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Core‑shell nanofiber mats for tactile pressuresensor and nanogenerator applications

Lin, Meng‑Fang; Xiong, Jiaqing; Wang, Jiangxin; Parida, Kaushik; Lee, Pooi See

2018

Lin, M.‑F., Xiong, J., Wang, J., Parida, K., & Lee, P. S. (2018). Core‑shell nanofiber mats fortactile pressure sensor and nanogenerator applications. Nano Energy, 44, 248‑255.doi:10.1016/j.nanoen.2017.12.004

https://hdl.handle.net/10356/138603

https://doi.org/10.1016/j.nanoen.2017.12.004

© 2017 The Author(s). Published by Elsevier Ltd. This is an open‑access article under the CCBY‑NC‑ND license (http://creativecommons.org/licenses/BY‑NC‑ND/4.0/).

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Nano Energy

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Core-shell nanofiber mats for tactile pressure sensor and nanogeneratorapplications

Meng-Fang Lin, Jiaqing Xiong, Jiangxin Wang, Kaushik Parida, Pooi See Lee⁎

School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, 639798, Singapore

A R T I C L E I N F O

Keywords:Tactile pressure sensorNanogeneratorElectrospinningCore-shell nanofibers

A B S T R A C T

Core-shell nanofibers of PDMS ion gel /PVDF-HFP were successfully prepared by incorporating cross-linkingagent during electrospinning. The electrospun nanofiber mats were used to fabricate pressure sensors to detectthe static and dynamic pressures by harnessing the capacitance changes and triboelectric effects judiciously. Thecore-shell PDMS ion gel/PVDF-HFP nanofiber sensor functions as a capacitive pressure sensor, which offers highsensitivity of 0.43 kPa−1 in the low pressure ranges from 0.01 kPa to 1.5 kPa. The sensitivity, flexibility, androbustness of our capacitive pressure sensor allows it to be utilized as a wrist-based pulse wave detector forheart-rate monitoring. In addition, the core-shell PDMS ion gel/PVDF-HFP nanofiber mat made a good tribo-electric based pressure sensor in the high pressure range with a linear pressure sensitivity 0.068 V kPa−1 from100 kPa to 700 kPa, one of the best reported at present. The increase in inductive charges and the enhanceddielectric capacitance of the core-shell nanofiber layer compared to the pure PVDF-HFP nanofiber layer allows itto function in the triboelectric nanogenerator (TENG) with the maximum power density reaching 0.9 W/m2,which is sufficient to light up several hundred light emitting diodes (LEDs) instantaneously.

1. Introduction

A vital component of consumer electronics technology is the pres-sure sensors, which can be found e.g. in touchpads, touchscreens, mi-crophones and biometric identification devices [1]. Several types offlexible pressure sensors including capacitive [2–5], piezoelectric[6–8], and resistive [9–12] are currently under assessment for market-ready applications. Among them, the capacitive pressure sensor hasachieved an enormous success, due to its high electrical sensitivity, fastresponse time, low power consumption, compact circuit layout, and arelatively simple device construction compared to their resistivecounterparts [13]. However, the capacitive pressure sensor is only op-erated at low pressure regimes (< 10 kPa) for touch screen, health careand medical diagnosis systems applications [1]. Recently, triboelectricnanogenerator (TENG) has been demonstrated as a pressure sensor[14–19] and a self-powered electronic device [20–26] in the higherpressure range (> 10 kPa) by converting mechanical energy into elec-trical energy. In contrast to the capacitive pressure sensor, which issuitable for measuring both static and low dynamic pressure changes,TENGs are suitable for dynamic pressure detection in the higher pres-sure regime. For further increasing the sensitivity and decreasing theresponse time of both the aforementioned types of pressure sensors,introducing microstructure on top of 2D polymeric surfaces turned out

to be critical in the device design. Pyramid [2,16,25,27], microfiber[28] and micropillar [4,15] shaped microstructures have been in-tegrated to capacitive and triboelectric pressure sensors to enhance thetactile sensitivity. In addition, it has been demonstrated that for thetriboelectric charges in TENGs are well separated in the micro-structured films and therefore a larger electric dipole moment can beformed between the electrodes [26]. Thus, surface microstructures playan important role in enhancing the output performance of triboelectricsystems. Despite achieving significant device sensitivity, the fabricationof surface microstructures largely relies on traditional lithographytechnique, which suffers from complex and time consuming procedures,limited scalability or incompatibility with roll to roll process. In con-trast, electrospinning is a straight forward, efficient and scalable tech-nique to fabricate the microstructured films consist of highly inter-connected, nonwoven fibers with diameters in the micro and nanometerrange [29]. For example, electrospun hydrogel has been widely used inbiomedical applications such as drug delivery, tissue engineering andbiosensor [30,31].

