Ultralow-Cost, Highly Sensitive, and Flexible Pressure Sensors Based

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Applications of Polymer, Composite, and Coating Materials

Ultralow-Cost, Highly Sensitive, and Flexible Pressure Sensors Based on Carbon Black and Airlaid Paper for Wearable Electronics Zhiyuan Han, Hangfei Li, Jianliang Xiao, Honglie Song, Bo Li, Shisheng Cai, Ying Chen, Yinji Ma, and Xue Feng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b12929 • Publication Date (Web): 13 Aug 2019 Downloaded from pubs.acs.org on August 15, 2019

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ACS Applied Materials & Interfaces

Ultralow-Cost, Highly Sensitive, and Flexible Pressure Sensors Based on Carbon Black and Airlaid Paper for Wearable Electronics Zhiyuan Han, †,‡ Hangfei Li, †,‡ Jianliang Xiao, § Honglie Song, †,‡ Bo Li, § Shisheng Cai, †,‡ Ying Chen, § Yinji Ma, †,‡ and Xue Feng* †,‡ †AML, Department of Engineering Mechanics, Tsinghua University, Beijing, 100084, China ‡Center for Flexible Electronics Technology, Tsinghua University, Beijing, 100084, China §Institute of Flexible Electronics Technology of THU, Zhejiang, Jiaxing, 314000, China

KEYWORDS: pressure sensors, low cost, simple fabrication, carbon black, wearable electronics, flexible electronics

ABSTRACT

Flexible pressure sensors have attracted considerable attention because of potential applications in healthcare monitoring and human-machine interaction. However, the complicated fabrication process and costly sensing materials limit their widespread applications in practice. Herein, a

ACS Paragon Plus Environment

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flexible pressure sensor with outstanding performances is presented through an extremely simple and cost-efficient fabrication process. The sensing materials of the sensor are based on low-cost carbon black (CB) @ airlaid paper (AP) composites, which are just prepared by drop casting CB solutions onto APs. Through simply stacking multiple CB@APs with irregular surface and fibernetwork structure, the obtained pressure sensor demonstrate an ultrahigh sensitivity of 51.23 kPa1

and an ultralow detection limit of 1 Pa. Additionally, the sensor exhibits fast response time, wide

working range, good stability, as well as excellent flexibility and biocompatibility. All the comprehensive and superior performances endow the sensor with abilities to precisely detect weak air flow, wrist pulse, phonation, and wrist bending in real time. In addition, an array electronic skin integrated with multiple CB@AP sensors has been designed to identify spatial pressure distribution and pressure magnitude. Through a biomimetic structure inspired by blooming flowers, a sensor with the open-petal structure has been designed to recognize the wind direction. Therefore, our study, which demonstrates a flexible pressure sensor with low cost, simple preparation, and superior performances, will open up for the exploration of cost-efficient pressure sensors in wearable devices.

1. INTRODUCTION With the development of wearable electronics and electronic skins, flexible pressure sensors have drawn considerable scientific, technological, and commercial attention in various applications, such as healthcare monitoring,1-5 human-machine interfacing,6-8 and robot prosthesis.9-10 In recent years, flexible pressure sensors have been widely studied by researchers due to their potential advantages, including mechanical flexibility, high sensitivity, fast response time, low cost, simple processing, and so on.11 Various sensing mechanisms have been developed


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to fabricate flexible and sensitive pressure sensors based on piezocapacitive,12-14 piezoresistive,1517

piezoelectric,18-19 and triboelectric effects.20-21 Among these types of sensors, piezoresistive

devices, which can transduce applied pressure into a resistance signal, have been widely investigated because of their low energy consumption, cost-efficient preparation, and simple signal acquisition.22 To realize piezoresistive pressure sensors with outstanding performances, two crucial factors need be considered. One is the active conductive material, which is used to construct the conductive paths. The other is the microstructure of the pressure sensor, which guarantees high sensitivity and other merits. To date, a wide range of expensive conductive materials,23 such as graphene,24 reduced graphene oxide (rGO),25 carbon nanotube (CNT),22 Au nanowires,26 and Ag nanowires,27 have been chosen as sensing elements of flexible pressure sensors. Various sophisticated microstructures, including micropyramid structure,28 micropillar structure,29 microdome structure,30 interlocked structure,31 porous structure,32 and so on,33 have been used to enhance the performances of pressure sensors, which can provide more contact points between conductive materials under loading and unloading. For instance, Pang et al. reported a highly flexible sensor based on two interlocked layers of Ptcoated polymeric nanofibers with flexible polydimethylsiloxane (PDMS) substrate.31 Shao et al. fabricated a high-performance pressure sensor that benefited from the Au-covered PDMS micropillar structure and the elasticity of conductive polypyrrole (PPy) film. The sensitivity of the sensor was 1.80 kPa-1 and the detection limit was as low as 2 Pa.34 Si and coworkers reported a highly pressure-sensitive honeycomb structured carbonaceous nanofibrous aerogels (CNFAs), which were fabricated through electrospinning, freeze drying, and thermal carbonization method. The CNFAs pressure sensor showed a high sensitivity of 1.02 kPa-1 and demonstrated a strong capability of monitoring human pulses in real time.35 The sensing materials used in aforementioned


