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. 2020 Dec 28;6(3):1960–1970. doi: 10.1021/acsomega.0c04792

Vertically and Horizontally Drawing Formation of Graphite Pencil Electrodes on Paper by Frictional Sliding for a Disposable and Foldable Electronic Device

Junseo Kim 1, Dahye Ahn 1, Jingzhe Sun 1, Siyong Park 1, Yujang Cho 1, Sangki Park 1, Sumin Ha 1, Seongcheol Ahn 1, Yoong Ahm Kim 1,*, Jong-Jin Park 1,*
PMCID: PMC7841772  PMID: 33521436

Abstract

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The objective of this study is to fabricate an electrode by frictional sliding caused by a rough paper surface. The pressure exerted during drawing induces adsorption of the graphite particles by the rough paper and simultaneously reduces the surface roughness of the paper electrode. Repetitive drawing in one-way direction reduced the roughness of the paper surface, decreasing the grain boundaries of graphite. This increases the electron pathway at the electrode, thus reducing the resistance to less than 50 Ω. At the same time, repetitive drawing could confirm that unstable errors caused by the hand could help converge within a certain margin of error. We quantified the relationship between pressure and resistance when drawing on the electrode using a pencil hardness tester. In addition, the electrodes formed by repeated drawing generated a new surface grain and boundary, parallel to the drawing direction, and changed the electrode characteristics with respect to the drawing direction. The grain boundary difference based on the drawing direction was measured via a heating test of the foldable device, a sound pressure level, and laser scattering vibrometer measurements of a linear speaker. The fabricated graphite electrodes can be used in disposable foldable paper electronics because they are prepared using inexpensive materials.

Introduction

Recent studies on paper electronics have reported the use of light and flexible paper as a substrate to manufacture various devices and electrodes.1−3 Paper is made of cellulose fibers with variable spacing based on the type and structure of cellulose, resulting in varying permeability and flexibility. The paper substrates are made into electrodes by various methods that do not require a high-temperature process, as follows: ink-jet printing by a drop-on-demand method to drop microdroplets,4,5 microfluidic devices,6−8 electrohydrodynamic technology using solution conductors,9−11 screen printing using conductive ink such as carbon nanotube (CNT) or silver nanowire,12−14 and direct pen writing, which forms an electrode by drawing on a paper with a pencil directly.15−17 Thus, the electrodes formed can be used as application devices, such as flexible displays,18−20 touchscreens,21 and supercapacitors.22−30 In addition, it is advantageous to manufacture environmentally friendly disposable devices for one time use because compared with other materials, it is a low-cost and biodegradable invention.2,31−33 The direct pencil writing by hand processing is advantageous in that the electrode is formed by simply drawing with a graphite pencil on paper, which is the simplest, cost-effective, and disposable method.34,35 Paper is produced from raw wood pulp softened with chemicals followed by separation of fibers from the pulp by mechanical actions that are energy intensive.36 The torn fibers are transformed into various types of papers via refining and filling processes.37 Paper fabricated by this process exhibits different physical properties and surface roughness, depending on the fiber material. The surface roughness of a paper substrate causes a frictional sliding when a graphite electrode is formed by drawing on paper with a pencil.38−40 Friction sliding refers to the force by which the motion of dry frictional sliding occurs between two contact interfaces with roughness. Frictional sliding is dependent on the difference between the coefficient of static friction and the coefficient of kinetic friction, the decrease in the coefficient of kinetic friction as the speed increases, and the degrees of roughness on the surface.38 Frictional sliding is usually used to explain the mechanical wear and to analyze the frictional force that occurs when rocks in the stratum move. Frictional sliding can be used to form pencil graphite electrodes by drawing on a paper substrate.40 When the roughness of the paper is varied, the frictional sliding force applied to the pencil varies, which in return varies the amount of the graphite transferred onto the paper. As such, the roughness of paper is an important factor in determining the resistance value when an electrode is formed by drawing with pencil graphite.42,43

This study examined the characteristics and applications of the foldable and disposable electrode made using the characteristics of frictional sliding differently and according to vertical and horizontal directions as drawing with a graphite pencil on the surface with different roughness depending on the type of cellulose fiber used. The difference in conductivity between the two electrodes was confirmed using a thermal imaging camera. Electrodes have different bending properties with respect to the drawing direction. Finally, we demonstrate that it is possible to form adequate electrodes for speaker application by hand drawing, and this is the first study of the characteristics and applications of foldable and disposable electrodes.

Results and Discussion

Figure 1 shows the concept of pencil graphite electrode formation and frictional sliding used in this experiment by hand drawing and mechanical drawing. As shown in Figure 1a, we confirmed that the surface morphology of kraft paper and printing paper is made of cellulose fiber, and each has different roughness corresponding to each paper. When drawing with the graphite pencil on the paper substrate, frictional sliding promoted adsorption on the paper, resulting in transferring the graphite on the paper based on the paper’s roughness (Figure S1). Stick slip, one of the frictional sliding, occurs when an object with a mass moves over a rigid substrate surface with irregularities,37 as shown in Figure 1a.

Figure 1.

Figure 1

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Schematic representation of the method of forming a graphite electrode by drawing on paper: (a) pristine paper and (inset) SEM image of printing paper and kraft paper. (b) Image drawing in a certain area using a pencil hardness tester. (c) Images of the surface measured with a 3D optical image in printing paper: (c-1) is for “before drawing”, (c-2) is for drawing 50 times with the hand, and (c-3) is for drawing 50 times with a pencil hardness tester. (c-4) SEM image that shows surface morphology of a printing paper-based graphite electrode. (d) Images of the surface measured with a 3D optical image in kraft paper: (d-1) is for “before drawing”, (d-2) is for drawing 50 times with the hand, and (d-3) is for drawing 50 times with a pencil hardness tester. (d-4) SEM image that shows surface morphology of a kraft paper-based graphite electrode.

