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Core-shell nanofiber mats for tactile pressure sensor and nanogenerator applications
Author’s Accepted Manuscript Core-Shell nanofiber mats for tactile pressure semmcnsor and nanogenerator applications Meng-Fang Lin, Jiaqing Xiong, Jiangxin Wang, Kaushik Parida, Pooi See Lee www.elsevier.com/locate/nanoenergy
PII: DOI: Reference:
S2211-2855(17)30771-1 https://doi.org/10.1016/j.nanoen.2017.12.004 NANOEN2385
To appear in: Nano Energy Received date: 9 November 2017 Revised date: 4 December 2017 Accepted date: 4 December 2017 Cite this article as: Meng-Fang Lin, Jiaqing Xiong, Jiangxin Wang, Kaushik Parida and Pooi See Lee, Core-Shell nanofiber mats for tactile pressure semmcnsor and nanogenerator applications, Nano Energy, https://doi.org/10.1016/j.nanoen.2017.12.004 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Core-Shell nanofiber mats for tactile pressure sensor and nanogenerator applications Meng-Fang Lin, Jiaqing Xiong, Jiangxin Wang, Kaushik Parida and Pooi See Lee*
Dr. M.-F. Lin, Dr. J. Xiong, Dr. J. Wang, K. Parida, Prof. P. S. Lee School of Materials Science and Engineering, 50 Nanyang Avenue, Nanyang Technological University, 639798, Singapore. E-mail: [email protected] Keywords: tactile pressure sensor, nanogenerator, electrospinning, core-shell nanofibers
Abstract Core-shell nanofibers of PDMS ion gel /PVDF-HFP were successfully prepared by incorporating cross-linking agent during electrospinning. The electrospun nanofiber mats were used to fabricate pressure sensors to detect the static and dynamic pressures by harnessing the capacitance changes and triboelectric effects judiciously. The core-shell PDMS ion gel/PVDF-HFP nanofiber sensor functions as a capacitive pressure sensor, which offers high sensitivity of 0.43 kPa-1 in the low pressure ranges from 0.01 kPa to 1.5 kPa. The sensitivity, flexibility, and robustness of our capacitive pressure sensor allows it to be utilized as a wrist-based pulse wave detector for heart-rate monitoring. In addition, the core-shell PDMS ion gel/PVDF-HFP nanofiber mat made a good triboelectric based pressure sensor in the high pressure range with a linear pressure sensitivity 0.068 V∙kPa-1 from 100 kPa to 700 kPa, one of the best reported at present. The increase in inductive charges and the enhanced dielectric capacitance of the core-shell nanofiber layer compared to the pure PVDF-HFP nanofiber layer allows it to 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. Graphical abstract: 1
Table of Contents Textual Information
A brief abstract
Title Authors’ Names
A simple fabrication rout for the core-shell PDMS ion gel / PVDF-HFP nanofibers are prepared by using cross-linking via electrospinning. The electrospun nanofiber mats are used to fabricate the tactile pressure sensing to detect the static and dynamic pressure which could be utilized as a heart-rate indicator and power generation in various pressure ranges. Core-shell nanofiber mats for tactile pressure sensor and nanogenerator applications Meng-Fang Lin, Jiaqing Xiong, Jiangxin Wang, Kaushik Parida, and Pooi See Lee＊
Graphical Information
2
1. Introduction A vital component of consumer electronics technology is the pressure sensors, which can be found e.g. in touchpads, touchscreens, microphones and biometric identification devices.[1] Several types of flexible 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 has achieved an enormous success, due to its high electrical sensitivity, fast response time, low power consumption, compact circuit layout, and a relatively simple device construction compared to their resistive counterparts.[13] However, the capacitive pressure sensor is only operated at low pressure regimes (<10 kPa) for touch screen, health care and medical diagnosis systems applications.[1] Recently, triboelectric nanogenerator (TENG) has been demonstrated as a pressure sensor[14-19] and a self-powered electronic device[20-26] in the higher pressure range (>10 kPa) by converting mechanical energy into electrical energy. In contrast to the capacitive pressure sensor, which is suitable for measuring both static and low dynamic pressure changes, TENGs are suitable for dynamic pressure detection in the higher pressure regime. For further increasing the sensitivity and decreasing the response 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 integrated to capacitive and triboelectric pressure sensors to enhance the tactile sensitivity. In addition, it has been demonstrated that for the triboelectric charges in TENGs are well separated in the microstructured films and therefore a larger electric dipole moment can be formed between the electrodes.[26] Thus, surface microstructures play an important role in enhancing the output performance of triboelectric systems. Despite achieving significant device sensitivity, the fabrication of surface microstructures largely relies on traditional lithography technique, which suffers from complex and time consuming procedures, limited scalability or incompatibility with roll to roll process. In contrast, electrospinning is a straight 3
forward, efficient and scalable technique to fabricate the microstructured films consist of highly interconnected, nonwoven fibers with diameters in the micro and nanometer range.[29] Electrospun hydrogel has been widely used in biomedical applications such as drug delivery, tissue engineering and biosensor.[30, 31] Recently it has been shown that ionic liquid based polymeric electrolyte attained by incorporating an ionic liquid into a crosslinkable gel matrix possesses not only high ionic conductivity of the ionic liquid but also exerts outstanding mechanical properties.[32, 33] The high capacitance of ionic liquid or ion gel can be attributed to the formation of electrical double layers (EDLs)[34-36], which have been utilized in energy storage cells, electrochromics and stretchable current collectors. The usage of the ionic hydrogel matrix as a thin film for interfacial capacitive sensors has been recently reported.[37] However, the hydrogel without the elastomeric coating is highly susceptible to dehydration. Thus, the hydrogel requires a protective layer to prevent evaporation of water from the hydrogel.[38] Furthermore it should be pointed out that the capacitance of the ionic hydrogel matrix thin film is directly related to its surface area, which poses a general limitation on the obtainable sensitivity in the previous study. Our pioneering concept herein provides the material design strategies to circumvent the aforementioned shortcomings that affect the stability of ionic hydrogel based device related to dehydration, and the limited pressure sensitivity due to low surface area. We incorporate an ion gel core within a nanofiber supported by a copolymer shell featuring an intrinsically larger surface area and a built in protection layer. In addition, the device design introduced in this work showcases an effective device fabrication strategy including cross functionality with tactile sensing for both static and dynamic pressure as well as power generation capability in a single integrated device system. We have fabricated the core-shell PDMS ion gel/ PVDF-HFP nanofiber mats via electrospinning, which can be used in various pressure ranges for the dual functional applications of tactile sensing for the static and dynamic pressure and power generation. 4
Utilizing the core-shell PDMS ion gel/PVDF-HFP nanofiber mats as a capacitive sensor offers high sensitivity of 0.43 kPa-1 in the pressure ranges from 0.01 kPa up to 1.5 kPa. Compared to a zinc oxide nanowire/poly(methylmethacrylate) film and a PDMS-coated conductive fiber, the sensitivity of the core-shell PDMS ion gel/PVDF-HFP nanofiber mats is 50 and 2 times higher, respectively.[28,
39]
Furthermore, our capacitive sensor has been
demonstrated as a wearable pulse rate detector that serves as a heart-rate indicator. In addition, the core-shell PDMS ion gel/PVDF-HFP nanofiber mats could be used as a self-powered device 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 to 700 kPa and 40 kPa to 100 kPa, respectively. The maximum power density of the core-shell PDMS ion gel/PVDF-HFP nanofiber mats could be brought up to 0.9 W/m2, which is sufficient to light up three hundred light emitting diodes (LEDs). The improvement in the electrical power generated by the core-shell PDMS ion gel/PVDF-HFP nanofiber mats can be explained by an increased amount of inductive charges and capacitance in the triboelectric layer.
