Structural conductive carbon nanotube nanocomposites for stretchable electronics

Carbon nanotube (CNT) nanocomposites have been widely used for electronic devices because of their high conductivity and ease of processing. However, these nanocomposites have limited functionality because of their rigid intrinsic mechanical properties. In this study, we fabricated a stretchable serpentine structure using a CNT nanocomposite with a carboxymethyl cellulose binder. For a flexible mold, a polydimethylsiloxane (PDMS) was cast by the stretchable serpentine structure fabricated by a 3D printer. The CNT nanocomposite slurry was squeegeed into the serpentine-patterned PDMS mold. Fourier-transform infra-red spectroscopy and scanning electron microscopy were used to analyze the material properties of the nanocomposites with 15–45 wt% CNTs. We analyzed the serpentine grid structure using current-voltage curves, strain resistance values, and the Joule heating effect. Next, we developed the structural CNT nanocomposite electrode (SCNE) that was insulated by PDMS, and induced a skin-warming effect by Joule heating. Furthermore, light emitting diodes (LEDs) were implanted in series into a T-shaped linear SCNE, which had greater stretchability. The nine LEDs embedded in the SCNE were successfully operated by applying 20 V during the bending of the structure. Finally, the serpentine-shaped linear SCNEs with serially-implanted LEDs were programmed to light the LEDs in unison with the beat of a song.


Introduction
Flexible electronics have been employed in the emerging electronics market for wearable devices, which include healthcare monitoring sensors [1,2], secondary rechargeable batteries [3,4], and electronic skin devices [5][6][7]. Flexible conductive materials are advantageous for interconnecting the electronic components with application circuits, while traditional metal wires such as copper restrict the stretchability of wearable technology. Metal wires easily become brittle when repeatedly stretched, making their use unsuitable for applications requiring changes in shape.
To provide a stretchable conductive layer, percolation of nanomaterials has been used to develop a nanomaterial network layer that typically is composed of stretchable polymers such as polydimethylsiloxane (PDMS), Ecoflex, styrene butadiene styrene (SBS), and polyurethane (PU) [8][9][10][11]. The junctions of high-aspectratio nanomaterials, such as Ag nanowires (NWs), CuNWs, AuNWs, and carbon nanotubes (CNTs), provide a conductivity of the layer until the network of nanomaterials is broken by stretching [12]. Stretchable nanocomposites have been widely developed for applications such as press touch sensors for tactic signals, Joule heating devices for the human body, body sensing devices for human body signals, sound pattern recognition devices for human voices, and flexible wire circuits for electronic components [13][14][15][16][17]. However, the nanocomposites have low conductivity due to the stretchable polymers. For high conductivity of the wiring, Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. waveform structural metal lines that connect with electronic components exhibit stretchability with seamless electrical performance [18][19][20]. Due to this stretchability, the waveform metal lines, formed by a semiconductor fabrication method [21], are widely used in flexible-skin disposable sensors, soft robot structures, and electronic logic circuit component implants [22][23][24]. For wearable skin electronics, the waveform metal lines are used for body heating devices by a Joule heating process [25], and are applied in disposable tattoo sensing devices that detect body signals through electrocardiogram (ECG) and electromyography (EMG) activity [26]. For soft robotics, the waveform of the metal electrodes has been demonstrated to control mechanical bending into a hydrogel by controlling the electronic signal [27]. In addition, the waveform metal electrodes have been widely used for highly stretchable electronic devices with a surface mount device (SMD)-type of commercial electronic components, including logic circuits [28].
