Highly Sensitive Bendable and Foldable Paper Sensors Based on

Jan 11, 2017 - Therefore, because it eliminates the need for complex processing steps and harmful chemicals, the key advantage of our rGO-paper sensor...
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Highly Sensitive Bendable and Foldable Paper Sensors Based on Reduced Graphene Oxide Biswajit Saha,*,†,‡ Sangwoong Baek,‡ and Junghoon Lee*,‡ †

BK21+ Transformative Training Program for Creative Mechanical and Aerospace Engineers and ‡Department of Mechanical and Aerospace Engineering, Seoul National University, Seoul 151744, South Korea S Supporting Information *

ABSTRACT: Over the past decade, the demand for high-performance wearable sensors has increased because of their capability for interaction with humans. Such sensors have typically been prepared on conventional substrates, such as silicon, PDMS, and copper mesh. In this work, we propose a class of wearable sensors fabricated from reduced graphene oxide (rGO) patterned paper substrates (rGO-paper). These rGO-paper sensors are highly sensitive to various deformations and capable of measuring bending and folding angles as small as 0.2° and 0.1°, respectively. We have demonstrated the applicability of these high-performance rGO-paper sensors by patterning rGO on kirigamis that can detect pulse and the motion of knees, wrists, and fingers. Finally, paper rings lined with rGO sensors were used to control a robotic hand, and an rGO-paper keyboard was used to light LEDs.

KEYWORDS: reduced graphene oxide, bendable-foldable strain sensor, paper sensor mechanical support and sustainable flexibility, which play critical roles in the performance of such devices. On the other hand, inexpensive paper-based devices have generated considerable interest because these devices are ubiquitous, light in weight, portable, flexible, foldable, and biodegradable.15,16 Whereas previous studies on paper-based devices have focused on laboratories-on-a-chip, supercapacitors, actuators, and transistors, no attempt has been made to develop wearable paper electronics because of the challenges involved in implementing variable resistors on paper substrates.16−19 Considering the performance of rGO and the abovementioned merits of the paper, herein, we have presented a novel and facile approach for developing bendable and foldable strain sensors based on paper substrates patterned with rGO (rGO-paper) that allow full-range operation of wearable electronics. On one hand, rGO-paper sensors maintain the simplicity of paper-based devices; on the other hand, these devices are inexpensive, scalable, and highly sensitive to tiny deflections over a broad sensing range. In this work, we have proposed a class of graphene oxide (GO) patterning processes that use desktop digital craft cutters for the fabrication of the masking layer, which is followed by the drop casting of an aqueous GO solution onto the exposed surfaces to form the desired patterns. After this step the dried GO patterns are reduced to rGO using photothermal energy by means of a commercial camera flash (see the Experimental

1. INTRODUCTION Recently, strain sensors have attracted considerable attention because of the remarkable developments in the fields of robotics, wearable electronics, energy harvesting, and healthcare.1−5 High flexibility, light weight, and low thickness are the prime requirements for the development of compact and intelligently designed sensors, whereas high sensitivity is required for the detection of small-scale movement.5−7 However, conventional metal- and semiconductor-based strain sensors cannot fulfill these requirements because of their limited flexibility and sensitivity.7,8 To address the above issues and attain the required performance for an ideal flexible strain sensor, many researchers have investigated the use of various nanomaterials, such as nanowires, nanoparticles, carbon nanotubes, and graphenes.6,7,9 Among them, graphene, graphene−metal nanowire hybrids, graphene foam, and graphene on woven copper mesh have been intensively studied because of their superior flexibility, high conductivity, and robust mechanical strength.10−12 However, the fabrication processes for these devices require either expensive chemical vapor deposition (CVD), harmful etching, complicated mixing processes, or a combination of these factors. Alternative lowcost reduced graphene oxide (rGO) mesh-like strain sensors show a high gauge factor (630), but their fabrication process includes harmful etching steps.13 Filtered crumpled graphene is an alternative option for addressing the increased demand for scalable and low-cost flexible devices. However, its fabrication requires additional nanocellulose binding, filtration, and PDMS compounding steps.14 Generally, graphene/rGO needs to be embedded into an elastomer base material to provide © 2017 American Chemical Society

Received: August 21, 2016 Accepted: January 11, 2017 Published: January 11, 2017 4658

DOI: 10.1021/acsami.6b10484 ACS Appl. Mater. Interfaces 2017, 9, 4658−4666

Research Article

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Figure 1. Schematic illustration of the rGO-paper sensor fabrication process and the fundamental characteristics of GO and rGO. (a) Key steps in fabricating rGO-paper sensors. (b) Photograph of I- and U-shaped rGO patterns on an A4 printing paper substrate. (c) Schematic presentation of the multilayer masking process. (d) Image of an rGO-paper sensor sample consisting of a U-shaped rGO pattern, silver coating at the contact pad and copper connection strips. (e) Micro-Raman spectroscopy results for GO (top) and rGO (below). (f) Comparison of the XPS spectra of GO (top) and rGO (below). Water contact angles on (g) GO and (h) rGO.

