Multilayer Methylcellulose Substrate-Based Wearable Touch Sensor and Display for Communication

In recent years, exible printed circuit boards (FPCBs) that have polyimide substrates have been widely used in electronic devices for industrial and academic research owing to their light weight, high dielectric constant, and exibility. However, these FPCBs have a critical limitation of recycling, as polyimide is not degradable or eco-friendly. To overcome this issue, we fabricated cellulose-based FPCBs. Transparent and exible methyl cellulose-based substrate was produced through a simple solvent evaporation process. The circuit layer was patterned of an Ag/carbon-nanotube composite fabricated using a stencil mask. The methyl cellulose-based FPCBs were evaluated for diverse mechanical stresses such as bending, torsional, and tensile stresses. In addition, their surface morphology was analyzed using optical microscopy and scanning electron microscopy. For the electrical properties, in addition to the current– voltage curves, their dielectric properties were analyzed. Finally, we reported the successful wearable communication device of the cellulose-based FPCBs in a 5 × 5 touch panel and a 5 × 5 light-emitting diode display.


Introduction
Cellulose-based substrates in electronic devices have been the focus of various circuit applications owing to their nontoxic and environmentally friendly nature (Steckl 2013;Alimenti et al. 2015;Jung et al. 2015).
Paper electronics based on cellulose have various bene ts such as disposability, low cost, easy handling, and degradability. For example, the conductive ink circuits on a standard copy paper, which have foldability and exibility, provide craft and origami with electrical properties (Shin et al. 2016). In addition, paper electronics have demonstrated their use in various applications, including supercapacitors, rechargeable batteries, sensors and actuators, diagnostic devices, and micro uidic systems (Ko et (Ham et al. 2016). A exible and transparent transistor has been fabricated using CNTs and a conductive polymer. Moreover, the mechanical properties of cellulose-based substrates makes them easy to handle for tasks such as cutting, bending, and folding (Nagashima et al. 2014).
In industries and academic research, the exibility of an electronic circuit has a signi cant role in applications such as bending smartphones and wearable devices Son et al. 2014). Polyimide (PI) substrates are widely used for fabricating exible printed circuit boards (FPCBs) (Kamiya et al. 2007). FPCBs are being broadly developed for wearable devices capable of sensing body signals to perform electrocardiography and electromyography (Kim et al. 2008;Yeon et al. 2020). Furthermore, the exibility of an FPCB provides stable electrical performance for soft robots . Currently, the high-resolution patterning of FPCBs is being performed via photolithography for the fabrication of FPCBs for these advanced applications (Chu et al. 2019). This fabrication process provides an advanced In this study, we report the fabrication of a wearable touch panel and display array based on multilayer MC substrates. The material properties of the MC substrate were investigated using X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR). The mechanical stability of the substrates was investigated by measuring the stress-strain curves and resistance ratio (R/R 0 ) with respect to strain. Furthermore, the dielectric constants of the MC substrates with different thicknesses were evaluated from their capacitances. To analyze the electrical performance of the substrates with Ag/CNT composite circuits under mechanical stresses, the current-voltage (I-V) curves and resistance ratio with respect to bending and torsional stresses were measured. The Ag/CNT circuitry was patterned onto the substrates using a shadow mask, and the LEDs were implanted on the circuit pattern. The LED circuit pattern was extended to a 5 × 5 LED array, which could operate to display an English letter. In order to fabricate the wearable devices, a 5 × 5 ring Ag/CNT circuit pattern on an MC hole cover layer was layered onto another 5 × 5 Ag/CNT circuit pattern. The wearable device consisted of a touch capacitance sensor panel, which was worn around a human wrist. Using a small via hole on the MC layer, the multilayer LED circuit was fabricated and then analyzed by Scanning electron microscopy (SEM). The via hole layer was aligned with the Ag/CNT composite circuit pattern to fabricate the multilayer 5 × 5 LED array. Finally, the touch panel and LED display were connected using a microprocessor and personal computer system.

