Facile Fabrication of a Flexible LiNbO3 Piezoelectric Sensor through

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Facile Fabrication of Flexible LiNbO3 Piezoelectric Sensor through Hot-Pressing for Biomechanical Monitoring Muzhen Xu, Hua Kang, Li Guan, Huayi Li, and Meining Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b10411 • Publication Date (Web): 13 Sep 2017 Downloaded from http://pubs.acs.org on September 15, 2017

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ACS Applied Materials & Interfaces

Facile Fabrication of Flexible LiNbO3 Piezoelectric Sensor through Hot-Pressing for Biomechanical Monitoring Muzhen Xu1, Hua Kang1, Li Guan1*, Huayi Li2, and Meining Zhang1* 1

Department of Chemistry, Renmin University of China, Beijing, 100872, China.

2

Institute of Chemistry, the Chinese Academy of Sciences (CAS), Beijing 100190, China

E-mail: [email protected] (M. Zhang); [email protected] (L. Guan)

KEYWORDS Pressure sensor; Piezoelectric composite; LiNbO3; Heart rate monitoring; Epilepsy seizure forecasting

ABSTRACT Wearable pressure sensors have attracted increasing attention for biomechanical monitoring due to their portability and flexibility. Although great advances have been made, there are no facile methods to produce sensors with good performance. Here, we present a simple method for manufacturing flexible and self-powered piezoelectric sensors based on LiNbO3 (LN) particles. The LN particles are dispersed in polypropylene (PP) doped with multi-walled carbon nanotubes (MWCNTs) by hot-pressing (200 °C) to form a flexible LN/MWCNTs/PP piezoelectric composite film (PCF) sensor. This cost-effective sensor has high sensitivity (8 Pa), fast response time (ca. 40 ms) and long-term stability (> 3000 cycles). Measurements of pressure

changes

from

peripheral

arteries

demonstrate

the

applicability

of

the

LN/MWCNTs/PP PCF sensor to biomechanical monitoring as well as its potential for biomechanics-related clinical diagnosis and forecasting.

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1. INTRODUCTION The continuous and effective monitoring of biomechanical properties, such as breathing rate, heart rate (HR), and human body movement, by wearable pressure sensors is essential to fitness tracking, remote manipulation, specific disease monitoring, and smart home detectors for people with disabilities and older adults due to the portability and flexibility of the sensors.1-3 However, the development of wearable pressure sensors for the personal persistent monitoring of pivotal vital signals remains challenging. To support this goal, a low-cost and facile technology is needed for producing flexible pressure sensors with not only high sensitivity, fast response time and good stability but also low energy consumption and convenient operation. Among the types of pressure sensors, piezoelectric sensors, which inherently convert mechanical/biomechanical oscillation, compression or bending to electric signals, have attracted increasing attention because of their low power consumption.4-6 Although great advances have been made in the development of wearable piezoelectric sensors,4-6 cost-effective sensors with good performance produced by facile approaches are insufficient. Over the past decades, various materials, including organics (polyvinylidene fluoride (PVDF), poly(vinylidene-fluoride-co-trifluoroethylene) (P(VDF-TrFE)), etc.) and inorganics (BaTiO3, PbTiO3, lead zirconate titanate (PZT), ZnO, etc.), have been employed in piezoelectric pressure sensors.7-13 Although inorganics generally have tens to hundreds of times higher piezoelectricity than organics, they are too rigid for wearable applications. The combination of inorganic piezoelectric materials with flexible polymers is expected to provide the best of both worlds: good flexibility and high piezoelectricity. To fabricate piezoelectric composite pressure sensors with high sensitivity, the composite materials are generally incorporated with an amplifier device, such as field-effect transistors (FETs),12,14,15 either working as a transducer connected to the transistor14 or a gate dielectric layer in the