Recently it has been shown that ionic liquid based polymeric elec-trolyte attained by incorporating an ionic liquid into a crosslinkable gelmatrix possesses not only high ionic conductivity but also exerts out-standing mechanical properties [32,33]. The high capacitance of ionicliquid or ion gel can be attributed to the formation of electrical double

https://doi.org/10.1016/j.nanoen.2017.12.004Received 9 November 2017; Received in revised form 4 December 2017; Accepted 4 December 2017

⁎ Corresponding author.E-mail address: pslee@ntu.edu.sg (P.S. Lee).

Nano Energy 44 (2018) 248–255

Available online 06 December 20172211-2855/ © 2017 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).

T

layers (EDLs) [34–36], which have been utilized in energy storage cells,electrochromics and stretchable current collectors. The usage of theionic hydrogel matrix as a thin film for interfacial capacitive sensors hasbeen recently reported [37]. However, the hydrogel without the elas-tomeric coating is highly susceptible to dehydration. Thus, the hydrogelrequires a protective layer to prevent evaporation of water from thehydrogel [38]. Furthermore it should be pointed out that the capaci-tance of the ionic hydrogel matrix thin film is directly related to itssurface area, which poses a general limitation on the obtainable sen-sitivity in the previous study. Our pioneering concept herein providesthe material design strategies to circumvent the aforementionedshortcomings that affect the stability of ionic hydrogel based devicerelated to dehydration, and the limited pressure sensitivity due to lowsurface area. We incorporate an ion gel core within a nanofiber sup-ported by a copolymer shell featuring an intrinsically larger surfacearea and a built-in protection layer. In addition, the device design in-troduced in this work showcases an effective device fabrication strategyincluding cross-functionality with tactile sensing for both static anddynamic pressure as well as power generation capability in a singleintegrated device system.

We have fabricated the core-shell PDMS ion gel/ PVDF-HFP nano-fiber mats via electrospinning, which can be used in various pressureranges for the dual functional applications of tactile sensing with re-spect to static and dynamic pressure, and power generation. Utilizingthe core-shell PDMS ion gel/PVDF-HFP nanofiber mats as a capacitivesensor offers high sensitivity of 0.43 kPa−1 in the pressure ranges from0.01 kPa up to 1.5 kPa. Compared to a zinc oxide nanowire/poly(me-thylmethacrylate) film and a PDMS-coated conductive fiber, the

sensitivity of the core-shell PDMS ion gel/PVDF-HFP nanofiber mats is50 and 2 times higher, respectively [28,39]. Furthermore, our capaci-tive sensor has been demonstrated as a wearable pulse rate detectorthat serves as a heart-rate indicator. In addition, the core-shell PDMSion gel/PVDF-HFP nanofiber mats could be used as a self-powereddevice and as a triboelectric sensor with sensitivities of 0.068 V kPa−1

and 0.102 V kPa−1 at higher pressures ranging from 100 kPa to700 kPa and 40 kPa to 100 kPa, respectively. The maximum powerdensity of the core-shell PDMS ion gel/PVDF-HFP nanofiber mats couldbe brought up to 0.9 W/m2, which is sufficient to light up three hun-dred light emitting diodes (LEDs). The improvement in the electricalpower generated by the core-shell PDMS ion gel/PVDF-HFP nanofibermats can be explained by an increased amount of inductive charges andcapacitance in the triboelectric layer.

2. Material and methods

2.1. Preparation of PDMS ion gels

Poly(dimethylsiloxane) with hydroxyl terminated end group(PDMS-OH, MW~500) was mixed with TEOS solution and 1-Butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [BMIM][TFSI]by 1:1:1 weight percentage. The solution was stirred for 30 mins atroom temperature. Subsequently, the PDMS (Sylgard 184) which con-sisted of liquid components (a mixture of catalyst Pt and prepolymerdimethylsiloxane with vinyl groups) and curing agent (prepolymer di-methylsiloxane with vinyl groups and Si-H groups) (10:1) were addedinto ionic liquid solution. Finally, tetrahydrofuran (THF) solvent was

Fig. 1. (a) Schematic diagram of the PDMS ion gel preparation (b) illustration of the process for preparing core-shell PDMS ion gel / PVDF-HFP nanofibers by electrospinning method. (c)FESEM image of core-shell PDMS ion gel /PVDF-HFP nanofiber mat (d) TEM image of core-shell PDMS ion gel / PVDF-HFP nanofibers.