Inspired by the waveform metal electrodes with stretchable functionality, CNT-based conductive nanomaterial networks have been used as sensors for body signals, and are fabricated by transfer-printing hydrogel templated serpentine patterns onto the skin [29]. There are several research for CNT-based conductive nanomaterial networks with cellulose materials to thin film research with 600 bending cycles [30], 3D printing materials [31], and energy materials [32]. However, without binder materials such as carboxymethyl cellulose (CMC), the structure of the electronic device incorporating the CNT wave patterned electrode can be easily deformed by external forces or solutions. Here, we fabricated rigid CNT nanocomposites with CMC using a PDMS mold that was transferred from a 3D printed template. The structural CNT nanocomposite electrode (SCNE), which has a serpentine structure, has been shown to exhibit stretchability with high conductivity. The CNT nanocomposites, which were fabricated with 15, 25, 35, and 45 wt% CNTs in water with CMC, were analyzed by Fourier-transform infrared spectroscopy (FT-IR) and the surface morphology was analyzed by scanning electron microscopy (SEM). The serpentine grid SCNE had a parallel serpentine structure for measuring high conductivity, and the values of strain-stress as a function of resistance were evaluated to monitor stability under stretching. The serpentine grid SCNE was also assessed for its skin heating effect by the Joule heating process. A T-shaped SCNE, which had a linear serpentine structure for high stretchability, had stable conductivity as a function of strain was analyzed. Furthermore, light emitting diode (LED) electronic components were implanted in series in the T-shaped SCNE and operated under stretching. Finally, the LED implanted SCNEs, which stretched into diverse shapes such as triangles and T shapes, were connected to a relay switchboard and programmable Arduino board, and were operated in sequence with the beat of a song.

Materials and methods
Preparation of nanocomposite slurry and PDMS mold Multi walls CNTs (7-15 nm diameter and 0.5-10 μm lengths) were obtained from Cheap Carbon, Inc., USA. CMC was obtained from Sigma-Aldrich, Inc. (avg. M w ∼250,000, Germany). A CNT aqueous slurry (15,25,35, and 45 wt% CNTs) was carefully mixed with CMC in distilled (DI) water using a mortar and pestle for 30 min. The 3D printed acrylonitrile butadiene styrene (ABS) structures for the SNCEs were designed with AutoCAD 2020 (Autodesk, Inc., USA) and printed using a 3D printer (Qidi X Smart, Qidi Technology, Co., China). For the flexible mold, the PDMS (Dow Corning, Co., USA) was cast onto 3D printed structures. After baking in an oven at 50°C for 12 h, the patterned PDMS mold was detached from the 3D printed structure.
Fabrication of the SCNE The CNT nanocomposite slurry was squeegeed into the serpentine pattern of the PDMS mold and then placed on a hot plate. The CNT nanocomposite in the serpentine pattern of the PDMS mold was carefully squeegeed to prevent trapped bubbles in the slurry from forming cavities when dried. After completely drying on a hot plate at 65°C for 1 h, the SCNEs were detached from the PDMS mold.

Characterization of the CNT nanocomposites
The FT-IR spectra of the 15 wt%-45 wt% CNT nanocomposites were analyzed from 800 cm −1 to 4000 cm −1 using an FT-IR microscope (HYPERION 2000, Bruker, USA). The surface and cross-sectional morphology of the CNT nanocomposites and SCNEs were observed by SEM (AIS2500C, Seron Technology Inc., South Korea).
The serpentine grid SCNE The serpentine grid SCNE was cast from the PDMS mold. For the casting of the CNT nanocomposite, copper wires were inserted into the linear edges of the serpentine grid SCNE. The IV curves of the SCNE were measured from −3 V to 3 V using a source meter (2450 Keithley, Tektronix, USA). The real-time Joule heat of the serpentine grid SCNE was tracked by infra-red (IR) video using a visual IR thermometer (VT02, Fluker, USA) for 15 V, 18 V, and 21 V. For the serpentine grid SCNE, the resistance values were measured using the source meter.
For Joule heating of the skin, the serpentine grid SCNE was insulated by PDMS, which was fabricated at 65°C for 1 h. For measuring the heating of the skin, 21 V was applied through the serpentine grid SCNE encapsulated by PDMS on a human arm for 3 min. The skin was monitored using a visual IR thermometer. Written consent was obtained from all research participants prior to this process.

