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A new approach for ultrahigh-performance piezoresistive sensor based onwrinkled PPy film with electrospun PVA nanowires as spacer

Cheng Luoa, Nishuang Liua,⁎, Hang Zhanga, Weijie Liua, Yang Yuea, Siliang Wanga, Jiangyu Raoa,Congxing Yanga, Jun Sua, Xueliang Jiangb, Yihua Gaoa,c,⁎

a Center for Nanoscale Characterization & Devices (CNCD), Wuhan National Laboratory for Optoelectronics (WNLO) & School of Physics, Huazhong University of Scienceand Technology (HUST), Luoyu Road 1037, Wuhan 430074, PR Chinab School of Material Science and Engineering, Wuhan Institute of Technology, Wuhan 430205, PR Chinac College of Physics Science & Technology, Guangxi University, Daxue Road 100, Nanning 530004, PR China

A R T I C L E I N F O

Keywords:PVA nanowiresSpacerIsolation effectConductive pathPressure sensor

A B S T R A C T

Pressure sensor with high sensitivity is desirable for it's vast application in fields like wearable electronics andhuman-machine interface. Here, we report a simple but rarely investigated approach to fabricate amazing highsensitivity pressure sensors by using PVA nanowires as spacer between active materials. On one hand, PolyvinylAlcohol (PVA) nanowire (PVANW) serves as contact spacer between polypyrrole (PPy) film and PET/ITO sheet,where the electrical transportation between PPy film and PET/ITO sheet can be altered with different density ofPVANW; on the other hand, the electroplating-obtained PPy film with intrinsic wrinkled and fluffy structure,shows various electrical conductivities under various external pressures. By virtue of these two traits, the PPy-PVANW-PET/ITO (PPPI) sensor distinguishes itself with superior performances of ultrahigh sensitivity(109.9 kPa−1, 0–0.17 kPa; 228.5 kPa−1, 0.5–2 kPa; 11.9 kPa−1, 5–9 kPa), low detection limit (2.97 Pa) andgood stability (10,000 loading-unloading cycles). The excellent advantages of the sensor make it a promisingcandidate in today's pressure sensor industry, and the fabrication procedure of PPPI sensor also provides a simplemethod to improve the performance of pressure sensors.

1. Introduction

Flexible pressure sensor is undergoing dramatic development for itsgreat potential application in wearable electronic, human-machine in-teractivity and artificial electronic skin (E-skin) fields [1–3]. To getpressure sensor with high sensitivity, low detectable limit, quick re-sponse capability and high stability, capacitive [4–6], piezoelectric[7–9], triboelectric [10,11] and piezoresistive [12–16] type sensorshave been intensively studied, which function by transducing me-chanical deformation into an electrical or other visible signals whensubjected to external force. Among these sensors, piezoresistive pres-sure sensors distinguish itself due to its simple fabrication process, lowcost and easy signal collection [17].

In general, the sensitivities, a vital index of a piezoresistive pressuresensor's performance, can be raised by two approaches of tuning thecontact resistance of active materials. The first approach is to roughenthe surface of active materials by shaping microstructures so that asmall preload will not be harmful to the device's sensing properties[18], which can be achieved by lithography technique and molding

method [19–21]. Despite promising sensitivity has been achieved, thecomplexity and economic inefficiency of this method limit its scal-ability. The other promising but rarely investigated approach usesspacer to control the contact between the electrodes, which also aims atavoiding side effect of the preload on sensitivity. Generally, insulatingpolymer bars, dots or a mesh can serve as spacer to pull the conductinglayers apart when pressure is released and produce promising sensi-tivity [22]. In fact, the application of spacer have been studied in tactilesensor to realize high spatial resolution in human-robot interaction field[23,24]. But up to now, very few works have been done to utilize spacerto raise the sensitivity of the piezoresistive pressure sensor because it ishard to control the spacer to appropriately match the active layers toeliminate the preloaded pressure and keep a well-off contact betweenthe two electrodes at the same time. Moreover, an inadequate workingrange, large threshold pressure values and unstable contacts betweenelectrodes are likely to occur if the spacer is not properly selected. Moreimportantly, the complicated fabrication process of the spacer furtherseverely hinder it's application in pressure sensors.