Section). The camera flash reduction process is rapid, clean, and free from harmful chemicals. At the same time, this reduction process is suitable for paper substrates. Compared with conventional graphene/GO patterning approaches, our proposed method not only is simple, rapid, and straightforward but also allows freedom in the choice of creative and delicate designs. Therefore, because it eliminates the need for complex processing steps and harmful chemicals, the key advantage of

our rGO-paper sensor fabrication process is that it supports sensor production in remote locations with limited laboratory facilities. The proposed rGO-paper sensors exhibit high sensitivity to tensile strain with a gauge factor of about 66.6 ± 5. Most importantly, these paper-based sensors maintain their high flexibility even after rGO patterning and show consistency in their resistance response to changes in their bending and 4659

DOI: 10.1021/acsami.6b10484 ACS Appl. Mater. Interfaces 2017, 9, 4658−4666

Research Article

ACS Applied Materials & Interfaces

The electrical properties of the rGO-paper sensors were measured using an IviumStat electrochemical interface (Ivium Technologies, USA) having a sensitivity current range of ±1 pA and current accuracy of 0.2%. The starching, bending, and folding tests were performed by MTS (MTS Criterion, model 43).

folding angles. The rGO-paper sensors possess excellent bending and folding sensitivity over a wide detection range from 0° to ±180° with detection limits of 0.2° and 0.1° for the bending and folding angles, respectively. We have demonstrated the use of rGO-paper sensors as wearable devices for detecting human body moments and controlling robotic hands. Furthermore, highly sensitive rGO-paper sensors were used to develop a paper-based keyboard.

3. RESULTS AND DISCUSSION 3.1. Physical and Chemical Properties of rGO-Paper Sensors. During patterning, the solvent of the aqueous GO solution easily penetrates deeply into the paper but most of the GO flakes are adsorbed onto the top fibers of the paper. Therefore, although the resistance of the rGO patterns decreases with increasing amounts of GO solution, excessively large volumes of GO solution were found to be detrimental as the water causes the paper substrate to rupture. Additionally, high stress along the rGO patterns causes bending of the paper strips. The amount of GO solution was optimized to the minimum amount of solution required to create conductive patterns with acceptable bending performance. A volume of 30−40 μL of an aqueous solution with a GO concentration of 0.005 g/cm3 was found to be optimal for the formation of a U pattern of 3.3 cm in length and 0.1 cm in width on a 5 × 1.5 cm2 paper strip, resulting in a resistance in the range of 140− 160 Kohm (Table S1, Supporting Information). Although we studied the effects of GO amounts on various paper substrates (Figure S1, Supporting Information) and GO concentration, our discussion focuses on commercial A4 printing paper, as it is the most readily available type of paper. The proposed method can produce not only rGO circuits with a clear edge, as shown in Figure 1b, but also complicated and delicate designs on various substrates (Figure S2, Supporting Information). Figure 1d shows a complete Ushaped rGO-paper sensor with connective copper strips. The micro-Raman spectra of GO-paper and rGO-paper exhibit two peaks: one at about 1500 cm−1 (G peak), which corresponds to an in-plane lattice vibration of the sp2 bond, and one at 1350 cm−1 (D peak), which is due to the disorder zone, as shown in Figure 1e.21−23 The decrease in the intensity ratio (ID/IG) after exposure to a camera flash suggests that disorder associated with the oxygen defects diminishes and sp2 sites are partially restored.20,23 A decrease in the percentages of CO and C−O bond in rGO compared with GO is observed in the XPS spectra, which further implies the successful removal of exogenous functional groups by the photothermal reduction process, as shown in Figure 1f. Reduction of GO to rGO was further assessed by their hydrophobicity as shown in Figure 1g and 1h. rGO surfaces show higher hydrophobicity compared to GO surfaces because of their lower percentage of oxygen, indicating reduction of GO.24,25 The field-effect scanning electron microscopy (FESEM) micrographs shown in Figure 2 reveal the surface morphologies of GO-patterned papers before and after reduction. The topview FESEM micrographs in Figure 2a and corresponding optical images (Figure S3a, Supporting Information) of GOpaper clearly illustrate that the top cellulose fibers of the paper are completely coated with GO. Furthermore, magnified views of GO-paper, as shown in the inset of Figure 2a and Figure S3b in the Supporting Information, reveal that net-like integrated networks are formed by the GO-coated cellulose fibers. Meanwhile, the cross-sectional views of GO-paper and its magnified view in Figure 2b and 2c illustrate that most of the GO flakes remain at the top surface of the paper. The hydrodynamic size of the GO flakes varies from 0.2 to 6 μm