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pressure sensors have achieved good performances and attracted great amount of scientific attention,23 however, the cost is relatively expensive, which may limit their widespread practical applications in the future. In addition, some complicated, costly, and time-consuming fabrication processes, such as traditional lithography, ion etching, or thermal treatment, are inevitably used to obtain the aforementioned microstructures, which may block their large-scale and cost-efficient production. Therefore, it is still a great challenge to manufacture high-performance flexible pressure sensors through a simple and low cost method. As is well known, paper is an abundant and renewable material, which possesses advantages of ultralow cost and mechanical flexibility. Recently, paper-based electronic devices have drawn considerable

attention

in

various

applications,

such

as

microfluidic

devices

(electrochemical/biochemical sensors),36-39 nanogenerators,40-42 supercapacitors,43-46 solar-driven evaporation systems,47-49 and on-paper electronic circuits.50-52 For instance, George M. Whitesides has done numerous researches about paper-based devices. He reported a paper-based multiplexed transaminase test that can measure rapidly and semiquantitatively aspartate aminotransferase (AST) and alanine aminotransferase (ALT) in a fingerstick whole-blood specimen.36 This low-cost and point-of-care liver function testing exhibited excellent stability for 80 days. He also proposed a foldable printed circuit boards on paper substrates52 and a paper-based electrical respiration sensor53 in a low-cost method. In addition, Zhonglin Wang has applied paper to supercapacitors and nanogenerators. A flexible supercapacitor was fabricated based polyaniline/Au/paper electrodes.43 The supercapacitor showed a high energy density of ~ 0.01 Wh cm-3 and exhibited a long-term stability after 10000 charge-discharge cycles at 1mA cm-1. He also designed and fabricated a polytetrafluoroethylene-Ag-paper-based nanogenerator (pNG) using electrostatic effect.40 For paper-based super capacitors, Wei Huang has written a comprehensive overview to


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highlight the importance of this exciting direction,45 in which the stability of the paper-based devices was confirmed through a lot of related literatures. Apart from the above applications, paper-based strain/pressure sensors have been explored as well.26, 54-55 Wenlong Cheng reported a wearable and highly sensitive pressure sensor based on gold nanowire (AuNW)-impregnated tissue paper, which was prepared through dip-coating AuNWs into tissue paper.26 Combined with photolithographic Ti/Au interdigitated electrodes and sandwiched between two PDMS sheets, the paper-based pressure sensor exhibited a sensitivity of > 1.14 kPa-1, a detection limit of 13 Pa, and a high durability of > 50000 cycles. Zhan et al. proposed a flexible pressure sensors based on SWNT/paper through a simple dip-coating process.54 Similarity, integrated with photolithographic Au interdigitated electrodes and encapsulated with a PDMS sheet, the sensor show good performances with a high sensitivity of 2.2 kPa-1 and a detection limit of < 35 Pa. Despite these achievements, the cost of the conductive materials is relatively high, and the fabrication process is not simple enough due to the use of photolithography technique. Hence, a paper-based pressure with low cost, simple preparation, and high performance is urgently needed and of great research value in wearable electronics. In this paper, a highly sensitive flexible pressure sensor is fabricated based on carbon black (CB) and airlaid paper (AP) through an extremely simple and cost-efficient fabrication process. CB and AP are ultra-cheap raw materials for designing pressure sensors compared with related sensors based on graphene, rGO, and noble metals. Conductive CB is uniformly wrapped onto AP fibers via the convenient solution drop-casting technique to construct conductive CB@AP composites. By stacking multilayer CB@APs with irregular surface and fiber-network structure, the sensor can increase the number of electric contact points under pressure, which contributes to prominent performances for the pressure sensors. Apart from the achievements of facile and low-cost


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preparation, the CB@AP pressure sensor exhibits an ultrahigh sensitivity of 51.23 kPa-1 and an ultralow detection limit of 1 Pa, which are obviously better than those of reported pressure sensors. Additionally, the sensor demonstrates a fast response time (< 200 ms), a large working range (030 kPa), a good repeatability (>3000 cycles), as well as excellent flexibility and biocompatibility. Benefiting from these comprehensive sensing performances, the sensor can accurately perceive weak air blow, wrist pulse, phonation, and wrist bending in real time. In addition, an array electronic skin integrated with multiple CB@AP sensors has been designed to map and identify spatial pressure distribution and pressure magnitude. Bioinspired by blooming flowers, a sensor with the structure like open petals has been designed to recognize the direction of blowing wind. Compared with previously reported flexible pressure sensors, the CB@AP sensor has obvious advantages in its use of cheap sensing materials and an easy preparation process, while simultaneously possessing a high sensitivity and a low detection limit to monitor human physiological signals. Therefore, our study will contribute effectually toward the development of cost-efficient pressure sensors with great potential applications in the field of healthcare, electronic skin, and smart wearable devices. 2. RESULTS AND DISCUSSION 2.1. Preparation and Characterization of CB@AP Composites. Figure 1a illustrates the schematic fabrication procedure of the CB@AP composite via dropcasting method. The CB solution was evenly dropped onto the neat AP and dried to leave the conductive particles coating on the AP fibers, which constructed the CB@AP composite. Through multiple dropping and drying, the CB@AP gradually turned blacker and exhibited a good electrical conductivity, which was used as the sensing materials of the flexible pressure sensor. Due to the