During the stick-slip phenomenon, the force generated by the roughness of the paper surface can be determined by the following equation40

graphic file with name ao0c04792_m001.jpg 1

where m is the mass of the slider, K denotes a constant in the frictional sliding phenomenon, v represents the drive velocity, t denotes the time, x denotes the pencil drawing displacement, Inline graphic is the static frictional force, and Inline graphic is the kinetic frictional force. Details about derivation of the equation can be seen in the Supporting Information. When the sliding force matches the frictional force Inline graphic, sliding occurs; and frictional force Inline graphic is defined as Inline graphic, where Inline graphic is the force from the rough surface and Inline graphic is the frictional force generating when applying the force to the pencil on the rough surface. When Inline graphic is greater than Inline graphic, the sliding accelerates and slides at a higher velocity.36 Increased roughness raises the value of Inline graphic, which indicates a stronger force at the start of frictional sliding, and a strong slip induces additional adsorption of graphite. Therefore, a paper with roughness adsorbs additional graphite compared with a general polymer film, and a kraft paper with higher roughness adsorbs more graphite than a printing paper (Table S1).

Figure 1b is an image when a graphite electrode is formed using a pencil hardness tester within a determined area with a constant speed as quantifying a constant pressure by a weight. After quantifying, the relationship between resistance and pressure when drawing electrodes using a pencil hardness tester is shown in Figure 3b.

Figure 3.

Figure 3

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Feature that occurs when a graphite pencil is repeatedly drawn in one direction. (a) Resistance value according to the number of times of drawing when a graphite pencil is used to draw in one direction on each paper substrate. (b) Variations in resistance when drawing time and pressure are varying in a pencil hardness tester. (c) Temperature change according to time when voltage is applied to electrodes drawn in the horizontal direction and electrodes drawn in the vertical direction. (d) An image of the heat generated by the electrode with an infrared camera (Movies S1 and S2 of the Supporting Information). (e) Resistance change by electrode type according to bending cycle. (f) Graph showing the value as R/R0.

Figure 1c shows images measured with a 3D optical profiler for the surface roughness on the printing paper: before drawing (c-1), for the electrode drawn 50 times with the hand (c-2), and for the electrode drawn 50 times with a pencil hardness tester (c-3). (c-4) is the SEM image of the graphite electrode drawn 50 times on the printing paper. Figure 1d shows images measured with a 3D optical profiler for the surface roughness on the kraft paper: before drawing (d-1), for the electrode drawn 50 times with the hand (d-2), and for the electrode drawn 50 times with the pencil hardness tester (d-3). (d-4) is the SEM image of the graphite electrode drawn 50 times on the kraft paper.

Before drawing, the paper surface is irregular due to the network of cellulose fibers, as shown in Figure 1c-1,d-1. At its peak, the slider descends quickly into the valley, resulting in a slip.39−41 Here, valley means the depth between the fibers. On a surface with larger roughness, a stronger slip induces additional graphite adsorption on the paper. When the electrode is drawn 20 times, there were some graphite particles unabsorbed on the surface, making the surface less smooth (Figure S2). At the same time, after drawing 50 times, the surface roughness is gradually reduced by the pressure on the paper and the frictional force generated by sliding, as shown in Figure 1c-2,c-3,d-2,d-3. Also, it was confirmed that the graphite particles are formed on the rough paper by frictional sliding and accumulated in the valley by repeated drawing, resulting in a smooth graphite layer, as shown in Figure 1c-4,d-4.

The pressure from hand drawing induced adsorption of the graphite particles on the rough paper, and the Rq (root-mean-square surface roughness) value of the paper surface decreased from 5.19 to 1.29 μm in the case of printing paper and from 5.99 to 1.83 μm in the case of kraft paper (Table S1). The difference in the surface roughness of the electrode is relative to the pathway of electron, as reported in previous studies.40,41 The paper electrode fabricated by drawing with a graphite pencil has low surface roughness due to frictional sliding, which increases the electron pathway of the electrode, resulting in a lower resistance.42

By contrast, graphite electrodes formed on paper exhibit different levels of resistance, depending not only on the roughness of paper but also on the grade of the graphite pencil. Based on the roughness of various kinds of papers, we selected the A4 size printing paper and kraft papers that are economical and easily available commercially. The graphite pencil leads are divided into 1H–9H, HB, and 1B–9B, depending on the content of fine graphite particles and clay binder. Approaching 9B, the graphite content increases, and darker pencil traces occur. As the amount of binder is reduced and the graphite content is increased, the resistance of the pencil is reduced, and the lead is softened as the pencil grade approaches 9B.17Figure 2a shows the resistance of the electrodes formed after drawing 50 times using the pencil grades HB, 2B, 4B, 6B, and 8B. The resistance of the pencil lead is the lowest in 8B, which has a high graphite content; however, a higher resistance was observed with 8B than 6B when used on paper. The 8B pencil contains a lower ratio of binder compared with 6B, yielding a very low hardness, preventing the application of adequate force to make frictional sliding. Therefore, the best conductivity by frictional sliding resulted from 6B pencil hardness. Figure 2b shows the roughness variation (Rq) when 6B and 8B graphite pencils were used to draw 20 and 50 times, respectively. The use of the 8B graphite pencil leads to a minor decrease in roughness (Figure S2 and Tables S1 and S2). The disadvantage of the 8B graphite pencil was once again confirmed with XRD based on the surface intensity of the electrode (Figure 2c,d). The electrodes were formed on paper by drawing 50 times with a graphite pencil of different graphite grades. A 2θ value of 26° represents a normal hexagonal (2H) graphite (002) peak,44,45 and as the pencil grade varied from HB to 6B, the surface (2H) graphite (002) peak increased and then declined rapidly at 8B. In Figure 2c,d, the XRD data of the 8B pencil on the printing paper shows that the peak intensity of graphite is 12,600, while that of 6B shows that it is 27,900. Although the 8B pencil has more amount of graphite than the 6B pencil, it has less XRD intensity than the 6B pencil. Similar to the XRD result of printing paper, for the kraft paper, the intensity of the 8B pencil is lower than that of the 6B pencil. Therefore, in this study, the 6B graphite pencil, which exhibits the lowest resistance on a paper substrate among any other pencil grades, was selected as the best pencil grade for further analysis. Figure 2e shows the XRD results of the surface intensity values of the electrodes when a 6B graphite pencil was used to draw on printing paper and kraft paper 20 and 50 times, respectively. Using the 6B graphite pencil, we confirmed that each graphite electrode layer was smoothly formed by filling the empty space. However, when using the 8B graphite pencil, we confirmed that the graphite particle was not adequately adsorbed, and the graphite layer was not formed fully. This was also confirmed through the cross section of the paper with the optical microscope shown in Figure 2f,g. In both kraft paper and printing paper, the graphite layer was formed thicker with the 6B pencil than with the 8B pencil.