2. Materail 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 at room temperature. Subsequently, the PDMS (Sylgard 184) which consisted of liquid components (a mixture of catalyst Pt and prepolymer
dimethylsiloxane
with
vinyl
groups)
and
curing
agent
(prepolymer
dimethylsiloxane with vinyl groups and Si-H groups) (10:1) were added into ionic liquid solution. Finally, tetrahydrofuran (THF) solvent was added to the PDMS with ionic liquid solution in order to form the PDMS ion gel.
5
2.2 Core-shell electrospun PDMS ion gel /PVDF-HFP nanofiber mats: PVDF-HFP (SigmaAldrich, Mw~455,000) was dissolved in a mix solvent DMF/THF (volume ratio=1:1). The concentration of PVDF-HFP was 100 mg/mL. The PVDF-HFP solution was stirred at 60 oC for 6 hrs to obtain a transparent solution. Fig. 1b illustrates the setup for core-shell electrospinning. PVDF-HFP and PDMS ionogel solutions were fed into the outer channel (15 G, OD 1.8 mm, ID 1.4 mm) and inner channel (21G, OD 0.8 mm, ID 1.4 mm) of the spinneret, respectively. At the nozzle tip, the inner channel protruded out of the outer channel by about 0.5 mm. The flow rates of the PDMS ionogel and PVDF-HFP solutions were controlled using syringe pumps at 0.7 mL/hr and 1 mL/hr, respectively. The aluminum foil was used as a collector, which has been placed 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 illustrated in Fig. 2 (a). The device is constructed by a metalinsulator-metal (MIM) capacitor structure where the metal is Cu electrode and the capacitive layer is the core-shell PDMS ion gel/PVDF-HFP nanofiber mat. The device size is 1 cm x1 cm. 2.4 Fabrication of the flexible triboelectric device: As illustrated in Fig. 4 (a), the core-shell PDMS ion gel/PVDF-HFP nanofiber mat with Cu electrode was placed on the polyethylene terephthalate (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 nanofiber 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 2o min-1 from 10 to 80o (2θ). Field emission scanning electron microscopy (FESEM, JEOL 7600F) operating at 5 kV was employed to determine the core-shell PDMS ion gel/PVDF-HFP nanofibers morphology. Detailed structure analyses were performed using transmission electron microscopy (TEM, JEOL2010) operated at 200 kV. The compression 6
test was performed on the core-shell PDMS ion gel/PVDF-HFP nanofiber mats by using Instron 5567. The capacitance measurement from the capacitive pressure sensor was obtained using the LCR meter (Agilent E4980A). To analyze the device sensitivity, the LCR meter was used to detect capacitive changes under various mechanical loads on the device. The voltage outputs from the triboelectric pressure sensor were measured by an oscilloscope (Trektronix, MDO 3024, input resistance = 10 MΩ), the current output was measured by a low-noise current pre-amplifier (Stanford Research System, Model SR570, input resistance = 4 Ω). The 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 prepared using electrospinning. Although PDMS is immiscible with many other ionic liquids, earlier studies indicate the feasibility of using tetraethylorthosilicate (TEOS) as the cross-linking agent to form the hybrid ion gel with various polymers such as PDMS and poly(methyl methacrylate) PMMA through sol-gel reaction.[40, 41] In this study, the PDMS ion gel has been prepared by sol-gel reaction to create the cross-linked PDMS with TEOS and PDMS-OH. The TEOS and PDMSOH are employed as the cross-linking agents between ionic liquid and PDMS polymer. The schematic diagram of the PDMS ion gel preparation is shown in Fig. 1a. The process of preparing the core-shell PDMS ion gel / PVDF-HFP nanofiber mats is found in Fig. 1b. The PVDF-HFP is configured as the shell during electrospinning due to its high molecular weight. The electrospinning of PDMS ion gel nanofibers has been realized through the PDMS prepolymer and the curing agent to form a stable 3D network via covalent bonds.[42] The morphology of electrospinning of core-shell PDMS ion gel / PVDF-HFP nanofiber mats is shown in Fig. 1c. In order to ascertain the formation of core-shell nanofiber structure, transmission electron microscopy (TEM) is employed as shown in Fig. 1d. The total diameter of core-shell PDMS ion gel / PVDF-HFP nanofibers and core PDMS ion gel nanofibers are 7
around 235 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-shell PDMS ion gel / PVDF-HFP nanofibers layer is around 100 μm as shown in Fig. S1b. Infrared spectroscopy has been used to characterize the coreshell PDMS ion gel / PVDF-HFP nanofiber mats as shown in Fig. S1c. Two strong bending bands are observed in the region between 500 and 750 cm-1, corresponding to a SO2 antisymmetric bending (610 cm-1) and another SO2 antisymmetric bending (612 cm-1) from the [EMIM][TFSI].[43] The peaks at 1090 and 1022 cm-1 were assigned to the Si-O-Si of siloxane, which showed evident vibration peaks for the PDMS fiber. The –CH3 rocking and Si-C vibrations appeared at around 802 cm-1.[44] The electrospun core-shell PDMS ion gel / PVDF-HFP nanofibers have a crystalline structure of α and β phases in coexistence. The vibrational bands at 1264 and 842 cm-1 are attributed to the β phases of PVDF-HFP. Several peaks in the region between 1197 and 762 cm-1 are assigned to α phases of PVDF-HFP.[45] The core-shell PDMS ion gel / PVDF-HFP nanofiber mats were made into capacitive pressure sensor in the metal-insulator-metal (MIM) configuration with Cu as the top and bottom electrode as shown in Fig. 2a. Fig. 2b shows the capacitance response of core-shell PDMS ion gel / PVDF-HFP nanofiber mats that are able to sense very small pressure of 0.01 kPa. The capacitance is altered by placing and removing a load cell of 0.01 kPa in Fig. 2b. The mechanism of the capacitance change of the core-shell PDMS ion gel / PVDF-HFP nanofiber mats when the pressure is applied is shown in Fig. 2c. The capacitance of a parallel plate capacitor with area A and thickness d can be written as: , where C is the capacitance, in Farads; A is the area of overlap of the two plates, in square meters; εo is the vacuum dielectric constant (εo~8.54×10-12 F/m); εr is the dielectric constant of
8
the material between the plates; and d is the separation between the plates. The change in the capacitance in the case that a constant pressure is applied allows it to measure static pressure. The dramatic increase of the capacitance of the core-shell nanofiber mats when the external pressure is applied can be attributed to three main factors: (1) the dielectric constant εr is changed when the PDMS ion gel / PVDF-HFP nanofiber mats are compressed since the displaced air has a lower dielectric constant (εr=1.0) than PDMS (εr=3.0) and PVDF-HFP (εr=11.38) as shown in Fig. 2c-(I), (2) when the distance between the top and bottom electrode is reduced (d to d’), the thickness of the elastomeric PDMS inside the core-shell PDMS ion gel / PVDF-HFP nanofibers can be reduced because PDMS possesses good elastic properties 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 (A to A’) due to the reduced spacings as a result of the highly deformable properties of PDMS, which results in an enhanced interfacial capacitance as shown in Fig. 2c-(III). The increase of the dielectric constant and the fiber contact area as well as the reduced distance between the separated electrodes leads to a significant increase of capacitance of the pressure sensor upon compression and therefore a high pressure sensitivity (as shown in next paragraph) in the low pressure range considered in this work. The pressure response curves for varying amounts of ionic liquid loaded core-shell PDMS ion gel / PVDF-HFP nanofiber mats are presented in Fig. 3a. The pressure sensitivity S can be defined as the slope of the curves in Fig. 3a. (S = δ (ΔC /Co) / δp = (1/Co) * δC / δp), where p denotes the applied pressure, and C and Co denote the capacitance with and without applied pressure, respectively. The 40 wt% ionic liquid loaded core-shell PDMS ion gel / PVDF-HFP nanofiber mat exhibits much higher pressure sensitivity than others. In the pressure range of <1.5 kPa, the sensitivity of the sample with 40 wt% ionic liquid loaded core-shell PDMS ion gel / PVDF-HFP nanofiber mat is 71 times higher than pristine PVDF-HFP nanofiber mat due to a larger interfacial capacitance produced at high amount of PDMS ion gel as shown in the 9