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MC powder (4-12% hydroxypropyl) and dimethyl sulfoxide (DMSO) were purchased from Sigma-Aldrich (USA). For a 20 wt.% suspension, MC and DMSO were mixed in the ratio 1:5, respectively, and sonicated for 1 h in a Powersonic 510 sonicator (Hwashin Tech, South Korea). The solubility of MC in various suspensions was monitored using diverse solvents, such as 99% grade EtOH (Sigma-Aldrich, USA) and isopropyl alcohol (IPA, ACS Material, USA)), through the solvent mixing process. After the suspensions were poured into at Petri dishes (4science, South Korea) of diameter 70 mm, the samples were dried on an MS300HS hotplate (iNtRON Biotechnology, South Korea) at 85°C for 24 h. After the samples were completely dried, they were shifted from the Petri dishes to the lab desk and maintained at room temperature for 1 h. For the electric circuits, the Ag/CNT composites were composed of Elcoat P-100 Ag conductive ink (Cans Co., USA) and CNTs of diameter 7-15 nm and length 0.5-10 µm (Sigma-Aldrich, USA). Ag and CNTs were mixed together in a 20:1 ratio in IPA and sonicated for 1 h. Then, the composite solutions were dried on a hotplate at 85°C for 30 min.

Characterization
The thicknesses of the MC substrates were measured using a BD500-300 digital caliper (Blue Tec, USA). The stress-strain curves of the substrates with varying thicknesses were measured using a tensile strength tester (Hand PI instrument, China). The resistances of the conductive material on the substrates were measured using a Keithley 2450 source meter (Keithley Instruments, USA), and the I-V curves were measured using the Keithley 2450 source meter with a custom software. The capacitances of the substrates were measured using an Agilent E4980A LCR meter (Keysight, USA). FTIR was performed using a Vertex 80v vacuum FTIR (Bruker, USA) and a Hyperion 2000 FTIR microscope (Bruker, USA). The

Circuit Fabrication
Red, orange, and blue surface mount device (SMD) type light-emitting diode (LED) components were purchased from Device Mart (South Korea). The circuit pattern was designed using AutoCAD 2020 (Autodesk, USA). For the shadow mask, a polyethylene terephthalate (PET) lm (4science, South Korea) was cut using L3020 laser cutters (Rexbot, South Korea). For the circuits, the Ag/CNT composite was deposited onto the MC substrates using the PET mask. To enhance the pattern resolution, the mask was attached to the substrate using a double-sided tape (3M, USA). After the PET mask was removed from the substrate, the Ag/CNT composite was cured for 1 h at room temperature. The SMD LEDs were assembled on the circuit pattern.

FPCB operation
The MC FPCB was connected to the Uno Arduino development board system (Arduino, Italy) and the development board was connected to a Microsoft foundation class (MFC visual C++). The MFC visual C++ was con gured as the MC FPCB's control and display. A customized program was developed to control the FPCB, as shown in Figure S4.

Result
The fabrication process of transparent MC FPCB Wearable and deformed electronic devices consisting of FPCBs that are based on polyimide (PI) are widely used in industries and academic research. Although PI has advantages, such as mechanical strength, high dielectric constant, and exibility, it is not an environmentally friendly material. To overcome this limitation, cellulose, which is found in wood and plants, has been used to replace PI as an FPCB substrate (Kim et al. 2006). Figure 1 shows the fabrication process and images of the exible circuits with MC substrates. In MC, 27-32% of the hydroxyl groups exist in the form of methoxy (Gustafsson et al. 1999) and thus MC dissolved in the aprotic polar solvent DMSO. Owing to the degree of substitution (DOS), MC was insoluble in the other solvents such as ethanol and alcohol as shown in Figure S1. Figure 1 (a) shows a schematic of the fabrication process of the MC substrates. First, 2% w/v of MC in DMSO was sonicated for 1 h to increase its solubility. The MC suspension was poured into a Petri dish and DMSO was evaporated at 85°C for 24 h using a hotplate. The evaporation of DMSO caused the completely dissolved MC (2% w/v) to change from a transparent gel state to a thin lm state (Figure 1 (b)). For controlling the thickness, the at MC substrates were fabricated in the Petri dishes of diameter 70 mm using the amounts of suspension as shown in Figure S2. By the dissolution in DMSO and the subsequent evaporation of DMSO, transparent and non-porous MC substrates for exible electronic devices were fabricated. Their surface morphology was determined using SEM (Figure 1 (c)). The thin and exible MC substrate could be used as an electronic circuit substrate in wearable devices or exible displays. To improve the mechanical endurance of the circuits, high aspect ratio CNTs were used for the Ag/CNT composites. The composite suspensions, which had various concentrations of Ag and CNTs in IPA, were sonicated for 1 h. Then, their viscosities were increased by heating them at 85°C for 30 min. The Ag/CNT composite circuits were deposited onto the MC substrates via stencil masks. Then, the circuits were completely dried at room temperature for 30 min.