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simplicity of the fabrication, the CB@AP can be manufactured on a large scale (Figure S1, Supporting Information). Additionally, the raw materials used for the sensing element are relatively inexpensive in contrast with graphene, rGO, CNT, and Au/Ag nanowires.16, 24, 26-27 CB is an abundant material that simultaneously possesses a desirable conductivity and cost advantage, which has been widely applied in conductive polymer composites.56 However, CB usually tends to agglomerate and settle in aqueous solution. To improve the dispersibility of CB in water, one type macromolecular of polyvinyl alcohol (PVA) has been widely used as a stabilizer in CB suspensions.57-58 PVA chains that are adsorbed onto the surface of CB can effectively prevent aggregation and improve the stability of suspensions. As shown in Figure S2, the CB suspension added with PVA demonstrated good stability compared to the CB solution without PVA after 24 h, which can make it feasible to form uniform CB conductive layers on the AP fibers. The AP is a kind of low-cost nonwoven fabric, which is commonly used in clean room. It was selected as the frame structure to integrate with CB particles because of its comprehensive advantages. The AP offers two naturally hierarchical microstructures: one is the irregular and rugged surface as shown in the 3D morphology of AP (Figure 1b, top), the other is the intricate fiber-network structure as shown in Figure 1c. The hierarchical microstructures can contribute to a very high sensitivity (51.23 kPa-1), a very low detection limit (1 Pa), and a large working range (0-30 kPa),11, 59 which is demonstrated experimentally in the following section. The porous network structure of the AP provides an outstanding breathability, which indicates a potential biocompatibility. The AP demonstrates sufficient elasticity and flexibility as well. The transition from white AP to black CB@AP preliminarily demonstrated the combination of CB and AP (Figure 1a, bottom). The scanning electron microscopy (SEM) images in Figure 1c and 1d show that the smooth AP fibers become rough after drop-casting, and the CB particles have


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been coated onto the surface of fibers, thus forming the conductive CB@AP composites. SEM images in Figure S3 (Supporting Information) display the opposite surfaces of AP and CB@AP. To indicate the uniformity of the CB@AP, we cut out three samples from three different parts on one whole piece of CB@AP (19.5 × 21 cm) for SEM. As shown in Figure S4, the distribution of CB particles on the AP fibers are almost the same for the three samples, and the sheet resistances measured by a four-point-probe resistance testing system for the three samples are almost the same, which both indicated the uniformity of the CB@AP. To further confirm that CB was successfully wrapped onto the surface of AP, characterizations including Fourier-transform infrared (FTIR) spectra and X-ray photoelectron spectrometry (XPS) were conducted. As shown in the FTIR spectra (Figure 1e), the peak at 3419 cm-1 that corresponds to the O-H stretching vibration indicates that there are hydroxyl groups on the surface of CB. Moreover, the typical peaks in AP located at 3334 cm-1, 2906 cm-1, and 1030 cm-1, which correspond to -OH, -CH, and C-O-C vibrations that are redshifted to 3329 cm-1, 2902 cm-1 and 1028 cm-1 respectively in CB@AP, suggesting the interaction between CB particles and AP.60 The XPS spectra in Figure 1f show that the C/O atom ratio of 1.6 in AP increases to 10.3 after loading CB due to the higher C/O atom ratio of 49 in CB, representing a CB content of approximately 80% in the composite. Moreover, after the coating of CB, the 3D surface morphology of CB@AP showed no significant changes compared with that of the clean AP (Figure 1b, the corresponding 2D view in Figure S5). The CB@AP presented the same superior flexibility as AP during bending and twisting as shown in Figure 1g. To optimize the electrical conductivity of CB@AP, a systematic study has been investigated about the relationship between the titration times and the sheet resistance (Figure S6, Supporting Information). If the number of titration was not enough, the conductivity would be too poor to achieve a desirable capability for pressure sensing. In contrast, overmuch drop-casting would


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cause the redundant CB particles to fall off the AP fibers, which resulted in an inferior robustness and durability of the pressure sensor. The sheet resistance could decrease to a desirable value of ~35 kΩ/sq by proper multiple drop-casting. Besides, the sheet resistance value of the corresponding CB@AP had almost no change after being stored for one month, which proves the conductive stability of the material. 2.2. Pressure-Sensing Mechanism and Electromechanical performance of the Flexible CB@AP Pressure Sensors. Taking advantages of the ultralow cost, extremely easy fabrication process, and other comprehensive superiorities, CB@AP was utilized as the sensing material to prepare the pressure sensor. Figure 2a shows the schematic illustration of the flexible CB@AP pressure sensor. Firstly, multiple CB@AP layers were stacked to form the piezoresistive structure. Then, the top and bottom layers were respectively connected to a copper foil electrode glued by conductive silver paste. Finally, the piezoresistive structure and electrodes were encapsulated by two pieces of surgical semipermeable polyurethane film, which possesses flexibility, water-proofness, and gas permeability.61 This simple encapsulation can not only protect human skin from direct contact with the CB, but also further stick CB@AP and electrode firmly to promote the mechanical strength of the sensor. It is worth noting that there was no complicated processing, such as lithography, during the entire preparation. Since all the components of the sensor are flexible, the CB@AP sensor exhibits excellent flexibility, which can be conformally attached to a glass rod with a radius of 8 mm (Figure 2b). To explain the piezoresistive mechanism of the CB@AP pressure sensor, the schematic illustration of two-layer sensor’s structural changes under compression is shown in Figure 2c. In the initial state without compression, there are much air gaps and few contact points of the two CB@AP layers, and the whole resistance of the sensor is relatively large. When a small


