Flexible and Washable Poly(Ionic Liquid) Nanofibrous Membrane with

Jul 8, 2019 - Real-life wearable electronics with long-term stable sensing performance are of significant practical interest to public. Wearable press...
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Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Flexible and Washable Poly(Ionic Liquid) Nanofibrous Membrane with Moisture Proof Pressure Sensing for Real-Life Wearable Electronics Zehong Wang,†,‡ Yang Si,§ Cunyi Zhao,† Dan Yu,‡ Wei Wang,‡ and Gang Sun*,† †

Fiber and Polymer Science, University of California, Davis, California 95616, United States Key Laboratory of High-Performance Fibers & Products, Ministry of Education, College of Chemistry, Chemical Engineering and Biotechnology and §Key Laboratory of Textile Science and Technology, Ministry of Education, College of Textiles, Donghua University, Shanghai 201620, China

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‡

S Supporting Information *

ABSTRACT: Real-life wearable electronics with long-term stable sensing performance are of significant practical interest to public. Wearable pressure sensors with washable, comfortable, breathable, and stable sensing ability are a key requirement to meet the desire. However, effects of ubiquitous ambient moisture and intrinsic defects of current capacitive sensing materials are two factors leading to unstable sensing performance of current pressure sensors. Existing ionic liquid-based materials (i.e., ionic hydrogel, ionic film, or ionic/ elastomers composite) have been used for efficient capacitive pressure sensing but are highly sensitive and especially affected by moisture. In this work, we introduce a washable capacitive pressure-sensing textile based on the use of a hydrophobic poly(ionic liquid) nanofibrous membrane (PILNM) with good mechanical properties and satisfactory moisture proof sensing performance. The PILNM membranes possessing rich ions and microporous structures are novel ideal polymeric dielectric materials for amplification of signals with negligible stimulations. Moreover, the PILNMs exhibit very high stable sensing signals under moisture interference (up to 70% relative humidity) and repeated washings (more than 10 washings), especially suitable for wearable electronics. Notably, the PILNM-based wearable pressure-sensing textiles offer high sensitivity for low pressure and bent chord length changes with a low-pressure detection limit even under harsh deformations. Owing to the superior performance, the PILNM-based wearable pressure-sensing textiles are comfortable to wear and suitable for monitoring different human motions and pulse vibrations at various body positions. Meanwhile, the assembled multiple wearable pressure-sensing array can spatially map the contact area of the pressure stimuli and synchronously reflect finger movements. KEYWORDS: poly(ionic liquid), dielectric, nanofibrous membrane, pressure sensing, textiles

1. INTRODUCTION The emerging wearable electronics have been studied for several decades and utilized in monitoring human health and flexible or soft machines,1−3 benefiting from their fascinating features of good flexibility,4 imperceptible weight,5 high sensitivity,6 biocompatibility,7 and multiple sensory capabilities.8 The key component of wearable electronics is a flexible pressure sensor, which can feel and respond to surrounding stimuli so well as to enable mimicking characteristics of human skin. Existing pressure-sensing mechanisms include triboelectric,9,10 piezoelectric,11,12 piezoresistive,13,14 and capacitive types.15,16 Among them, the capacitive pressure sensors exhibited high sensitivity, fast response time, and facile device construction as well as low power consumption,14,17,18 advantageous than both piezoresistive19 and piezoelectric sensors.20 A basic capacitive pressure sensor is typically made from parallel conductive electrodes and a dielectric layer © XXXX American Chemical Society

accumulating charges. The sensing mechanism is based on variation of capacitance under external forces, which cause changes of contact area, thickness, and dielectric constant of the dielectric layer between the electrodes. Meanwhile, as wearable capacitive pressure-sensing products, the sensors should be flexible, comfortable, breathable, washable, and durable. Moreover, ubiquitous noise sources from environment, such as humidity influence and intrinsic defects of dielectric materials, including poor mechanical property and low polarized performance, could greatly impede accuracy and reliability of the capacitance measurements, bringing a huge hindrance to the successful development and application of wearable capacitive pressure sensors. Thus, design and Received: May 4, 2019 Accepted: July 8, 2019 Published: July 8, 2019 A

DOI: 10.1021/acsami.9b07786 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

imidazolium bis(trifluoromethanesulfonyl)imide) ([PBVIm] [TFSI]) was used to serve as the main ion-conductive component due to the feature of a substantial number of polarizable ions and potential to form high capacitance and high sensitivity for pressure sensing. In addition, [PBVIm] [TFSI] is water insoluble and more hydrophobic than regular poly(ionic liquids). Thus, the fabricated microporous and nanofibrous PILNMs will be immune to ambient interference. Finally, PILNMs with high dielectric property and robust mechanical performance were developed and wearable capacitive pressure-sensing textiles were assembled through flexible electrodes sandwiched with the PILNM inside. This work opens a new way to use poly(ionic liquid) nanofibers in various wearable electronic devices.