Figure 2.

Figure 2

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Graph showing characteristics that depend on the graphite grade and type of substrate. (a) Resistance of electrodes formed when drawing 50 times using a graphite pencil with different grades on two types of papers. Inset: top view of blackness, which varies depending on the grade of graphite, using an optical microscope. (b) Surface roughness value of the substrate when drawing 0, 20, and 50 times using graphite pencils 6B and 8B. (c, d) Surface intensity value of the electrode measured by XRD on printing paper and kraft paper, respectively. (e) Surface intensity value of the electrode formed with 0, 20, and 50 times drawing using a 6B graphite pencil on the two papers. (f, g) Optical images of the graphite electrode formed by 50 times drawing of 6B and 8B graphite pencils on printing paper and kraft paper, respectively.

In Figure 3a,b, as a method to quantify the pressure when drawing by the hand, the number of drawings and the applied pressure were quantified using a pencil hardness tester, by matching with the results obtained by drawing with the hand.

Figure 3a shows the resistance of an electrode generated by drawing with pencils in one direction on paper with different levels of roughness. When the graphite produced by frictional sliding is adsorbed on the surface of the paper resulting from the differences in roughness, the rougher paper may adsorb more graphite.41 Therefore, the larger the surface roughness of the paper, the thicker the graphite layer containing the π–π stacking. However, since it was difficult to fabricate networks caused by the deep valleys, resistance was measured high despite the reduced roughness in early stage. The repetitive drawing reduced the roughness of the substrate itself, and the graphite layer was deposited, facilitating a better network formation. Drawing for more than 30 times reduced the surface roughness of kraft paper compared with that of printing paper and increased the graphite particle adsorption on the electrodes. Also, drawing in a specific direction increased the adsorption of graphite on the paper depending on the number of repetitions, with little decrease in resistance after repeating 50 times. When the drawing frequency was higher than 50, the roughness of the paper substrate was sufficiently reduced and interfered with frictional sliding. In addition, as the graphite particles filled the depths (valleys) of the rough paper and crossed the saturation point for graphite adsorption, the resistance was reduced.

In the case of printing paper, the resistance was about 400 Ω during the first 10 times with a large error range of about 46 Ω, with larger errors accompanying fewer strokes. However, as the frequency of drawing increased to over 50, the resistance was 83–106 Ω, and the error was greatly reduced to approximately 7 Ω. In the case of kraft paper, the error range was 70 Ω during the first 10 times. However, when the drawing frequency was increased to over 50, the resistance was 62.5–48.7 Ω, and the error was approximately 7 Ω. Therefore, this experiment confirmed that the electrodes obtained at a drawing frequency greater than 50 with almost constant force showed nearly constant resistance. As shown in Figure S3 of the Supporting Information, the surface electrode exhibits similar resistance at almost all the drawing ranges exceeding a frequency of 50. Initial drawing induced a large error in resistance due to fewer connections of the graphite layer and a lot of boundaries. However, repeated drawing lowered the resistance and the error because of the grain overlap, and the number of boundary lines is also reduced by repeated drawing.

In addition, Figure 3a shows that the electrodes drawn in the horizontal direction with respect to the current yielded a lower resistance compared with the electrodes drawn in the vertical direction. The electrode that is drawn in the perpendicular direction to the current flow shows a relatively higher number of boundaries than the electrode boundary generated by drawing in the horizontal direction. As the number of boundaries increases, the current flow is disturbed, the electron conductive pathway decreases, and the resistance becomes higher. This phenomenon was confirmed by the heat test (infrared thermal camera, Ti400) using the power supply in Figure 3e,f.

Since it is difficult to quantify the pressure when drawing on the paper substrate by the hand, the experiment shown in Figure 3b was conducted by quantifying the pressure with a pencil hardness tester. Among the various conditions, as shown in Figure 3a, the resistance result drawn on an area of 4 cm × 1 cm with an acceleration of 1 mm/s2 in the horizontal direction on the kraft paper was the best; therefore, the experiment was conducted with this method.

Using the area and acceleration of the fabricated electrode, the pressure could be calculated as follows

graphic file with name ao0c04792_m012.jpg 2

where “P” refers to the pressure, “F” refers to the force, “A” refers to the area of the drawn pencil lead, “m” refers to the mass of the weight, and “a” refers to the acceleration.

The acceleration required when calculating the pressure and the area of the pencil lead are described in detail in the Supporting Information and eq 2.

As shown in Figure 3b, from the experimental results of the drawings, with the pressures of 3.8 kPa (50 g of weight), 7.8 kPa (100 g of weight), 11.8 kPa (50 g of weight), 15.8 kPa (200 g of weight), 19.8 kPa (250 g of weight), 23.8 kPa (300 g of weight), and 27.8 kPa (350 g of weight), 10, 30, and 50 times, it was confirmed that the resistance decreased as the number of drawing and pressure increased. In particular, it was confirmed from the graph that the resistance (62.6 Ω) obtained by drawing 50 times at a pressure corresponding to 26.1 kPa is a resistance value within the error range of ±0.1 Ω from the resistance (62.5 Ω) of the electrode formed by repeated drawing 50 times on kraft paper by the hand. That is, the pressure applied when drawing 50 times on kraft paper by the hand was 26.1 kPa within the error range from the resistance variation graph obtained when drawing 50 times with a pencil hardness tester.