Here we demonstrate a simple and cost-efficient approach to

http://dx.doi.org/10.1016/j.nanoen.2017.10.007Received 10 August 2017; Received in revised form 20 September 2017; Accepted 2 October 2017

⁎ Corresponding authors at: Center for Nanoscale Characterization &Devices (CNCD), Wuhan National Laboratory for Optoelectronics (WNLO) & School of Physics, HuazhongUniversity of Science and Technology (HUST), Luoyu Road 1037, Wuhan 430074, PR China.

E-mail addresses: nishuang_liu@foxmail.com (N. Liu), gaoyihua@hust.edu.cn (Y. Gao).

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Available online 03 October 20172211-2855/ © 2017 Elsevier Ltd. All rights reserved.

MARK

fabricate an ultrahigh-sensitive and high-detectable piezoresistivepressure sensor based on wrinkled polypyrrole (PPy) film with in-sulating Polyvinyl Alcohol (PVA) nanowire (PVANW) as spacer. WherePPy, as a conducting polymers, is easy-to-get just by polymerizingpyrrole monomer with electrochemical method and has been used asactive material in sensors [25], supercapacitor [26–29] for its excellentflexibility, good conductivity and porous structure. A simple and ma-ture process, electrospinning, which have been vastly used to producenano-structured functional devices [30], is adopted to weave insulatingPVANW as spacer to realize subtly controlling the conductive path ofthe active electrodes. In brief, above a PET/ITO sheet, a piece of PPyfilm is supported by insulating PVANW, where PVANW serving as acontact spacer between PPy film and PET/ITO sheet can regulate theconductive path between PPy film and PET/ITO sheet simply by al-tering electrospinning time. Thanks to the surface wrinkling and fluffystructure of PPy film and the contact isolation effect of PVANW spacer,an amazing high sensitivity of 228.5 kPa−1 was achieved with the PPy-PVANW-PET/ITO (PPPI) pressure sensor, which is far higher than thesensitivities of the most of reported pressure sensors. Our pressuresensor also showed a good stability of 10,000 load-unload cycles. Moreimportantly, a small pressure 2.97 Pa was detected with a PPPI pressuresensor. The working mechanism was also investigated by an in-depthexploration of the contact situation within a PPPI sensor before, underand after pressing. Furthermore, the excellent performance of theflexible PPPI pressure sensor enabled us to succeed in monitoring anadult's wrist pulse signal and the movements of fingers. Compared withother methods to get high-performance pressure sensors [31,32], ourfabrication method is simpler, quicker and cheaper.

2. Experimental section

2.1. Materials preparation

PVANW was obtained by electrospinning aqueous solution of 10 wt% PVA (Alfa Aesar, 87–89% purity) on PET/ITO sheet, before which thesheet was washed by acetone, absolute ethyl alcohol and deionizedwater successively, and then been blew dry under continuous stream ofnitrogen. The PVANW electrospinning was carried out at a high voltageof 15 kV, with a collecting distance of 15 cm, and PVA solution wassubsequently electrospun at a constant flow rate of 2 mL h−1. Thedensity of PVA nanowires was controlled by electrospinning time.Finally, the PVA nanowires were heated at 160 °C for 5 min by a warmtable after being electrospun on PET/ITO sheet to form stable PVANW-PET/ITO structure.

Three-electrode system was adopted to electrochemically synthesisof PPy, where an PET/ITO sheet act as working electrode, Ag/AgCl as areference electrode and carbon rod as the counter electrode. PPy filmelectroplating was performed in an aqueous solution of 5 vol% pyrrolemonomer (Aladdin) and 0.2 M of sodium perchlorate (NaClO4, WanhuaChemical Group Co., Ltd) with respect to the total volume of the so-lution at a constant current of 2.5 A m−2 for 5 h at room temperature.Afterwards, the film was carefully peeled off from working electrodeand washed by deionized water to remove residual monomer and theelectrolyte, and dried in ambient air.