Fig. 3a. Compared with previous fiber-based pressure sensor, our proposed pressure sensor exhibits better sensitivity and improved detection limitation.[28, 46-48] Our device shows 12.5 times lower 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 higher than the previously reported value for ZnO-PMMA composite.[39] In addition, our device is able to extend the range of possible measurements up to 10 kPa as shown in Fig. 3b. However, there is a reduction in sensitivity when the pressures are higher than 1.5 kPa. The sensitivity reduction is attributed to the increasing elastic resistance with increasing compression.[2] For real world applications this progressive damping could be desirable, since it increases the range of detectable pressure to the case of high loads in which high sensitivity is not required for the capacitive pressure sensor, and therefore results in a more versatile pressure sensor.[2] The mechanical properties of the pristine PVDF-HFP and different weight percentage of ionic liquid loading PDMS ion gel / PVDF-HFP nanofiber mats are shown in Fig. 3c. Compared to pristine PVDF-HFP nanofiber mat, the 40 wt% ionic liquid loaded core-shell PDMS ion gel / PVF-HFP nanofiber mat exhibits a higher compressive strain at the same compressive stress. It indicates that the higher amount of ionic liquid in the core-shell nanofiber mat leads to a larger deformation upon applied pressure. The nanofibers with a higher amount of ionic liquid loaded PDMS ion gel could have a lower elastic modulus as shown in Fig. S2a. It is noted that the elastic modulus of 10 wt% ionic liquid loaded core-shell PDMS ion gel / PVF-HFP nanofiber mat is dramatically reduced compared to PVF-HFP nanofiber 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 ionic conductivity of the solution could cause large instabilities during the electrospinning process[50] as a high voltage operation (25 kV) is required to fabricate the core-shell PDMS ion gel / PVF-HFP nanofiber mat. The ionic 10
conductivity of different weight percentage of PDMS ion gel was measured by electrochemcial impedance spectroscopy (EIS) with a frequency range of 0.01 Hz-100 kHz. Fig. S2b shows a typical Nyquist plot of the impedance analysis on different weight percentage of PDMS ion gel. At high frequency (~100 KHz), the corresponding value of the intercept on the real axis represents the instrinsic resistance of the ion gel as the ohmic resistance of the testing device is negligible as shown in Fig. S2c.[51] Therefore, the ionic conductivity can be calculated according to the formula:
Where σ is the ionic conductivity, L is the distance between the two electrodes, R is the resistance of ion gel, and S is the geometric area of the electode interface. The ionic conductivity is increased by increasing amount of the ionic liquid as shown in Fig. S2d. The sensitivity, flexibility, and robustness of our device allows it to be utilized as a wrist-based heart-rate monitor. The device was attached directly to the wrist of a living subject to measure the radial arterial pulse wave as shown in Fig. 3d. It is demonstrated that the device can track the number of pulses through the relative capacitance change. The test subject has a stable heartrate of 75 times per minute which is consistent with the counting results and shows that the subject is currently healthy and relax. The video record for the wrist-based pulse wave detector for heart-rate monitoring as shown in the Movie S2 in the Supporting Information. A higher, arrhythmic or otherwise altered heartrate can indicate states of stress and physical exercise or detect early signs of potentially lethal heart defects and diseases. Fig. 3d shows the close up view on a pulse having the characteristic peak typically measured at the radial artery. There are two peaks per pulse. The first peak corresponds to shutting the valves that let blood flow into the heart and the second peak is shutting the valves that let the blood flow from the heart. It has been reported that the radial pulse wave could be a useful index of the arterial
11
stiffness.[52] Therefore, our device has potential for applications in health diagnostics e.g. hypertension, atherosclerosis, heart failure and etc. The PDMS core-shell PDMS ion gel / PVDF-HFP nanofiber mats can be configured into a triboelectric sensor and a self-powered device by the triboelectric effect at high dynamic mechanical pressure. The triboelectric nanogenerator (TENG) device is composed of a polyethylene terephthalate (PET) sheet as a substrate, a spacer, Cu electrode, a Kapton layer and the core-shell PDMS ion gel / PVDF-HFP nanofiber mats as shown in Fig. 4a. The operation principle of a TENG is based on the 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 is no charge induced by the electric potential difference between two electrodes of the core-shell PDMS ion gel / PVDF-HFP nanofiber mats TENG in the initial state. When the external compressive force is applied, the core-shell PDMS ion gel / PVDF-HFP nanofibers and Kapton are brought into contact with each other in the pushed state. Surface charge transfer occurs at the interface as a result of the triboelectric effect. The Kapton film possesses strong negative triboelectric polarity according to the triboelectric series.[53] The positive and negative charges are induced at the core-shell PDMS ion gel / PVDF-HFP nanofiber mats and the Kapton surface, respectively. When the pressure is released, the Kapton and the core-shell PDMS ion gel / PVDF-HFP nanofiber mats surface separate from each other. The dipole moment becomes stronger at this stage. Thus, a strong electric potential difference is generated between the electrodes. The connection to the device (switching polarity) with the measurement equipment is reversed in order to confirm the measured output performance is originated from the TENG and to eliminate the influence of the noise caused by the measurement system as shown in Fig. S3. The connecting configuration that the PDMS core-shell PDMS ion gel / PVDF-HFP nanofiber mats is connected to a positive probe and the Kapton film is connected to a negative probe which is defined as a forward connection. On the other hand, the inverted 12
connection is defined as the device measured with the reverse connection.[54] The output signals were reversed when the device is reversely connected which implies that the output signals were generated by a triboelectric effect. Fig. 4b and c show the generated output voltage and current density for different amounts of ionic liquid loaded in the core-shell PDMS ion gel / PVDF-HFP nanofiber mat under a dynamic mechanical pressure up to 700 kPa at an applied frequency of 5 Hz. The output voltage and current density increase with an increasing amount of ionic liquid loading in the PDMS ion gel, which implies that the electrical double layer formed by electrochemical effect in the ion gel nanofibers plays an important role in increasing the amount of induced charge and the capacitance of the triboelectric layers, resulting in an improvement of the output performance of TENGs.[45] At higher amount of ion gel, the two interfaces could stick together, which causes the charge to rebalance during the releasing state. Therefore, we have selected 40 wt% ionic liquid loaded core-shell PDMS ion gel / PVDF-HFP nanofiber mats for further study. Several reports have demonstrated that triboelectric nanogenerator can be utilized as dynamic pressure sensing.[14-19] However, the tribo based sensors encounter the limit of saturated output voltage at high pressure range. Our previous study has proved that selfpolarized polyvinydifluoride-trifluoroethylene (PVDF-TrFE) sponge could be utilized for ultra large range pressure detection.[17] It is interesting to investigate the triboelectric pressure sensor at higher pressure range. The output voltage signals of TENG under higher pressure range are shown in Fig. 5a. The output voltage is increased by increasing the dynamic mechanical pressure. The pressure sensitivity of the triboelectric nanogenerator-based pressure sensor was calculated from the slope of the voltage response curve (S=d(ΔV/Vs)/dP, where ΔV is the relative change in the output voltage, Vs is the final saturation voltage, and P denotes the applied pressure. The sensitivity of the 40 wt% ionic liquid loaded core-shell PDMS ion gel / PVDF-HFP nanofiber mats is 0.068 V∙kPa-1 from 100 kPa to 700 kPa as shown in Fig. 5b. The sensitivity of the core-shell PDMS ion gel / PVDF-HFP nanofiber mats 13