fabrication of an all fabric-based wearable pressure-sensing device with high sensitivity and immunity from external distractions are challenging and a goal of the research. Ionic conducting materials use ions as current carriers, such as ion gels and ionic liquids,21,22 and have found applications in the wearable sensing devices. Incorporation of ionic transporting structures into polymer chains can achieve an ionic conducting material with good dielectricity and desirable mechanical performance. The same idea has been explored by researchers23−25 to develop hydrogel- or film-based ionic elastic polymers to meet requirements of high mechanical stability and high conducting property. Others26−29 directly combined single, binary, or trinary networked polymer matrixes with mobile ionic liquid monomer into elastic dielectric materials to fabricate hybrid ionic dielectrics. However, hydrogel-based ionic conductive materials are susceptible to dehydration without good encapsulation, leading to nondurable sensing. In addition, mobile ionic liquid monomers added in polymer matrix are toxic and miscible with water and organic solvents30 and easily run away, resulting in reduced sensitivity and durability. Furthermore, hydrogel- or film-based ionic conductive materials are thick or stiff and uncomfortable when used in wearable sensing devices. Recently, poly(ionic liquid) materials have been prepared,31 possessing good ionic conducting performance due to substantial amounts of mobile anions and polarizable cations, as well as potential desirable mechanical properties. The remarkable ionic structures and unique features of the polymers provide a promising base as capacitive-based sensing materials. Moreover, if the ionic materials could possess additional structural features, including microporous,32 microarrays,6,16 and hierarchical microarray,33 high specific area polarization and consequently high sensitivity could be achieved on the materials. Thus, we designed and speculated a novel poly(ionic liquids) microporous nanofibrous membrane structure as the sensing component for novel pressure sensors. Currently, some nanofibers containing ionic liquid monomers have been reported, such as a core−shell nanofiber sensor,34 by incorporation of imidazole-based ionogel in poly(vinylidene fluoride-co-hexafluoropropylene) (PVDFHFP) nanofiber matrix to obtain a capacitive-based pressure sensor with sensitivity of 0.43 kPa−1 and an iontronic nanofiber sensor15 fabricated by blending imidazole-based ionic liquid monomer with PVDF-HFP, generating a wearable pressure sensor with a sensitivity of 114 nF kPa−1. To easily prototype the designed membranes, electrospinning of the poly(ionic liquid) materials was considered as an optional process.35 Herein, we present first the fabrication of poly(ionic liquid) nanofibrous membranes (denoted as PILNM) and subsequently application in development of durable wearable pressure-sensing devices. In the fabrication process, variation of anionic structures of poly(ionic liquid) will result in changes of hydrophilicity characteristics of the electrospun PILNMs. Moreover, the polymerized ionic liquids consist of rigid molecular chains36 and lack a unique viscoelastic behavior due to electrostatic interactions between ionic groups,37 necessary to form fine and soft fibers. The flexible and linear macromolecular polyacrylonitrile (PAN) was used to assist the formation of the poly(ionic liquids) to nanofibers. The addition of PAN into the poly(ionic liquids) could alter ratios of ionic liquids in the fibers and vary morphologies of the membranes, depending on the weight ratio of poly(ionic liquid) and PAN. In this work, poly (1-butyl-3-vinyl-

2. EXPERIMENTAL SECTION 2.1. Fabrication of Poly(ionic liquid) Nanofibrous Membranes (PILNM). Poly(ionic liquid) nanofibrous membranes were prepared by electrospinning of polymeric ionic liquids. The poly(ionic liquids) were synthesized through a free radical polymerization process. Details can be found in Figures S1 and S2 and the Supplement Methods section in the Supporting Information. To prepare PILNMs with maximum ion content, poly(1-butyl-3-vinylimidazolium bromide) ([PBVIm][Br]) and poly (1-butyl-3-vinylimidazolium bis(trifluoromethanesulfonyl)imide) ([PBVIm] [TFSI]) were dissolved in dimethylformamide (DMF, Sigma-Aldrich) for 30 min at 70 °C with different concentrations (5, 10, 20, and 30 wt %), in which only [PBVIm][Br] solution with 30 wt % can generate nanofibrous membranes. Therefore, polyacrylonitrile (PAN, SigmaAldrich) was added into the [PBVIm] [TFSI]/DMF solution as an electrospinning auxiliary. The weight ratio between [PBVIm][TFSI] and the solvent DMF was maintained at 1:12 (wt/wt). PAN was then added to the solution in weight ratios of 0.5:1, 1:1, and 1.5:1 relative to [PBVIm][TFSI], in which both [PBVIm][TFSI] and PAN were dissolved completely in DMF for 4 h in the following experiments. The electrospinning process was performed with an applied high voltage of 23 kV and a syringe capped with an 18-G metal needle with a constant feed rate of 0.5 mL h−1. The as-spun nanofibers were collected as a membrane form on an aluminum foil-covered metallic rotating roller at a rotation rate of 10 rpm with a 14 cm spinneret− collector distance. 2.2. Fabrication of Single Wearable Pressure-Sensing Textiles and Multipixel Arrays. A basic assembly method for parallel capacitors was used to fabricate wearable pressure-sensing textiles. Conductive fabrics were used as flexible electrodes, which were prepared by electroless plating of copper & nickel onto polyester fabrics (see details (Table S2) in the Supplement Methods section in the Supporting Information). For single pressure-sensing textiles, two pieces of the conductive fabrics (thickness in 150 μm, size of 2.5 × 2.5 cm2) and cotton fabrics (size of 3 × 3 cm2) were cut, respectively. Then, the conductive fabrics were separately glued on the cotton fabrics by Elmer’s bundle liquid school glue. Afterward, two copper wires were tightly adhered to the opposite ends of the conductive fabrics by Tensive conductive adhesive gel (Parker Laboratories, 50 g) and the fabrics were assembled as flexible electrodes. Meanwhile, the prepared [PBVIm][TFSI]-based PILNMs (thickness in 76 μm) were cut into square shapes (3 × 3 cm2) as dielectric layer materials of the pressure-sensing device and placed between the two flexible electrodes into a sandwich-like pressure sensor. Finally, 3M semitransparent adhesive tapes were utilized to encapsulate the device to minimize the ambient environment effect on the capacitance signals. Preparations of multipixel sensor arrays were same as that of the single-unit pressure-sensing device. The only difference was the small size of every single sensor (1.5 × 1.5 cm2) unit in the arrays. We utilized nine of the sensing units in the same size to integrate into a 3 × 3 pixel array, serving in measurements of large area and pressure mapping. B