2.2. Device assembling and signal testing

First, cover the as-prepared PPy film on the as-prepared PVANW-PET/ITO, with the wrinkled surface (the right side) facing to PET/ITOsheet. Second, two carbon wires were firmly attached to PET/ITO sheetand a carbon paper with silver paste and scotch tape, respectively,while the unconnected ends of two wires was left to connect the sensorto the external testing system. Third, carbon paper was covered on PPyfilm with the aid of scotch tape to promise a uniform contact betweenPPy film and PVANW-PET/ITO. All the active part of PPPI sensors werefabricated in a 1 × 1 cm2 size in our test. The testing system consists of

a computer collector, single-axe motion controller (Beijing 7-s OpticalInstruments Co., Ltd., 7SC301) and a source measure unit (AgilentB2901A). The testing process was conducted as follows. First, we con-nected the carbon wires with Agilent B2901A, and then put the PPPIsensor perpendicularly between the single-axe stakes. The pressure ofthe single-axe stakes can be recorded in the computer collector. Second,we controlled the single-axe motion controller to press the sensor atvarious pressure, and test I-V curves corresponding to various pressureswith Agilent B2901A at sweep mode from − 1 V to 1 V, which helps usfind out at what voltage the PPPI sensor exhibits best performance.Third, with setting the Agilent B2901A at the constant voltage that wehave found out in step two, we controlled the single-axe motion con-troller to press the sensor, and the sensor showed various current undervarious pressure. Finally, we controlled the single-axe motion controllermove back and forth, detaching from and pressing at the sensor, re-spectively. In this dynamic process, we got the switch-off ratio at var-ious pressures by changing the press distance. In addition, the stabilityof the sensor also can be tested in this process by maintaining the pressdistance at a certain value.

2.3. Characterization

The microstructures of PVANW, PPy film were investigated by athermal field scanning electron microscope (SEM, Nova NanoSEM 450,10 kV). Raman spectroscopy of PPy film was recorded by a LabRAMHR800 spectrometer with 514 nm laser light (HORIBA Jobin Yvon).

3. Results and discussion

3.1. Preparation of PPPI pressure sensor

Fig. 1(a) illustrates the fabrication process of PPPI pressure sensor.Specifically, Polyvinyl Alcohol (PVA) aqueous solution (10 wt%) waspumped into PVA nanowire network on transparent PET/ITO sheet byelectrospinning method, as previously reported method [33]. The fivedensities of the network was obtained by varying the electrospinningtime with 0, 0.5, 1, 2 and 4 min, respectively, and then the as-preparednetworks were cured at 160 °C for 5 min by a warm table to form stablePVA nanowire networks firmly patterned on PET/ITO sheets. Finally,the devices were assembled simply via successively covering a piece ofPPy film and a piece of carbon paper on PVANW and followed byelectronic wiring. Here, carbon paper serves as an electrode and carbonwire is adopted as the electronic wiring material with the ohmic con-tacts between carbon wires and carbon paper/PPy film, carbon wiresand PET/ITO sheet, as shown in Fig. S1. Fig. 1(b) shows a well fabri-cated PPPI pressure sensor in bending state under a longitudinal pres-sure by hand, which exhibits good flexibility of the sensor. It's worthnoting that the PPy film electroplated on the PET/ITO working elec-trode with the dramatically different morphologies of two surfaces. Asshown in Fig. S2, the surface closely attaching to the working electrode(the reverse side) is restricted by the flat PET/ITO sheet electrodeduring electroplating, showing a smoother morphology with smallwrinkles. Whereas, being free of restriction, the other surface (the rightside) polymerized in electroplating solution environment, forms largewrinkled structure for different sizes of globules aggregates, leading tothe PPy film with very different homogeneity and porosity [34]. Thecross-section of PPy film, as shown in Fig. 1(c), also illustrates themorphological difference of the two surface of PPy film. Fig. S3 showsthe Raman spectra of PPy film. Typically, the stretching of the π con-jugated structure and the ring stretching mode of PPy form the twocharacteristic peaks at 1576 and 1382 cm−1, respectively [35]. Asschematically shown in Fig. 1(d), a forward voltage is applied on thedevice from PPy film to PET/ITO electrode, and the pressure-dependentcontact within PPy film and PET/ITO sheet gets more closely underexternal pressure, which causes an increase in current (Fig. 1(e)).

