Flexible and Stretchable Piezoelectric Sensor with Thickness-Tunable

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Flexible and Stretchable Piezoelectric Sensor with Thickness-Tunable Configuration of Electrospun Nanofiber Mat and Elastomeric Substrates Suk Hee Park, Han Bit Lee, Si-Mo Yeon, Jean Ho Park, and Nak Kyu Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b07833 • Publication Date (Web): 29 Aug 2016 Downloaded from http://pubs.acs.org on August 30, 2016

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ACS Applied Materials & Interfaces

Submitted to ACS Applied Materials & Interfaces (Revised manuscript)

Flexible and Stretchable Piezoelectric Sensor with Thickness-Tunable Configuration of Electrospun Nanofiber Mat and Elastomeric Substrates

Suk-Hee Park1*, Han Bit Lee1, Si-Mo Yeon1, Jean Ho Park1, Nak Kyu Lee1*

1

Micro/Nano Scale Manufacturing R&D Group,

Korea Institute of Industrial Technology, Ansan-si, Gyeonggi-do, 426-910, Korea

*

To whom correspondence should be addressed.

Tel: +82-31-8040-6829; Fax: +82-31-8040-6820; E-mail: [email protected] Tel: +82-31-8040-6821; Fax: +82-31-8040-6820; E-mail: [email protected]

ACS Paragon Plus Environment


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ABSTRACT Here, we developed highly sensitive piezoelectric sensors in which flexible membrane components

were

harmoniously

integrated.

An

electrospun

nanofiber

mat

of

poly(vinylidenefluoride-co-trifluoroethylene) was sandwiched between two elastomer sheets with sputtered electrodes as an active layer for piezoelectricity. The developed sensory system was ultrasensitive in response to various microscale mechanical stimuli and was able to perceive the corresponding deformation at a resolution of 1 µm. Owing to the highly flexible and resilient properties of the components, the durability of the device was sufficiently stable so that the measuring performance could still be effective under harsh conditions of repetitive stretching and folding. When employing spin-coated thin elastomer films, the thickness of the entire sandwich architecture could be less than 100 µm, thereby achieving sufficient compliance of mechanical deformation to accommodate artery-skin motion of the heart pulse. These skin-attachable film- or sheet-type mechanical sensors with high flexibility are expected to enable various applications in the field of wearable devices, medical monitoring systems and electronic skin.

Keywords : piezoelectric nanofiber, electrospinning, elastomer substrate, flexible sensor, pulse monitoring


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1. INTRODUCTION Recently, flexible electronic devices have enabled significant advancements in a wide range of future technology fields, such as displays, robotics, energy harvesters, and physical/chemical sensors.1-2 Among these, artificial sensory systems for biomedical purposes have been extensively studied in the context of health monitoring systems, which could be developed into wearable or skin-attachable devices.3-4 With recent advances in material science and engineering, highly functional materials that are capable of mechanical and electrical activation have been processed and developed into precision pressure sensors. Previous striking examples involve the employment of inorganic semiconductor/metal mats, nanomaterial-based composites, graphene, and various organic nano/micro-structures.2 To be able to work with tiny mechanical stimuli, these materials tended to be processed into microor nanometer-scale structures, thereby enhancing the precision of capacitance change,5-6 triboelectricity,7-8 piezoresistivity9-11 and piezoelectricity,12-13 which have been the main mechanisms for providing high sensitivity. In terms of energy consumption, piezoelectrically driven systems, particularly those using inorganic materials, such as ZnO, Pb(Zr,Ti)O3 (PZT) and BaTiO3, have been studied extensively to implement self-powered systems.14 Compared with these inorganic materials, organic-based materials are generally considered to have inferior piezoelectric properties. However, they have still attracted considerable attention owing to the many advantages of polymer-based fabrication, such as its facile process, cost effectiveness, and large-area processability. Furthermore, the flexibility, stretchability, durability and transformability of inherent

polymeric

characterization

could

provide

outstanding

performance

and

multifunctionality to the practically integrated devices. Previously, the functionality of poly(vinylidene

fluoride)

(PVDF)

or

copolymerized

poly(vinylidene

fluoride-

trifluoroethylene) (PVDF-TrFE), both representative piezoelectric polymers, for electronic



