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Multilayer Graphene Epidermal Electronic Skin Yancong Qiao,†,‡,# Yunfan Wang,§,# He Tian,*,†,‡,# Mingrui Li,§ Jinming Jian,†,‡ Yuhong Wei,†,‡ Ye Tian,†,‡ Dan-Yang Wang,†,‡ Yu Pang,†,‡ Xiangshun Geng,†,‡ Xuefeng Wang,†,‡ Yunfei Zhao,†,‡ Huimin Wang,⊥ Ningqin Deng,†,‡ Muqiang Jian,⊥ Yingying Zhang,⊥ Renrong Liang,*,†,‡ Yi Yang,*,†,‡ and Tian-Ling Ren*,†,‡ Downloaded via UNIV OF SUSSEX on August 8, 2018 at 14:13:29 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
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Institute of Microelectronics, Tsinghua University, Beijing 100084, China Beijing National Research Center for Information Science and Technology (BNRist), Tsinghua University, Beijing 100084, China § Institute of Physics, Tsinghua University, Beijing 100084, China ⊥ Department of Chemistry and Center for Nano and Micro Mechanics (CNMM), Tsinghua University, Beijing 100084, China ‡
S Supporting Information *
ABSTRACT: Due to its excellent flexibility, graphene has an important application prospect in epidermal electronic sensors. However, there are drawbacks in current devices, such as sensitivity, range, lamination, and artistry. In this work, we have demonstrated a multilayer graphene epidermal electronic skin based on laser scribing graphene, whose patterns are programmable. A process has been developed to remove the unreduced graphene oxide. This method makes the epidermal electronic skin not only transferable to butterflies, human bodies, and any other objects inseparably and elegantly, merely with the assistance of water, but also have better sensitivity and stability. Therefore, pattern electronic skin could attach to every object like artwork. When packed in Ecoflex, electronic skin exhibits excellent performance, including ultrahigh sensitivity (gauge factor up to 673), large strain range (as high as 10%), and long-term stability. Therefore, many subtle physiological signals can be detected based on epidermal electronic skin with a single graphene line. Electronic skin with multiple graphene lines is employed to detect large-range human motion. To provide a deeper understanding of the resistance variation mechanism, a physical model is established to explain the relationship between the crack directions and electrical characteristics. These results show that graphene epidermal electronic skin has huge potential in health care and intelligent systems. KEYWORDS: laser scribed graphene, programmable pattern, epidermal skin, GO lift-off, crack simulation
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tattoo sensors based on graphene grown by chemical vapor deposition (CVD),26 which could monitor human bioelectrical signals. However, tattoo-like epidermal strain sensors have not been realized yet. Moreover, tattoos are essentially a kind of decoration, whose patterns should be customizable and artistic. Since CVD graphene layers are vulnerable, it is difficult to obtain the desired pattern and good durability. In addition, the design is determined by the photolithography mask. Therefore, there is an urgent need to realize mechanical graphene epidermal sensors with excellent performance, good lamination, and favorable artistry. In this work, we demonstrate a full process to fabricate graphene epidermal artwork sensors based on laser scribed gra-
pidermal sensors have a significant application prospect in electronic skins, which has attracted much attention in recent years.1−13 Compared with traditional sensors, epidermal sensors should have many special characteristics, such as high sensitivity, superior flexibility, good adhesion, and comfort. Traditional materials, such as metals and semiconductors, have been used as epidermal sensors due to their excellent characteristics.12,14,15 However, these epidermal sensors still have several weaknesses that hinder their further application. For example, the pattern is complicated to design and complex to fabricate. In addition to traditional materials, many materials, such as carbon nanotubes,16,17 silver nanowires,18,19 and polymers,20−22 have been employed to improve the performance of epidermal sensors. Graphene, a two-dimensional material with excellent characteristics,23,24 has attracted great attention since its discovery in 2004.25 Recently, Ameri et al. developed electronic © XXXX American Chemical Society
Received: March 22, 2018 Accepted: July 24, 2018 Published: July 24, 2018 A
DOI: 10.1021/acsnano.8b02162 ACS Nano XXXX, XXX, XXX−XXX
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Figure 1. Schematic illustration of the fabrication of graphene epidermal electronic skin. (a) The process of fabricating graphene with the Schrodinger equation on transfer paper and contact angles of GO and LSG. (b) The graphene pattern can be compressed and stretched without irreparably destroying it. All scale bars represent 1 cm.