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3.2. Characterization of piezoresistance properties

The performance of PPPI pressure sensor was investigated withvarious PVA nanowire electrospinning time. Fig. 2(c–g) is the scanning

electron microscope (SEM) images of various PVANW with an averagediameter ~ 300 nm of PVA nanowire. PVANW-0, PVANW-0.5, PVANW-1, PVANW-2 and PVANW-4 corresponding to electrospinning time of 0,0.5, 1, 2 and 4 min represent the five sensors. The images show that the

Fig. 1. Preparation of PPPI pressure sensor and the sketch map ofthe testing mechanism. (a) Schematic illustration of fabrication ofa flexible sensor. (b) Photograph of a well fabricated sensor. (c)Scanning electron microscopy image of the cross-section of PPyfilm. (Scale bar: 100 µm). (d) Schematic illustration of the sensingmechanism. (e) Current change in response to loading and un-loading (I: loading, I0: unloading).

Fig. 2. Pressure-dependent current and switch-off ratio trend of PPPI sensor and morphology of various PVANWs. (a) Current changes at bias of 0.2 V and (b) Switch-off ratio (I/I0)changes of various PPPI pressure sensors under pressures. Inset in (b): enlarged diagram of the sensitivity of PVANW-1. (c–g) SEM images of PVANW of various electrospinning time (scalebar, 5 µm).

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density of PVANW gets thicker as the electrospinning time increase.Fig. 2(a) and (b) show that the density of PVANW has a regulating

effect on the performance of PPPI pressure sensor. Fig. 2(a) showsvarious changes in current of the sensors of five various electrospinningtimes under external pressure at bias of 0.2 V. It is observed that thesparser the PVANW, the larger the initial current (with pressure equalsto 0) of the device, which is due to a weaker isolation effect of thesparser PVANW on the sensor. PVANW-4 shows irregular tendencybecause 4 min electrospinning time is too long to leave large interspaceto promise a steady contact between PPy film and PET/ITO, whichcauses disordered current. For the devices of shorter electrospinningtime, the contact is steady, and with the external pressure increasing,the current shows an ever-increase as the conductive path increase.

Fig. 2(b) illustrates the PPPI sensor of five various electrospinningtimes with various changes in switch-off ratio under dynamicallyloading on and unloading pressure. The sensitivity S corresponding tothe slope of curves in Fig. 2(b) is defined as:

=S δ I I δP( / )/0

where I is the current when pressure is applied on the device, I0 is thecurrent of device free of pressure, I/I0 is switch-off ratio, P is the appliedpressure. The switch-off ratios under the same pressure firstly increaseand then decrease with the density of PVANW increasing, which is dueto a contact isolation effect of PVANW. An external pressure can

promote a closer contact between PPy film and PET/ITO sheet thusincreasing the current of device. With the PVANW getting thicker fromnone (from PVANW-0 to PVANW-0.5), the device has a large initialcurrent and won’t change much under various pressure, because asparse PVANW still has large interspaces (Fig. 2(c) and (d)) and has alimited impact on the contact between PPy film and PET/ITO sheet. Asthe PVANW gets thicker (PVANW-1), the interspaces get smaller(Fig. 2(e)) and become a vital factor decreasing the initial current ofdevice. The current changes greatly as external pressure loads on,which contributes to a high switch-off ratio and a high sensitivity valueand will be discussed later in detail. As the PVANW continues gettingthicker (Fig. 2(f)), isolation effect of PVANW more strongly decreasesthe initial current. But the current changes a little under increasingpressure (PVANW-2) and even totally maintain around a low value(PVANW-4, Fig. 2(g)) because the thick PVANW prevents a good con-tact between PPy film and PET/ITO sheet. Among the five sensors,PVANW-1 has the highest sensitivities of 109.9 kPa−1 in the pressurerange of 0–0.17 kPa, 228.5 kPa−1 in 0.5–2 kPa and 11.9 kPa−1 in5–9 kPa. The detail sensitivities of the five sensors are shown in the Fig.S4. The transient stability of switch-off ratio, as can be seen in the insetof Fig. 2(b), is probably because of an unavoidable adaptive phase ofthe interaction between PPy film and PVANW. PVANW-0 shows asensitivity of 7.14 kPa−1 in 0–2 kPa (Fig. S4(a)), which indicates thegood sensing property of the sole fluffy and wrinkled PPy film. The