Figure 3c,d shows the changes in resistance when the electrode underwent the bending cycle. Figure S4a shows the schematic of the electrode in the bent shape, and Figure S4b,c shows the resistance change of each electrode under bending and respreading. The resistance change of the vertical electrode was smaller than that of the horizontal electrode, regardless of the paper type. When paper graphite electrodes are bent, the graphite electrodes receive the outward direction bending moment.46 The electrode drawn in the horizontal direction showed few boundaries that interfere with the flow of electrons in the horizontal direction initially. When bending, cracks were further generated on the grain upon, thereby increasing the resistance. Even when the electrode returned to the initial state where the electrode was bent and flattened again, the crack acted as a new boundary, and the resistance of the electrode was relatively increased. On the other hand, the graphite electrodes drawn in the vertical direction exhibited a relatively larger number of boundaries than those drawn in the horizontal direction. When the vertical direction electrode was bent, the graphite particles were separated bilaterally based on the boundary followed by readherence, which inhibited the formation of new cracks (Figure S5).

Also, the resistance in the electrode varied depending on the thickness of the paper. When an object is bent, the inner side of the object exhibits a smaller variation in length, and the outer side of the object shows a longer length change (Figure S6). Therefore, when bending, the thicker the object, the more the change in the length of the outer surface. As the thickness increases, the graphite electrode undergoes stronger tensions in both directions, and larger defects generated in the graphite electrode increase the resistance of the graphite electrode further. The kraft paper (220 μm) used was about twice as thick as the printing paper (110 μm), showing a significant resistance change in the bending test of the kraft paper graphite electrode (Figure 3c,d).

Figure 3e shows the heat temperature over time under a force of 20 V and 0.05 A for a certain period. A current of 0.0485 A was applied at 20 V in the electrode drawn in the horizontal direction, and a current of 0.0389 A was used at 20 V in the electrode drawn in the vertical direction (based on 4 cm).

In the case of the horizontal sample, the interference with the flow of electron was minimal when compared with that of the vertical sample. During the first 10 s, the temperature initially showed a high slope, causing a sharp increase in temperature and gradually increasing over time, and then stabilized at constant values at 62 and 54 °C.

Figure 3e,f shows the images, from a thermal imaging camera, of the heat generated when voltage (20 V) is applied to the electrode as a power supply. The temperature of the electrode starts to heat from 10 s, and the quantity of heat at 30 s is calculated using the Joule heating method. The Joule quantity of heat (heating energy) Q is proportional to the square of the current, resistance, and time, and the equation is as follows

graphic file with name ao0c04792_m013.jpg 3

Here, “Q” refers to the quantity of heat, “I” refers to the current, “R” refers to the resistance, and “V” refers to the voltage.

The induction process of the equation was described in detail in Joule’s law in the Supporting Information. Therefore, according to eq 3, the horizontal direction drawing with low resistance and high current has a larger current value than the vertical direction drawing, resulting in more heat generation. Here, the horizontal direction electrode has a Joule heat of H = 4.41 J, and the vertical direction electrode has a Joule heat of H = 3.22 J. In addition, the temperature gradually increased until 30 s after applying a constant voltage, and after 30 s, the temperature was constantly and slowly increased.

It was confirmed that a grain boundary existed based on the result of adsorbing graphite in the electrode formed by drawing in the horizontal direction and in the vertical direction.

Since the grain boundary is a barrier that obstructs the flow of electric current, when the grain boundary increases, barriers hindering the flow of electric current increase and then the resistance increases. It shows that the electrode drawn in the horizontal direction has lower resistance than the electrode drawn in the vertical direction so that it has higher power and thus, the heat generation increases.

Figure 4 shows the differences in electric characteristics according to the drawing direction based on the sound pressure level (SPL) and laser scattering vibrometer (LSV), using paper electrodes to manufacture a linear speaker. First, the electrodes formed by drawing in the horizontal and the vertical directions on the printing paper and the kraft paper were fabricated and placed on both upper and lower sides. Then, the PVDF piezoelectric film was put between the graphite electrodes to produce a paper speaker. Figure 4a shows the structure of the speaker used in the experiment. Figure 4b–e shows the speaker vibrations at 10 kHz using an LSV. Based on the Doppler effect, the LSV continuously calculates the reflection time of the laser, to monitor and image the vibrations. As shown in the legend variation width in the figure, the speaker using the electrode in the horizontal direction generates a higher difference in vibration displacement than the speaker using the electrode drawn in the vertical direction. The difference was attributed to fewer graphite boundaries in the case of the speaker with electrodes in the horizontal direction compared with those in the vertical direction, resulting in the smooth transfer of electrons. Figure 4b,c shows that in the case of printing paper, the speaker with electrodes in the horizontal direction vibrates with a maximum vibration of 20 μm/s, i.e., −10 to 10 μm/s, while the speaker with electrodes in the vertical direction vibrates at a maximum of 7 μm/s, i.e., 0 to 7 μm/s. Figure 4d,e shows that in the case of kraft paper, the speaker with electrodes in the horizontal direction revealed a maximum vibration of 10 μm/s, i.e., −5 to 5 μm/s, while the speaker with electrodes in the vertical direction showed vibrations of up to 8 μm/s, i.e., −4 to 4 μm/s. The results confirm that the speaker with horizontal electrodes in both paper electrodes showed greater vibration. The amplitude of the vibrations per second determines the SPL of the speaker. Figure 4f,g shows that the speaker with the electrode drawn in the horizontal direction exhibited a larger SPL than the speaker with the electrode drawn in the vertical direction. The sound source used for the measurement produced a pink noise using a mobile phone (Samsung Galaxy S9), and a commercial amplifier was used. The maximum sound pressure was increased from 40 dB (vertical) to 50 dB (horizontal) for printing paper and from 40 dB (vertical) to 48 dB (horizontal) for kraft paper. In the case of printing paper, the paper was relatively thin and, therefore, exhibited a strong SPL in the high tone region. On the other hand, the kraft paper was relatively thicker, and the overall sound pressure was reduced. Instead, its spectrum showed wide frequency by oscillating in a region of relatively low frequency (Hz). Kraft paper with high conductivity exhibited lower vibration and SPL than the printing paper. Based on the thickness of the paper used as the substrate, the kraft paper was twice as thick, suggesting that the vibration characteristics of the speaker varied accordingly.47

Figure 4.