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added to the PDMS with ionic liquid solution in order to form the PDMSion gel.

2.2. Core-shell electrospun PDMS ion gel /PVDF-HFP nanofiber mats

PVDF-HFP (Sigma-Aldrich, Mw ~ 455,000) was dissolved in a mixsolvent DMF/THF (volume ratio = 1:1). The concentration of PVDF-HFP was 100 mg/mL. The PVDF-HFP solution was stirred at 60 °C for6 h to obtain a transparent solution. Fig. 1b illustrates the setup forcore-shell electrospinning. PVDF-HFP and PDMS ionogel solutions werefed into the outer channel (15 G, OD 1.8 mm, ID 1.4 mm) and innerchannel (21 G, OD 0.8 mm, ID 1.4 mm) of the spinneret, respectively.At the nozzle tip, the inner channel protruded out of the outer channelby about 0.5 mm. The flow rates of the PDMS ionogel and PVDF-HFPsolutions were controlled using syringe pumps at 0.7 mL/h and 1 mL/h,respectively. The aluminum foil was used as a collector, which has beenplaced on the ground. The tip to collector distance was fixed at 15 cm,and the applied voltage was 25 kV.

2.3. Fabrication of the flexible capacitive pressure sensor device

The device configuration of capacitive pressure sensor is illustratedin Fig. 2(a). The device is constructed by a metal-insulator-metal (MIM)capacitor structure where the metal is Cu electrode and the capacitivelayer is the core-shell PDMS ion gel/PVDF-HFP nanofiber mat. Thedevice size is 1 cm × 1 cm.

2.4. Fabrication of the flexible triboelectric device

As illustrated in Fig. 4(a), the core-shell PDMS ion gel/PVDF-HFPnanofiber mat with Cu electrode was placed on the polyethylene ter-ephthalate (PET). VHB tape (3 M) was used for a separation layer.Kapton film was used as a strong negative triboelectric material.

2.5. Characterization

The crystallinity of the core-shell PDMS ion gel/PVDF-HFP nano-fiber mat was identified by a XRD with Cu Kα radiation (λ = 1.5418 Å)at 30 kV and 20 mA and at a scan rate of 2° min−1 from 10° to 80° (2θ).Field emission scanning electron microscopy (FE-SEM, JEOL 7600F)operating at 5 kV was employed to determine the core-shell PDMS iongel/PVDF-HFP nanofibers morphology. Detailed structure analyseswere performed using transmission electron microscopy (TEM,JEOL2010) operated at 200 kV. The compression test was performed onthe core-shell PDMS ion gel/PVDF-HFP nanofiber mat by using Instron5567. The capacitance measurement from the capacitive pressuresensor was obtained using the LCR meter (Agilent E4980A). To analyzethe device sensitivity, the LCR meter was used to detect capacitivechanges under various mechanical loads on the device. The voltageoutputs from the triboelectric pressure sensor were measured by anoscilloscope (Trektronix, MDO 3024, input resistance = 10 MΩ), thecurrent output was measured by a low-noise current pre-amplifier(Stanford Research System, Model SR570, input resistance = 4 Ω). The

Fig. 2. (a) Device configuration of pressure sensor for the core-shell PDMS ion gel / PVDF-HFP nanofiber mat (b) pressure sensor repeated real-time responses at 0.01 kPa (c) Themechanism of the core-shell PDMS ion gel / PVDF-HFP nanofiber mat when the pressure is applied (I) the dielectric constant is changed when the nanofiber mat is compressed (II) thetop/bottom electrode distance is reduced (III) the fiber contact area is increased.

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dynamic mechanical pressure was applied by a magnetic shaker(Sinocera, Model JZK-20).