Characteristics of MC substrate
The XRD pattern of the MC substrate is shown in Figure 2 (a). The diffraction curve exhibited high intensity peaks at 2θ values of approximately 13.4° and 20°; the strongest peak was at 20°. The FTIR spectrum of the MC substrate is shown in Figure 2

Stress-strain curves of the MC substrate
An FPCB should be robust in order to maintain a stable electrical performance under an external mechanical force. Figure 3 shows the force-strain curves and resistance ratios with respect to strain of the MC substrates with various thicknesses. The stress-strain curves of the 7.5 × 2.5 mm MC substrates were measured until they underwent failure as shown in Figure 3  We measured the resistance ratio (R/R0) with respect to the strain (mm/mm) of the MC substrates with various thicknesses as shown in Figure 2 (b). For these tests, Ag/CNT composites with a CNT concentration of 4 wt. % were used as the conductive material. The resistance of the Ag/CNT composite circuit on the MC substrate increased when stress was applied. Under a strain of 0.08 mm/mm, the electrical performance (R/R 0 ) of the circuits on the MC substrates with various thicknesses was stable.
However, from a strain of 0.08 mm/mm onwards, the resistance ratio (R/R 0 ) increased with the thickness of the MC substrate. At a strain of 0.266 mm/mm, the resistance ratio for the 100 µm substrate was 4.12, whereas that for the 300 µm substrate was 8.93.

Dielectric constant of MC substrate
We evaluated the dielectric constants of the MC substrates. Figure 4  where C is the capacitance, ε 0 is the dielectric constant of vacuum (ε 0 = 8.854 × 10 −12 Fm −1 ), A is the area of the electrodes, and d is thickness of the MC substrate. Figure 4 (b) shows a graph of the capacitances and dielectric constants with respect to the thicknesses of the MC substrates. The average dielectric constant of the substrates was approximately 2.87, with a minimum value of 2.81 (150 µm thickness) and maximum of 3.12 (200 µm thickness) owing to the surface uniformity and non-uniform thickness caused by the evaporation process. However, majority of the dielectric constant values were stable with thickness.