Figure 4

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Measured data to compare the electric characteristic difference according to drawing direction. (a) Schematic representation of a linear speaker manufactured using a paper substrate and graphite pencil. (b–e) Vibration of a speaker taken with a laser scattering vibrometer (LSV). (b) Horizontal direction, printing paper (Movie S3). (c) Vertical direction, printing paper (Movie S4). (d) Horizontal direction, kraft paper (Movie S5). (e) Vertical direction, kraft paper (Movie S6). (f, g) Graphs showing the SPL output value of the speaker for each frequency (Hz) for printing paper and kraft paper, respectively.

Figure 5 shows various devices that were manufactured using the graphite electrodes. Handwritten graphite electrodes are difficult to use as permanent electrodes because of the durability of paper electrodes; however, they can be easily disposed in everyday life. Figure 5a shows an expanded view of the manufactured touchscreen. The touchscreen used here was piezoresistive and based on the recognition and contact with two graphite electrodes separated by pressure. The two graphite electrodes transmit signals in the x and y planes, and the voltage in each direction is determined when the two electrodes are in contact with each other. The electrodes were fabricated by drawing in one direction, to facilitate smooth signal transmission in each direction. It is also important to have a spacer in the middle to separate the upper and lower electrodes. Due to the enormous roughness of ordinary paper, dot spacers do not work efficiently. However, due to repetitive drawing, the roughness of the paper substrate was reduced, and the spacer effectively separated the two graphite electrodes. Figure 5b shows the operation of the touchscreen manufactured using this technique. Figure 5c shows a foldable paper model with LED bulbs. In this case, the electrode was formed by drawing in the horizontal direction, and in the folding portion, the electrode was drawn in the vertical direction to enhance the bending strength and durability. As a result, the device performed well during folding, which was enhanced by supplemental drawing in the vertical direction. Figure 5d shows that the transformable speaker performed adequately even when it was bent and spread; the measured sound varied from 49.8 dB at silence to a maximum volume of 74.4 dB. Also, Figure 5e shows a paper folded in a lion shape with a foldable speaker inside the model. Our paper speaker was used to measure up to 69.5 dB of sound when playing a low-pitched animal roar.

Figure 5.

Figure 5

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Various devices manufactured using the electrodes formed in this experiment. (a) Exploded view of a piezoresistive touchscreen made with paper and graphite electrodes. (b) Appearance of working (a). (c) Working of foldable paper models attached with LED light bulbs. (d) Transformable speaker made of graphite electrodes in the inside to produce sound even when the shape changes. (e) Paper speaker folded as a lion shape and inserted a graphite electrode.

Conclusions

In this study, when a graphite pencil was hand drawn on paper, frictional sliding caused by the roughness of the paper surface was used to form electrodes for foldable paper electronics. When a graphite pencil was used to draw on printing paper and kraft paper, the roughness was reduced by hand pressure and formed high conductivity. The pressure by hand drawing caused graphite particles to be adsorbed by the roughness of paper. The Rq value of the paper surface was decreased from 5.19 to 1.29 μm for printing paper and from 5.99 to 1.83 μm for kraft paper. The larger the roughness of the paper substrate, the stronger the frictional sliding observed. Also, a new boundary was formed in the direction of horizontal and vertical drawing on the paper. Using a thermal imaging camera, we measured the heat generated by applying the same voltage to the electrodes drawn in vertical and horizontal directions as 54 and 62 °C, respectively. Repetitive hand drawing showed that resistance of the electrode was reduced to within 70 Ω for kraft paper and the error value converged to a level of 9 Ω. SPL and LSV results are found to be different due to the conductivity difference of the electrode formed by drawing in vertical and horizontal directions. The maximum sound pressure was increased from 40 dB (vertical) to 50 dB (horizontal) for printing paper and from 40 dB (vertical) to 48 dB (horizontal) for kraft paper. In addition, electrodes drawn in the vertical direction have higher durability when bent because the boundary of electrodes prevents the formation of cracks due to pulling of graphite particles on both sides based on the boundary. We demonstrated that this hand drawing process can be applied to foldable LED electrodes and foldable speakers for disposable paper electronics.

Experimental Section

Fabrication of Graphite Electrode

Graphite electrodes were made using a graphite pencil purchased from Faber-Castell (Germany), printing paper, and kraft paper (Hana Paper, Korea). Paper thicknesses of printing paper and kraft paper were 0.1 and 0.15 mm, respectively. The electrode size was 4 cm × 1 cm, and we drew on the papers with a graphite pencil at intervals of 1 mm every 50 times. Surface roughness was observed using a 3D optical profiler (NV-E1000, NanoSystem). The amount of graphite on the paper surface was determined at room temperature using an XRD system (D8 ADVANCE, Bruker AXS GmbH, Karlsruhe, Germany).

Fabrication of Foldable Speaker

A foldable PVDF film loudspeaker is a sandwiched membrane structure that contains a PVDF film with sandwiched papers drawn with a graphite electrode. Local acoustic performance of the speaker was characterized using a 3D laser vibrometer (Polytec, PSV-500) and a sound meter (CLIO 11, Audiomatica). Performance of the speaker was investigated by measuring the sound pressure level (SPL) in an anechoic chamber (to eliminate environmental noise from surroundings). A commercial PVDF film was obtained from Fils Corporation (Korea). The paper lion model was obtained from Nicole paper (Korea).

Fabrication of Paper Touch Panel

The graphite electrode was drawn on the paper at intervals of 0.2 mm every 50 times. We drew on the paper with a graphite pencil at intervals of 0.2 mm every 50 times. We dotted UV gel as a dot spacer on a large area of square-shaped graphite electrodes at intervals of 5 mm and cured it using a UV lamp. We used pristine paper as the spacer of the edge. To create a connection point, two Au textile electrodes were attached on the bottom of the paper substrate and one on the top of the paper substrate. We covered the paper electrode with an electrode facing it. The manufactured touch panel was used in connection with a computer through a specialized bluetooth module.