3. Results and discussion

The core-shell nanofibers of PDMS ion gel /PVDF-HFP were pre-pared using electrospinning. Although PDMS is immiscible with manyother ionic liquids, earlier studies indicate the feasibility of using tet-raethylorthosilicate (TEOS) as the cross-linking agent to form the hy-brid ion gel with various polymers such as PDMS and poly(methylmethacrylate) PMMA through sol-gel reaction [40,41]. In this study,the PDMS ion gel has been prepared by sol-gel reaction to create thecross-linked PDMS with TEOS and PDMS-OH. The TEOS and PDMS-OHare employed as the cross-linking agents between ionic liquid andPDMS polymer. The schematic diagram of the PDMS ion gel preparationis shown in Fig. 1a. The process of preparing the core-shell PDMS iongel / PVDF-HFP nanofiber mat is found in Fig. 1b. The PVDF-HFP isconfigured as the shell during electrospinning due to its high molecularweight. The electrospinning of PDMS ion gel nanofibers has been rea-lized through the PDMS prepolymer and the curing agent to form astable 3D network via covalent bonds [42]. The morphology of elec-trospinning of core-shell PDMS ion gel / PVDF-HFP nanofiber mat isshown in Fig. 1c. In order to ascertain the formation of core-shell na-nofiber structure, transmission electron microscopy (TEM) is employedas shown in Fig. 1d. The total diameter of core-shell PDMS ion gel /PVDF-HFP nanofibers and core PDMS ion gel nanofibers are around235 nm and 35 nm, respectively. On the other hand, the pristine PVDF-HFP nanofibers do not form the core-shell structure as shown in Fig.S1a. From the cross sectional image, the thickness of the core-shellPDMS ion gel / PVDF-HFP nanofibers layer is around 100 µm as shownin Fig. S1b. Infrared spectroscopy has been used to characterize the

core-shell PDMS ion gel / PVDF-HFP nanofiber mats as shown in Fig.S1c. Two strong bending bands are observed in the region between 500and 750 cm−1, corresponding to a SO2 antisymmetric bending(610 cm−1) and another SO2 antisymmetric bending (612 cm−1) fromthe [EMIM][TFSI] [43]. The peaks at 1090 and 1022 cm−1 were as-signed to the Si-O-Si of siloxane, which showed evident vibration peaksfor the PDMS fiber. The –CH3 rocking and Si-C vibrations appeared ataround 802 cm−1 [44]. The electrospun core-shell PDMS ion gel /PVDF-HFP nanofibers have a crystalline structure of α and β phases incoexistence. The vibrational bands at 1264 and 842 cm−1 are attrib-uted to the β phases of PVDF-HFP. Several peaks in the region between1197 and 762 cm−1 are assigned to α phases of PVDF-HFP [45].

The core-shell PDMS ion gel / PVDF-HFP nanofiber mats were madeinto capacitive pressure sensor in the metal-insulator-metal (MIM)configuration with Cu as the top and bottom electrode as shown inFig. 2a. Fig. 2b shows the capacitance response of core-shell PDMS iongel / PVDF-HFP nanofiber mats that are able to sense very small pres-sure of 0.01 kPa. The capacitance is altered by placing and removing aload cell of 0.01 kPa in Fig. 2b. The mechanism of the capacitancechange of the core-shell PDMS ion gel / PVDF-HFP nanofiber mat whenthe pressure is applied is shown in Fig. 2c. The capacitance of a parallelplate capacitor with area A and thickness d can be written as:

=C ε εd

A,o r

where C is the capacitance, in Farads; A is the area of overlap of the twoplates, in square meters; εo is the vacuum dielectric constant (εo ~ 8.54× 10–12 F/m); εr is the dielectric constant of the material between theplates; and d is the separation between the plates. The change in thecapacitance in the case that a constant pressure is applied allows it tomeasure static pressure.

Fig. 3. (a) The maximum slope of the relative capacitance changes of the core-shell PDMS ion gel / PVDF-HFP nanofiber mats in the pressure ranging from 0.01 kPa to 1.5 kPa. (b)Pressure-response curves for different amount of core-shell PDMS ion gel / PVDF-HFP nanofiber mats. (c) The stress-strain curves of the pristine PVDF-HFP nanofibers and differentamount to PDMS ion gel / PVDF-HFP nanofiber mats (d) the real time pressure waveforms of the measured heart rate. The inset shows that the device has been mounted onto the wristand the zoomed view on a pulse having the characteristic peak typically measured at the radial artery.