is comparable to the polarized PVDF-TrFE sponge.[17] The higher sensitivity of core-shell PDMS ion gel / PVDF-HFP nanofiber mat is attributed to the increasing inductive charges, capacitance and nanofibers surface area. In the pressure range from 40 kPa to 100 kPa, the sensitivity 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/PVDFHFP nanofiber mat could be used not only as a triboelectric sensor but also as a self-powered device. External loads with varying resistance can be connected to TENGs for different applications. The systematical study of output performance with different external loads of varying resistance is shown in Fig. 4Sd. The output voltage increases from 0.1 to 31 V by increasing the resistance of the load from 1 KΩ to 100 MΩ. The output current decreases slightly from 3.92 to 3.06 μA. The instantaneous power density can be calculated by P=V2/R. The maximum output power obtained at different resistance is shown in Fig. 5c. The maximum power density can reach up to 0.9 W/m2 at a load of 10 KΩ. The power density of the core-shell PDMS ion gel / PVDF-HFP nanofiber mat is 3 times higher than those reported on silicone rubber based TENGs.[55, 56] The core-shell PDMS ion gel / PVDF-HFP nanofiber mat allows a scalable design of the device sample. An array of 300-LED can be powered instantaneously by the output generated from the TENG as shown in Fig. 5d. The video record for lighting up the LED arrays is shown in the movie (Movie S2 in the Supporting Information). It is noteworthy that our device operates in a stable output performance even after a shelf life of six months, indicating the robustness of the materials and device configuration as shown in Fig. 5S.
4. Conclusions In summary, the core-shell PDMS ion gel / PVDF-HFP nanofiber mats have been fabricated successfully by electrospinning. The core-shell PDMS ion gel / PVDF-HFP nanofiber mats have been demonstrated as tactile pressure sensor to detect static and dynamic pressure. The 14
core-shell PDMS ion gel/PVDF-HFP nanofiber mats can be used as a capacitive tactile sensor which offers higher sensitivity of 0.43 kPa-1 in the low pressure regime range up to 1.5 kPa, which has been demonstrated as a wearable heart-rate monitor. Furthermore, the core-shell PDMS ion gel/PVDF-HFP nanofiber mats can be used in a triboelectric sensor and selfpowered 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 sensor exhibits high sensitivity of 0.102 V∙kPa-1 and 0.068 V∙kPa-1 at higher pressure 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-HFP nanofiber mat could be brought up to 0.9 W/m2, which is sufficient to light up hundreds of light emitting diodes (LEDs).
Acknowledgement This work was supported by the National Research Foundation Competitive 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. L wrote the paper.
Apprendix A. Supporting Information Supplementary data associated with this article can be found in the online version at
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Fig. 1. (a) Schematic diagram of the PDMS ion gel preparation (b) illustration of 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 nanofibers mats (d) TEM image of core-shell PDMS ion gel / PVDF-HFP nanofibers
19
Fig. 2. (a) Device configuration of pressure sensor for the core-shell PDMS ion gel / PVDFHFP nanofibers mats (b) pressure sensor repeated real-time responses at 0.01 kPa (c) The mechanism of the core-shell PDMS ion gel / PVDF-HFP nanofibers mats when the pressure is applied (I) the dielectric constant is changed when the nanofiber mats are compressed (II) the top/bottom electrode distance is reduced (III) the fiber contact area is increased.
20
Fig. 3. (a) The maximum slope of the relative capacitance changes of the core-shell PDMS ion gel / PVDF-HFP nanofibers 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 nanofibers mats. (c) The stress-strain curves of the pristine PVDF-HFP nanofibers and different amount to PDMS ion gel / PVDF-HFP nanofibers mats (d) the real time pressure waveforms of the measured heart rate. The inset shows that the device has been mounted onto the wrist and the zoomed view on a pulse having the characteristic peak typically measured at the radial artery.
21
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 for electric 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
22
Fig. 5. (a) The output voltage of TENG at pressure range 100 to 700 kPa (b) Voltageresponse curves of the 40 wt% ionic liquid loaded core-shell PDMS ion gel / PVF-HFP nanofiber mats output voltage at high pressure range (100 to 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 PDMS core-shell PDMS ion gel / PVDF-HFP nanofiber mats (device size: 5 cm*5 cm).
23
Dr. Meng-Fang Lin is a senior research fellow in the School of Materials Science and Engineering, Nanyang Technological University (NTU), Singapore. She received her Ph. D. degree in School of Materials Science and Engineering, Nanyang Technological University (NTU), Singapore. She did postdoctoral research with Dr. Tsukagoshi Kazuhito at National Institute for Materials Science (NIMS), Japan (2013-2014). Her current research interests focus on pressure sensor and triboelectric energy harvesting.
Dr. Jiaqing Xiong received his Ph.D. degree from Soochow University in Department of Textile Engineering in 2015. Now he is a research fellow in Prof. Pooi See Lee’s group at the School of Materials Science and Engineering, Nanyang Technological University. His recent research interest focuses on the biomass materials functionalization, smart materials and wearable devices, energy conversion and harvesting. Especially, eco-friendly hydrophilic/hydrophobic materials, triboelectric energy harvesting.
Dr. Jiangxin Wang is a research fellow in the School of Materials Science and Engineering, Nangyang Technological University (NTU), Singapore. He obtained his 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 Science and Engineering, Nangyang Technological University (NTU), Singapore. His current research interests focus on deformable optoelectronic devices.
24
Mr. Kaushik Parida received his master's degree from the School of Metallurgical Engineering and Materials Science of Indian Institute of Technology Bombay, India in 2013. He is currently pursuing his doctoral degree under the supervision of Prof. P.S. Lee at the School of Materials Science and Engineering in Nanyang Technological University, Singapore. His research focuses on piezoelectric polymers for application in piezoelectric and triboelectric energy harvester, self-powered devices and soft electronics.
Prof. Pooi See Lee received her Ph.D. from National University of Singapore in 2002. She joined the School of Materials Science and Engineering, Nanyang Technological University as an Assistant Professor in 2004. She was promoted to tenured Associate Professor in 2009 and full Professor in 2015. Her research focuses on nanomaterials for energy and electronics applications, flexible and stretchable devices, electrochemical inspired devices, and human-machine interface. She received the National Research Foundation Investigatorship Award and the Nanyang Research Excellence Award in 2015.
Highlights
Core-shell nanofiber mats of PDMS ion gel /PVDF-HFP were successfully prepared by incorporating cross-linking agent during electrospinning.
The electrospun nanofibers were used to fabricate pressure sensors to detect the static and dynamic pressures by harnessing the capacitance changes and triboelectric effects judiciously.
The sensitivity, flexibility, and robustness of our capacitive pressure sensor allow it to be utilized as a wrist-based pulse wave detector for heart-rate monitoring.