The LED implanted SCNE circuits
For high stretchability, a T-shaped linear serpentine SCNE was cast from the PDMS mold. To measure the strain of the T-shaped SCNE, resistance values were measured using a source meter. Linearly 3216 SMD LEDs (UNEEDS, Co., South Korea) that were soldered with copper wires were inserted into the T-and triangularshaped SCNEs for casting the CNT nanocomposites. The LED-implanted SCNEs were connected to a 2 cm × 8 cm HSE-0208D PCB board (Hypshin Electronics, Co., South Korea), which consisted of circuits with an Arduino Mega 2560 R3 (Mechasolution, Co., South Korea), a 5 V 16 channel relay switchboard (Mechasolution, Co., South Korea), and an SZH-EK033 sound sensor module (Mechasolution, Co., South Korea). A Christmas LED tree device was operated in response to a Christmas song, according to the encoded Arduino code.

Results and discussion
The serpentine SCNE fabrication Figure 1 shows the fabrication process and images for the serpentine SCNE, which consisted of CNTs for conductivity and mechanical strength and CMC as a binder. The nanocomposite surface, which was a mixture of CNTs, CMC, and DI water (figure 1(a)), was dried to form the structure of the PDMS mold. The SEM image of the completely dried nanocomposite surface is shown in figure 1(a) (iv). Figures 1(b)-(e) shows the process diagram and images of the mold fabrication and casting of the nanocomposite structure. The flexible PDMS mold was fabricated from a 3D printed ABS structure so that the SCNE could be easily removed from the mold. The CNT/CMC slurry with wiring was squeegeed into the PDMS mold, which was 2 mm wide with a tight serpentine pattern, as shown in figure 1(d) (iii). To prevent cavities from forming during drying, the slurry was carefully squeegeed into the PDMS mold prior to the heating process. After drying completely and removing the mold, figure 1(e) (i) shows the serpentine SCNE with the copper wires. Figure 1(e) (ii) shows high magnification images of the nanocomposite after casting into the PDMS mold. The serpentine structure of the nanocomposite is defined by the material properties of the CNTs and CMC. Due to the flexibility of PDMS mold, the SCNE was smoothly detached from the mold. For the repeatable process, the SCNE was reproduced by PDMS mold. The CNT nanocomposites CNTs have a unique property of high conductivity and high mechanical strength, owing to their high aspect ratio. Because of its binding properties, CMC is widely used for carbon-based nanocomposites as well as energy storage materials [33,34]. However, the CMC in the structure reduces its conductivity by reducing the direct connection between CNTs [35].
The conductivity of three-dimensional nanocomposites follows the power-law [36][37][38]: where σ DC is the conductivity of the CNT nanocomposite, σ 0 is a constant parameter, m is the volume fraction of the conducting filler, m c is the percolation threshold, and β is the critical exponent. In order to define the conductivity of the nanocomposite depending on the CNT concentration, we constructed the nanocomposite with a 15−45 wt% concentration of CNTs. After the nanocomposite fabrication process, the 15 wt% CNT nanocomposite exhibited the lowest conductivity based on multimeter readings. At lower concentrations, resistance values of the CNT nanocomposite could not be measured due to the low conductivity. Nanocomposites with the highest conductivity were the 45 wt% CNTs. At higher concentrations of CNTs, we were unable to suitably mix the nanocomposites due to the low ratio of the CMC binder. For elemental analysis of the nanocomposite, figure 2(a) shows the FT-IR results for all compositions of the nanocomposites. The FT-IR spectrum of the nanocomposite showed a broad absorption band at 3353.54 cm −1 , confirming the stretching frequency of the OH group, while the band at 2923.5 cm −1 was associated with a CH stretching vibration. In addition, the spectrum of the nanocomposite had a strong absorption at 1581.31 cm −1 , which is attributable to the stretching vibration of carboxyl groups (COO−), at 1423.81 cm −1 , which is assigned to carboxyl groups. The bands at ∼1329.32 cm −1 were attributed to −OH bending vibrations and at 1112.65 cm −1 to C−O−C stretching. The peaks that represented the CMC molecules [39] decreased with increasing CNT content (15 wt %, 25 wt %, 35 wt %, and 45 wt %). Furthermore, we provided that the Raman spectra were To analyze the morphology and inner structure of the nanocomposite, figure 3 shows the cross-section and surface SEM images of the nanocomposites with various CNT concentrations. On the surface, the abundant CNTs are mostly connected together by CMC. However, figure 3(a) shows the aggregation of the 15 wt% CNTs, and this composition exhibited reduced conductivity and mechanical