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The dramatic increase of the capacitance of the core-shell nanofibermats when the external pressure is applied can be attributed to threemain factors: (1) the dielectric constant εr is changed when the PDMSion gel / PVDF-HFP nanofiber mats are compressed since the displacedair has a lower dielectric constant (εr = 1.0) than PDMS (εr = 3.0) andPVDF-HFP (εr = 11.38) as shown in Fig. 2c-(I), (2) when the distancebetween the top and bottom electrode is reduced (d to d′), the thicknessof the elastomeric PDMS inside the core-shell PDMS ion gel / PVDF-HFPnanofibers can be reduced because PDMS possesses good elastic prop-erties in the< 100 kPa pressure regime [2] as shown in Fig. 2c-(II), and(3) the contact area of the formed electrical double layer is increased (Ato A′) due to the reduced spacings as a result of the highly deformableproperties of PDMS, which results in an enhanced interfacial capaci-tance as shown in Fig. 2c-(III). The increase of the dielectric constantand the fiber contact area as well as the reduced distance between theseparated electrodes leads to a significant increase of capacitance of thepressure sensor upon compression and therefore a high pressure sen-sitivity (as shown in next paragraph) in the low pressure range con-sidered in this work.

The pressure response curves for varying amounts of ionic liquidloaded core-shell PDMS ion gel / PVDF-HFP nanofiber mats are pre-sented in Fig. 3a. The pressure sensitivity S can be defined as the slopeof the curves in Fig. 3a. (S = δ (ΔC /Co) / δp= (1/Co) * δC / δp), wherep denotes the applied pressure, and C and Co denote the capacitancewith and without applied pressure, respectively. The 40 wt% ionic li-quid loaded core-shell PDMS ion gel / PVDF-HFP nanofiber mat ex-hibits much higher pressure sensitivity than others. In the pressurerange of< 1.5 kPa, the sensitivity of the sample with 40 wt% ionic li-quid loaded core-shell PDMS ion gel / PVDF-HFP nanofiber mat is 71times higher than pristine PVDF-HFP nanofiber mat due to a largerinterfacial capacitance produced at high amount of PDMS ion gel asshown in the Fig. 3a. Compared with previous fiber-based pressuresensor, our proposed pressure sensor exhibits better sensitivity andimproved detection limitation [28,46–48]. Our device shows 12.5 timeslower detection limit compared to the PDMS-coated conductive fibers[28]. It is noteworthy that sensitivity of core-shell PDMS ion gel /PVDF-HFP nanofiber mat in the low pressure regime is 50 times higherthan the previously reported value for ZnO-PMMA composite [39]. Inaddition, our device is able to extend the range of possible measure-ments up to 10 kPa as shown in Fig. 3b. However, there is a reduction insensitivity when the pressures are higher than 1.5 kPa. The sensitivity

reduction is attributed to the increasing elastic resistance with in-creasing compression [2]. For real world applications this progressivedamping could be desirable, since it increases the range of detectablepressure to the case of high loads in which high sensitivity is not re-quired for the capacitive pressure sensor, and therefore results in amore versatile pressure sensor [2]. The mechanical properties of thepristine PVDF-HFP and different weight percentage of ionic liquidloading PDMS ion gel / PVDF-HFP nanofiber mats are shown in Fig. 3c.Compared to pristine PVDF-HFP nanofiber mat, the 40 wt% ionic liquidloaded core-shell PDMS ion gel / PVF-HFP nanofiber mat exhibits ahigher compressive strain at the same compressive stress. It indicatesthat the higher amount of ionic liquid in the core-shell nanofiber matleads to a larger deformation upon applied pressure. The nanofiberswith a higher amount of ionic liquid loaded PDMS ion gel could have alower elastic modulus as shown in Fig. S2a. It is noted that the elasticmodulus of 10 wt% ionic liquid loaded core-shell PDMS ion gel / PVF-HFP nanofiber mat is dramatically reduced compared to PVF-HFP na-nofiber mat, which results in 6 times higher pressure sensitivity(0.04 kPa−1) compared to pristine PVDF-HFP as shown in Fig. 3a.However, nanofibers with a high amount of PDMS ion gel beyond 40%cannot be formed due to high ionic conductivity [49]. The high ionicconductivity of the solution could cause large instabilities during theelectrospinning process [50] as a high voltage operation (25 kV) is re-quired to fabricate the core-shell PDMS ion gel / PVDF-HFP nanofibermat. The ionic conductivity of different weight percentage of PDMS iongel was measured by electrochemical impedance spectroscopy (EIS)with a frequency range of 0.01 Hz–100 kHz. Fig. S2b shows a typicalNyquist plot of the impedance analysis on different weight percentageof PDMS ion gel. At high frequency (~ 100 kHz), the correspondingvalue of the intercept on the real axis represents the instrinsic resistanceof the ion gel as the ohmic resistance of the testing device is negligibleas shown in Fig. S2c [51]. Therefore, the ionic conductivity can becalculated according to the formula:

=σ LRS

Where σ is the ionic conductivity, L is the distance between the twoelectrodes, R is the resistance of ion gel, and S is the geometric area ofthe electrode interface. The ionic conductivity is increased by in-creasing amount of the ionic liquid as shown in Fig. S2d. The sensi-tivity, flexibility, and robustness of our device allows it to be utilized as

Fig. 4. (a) Device configuration of TENG for the core-shell PDMS ion gel / PVDF-HFP nanofiber mats and the mechanism of the core-shell PDMS ion gel / PVDF-HFP nanofibers mats forelectric power generation process (b) output voltage (c) current density of the TENG for the core-shell PDMS ion gel / PVDF-HFP nanofiber mats under the pressure of 700 kPa at 5 Hz.

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a wrist-based heart-rate monitor. The device was attached directly tothe wrist of a living subject to measure the radial arterial pulse wave asshown in Fig. 3d. It is demonstrated that the device can track thenumber of pulses through the relative capacitance change. The testsubject has a stable heartrate of 75 times per minute which is consistentwith the counting results and shows that the subject is currently healthyand relax. The video record for the wrist-based pulse wave detector forheart-rate monitoring as shown in the Movie S2 in the Supporting in-formation. A higher, arrhythmic or otherwise altered heartrate can in-dicate states of stress and physical exercise or detect early signs ofpotentially lethal heart defects and diseases. Fig. 3d shows the close upview on a pulse having the characteristic peak typically measured at theradial artery. There are two peaks per pulse. The first peak correspondsto shutting the valves that let blood flow into the heart and the secondpeak is shutting the valves that let the blood flow from the heart. It hasbeen reported that the radial pulse wave could be a useful index of thearterial stiffness [52]. Therefore, our device has potential for applica-tions in health diagnostics e.g. hypertension, atherosclerosis, heartfailure and etc.

Supplementary material related to this article can be found online athttp://dx.doi.org/10.1016/j.nanoen.2017.12.004.

The PDMS core-shell PDMS ion gel / PVDF-HFP nanofiber mats canbe configured into a triboelectric sensor and a self-powered device bythe triboelectric effect at high dynamic mechanical pressure. The tri-boelectric nanogenerator (TENG) device is composed of a polyethyleneterephthalate (PET) sheet as a substrate, a spacer, Cu electrode, aKapton layer and the core-shell PDMS ion gel / PVDF-HFP nanofibermats as shown in Fig. 4a. The operation principle of a TENG is based onthe coupling of the electrostatic induction and the triboelectric effect.The mechanism for electric power generation using the core-shell PDMS

ion gel / PVDF-HFP nanofiber mats is shown in Fig. 4a. Initially, there isno charge induced by the electric potential difference between twoelectrodes of the core-shell PDMS ion gel / PVDF-HFP nanofiber matsTENG in the initial state. When the external compressive force is ap-plied, the core-shell PDMS ion gel / PVDF-HFP nanofibers and Kaptonare brought into contact with each other in the pushed state. Surfacecharge transfer occurs at the interface as a result of the triboelectriceffect. The Kapton film possesses strong negative triboelectric polarityaccording to the triboelectric series [53]. The positive and negativecharges are induced at the core-shell PDMS ion gel / PVDF-HFP na-nofiber mats and the Kapton surface, respectively. When the pressure isreleased, the Kapton and the core-shell PDMS ion gel / PVDF-HFP na-nofiber mats surface separate from each other. The dipole momentbecomes stronger at this stage. Thus, a strong electric potential differ-ence is generated between the electrodes. The connection to the device(switching polarity) with the measurement equipment is reversed inorder to confirm the measured output performance is originated fromthe TENG and to eliminate the influence of the noise caused by themeasurement system as shown in Fig. S3. The connecting configurationthat the PDMS core-shell PDMS ion gel / PVDF-HFP nanofiber mats isconnected to a positive probe and the Kapton film is connected to anegative probe which is defined as a forward connection. On the otherhand, the inverted connection is defined as the device measured withthe reverse connection [54]. The output signals were reversed when thedevice is reversely connected which implies that the output signalswere generated by a triboelectric effect. Fig. 4b and c show the gen-erated output voltage and current density for different amounts of ionicliquid loaded in the core-shell PDMS ion gel / PVDF-HFP nanofiber matunder a dynamic mechanical pressure up to 700 kPa at an appliedfrequency of 5 Hz. The output voltage and current density increase with