Fig. 3. Property characterization of PPPI pressure sensor. (a) I-V curves of the device under various pressures. Inset: I-V curve for zero pressure, the unit of current is µA. (b) I/I0-V curvesof PVANW-0 under various pressures. (c) 4 cycles of pressure response under various pressures ranging from 0.37 kPa to 4.21 kPa at− 0.2 V. (d) The durability test of the pressure sensor(10,000 loading-unloading cycles). (e) Response time under various press velocity with the maximum one-way press distance 0.1 mm. (f) Pressure response at a press speed of 80 mm/swith a response time of 66.8 ms. (g) Transient response to the loading and removal of glass sheet of various mass from small to large, corresponding to pressure of 61, 96, 129, 175 and217 Pa. (h) 3 cycles loading and removing petal with a mass of about 30 mg, corresponding to ~ 2.97 Pa pressure.

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PVANW sensors discussed in the following are PVANW-1 except forspecifically illustrated.

Fig. 3 shows the properties of the PVANW-1 pressure sensor. Asshown in Fig. 3(a), the testing voltage was set by a step of 0.2 V from −1 V to 1 V. The current-voltage (I-V) curves of the device shows atendency from a nonlinear characteristics to a linear ohmic one as ex-ternal pressure increases. The nonlinearity of I-V curve is due to theswitching effect depended on “point-to-face” mode between the wrin-kled part of PPy film and the flat PET/ITO sheet, which will diminishwhen the sensor is under an ever-increasing pressure with the “point-to-face” contact mode transforming into “face-to-face” contact mode [36].As shown in Fig. 3(b), we get the I/I0-V curve based on the dates ofFig. 3(a), where I0 is the current without external pressure. The I-Vcurves and I/I0-V curves of the other four various sensors are also ap-pended in Fig. S5. It is concluded that the devices show better perfor-mance in the applied voltage range from− 0.4 V to 0.4 V (exclude 0 V).Typically, the value of I/I0 of PVANW-1 reaches the highest point whenthe voltage is − 0.2 V. The testing voltages discussed in the followingare − 0.2 V except for specifically illustrated.

Further, we verified that the PVANW-1 has good performance bydynamically loading and unloading a pressure ranging from 0.37 kPa to4.21 kPa for 4 cycles, and a higher pressure brings about a higherswitch-off ratio, as shown in Fig. 3(c). The relative change of currentshowing a stable switch-off ratio signal indicates that the pressuresensor is reliable at various pressures.

Fig. 3(d) confirms that the PPPI sensor exhibits good stability anddurability in the test of 10,000 press-release cycles with a pressure of0.37 kPa, and the enlarged image in Fig. 3(d) clearly illustrates that thePPPI pressure sensor keeps a good performance after thousands ofpress-release cycles. In addition, as shown in Fig. S6, the cyclic changeof the current under reciprocating increase and decrease pressure for 4cycles shows good overlaps, indicating the stability of PPPI sensor. It isbecause the curing process of PVA nanowires promises a firmly at-tachment of PVANW on PET/ITO sheet, and the PVANW almost keepsthe initial appearance after 10,000 testing cycles (Fig. S7).

In our study, we found that the response time of PPPI sensor is onlyrelated to the press speed of external pressure on the device. The re-lationship between response time and press velocity is shown inFig. 3(e). When the press velocity reaches 10 mm/s, the response timedecreases from hundreds of milliseconds to about 80 ms. Once the pressspeed surpass 10 mm/s, the response times maintain at 63–70 ms. Arepresentative response time of 66.8 ms is shown in Fig. 3(f) under apress velocity of 80 mm/s. Considering that it takes time to finish the0.1 mm one-way press distance, it is concluded that the intrinsic re-sponse time of PPPI pressure sensor is far below 70 ms.