The maximum power density of the core-shell PDMS ion gel/PVDF-HFP nanofiber mats could be brought up to 0.9 W/m2, which is sufficient to light up hundreds of light emitting diodes (LEDs).
25
PII: DOI: Reference:
S2211-2855(17)30771-1 https://doi.org/10.1016/j.nanoen.2017.12.004 NANOEN2385
To appear in: Nano Energy Received date: 9 November 2017 Revised date: 4 December 2017 Accepted date: 4 December 2017 Cite this article as: Meng-Fang Lin, Jiaqing Xiong, Jiangxin Wang, Kaushik Parida and Pooi See Lee, Core-Shell nanofiber mats for tactile pressure semmcnsor and nanogenerator applications, Nano Energy, https://doi.org/10.1016/j.nanoen.2017.12.004 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Core-Shell nanofiber mats for tactile pressure sensor and nanogenerator applications Meng-Fang Lin, Jiaqing Xiong, Jiangxin Wang, Kaushik Parida and Pooi See Lee*
Dr. M.-F. Lin, Dr. J. Xiong, Dr. J. Wang, K. Parida, Prof. P. S. Lee School of Materials Science and Engineering, 50 Nanyang Avenue, Nanyang Technological University, 639798, Singapore. E-mail: [email protected] Keywords: tactile pressure sensor, nanogenerator, electrospinning, core-shell nanofibers
Abstract Core-shell nanofibers of PDMS ion gel /PVDF-HFP were successfully prepared by incorporating cross-linking agent during electrospinning. The electrospun nanofiber mats were used to fabricate pressure sensors to detect the static and dynamic pressures by harnessing the capacitance changes and triboelectric effects judiciously. The core-shell PDMS ion gel/PVDF-HFP nanofiber sensor functions as a capacitive pressure sensor, which offers high sensitivity of 0.43 kPa-1 in the low pressure ranges from 0.01 kPa to 1.5 kPa. The sensitivity, flexibility, and robustness of our capacitive pressure sensor allows it to be utilized as a wrist-based pulse wave detector for heart-rate monitoring. In addition, the core-shell PDMS ion gel/PVDF-HFP nanofiber mat made a good triboelectric based pressure sensor in the high pressure range with a linear pressure sensitivity 0.068 V∙kPa-1 from 100 kPa to 700 kPa, one of the best reported at present. The increase in inductive charges and the enhanced dielectric capacitance of the core-shell nanofiber layer compared to the pure PVDF-HFP nanofiber layer allows it to 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. Graphical abstract: 1
Table of Contents Textual Information
A brief abstract
Title Authors’ Names
A simple fabrication rout for the core-shell PDMS ion gel / PVDF-HFP nanofibers are prepared by using cross-linking via electrospinning. The electrospun nanofiber mats are used to fabricate the tactile pressure sensing to detect the static and dynamic pressure which could be utilized as a heart-rate indicator and power generation in various pressure ranges. Core-shell nanofiber mats for tactile pressure sensor and nanogenerator applications Meng-Fang Lin, Jiaqing Xiong, Jiangxin Wang, Kaushik Parida, and Pooi See Lee＊
Graphical Information
2
1. Introduction A vital component of consumer electronics technology is the pressure sensors, which can be found e.g. in touchpads, touchscreens, microphones and biometric identification devices.[1] Several types of flexible 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 has achieved an enormous success, due to its high electrical sensitivity, fast response time, low power consumption, compact circuit layout, and a relatively simple device construction compared to their resistive counterparts.[13] However, the capacitive pressure sensor is only operated at low pressure regimes (<10 kPa) for touch screen, health care and medical diagnosis systems applications.[1] Recently, triboelectric nanogenerator (TENG) has been demonstrated as a pressure sensor[14-19] and a self-powered electronic device[20-26] in the higher pressure range (>10 kPa) by converting mechanical energy into electrical energy. In contrast to the capacitive pressure sensor, which is suitable for measuring both static and low dynamic pressure changes, TENGs are suitable for dynamic pressure detection in the higher pressure regime. For further increasing the sensitivity and decreasing the response 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 integrated to capacitive and triboelectric pressure sensors to enhance the tactile sensitivity. In addition, it has been demonstrated that for the triboelectric charges in TENGs are well separated in the microstructured films and therefore a larger electric dipole moment can be formed between the electrodes.[26] Thus, surface microstructures play an important role in enhancing the output performance of triboelectric systems. Despite achieving significant device sensitivity, the fabrication of surface microstructures largely relies on traditional lithography technique, which suffers from complex and time consuming procedures, limited scalability or incompatibility with roll to roll process. In contrast, electrospinning is a straight 3
forward, efficient and scalable technique to fabricate the microstructured films consist of highly interconnected, nonwoven fibers with diameters in the micro and nanometer range.[29] Electrospun hydrogel has been widely used in biomedical applications such as drug delivery, tissue engineering and biosensor.[30, 31] Recently it has been shown that ionic liquid based polymeric electrolyte attained by incorporating an ionic liquid into a crosslinkable gel matrix possesses not only high ionic conductivity of the ionic liquid but also exerts outstanding mechanical properties.[32, 33] The high capacitance of ionic liquid or ion gel can be attributed to the formation of electrical double layers (EDLs)[34-36], which have been utilized in energy storage cells, electrochromics and stretchable current collectors. The usage of the ionic hydrogel matrix as a thin film for interfacial capacitive sensors has been recently reported.[37] However, the hydrogel without the elastomeric coating is highly susceptible to dehydration. Thus, the hydrogel requires a protective layer to prevent evaporation of water from the hydrogel.[38] Furthermore it should be pointed out that the capacitance of the ionic hydrogel matrix thin film is directly related to its surface area, which poses a general limitation on the obtainable sensitivity in the previous study. Our pioneering concept herein provides the material design strategies to circumvent the aforementioned shortcomings that affect the stability of ionic hydrogel based device related to dehydration, and the limited pressure sensitivity due to low surface area. We incorporate an ion gel core within a nanofiber supported by a copolymer shell featuring an intrinsically larger surface area and a built in protection layer. In addition, the device design introduced in this work showcases an effective device fabrication strategy including cross functionality with tactile sensing for both static and dynamic pressure as well as power generation capability in a single integrated device system. We have fabricated the core-shell PDMS ion gel/ PVDF-HFP nanofiber mats via electrospinning, which can be used in various pressure ranges for the dual functional applications of tactile sensing for the static and dynamic pressure and power generation. 4
Utilizing the core-shell PDMS ion gel/PVDF-HFP nanofiber mats as a capacitive sensor offers high sensitivity of 0.43 kPa-1 in the pressure ranges from 0.01 kPa up to 1.5 kPa. Compared to a zinc oxide nanowire/poly(methylmethacrylate) film and a PDMS-coated conductive fiber, the sensitivity of the core-shell PDMS ion gel/PVDF-HFP nanofiber mats is 50 and 2 times higher, respectively.[28,
39]
Furthermore, our capacitive sensor has been
demonstrated as a wearable pulse rate detector that serves as a heart-rate indicator. In addition, the core-shell PDMS ion gel/PVDF-HFP nanofiber mats could be used as a self-powered device 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 to 700 kPa and 40 kPa to 100 kPa, respectively. The maximum power density of the core-shell PDMS ion gel/PVDF-HFP nanofiber mats could be brought up to 0.9 W/m2, which is sufficient to light up three hundred light emitting diodes (LEDs). The improvement in the electrical power generated by the core-shell PDMS ion gel/PVDF-HFP nanofiber mats can be explained by an increased amount of inductive charges and capacitance in the triboelectric layer.