Fig. 5. (a) The output voltage of TENG at pressure range 100–700 kPa (b) Voltage-response curves of the 40 wt% ionic liquid loaded core-shell PDMS ion gel / PVF-HFP nanofiber matsoutput voltage at high pressure range 100–700 kPa) (c) output power density of the TENG for the core-shell PDMS ion gel / PVDF-HFP nanofiber mats on the resistance of an external load(d) (I) Schematic diagram of LED bulbs operation circuit with a full-wave bridge rectifier (II) Photograph of the serial connections of 300 LED bulbs driven by the TENG for the PDMScore-shell PDMS ion gel / PVDF-HFP nanofiber mats (device size: 5 cm*5 cm).

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an increasing amount of ionic liquid loading in the PDMS ion gel, whichimplies that the electrical double layer formed by electrochemical effectin the ion gel nanofibers plays an important role in increasing theamount of induced charge and the capacitance of the triboelectriclayers, resulting in an improvement of the output performance ofTENGs [45]. At higher amount of ion gel, the two interfaces could sticktogether, which causes the charge to rebalance during the releasingstate. Therefore, we have selected 40 wt% ionic liquid loaded core-shellPDMS ion gel / PVDF-HFP nanofiber mats for further study.

Several reports have demonstrated that triboelectric nanogeneratorcan be utilized as dynamic pressure sensing [14–19]. However, thetribo based sensors encounter the limit of saturated output voltage athigh pressure range. Our previous study has proved that self-polarizedpolyvinydifluoride-trifluoroethylene (PVDF-TrFE) sponge could be uti-lized for ultra large range pressure detection [17]. It is interesting toinvestigate the triboelectric pressure sensor at higher pressure range.The output voltage signals of TENG under higher pressure range areshown in Fig. 5a. The output voltage is increased by increasing thedynamic mechanical pressure. The pressure sensitivity of the tribo-electric nanogenerator-based pressure sensor was calculated from theslope of the voltage response curve (S = d(ΔV/Vs)/dP, where ΔV is therelative change in the output voltage, Vs is the final saturation voltage,and P denotes the applied pressure. The sensitivity of the 40 wt% ionicliquid loaded core-shell PDMS ion gel / PVDF-HFP nanofiber mats is0.068 V kPa−1 from 100 kPa to 700 kPa as shown in Fig. 5b. The sen-sitivity of the core-shell PDMS ion gel / PVDF-HFP nanofiber mats iscomparable to the polarized PVDF-TrFE sponge [17]. The higher sen-sitivity of core-shell PDMS ion gel / PVDF-HFP nanofiber mat is at-tributed to the increasing inductive charges, capacitance and nanofiberssurface area. In the pressure range from 40 kPa to 100 kPa, the sensi-tivity of the 40 wt% ionic liquid loaded core-shell PDMS ion gel / PVDF-HFP nanofiber mat is 0.102 V kPa−1 as shown in Fig. 4Sb. In our work,the core-shell PDMS ion gel/PVDF-HFP nanofiber mat could be used notonly as a triboelectric sensor but also as a self-powered device. Externalloads with varying resistance can be connected to TENGs for differentapplications. The systematical study of output performance with dif-ferent external loads of varying resistance is shown in Fig. 4Sd. Theoutput voltage increases from 0.1 to 31 V by increasing the resistance ofthe load from 1 KΩ to 100 MΩ. The output current decreases slightlyfrom 3.92 to 3.06 μA. The instantaneous power density can be calcu-lated by P˭V2/R. The maximum output power obtained at differentresistance is shown in Fig. 5c. The maximum power density can reachup to 0.9 W/m2 at a load of 10 KΩ. The power density of the core-shellPDMS ion gel / PVDF-HFP nanofiber mat is 3 times higher than thosereported on silicone rubber based TENGs [55,56]. The core-shell PDMSion gel / PVDF-HFP nanofiber mat allows a scalable design of the devicesample. An array of 300-LED can be powered instantaneously by theoutput generated from the TENG as shown in Fig. 5d. The video recordfor lighting up the LED arrays is shown in the movie (Movie S2 in theSupporting information). It is noteworthy that our device operates in astable output performance even after a shelf life of six months, in-dicating the robustness of the materials and device configuration asshown in Fig. 5S.