strength. When the CNT content increased, the number of wrinkled CNTs on the surface increased, as shown in figures 3(a)-(d) (i). At the 45 wt% CNT composition, the largest amount of CNTs, the morphology had a wrinkled CNT surface. In order to investigate the interior connectivity, the cross-section surfaces were monitored, as shown in figures 3(a)-(d) (ii). The 15 wt% nanocomposites had a mostly smooth surface when viewing the cross-sections. Owing to the abundant amount of CMC, the nanocomposite had low conductivity. However, with increasing CNT content, the cross-sections of the nanocomposite appeared to consist of CNT particles. At 45 wt% CNT content, the cross-section of the nanocomposite displayed an abundant amount of CNTs, with an increasing number of electrical connections. As electrical components with mechanical strength, the ratio of CNTs to the nanocomposite plays an important role that determines the functionality of the material. As expected, the 45 wt% CNT slurry, which was the highest ratio that could be mixed, the nanocomposite exhibited the best electrical performance. The SCNE By fabricating a structural nanocomposite with a binding polymer such as CMC, the good electrical properties with favorable mechanical rigidity were well-suited for use with electronic components [40]. However, mechanically rigid CNT nanocomposites have limited performance in wearable devices. Serpentine electrodes have stretchable functionality, meaning that they are useful for preventing brittle electrodes and deformation by external forces that deteriorate the electrical performance. Recently, serpentine electrodes have been applied to diverse applications such as joule heating devices, body sensing devices, and voice pattern recognition devices [41,42]. To impart stretchability to a rigid nanocomposite, a 15 wt% to 45 wt% CNT slurry was structurally fabricated into a serpentine-shaped PDMS mold. The structural nanocomposites exhibited various results, including diverse mechanical and electrical properties. Figure 4(a) shows an image of the fabricated structural nanocomposite component with wiring, and figure 4(b) shows SEM images of its details. The top surface of the structural nanocomposite had a smooth morphology (figure 4(b) (i)) owing to the water evaporation process. The side surface shows the layered imprint ( figure 4(b) (ii)) that was transferred from the original 3D printed shape. Due to the layered shape, the structural nanocomposite had to be carefully removed from the PDMS mold. In addition, a few empty cavities were created (figure 4(b) (iii-iv)) due to the micron-sized bubbles of the slurry. However, the structural nanocomposite was mostly fabricated without cavities and demonstrated uniform electrical and mechanical performance.
The electrical performance of the serpentine grid SCNEs was measured for the various compositions. The SCNEs showed linear current-voltage curves from −3 V to 3 V. At the lowest CNT content of 15 wt%, the resistance was 6.7 MΩ, whereas the resistance was the lowest at 138 Ω when the CNT content was highest, at 45 wt%. The SCNEs, which had electrical stability, were Joule-heated by different voltages (15 V, 18 V, and 21 V). Joule heating of the 15 wt% and 25 wt% CNT composition was not detected between 15 V and 21 V. However, the SCNEs with the higher percentage of CNTs (35 wt% and 45 wt%) did exhibit the Joule heating effect. While the Joule heating effect was greater for the 35 wt% nanocomposites for 15 V and 21 V, as shown in figure 4(e), the 45 wt% nanocomposites had a constant Joule heating effect from 15 V to 21 V, as shown in figure 4(f). At 21 V, the 35 wt% SCNE with a resistance of 331 Ω was saturated at 43.0°C after 227 s. At ∼43.3°C, the 45 wt% SCNE with a resistance of 69 Ω was saturated after 158 s at 18 V and 21 V, while the 45 wt% SCNE was saturated ∼42.0°C after 100 s at 15 V. Owing to the spring-shaped structure, the SCNEs were very stretchable, as shown in figure 4(g) and Supporting Video 1. For 100 bending cycles, the SCNE has stable resistance values, as shown in figure 4(g) (iii). When stretched, the SCNE Joule heated devices demonstrated electrical stability for maximized strain (%), which was determined by the CNT content, as shown in figures 4(h)-(k). The resistance values of the SCNEs were similar before breaking. The rate of stretching and the resistance values of the various CNT compositions of the SCNEs had a trade-off relationship. The 15 wt% SCNE demonstrated high resistance values, including instability, at 35.6% strain. The 25 wt% SCNE had a stable resistance value for stretching, which dramatically reduced the strain rate (18.9%). The 45 wt% SCNE maintained a stable resistance of 138 Ω for 6.81% strain. This is because the composite becomes more rigid as the CNT ratio increases.