2. Materail 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 at room temperature. Subsequently, the PDMS (Sylgard 184) which consisted of liquid components (a mixture of catalyst Pt and prepolymer
dimethylsiloxane
with
vinyl
groups)
and
curing
agent
(prepolymer
dimethylsiloxane with vinyl groups and Si-H groups) (10:1) were added into ionic liquid solution. Finally, tetrahydrofuran (THF) solvent was added to the PDMS with ionic liquid solution in order to form the PDMS ion gel.
5
2.2 Core-shell electrospun PDMS ion gel /PVDF-HFP nanofiber mats: PVDF-HFP (SigmaAldrich, Mw~455,000) was dissolved in a mix solvent DMF/THF (volume ratio=1:1). The concentration of PVDF-HFP was 100 mg/mL. The PVDF-HFP solution was stirred at 60 oC for 6 hrs to obtain a transparent solution. Fig. 1b illustrates the setup for core-shell electrospinning. PVDF-HFP and PDMS ionogel solutions were fed into the outer channel (15 G, OD 1.8 mm, ID 1.4 mm) and inner channel (21G, OD 0.8 mm, ID 1.4 mm) of the spinneret, respectively. At the nozzle tip, the inner channel protruded out of the outer channel by about 0.5 mm. The flow rates of the PDMS ionogel and PVDF-HFP solutions were controlled using syringe pumps at 0.7 mL/hr and 1 mL/hr, respectively. The aluminum foil was used as a collector, which has been placed 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 illustrated in Fig. 2 (a). The device is constructed by a metalinsulator-metal (MIM) capacitor structure where the metal is Cu electrode and the capacitive layer is the core-shell PDMS ion gel/PVDF-HFP nanofiber mat. The device size is 1 cm x1 cm. 2.4 Fabrication of the flexible triboelectric device: As illustrated in Fig. 4 (a), the core-shell PDMS ion gel/PVDF-HFP nanofiber mat with Cu electrode was placed on the polyethylene terephthalate (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 nanofiber 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 2o min-1 from 10 to 80o (2θ). Field emission scanning electron microscopy (FESEM, JEOL 7600F) operating at 5 kV was employed to determine the core-shell PDMS ion gel/PVDF-HFP nanofibers morphology. Detailed structure analyses were performed using transmission electron microscopy (TEM, JEOL2010) operated at 200 kV. The compression 6
test was performed on the core-shell PDMS ion gel/PVDF-HFP nanofiber mats by using Instron 5567. The capacitance measurement from the capacitive pressure sensor was obtained using the LCR meter (Agilent E4980A). To analyze the device sensitivity, the LCR meter was used to detect capacitive changes under various mechanical loads on the device. The voltage outputs from the triboelectric pressure sensor were measured by an oscilloscope (Trektronix, MDO 3024, input resistance = 10 MΩ), the current output was measured by a low-noise current pre-amplifier (Stanford Research System, Model SR570, input resistance = 4 Ω). The 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 prepared using electrospinning. Although PDMS is immiscible with many other ionic liquids, earlier studies indicate the feasibility of using tetraethylorthosilicate (TEOS) as the cross-linking agent to form the hybrid ion gel with various polymers such as PDMS and poly(methyl methacrylate) PMMA through sol-gel reaction.[40, 41] In this study, the PDMS ion gel has been prepared by sol-gel reaction to create the cross-linked PDMS with TEOS and PDMS-OH. The TEOS and PDMSOH are employed as the cross-linking agents between ionic liquid and PDMS polymer. The schematic diagram of the PDMS ion gel preparation is shown in Fig. 1a. The process of preparing the core-shell PDMS ion gel / PVDF-HFP nanofiber mats is found in Fig. 1b. The PVDF-HFP is configured as the shell during electrospinning due to its high molecular weight. The electrospinning of PDMS ion gel nanofibers has been realized through the PDMS prepolymer and the curing agent to form a stable 3D network via covalent bonds.[42] The morphology of electrospinning of core-shell PDMS ion gel / PVDF-HFP nanofiber mats is shown in Fig. 1c. In order to ascertain the formation of core-shell nanofiber structure, transmission electron microscopy (TEM) is employed as shown in Fig. 1d. The total diameter of core-shell PDMS ion gel / PVDF-HFP nanofibers and core PDMS ion gel nanofibers are 7
around 235 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-shell PDMS ion gel / PVDF-HFP nanofibers layer is around 100 μm as shown in Fig. S1b. Infrared spectroscopy has been used to characterize the coreshell PDMS ion gel / PVDF-HFP nanofiber mats as shown in Fig. S1c. Two strong bending bands are observed in the region between 500 and 750 cm-1, corresponding to a SO2 antisymmetric bending (610 cm-1) and another SO2 antisymmetric bending (612 cm-1) from the [EMIM][TFSI].[43] The peaks at 1090 and 1022 cm-1 were assigned to the Si-O-Si of siloxane, which showed evident vibration peaks for the PDMS fiber. The –CH3 rocking and Si-C vibrations appeared at around 802 cm-1.[44] The electrospun core-shell PDMS ion gel / PVDF-HFP nanofibers have a crystalline structure of α and β phases in coexistence. The vibrational bands at 1264 and 842 cm-1 are attributed to the β phases of PVDF-HFP. Several peaks in the region between 1197 and 762 cm-1 are assigned to α phases of PVDF-HFP.[45] The core-shell PDMS ion gel / PVDF-HFP nanofiber mats were made into capacitive pressure sensor in the metal-insulator-metal (MIM) configuration with Cu as the top and bottom electrode as shown in Fig. 2a. Fig. 2b shows the capacitance response of core-shell PDMS ion gel / PVDF-HFP nanofiber mats that are able to sense very small pressure of 0.01 kPa. The capacitance is altered by placing and removing a load cell of 0.01 kPa in Fig. 2b. The mechanism of the capacitance change of the core-shell PDMS ion gel / PVDF-HFP nanofiber mats when the pressure is applied is shown in Fig. 2c. The capacitance of a parallel plate capacitor with area A and thickness d can be written as: , where C is the capacitance, in Farads; A is the area of overlap of the two plates, in square meters; εo is the vacuum dielectric constant (εo~8.54×10-12 F/m); εr is the dielectric constant of
8
the material between the plates; and d is the separation between the plates. The change in the capacitance in the case that a constant pressure is applied allows it to measure static pressure. The dramatic increase of the capacitance of the core-shell nanofiber mats when the external pressure is applied can be attributed to three main factors: (1) the dielectric constant εr is changed when the PDMS ion gel / PVDF-HFP nanofiber mats are compressed since the displaced air has a lower dielectric constant (εr=1.0) than PDMS (εr=3.0) and PVDF-HFP (εr=11.38) as shown in Fig. 2c-(I), (2) when the distance between the top and bottom electrode is reduced (d to d’), the thickness of the elastomeric PDMS inside the core-shell PDMS ion gel / PVDF-HFP nanofibers can be reduced because PDMS possesses good elastic properties 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 (A to A’) due to the reduced spacings as a result of the highly deformable properties of PDMS, which results in an enhanced interfacial capacitance as shown in Fig. 2c-(III). The increase of the dielectric constant and the fiber contact area as well as the reduced distance between the separated electrodes leads to a significant increase of capacitance of the pressure sensor upon compression and therefore a high pressure sensitivity (as shown in next paragraph) in the low pressure range considered in this work. The pressure response curves for varying amounts of ionic liquid loaded core-shell PDMS ion gel / PVDF-HFP nanofiber mats are presented in Fig. 3a. The pressure sensitivity S can be defined as the slope of the curves in Fig. 3a. (S = δ (ΔC /Co) / δp = (1/Co) * δC / δp), where p denotes the applied pressure, and C and Co denote the capacitance with and without applied pressure, respectively. The 40 wt% ionic liquid loaded core-shell PDMS ion gel / PVDF-HFP nanofiber mat exhibits much higher pressure sensitivity than others. In the pressure range of <1.5 kPa, the sensitivity of the sample with 40 wt% ionic liquid loaded core-shell PDMS ion gel / PVDF-HFP nanofiber mat is 71 times higher than pristine PVDF-HFP nanofiber mat due to a larger interfacial capacitance produced at high amount of PDMS ion gel as shown in the 9