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transistor12,15. However, the introduction of an amplifier device trades the advantageous self-powering ability for more complexity and higher cost. Herein, for the first time, we report the fabrication of a piezoelectric composite film (PCF) sensor composed of LiNbO3 (LN) particles, polypropylene (PP), and multi-walled carbon nanotubes (MWCNTs) by hot-pressing to achieve flexibility, highly sensitivity, fast response time and long-term stability (as shown in Figure 1). We chose LN as the key piezoelectric component because LN is a promising material for high-performance pressure sensors.16-18 First, LN is nontoxic, in contrast to lead-containing materials, making it suitable for long-term skin contact.19,20 In addition, as a single crystal with a high Curie temperature (1210 °C)21, LN is able to maintain stable piezoelectricity over a long time and a large temperature range without suffering from issues of grain and porosity.16 PP is an ideal polymer material because of its low cost, nontoxicity, durability, flexibility, light weight and low dielectric loss.22-24 As a thermoplastic material, PP can be quickly and easily processed by hot-pressing. These merits of PP lay a solid foundation for the facile fabrication of PCF sensors. Moreover, the relatively large modulus of PP (ca. 1 GPa) is expected to reduce the negative effect of polymer deformation on the sensor sensitivity and afford fast response times, as an increase in the elastic modulus of a composite material can lead to enhanced piezoelectricity.25,26 The multi-walled carbon nanotubes (MWCNTs) dispersed in PP are expected to play an important role in improving the piezoelectric properties because of their conductivity and good mechanical properties. Therefore, LN/MWCNTs/PP PCF sensors are expected to be self-powering with high sensitivity, fast response time and good stability.



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Figure 1. Schematic of the LN/MWCNTs/PP piezoelectric composite film (PCF) sensor fabrication process.

2. EXPERIMENTAL SECTION 2.1 Materials. A polarized LN wafer (Shanghai Institute of Optics and Fine Mechanics, the Chinese Academy of Sciences) was ground into powder before use. MWCNTs (Shenzhen Nanotech Port Co., Ltd.) and PP pellets (Shanghai Kai Sheng Plastic Co., Ltd.) were used as received. Chloral hydrate and kainic acid (KA) were purchased from Sigma. 2.2 Fabrication of the LN PCF Sensor. PP pellets, MWCNTs (1 wt%) and LN powder (1 wt%, 3 wt%, 5 wt%, and 8 wt%) were thoroughly pre-mixed at room temperature. The mixtures were added into a Dolylab OS Reactive Twin Screw Extruder System (Yantai Lingyu Powder Machinery Co., Ltd.) heated at 200 °C, followed by blending for 5 min. Then, ca. 0.6 g of the homogenous mixture was extruded and sandwiched between two splints on a XH-407 laminator (Dongguan City Xinhua Testing Machines Co., Ltd.), and a pressure of 3 MPa was applied at 200 °C for 5 min. After cooling to room temperature, the


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LN/MWCNTs/PP film was formed. The control PP and LN/PP films were prepared in the same manner as the LN/MWCNTs/PP film. The fabrication process of the LN/MWCNTs/PP PCF sensor is shown in Figure 1. The LN/MWCNTs/PP film (2 cm × 3 cm) was sandwiched between two pieces of tin foil paper and sealed by the adhesive tape. Cu wires were fixed to the adhesive tape in advance to connect the PCF sensor with the measurement devices. 2.3 Characterization of the LN Particles, MWCNTs and LN/MWCNTs/PP Film. The morphologies of the LN powder and composite films were characterized by a JEOL 7401 scanning electron microscopy (SEM). The tensile moduli of the composite films were tested by an electronic universal material testing machine (Instron Legend 2367). 2.4 Characterization of the PCF Sensor. The PCF sensors were fixed to the testing area of an HP 10 digital force gauge (Yueqing Handpi Instruments Co., Ltd.) and connected to a computer-controlled CHI 660D electrochemical analyzer (Shanghai Chenhua Instrument Corporation, China). When a pressure was loaded on the PCF sensors by the force gauge, the electric current generated in the circuit was recorded by the electrochemical analyzer with the amperometric i-t technique. 2.5 Rat Epilepsy Seizure Model. Adult male Sprague-Dawley rats (300-350 g) were purchased from Health Science Center, Peking University. All animal procedures were approved by the Animal Care and Use Committee at the National Center for Nanoscience and Technology of China and performed according to their guidelines. The rat epilepsy seizure model experiments were performed following the reported method.27 Briefly, the rats were anaesthetized with chloral hydrate (345 mg/kg, i.p.). A guide cannula for the microinjection of KA was implanted in the left ventral hippocampus area (AP = -5.6 mm; L = -5 mm; V = -5.5 mm). After monitoring the HR for 4 min, KA (2 µg/µL normal saline) was microinjected unilaterally in the left ventral hippocampus through the guide cannula at a rate of 0.4 µL/min over a period of 6 min. 5 Environment ACS Paragon Plus