Characterization of the Performance of Graphite Electrode

Voltage was applied to the graphite electrode using a power supply (2200-72-1 programmable DC power supply, Keithley) to generate heat and observe the thermal change visually through an infrared thermal camera (Ti400, Fluke). Resistance was measured using a noncontact resistance measurement (TF map 2525 SR and TF lab 4040, EddyCus, SURAGUS), a multimeter (Card HiTester 3244-60, Hioki), and a source meter (Keithley 2450 SourceMeter, SMU Instrument, Tektronix). Also, the surface of the graphite electrode was monitored by field emission SEM (FE-SEM, S-4700, Hitachi).

Acknowledgments

This research was financially supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (nos. NRF-2017R1A2A1A17069771 and NRF-2018R1A2B6005220). Additional support was provided by the Ministry of Trade, Industry and Energy (MOTIE, Korea) under the Industrial Technology Innovation Program 20003970, smart device connected, and textile-based smart function mounted outdoor garment for smart life with ergonomic design.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.0c04792.

  • Figure S1, schematic image of the fabrication of the electrode; Figure S2, SEM image of graphite particle when drawing on the paper; stick-slip equation; Figure S3, surface roughness of a paper substrate measured using a 3D optical profiler; Figure S4, noncontact sheet resistance mapping results measured by TF map 2525SR; pressure equation; Joule’s equation; Figure S5, resistance change according to the bending radius and bending cycle of the electrode formed on paper; Figure S6, SEM image obtained by measuring cracks occurred in the bending test to the electrodes in the horizontal and vertical drawing directions; Figure S7, simple schematic to show the amount of change in length of the outer surface with respect to the thickness of the object when the object is bent; Table S1, surface roughness values of paper measured using a 3D optical profiler with 0, 20, and 50 times drawing of a graphite pencil (6B) in printing paper and kraft paper; and Table S2, surface roughness values of paper measured using a 3D optical profiler with 0, 20, and 50 times drawing of a graphite pencil (8B) in printing paper and kraft paper (PDF)

  • Movie S1, heating experiment of the horizontally drawn electrode (MP4)

  • Movie S2, heating experiment of the vertically drawn electrode (MP4)

  • Movie S3, LSV data of the horizontally drawn electrode on the printing paper (MP4)

  • Movie S4, LSV data of the vertically drawn electrode on the printing paper (MP4)

  • Movie S5, LSV data of the horizontally drawn electrode on the kraft paper (MP4)

  • Movie S6, LSV data of the vertically drawn electrode on the kraft paper (MP4)

  • Movie S7, touchscreen pad test (MP4)

  • Movie S8, paper speaker folded as a lion shape (MP4)

Author Contributions

† J.K. and D.A. contributed equally to this work.

The authors declare no competing financial interest.

Supplementary Material

ao0c04792_si_001.pdf (827.9KB, pdf)
ao0c04792_si_002.mp4 (4MB, mp4)
ao0c04792_si_003.mp4 (4.6MB, mp4)
ao0c04792_si_004.mp4 (4MB, mp4)
ao0c04792_si_005.mp4 (3MB, mp4)
ao0c04792_si_006.mp4 (2.3MB, mp4)
ao0c04792_si_007.mp4 (2.2MB, mp4)
ao0c04792_si_008.mp4 (3.5MB, mp4)
ao0c04792_si_009.mp4 (2.5MB, mp4)