DOI: 10.1021/acsami.9b07786 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Figure 1. (a) Fabrication process of hydrophobic poly(ionic liquid) nanofibrous membrane: first is the water-soluble polymeric ionic liquid [PBVIm][Br], second is the precipitate of water insoluble ionic polymer [PBVIm][TFSI]. (b) The optical image of 67% [PBVIm][TFSI]-based nanofibrous membrane and its water contact angle. Scanning electron microscopy (SEM) images of electrospun nanofibrous membranes with weight contents of [PBVIm][TFSI] from (c) 67% to (d) 50% to (e) 40%. 2.3. Characterization. The microscopic structure and thermal properties of PILNMs were examined by an electron scanning microscope (Philips FEI XL30) and a thermal gravimetric analyzer (TGA-60, Shimadzu). Dynamic strain characteristics were evaluated by a PerkinElmer dynamic mechanical analyzer (DMA 8000). Fourier transform infrared spectroscopy spectra (FTIR) were obtained by using Fourier transform infrared spectroscopy (Nicolet 6700, Thermo Scientific). A resistivity Meter (Portable - Surface - model 272A) was used to measure the surface resistance of PILNMs at different temperatures. All electrochemical measurements were carried out in a three-electrode system via a screen-printed electrode (SPE, SPEnsor CL 10) involving the immersion of PILNM@SPE into a 1 M NaCl solution, in which, the working electrode was prepared by coating the PILNM on the carbon electrode of the SPE through ion-blocking conductive adhesive (Silver conductive adhesive 478SS 12685-15). Ag/AgCl was the reference electrode, and carbon served as a counter electrode. Electrochemical impedance spectroscopy (EIS) tests were carried out in a frequency range of 0.01 Hz−10 kHz with an AC amplitude of 50 mV. The capacitance characters and output voltages of the PILNM-based wearable pressure-sensing device were determined by an LCR meter (M4070 Auto Ranging LCR Meter) and a potentiostat/galvanostat meter (263A, EG&G Princeton Applied Research).

ambient moisture on the surface of the electrospun [PBVIm][Br]-based microfibrous membrane and subsequent decrease in surface resistance with the increase of relative humidity, as shown in Figure S3e. Moreover, the [PBVIm][Br]-based microfibrous membrane could become partially dissolved (swollen) by increasing moisture, as shown in Figure S3f, and the surface resistance decreased by 3 magnitudes of order if changing relative humidity of the environment from 30 to 70%. In addition, the [PBVIm][Br]-based microfibrous membrane could be dissolved in water in 4 s (Figure S3g). To further improve the moisture proof performance of the nanofibrous membrane produced from the poly(ionic liquid), exchange of Br− in water-soluble [PBVIm] [Br] with hydrophobic trifluomethanesulfonic anion [TFSI] in lithium (trifluoromethanesulfonyl) imide (LiTFSI) can prepare water insoluble poly (1-butyl-3-vinylimidazolium bis(trifluoromethanesulfonyl)imide) (denoted as [PBVIm] [TFSI]),38 as shown in Figure 1a. Subsequently, hydrophobic PILNMs were made from electrospinning of a mixture of the water insoluble [PBVIm][TFSI] with polyacrylonitrile (PAN) in DMF solution. Finally, the electrospun [PBVIm][TFSI]based nanofibrous membranes exhibited hydrophobic performance and were used to assemble capacitive pressure-sensing textiles. Figure 1b shows optical photos of flexible and dried nanofibrous membranes made from 67% [PBVIm][TFSI] and water contact angles (Figure 1b) on it, revealing the hydrophobic nature of the PILNM. Figure 1c−e are SEM images of the electrospun nanofibrous membranes with weight contents of [PBVIm][TFSI] varied from 67% to 50% to 40%, respectively. The microstructure of the PILNM containing 67% [PBVIm][TFSI] (Figure 1c) is very fine (213 nm) and uniform with interconnected network structure, and the one containing 50% of the ionic polymer (Figure 1d) exhibits a

3. RESULTS AND DISCUSSION 3.1. Preparation of Poly(ionic liquid) Nanofibrous Membranes (PILNMs). To fabricate ionic nanofibrous materials, pure water-soluble poly (1-butyl-3-vinylimidazolium bromide) (denoted as [PBVIm] [Br]) was dissolved in DMF to prepare electrospinning solutions. Pure [PBVIm][Br]-based microfibers membrane was fabricated by electrospinning of 30 wt % [PBVIm][Br]/DMF solution, as shown in Figure S3a,b in the Supporting Information. The average diameter of the [PBVIm][Br]-based microfiber membrane was 1.12 μm (Figure S3c,d), exhibiting a resistance of 59.8 MΩ cm−1 at room temperature. This was caused by the absorption of C

DOI: 10.1021/acsami.9b07786 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 2. Characterizations of the hydrophobic PILNMs: (a) FTIR spectra, (b) thermogravimetric analysis (TGA) results, and (c) frequency dependencies of storage modulus results. (d) Plot of surface resistance of different hydrophobic PILNMs versus temperature variation. (e) Nyquist plot of the PILNMs from 0.1 Hz to 10 kHz at room temperature. (f) Schematic illustration of the PILNM@screen-printed electrode involved in the electrolyte and its corresponding equivalent electrical circuit. (g) density functional theory (DFT)-optimized structures and electrostatic potential maps (ESP) of ionic liquid [BVIm][Br], [BVIm][TFSI], and acrylonitrile ([AN]). All DFT calculations were carried at the B3LYP/6-31G(d,p) level of theory. (h) Schematic illustration of a self-assembled PILNM-based wearable capacitor and its equivalent electrical circuit. (i) The parallel capacitance of the PILNM-based flexible capacitors with different [PBVIm][TFSI] contents versus density of the PILNM.