2. EXPERIMENTAL SECTION 2.1. rGO Patterning. First, we adhered one sheet of A4 printing paper (Double-A) on top of another using spray adhesive (75Graphic Arts, 3M). The top sheet of paper functioned as a masking layer for the aqueous GO solution, whereas the bottom sheet of paper served as a substrate for rGO patterns. The adhesiveness of 75Graphic Arts is just sufficient to fix two sheets of papers together firmly enough to prevent sliding during the cutting process while still allowing the top masking paper to be easily removed afterward without causing any damage. Blueprints for the rGO patterns were created using computeraided design software (AutoCAD 2015), and the top sheets were then cut using a digital craft cutter (Silhouette CAMEO2). The unwanted patterns cut from the top sheet were carefully removed using tweezers, and an aqueous GO solution (Graphene Supermarket) was drop cast onto the surfaces exposed by the patterns. The solution was then dried under ambient conditions for 1−3 h. As the GO flakes were mostly adsorbed by the cellulose fibers, no GO flakes passed through the top mask, and therefore, clear patterns were transferred to the bottom substrate. After separation from the mask, the GO patterns were reduced to rGO using a camera flash with stepwise increases in light intensity. Details of the camera flash reduction process and its optimization were described in previous work.20 The individual processing steps of the rGO patterning process are outlined schematically in Figure 1a, and clear edge rGO patterns are shown in Figure 1b. 2.2. Multilayer Masking. To fabricate rGO-paper-based sensors with silver patterns, we used a multilayer masking method, consisting of an alternating cutting and drop-casting process for each material. First, the paper substrates were cut in accordance with the sensor design using the digital craft cutter. After that silver (Ted Pella, Inc.) connecting lines were cut on Ag-masking paper, which was followed by patterning of silver on the paper substrates. Finally, strain-sensitive rGOs were cut on rGO-masking paper and then patterned on paper substrates. Individual process steps of multilayer masking method are shown in Figure 1c. The paper substrates and the masking layers were patterned with alignment keys to guide the adjustment of the individual designs on a glass light table. 2.3. Electrical Connections. To create the electrical connections, the ends of the rGO patterns (connection pads) were connected to copper strips and laminated with transparent polymer sheets, as presented in Figure 1d. The connection pads were painted with silver paste to minimize the contact resistance between the rGO patterns and the connecting copper strips. The Arduino UNO microcontroller was used to interface the rGOpaper sensors to the servo motors, which control the movement of robotic fingers. The change in voltage due to change in resistance was used as an input signal to control the degree of rotation of servo motors. Arduino UNO-controlled servo motor was used to study the durability of rGO-paper sensors. 2.4. Characterization. The Raman spectra of GO and rGO from 1000 to 3000 cm−1 were recorded using micro-Raman spectroscopy (Horiba, T64000) and fitted with Gaussian profiles to determine the intensity ratio between the D peak and the G peak (ID/IG). X-ray photoelectron spectroscopy (XPS, Kratos, AXIS-Hsi) analysis was performed to study the chemical compositions of GO and rGO. Static contact angles of DI water on GO and rGO were measured at five different randomly selected locations on each sample to confirm the reduction process. Field effect scanning electron microscopy (FESEM; Hitachi, S-4800) was used to evaluate the microstructures of the GO before and after the reduction process. 4660