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pressure is applied on the sensor, the air space between the two layers will decrease, or even disappear, which can increase the number of contact points. In this stage, the whole resistance decreases sharply, so the sensor exhibits a high sensitivity. As the pressure continues to increase, the interwoven CB@AP fibers become denser and deform elastically, which results in more new contact area. The whole resistance furtherly declines to endow the pressure sensor with the capability of pressure perception in a large pressure range. To make the best of the rugged CB@AP surface structure and the sensing mechanism of the pressure sensor, multiple-layer CB@AP sensors were designed to improve the sensitivity.30 The more layers in the sensor, the more resistance change were due to the increased number of deformable air gaps. To systematically investigate the high sensitivity of the pressure sensors, a series of compression tests were set up. Figure 2d and 2e show the response of the relative resistance change ratios (ΔR/R0=(R0 - R)/ R0, where R0 and R refer to the resistance value without and with applied pressure, respectively) with the applied pressure for the flexible pressure sensors with two-layer, three-layer, and four-layer CB@AP composites. As the number of the layers increases, the sensitivity has been gradually enhanced in the small-pressure range. The pressure sensitivity (S) can be defined by the formula S=δ(ΔR/R0)/ δP, where P is the applied pressure. As shown in the enlarged view of Figure 2e (Figure S7a, Supporting Information), the sensitivity of the four-layer sensor is ultrahigh with a value of 51.23 kPa-1 in the range of 0-10 Pa. The calculated sensitivities for the three-layer and two-layer sensors are 9.15 kPa-1 in the range of 0-70 Pa and 7.12 kPa-1 in the range of 0-90 Pa, respectively. Compared with previously reported researches,62-64 which calculated the sensitivity based on the same formula, the CB@AP pressure sensors achieve a relatively high sensitivity (Table S1, Supporting Information). Apart from the high sensitivity, the three-layer and four-layer sensors demonstrated a wide working range of 0-30 kPa (Figure S7b), which could contribute to


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monitoring human physiological signals in the low-pressure and medium-pressure regimes.65 However, the more layers of the sensor can lead to a thicker thickness. The cube of the thickness is proportional to the bending stiffness, in which the bending stiffness (Ebh3/12, where E refers to Yong’s modulus, b is the width of the cross section, and h refers to the thickness) is a mechanical parameter to quantify the flexibility of films. So the bending stiffness for four-layer sensor is twice larger than that of three-layer sensor. That is, the three-layer sensor is much more flexible than the four-layer sensor. Besides, the three-layer sensor exhibited a very high sensitivity (9.15 kPa-1), which is much larger than that of recently reported pressure sensors (Table S1, Supporting Information). On the whole, by making a comprehensive tradeoff between flexibility and sensitivity, the three-layer CB@AP sensor is chosen to be mainly investigated in the following experiments. Because of the ultrahigh sensitivity, the CB@AP sensor presented an ultralow detection limit. As shown in the inset of Figure 2f, a tiny weight (20 mg) was cyclically loaded and unloaded on a sensor (15 mm × 15 mm), giving a pressure of ~1 Pa. The elicited resistance response signal was distinct and reproducible (Figure 2f). The ultralow detection limit of the CB@AP pressure sensor is superior in contrast to that reported previously for pressure sensors (Table S1, Supporting Information), which enables the sensor to detect weak airflow and human pulses. Figure S8 (Supporting Information) shows that the response and recovery time of the sensor are both less than 200 ms, indicating the adequate capability of monitoring human physiological signals.6, 15, 2728

To evaluate the outstanding stability and reliability of the pressure sensor, various compression

tests were performed. As shown in Figure 2g, when the sensor was applied with multiple cyclic loading and unloading tests under different pressure values of 25, 56, 80, 122, 200, and 433 Pa, the relative resistance change ratios increased monotonically with the incremental applied


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pressures, and the response was steady and repeatable. The recorded compressive stress-strain (σ– ε) curve for the CB@AP sensor is plotted in Figure S9a (Supporting Information). There are two distinct stages: one relatively flat regime corresponding to the compression of air gaps, and one rising stage corresponding to the densification and elastic deformation of the fibers. As shown in Figure S9b (Supporting Information), the σ–ε curves for 10 compressive cycles at a strain of 60% are almost identical, which confirms that the sensor can fully return to the initial state without plastic deformation. Figure 2h presents the relative resistance variation of the CB@AP sensor under cyclic test with different working frequencies. The response signal is highly steady and is independent of the loading rate, which is important for the practical applications of the sensor. Fatigue test was carried out to demonstrate the excellent durability as well. As shown in Figure 2i and Movie S1 (Supporting Information), the resistance response of the sensor was nearly invariant during more than 3000 loading-unloading cycles (1 s for each cycle). Furthermore, there were no significant changes in the CB@AP structures after the long time working (Figure S10, Supporting Information), indicating the stability of the CB@AP sensor.26, 56 In order to further confirm the stability of the CB@AP pressure sensor, another cyclic bending test was performed as shown in Figure S11. The two ends of the long side of the CB@AP composite (2 × 7 cm), which has been stored for more than four months, were respectively clamped by the two collets of the fatigue testing machine. The CB@AP would be subjected to large bending deformation under cyclic loading and unloading. By comparing the SEM images of the samples before (Figure S12a) and after (Figure S12b) the cyclic bending experiment for 10000 times, we found that, although there are a very small number of CB particles falling off the fibers of AP after the bending test, no significant changes were found for the structure of the CB@AP, and the two-point resistances of the same CB@AP before and after the bending test were almost identical through measuring the