Figures 4(l)-(n) shows the Joule heating effect of the SCNE on the skin of a human arm. To protect the skin from the current through the 45 wt% SCNE, which had the highest Joule heating effect device, the SCNE was insulated by PDMS. The voltage was applied to the SCNE device to demonstrate the Joule heating effect on the arm skin (figure 4(m) (ii)). The arm, which was laid on the SCNE, was monitored by an IR imaging camera. The ambient temperature of the arm skin (34°C) rose to 38.4°C immediately after removing the SCNE, and fell slightly to 37.2 o C after being removed for 30 s. Furthermore, the Joule heating of SCNEs was observed with current monitoring for bending (58°). Even though the current values were different at 45% SCNEs, stable currents and temperatures were monitored, as shown in figure S1. Here, we demonstrate that the serpentine CNT nanocomposites are connected at the edge of the structure with copper wires to show bending. However, for heat applications, the line source should provide more stable and enhanced joule heating, including low resistance. Because we used the MWCNT with high resistance for the SCNE, low-resistance nanoparticles, such as single-wall CNTs, Cu nanowires, and Ag nanowires could be useful for uniform Joule heating.

Highly stretchable LED SCNE circuits
To provide a mechanically stretchable device, a rigid CNT nanocomposite was fabricated into a serpentine structure, similar to a gold serpentine patterned thin electrode. Moreover, the electrode of the SCNE could be extended to diverse circuits, including electronic components such as LEDs, owing to the 2 mm thickness of the SCNE, which was stable without the need for a supporting substrate. Since the electrical properties of the serpentine shaped nanocomposites were constant under mechanical stress, we designed a series structure of the serpentine SCNE to easily test for high stretchability and LED implantation. Figure 5 demonstrates the fabrication and analysis of the highly stretchable T-shaped serpentine SCNE and the linear LEDs implanted in the SCNE circuits. The T shape of the SCNE consisted of a contact that was connected to an electronic component or wiring and stretching component, which has the serpentine structure. The T shape of the 45 wt% SCNE had high stretchability due to the serpentine part of the SCNE. (figure 5(b) and Supporting Video 2) To measure the resistance values for the stretching, the end of the serpentine part was carefully stretched using a finger after the end of the linear part was fixed. The total strain rate of the SCNE was measured as 357%, including the linear section, which was the non-stretchable part. The resistance values were measured during stretching, as shown in figure 5(c). At the beginning of the stretching, the resistance of the T-shaped SCNE fluctuated slightly, owing to the initial wiring contact movement. However, the conductivity was mostly stable for the high strain rate (357%), and eventually was disconnected due to breaking.