discussed in the Experimental Methods part. There are two reasons that GO can be removed while LSG remains. First, after the laser scribing process, the surface becomes less hydrophilic. Therefore, during the lift-off process, there will be a protective gas layer on the LSG films, which is not found on GO. Second, laser scribing is essentially a thermal reducing process. The relatively high temperature makes the contact between the substrate and LSG tighter. Graphene patterns can be attached even on motorial, wrinkled, and hairy skin (see Figure 1b). Original, compressed, and stretched patterns on a hand have all been exhibited. Besides transferring paper, the lift-off process is also suitable for many other flexible substrates with different washing force. Total graphene patterns on Ecoflex, PDMS (10% cross-linker concentration), PET, PEN, and PVA are shown in the inset of Figure S3. The mechanical properties of these substrates, GO, and LSG are listed in Table S1. LSG on these substrates can also be used as strain sensors (see Figure S3). Graphene has three characteristic peaks, D, G, and 2D. The Raman shifts of GO and LSG are demonstrated in Figure S4a. The Raman shift of LSG is similar to that of graphene grown by CVD (see Figure S4b). After the laser scribing, the ratio between G and D amplitude was obviously increased. The 2D peak, which was missing in the GO, appeared in LSG, which originated from a secondary phonon vibration of the C−C bonding, suggesting the presence of fewlayer graphene. Half-widths of LSG are clearly much smaller than that of GO. Characteristics of LSG and GO are further confirmed by X-ray photoemission spectroscopy (XPS) (see Figure S5). The XPS of GO is dominated by CO, C−O, and OC−O peaks, all of which are almost invisible for the LSG sample. The C/O ratios of LSG and GO are 10.72 and 0.88, respectively. This means most functional groups disappeared during the laser scribing process. In all, the laser scribing process successfully converted GO to multilayer graphene. In order to measure the electromechanical character of TGASSs, patterned sensors (2 × 6 meshes with a square width of 1 mm on transfer papers) were packed between two Ecoflex layers. The TGASSs showed excellent performances, including superior sensitivity, large strain range, and long-term stability. GF is the ratio between the relative resistance change and strain.
phene (LSG), which can grow and pattern graphene at the same time without a shield mask. The process is also compatible with many flexible substrates. Compared with the previous LSG work,27−30 a simple lift-off process to remove the unreduced graphene oxide (GO) film has been developed, merely with the help of water. The total graphene structure can increase the sensitivity and artistry of the devices. In addition, compared with patterns with GO, the total graphene structure could better withstand high temperature (as shown in Figure S1). The I−V curve of the GO film indicates its resistance is around 10 GΩ. Therefore, GO can be considered as an insulator. After the laser scribing, the resistance is reduced to 2.1 kΩ with a square resistance of 700 Ω/square (Figure S2). The resistance is decreased by about 5 × 107 times. In addition, after being heated to 200 °C, the conductivity of GO is around 1 kΩ/square. Therefore, sensors could withstand higher temperatures, after removing GO. When combined with wet transfer papers, total graphene artwork strain sensors (TGASSs) can be transferred onto any object, including the human body, perfectly and elegantly without any tape and can be used to detect physiological signals, such as pulses, respiration, and voices. When packed into elastic polymers Ecoflex, TGASSs have an ultrahigh gauge factor (GF) of 673. Besides, the sensitivity of TGASSs is much higher than that of strain sensors with GO (SSGs) at the same range. The sensitivity and strain range (up to 10%) can be adjusted by designing the pattern. To analyze the mechanism of resistive devices, two types of cracks have been discovered, which are simulated with a model. The simulation result is consistent with the experimental data. In addition, the removal process of GO is similar to the lift-off process in the traditional silicon-based process, which is an important step for the industrial production of carbon-based device systems. These epidermal sensors have significant potential in the area of health monitoring, agricultural monitoring, electronic tags, and so on.
RESULTS The schematic diagram of fabricating TGASSs is shown in Figure 1a, which consists of coating GO, laser scribing, a lift-off process, and water transferring. The detailed process will be B
DOI: 10.1021/acsnano.8b02162 ACS Nano XXXX, XXX, XXX−XXX
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Figure 2. Electromechanical performance of TGASSs and SSGs. (a) Relative resistance variation of TGASSs as a function of tensile strain. (b) Relative resistance variation of SSGs as a function of tensile strain. (Inset) Digital photographs showing the difference between TGASSs and SSGs. Scale bars in both figures represent 1 cm. (c) Comparison of relative resistance change between TGASSs and SSGs under various cyclic strains at a frequency of 0.5 Hz. (d) Relative resistance change of TGASSs under cyclic stretching−releasing with a strain of 2% at frequencies of 0.1, 0.25, 0.5, and 1.25 Hz. (e) Relative resistance variation with a cyclic strain of 2% for 1000 cycles.