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resistance between the same two farthest points of the CB@AP sample. For the same one threelayer CB@AP sensor that has been stored for more than four months, another compression test has been carried out to show the response of the relative resistance change ratios with the applied pressure. As shown in Figure S13, the two curves for the relative resistance change ratios with the applied pressure before and after being stored for more than four months almost overlapped. In addition, a series of simple controlled trials were carried out to investigate the stability of the CB@AP sensor in different environments (see details in Figure S14, Supporting Information). All the results indicated the superior performances and excellent stability of the CB@AP pressure sensors, which would have potential practical applications in the future. 2.3. Applications of the Flexible CB@AP Pressure Sensor. In consideration of the low cost, simple preparation, ultrahigh sensitivity, and other outstanding properties of the flexible CB@AP pressure sensor, a variety of practical applications have been explored. For instance, the sensor can detect weak airflow blown by a rubber suction bulb (Figure 3a), benefiting from the ultralow detection limit and ultrahigh sensitivity. The stable and repeatable response of the sensor is shown in Figure 3b and Movie S2 (Supporting Information). Similarly, the sensor can also precisely perceive the gentle air movements caused by the mouth breathing (Movie S3 in the Supporting Information, and Figure 3c), which may endow the sensor with the ability to monitor human’s respiration state in real time. Figure 3d shows the flexible CB@AP sensor conformally attached to a human neck by a surgical semipermeable polyurethane film. The sensor can be effective in detecting the vocal muscle motions when the tester is speaking. The response signals of the sensor demonstrate apparently distinct patterns when pronouncing the words “wave” and “science”, and the signal curves for each word have obvious and identical characteristic peaks (Figure 3e and 3f). These results reveal a promising possibility of voice


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recognition. As one of the four primary vital signs of human life (respiration rate, body temperature, pulse rate, and blood pressure), pulse signal monitoring can bring helpful medical information for the diagnosis and prevention of cardiovascular diseases.66 Figure 3g shows the sensor attached onto the wrist to noninvasively monitor the pulse signal of human radial artery in real time. The pulse waveforms are clearly presented in Figure 3h and Movie S4 (Supporting Information), and the pulse rate of 78 beats/min can be accurately read out. For each pulse waveform, there are three distinguishable characteristic peaks, which correspond to percussion wave (P-wave), tidal wave (T-wave), and diastolic wave (D-wave), respectively (Figure S15, Supporting Information). Deep analysis of the radial augmentation index, diastolic augmentation index, and digital volume pulse time extracted from the pulse waveform can be utilized to assess the health state of human body.14, 30, 67

In addition to abilities for the precise perception of subtle pressure, the CB@AP sensor can

detect large pressure generated by human activities because of the wide working range. The sensor mounted onto the back of wrist joint is able to identify bending deformation with different angles (Figure 3i), which provides a promising application in monitoring the athlete’s exercise state during training. Notably, there was no inflammation, irritation, or discomfort for the tester’s skin after wearing the sensor for 24 hours (Figure S16, Supporting Information) benefiting from the fiber-network structure of CB@AP and the waterproofness and breathability of the surgical semipermeable polyurethane film, showing the excellent biocompatibility. On the whole, all the aforementioned applications indicate that the CB@AP sensor can be highly practical in wearable devices and medical healthcare. In order to open up more potential applications, we have skillfully designed a petal-inspired sensor (Figure 4a) for distinguishing the direction of wind blow and an array electronic skin for mapping and identifying the spatial pressure distribution. Bioinspired by the open petals of


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blooming flowers (Figure 4b), in which the petals will open and close blown by breezes with different directions, an open-structure CB@AP sensor was prepared to recognize the direction of blowing wind. As can be seen in Figure 4a, compared with the normal CB@AP sensor (Figure 2a), in the biomimetic sensor, the upper polyurethane encapsulation layer was just removed, and the upper layer of CB@AP was in partial contact with the bottom CB@AP through pressing the upper copper foil electrode, thus forming the open structure like open petals. Figure 4c and Movie S4 (Supporting Information) show the opposite resistance response of the biomimetic sensor to wind blow with two different directions, exhibiting a remarkable capability of wind direction identification. When the wind was blown from the direction in which the upper and bottom CB@AP layers were not in contact (right direction of the inset in Figure 4c and of the Movie S5 in the Supporting Information), the structure of the sensor would be more open to decrease the conductive contact area, resulting in the increase of the resistance. On the contrary, the resistance of the sensor would decrease due to the gain of the contact area between the upper and bottom CB@AP layers, when the wind was blown from the opposite direction. This simple structural design will develop a promising and efficient idea for wind direction recognition or even wind magnitude detection. The aforementioned CB@AP sensors possess a facile and cost-efficient manufacturing method without involving complicated micro/nano processing. Nevertheless, there will be versatile and better performances, when conventional lithography is applied in the fabrication of the CB@AP sensor. Herein, a flexible array electronic skin was designed to measure the spatial distribution and magnitude of applied pressure, which is critical for the applications of wearable electronics, intelligent robots, and human-machine interface. Figure 4d shows the photographs and schematic illustrations of the 3 × 3 pixelated array of the CB@AP pressure sensors, and the sensor array can