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3. RESULTS AND DISCUSSION 3.1 Structural and Mechanical Properties of the PCF Sensor. Figure 1 shows a schematic diagram of the simple fabrication process of the flexible and self-powered PCF sensor and its functional mechanism. The advantage of hot-pressing is its ability to produce tens of films without size limitation. The size and thickness of the film can be simply manipulated by the amount of input materials and pressure. In our study, the LN/MWCNTs/PP composites were pressed under a pressure of 3 MPa to films ca. 12 cm in diameter (Figure 2a) and ca. 70 µm in thickness (Figure 2c). The film could be bended and distorted arbitrarily, exhibiting good flexibility and translucency (Figure 2b). LN particles ranging from sub-micrometers to micrometers in size (Figure 2e) were well dispersed in the LN/MWCNTs/PP film (Figure 2g). As shown in Figure 2d, MWCNTs were also homogeneously dispersed in the LN/MWCNTs/PP film. The MWCNTs, as conducting phases, played a role in regulating the electrical properties of the LN/MWCNTs/PP PCF sensor (ca. 1.0 × 1014 Ω cm) in comparison with the LN/PP film sensor (ca. 6.7 × 1014 Ω cm) and maintained good dielectricity (Figure S1). Moreover, well-dispersed MWCNTs enhanced the mechanical strength of the PCF sensor due to their high Young’s modulus.28 As shown in Figure 2h, the addition of LN did not obviously change the mechanical strength of the PP film, while the LN/PP films doped with MWCNTs (1 wt%) showed a higher tensile modulus. This result indicates that the LN/MWCNTs/PP film has better mechanical strength than the LN/PP film, which may be because MWCNTs act as reinforcing agents in composite materials, as reported previously.28,29 The improved mechanical strength not only makes the PCF sensor more durable but also increases its piezoelectricity, because MWCNTs can reinforce the stress applied to LN particles in the PCF sensor due to their enhanced tensile modulus,30,31


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effectively reducing the negative impact of polymer deformation on the piezoelectricity of the PCF sensor.26

Figure 2. Photographs of (a) an intact piece of the LN/MWCNTs/PP film and (b) the LN/MWCNTs/PP film being distorted by manual manipulation. The inset photograph in (b) is the LN/MWCNTs/PP film over our university’s logo to demonstrate its translucency. SEM images of (c) a cross section of the LN/MWCNTs/PP film, (d) the magnified cross section of (c) where MWCNTs are indicated by white arrows, (e) the LN particles and (f) the MWCNTs. 7 Environment ACS Paragon Plus


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(g) Microscopic photograph of the LN/MWCNTs/PP film containing 3 wt% LN. The LN particles are indicated by red arrows. (h) Typical stress-strain curves of the pure PP film (red curve), the LN (5 wt%)/PP film (dark green curve) and LN/MWCNTs/PP films containing 1 wt% MWCNTs and 1 wt% (black curve), 3 wt% (light green curve), 5 wt% (blue curve) and 8 wt% LN (pink curve).

3.2. Piezoelectric Properties of the PCF Sensor. To investigate the piezoelectric performance of the PCF sensors, we measured the output current generated in the circuit during repeated press and release cycles (Figure 3a). The corresponding current changes are illustrated in Figure 3b. In the original state, there is no current in the circuit (Figure 3b-i). When a normal force is applied, the polarization of the LN particles changes, resulting in electron flow in the external circuit from the bottom electrode to the top electrode and generating an output current signal (Figure 3b-ii). Upon removing the force, the polarization of the LN particles returns, and the electrons accumulated at the top electrode will move back to the bottom electrode, leading to an output current signal in the opposite direction (Figure 3b-iii). To confirm that the current signals in the circuit were completely generated by the LN/MWCNTs/PP PCF sensor, a polarity-switching test was conducted. When the LN/MWCNTs/PP PCF sensor was forward connected to the measurement instrument, a negative current signal was generated upon the application of pressure, and a positive signal upon release (Figure 3c). In contrast, when the PCF sensor was backward connected to the measurement instrument, the opposite behavior was observed, where a positive current signal was generated upon the application of pressure and a negative signal upon release (Figure 3d). These results demonstrate that the recorded current signals are truly generated by the LN/MWCNTs/PP PCF sensor without the interference of electrostatic charge. The piezoelectric performances of the LN/MWCNTs/PP PCF sensor, PP film sensor and LN/PP film sensor were compared to clarify the role of LN and MWCNTs in the PCF sensor. The films used in these three sensors were of same size and thickness (Figure S2). Compared 8 Environment ACS Paragon Plus