To confirm the detect limitation of PPPI pressure, we horizontallyload on and remove from the device with five glass sheets with mass of666.4, 979.6, 1316, 1785.7, and 2214 mg successively, correspondingto pressure of 61, 96, 129, 175 and 217 Pa, respectively, where all thecontact area between the glass sheets and the active part of the deviceare kept at roughly 1 cm2. As shown in Fig. 3(g), with the mass of glasssheets increasing, an increasing load current was observed. The currentat 61 Pa is clearly distinguished from that at 0 Pa, which indicates thesensor can detect a smaller pressure. Further, as seen in Fig. 3(h), whena petal about 30 mg (~ 2.97 Pa) was carefully placed on and removedfrom the PPPI sensor for 3 cycles, a variation in current was observed,which shows us the ultralow pressure detectable property of PPPIpressure sensor.

3.3. Working mechanism

Further investigation shows that the reason of the high performancePPPI pressure sensor lies in the variation of contact state within PPyfilm and PET/ITO. As shown in Fig. 4(a), PPy film is placed belowPVANW-PET/ITO. We photographed the interface between PPy filmand PVANW-PET/ITO with SEM corresponding to three states of before

being given an external pressure, under the pressure and recoveringfrom the pressure. The places marked “1” in the red boxes show thecontact situations between the right side of PPy film and PVANW-PET/ITO, “2” and “3” show the contact situations of the two layers of thePPy film, “4” shows the gaps between the right side of PPy film andPVANW-PET/ITO. Under a pressure, as Fig. 4(b) shows, the four placesall witnessed a compacter contact within them than the free state(Fig. 4(a)), which is manifested as “1” undergoing a movement of PPyfilm get contact with PET/ITO sheet from a separate state, “2”, “3”going through movements of the right side of PPy film get contact withthe reverse side and “4” experiencing a diminution. When the externalpressure is removed, as shown in Fig. 4(c), the contacted places in “1”,“2” and “3” get detached and nearly recover to the original states.

Meanwhile, the cross-section thickness distribution situation of PPyfilm is also investigated, where the cross-section of PPy film was ob-tained by carefully cutting a piece of PPy film into two parts with ablade. The thickness was measured by SEM along the cross-section at aninterval of 50 µm, and 187 values of thicknesses were measured in ourstudy. We count the number of thickness within every 10 µm and cal-culate it's ratio to the total number, as shown in Fig. 4(d)–(f). As can beseen from the graph, the large thickness ratio is much smaller than thatof the small thickness. With an external pressure loading on, the ratio ofsmall thickness increases and the ratio of large thickness decreases.Before pressing, the thickness values distribute in a large range from 0to 170 µm (Fig. 4(d)). A smaller distribution range from 0 to 130 µmappeared after pressing, and the average thickness shrink from35.08 µm to 28.47 µm after pressing. This is because that the higherplaces of PPy film are suppressed by PVANW-PET/ITO side but thelower places are free of suppression. During the pressing process, thePVANW-PET/ITO contact the highest places of PPy film, and then thenext highest, then the third highest and so on. The initial contact of PPyfilm and the PVANW-PET/ITO is determined by a small number of thehighest places of PPy film and the appropriate density of PVANW. Whena certain pressure is imposed on the device, the external pressure helpsovercome the isolation function of PVANW on PPy film and a largeramount of the next highest wrinkles of PPy began to contact with thePVANW-PET/ITO, and the former contacted area expand (as indicatedby the change of place “4”), which contributes to a sharp increase incurrent and leads to high sensitivity. As the external pressure increasescontinuously, there is still much room for the current to increase asthere being large amount PPy wrinkles of low height. This could explainwhy the sensor still shows high sensitivity of 11.9 kPa−1 at high pres-sure range of 5–9 kPa. As the external pressure removes, the contactsituation recovers nearly to the origin state, and the thickness dis-tribution of PPy film changes a little with a small group of highestthickness decreasing and the mean thickness rebounding to 34.84 µm(Fig. 4(f)), which shows that the PPy film can elastically deform andrecover upon imposing and releasing external pressure and explainswhy the PPPI pressure have good stability.

