, the size of CF electrodes can be miniaturized up to microscale and even less. Through the surface modification of a CF electrode, it can be used not only with the elastomers such as Ecoflex and PDMS but also with various classes of polymers. Furthermore, the fast response time and the excellent biocompatibility of these electrodes make them a powerful tool for human/machine interface with high temporal and spatial resolutions. It should also be noted that the high surface area of a CF electrode would allow the nanoconductive fillers to move deep into a CF electrode, which would provide a better conductive interface between the nanocomposite and the electrode.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b00464. Sample fabrication process; resistance of sensors with respect to ST wt %; FTIR spectra of the nanocomposite; schematic model of a piezo-resistive mechanism; piezoresistance response of ST-0 under dynamic straining conditions; hysteresis model based on AI; resistance of sensors accompanied by the copper electrode and the CF electrode; dispersion of CF filaments in the nanocomposite and the AFM image of the CF filament surface; FTIR spectra of the CF and the nanocomposite; inherent piezo-resistance of the CF; illustration of stretchability of CF via different geometric designs; and training and test accuracies (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jeong Heon Lee: 0000-0002-6451-8170 Seong-Hoon Kim: 0000-0003-2962-3722 Ji Sik Kim: 0000-0002-1432-8176 Suman Timilsina: 0000-0003-3362-9369 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean government (MPIS) (grant numbers: 2015M3D1A1069719 and 2017R1A2B4001877).

5. CONCLUSIONS The compatibility of extremely inexpensive, simple, and flexible CF electrodes with tunable elastomeric piezo-resistive sensors was investigated. Piezo-resistive sensors were constructed from a highly flexible, superstretchable, and ultrasoft silicone rubber known as Ecoflex. The sensors were blended with CNT, and two CF electrodes were inserted before solidification. The sensitivity and softness of the sensors were tuned by incorporating ST, and their responses were assessed by subjecting them to different levels of strain and to dynamic straining conditions. The responses were clear and noise-free positive-pressure effects with rate-dependent asymmetric non-linear hysteresis characteristics. The hysteresis model based on LSTM RNN described the relationship between the input strain and the output resistance with sufficiently high precision, and such a model is very important to realize the piezo-resistive sensor. The bonding strength (CF = 19 N and CW = 12 N), the displacement to reach the bond strength (CF = 19 mm and CW = 7.5 mm), and the resistance of the sensor (CF = 280 kΩ and CW = 12 MΩ) supported the robust interfacial strength and conductivity of the CF and the nanocomposite sensor. The in situ resistance profile from the pull-off test was also helpful in elucidating the bonding mechanism. We also demonstrated how the contact resistance might affect the piezo-resistance of a nanocomposite if the interface between the CF and a nanocomposite is not robust and conductive. Furthermore, we found that the inherent resistance

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