GF = (ΔR /R 0)/(ΔL /L0)
there is more than one effect influencing the devices, which will be discussed below. As shown in Figure 2d, the relative resistance variation of the TGASSs is almost independent of the frequency under the strain range of 2% within a relatively large frequency range (0.1−1.25 Hz). A cycling test was performed under the condition of 2% strain, which illustrated that TGASSs also exhibited good durability and long-term stability (as shown in Figure 2e). Overshoots of TGASSs in the incipient 200 cycles are caused by Ecoflex layers. When the device became stable, TGASSs were able to remain stable for at least 1000 cycles. The performance of TGASSs can be tuned by changing the graphene patterns. As shown in Figure 3a, TGASSs with a single graphene line show great sensitivity in a small range
(1)
The curve shown in Figure 2a illustrates two linear parts, 0− 2% and 2.5−3.5%, with extracted GF values of 58 and 521, respectively. The two parts correspond to two different processes (crack formation and expansion). Compared with SSGs, the GF of TGASSs is higher (see Figure 2b) because the total graphene pattern enables the stress to load on graphene totally. Relative resistance variations of TGASSs and SSGs are tested under different strain ranges (0.5−2%) with a frequency of 0.5 Hz (as shown in Figure 2c). It is clear that both TGASSs and SSGs demonstrate good linearity at the low range, but TGASSs have a better sensitivity. Both TGASSs and SSGs show nonlinear characteristics at the high range, reflecting that C
DOI: 10.1021/acsnano.8b02162 ACS Nano XXXX, XXX, XXX−XXX
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Figure 3. Detection of subtle and violent human motions. (a) Relative resistance change of TGASSs with one, two, and four graphene lines as a function of tensile strain. (Inset) Digital photographs of TGASSs with one, two, and four graphene lines. (b) Respiration signal detected by a TGASS attached on a mask. (Inset) Tester wearing the mask with a TGASS attached. (c) Respiration signal detected by a TGASS attached to the philtrum. (Inset) The tester with a TGASS attched to the philtrum. (d) Respiration signal detected by a TGASS attached to the throat. (Inset) Tester with a TGASS attached to the throat. (e) Pulse signal detected with a TGASS attached on the wrist. (f) Magnified image of a single pulse. (Inset) Wrist with an attached TGASS. (g) Response toward the sound “graphene” and its magnified image. (Inset) Throat with an attached TGASS. (h) Response toward the sound “sensor” and its magnified image. (i) Response toward the sound “happy” and its magnified image. (j) Relative resistance variation at different finger bending degrees. (Inset) Finger displaying different bending degrees. The scale bars in all figures represent 2 cm.
detection (1.5%). Meanwhile, TGASSs with four graphene lines exhibit excellent sensitivity for large range detection (7.5%). According to the requirements of the detection range and sensitivity, TGASSs with different numbers of graphene lines are suitable for different circumstances. Respiration is an important health indicator. However, traditional devices for respiration detection are neither wearer-friendly nor attractive. On the contrary, TGASSs, with a programmed pattern, are wearable without any discomfort and are capable of detecting respiration when attached on a mask, philtrum, and throat (as shown in Figure 3b−d). Due to the screen of the mask, the sensitivity of TGASSs on the mask is much lower than that on the philtrum and throat. As a result of the ultrahigh sensitivity, the ΔR/R0 of TGASSs on the throat can be as high as 50%, which is around 10 times that of TGASSs on the philtrum (5%). These devices have great potential in detecting snoring
and athletics. In addition to respiration, the pulse is another physiological signal that reflects the body’s condition. Due to the excellent GF of TGASSs, clear pulse waves are detected including the percussion wave (P-wave), tidal wave (T-wave), and diastolic wave (D-wave) when attached on the wrist. Regular pulse shapes and 72 beats per minute of pulses reveal a healthy condition of the tester (as shown in Figure 3e and f). Besides respiration, the voice is also detected by a TGASS attached on the throat. By virtue of the tight contact, ambient noise is well filtered. Therefore, word signals with obvious characteristic waves can be detected at a regular speaking speed. Moreover, the signal could clearly respond to every syllable the tester pronounced. For instance, the phonetic alphabet of “graphene” is [’grafi:n], which is decomposed into three parts, [’g], [ra], and [f i:n]. Waves of three parts are shown in Figure 3g for the words “happy” and “sensor” in D