DOI: 10.1021/acsami.6b10484 ACS Appl. Mater. Interfaces 2017, 9, 4658−4666

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6% strain, which is much higher than the gauge factors of CNT/polymer composites (0.06−0.82),6 conventional metal strain gauges (1−5), nanowires (0.65−9.9),26,27 carbon black/ polymer composites (∼20),28 and measured Ag-papers (∼30). Although graphene−elastomer-based strain sensors, which exhibit stretchability, have been widely studied as wearable sensors, their limited GFs (3.7−10) restrict their application for measuring ultrasmall deflections.14,29 A recently developed graphene−rubber composite with a high GF of 35 permits the detection of subtle motions, such as breathing, pulse, and speech.30 The strain detection limit of the rGO-papers was determined under gradually increasing step strain as presented in Figure 3b. Limit of detection was quantified using the standard criteria of a signal-to-noise ratio larger than three.31 Highly sensitive rGO-papers can detect a strain change as minute as 0.001 having a signal-to-noise ratio of 11 ± 1.5, and the output signal is highly repeatable. The same criteria were used to find the bending/folding limit of detection in the later part of this work. Thus, the superior piezoresistive response and high GF of our rGO-paper sensors reinforce their suitability for detecting of minute motions. In addition, the highly stable and low-noise response of these rGO-paper sensors makes them promising candidates for precise sensing. 3.3. Piezoresistive Properties of rGO-Paper Sensors against Bending. To reveal the potential of rGO-paper for all-directional sensing, we studied its relative resistance over compressive and tensile bending angles from 0° to ±40°, as shown in Figure 3c. Although rGO-paper exhibits a sensing range over ±180° for bending, a detection range of ±40° was selected in this experiment to avoid the risk of permanently creasing the paper substrates and to eliminate the effects of folding. The recorded relative resistance of rGO-paper is found to be a strong function of the bending angle with an average slope of about 0.12/degree. Different deformation mechanisms correspond to compressive and tensile bending, leading to decreases and increases in resistance, respectively. As most of the graphene flakes remain on the top surface of the paper (Figure 2e and 2f), a compressive (concave) bending angle increases the compressive stress on the rGO patterns and the packing compactness of the rGO flakes (Figure S4b,c, Supporting Information), causing a decrease in resistance. Conversely, a tensile (convex) bending angle stretches the rGO and increases the relative distances among rGO flakes, leading to an increase in resistance.32,33 Additionally, the FESEM micrograph of rGO patterns under bending that is presented in Figure S4d (Supporting Information) shows that even a small bending angle of 20° causes microcracks to form in the rGO patterns. The increase in the crack density and the widening of the crack openings with an increase in the bending angle cause a further increase in resistance. By contrast, no significant change is observed in the case of conductive silver patterns, and their relative resistances vary randomly without any clear trend caused by the movement of the paper substrates during bending tests, as presented in the top inset of Figure 3c. Although few reports have mentioned changes in the resistance of graphene-based sensors versus bending angle, the detection limit of rGO-paper sensors is superior to those of various other forms of graphene sensors.10,29,34 Apart from the remarkable sensitivity to bending, the experimentally measured relative resistances with respect to incremental steps in the bending angles indicate that the limits of detection are as small as −0.2° and 1° for compressive and tensile bending, respectively, as shown in Figure 3d and 3e. Interestingly, the

Figure 2. Structural characterization of GO and rGO on A4 printing paper. (a) Top-view FESEM images of the GO layers staged on paper. Highly magnified top view is shown as an inset. (b and c) Crosssectional view showing filtered GO flakes on top of A4 printing paper. (d) Top-view FESEM images of rGO. (e and f) Image showing wavy rGO flakes throughout the entire surface and thickness.

with a distribution center of 1.07 μm (Figure S1g, Supporting Information), and they are highly hydrophilic in nature (Figure 1g). Therefore, these flakes are easily adsorbed by the paper fibers as shown in Figure 2c. The adsorbed GO flakes appear and form wavy layered structures throughout the entire surface and thickness after photothermal reduction, as shown in Figure 2d−f (Figure S3c,d, Supporting Information). The thickness of the GO layer, which was deposited at previously mentioned conditions, is 10 ± 1 μm as shown in Figure 2b. Relative movements of these wavy flakes play a crucial role in the piezoresistive behavior of rGO-paper sensors. 3.2. Piezoresistive Properties of rGO-Paper Sensors against Strain. To investigate the resistive response of the sensors, the changes in relative resistance (ΔR/R0) of rGOpaper and Ag-paper were measured under varying strain while maintaining a constant voltage of 1 V across the sensors. Here, ΔR = R − R0, where R0 is the initial electrical resistance and R is the electrical resistance under a strain ε. The piezoresistive performances presented in Figure 3a show that the electrical resistances of rGO-paper and Ag-paper increase as applied strain increases, behaving similarly to variable resistors, but the slope of the relative resistance curve of rGO-paper is significantly higher than that of the Ag-paper. At the same time, a steeper slope in the nonelastic region compared with the elastic region indicates that the piezoresistive property in the stretching mode is a combined effect of the increase in the relative distances among the conductive rGO flakes and the breakage of their contact. To evaluate the characteristic performance of the sensors, the gauge factor (GF) was calculated as (ΔR/R0ε). The calculated GF of our flexible rGO-paper sensors is about 66.6 ± 5 within 4661