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with the LN/PP film sensor (Figure 3f) and LN/MWCNTs/PP PCF sensor (Figure 3g), the PP film sensor generated imperceptible current signals (Figure 3e). This result confirms that the current signals of the LN/PP film sensor are mainly contributed by LN particles. As shown in Figure 3f and 3g, the current signals generated by the LN/MWCNTs/PP PCF sensor with 1 wt% MWCNTs were seven times larger than those generated by the LN/PP film sensor, which indicated that MWCNTs played an effective role in improving the piezoelectric ability of the sensor. This result may arise because MWCNTs can improve the charge transfer ability of the LN/MWCNTs/PP PCF sensor by forming conductive paths, as well as serve as electron collectors and transfer mediators, thus generating higher output current.32 This assumption was confirmed, as an increased content of MWCNTs in the LN/MWCNTs/PP PCF sensor enhanced the output current of the PCF sensor (Figure S3), which is similar to the results of previous reports.32 Notably, the incorporation of over 1 wt% MWCNTs led to decreased piezoelectricity (Figure S3), possibly due to the higher dielectric loss.32 Therefore, in our work, we chose 1 wt% MWCNTs to fabricate the LN/MWCNTs/PP sensor.



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Figure 3. (a) Photograph of the PCF sensor in during repeated press and release cycles. (b) Schematic piezoelectric mechanism of the PCF sensor during repeated press and release cycles. The measured current signals from the PCF sensor in the (c) forward connection circuit and (d) backward connection circuit during repeated press and release cycles at a pressure of 2.00 kPa. The bottom left insets show the circuit diagrams, and the bottom right insets are the magnified current signals. The output current signals obtained from the (e) pure PP film sensor, (f) LN/PP film sensor and (g) LN/MWCNTs/PP PCF sensor. The bottom right insets are the magnified current signals.

LN/MWCNTs/PP PCF sensors with different mass fractions (1 wt%, 3 wt%, 5 wt%, and 8 wt%) of LN particles with the optimized MWCNT doping content (1 wt%) were fabricated, and the results are shown in Figure 4a. Under a pressure of 2.00 kPa (applied force of 0.40 N, active area of 2 cm2), the LN/MWCNTs/PP PCF sensors containing 3 wt% LN and 5 wt% LN 10 Environment ACS Paragon Plus

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exhibited stronger current generation ability, with peak currents of ca. 50 nA (Figure 4a). LN/MWCNTs/PP films doped with more LN particles (10 wt%, 30 wt% and 50 wt%) were also fabricated but not tested here. First, the piezoelectric properties began to decrease when over 5 wt% LN particles was incorporated, probably owing to disorganization in the copolymer matrix caused by the LN particles.33 Second, excess LN particles in PP make the composite film fragile. Therefore, we chose the film containing 3 wt% LN for further experiments due to its better piezoelectric property and lower cost. The output peak currents of the LN/MWCNTs/PP PCF sensor under different pressures are shown in Figure 4b. The peak current (I) and applied pressure (P) have a linear relationship in the range of 0-1.50 kPa (I (nA) = 28.8P (kPa)), n = 12, Figure 4b). Moreover, our LN/MWCNTs/PP PCF sensor was very sensitive to low pressures. As shown in Figure 4c, the pressure from a light plastic foam (P = 8 Pa, F = 0.8 mN and 1 cm × 1 cm × 1 cm in size) could be easily detected. Three small water droplets with increasing volume, dropped from different heights could also be detected and distinguished, as shown in Figure S4. The detection limit of the LN/MWCNTs/PP PCF sensor was superior to those of many reported pressure sensors (Table 1). For instance, the detection limit of our LN/MWCNTs/PP PCF sensor was over twenty times lower than that of a reduced graphene oxide foam, whose detection limit was 163 Pa.34 The response time, defined as the time required for the output current to rise from the baseline to the maximum level, was estimated to be ca. 7 ms upon the application of pressure and ca. 40 ms upon release (Figure 4d). The response time upon release (ca. 40 ms) was slightly longer than that upon the application of pressure (ca. 7 ms), possibly because the sensor required more time to recover from its shape change. A vibration motor (180 Hz) with 0.7 s on and 1 s off was placed above the LN/MWCNTs/PP PCF sensor to further examine the sensor response under high frequency (Figure 4e). The result demonstrated the excellent performance of the LN/MWCNTs/PP PCF sensor in high-frequency perception. Moreover, the developed LN/MWCNTs/PP PCF sensor exhibited 11 Environment ACS Paragon Plus