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be conformally attached to the forearm, demonstrating good flexibility. The preparation of the array electronic skin is basically similar to that of the CB@AP pressure sensor except the electrodes, which were fabricated by conventional lithography for flexible electronics.68 The 9 sensing units share one bottom electrode (Figure S17a), and each sensor leads one top electrode (Figure S17b). It has been experimentally verified that the 10 electrodes for the 9-pixel sensing array can eliminate mutual interference between the sensors. By using a microcontroller and a homemade program (Figure S18a, Supporting Information), the array e-skin can monitor the dynamic response of pressure distribution in real time. As shown in Figure 4e, the 3D-printed modes “T”, “H”, “U”, which is the abbreviation of “Tsinghua University”, are positioned over the array e-skin. The 3D bar graphs exhibit corresponding resistance response, which reveal the pressure distributions. Additionally, Movie S6 and Figure S18b, c, d in the Supporting Information show that the array e-skin can identify the pressure magnitude of each sensor for loading three different weights (50, 200, and 500 g). Movie S7 (Supporting Information) shows that the number, position, and pressure magnitude of the fingers touching onto the array e-skin can be accurately recognized, which could be used in human-machine interaction and intelligent prosthesis. 3. CONCLUSIONS In summary, we proposed an extremely simple and cost-efficient strategy to prepare highly sensitive and flexible pressure sensors. The sensing material of the sensor is based on low-cost CB@AP composites, which were just synthesized by drop casting carbon black solutions onto airlaid papers and could be produced on a large scale. Followed by stacking multilayer CB@APs, the simple-fabricated pressure sensor exhibited outstanding performances, due to the unique structure and compressive contact of the CB@AP conductive fiber backbones and that of the multi irregular layers. Notably, the pressure sensitivity of the sensor can reach 51.23 kPa-1, which is


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obviously higher than that of the reported pressure sensors fabricated based on expensive conductive materials. The flexible sensor exhibited other merits as well, including a low detection limit of 1 Pa, a wide sensing range of 0-30000 Pa, a fast response time of < 200 ms, and a high durability for over 3000 cycles. All the outstanding characteristics endowed the CB@AP sensor with capabilities of precisely detecting both subtle (such as weak gas flow detection, pulse monitoring, and voice recognition) and large (such as wrist bending monitoring) pressures in realtime. In addition, the sensor can be integrated into an array electronic skin, which can simultaneously identify spatial pressure distribution and pressure magnitude. Through the design of a biomimetic structure, the sensor can distinguish the direction of wind blow. We believe that, based on such cheap raw materials, facile fabrication process, and superior performance, the CB@AP sensor will open up new opportunities for low-cost pressure sensors with practical applications in e-skin and smart wearable electronics. 4. EXPERIMENTAL SECTION 4.1. Preparation of CB@AP Composites. The CB@AP composites were fabricated by dropcasting CB dispersions onto APs. AP (Cleanroom Wipers, Alibaba, China) was cut into rectangular pieces of 3 × 6 cm. Conductive CB (XFI15, XFNANO Materials Tech Co., Ltd, China) with a mass of 0.1 g was added into 100 ml of water to prepare the CB solution (1 mg/ml), in which the diameter of CB particles was ~30 nm in the transmission electron microscopy (TEM) images (Figure S19). To improve the dispersity of the CB suspension, 2 ml PVA solution (1 mg/ml) was added and the mixture was sonicated for 30 min. Then, 0.6 ml PVA stabilized CB dispersion was dropped into the rectangular AP by a pipette. Next, the CB@AP was put on a heating plate with 50°C until it was dry. Finally, the above drop-casting and drying processes were repeated several


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times to endow the CB@AP composite with an appropriate conductivity (sheet resistance of ~35 kΩ/sq). 4.2. Fabrication of the Flexible CB@AP Pressure Sensors. Firstly, the CB@AP was cut into squares, and multiple pieces of squares were stacked to make the pressure sensitive structure. Then the top and bottom layers were respectively connected to a copper foil electrode (1181, 3M, USA). A small amount of silver paste was used to improve the strength and electrical conductivity of the connection. Finally, two pieces of surgical semipermeable polyurethane film (Tegaderm-Film 1624W, 3M, USA) were utilized to encapsulate the piezoresistive structure and electrodes, thus producing the CB@AP pressure sensor with excellent flexibility and compatibility. 4.3. Characterization. SEM images were performed by a GeminiSEM 500 instrument at a voltage of 3 kV. The 3D morphologies were collected using a Hirox RH-2000 digital microscope. The CB particle size was characterized by a TEM (JEM-2100F). FTIR spectra was measured by a VERTEX 70v spectrometer. XPS spectra was collected on a Thermo Scientific ESCALAB 250Xi. The sheet resistance was measured by a four-point-probe resistance testing system (Napson RT8S). To investigate piezoresistive performances of the sensor, compression tests were performed by a universal testing machine (Zwick-Z005) and a fatigue testing machine (Instron E3000). The responsive electrical signals were recorded by a resistance meters (TH2515), a digital multimeter (Keysight 34461A), and a voltage acquisition card (NI USB-4431), in which the suitable test instrument was chosen for the corresponding application scenario. To collect the low-frequency signals (air flow, mouth breathing, and wrist bending), the resistance meter (TH2515 or Keysight 34461A), which has a relatively low sampling frequency, was used. A voltage acquisition card (NI USB-4431) with a high date rate from 2 to 102.4 kS/s was chosen to monitor the signals (phonation, pulse) that have high frequency or many features. The pressure distribution and magnitude of the


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3 ×3 pixelated array e-skin were identified by a microcontroller (Arduino Mega 2560), which was connected to a computer via a homemade program.

ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. Fatigue test of the CB@AP sensor corresponding to Figure 2i (mp4) Detection of the weak airflow blown by a rubber suction bulb (mp4) Detection of the gentle air movements generated by the mouth breathing (mp4) Wrist pulse monitoring in real time (mp4) Demonstration of the bioinspired CB@AP sensor’ ability of wind direction identification (mp4) Detection of the pressure distribution and magnitude by the array e-skin (mp4) Finger touching detection by the array e-skin (mp4) Figure S1-S19 and Table S1 (PDF) AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ORCID Xue Feng: 0000-0001-9242-8474 Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENTS This work is supported by the National Basic Research Program of China (Grant No. 2015CB351900) and National Natural Science Foundation of China (Grant Nos. 11625207, 11320101001 and 11222220). REFERENCES (1) Chung, H. U.; Kim, B. H.; Lee, J. Y.; Lee, J.; Xie, Z.; Ibler, E. M.; Lee, K.; Banks, A.; Jeong, J. Y.; Kim, J.; Ogle, C.; Grande, D.; Yu, Y.; Jang, H.; Assem, P.; Ryu, D.; Kwak, J. W.; Namkoong, M.; Park, J. B.; Lee, Y.; Kim, D. H.; Ryu, A.; Jeong, J.; You, K.; Ji, B.; Liu, Z.; Huo, Q.; Feng, X.; Deng, Y.; Xu, Y.; Jang, K.-I.; Kim, J.; Zhang, Y.; Ghaffari, R.; Rand, C. M.; Schau, M.; Hamvas, A.; Weese-Mayer, D. E.; Huang, Y.; Lee, S. M.; Lee, C. H.; Shanbhag, N. R.; Paller, A. S.; Xu, S.; Rogers, J. A. Binodal, Wireless Epidermal Electronic Systems with in-Sensor Analytics for Neonatal Intensive Care. Science 2019, 363 (6430), eaau0780. (2) Boutry, C. M.; Beker, L.; Kaizawa, Y.; Vassos, C.; Tran, H.; Hinckley, A. C.; Pfattner, R.; Niu, S.; Li, J.; Claverie, J.; Wang, Z.; Chang, J.; Fox, P. M.; Bao, Z. Biodegradable and Flexible Arterial-Pulse Sensor for the Wireless Monitoring of Blood Flow. Nat. Biomed. Eng. 2019, 3 (1), 47-57. (3) Lee, S.; Sasaki, D.; Kim, D.; Mori, M.; Yokota, T.; Lee, H.; Park, S.; Fukuda, K.; Sekino, M.; Matsuura, K.; Shimizu, T.; Someya, T. Ultrasoft Electronics to Monitor Dynamically Pulsing Cardiomyocytes. Nat. Nanotechnol. 2019, 14 (2), 156-160. (4) Hong, Y. J.; Jeong, H.; Cho, K. W.; Lu, N.; Kim, D.-H. Wearable and Implantable Devices for Cardiovascular Healthcare: from Monitoring to Therapy Based on Flexible and Stretchable Electronics. Adv. Funct. Mater. 2019, 29 (19), 1808247. (5) Kabiri Ameri, S.; Ho, R.; Jang, H.; Tao, L.; Wang, Y.; Wang, L.; Schnyer, D. M.; Akinwande, D.; Lu, N. Graphene Electronic Tattoo Sensors. ACS Nano 2017, 11 (8), 7634-7641. (6) Tsai, Y.-J.; Wang, C.-M.; Chang, T.-S.; Sutradhar, S.; Chang, C.-W.; Chen, C.-Y.; Hsieh, C.H.; Liao, W.-S. Multilayered Ag NP–PEDOT–Paper Composite Device for Human–Machine Interfacing. ACS Appl. Mater. Interfaces 2019, 11 (10), 10380-10388. (7) Song, J.-K.; Son, D.; Kim, J.; Yoo, Y. J.; Lee, G. J.; Wang, L.; Choi, M. K.; Yang, J.; Lee, M.; Do, K.; Koo, J. H.; Lu, N.; Kim, J. H.; Hyeon, T.; Song, Y. M.; Kim, D.-H. Wearable Force Touch


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FIGURES Figure 1. Fabrication and characterization of the flexible CB@AP. (a) Schematic illustration showing the preparation of the CB@AP composite through drop-casting method and photographs of the AP and CB@AP. (b) 3D morphologies of AP and CB@AP with irregular surface. (c, d) Scanning electron microscopy (SEM) images of the pure AP and CB@AP composite with fiber network structures, respectively. (e) FTIR spectra of CB, AP, and CB@AP. (f) XPS survey scans of (wide-scan XPS survey spectrum of) CB, AP, and CB@AP. (g) Photographs showing the excellent flexibility of the CB@AP composite during bending and twisting.