DOI: 10.1021/acsami.6b10484 ACS Appl. Mater. Interfaces 2017, 9, 4658−4666

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Figure 3. Electromechanical behavior of the rGO-paper sensors. (a) Relative changes in the resistance and stress of the rGO-paper sensors and Agpaper as a function of the applied strain. (b) Variation in resistance under gradually stepwise increases of 0.0005, 0.001, and 0.002 strain. (c) Resistance variations of the rGO-paper sensors under ±40° compressive and tensile bending. For comparison, the top inset shows the resistance variation of silver under compressive and tensile bending. Bottom inset depicts bending test setup mounted on the MTS. The resistance variations under gradual stepwise increases of (d) −0.2°, −0.7°, and −1.2° in the compressive bending angle and (e) 0.5°, 1°, and 1.5° in the tensile bending angle. Resistance variations over multiple cycles of (f) compressive bending, (g) tensile bending, (h) compressive folding, and (i) tensile folding. Inset images in (i) show the experimental setup for the folding test. rGO-paper sensor was adhered to a foldable polymer clip to clamp onto the MTS. Resistance variations under gradual stepwise increases of (j) −0.1°, −0.3°, and −0.5° in the compressive folding angle and (k) 0.1°, 0.3°, and 0.5° in the tensile folding angle. 4662

DOI: 10.1021/acsami.6b10484 ACS Appl. Mater. Interfaces 2017, 9, 4658−4666

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ACS Applied Materials & Interfaces sensitivity of the rGO-paper sensors as a function of substrate porosity (Figure S1f, Supporting Information) allows further freedom in tailoring their performance. The response profiles over multiple compressive and tensile bending cycles in Figure 3f and 3g show that the resistance changes during the first and second measurements because of permanent deformations induced in the paper substrates; thereafter, the variations between two consecutive cycles decrease, and the curve profiles remain nearly the same during subsequent cycles. 3.4. Piezoresistive Properties of rGO-Paper Sensors against Folding. To study the performance of the rGO-paper sensors having permanent creases, the relative resistance was measured with respect to the folding angle, while rGO-paper sensors were placed either above (tensile folding) or underneath (compressive folding) the foldable polymer clip, as shown in the inset of Figure 3i. Folding causes a concentration of stress along the line of the crease and a change in the resistance, which allows rGO-papers to act as a highly sensitive foldable sensor for measurement of continuous changes in the folding angle. Figure 3h shows a continuous decrease in the resistance as the compressive folding angle increases, and the curves show two distinct regimes. In the first phase, the resistance decreases exponentially up to 70°, whereas at higher folding angles (70°−180°) it decreases linearly at a rate of about 4/degree. Similar to the compressive folding case, an exponential resistance profile followed by a linear change is observed in tensile folding, as shown in Figure 3i. FESEM images (Figure S4f−-m, Supporting Information) show a large number of folds and cracks across the rGO patterns near permanent creases. Compressive folding creates a number of folds on rGO-paper that compress rGO flakes and reduce the distance of the electrical path (Figure S4f−i, Supporting Information), causing a decrease in resistance. The initiation and fast propagation of these folds and cracks at the beginning of folding cause exponential changes in resistance. Benefiting from their high sensitivity, foldable rGO-paper sensors not only have a wide detection range but can also reach an extremely small detection limit of 0.1°, as presented in Figure 3j and 3k, which is smaller than that of any previously reported foldable sensor.10,29 The response curves presented in Figure 3h and 3i show that the folding induces permanent deformations in the paper substrates and prevents the polymer clips from returning to its original position, leading to the shifts in the starting angles and changes in the response profile observed in the first two runs. Similar to the bending test, the resistance profile remained stable after the second run. Because the hydrophilic surface property of GO (Figure 1g) causes favorable adsorption on paper substrates, no delamination of the rGO is observed even after complete folding of the paper. 3.5. Durability of rGO-Paper Sensors. To further examine the durability of the rGO-paper sensors, the current (I) was measured under a cyclic bending and folding angle of ±70° at a constant potential of 1 V using a servo motor as shown in Figure S5 (Supporting Information). During the cyclic bending test, the stable and reproducible electrical signals are observed for 250 s (i.e., up to ∼100 cycles) as shown in Figure 4a. Even though total current decreases after 250 s, the rGO-papers show a negligible change in ΔI (ΔI = I−70° − I+70°) and current profile after the cyclic tensile/compressive bending test as shown in the insets of Figure 4a. The bending-induced strain (ε = t/2r; t is the thickness of the paper substrate, and r is the bending radius) on the rGO was calculated from the bending geometry as defined in the inset of Figure 5S