Here we can speculate that: First, with increasing the externalpressure, the isolation function of PVANW is overcome, at the sametime the contact area of the wrinkled PPy film and the substrate elec-trode increase. Second, the fluffy and wrinkled PPy film gets compactunder external pressure, which causes a contact area increase withinPPy film. The above two processes form new conductive paths withinthe sensor and promote a shoot up in current. Compared with otherpiezoresistive pressure sensors that work with two electrodes contactdirectly, PPPI pressure sensor shows a higher sensitivity with the helpof PVANW spacer, and their performance comparison is shown in TableS1.

3.4. Application in monitoring wrist pulses and finger movements

High sensitivity and low pressure detectable limit of the PPPIpressure sensors promise us a lot of applications ranging from mon-itoring tiny signal of human's pulse beat to recording hard mechanical

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movement of human organs. Fig. 5(a) displays the pulse waves detectedby PPPI pressure sensor. As shown in the inset of Fig. 5(a), the devicewas fixed at wrist with the aid of polyimide tape. The test was carriedout on a 25-year-old adult before and after 2 min jogging exercise.According to the test dates, the heart rates are about 75 and 93 bpm

(beats per minute) before and after exercise, respectively, as directlyshown in Fig. 5(b). The pulse intensity of the after exercise state wasevidently higher than the calm state one. At the same time, the threecharacteristic peaks of pulse, “P”, “T”, “D” type wave (refer to percus-sion, tidal and diastolic, respectively), are both visible before and after

Fig. 4. Microstructure characterization of the contact situation between the PPy film and the PET/ITO electrode at three states: (a) Before being given an external pressure. (b) Under thepressure. (c) Recover from the pressure. Scale bar: 50 µm. (d)–(f) Cross-section thickness distribution of PPy film corresponding to (a)–(c).

Fig. 5. Various applications of PVANW-1 pressure sensor. (a) The measurement of real-time arterial pulse waves of an adult before and after exercise. (b) Local enlarged view of (a) for asignal pulse cycle. (c) Real-time record of the signal amplitude output from finger motion: bending and unbending. (d) Real-time record of the signal amplitude output of finger motionfrom the sensor under/without pressure.

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exercise. But after exercise, the T-wave is much smaller than the other,which could be caused by exercise-induced vasodilation, dilation ofmuscular arteries and enhanced ventricular-vascular coupling becausethey weaken the load on the left ventricle thus suppress the T-wavefrom coming out [37,38].

To test a mechanical movement of finger, a PPPI pressure sensorwas located in the back side of index finger joint, as pictured in the insetof Fig. 5(c). The output signal presented in Fig. 5(c) shows a real-timeresponse current and a stable current waveform under a periodicbending and straightening movement of index finger. It is worth notingthat the switch-off ratio reaches as high as 120 at roughly 20 bendingdegree, indicating a high sensitivity and good stability performance ofPPPI sensor.

Finally, sharp summit of current was recorded when the PPPI sensorsubjected to a repeated process of loading and unloading a mediumstrength pressure of an index finger. As given in Fig. 5(d), the sharpsummit of current is caused by a short finger press time on the device,and the variation of peak is due to the unmanageable finger pressure.The switch-off ratios hitting nearly 600 shows that the PPPI sensor iscompetent to differentiate a large pressure from a small one.