[PBVIm][TFSI]. Among them, the peak at 1633 cm−1 is assigned to C−N of [PBVIm][TFSI] and bands located at 1357 and 1198 cm−1 are attributed to asymmetric and symmetric stretching vibrations of OSO group of TFSI−, respectively.39,40 Besides, the peak at 1051 cm−1 is the characteristic stretching vibration of S−N−S in TFSI anions.39 The FTIR results confirmed retention of a mixture of both [PBVIm][TFSI]/PAN solution in the electrospun hydrophobic PILNMs. Moreover, TGA results in Figure 2b show different thermal decomposition temperatures under a N2 atmosphere for the electrospun hydrophobic PILNMs with different [PBVIm][TFSI] contents. The detailed thermal decomposition process and the weight ratios of [PBVIm][TFSI] with PAN can be divided and calculated from DrTGA, as shown in Figure S5 and Table S1 in the Supporting Information. The membranes containing 67 and 40% [PBVIm][TFSI] exhibit two obvious decomposition steps, but the one with 50% [PBVIm][TFSI] shows a tangled DrTGA result and higher thermal stability than other PILNMs. This was due to the immiscible mixtures of [PBVIm][TFSI] with PAN at the weight ratio of 1:1. These results indicate that PAN could affect the orientation arrangement and decrease the rigidity of [PBVIm][TFSI] molecular chains and thereby reduce thermal stability of the hydrophobic PILNMs, as well as improve the softness of the PILNMs. Moreover, Figure 2c

more open microstructure (674 nm), whereas the one with 40% of the ionic polymer (Figure 1e) shows uniform but very coarse fibers (2.4 μm). Details of fiber diameter distributions are shown in Figure S4. Obviously, different concentrations and contents of [PBVIm][TFSI] in its mixtures with PAN in DMF solutions caused the entirely different microstructures of the PILNMs. Utilization of the water insoluble [PBVIm][TFSI] to fabricate hydrophobic and highly polarized nanofibrous membranes could present an ideal high dielectric layer with moisture proof properties. Besides, the electrospinning process brings in several unique advantages in fabrication of the PILNM-based dielectric layer and pressure-sensing textiles. First, this method can directly produce microporous and nanofibrous structures with a highly polarizable ionic dielectric layer with ultrahigh surface charges, tunable composition, and moisture immunity. Second, the produced nanofibrous membranes can be easily embedded into clothing structures to provide stability and robust wearable pressure-sensing performance. 3.2. Characterization and Basic Conducting Performance of Electrospun Hydrophobic PILNMs. Figure 2a shows the FTIR spectra of the electrospun hydrophobic PILNMs. The peak at 2241 cm−1 is assigned to the −CN of PAN. Moreover, the infrared bands at 1633, 1357, 1198, and 1051 cm−1 are considered as the characteristic peaks of D

DOI: 10.1021/acsami.9b07786 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces Table 1. Fitting Parameters for the Equivalent Circuit by the Demo of the EIS Results 67% PILNM 50% PILNM 40% PILNM

2

Rs (Ω cm2)

Cdl (μF cm‑2)

Rct (Ω cm2)

Q (CPE) (10−7 s secn/cm )

n

Ri (103 Ω cm2)

W (10−5 Ω cm2)

χ-squared error (10−3)

807.8 282.6 70.74

0.94 0.40 0.80

6359 17 730 8662

20.16 8.92 15.53

0.67 0.83 0.76

208.7 48.75 61.66

3.709 5.16 0.57

1.73 2.04 2.75

Figure 3. Evaluation of sensitivity of PILNM-based wearable pressure-sensing textiles. (a) Schematic illustration of the configuration of wearable pressure-sensing textiles and the real-time measurement system. (b) Recorded capacitance variation (ΔC/C0)−pressure relationship of the PILNM-based wearable pressure-sensing textiles. (c) Capacitance variation as a function of time at different pressures for a range of low pressures (20 Pa) to high pressures (5 kPa). (d) Comparison of recently reported fiber-based capacitive pressure sensors. (e) Durability test under a pressure of 0.5 kPa for more than 300 times. (f) Quantitative analysis of capacitance variation under different bending chord lengths from a flat to folding state and then recovery to the flat state. (g) Capacitance changes under multiple cycles of different bending chords. (h) Frequency-dependent behavior of the wearable pressure-sensing textiles under various elbow motion frequencies (0.2, 0.5, 1, and 2 Hz). (i) Response time of pressuresensing textiles upon repeated pressure on/off.