4. Conclusions

In summary, the core-shell PDMS ion gel / PVDF-HFP nanofibermats have been fabricated successfully by electrospinning. The core-shell PDMS ion gel / PVDF-HFP nanofiber mats have been demon-strated as tactile pressure sensor to detect static and dynamic pressure.The core-shell PDMS ion gel/PVDF-HFP nanofiber mats can be used as acapacitive tactile sensor which offers higher sensitivity of 0.43 kPa−1 inthe low pressure regime range up to 1.5 kPa, which has been demon-strated as a wearable heart-rate monitor. Furthermore, the core-shellPDMS ion gel/PVDF-HFP nanofiber mats can be used in a triboelectricsensor and self-powered energy generating device when the high

dynamic mechanical pressures are applied. The core-shell PDMS ion gel/ PVDF-HFP nanofiber mat based triboelectric dynamic pressure sensorexhibits high sensitivity of 0.102 V kPa−1 and 0.068 V kPa−1 at higherpressure from 40 kPa to 100 kPa and 100 kPa to 700 kPa, respectively.The maximum power density of the core-shell PDMS ion gel/PVDF-HFPnanofiber mat could be brought up to 0.9 W/m2, which is sufficient tolight up hundreds of light emitting diodes (LEDs).

Acknowledgements

This work was supported by the National Research FoundationCompetitive Research Program (Award No. NRF-CRP13-2014-02).

Competing interests

The authors declare no competing financial interests.

Author contributions

M.-F. L., J. Q. X. and P. S. L. designed the experiments. M.-F. L., J. X.W., and K. P. performed the experiments. M.-F. L., J. Q. X. and P. S. Lwrote the paper.

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.2017.12.004.

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22233.

Dr. Meng-Fang Lin is a senior research fellow in the Schoolof Materials Science and Engineering, NanyangTechnological University (NTU), Singapore. She receivedher Ph. D. degree in School of Materials Science andEngineering, Nanyang Technological University (NTU),Singapore. She did postdoctoral research with Dr.Tsukagoshi Kazuhito at National Institute for MaterialsScience (NIMS), Japan (2013–2014). Her current researchinterests focus on pressure sensor and triboelectric energyharvesting.

Dr. Jiaqing Xiong received his Ph.D. degree from SoochowUniversity in Department of Textile Engineering in 2015.Now he is a research fellow in Prof. Pooi See Lee's group atthe School of Materials Science and Engineering, NanyangTechnological University. His recent research interest fo-cuses on the biomass materials functionalization, smartmaterials and wearable devices, energy conversion andharvesting. Especially, eco-friendly hydrophilic/hydro-phobic materials, triboelectric energy harvesting.

Dr. Jiangxin Wang is a research fellow in the School ofMaterials Science and Engineering, NangyangTechnological University (NTU), Singapore. He obtainedhis B.S. degree in the School of Physical Electronics,University of Electronic Science and Technology of China(UESTC) in 2010 and Ph.D. in School of Materials Scienceand Engineering, Nangyang Technological University(NTU), Singapore. His current research interests focus ondeformable optoelectronic devices.

Mr. Kaushik Parida received his master's degree from theSchool of Metallurgical Engineering and Materials Scienceof Indian Institute of Technology Bombay, India in 2013.He is currently pursuing his doctoral degree under the su-pervision of Prof. P.S. Lee at the School of Materials Scienceand Engineering in Nanyang Technological University,Singapore. His research focuses on piezoelectric polymersfor application in piezoelectric and triboelectric energyharvester, self-powered devices and soft electronics.

Prof. Pooi See Lee received her Ph.D. from NationalUniversity of Singapore in 2002. She joined the School ofMaterials Science and Engineering, Nanyang TechnologicalUniversity as an Assistant Professor in 2004. She was pro-moted to tenured Associate Professor in 2009 and fullProfessor in 2015. Her research focuses on nanomaterialsfor energy and electronics applications, flexible andstretchable devices, electrochemical inspired devices, andhuman-machine interface. She received the NationalResearch Foundation Investigatorship Award and theNanyang Research Excellence Award in 2015.

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