4. Conclusion

To sum up, we present a new and simple approach for a high per-formance PPPI pressure sensor with the help of PVA nanowires, whichserve as a spacer between PPy film and PET/ITO. When the PPPI sensoris subjected to an external pressure, the PPy film deforms and over-comes the isolation function of PVANW, and the conductive path in-crease hugely within PPy film, between PPy film and PET/ITO, whichleads to an ultrahigh sensitivity of 228.5 kPa−1. At the same time, thePPPI sensor shows low detectable pressure of 2.97 Pa, good stability of10,000 cycles, fast response time of about 66.8 ms and low operatingvoltage of 0.2 V. Furthermore, the PPPI sensor was successfully used tomonitor pulse wave, and record mechanical movement of finger likebending and unbending. We believe that the above mentioned prop-erties make PPPI pressure sensor a potential challenger of presentpressure sensor and pave the way to get new type pressure sensors ofhigh performance. Importantly, the expensive-equipment-free andsimple fabrication process promise a bright future that the PPPI pres-sure sensor or it's analogues will be largely used in our daily life.

Acknowledgements

This work was supported by the National Natural ScienceFoundation of China (11374110, 11674113 and 51371085) and theNatural Science Foundation of Hubei Province (2016CFB432). Y.H.G.would like to thank the facility support from Prof. Zhong Lin Wang.

Appendix A. Supplementary material

Supplementary data associated with this article can be found in theonline version at http://dx.doi.org/10.1016/j.nanoen.2017.10.007.

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Cheng Luo obtained his B.S. degree from ChongqingTechnology and Business University in 2015. From 2015,he is a master degree candidate in Physics School ofHuazhong University of Science and Technology (HUST).His main research interest focuses on sensors and micro/nano-devices.

Dr. Nishuang Liu received his B.S. degree in ElectronicScience and Technology (2006) and his Ph.D. degree inMicroelectronics and Slide State Electronics (2011) fromWuhan University, P.R. China. Then, he joined School ofPhysics in HUST in 2011 as an Assistant Professor, andbecame an Associate Professor in 2014. His main researchinterests include sensors, energy storage device and self-powered nano-systems.

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Hang Zhang obtained his B.S. degree from YangtzeUniversity in 2015. From 2015, he is a master degree can-didate in Wuhan National Laboratory for Optoelectronics(WNLO) of HUST. His main research interest is focusing onpressure sensor.

Weijie Liu obtained her B.S. degree in Physics fromXinyang Normal University in 2013. From 2016, she is aPh.D. candidate in School of Physics, HUST. Her main re-search interests are focusing on energy storage device andsensors.

Yang Yue obtained his B.S. degree from Fuyang NormalUniversity in 2014. From 2014, he is a Ph.D. candidate inSchool of Physics, HUST. His main research interest is fo-cusing on printed and functional energy storage devices.

Siliang Wang obtained his B.S. degree from WuhanInstitute of Technology in 2013. From 2013, he is a Ph.D.candidate in WNLO of HUST. His main research interest isfocusing on printed and functional energy storage devices.

Jiangyu Rao is now a Ph.D. candidate in WNLO of HUSTunder the supervision of Prof. Yihua Gao. He obtained B.S.degree from College of Materials Science & Engineering,Hunan University. His main research interests focus onflexible lithium-ion battery.

Congxing Yang obtained his B.S. degree from YangtzeUniversity in 2011. From 2013, he is a Ph.D. candidate inWNLO of HUST. His main research interest is focusing ongraphene energy storage and micro/nano-devices.

Dr. Jun Su is an engineer of electron microscope in WNLOof HUST. He received his Ph.D. degree in Institute ofPhysics, Chinese Academy of Science in 2008.

Xueliang Jiang, Ph.D., Professor in the Department ofPolymer Materials and Engineering, School of MaterialsScience and Engineering, Wuhan Institute of Technology.Dr. Jiang's research interests are in areas associated withorganic-inorganic hybrid materials, functionalized poly-mers and nanocomposites.

Prof. Yihua Gao received his Ph.D. degree in Institute ofPhysics, Chinese Academy of Sciences (1998). Next he be-came a special researcher in the National Institute forMaterials Science (NIMS) in Japan. He and Prof. YoshioBando invented carbon nanothermometer (Nature 415, 599(2002)) in 2002 and obtain the 16th Tsukuba Prize in 2005together with Prof. Y. Bando and Prof. D. Golberg. In 2006,Dr. Gao accepted a professorship in HUST. His researchinterests include microstructure, energy devices and sensorsbased on nanomaterials.

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