× 108, 2.3 × 108, and 2.1 × 108 Ω sq−1, respectively, at room temperature (23 °C), indicating that the hydrophobic PILNMs are dielectrics. With the temperature increased, the electric conductive performance of the PILNMs became better and the surface sheet resistance turned out to be an order of magnitude smaller. When the temperature was increased up to 110 °C, the 67% [PBVIm][TFSI] PILNM showed the lowest surface resistance of 3 × 106 Ω sq−1, while both 50 and 40% [PBVIm][TFSI] PILNMs exhibited higher values of 1.3 × 108 and 7 × 107 Ω sq−1, respectively. Higher temperature will contribute to more intense movements of polymer chains and ions, resulting in easier ion transport and higher electric conducting performance. Moreover, this result also proves that the PILNMs are ion-conducting materials and the ions inside of the membrane could be polarized or transported along the main chain under an external field. To further understand the influence of [PBVIm][TFSI] concentrations on morphology of the hydrophobic PILNMs,

displays that the 67% [PBVIm][TFSI] PILNM exhibited a high and stable Young modulus, above 30 MPa, due to its possible network structure, whereas PILNMs containing both 50 and 40% [PBVIm][TFSI] showed lower Young’s modulus of about 18 and 3 MPa, respectively. The loss of the modulus and damping ratio of the PILNMs can be found in Figure S4 in the Supporting Information, revealing that the increased PAN content will contribute to the softness of the electrospun hydrophobic PILNMs. But excess PAN will cause a coarse fiber and lower polarization of the PILNMs. Large amounts of ions existing in the polymeric materials are good for ionic conduction and high polarization when an external electric field is applied at room temperature. The prepared PILNMs consist of ultrahigh ion contents, which are easily polarized to show a good dielectric property. As shown in Figure 2d, the surface resistances of the PILNMs have been measured at different temperatures. The surface sheet resistances of the PILNMs containing 67, 50, and 40% [PBVIm][TFSI] are 3.9 E

DOI: 10.1021/acsami.9b07786 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

of the PILNMs in wearable pressure-sensing textiles were evaluated by a facile assembly of cotton fabric-and conductive PET fabric-sandwiched PILNM (67% [PBVIm][TFSI]), as shown in Figure 3a. The electric conductive performances of the conductive PET fabrics are showed in Table S2 in the Supporting Information. Owing to the softness of all layers in the wearable pressure-sensing textiles, it can be worn like comfortable cloth, as shown in the optical picture of Figure 3a. To explore the response of the prepared sensors to external pressures, the capacitances of the wearable device to varied applied pressures were measured. The pressure applied on the device was a random value ranging from 0 to 5 kPa given by Instron 5566, and the corresponding capacitances were continuously recorded. The capacitance (C)-to-pressure (P) curves at different external pressures are shown in Figure 3b. The sensitivity of the PILNM-based pressure sensor is defined by an equation S = δ(ΔC/C0)/δP, where C0 is the initial capacitance (∼44.5 pF at the test frequency of 0.5 MHz), ΔC is the relative capacitance change (C − C0), and P is the applied external pressure. The sensitivities of the PILNMbased pressure sensor are 0.49 kPa−1 below 0.2 kPa (R2 = 0.99) and 0.18 kPa−1 in a pressure range of 0.2−10 kPa (R2 = 0.98), as shown in Figure 3b. These results demonstrate that the PILNM-based pressure sensors have high sensitivity for tiny amounts of pressure changes and can be possibly used to monitor human skin deformations. The excellent performances of this sensor and comparison with recently reported fiberbased capacitive wearable pressure sensors are summarized in Figure 3d and Table S4. The points (blue triangle) closer to the upper-left area of Figure 3d exhibit a higher sensitivity and fast response time of the pressure sensor, showing the performances of the PILNM-based pressure sensor (this work) are better than these recently reported fibers-based pressure sensors at the pressure range lower than 0.2 kPa. Figure 3c depicts the capacitance response of the PILNMbased pressure sensor to different pressure at 0.2 Hz, which can accurately recognize the low pressure (20 Pa) to high pressure (4.9 kPa) with a quiet similar stable waveform. The durable cyclic pressure test results are shown in Figure 3e, revealing that no obvious degradation of capacitance change (ΔC) occurred after more than 300 cycles of repeated loading on/off under a pressure 0.5 kPa, confirming the reproducibility and durability of the PILNM-based pressure sensor. To further investigate the wearable applications, the pressure sensor was subjected to static bending and the corresponding capacitance variation to different bending chord lengths is shown in Figure 3f. The details of the bending process are shown in Figure S6. Under static bending at a bending chord length range from the initial flat state (chord length at 4.2 cm, width at 1 cm, and thickness at 376 μm) to extreme bending state (chord length at 1 cm) and then recovery to flat state, the capacitance responses to the different bending chord lengths were significantly different, as shown in Figure 3f. According to the sensitivity of capacitance to pressure, the bending sensitivity can be measured by using the same equation mentioned above with the pressure applied (P) replaced by the bending chord length (d), defined as: S = δ(ΔC/C0)/δd. The results revealed a high sensitivity of 0.9 cm−1 for bending and 0.85 cm−1 for recovery for the sensor. Figure 3g shows the capacitance increase as the bending chord length decreases from 3.4 to 2 cm under static bending. At a given bending chord length, the capacitance waveform after three bending cycles was stable. The capacitance change to bending was