Mechanical test for MC FPCB
The substrates and circuits of MC FPCBs exhibit exibility. To improve the mechanical properties and electrical conductivity of the composite circuits, CNTs were mixed in the composites. Ag/CNT composite circuits provide mechanical endurance toward bending and torsional stresses. To determine the best electrical performance with mechanical bending and torsion, Ag/CNT composite circuits with various CNT concentrations were investigated via various methods as shown in Figure 5. Composites with four different CNT concentrations (0 wt. %, 4.3 wt. %, 9.9 wt. %, and 16.7 wt. %) were deposited onto the MC substrates in a 2 × 2 inch area. Figure S3 shows the images of the composites with four different CNT concentrations layered onto the MC substrates. Figure 5 (b) shows the SEM images of the A composites layered onto the substrates. Figures 5 (b) ( -) show the SEM images of the composite with the highest CNT concentration (16.7 wt. %). The morphology of the composite with a CNT concentration of 16.7 wt. % was larger than that of the other composites, which means that higher the concentration of the CNTs the higher the degree of particle bonding. We analyzed the I-V curves of the composites with four different CNT concentrations ( Figure 5(c)). The curves showed linear and symmetric current ow from -1.1 V to 1.1 V. The slope of the curves increased with the CNT concentration owing to the higher resistance of CNT than that of Ag. In addition, we tested the resistance ratio (R/R0) with respect to mechanical stresses such as bending and torsional stresses ( Figure 5(e)). The resistances of majority of the composites were stable during bending and torsion. However, during the bending test the resistance ratio of the composite with the 4.3 wt. % of CNTs was 1.02, whereas that of the composite with 16.7 wt. % of CNTs was 1.82 (green dot in Figure 5(d)). Furthermore, during the torsional test the resistance ratio of the composite with 4.3 wt. % of CNTs was 1.04, whereas that of the composite with 16.7 wt. % of CNTs was 2.36 (green dot in Figure 5 (e)). The resistance ratio, which increased by a factor of 4 for the composite with 16.7 wt. % of CNTs, indicated that the composites with 4.3 wt. % of CNTs provided a higher electrical performance.

LED circuits
Using the Ag/CNT composites, we fabricated SMD LED circuits on the MC substrates. A diagram of the fabrication process is shown in Figure 6 (a). The Ag/CNT composites were deposited with a 500 µm linewidth circuit pattern onto the MC substrates through the PET shadow mask, which was created using a laser cutter. The SMD LEDs were implanted on the circuit pattern. Then, the LED circuits were dried at room temperature for 0.5 h to completely evaporate the IPA from the Ag/CNT composites. Figure 6 (b) shows the LED circuit that was operated at 2.5 V to switch on and off. It shows three LEDs con gured in parallel in the circuit operated under similar conditions of LED light intensity and current ow. Figures 6  (c-f) show the optical microscopy images of the LED circuit that was deposited on the MC substrate.
The Ag/CNT circuitry, which had a thickness of approximately 50 µm because of the PET mask thickness, was deposited on the surface. The Ag/CNT composite circuit pattern was spread out compared to that of the design in the circuitry pattern process. The circuit was strongly bound to the surface of the MC substrate owing to the IPA suspension. In addition, the circuitry was in good contact with the contact pads of the SMD LEDs as shown in Figure 6 (e). The surfaces of the Ag/CNT composites circuitry were clearly observed in the SEM images as shown in Figures 6 (g-j). Despite the low clarity of the circuit line due to the shadow masking, it was observed that the 500 µm linewidth of the circuit was well patterned onto the MC substrate.

Single layer 5 x 5 LED display
Ag/CNT composite circuits with complex patterns could be deposited on the MC substrates. We fabricated a 5 × 5 LED display on the MC substrate using the PET shadow mask process (Figure 7 (a)). A thicker size of the composite was deposited on both ends of the LEDs to increase the contact between the LEDs and circuits. After the fabrication of the 5 × 5 LED display onto the MC substrate (Figure 7 (b)), the LED display was connected to the Arduino Mega board, which could directly control the 25 LEDs using a programming language. Each LED was operated individually using the Arduino board. To demonstrate the control of individual LEDs, we displayed the letters M and C as shown in gures 7 (c) and (d).