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very good stability and durability (Figure 4f), because the mechanical properties of LN single crystals and PP are both extremely stable and PP has low dielectric loss.21,22,24 The current did not obviously change after 600 press and release cycles in the first day and remained very stable over 2000 cycles on the second day and 600 cycles on the fourth day. Moreover, in each part of the durability test, the sensor showed good repetition, as presented in the insets of Figure 4f. Considering the overall performance, our PCF sensor is superior to many reported pressure sensors in terms of sensitivity, stability, response time, energy consumption, etc., as shown in Table 1. These properties enable the LN/MWCNTs/PP PCF sensor to record biomechanical properties, such as voice frequency from an audio speaker (Figure S5) and human HR, as demonstrated in the next section.

Figure 4. (a) The measured output current signals of the PCF sensors containing 1 wt% (red line), 3 wt% (orange line), 5 wt% (green line), and 8 wt% LN (blue line). (b) The measured 12 Environment ACS Paragon Plus

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output current signals of the PCF sensors as a function of pressure. (c) The measured output current signals of the PCF sensors upon application of a plastic foam (1 cm × 1 cm × 1 cm). (d) The measured output current signals of the PCF sensors upon pressing and releasing. The insets show a response time of ca. 7 ms upon the application of pressure and ca. 40 ms upon release. (e) The measured output current signals of the PCF sensors as a function of time during attachment to a vibration motor with 0.7 s on and 1 s off. The inset shows the magnified photograph of the area in the blue dashed box in (e). (f) The durability test conducted over three days. The bottom insets show magnified images of the stable current signals measured on the first, second and fourth day. The MWCNT content in all figures is 1 wt%, and the LN content in (b-f) is 3 wt%.

Table 1. Summary of the performance of flexible pressure sensors reported in the literature. Sensing mechanism Capacitance Capacitance Capacitance Capacitance Piezoresistive Piezoresistive Piezoresistive Piezoresistive Piezoresistive Piezoresistive Triboelectric Triboelectric Piezoelectric Piezoelectric Piezoelectric Piezoelectric

Piezoelectric

Active materials Fluorosilicane Ecoflex Suspended-gate OTFT Microstructured PDMS/OFET Tissue paper/AuNWs Coplanar-gate graphene FET Sea-urchin-shaped metal NPs Microstructured gold thin film Microstructured PDMS/SWCNTs R-GO foam Micro/nanostructured PDMS PDMS/Ag NWs P(VDF-TrFE)/BaTiO3 FET Microstructured P(VDF-TrFE) FET P(VDF-TrFE)/PbTiO3 FET ZnO NWs strain-gated vertical piezotronic transistor LiNbO3/MWCNTs