References

  1. Lee K.; Lee J.; Kim G.; Kim Y.; Kang S.; Cho S.; Kim S.; Kim J. K.; Lee W.; Kim D. E.; Kang S.; Kim D.; Lee T.; Shim W. Rough-Surface-Enabled Capacitive Pressure Sensors with 3D Touch Capability. Small 2017, 13, 1700368. 10.1002/smll.201700368. [DOI] [PubMed] [Google Scholar]
  2. Tobjörk D.; Österbacka R. Paper electronics. Adv. Mater. 2011, 23, 1935–1961. 10.1002/adma.201004692. [DOI] [PubMed] [Google Scholar]
  3. Güder F.; Ainla A.; Redston J.; Mosadegh B.; Glavan A.; Martin T. J.; Whitesides G. M. Paper-Based Electrical Respiration Sensor. Angew. Chem., Int. Ed. Engl. 2016, 55, 5727–5732. 10.1002/anie.201511805. [DOI] [PubMed] [Google Scholar]
  4. Yang L.; Rida A.; Vyas R.; Tentzeris M. M. RFID Tag and RF Structures on a Paper Substrate Using Inkjet-Printing Technology. IEEE Trans. Microwave Theory Tech. 2007, 55, 2894–2901. 10.1109/TMTT.2007.909886. [DOI] [Google Scholar]
  5. Lessing J.; Glavan A. C.; Walker S. B.; Keplinger C.; Lewis J. A.; Whitesides G. M. Inkjet printing of conductive inks with high lateral resolution on omniphobic ″R(F) paper″ for paper-based electronics and MEMS. Adv. Mater. 2014, 26, 4677–4682. 10.1002/adma.201401053. [DOI] [PubMed] [Google Scholar]
  6. Martinez A. W.; Phillips S. T.; Whitesides G. M.; Carrilho E. Diagnostics for the Developing World Microfluidic Paper-Based Analytical Devices. Anal. Chem. 2010, 82, 3–10. 10.1021/ac9013989. [DOI] [PubMed] [Google Scholar]
  7. Martinez A. W.; Phillips S. T.; Butte M. J.; Whitesides G. M. Patterned paper as a platform for inexpensive, low-volume, portable bioassays. Angew. Chem., Int. Ed. Engl. 2007, 46, 1318–1320. 10.1002/anie.200603817. [DOI] [PMC free article] [PubMed] [Google Scholar]
  8. Hamedi M. M.; Ainla A.; Güder F.; Christodouleas D. C.; Fernández-Abedul M. T.; Whitesides G. M. Integrating Electronics and Microfluidics on Paper. Adv. Mater. 2016, 28, 5054–5063. 10.1002/adma.201505823. [DOI] [PubMed] [Google Scholar]
  9. Mandal P.; Dey R.; Chakraborty S. Electrokinetics with ″paper-and-pencil″ devices. Lab Chip 2012, 12, 4026–4028. 10.1039/c2lc40681k. [DOI] [PubMed] [Google Scholar]
  10. Dey R.; Kar S.; Joshi S.; Maiti T. K.; Chakraborty S. Ultra-low-cost ‘paper-and-pencil’ device for electrically controlled micromixing of analytes. Microfluid. Nanofluid. 2015, 19, 375–383. 10.1007/s10404-015-1567-3. [DOI] [Google Scholar]
  11. Park S.; Kim M.; Kwak D.; Im G.; Park J. J. Skinlike Disposable Tattoo on Elastic Rubber Adhesive with Silver Particles Penetrated Electrode for Multipurpose Applications. ACS Appl. Mater. Interfaces 2018, 10, 16932–16938. 10.1021/acsami.8b03057. [DOI] [PubMed] [Google Scholar]
  12. Wang J.; Musameh M. Carbon nanotube screen-printed electrochemical sensors. Analyst 2004, 129, 1–2. 10.1039/b313431h. [DOI] [PubMed] [Google Scholar]
  13. Liana D. D.; Raguse B.; Gooding J. J.; Chow E. Recent advances in paper-based sensors. Sensors 2012, 12, 11505–11526. 10.3390/s120911505. [DOI] [PMC free article] [PubMed] [Google Scholar]
  14. Parolo C.; Medina-Sánchez M.; Montón H.; de la Escosura-Muñiz A.; Merkoçi A. Paper-Based Electrodes for Nanoparticles Detection. Part. Part. Syst. Charact. 2013, 30, 662–666. 10.1002/ppsc.201200124. [DOI] [Google Scholar]
  15. Yu Y.; Zhang J. Pencil-drawing assembly to prepare graphite/MWNT hybrids for high performance integrated paper supercapacitors. J. Mater. Chem. A 2017, 5, 4719–4725. 10.1039/C6TA10076G. [DOI] [Google Scholar]
  16. Brus V. V.; Collins S. D.; Mikhnenko O. V.; Wang M.; Bazan G. C.; Nguyen T.-Q. Fabricating Low-Cost Ionic-Organic Electronic Ratchets with Graphite Pencil and Adhesive Tape. Adv. Electron. Mater. 2016, 2, 1500344. 10.1002/aelm.201500344. [DOI] [Google Scholar]
  17. Lin C. W.; Zhao Z.; Kim J.; Huang J. Pencil drawn strain gauges and chemiresistors on paper. Sci. Rep. 2014, 4, 3812. [DOI] [PMC free article] [PubMed] [Google Scholar]
  18. Siegel A. C.; Phillips S. T.; Wiley B. J.; Whitesides G. M. Thin, lightweight, foldable thermochromic displays on paper. Lab Chip 2009, 9, 2775–2781. 10.1039/b905832j. [DOI] [PubMed] [Google Scholar]
  19. Kim D. Y.; Steckl A. J. Electrowetting on paper for electronic paper display. ACS Appl. Mater. Interfaces 2010, 2, 3318–3323. 10.1021/am100757g. [DOI] [PubMed] [Google Scholar]
  20. Yao B.; Zhang J.; Kou T.; Song Y.; Liu T.; Li Y. Paper-Based Electrodes for Flexible Energy Storage Devices. Adv. Sci. 2017, 4, 1700107. 10.1002/advs.201700107. [DOI] [PMC free article] [PubMed] [Google Scholar]
  21. Mazzeo A. D.; Kalb W. B.; Chan L.; Killian M. G.; Bloch J. F.; Mazzeo B. A.; Whitesides G. M. Paper-based, capacitive touch pads. Adv. Mater. 2012, 24, 2850–2856. 10.1002/adma.201200137. [DOI] [PubMed] [Google Scholar]
  22. Zheng G.; Hu L.; Wu H.; Xie X.; Cui Y. Paper supercapacitors by a solvent-free drawing method. Energy Environ. Sci. 2011, 4, 3368–3373. 10.1039/c1ee01853a. [DOI] [Google Scholar]
  23. Wang Y.; Zhou H. To draw an air electrode of a Li–air battery by pencil. Energy Environ. Sci. 2011, 4, 1704–1707. 10.1039/c0ee00759e. [DOI] [Google Scholar]
  24. Yao B.; Yuan L.; Xiao X.; Zhang J.; Qi Y.; Zhou J.; Zhou J.; Hu B.; Chen W. Paper-based solid-state supercapacitors with pencil-drawing graphite/polyaniline networks hybrid electrodes. Nano Energy 2013, 2, 1071–1078. 10.1016/j.nanoen.2013.09.002. [DOI] [Google Scholar]