For the SCNE circuit with the electronic component, SMD-type LEDs, which are soldered with copper wires to enhance bonding with the nanocomposite, were serially connected, as shown in figure 5(f). Because electrical components such as copper wire are embedded within the CNT nanocomposite slurry, the SCNE has electrical and mechanical stability with a copper wire connection. The nine red, green, and yellow LEDs, which have 1.5 V turn-on voltage between the anode and cathode, were implanted into the T-shaped SCNE. The load resistances of the SCNEs between LEDs components (figures 5(d), (e)) were approximately 200-400 Ω, owing to the nonuniform length. Due to the uniform ratio of materials (nanocomposites) and shape of the serpentine grid, the electrical properties have similar values. The T-shaped SCNE with red, green, and yellow LEDs was successfully operated by 20 V, which is the dropped series voltage of the nine LEDs. In addition, the three-colored LEDs operated well when stretched, as shown in figure 5(g) (ii).
Finally, we changed the shape of the SCNEs with implanted LEDs connected to a commercially available printed circuit board (PCB). Figure 5(h) and Supporting Video 3 show that the T and triangular-shaped SCNEs with LEDs could be operated by the Arduino programming board with a relay board. The Arduino board was programmed to turn the LEDs in the SCNEs on and off in sequence with a song, creating a Christmas LED tree device. Since we demonstrated simple circuit applications with stretchability, the circuits of the SCNE could possibly be used in diverse electronics applications. Furthermore, commercially available electronics, such as SMD-type LEDs, resistors, capacitors, and PCBs could easily be implanted with SCNE devices.

Conclusion
For stretchable and wearable electronics, the serpentine structure is a feasible solution that provides the desired stretchability with stable electrical properties, instead of intrinsic brittle and stiff properties. In this study, we fabricated a serpentine-shaped CNT nanocomposite, which provided flexibility through a structural approach, overcoming the typical stiffness of the nanocomposite. The nanocomposites were a mixture of 15, 25, 35, and 45 wt% CNTs in water and CMC, and all compositions were analyzed by FT-IR and SEM. Serpentine grid SCNEs were fabricated using a thick PDMS mold, which was transferred from a 3D printed structure, and the mechanical and electrical properties were observed. The serpentine grid SCNE, which was insulated with PDMS, demonstrated the warming of the skin on an arm by Joule heating. Furthermore, we fabricated a T-shaped linear serpentine SCNE that was highly stretchable. LEDs were implanted in the T-shaped serpentine SCNE and Figure 5. Tree-shaped SCNE with implanted LEDs and printed circuit board. (a) T tree-shape SCNE, (b) the stretching images of the T tree-shaped SCNE, (c) resistance vs. strain graph of the T tree-shaped SCNE, (d) the operating images of the T tree-shaped SCNE implanted with three-colored LEDs at 0 V (i) and 20 V (ii), (e) the resistance values of the SCNE between LEDs, (f) SEM images of the LED connection with the SCNE, (g) stretching the LED tree SCNE, and (h) Scheme of the operating process LED tree SCNEs (i), (ii-iv) optical images of Christmas tree SCNEs with LEDs in a PCB operated by a Christmas song. operated while stretching the structure. Finally, T and triangular-shaped SCNEs with LEDs were connected to a PCB board and the LEDs were turned on and off by a sound sensor that was operated by an Arduino circuit with a relay switchboard.
The CNT-based nanocomposite had several benefits such as lightweight, thermal stability, chemical stability, simple processing, and low cost [43,44]. Owing to these benefits, CNT-based electronics have been investigated for applications in diverse areas, including military craft devices [45], energy applications [46,47], and biosensors [48]. For applications requiring mechanical deformation with electrical stability, the serpentine SCNE could be used for wearable devices [49], energy harvesting devices [50], and electronic circuits [51], and could also include the implantation of electronic components such as transistors, LEDs, resistors, inductors, and capacitors. In addition, highly conductive nanomaterial-based structures such as Cu nanowires and Ag nanowires enhance material properties, such as uniform Joule heating with new functionalities such as transparency [52][53][54]. Even though we simply demonstrated the LED circuits responding to sound, we believe that future work will demonstrate that SCNE devices can be applied to other diverse applications, such as wearable biosignal detection devices [55], soft robots [56], nanocomposite energy harvesting [57,58], radio frequency electronic circuits [59], and micro-electromechanical systems [60].