Detection limit 0.5 kPa < 0.5 Pa

Sensitivity

Operating voltage

Reference

0.91 kPa-1 1.62 MPa-1 192 kPa-1

45 46 47

3 Pa

0.55 kPa-1

VD = -60 V VG = -60 V 1V

13 Pa 5 kPa

1.14 kPa-1 0.12 kPa-1

0.3 kPa 10.4 Pa

48

2.46 kPa-1

1.5 V VD = 0.3 V VG = -1 V 1V

51

50.17 kPa-1

0.01 V

52

-1

49 50

0.6 Pa

1.80 kPa

2V

53

163 Pa 1 kPa

15.2 kPa-1 0.06 kPa-1

None None

34 54

2.1 Pa 0.1 MPa

0.31 kPa-1 -

None VD = -5 V

55 12

20 Pa

1.02 kPa-1

VD = -20 V

56

2 MPa

-

14

3.5 kPa

2.1 µS kPa-1

VD = 10 V VG = 15 V VD = 1 V

11

8 Pa

28.8 nA kPa-1

None

This work

3.3. Wearable Measurement of Human HR HR is a vital signal that is closely related to body movement, mood, illness, etc. and is broadly adopted as an indicator in many wearable sensors not only for personal health



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monitoring, particularly for athletes and people with disabilities, but also for special disease forecasting, such as epileptic seizures35,36 and brain ischemia (strokes)37. As a proof of principle, HR monitoring was chosen to demonstrate the properties of our developed LN/MWCNTs/PP PCF sensor. The LN/MWCNTs/PP PCF sensor was able to monitor the HR of a young woman’s radial artery (Figure 5a) and carotid artery (Figure 5b) in real time. The current waveforms showed periodic and stable peak current with a high S/N ratio (ca. 140 in Figure 5a and ca. 250 in Figure 5b). The radial artery pulse generated an average peak current of 0.36 nA, and the carotid artery pulse generated an average peak current of 0.62 nA, corresponding to the fact that the carotid artery has a stronger pulse than the radial artery. According to the fitting formula I (nA) = 28.8P (kPa), the radial artery was estimated to have a pulse pressure change of ca. 13 Pa, and the carotid artery had a pulse pressure change of ca. 22 Pa, which are similar to the values in reported literatures.38-40 These results indicate that the LN/MWCNTs/PP PCF sensor is sensitive and stable enough to detect tiny pressure changes induced by biomechanical movement. The LN/MWCNTs/PP PCF sensor was also capable of measuring pre- and post-exercise HR and pulse pressure changes of the same person. As shown in Figure 5c, the post-exercise current waveform (red curve) had increased frequency, corresponding to the increased HR after exercise. Compared to a typical single waveform, the post-exercise waveform (Figure 5d, red curve) was taller than the pre-exercise waveform (Figure 5d, blue curve), which was attributed to the increased pulse pressure. Figure S6 shows a more obvious comparison in which the frequency and average peak current changed from 68 bpm and 0.62 nA (pre-exercise) to 86 bpm and ca. 0.74 nA (post-exercise). Furthermore, Figure 5e shows the waveform caused by a single heartbeat with higher temporal resolution (0.02 s), and three different peaks P1 (t1), P2 (t2) and P3 (t3) could be distinguished, where P1 (t1), P2 (t2) and P3 (t3) correspond to the early systolic pressure, late systolic pressure and early diastole pressure, respectively.41,42 These three peaks are important indicators for arterial stiffness diagnosis, left 14 Environment ACS Paragon Plus

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ventricular load, etc.41,42 ∆TDVP = t2 – t1 is the time delay between P1 and P2, AIr = P2/P1 is the radial augmentation index, and DAI = P3/P1 is the diastolic augmentation index.41,42 In Figure 5d, AIr and DAI both decreased after exercise, from ca. 0.47 and ca. 0.28 to ca. 0.41 and ca. 0.25, respectively, which corresponds to a reduced load on the left ventricle.42 In Figure 5e, ∆TDVP = 0.13 s and AIr = 0.47, which are characteristic for a healthy person in their early twenties.41 These results demonstrate that our LN/MWCNTs/PP PCF sensor has excellent performance in terms of high sensitivity, good stability and fast response and therefore can be employed in HR monitoring and HR-related health diagnosis.



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Figure 5. Current signals measured by the LN/MWCNTs/PP PCF sensor on the (a) radial artery of a sedentary person, (b) carotid artery of the same sedentary person, and (c) carotid artery of the same person before (blue curve) and after (red curve) exercise. (d) Comparison of typical single current waveform before (blue curve) and after (red curve) exercise extracted from (c). (f) A single current waveform extracted from (c) labeled by a dashed box.