an alternating-current impedance (EIS) measurement was carried out through a PILNM@ screen-printed electrode (SPE). The area of the PILNM is 0.2 cm2, and it was glued onto the screen-printed electrode (SPE) by an ion-blocking Ag adhesive (Figure 2f). As shown in Figure 2e, the PILNMincorporated SPE (PILNM@SPE) exhibited impedance frequency dependency, reflecting a Nyquist plot with semicircles. By the analysis of an impedance Demo program (ZSimDemo 3.30d), the results of the impedance of the SPE fitted very well to a charge-transfer process and equivalent circuit model, shown in Figure 2f. As shown in Table 1, Rs is the solution resistance of the electrolyte filling in pores of the microporous PILNMs. With the content of [PBVIm][TFSI] in the PILNMs decreased from 67 to 40%, the density of the PILNMs decreased from 0.21 to 0.073 g cm−3 (Figure 2h), causing higher electrolyte adsorption on the surfaces and therefore reduced Rs. Moreover, Cdl is the double layer capacitance caused by the ions of the electrolyte aligned along the fibrous surfaces of the PILNM. The PILNM containing 67% [PBVIm][TFSI] exhibited a higher Cdl value of 0.94 μF due to the higher amount of poly cations and anions inside PILNM. In addition, Ri and W represent the Warburg impedance, attribute of the diffusion of ions. The interconnected PILNMs (67% [PBVIm][TFSI]) showed a highly microporous structure and resistance to the ion diffusion, resulting in a high Ri (208.7E3 Ω cm2). Electrochemical impedance spectrometry (EIS) and the equivalent circuit demo indicated that the PILNM containing 67% [PBVIm][TFSI] was the most efficient polarizable nanofiber due to the high amount of ions inside and on the large specific surface area of the membranes. The dielectric performance of the PILNM (67% [PBVIm][TFSI]) can be characterized by assembling a flexible capacitor by using PILNM as a dielectric layer in a sandwiched structure consisting of conductive fabrics as the electrodes, shown in Figure 2h. The parallel capacitance of every PILNM-based capacitor was tested by an LCR meter at 0.5 MHz. It can be seen from the results (Figure 2i) that the capacitance of the capacitor made from using the PILNM (67% [PBVIm][TFSI]) is 44.46 pF, which is 6 times larger than the one containing 40% [PBVIm][TFSI] (8.5 pF), indicating that the ion-rich PILNMs are more conducive to achieving a highly polarized dielectric material and can improve the capacitance of the assembled flexible capacitor. Moreover, the electrostatic potential maps (ESP) and dipole moment of the ionic monomers were estimated by using Gaussian 09W program, as shown in Figure 2g. Due to a delocalized electron effect of the (trifluoromethanesulfonyl) imide ion (TFSI−), as shown in the ESP picture of the [BVIm][TFSI], the molecule exhibits a larger dipole moment, 12.485 D than that of 10.4817 D for [BVIm][Br]. The net dipolar repeating units of the PILNM offer high orientational polarization under an external electric field and achieve good dielectric performance. In summary, the PILNM containing 67% [PBVIm][TFSI] exhibited more ionic features inside and on surfaces of the uniformly interconnected nanofibers, helping it to achieve a higher capacitance. Finally, provided with good ion-conducting performance, mechanical properties, and dielectric properties, the microporous nanofibrous membrane containing 67% [PBVIm][TFSI] was chosen as a dielectric layer to assemble wearable pressuresensing devices. 3.3. Capacitive Pressure-Sensing Performance of PILNM-based pressure sensor. The sensing performances F

DOI: 10.1021/acsami.9b07786 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Figure 4. Real-time monitoring of different human motions with the PILNM-based wearable pressure-sensing textiles. Photographs of the capacitance variation in response to finger movement: (a) pressing, (b) stretching, and (c) twisting. (d) Recognition of real-time sound signal to go and pressure sensor. (e) Monitoring of the real-time human neck pulse under normal conditions. Inserts are the photograph and an enlarged signal. (f) Hydrophobic performance of PILINM and an illustration of the washing process. (g) PILNM-based wearable pressure-sensing performance to different relative humidities. (h) The waveform of the PILNM-based pressure-sensing textile to wrist bending after 10 washings. (i) PILNM-based wearable pressure-sensing performance to a given pressure under different washing times.

3.4. Stable and Durable Human Motion Detection. On the basis of the superior flexibility, high sensitivity, and fast response of the PILNM-based pressure-sensing textiles, they were employed in real-time detection of human motions. First, the prepared pressure sensor was used for simulating finger movements, such as pressing, stretching, and twisting, as shown in Figure 4a−c. The distinctive curves of capacitances versus time in a high signal-to-noise ratio demonstrate high consistency and high sensitivity of the pressure sensor in detection of the three finger motions. Then, the pressure sensor was attached to the first author’s neck, joint, and throat to monitor human movement signals. Figure 4d shows voltage signals when a person speaks “go” and “pressure sensor” with a PILNM-based pressure sensor attached onto the throat area to monitor muscle movement during speaking. The wearable pressure-sensing textiles demonstrated high sensitivity and distinct voltage signals when the different words were spoken. Figure 4e shows a record of voltage changes under a neck pulse detection with two distinguished peaks, which were caused by superposition of the incoming blood wave: percussion wave, tidal wave, and diastolic wave.42,43 These two peaks, peak 1 and peak 2, could easily yield two of the parameters that are mostly utilized to derive the radial augmentation index (AIr), which is related to arterial stiffness, defined as peak 2/peak 1.44 From the obtained human test results, we calculated an average

larger than that to press, possibly caused by the deformation of the pressure sensor sandwiched structure since bending causes in-plane tensile and compressive stresses to the membranes compared with only compressive stress from the press. Bending will greatly increase contact areas of the microporous nanofibrous membranes with electrodes and thus generate a stronger response in capacitance. The results in Figure 3h show the frequency-dependent behavior of the pressure-sensing textiles under various elbow motion frequencies (0.2, 0.5, 1, and 2 Hz). Durable and stable signals can be seen from the voltage signals and different waveforms for different frequencies, proving the high reliability and fast response of the PILNM-based pressure-sensing device. Moreover, the response/relaxation time of the PILNM-based pressure sensor is analyzed in Figure 3i. The prepared pressure-sensing textiles immediately responded to a tiny amount of pressure (20 Pa), as shown in the corresponding output signals and their enlarged picture (Figure 3i). The exhibited delta-t (Δt) of the first step of the output signal change is 30 ms, which exceeds the response limit of human skin (30−50 ms),41 and then the sensor output voltage recovered to the initial state in 30 ms. These extremely fast response and recovery times are attributed to the good mechanical properties of the microporous nanofibrous structure of the PILNM. G

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Figure 5. Model of PILNM-based wearable pressure-sensing matrix and performance for pressure mapping. (a) schematic illustration of assembling a single pressure-sensing unit. (b) Schematic description of a wearable pressure-sensing array with 3 × 3 pixels. Inserts are the photographs of the wearable pressure-sensing array pasted on wrist and hand. (c) Left: a photograph of the sensor array pressed with “U” and the corresponding capacitance mapping; right: pressure value and corresponding color changes; (d) schematic illustration of the motion of an index finger moved over and the corresponding pressure mapping.