Figure 4. Durability test rGO-paper sensors; 700 cycling test at an input voltage of 1 V under ±70° (a) bending and (b) folding tests. (Insets) Enlarged views of current vs time curves. (Left and right insets) Beginning and ending 10 cycles of the test, respectively.

(Supporting Information).35,36 Calculated bending strain for the bending angle of ±70° is about ±0.0035. On the other hand, Figure 4b shows a stable electrical current for 150 s (i.e., up to 60 cycles) during the cyclic folding test. A continuous increase in ΔI and change in current profiles are observed at long run cyclic folding tests. It is expected that repeated folding would lead to permanent deformation of rGOpapers, which causes an increase in ΔI and change in current profiles of the sensors as shown in the insets of Figure 4b. However, the rGO-paper sensors provide a stable change in conductance during initial bending/folding cycles and allow us to plan for a low-cost, biodegradable, and use-and-through paper-based sensor, which is hard to achieve with metallic sensors. In order to further improve the durability of the rGOpaper sensors, selection of suitable protective materials and methods that fulfill the above functions will be required, which is under extensive research in our group. 3.6. Application of rGO-Paper Sensors. Although rGOpapers have limited stretchability, their high folding and bending sensitivity (Videos S1 and S2, Supporting Information) enables the design of paper-based wearable sensors for the real-time monitoring of human movements. A proof-of-concept device for the sensing of vigorous finger motions was created, consisting of a double-ring paper kirigami with a U-shaped rGO pattern on the connecting strip (rGO-paper ring). An overview of the rGO-paper ring and its positions on a bent and extended finger is presented in Figure 5a. The inside ring is tightly fixed onto the knuckle, whereas the outside ring moves freely to allow the rGO-paper ring to be bent or straightened following the position of the finger. Figure 5b illustrates the response behaviors of rGO-paper rings when various fingers are bent and extended. We demonstrated the real-time control of a 3Dprinted robotic hand using smart rGO-paper rings, as shown in Figure 5c (Video S3, Supporting Information). The changes in compressive and tensile resistances of the rGO-paper rings were used as single inputs to control the motions of the middle finger and thumb, respectively. Flexible paper substrates combined with a precise digital craft cutter enable the preparation of intricately designed rGO-paper strain sensors, which can bend/fold and detect a large range of human motions. To detect the motions of knee and wrist joints, which involve the stretching of the skin by as much as 55%,6 parallel rGO lines and conductive silver connections were patterned sequentially on a stretchable paper kirigami using the multilayer masking method. When the two ends are pulled, two consecutive paper strips of the stretchable kirigami bend in opposite directions: concave and convex. Therefore, the strainsensitive rGO was patterned on alternating paper strips to avoid the nullification of one signal by the other, and the rGO 4663

DOI: 10.1021/acsami.6b10484 ACS Appl. Mater. Interfaces 2017, 9, 4658−4666

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Figure 5. Wearable electronics and keyboard applications. (a) Photographs of an rGO-paper ring and its extended and bent positions. (b) Response signals of five independent rGO-paper rings monitoring the positions of various fingers. (Top insets ) Photographs of the hand in the four different positions corresponding to the plotted signals. (c) Control of the middle finger and thumb of a 3D-printed robotic hand using rGO-paper rings. (d) Photograph of an rGO-paper kirigami prepared using the multilayer masking process. (e) Photographs of a knee in extended and bent positions while wearing rGO-paper sensors. Changes in the resistance of an rGO-paper sensor on a knee during (f) sitting and (g) walking. (h) Response curve due to the movement of a wrist. (i) rGO-paper sensor attached to the wrist for detection of pulse, and (j) corresponding signal from the rGOpaper sensor. (k) Photographs of the top and bottom sides of a paper keyboard. (l and m) Touching a paper key to light an LED and the corresponding signal from the paper key.