3.4. Wearable HR Monitor in Rat Epilepsy Seizure. The LN/MWCNTs/PP PCF sensor further demonstrated its potential utilization in disease forecasting. For diseases such as epilepsy, an early warning several minutes before the seizure is of great importance. HR is regarded as a forecasting candidate because epilepsy seizures can affect the autonomic nervous system and result in prior tachycardia.43 Here, the rat epilepsy model was chosen for this experiment. To prevent the rat from removing the film sensor and to alleviate the pain from epilepsy seizure, the rat was anaesthetized during measurement. The LN/MWCNTs/PP PCF sensor was fixed on the rat’s right hind leg. Figure 6 and Figure S8 show the HR changes of the rat before and after epilepsy seizure. In contrast to the HR measurements of human peripheral arteries, the baseline of the current-time curve recorded for the rat changed periodically (Figure S7, i-iv), which may be attributed to the breathing of the rat. The HR was approximately 380 bpm before and during injection, which is in the range of the normal rat HR.44 Two minutes after KA injection, the HR changed to 400-410 bpm. Seven minutes later, tachycardia became much more obvious, and the heartbeat was very unstable – the HR dramatically fluctuated between 420 bpm and 480 bpm. In contrast, it wasn’t until 19 min after KA injection that the rat’s head was observed to move slightly over a very short time period (within a second, once several minutes), which is typical for seizure onset. It has been reported that status epilepticus generally begins 5-20 min after KA injection.27 These results suggested that the LN/MWCNTs/PP PCF sensor succeeds in the real-time monitoring of HR and is promising for early disease prediction.


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Figure 6. HR as a function of time measured by the LN/MWCNTs/PP PCF sensor before, during and after kainic acid (KA) delivery in the left ventral hippocampus area of a rat. The inset scheme shows the wearable LN/MWCNTs/PP PCF sensor that measures the HR on the right hind leg of a rat. The red arrow indicates the KA delivery position, and the blue arrow indicates the pressure-monitoring position.

4. CONCLUSION In summary, we successfully fabricated a flexible and highly sensitive LN/MWCNTs/PP PCF sensor with fast response, long-term stability and low energy consumption. The simple process of hot-pressing is a cost-effective way to produce PCF sensors, allowing personal usage use the sensors. The LN/MWCNTs/PP PCF sensor successfully measured HR for human health monitoring and epilepsy seizure prediction. Furthermore, the LN/MWCNTs/PP PCF sensor has sufficient resolution to detect the pressure change waves within a single heartbeat, which is promising for health diagnosis. Due to its good flexibility, superior performance, simple fabrication process and low cost, the LN/MWCNTs/PP PCF sensor has great potential in wearable e-skins and self-powered biomechanical devices. ASSOCIATED CONTENT 17 Environment ACS Paragon Plus


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Supporting Information Electric characteristics of the LN/PP film sensor and LN/MWCNTs/PP PCF sensor under pressures of 0 kPa and 0.5 kPa; cross-sectional SEM image of the PP film, LN/PP film and LN/MWCNTs/PP film; measured output current signals of the LN/MWCNTs/PP PCF sensors containing different contents of MWCNTs; measured output current response of the PCF sensor for water droplets of different volumes dropped from different heights; measured output current signals of the PCF sensor as a function of time during exposure to music generated by a loudspeaker; output current signals measured from the human carotid artery before and after exercise; rat HR as a function of time measured by the PCF sensor before, during and after KA delivery. AUTHOR INFORMATION Corresponding Authors *(M. Zhang) E-mail: [email protected] *(L. Guan) E-mail: [email protected] Notes The author declares no competing financial interest. ACKNOWLEDGEMENTS We gratefully acknowledge financial support from the National Natural Science Foundation of China (Grant Nos. 21475149 and 21522509) and the National Key Research and Development Program of China (Grant No. 2016YFC0207104). We also gratefully acknowledge Dr. Wenliang Ji and Tongfang Xiao for preparing the rat epilepsy model. REFERENCES (1) Zhao, S.; Li, J.; Cao, D.; Zhang, G.; Li, J.; Li, K.; Wong, C. P. Recent Advancements in


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Facile Fabrication of a Flexible LiNbO3 Piezoelectric Sensor through

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