3.5. Wearable Pressure Mapping. The function of a single sensing unit is limited by only detection of a point pressure. Integration of multiple single sensing units to form a wearable sensor array can enable a mapping of pressures in two-dimensional areas, which could provide more useful information. Figure 5a schematically illustrates the assembling process of a single pressure-sensing unit, in which PILNM could be tightly immobilized into the middle of both upper and lower flexible electrodes by using a liquid glue, which only binds their edges. Thereafter, 3 M semitransparent tapes were used to encapsulate the edges of a prepared pressure sensor. All of the materials used in the fabrication of the sensor were soft and in porous structures, leading to the desired flexibility and breathability of the assembled pressure-sensing textiles, shown in the insert pictures in Figure 5a. On the basis of the single pressure-sensing textile, a 3 × 3 capacitance parallel plate sensor array on cotton fabrics was fabricated, as shown in Figure 5b, which can be easily attached onto the skin of wrist and comfortably applied onto the back of hand. Moreover, a dynamic finger motion trajectory and object static contact position with the surface of the PILNM-based pressure-sensing textiles can be accurately monitored, as shown in Figure 5c. When a U-shaped plate was placed on the multiple sensing array, the corresponding capacitances of each pressure-sensing unit were recorded and then a capacitance matrix (3 × 3) was generated. A distribution of the external pressures can be displayed in a three-dimensional color-map of the measured capacitances. The pressure mapping can literally display the pressure values corresponding to the touching area, as shown in the left picture of Figure 5c. In addition, when an index finger moves over the surface, the motion pathway could be tracked, and the different capacitance mapping distributions could be clearly observed, as shown in the right picture in

AIr of 0.64, agreeing quite well with a healthy adult male in an age range of 25−30 years.45 These results proved that the tiny pulse motion could be detected by the PILNM-based pressure sensor, implying its potential uses in diagnosing vascular aging and diseases. To further demonstrate the long-term sensing performance with moisture and washing immunity of the PILNM-based pressure sensor, the pressure detections at different harsh environments have been tested. Different contaminations of liquid droplets on the PILNM surface were performed, as shown in Figure 4f. The prepared PILNM cannot be wetted by common liquids, such as coffee, milk, juice, etc., and exhibited washing stability. Moreover, the prepared PILNM can float on water in its high microporous structure. Since the electrospun PILNM is hydrophobic, the PILNM-based pressure-sensing textiles were very stable under increased humidity (up to 70%). The response waveforms were basically constant under the varied humidity range (30−70%) after a few cycles of pressuring on/off, as shown in Figure 4g. In addition, the influence of repeated water washings of the PILINM on its pressure-sensing performance is illustrated in Figure 4h,i. The water washing process was performed by immersion of PILNM into water with stirring at room temperature. After drying the PILNM, it was assembled into a pressure-sensing device to be tested. After 10 times repeated washing cycles, the waveform of the PILNM-based pressure-sensing textile to wrist bending (Figure 4h) were still consistent, and no noise signal appeared. Moreover, the washed PILNM-based pressure-sensing textiles can accurately detect the pressure like the original one (Figure 4i), indicating the stability and durability of the PILNM-based pressure-sensing textiles. This stability provides a guarantee for the application of the PILNM to wearable electronics. H