Fig. 3a. Compared with previous fiber-based pressure sensor, our proposed pressure sensor exhibits better sensitivity and improved detection limitation.[28, 46-48] Our device shows 12.5 times lower 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 higher than the previously reported value for ZnO-PMMA composite.[39] In addition, our device is able to extend the range of possible measurements up to 10 kPa as shown in Fig. 3b. However, there is a reduction in sensitivity when the pressures are higher than 1.5 kPa. The sensitivity reduction is attributed to the increasing elastic resistance with increasing compression.[2] For real world applications this progressive damping could be desirable, since it increases the range of detectable pressure to the case of high loads in which high sensitivity is not required for the capacitive pressure sensor, and therefore results in a more versatile pressure sensor.[2] The mechanical properties of the pristine PVDF-HFP and different weight percentage of ionic liquid loading PDMS ion gel / PVDF-HFP nanofiber mats are shown in Fig. 3c. Compared to pristine PVDF-HFP nanofiber mat, the 40 wt% ionic liquid loaded core-shell PDMS ion gel / PVF-HFP nanofiber mat exhibits a higher compressive strain at the same compressive stress. It indicates that the higher amount of ionic liquid in the core-shell nanofiber mat leads to a larger deformation upon applied pressure. The nanofibers with a higher amount of ionic liquid loaded PDMS ion gel could have a lower elastic modulus as shown in Fig. S2a. It is noted that the elastic modulus of 10 wt% ionic liquid loaded core-shell PDMS ion gel / PVF-HFP nanofiber mat is dramatically reduced compared to PVF-HFP nanofiber 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 ionic conductivity of the solution could cause large instabilities during the electrospinning process[50] as a high voltage operation (25 kV) is required to fabricate the core-shell PDMS ion gel / PVF-HFP nanofiber mat. The ionic 10
conductivity of different weight percentage of PDMS ion gel was measured by electrochemcial impedance spectroscopy (EIS) with a frequency range of 0.01 Hz-100 kHz. Fig. S2b shows a typical Nyquist plot of the impedance analysis on different weight percentage of PDMS ion gel. At high frequency (~100 KHz), the corresponding value of the intercept on the real axis represents the instrinsic resistance of the ion gel as the ohmic resistance of the testing device is negligible as shown in Fig. S2c.[51] Therefore, the ionic conductivity can be calculated according to the formula:
Where σ is the ionic conductivity, L is the distance between the two electrodes, R is the resistance of ion gel, and S is the geometric area of the electode interface. The ionic conductivity is increased by increasing amount of the ionic liquid as shown in Fig. S2d. The sensitivity, flexibility, and robustness of our device allows it to be utilized as a wrist-based heart-rate monitor. The device was attached directly to the wrist of a living subject to measure the radial arterial pulse wave as shown in Fig. 3d. It is demonstrated that the device can track the number of pulses through the relative capacitance change. The test subject has a stable heartrate of 75 times per minute which is consistent with the counting results and shows that the subject is currently healthy and relax. The video record for the wrist-based pulse wave detector for heart-rate monitoring as shown in the Movie S2 in the Supporting Information. A higher, arrhythmic or otherwise altered heartrate can indicate states of stress and physical exercise or detect early signs of potentially lethal heart defects and diseases. Fig. 3d shows the close up view on a pulse having the characteristic peak typically measured at the radial artery. There are two peaks per pulse. The first peak corresponds to shutting the valves that let blood flow into the heart and the second peak is shutting the valves that let the blood flow from the heart. It has been reported that the radial pulse wave could be a useful index of the arterial
11
stiffness.[52] Therefore, our device has potential for applications in health diagnostics e.g. hypertension, atherosclerosis, heart failure and etc. The PDMS core-shell PDMS ion gel / PVDF-HFP nanofiber mats can be configured into a triboelectric sensor and a self-powered device by the triboelectric effect at high dynamic mechanical pressure. The triboelectric nanogenerator (TENG) device is composed of a polyethylene terephthalate (PET) sheet as a substrate, a spacer, Cu electrode, a Kapton layer and the core-shell PDMS ion gel / PVDF-HFP nanofiber mats as shown in Fig. 4a. The operation principle of a TENG is based on the 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 is no charge induced by the electric potential difference between two electrodes of the core-shell PDMS ion gel / PVDF-HFP nanofiber mats TENG in the initial state. When the external compressive force is applied, the core-shell PDMS ion gel / PVDF-HFP nanofibers and Kapton are brought into contact with each other in the pushed state. Surface charge transfer occurs at the interface as a result of the triboelectric effect. The Kapton film possesses strong negative triboelectric polarity according to the triboelectric series.[53] The positive and negative charges are induced at the core-shell PDMS ion gel / PVDF-HFP nanofiber mats and the Kapton surface, respectively. When the pressure is released, the Kapton and the core-shell PDMS ion gel / PVDF-HFP nanofiber mats surface separate from each other. The dipole moment becomes stronger at this stage. Thus, a strong electric potential difference is generated between the electrodes. The connection to the device (switching polarity) with the measurement equipment is reversed in order to confirm the measured output performance is originated from the TENG and to eliminate the influence of the noise caused by the measurement system as shown in Fig. S3. The connecting configuration that the PDMS core-shell PDMS ion gel / PVDF-HFP nanofiber mats is connected to a positive probe and the Kapton film is connected to a negative probe which is defined as a forward connection. On the other hand, the inverted 12
connection is defined as the device measured with the reverse connection.[54] The output signals were reversed when the device is reversely connected which implies that the output signals were generated by a triboelectric effect. Fig. 4b and c show the generated output voltage and current density for different amounts of ionic liquid loaded in the core-shell PDMS ion gel / PVDF-HFP nanofiber mat under a dynamic mechanical pressure up to 700 kPa at an applied frequency of 5 Hz. The output voltage and current density increase with an increasing amount of ionic liquid loading in the PDMS ion gel, which implies that the electrical double layer formed by electrochemical effect in the ion gel nanofibers plays an important role in increasing the amount of induced charge and the capacitance of the triboelectric layers, resulting in an improvement of the output performance of TENGs.[45] At higher amount of ion gel, the two interfaces could stick together, which causes the charge to rebalance during the releasing state. Therefore, we have selected 40 wt% ionic liquid loaded core-shell PDMS ion gel / PVDF-HFP nanofiber mats for further study. Several reports have demonstrated that triboelectric nanogenerator can be utilized as dynamic pressure sensing.[14-19] However, the tribo based sensors encounter the limit of saturated output voltage at high pressure range. Our previous study has proved that selfpolarized polyvinydifluoride-trifluoroethylene (PVDF-TrFE) sponge could be utilized for ultra large range pressure detection.[17] It is interesting to investigate the triboelectric pressure sensor at higher pressure range. The output voltage signals of TENG under higher pressure range are shown in Fig. 5a. The output voltage is increased by increasing the dynamic mechanical pressure. The pressure sensitivity of the triboelectric nanogenerator-based pressure sensor was calculated from the slope of the voltage response curve (S=d(ΔV/Vs)/dP, where ΔV is the relative change in the output voltage, Vs is the final saturation voltage, and P denotes the applied pressure. The sensitivity of the 40 wt% ionic liquid loaded core-shell PDMS ion gel / PVDF-HFP nanofiber mats is 0.068 V∙kPa-1 from 100 kPa to 700 kPa as shown in Fig. 5b. The sensitivity of the core-shell PDMS ion gel / PVDF-HFP nanofiber mats 13
is comparable to the polarized PVDF-TrFE sponge.[17] The higher sensitivity of core-shell PDMS ion gel / PVDF-HFP nanofiber mat is attributed to the increasing inductive charges, capacitance and nanofibers surface area. In the pressure range from 40 kPa to 100 kPa, the sensitivity 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/PVDFHFP nanofiber mat could be used not only as a triboelectric sensor but also as a self-powered device. External loads with varying resistance can be connected to TENGs for different applications. The systematical study of output performance with different external loads of varying resistance is shown in Fig. 4Sd. The output voltage increases from 0.1 to 31 V by increasing the resistance of the load from 1 KΩ to 100 MΩ. The output current decreases slightly from 3.92 to 3.06 μA. The instantaneous power density can be calculated by P=V2/R. The maximum output power obtained at different resistance is shown in Fig. 5c. The maximum power density can reach up to 0.9 W/m2 at a load of 10 KΩ. The power density of the core-shell PDMS ion gel / PVDF-HFP nanofiber mat is 3 times higher than those reported on silicone rubber based TENGs.[55, 56] The core-shell PDMS ion gel / PVDF-HFP nanofiber mat allows a scalable design of the device sample. An array of 300-LED can be powered instantaneously by the output generated from the TENG as shown in Fig. 5d. The video record for lighting up the LED arrays is shown in the movie (Movie S2 in the Supporting Information). It is noteworthy that our device operates in a stable output performance even after a shelf life of six months, indicating the robustness of the materials and device configuration as shown in Fig. 5S.