  25. Choi K.-H.; Yoo J.; Lee C. K.; Lee S.-Y. All-inkjet-printed, solid-state flexible supercapacitors on paper. Energy Environ. Sci. 2016, 9, 2812–2821. 10.1039/C6EE00966B. [DOI] [Google Scholar]
  26. Zheng G.; Cui Y.; Karabulut E.; Wågberg L.; Zhu H.; Hu L. Nanostructured paper for flexible energy and electronic devices. MRS Bull. 2013, 38, 320–325. 10.1557/mrs.2013.59. [DOI] [Google Scholar]
  27. Huang J.; Zhu H.; Chen Y.; Preston C.; Rohrbach K.; Cumings J.; Hu L. Highly Transparent and Flexible Nanopaper Transistors. ACS Nano 2013, 7, 2106–2113. 10.1021/nn304407r. [DOI] [PubMed] [Google Scholar]
  28. Zhang Y.; Hao N.; Lin X.; Nie S. Emerging challenges in the thermal management of cellulose nanofibrilbased supercapacitors, lithium-ion batteries and solar cells: A review. Carbohydr. Polym. 2020, 234, 115888. [DOI] [PubMed] [Google Scholar]
  29. Liu Y.; Mo J.; Fu Q.; Lu Y.; Zhang N.; Wang S.; Nie S. Enhancement of Triboelectric Charge Density by Chemical Functionalization. Adv. Funct. Mater. 2020, 2004714. 10.1002/adfm.202004714. [DOI] [Google Scholar]
  30. Lu Y.; Tao P.; Zhang N.; Nie S. Preparation and thermal stability evaluation of cellulose nanofibrils from bagasse pulp with differing hemicelluloses contents. Carbohydr. Polym. 2020, 245, 116463. 10.1016/j.carbpol.2020.116463. [DOI] [PubMed] [Google Scholar]
  31. Huber T.; Müssig J.; Curnow O.; Pang S.; Bickerton S.; Staiger M. P. A critical review of all-cellulose composites. J. Mater. Sci. 2011, 47, 1171–1186. [Google Scholar]
  32. Klemm D.; Heublein B.; Fink H. P.; Bohn A. Cellulose: fascinating biopolymer and sustainable raw material. Angew. Chem., Int. Ed. Engl. 2005, 44, 3358–3393. 10.1002/anie.200460587. [DOI] [PubMed] [Google Scholar]
  33. Edgar K. J.; Buchanan C. M.; Debenham J. S.; Rundquist P. A.; Seiler B. D.; Shelton M. C.; Tindall D. Advances in cellulose ester performance and application. Prog. Polym. Sci. 2001, 26, 1605–1688. 10.1016/S0079-6700(01)00027-2. [DOI] [Google Scholar]
  34. Hua Q.; Liu H.; Zhao J.; Peng D.; Yang X.; Gu L.; Pan C. Bioinspired Electronic Whisker Arrays by Pencil-Drawn Paper for Adaptive Tactile Sensing. Adv. Electron Mater. 2016, 2, 1600093. 10.1002/aelm.201600093. [DOI] [Google Scholar]
  35. Yang L.; Qi K.; Chang L.; Xu A.; Hu Y.; Zhai H.; Lu P. A powerful dual-responsive soft actuator and photo-to-electric generator based on graphene micro-gasbags for bioinspired applications. J. Mater. Chem. B 2018, 6, 5031–5038. 10.1039/c8tb01222a. [DOI] [PubMed] [Google Scholar]
  36. Wong K. K. Y.; Richardson J. D.; Mansfield S. D. Enzymatic Treatment of Mechanical Pulp Fibers for improving Papermaking Properties. Biotechnol. Prog. 2000, 16, 1025–1029. 10.1021/bp000064d. [DOI] [PubMed] [Google Scholar]
  37. Hunter D.Papermaking: the history and technique of an ancient craft ;Dover Publications: New York, 1978; [Google Scholar]
  38. Byerlee J. D. THE MECHANICS OF STICK-SLIP. Tectonophysics 1970, 9, 475–486. 10.1016/0040-1951(70)90059-4. [DOI] [Google Scholar]
  39. Berman A. D.; Ducker W. A.; Israelachvili J. N. Origin and Characterization of Different Stick-Slip Friction Mechanisms†. Langmuir 1996, 12, 4559–4563. 10.1021/la950896z. [DOI] [Google Scholar]
  40. Yoshioka N. TEMPERATURE MEASUREMENT DURING FRICTIONAL SLIDING OF ROCKS. J.Phys. Erath 1985, 33, 295–322. [Google Scholar]
  41. Setiyana B.; Ismail R.; Jamari J.; Schipper D. J. Stick-Slip Behaviour of a Viscoelastic Flat Sliding along a Rigid Indenter. Tribology Online 2016, 11, 512–518. 10.2474/trol.11.512. [DOI] [Google Scholar]
  42. Siegel A. C.; Phillips S. T.; Dickey M. D.; Lu N.; Suo Z.; Whitesides G. M. Foldable Printed Circuit Boards on Paper Substrates. Adv. Funct. Mater. 2010, 20, 28–35. 10.1002/adfm.200901363. [DOI] [Google Scholar]
  43. Steudel S.; De Vusser S.; De Jonge S.; Janssen D.; Verlaak S.; Genoe J.; Heremans P. Influence of the dielectric roughness on the performance of pentacene transistors. Appl. Phys. Lett. 2004, 85, 4400. 10.1063/1.1815042. [DOI] [Google Scholar]
  44. Pokpas K.; Zbeda S.; Jahed N. Electrochemically Reduced Graphene Oxide Pencil-Graphite in situ Plated Bismuth-film Electrode for the Determination of Trace Metals by Anodic Stripping Voltammetry. Int. J. Electrochem. Sci. 2014, 9, 736–759. [Google Scholar]
  45. Ganguli S.; Roy A. K.; Anderson D. P. Improved thermal conductivity for chemically functionalized exfoliated graphite/epoxy composites. Carbon 2008, 46, 806–817. 10.1016/j.carbon.2008.02.008. [DOI] [Google Scholar]
  46. Gere James M.; Mechanics of Materials, 6th ed, Thomson: US, 2003; [Google Scholar]
  47. Eshleman R. L.;Basic machinery vibrations: An introduction to machine testing, analysis, and monitoring; VIPress: US, 1999 [Google Scholar]

Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

ao0c04792_si_001.pdf (827.9KB, pdf)
ao0c04792_si_002.mp4 (4MB, mp4)
ao0c04792_si_003.mp4 (4.6MB, mp4)
ao0c04792_si_004.mp4 (4MB, mp4)
ao0c04792_si_005.mp4 (3MB, mp4)
ao0c04792_si_006.mp4 (2.3MB, mp4)
ao0c04792_si_007.mp4 (2.2MB, mp4)
ao0c04792_si_008.mp4 (3.5MB, mp4)
ao0c04792_si_009.mp4 (2.5MB, mp4)

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