DOI: 10.1021/acsnano.8b02162 ACS Nano XXXX, XXX, XXX−XXX
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Figure 4. Schematic illustration of the two types of cracks. (a) Diagram of cracks I. (b) Diagram of cracks II. (c, d) SEM images of cracks I. (e, f) SEM images of cracks II. All scale bars in SEM images represent 100 μm. (g) Relative resistance variation of TGASSs with cracks I as a function of tensile strain. (Inset) Digital photographs showing cracks perpendicular to laser routes. (h) Relative resistance variation of TGASSs with cracks II as a function of tensile strain. (Inset) Digital photographs showing cracks perpendicular to laser routes. Scale bars in the inset (g) and (h) represent 1 cm. (i) Lumped component model of the graphene sensor. (j) Simulation of cracks I distribution at 0%, 2.5%, 4.5%, and 5% tensile strain and cracks II distribution at 0%, 2%, 4%, 6%, 8%, and 10% tensile strain. The constant current is set to 1 A.
Figure 3h and i. TGASSs with four graphene lines are capable of detecting large-range motion, for instance the bending of a finger. Figure 3j illustrates that the changes in TGASS resistance are different when a finger bends at various angles. The bending angles of TGASSs can be larger than 90° (Movie S3). In addition, TGASSs on fingers can work more than 2000 times, which can be sustained on the skin for more than 9 h (as shown in Figure S6 and Figure S7). In order to investigate the mechanism, different strain directions have been applied to TGASSs. The scanning electron microscopy (SEM) images, electrical performance, and simulation are combined to provide a deeper understanding. During laser scribing, the laser, with a spot size of 100 μm, moved along the y direction. Therefore, sensors are built up by many parallel and overlapped LSG tapes (as shown in Figure S8). Two different crack types are distinguished. Cracks perpendicular to laser routes are named “cracks I”, which are shown in Figure 4a. It is clear that cracks I cut off LSG tapes. Second, cracks parallel to laser routes are named “cracks II”, as
shown in Figure 4b. Cracks II are gaps between adjacent LSG tapes, while structures of LSG tapes are not destroyed. SEM images illustrate that the structures of the two cracks show obvious differences (see Figure 4c−f). To eliminate other factors, sensors without any patterns are fabricated to test the performance of two different devices. The curve shown in Figure 4g illustrates that sensors with cracks I have three linear ranges, 0−2.5%, 2.5−4.5%, and 4.5−5%, with a GF of 11, 92, and 673. Compared with sensors containing cracks I, sensors containing cracks II show a larger range (10%) and better linearity (as shown in Figure 4h). However, the sensitivity is worse and there is almost no resistance change in the low range (2%). What is more, the structures of the two cracks can be distinguished easily in digital photographs (see insets of Figure 4g and h). The results that sensors without patterns have a larger range and worse sensitivity at the same range agrees with previous work.31 Our devices exhibit excellent performance among carbon-based strain sensors (see Table 1), such as ease of wear, large GF, long-term E
DOI: 10.1021/acsnano.8b02162 ACS Nano XXXX, XXX, XXX−XXX
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Table 1. Comparison of the Sensitivities and Strain Tolerance between TGASSs and Previously Reported Carbon-Based Strain Sensors material
fixing method
total LSG with cracks I total LSG with cracks II single-walled carbon nanotube LSG with pattern LSG without pattern LSG LSG
without tape tape tape tape tape tape
LSG pencil-trace film graphite carbonized cotton fabric graphene−nanocellulose nanopaper nanographene by CVD monolayer graphene by CVD carbon nanotube carbonized silk fabric electrochemical exfoliated graphene flakes graphene-based fiber reduced graphene oxide reduced graphene oxide
tape tape tape tape tape tape tape tape tape tape tape tape tape
strain range
gauge factor
5% 10% 280% 35% 100% 7.5% 100% 10% N/A 0.62% 50% 140% 100% 1.6% 4.5% 410% 500% 2% 100% 2% 82%
673 23 15 457 268 400 20000