patterns were connected by silver lines, as shown in Figure 5d. As the knee or wrist stretches, all rGO-patterned paper strips are bent in the same direction and generate positive or negative signals depending on the bending direction. Figure 5e−h shows the response behavior of this rGO-paper sensor, which can easily detect and identify various motions, including knee bending, walking, and wrist bending. rGO-paper sensors could

also read pulse accurately in real time as shown in Figure 5i and 5j. Wearable electronics that do not restrict body motion have attracted considerable attention for a wide range of applications.2,3 In addition, rGO-paper sensors are highly sensitive to bending and folding and are easy to fabricate. At the same time, rGO-papers offer freedom in developing creative designs for a wide sensing range. 4664

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ACS Applied Materials & Interfaces Apart from their use as wearable electronics, the outstanding sensitivity of rGO-paper sensors enables their use as paper keyboards to detect the touching of keys, and thereafter, detected signals can be processed according to instruction. To verify this concept, a number of U-shaped rGO patterns and silver connecting lines were patterned on paper cantilevers using the multilayer masking method. Figure 5k shows the top and bottom sides of an rGO-paper keyboard. We used the signals from individual key to light various LEDs, as shown in Figure 5l and 5m (Video S4, Supporting Information). Even though the main objective of this work is to find the use-and-through sensors, our future work focuses on the improvement of lifetime of rGO-papers in harsh environments, which include humid weather or body sweat. Superhydrophobic spray coating on rGO paper could be one of the potential research directions for the above-mentioned requirements. To demonstrate the use of coating, water was spread on commercial superhydrophobic spray (NeverWet, Rust-oleum) coated rGO-paper and bare rGO-paper sensors. Coated rGOpaper repels water and maintains a constant resistance, whereas bare rGO-paper absorbs water and thereafter decreases resistance (Video S5, Supporting Information). A detailed study on this direction is beyond the scope of this paper.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Biswajit Saha: 0000-0002-6201-6628 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the BioNano Health-Guard Research Center funded by the Ministry of Science, ICT & Future Planning (MSIP) of Korea as Global Frontier Project (Grant number H-GUARD_2015M3A6B2068409). This research was also supported by a grant to the Bio-Mimetic Robot Research Center Funded by Defense Acquisition Program Administration and by the Agency for Defense Development (UD130070ID).

4. CONCLUSION In summary, we presented paper-based reduced graphene oxide sensors, which are biodegradable, low in cost, highly sensitive to various forms of deformation, and capable of bending and folding detection over a wide range. These rGO-paper sensors have a gauge factor of about 66.6 ± 5. They exhibit bending and folding detection limits of 0.2° and 0.1°, respectively, which are smaller than those of any other forms of graphene sensors. Capitalizing on their high bending and folding sensitivity, rGOpaper sensors were used for the detection of various human motions, including finger, knee, and wrist movements. In addition, rGO-paper sensors were used to develop a prototype device for tracking the gestures of a robotic hand and a touchsensitive paper keyboard. Another highlight of this study is the innovative method of rGO patterning, which is a facile method that opens the door for creative sensor design. This versatile and unique fabrication strategy could be applied to other substrate materials for the development of stretchable strain sensors.

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optical images of a GO pattern on A4 printing paper; effects of bending and folding on microstructure of rGO patterns; FESEM micrographs of rGO patterns; experimental setup for durability test; photograph of the rGOpaper sensor attached with the arm of a servo motor during the durability test (PDF)

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b10484. (AVI) (AVI) (AVI) (AVI) (AVI) Optimization of GO solution and paper substrate; resistances of U-shaped rGO patterns formed on various paper substrates; photographs of U-shaped rGOs patterns; patterning of delicate designs, photographs of cut paper, cut paper with light illuminating it from the backside, cut paper covered with aqueous GO solution, and rGO tree and elephants; photographs of the Ushaped rGO pattern and the SNU logo on PET substrates; optical images of GO-paper and rGO-paper; 4665

DOI: 10.1021/acsami.6b10484 ACS Appl. Mater. Interfaces 2017, 9, 4658−4666

Research Article

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