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Nanotube Strain Sensor for Human-Motion Detection. Nat. Nanotechnol. 2011, 6, 296−301. (3) Takei, K.; Takahashi, T.; Ho, J. C.; Ko, H.; Gillies, A. G.; Leu, P. W.; Fearing, R. S.; Javey, A. Nanowire Active-Matrix Circuitry for Low-Voltage Macroscale Artificial Skin. Nat. Mater. 2010, 9, 821− 826. (4) Rogers, J. A.; Someya, T.; Huang, Y. Materials and Mechanics for Stretchable Electronics. Science 2010, 327, 1603−1607. (5) Zhu, Z.; Li, R.; Pan, T. Imperceptible Epidermal−Iontronic Interface for Wearable Sensing. Adv. Mater. 2018, 30, No. 1705122. (6) Mannsfeld, S. C.; Tee, B. C.; Stoltenberg, R. M.; Chen, C. V. H.; Barman, S.; Muir, B. V.; Sokolov, A. N.; Reese, C.; Bao, Z. Highly Sensitive Flexible Pressure Sensors with Microstructured Rubber Dielectric Layers. Nat. Mater. 2010, 9, 859−864. (7) Trung, T. Q.; Ramasundaram, S.; Hwang, B. U.; Lee, N. E. An All-Elastomeric Transparent and Stretchable Temperature Sensor for Body-Attachable Wearable Electronics. Adv. Mater. 2016, 28, 502− 509. (8) Lei, Z.; Wu, P. A Supramolecular Biomimetic Skin Combining A Wide Spectrum of Mechanical Properties and Multiple Sensory Capabilities. Nat. Commun. 2018, 9, No. 1134. (9) Fan, F.-R.; Lin, L.; Zhu, G.; Wu, W.; Zhang, R.; Wang, Z. L. Transparent Triboelectric Nanogenerators and Self-Powered Pressure Sensors Based on Micropatterned Plastic Films. Nano Lett. 2012, 12, 3109−3114. (10) Lin, Z.; Chen, J.; Li, X.; Zhou, Z.; Meng, K.; Wei, W.; Yang, J.; Wang, Z. L. Triboelectric Nanogenerator Enabled Body Sensor Network for Self-Powered Human Heart-Rate Monitoring. ACS Nano 2017, 11, 8830−8837. (11) Tien, N. T.; Jeon, S.; Kim, D. I.; Trung, T. Q.; Jang, M.; Hwang, B. U.; Byun, K. E.; Bae, J.; Lee, E.; Tok, J. B. H.; et al. A Flexible Bimodal Sensor Array for Simultaneous Sensing of Pressure and Temperature. Adv. Mater. 2014, 26, 796−804. (12) Chen, Z.; Wang, Z.; Li, X.; Lin, Y.; Luo, N.; Long, M.; Zhao, N.; Xu, J.-B. Flexible Piezoelectric-Induced Pressure Sensors for Static Measurements Based on Nanowires/Graphene Heterostructures. ACS Nano 2017, 11, 4507−4513. (13) Jung, S.; Kim, J. H.; Kim, J.; Choi, S.; Lee, J.; Park, I.; Hyeon, T.; Kim, D. H. Reverse-Micelle-Induced Porous Pressure-Sensitive Rubber for Wearable Human−Machine Interfaces. Adv. Mater. 2014, 26, 4825−4830. (14) Hua, Q.; Sun, J.; Liu, H.; Bao, R.; Yu, R.; Zhai, J.; Pan, C.; Wang, Z. L. Skin-Inspired Highly Stretchable and Conformable Matrix Networks for Multifunctional Sensing. Nat. Commun. 2018, 9, No. 244. (15) Li, R.; Si, Y.; Zhu, Z.; Guo, Y.; Zhang, Y.; Pan, N.; Sun, G.; Pan, T. Supercapacitive Iontronic Nanofabric Sensing. Adv. Mater. 2017, 29, No. 1700253. (16) Li, T.; Luo, H.; Qin, L.; Wang, X.; Xiong, Z.; Ding, H.; Gu, Y.; Liu, Z.; Zhang, T. Flexible Capacitive Tactile Sensor Based on Micropatterned Dielectric Layer. Small 2016, 12, 5042−5048. (17) Wang, X.; Zhang, Y.; Zhang, X.; Huo, Z.; Li, X.; Que, M.; Peng, Z.; Wang, H.; Pan, C. A Highly Stretchable Transparent Self-Powered Triboelectric Tactile Sensor with Metallized Nanofibers for Wearable Electronics. Adv. Mater. 2018, 30, No. 1706738. (18) Wang, X.; Zhang, H.; Dong, L.; Han, X.; Du, W.; Zhai, J.; Pan, C.; Wang, Z. L. Self-Powered High-Resolution and Pressure-Sensitive Triboelectric Sensor Matrix for Real-Time Tactile Mapping. Adv. Mater. 2016, 28, 2896−2903. (19) Vural, M.; Behrens, A. M.; Ayyub, O. B.; Ayoub, J. J.; Kofinas, P. Sprayable Elastic Conductors Based on Block Copolymer Silver Nanoparticle Composites. ACS Nano 2015, 9, 336−344. (20) Zeng, W.; Shu, L.; Li, Q.; Chen, S.; Wang, F.; Tao, X. M. FiberBased Wearable Electronics: A Review of Materials, Fabrication, Devices, and Applications. Adv. Mater. 2014, 26, 5310−5336. (21) Kim, C.-C.; Lee, H.-H.; Oh, K. H.; Sun, J.-Y. Highly Stretchable, Transparent Ionic Touch Panel. Science 2016, 353, 682−687.

Figure 5b. These results demonstrate that the wearable multiple PILNM-based pressure-sensing device can perform skin sensing to mapping external stimulations and enabling potential applications of the wearable textiles in soft machines.

4. CONCLUSIONS In conclusion, we developed real-life wearable capacitive pressure-sensing textiles utilizing a novel poly(ionic liquid) nanofibrous membrane as the dielectric material. Owing to the microporous nanofibrous structure and good dielectric performance of the PILNM, the prepared pressure sensor could detect static and dynamic external pressures rapidly and sensitively with high moisture immunity. The high sensitivity of the PILNM-based pressure-sensing textiles is 0.49 kPa−1 under very low pressure, which could be used as a wearable neck pulse monitor. Moreover, the prepared capacitive type pressure-sensing textile achieved high bending chord length sensing ability (0.9 cm−1), low detection limit (20 Pa), fast response time (30 ms), and durable water washing sensing stability. Benefiting from these advantages, this sensor device can also be used in clothing to monitor various body movements under regular humidity environments without sacrifice of the comfortable nature of the textiles.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b07786.

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Synthesis of ionic monomer and poly(ionic liquids); characterization of prepared poly(ionic liquid); structure, morphology and thermal stability, and mechanical properties of as-spun PILNMs; performance of commercial conductive fabrics; comparison of a recent capacitive pressure sensor (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: 530-752-0840. Fax: 530752-7584. ORCID

Zehong Wang: 0000-0002-9236-3954 Yang Si: 0000-0001-7209-6206 Cunyi Zhao: 0000-0003-0360-2050 Dan Yu: 0000-0002-9790-4246 Gang Sun: 0000-0002-6608-9971 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Z.W. is grateful for a scholarship from China Scholarship Council to study at University of California, Davis. This work was supported by the National Science Foundation of China (NSFC) (No. 51403032)

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REFERENCES

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