4. Conclusions In summary, the core-shell PDMS ion gel / PVDF-HFP nanofiber mats have been fabricated successfully by electrospinning. The core-shell PDMS ion gel / PVDF-HFP nanofiber mats have been demonstrated as tactile pressure sensor to detect static and dynamic pressure. The 14
core-shell PDMS ion gel/PVDF-HFP nanofiber mats can be used as a capacitive tactile sensor which offers higher sensitivity of 0.43 kPa-1 in the low pressure regime range up to 1.5 kPa, which has been demonstrated as a wearable heart-rate monitor. Furthermore, the core-shell PDMS ion gel/PVDF-HFP nanofiber mats can be used in a triboelectric sensor and selfpowered 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 sensor exhibits high sensitivity of 0.102 V∙kPa-1 and 0.068 V∙kPa-1 at higher pressure 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-HFP nanofiber mat could be brought up to 0.9 W/m2, which is sufficient to light up hundreds of light emitting diodes (LEDs).
Acknowledgement This work was supported by the National Research Foundation Competitive 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. L wrote the paper.
Apprendix A. Supporting Information Supplementary data associated with this article can be found in the online version at
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Fig. 1. (a) Schematic diagram of the PDMS ion gel preparation (b) illustration of 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 nanofibers mats (d) TEM image of core-shell PDMS ion gel / PVDF-HFP nanofibers
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Fig. 2. (a) Device configuration of pressure sensor for the core-shell PDMS ion gel / PVDFHFP nanofibers mats (b) pressure sensor repeated real-time responses at 0.01 kPa (c) The mechanism of the core-shell PDMS ion gel / PVDF-HFP nanofibers mats when the pressure is applied (I) the dielectric constant is changed when the nanofiber mats are compressed (II) the top/bottom electrode distance is reduced (III) the fiber contact area is increased.
20
Fig. 3. (a) The maximum slope of the relative capacitance changes of the core-shell PDMS ion gel / PVDF-HFP nanofibers 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 nanofibers mats. (c) The stress-strain curves of the pristine PVDF-HFP nanofibers and different amount to PDMS ion gel / PVDF-HFP nanofibers mats (d) the real time pressure waveforms of the measured heart rate. The inset shows that the device has been mounted onto the wrist and the zoomed view on a pulse having the characteristic peak typically measured at the radial artery.
21
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 for electric 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
22
Fig. 5. (a) The output voltage of TENG at pressure range 100 to 700 kPa (b) Voltageresponse curves of the 40 wt% ionic liquid loaded core-shell PDMS ion gel / PVF-HFP nanofiber mats output voltage at high pressure range (100 to 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 PDMS core-shell PDMS ion gel / PVDF-HFP nanofiber mats (device size: 5 cm*5 cm).
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Dr. Meng-Fang Lin is a senior research fellow in the School of Materials Science and Engineering, Nanyang Technological University (NTU), Singapore. She received her Ph. D. degree in School of Materials Science and Engineering, Nanyang Technological University (NTU), Singapore. She did postdoctoral research with Dr. Tsukagoshi Kazuhito at National Institute for Materials Science (NIMS), Japan (2013-2014). Her current research interests focus on pressure sensor and triboelectric energy harvesting.
Dr. Jiaqing Xiong received his Ph.D. degree from Soochow University in Department of Textile Engineering in 2015. Now he is a research fellow in Prof. Pooi See Lee’s group at the School of Materials Science and Engineering, Nanyang Technological University. His recent research interest focuses on the biomass materials functionalization, smart materials and wearable devices, energy conversion and harvesting. Especially, eco-friendly hydrophilic/hydrophobic materials, triboelectric energy harvesting.
Dr. Jiangxin Wang is a research fellow in the School of Materials Science and Engineering, Nangyang Technological University (NTU), Singapore. He obtained his 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 Science and Engineering, Nangyang Technological University (NTU), Singapore. His current research interests focus on deformable optoelectronic devices.
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Mr. Kaushik Parida received his master's degree from the School of Metallurgical Engineering and Materials Science of Indian Institute of Technology Bombay, India in 2013. He is currently pursuing his doctoral degree under the supervision of Prof. P.S. Lee at the School of Materials Science and Engineering in Nanyang Technological University, Singapore. His research focuses on piezoelectric polymers for application in piezoelectric and triboelectric energy harvester, self-powered devices and soft electronics.
Prof. Pooi See Lee received her Ph.D. from National University of Singapore in 2002. She joined the School of Materials Science and Engineering, Nanyang Technological University as an Assistant Professor in 2004. She was promoted to tenured Associate Professor in 2009 and full Professor in 2015. Her research focuses on nanomaterials for energy and electronics applications, flexible and stretchable devices, electrochemical inspired devices, and human-machine interface. She received the National Research Foundation Investigatorship Award and the Nanyang Research Excellence Award in 2015.
Highlights
Core-shell nanofiber mats of PDMS ion gel /PVDF-HFP were successfully prepared by incorporating cross-linking agent during electrospinning.
The electrospun nanofibers were used to fabricate pressure sensors to detect the static and dynamic pressures by harnessing the capacitance changes and triboelectric effects judiciously.
The sensitivity, flexibility, and robustness of our capacitive pressure sensor allow it to be utilized as a wrist-based pulse wave detector for heart-rate monitoring.
The maximum power density of the core-shell PDMS ion gel/PVDF-HFP nanofiber mats could be brought up to 0.9 W/m2, which is sufficient to light up hundreds of light emitting diodes (LEDs).
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