Wearable touch sensor array panel
For the application of the transparent and exible properties of the MC substrates, we demonstrated the operation of a wearable touch sensor array, which had a 5 × 5 capacitance sensor. Figure 8 (a) shows a diagram of a 5 × 5 touch sensor composed of two layers of substrates. The bottom substrate had an Ag/CNT composite capacitance sensor with wiring circuits. The top substrate was the protection layer for the wiring circuits and was open to the sensors by via holes. The two substrates were assembled using DMSO. The individual touch sensor in the array showed a capacitance of approximately 3.31 pF when touched with the index nger as shown in Figure S5. For multi-touch sensing, the Arduino Mega board was used as the 5 × 5 capacitance sensor. In addition, to monitor touch sensing in real time, we developed a customized monitoring program. Figure 8 (b) shows a diagram of the touch monitoring system. The input data of the touch sensor received by the Arduino board were transmitted by the customized monitoring program on a computer (the MFC display). Figure 8 (c) shows the operation of the touch sensor. The touched array position was displayed on the customized monitoring program by highlighting the position with a green marker (for more details, the codes are shown in Figure S6). Figure  8 (d) shows the operation of the wearable on the wrist of a hand. Its operation was demonstrated by touching the area of the wearable to display the letter C on the customized monitoring program.
Via holes for multilayer MC substrates Multilayer circuits are being used for advanced circuits and applications. The bottom and top layers are connected using via holes to increase the circuit density and design complex circuits. The top MC substrate layer with a via hole and the bottom MC substrate layer were assembled using DMSO, which was similar to the fabrication of the wearable touch sensor array. An LED was implanted on the Ag/CNT circuit of the top layer. Finally, the Ag/CNT composite was deposited at the via hole to connect the two layers. Figures 9 (a) and (b) show a diagram of a multilayer LED circuit and an image of the circuit, respectively. Figure 9 (c) shows a SEM image of the via hole for the multilayer MC substrate. The Ag/CNT composite circuits were patterned onto a double-layer MC substrate. The top and bottom layers were connected by the Ag/CNT composite through a 500 µm diameter via hole as shown in gure 9 (e).  Figure S8. To show the LED pattern, the 5 × 5 LEDs were repeatedly activated using the Arduino board by applying 2.5 V at 20 Hz. Although the operation process of a two-layer LED display is more complex (Kang et al. 2017) than that of a single layer one, which applies an individual voltage, the two-layer 5 × 5 LED display has a simpler circuit. Figure  10 (h) shows the transparency of the 5 × 5 LED display using multilayer MC substrates for the LED operation.

Communications for the cellulose devices
Finally, we demonstrated the operation of a real-time communication system for a wearable touch panel and a 5 × 5 LED display using two Arduino modules and an MFC program as shown in Figure 11 (a). The data of the touch pattern was recognized by the wearable touch panel and transmitted to the customized pattern generation program in real time through the receiver (RX) Arduino board. The pattern generation program transmitted the data to the transmission (TX) Arduino board to create the pattern on the 5 × 5 LED display. Figure 11 (b) shows a single position of the array display based on the data received from the touch panel. Figure 11 (c) shows the LED pattern created from the data received for multiple positions. The LEDs of the 5 × 5 display were scanned at 20 Hz. Figure 11 (d) shows the pattern of the letter X recognized by the touch panel after several touch actions. Subsequently, the letter X was created on the LED display as shown in Figure 11 (e). Circuits made of cellulose-based substrates with conductive composites were fabricated for their use in wearable communication devices such as touch panels and LED displays.

Conclusion
In addition to unique properties, such as exibility, recyclability, high dielectric constant, and light weight, eco-friendly and renewable electronics based on cellulose provide a novel eld of academic research and commercial interest (Pandey et al. 2012;Pang et al. 2021). Because of their excellent recyclability and transparency, cellulose-based substrates can host more types of electronics than conventional FPCBs that are not environmentally friendly, although the process of creating a cellulose-based substrate can be extremely challenging. Here, we reported FPCBs consisting of transparent and exible cellulose-based substrates fabricated by a simple evaporation process. Ag/CNT composites were used for the circuits, which were patterned onto the cellulose-based substrates using a shadow mask. The electrical and mechanical properties of the MC substrate-based FPCBs were observed under diverse mechanical stresses such as bending, torsional, and tensile stresses. Excellent dielectric properties were observed using the capacitance measurements. In addition, the morphology of the MC substrate circuits were analyzed using optical microscopy, SEM, and XRD. To demonstrate the application of the FPCBs, we fabricated a 5 × 5 touch panel that successfully controlled a 5 × 5 LED display, which was fabricated using MC FPCBs as well.