Figure 2. Electromechanical performances of the flexible CB@AP pressure sensor. (a) Schematic illustration of the flexible multilayer CB@AP pressure sensor. (b) Photograph of the CB@AP sensor twined around a glass rod with a diameter of 16 mm. (c) Schematic diagrams of the structural change of two CB@AP layers under small and large pressures. (d) Relative resistance variation ratios as a function of pressure for the flexible pressure sensors composed of two-layer, three-layer, and four-layer CB@AP composites. (e) Enlarged view from (d) in a small range of 00.5 kPa. (f) Resistance change of the CB@AP sensor upon loading a 20 mg weight (indicating an ultralow detection limit of ≈ 1 Pa). (g) Response test of the sensor under different cycling pressures. (h) Relative resistance change of the sensor under cyclic loading and unloading at different frequencies. (i) Durability test of the CB@AP sensor under a repeated pressure of 80 Pa for over 3000 cycles.

Figure 3. Applications of the flexible CB@AP pressure sensor for various physiological signals monitoring in real time. (a)Photograph showing the sensor blown by a rubber suction bulb and (b) the corresponding resistance variation. (c) Resistance response of the sensor to the air movements generated by blowing from mouth. (d) Photograph showing a CB@AP sensor pasted on a human neck for voice recognition and the response curves induced by phonation of (e) “Wave” and (f) “Science”, respectively. (g) Photograph of the sensor attached to the wrist to monitor the pulse. (h) Original pulse signal with clear waveforms. (i) Resistance signal of the CB@AP sensor fixed on the back of the wrist under different degrees of cyclic bending.

Figure 4. A petal-inspired sensor for distinguishing the direction of wind blow and an array electronic skin for mapping and identifying the spatial pressure distribution and magnitude. (a) Schematic illustration and photograph of the bioinspired sensor that mimics (b) the open petals of a blooming flower. (c) Opposite resistance response of the bioinspired sensor to wind blow from two different directions. (d) Wearable 3×3 array electronic skin that can be conformally attached to a human arm. (e) Top view of the 3D-printed models “T”, “H”, “U” positioned over the array electronic skin and the corresponding response mapping of the pressure distribution.


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(b)

(a)

AP CB drop-casting

Airlaid paper

CB@AP

CB@AP

(c)

100 μm

30 μm

4 μm

100 μm

30 μm

4 μm

(d)

(e)

(f) AP

2902

1028

3329 2906

CB

1030

3334 3419

4000 3400 2800 2200 1600 1000

Wavenumber (cm-1)

(g)

C1s

CB@AP

Intensity (a.u.)

Transmittance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 33

O1s

CB C/O=49

O1s

AP C/O=1.6 CB@AP C/O=10.3

1500 1200 900

600

O1s

Bending C1s C1s

300

0

Twisting

Binding energy (eV)


Figure 01

Page 29 of 33

(a)

(b)

Semipermeable film CB@AP Copper electrode

2 cm

CB AP fiber

(e) 100 S=51.23 kPa

100

-1

75 50

two-layer three-layer four-layer

25 0

0

6

12

18

Pressure (kPa)

△R/R0 (%)

1 Pa

58 57

24

25 Pa 56 Pa 80 Pa 122 Pa 200 Pa 433 Pa

40 20

20

40

Time (s)

60

80

0

1

2

3

4

5

Compression cycles

15 0

△R/R0 (%)

30

60 40 20

55

60

65

Time (s)

70

75

0.3

0.4

0.5

0.05 Hz 0.1 Hz 0.15 Hz 0.2 Hz

40 20 0

50

100

150

Time (s)

200

250

45 30 15 0

0 50

0.2

Pressure (kPa)

60

80

45

0.1

two-layer three-layer four-layer

80

0

6

100

60

25

60

0 0

S=9.15 kPa-1

50

S=7.12 kPa-1

(h)

60

75

0 0.0

30

100 80

59

56

(i)

(g)

60

△R/R0 (%)

Resistance (kΩ)

(f)

Under small pressure Under large pressure

△R/R0 (%)

Initial state

△R/R0 (%)

(d) △R/R0 (%)

(c)

△R/R0 (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0

600

1200

1800

2400

Time (s)


3000

3600

3550 3555 3560 3565 3570 3575

Time (s)

Figure 02

(b) 32

(c) 32 Resistance (kΩ)

(a)

31 30 29 28

5

(e)

10

Time (s)

15

31 30 29 28

20

0

3

6

12

15

18

6

7

‘Science’

Voltage (a.u.) 0

1

2

3

4

5

6

0

7

1

2

4

5

(i) Resistance (a.u.)

(h)

3

Time (s)

Time (s)

(g)

9

Time (s)

(f)

‘Wave’

Voltage (a.u.)

(d)

0

Voltage (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 33

Resistance (kΩ)


0

1

2

3

4

5

6

7

Time (s)


0°

θ° 0

8

16

24

32

Time (s)

Figure 03

Page 31 of 33

(a)

(b)

(c)

Blowing

Resistance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Left Right

1 cm 0

(d)

(e)

7

14

21

28

Time (s)

1 cm


Figure 04

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 33

Voltage (a.u.)


0

CB

1

2

3

4

5

AP fiber

Initial state

Under small pressure

Under large pressure


TOC

Page 33 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



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