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devices has been demonstrated as processed into mechanically stretched films,15-17 micropatterned layered assemblies,18 and electrospun fiber mats.19-27 Here, we developed novel piezoelectric nanofiber-based sensors that were implemented by packing with flexible elastomer films, thereafter demonstrating their functionalities as high-precision sensors and skin-attachable pulse monitoring devices. Piezoelectric polymers are generally processed into thin films, thereafter being polarized by electric poling process to obtain higher crystallinity and superior ferroelectric properties.15-17 Unlike the thin-film processes, the electrospinning does not require an additional electrical poling process owing to its own high-voltage driven mechanism. Furthermore, owing to the ultrathin nanoscale geometry for effective polarization, the electrospun nanofibers have shown superior piezoelectric properties over the simple thin films, thereby being applied to high efficiency energy generators and sensors using various materials.26-41 In our method, PVDF-TrFE was electrospun into a nonwoven nanofiber mat with a fiber diameter of only a few hundred nanometers, which were demonstrated for piezoelectricity. For supportive foundations and electrodes, polydimethylsiloxane (PDMS) sheets sputtered with gold were used to provide both stability and flexibility to the sandwich-like configured assembly. We showed that the PDMS sheets and an electrospun mat were integrated as physically manipulable devices with harmonious flexibility, thereby exhibiting a great sensing ability to detect minute deformation as small as 1 µm. In concurrence with the flexible property of all components in this device, the large contact area between the constituent layers could provide reliable and durable sensitivity performance under harsh deformation conditions, such as repetitive stretching and folding. PDMS could be processed into ultrathin films by spincoating with a thickness of tens of micrometers, which has shown great potential in applications of wearable and patch-like electronic systems.42-44 As we employed the spincoated PDMS film, a total thickness of the integrated device of less than 100 µm could be


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realized, which was conformally skin-attachable and sufficiently compliable with the tiny skin deformation incurred by the heart pulse. The potential of ultrathin sensors for health monitoring was confirmed by measuring changes in the heart pulse according to several body conditions before, immediately after, and a period of time after exercise. Taken together, owing to their simple, low-cost fabrication and high performance, the developed sensory devices are expected to contribute to the field of flexible electronics, particularly in soft, freely deformable, stretchable and foldable systems.

2. MATERIALS AND METHODS 2.1. Preparation and Characterization of the Electrospun Nanofiber Mat. A PVDF-TrFE (70/30 Mol%, Piezotech, France) solution dissolved in a 60/40 (volume ratio) mixture of acetone (OCI Company, Korea) and dimethylformamide (Junsei Chemical, Japan) was used as a material for electrospinning. The PVDF-TrFE solutions were used at concentrations of 14, 18, and 22% (w/v) to obtain an optimized piezoelectric nanofiber condition. The polymer solution was infused at a rate of 0.2 mL/min through a 25-gauge (0.26-mm inner diameter) metal needle, which was subjected to a voltage of 15 kV with a 20 cm distance between the needle and collector surface. The electrospinning time was adjusted to 1, 5, and 10 min to study the relationship between input deformation or displacement and output signals. The asspun fibers were observed using scanning electron microscopy (SEM; SU8010, Hitachi, Japan). IR spectroscopy was carried out with an FTIR spectrophotometer (Nicolet 6700, Thermo Electron Co., USA) to confirm the dipolar crystalline structures of the electrospun fibers.

2.2. Fabrication and Characterization of the Piezoelectric Sensor. The piezoelectric



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tactile sensors were fabricated using PVDF-TrFE nanofiber mats and Au-sputtered PDMS sheets (Figure 1g). PDMS sheets with a thickness of approximately 500 µm were prepared by pouring a certain amount of PDMS prepolymer (resin:curing agent = 10:1) into a flatbottomed dish. After degassing and curing for 4 h at 60 °C, it was sliced into square sheets with a side length of 30 mm. Masked by another PDMS slab with a square puncture having a side length of 15 mm, PDMS-based electrodes were prepared by 180-nm-thick Au-sputtering. All components, including a nanofiber mat and a pair of copper wires, were packed with the flexible electrodes, of which surfaces were exposed to oxygen plasma treatment in advance. The masked, un-sputtered margin surfaces of the upper and lower PDMS electrodes were bonded with each other. In addition to the oxygen plasma-aided bonding, the bond was reinforced by side-sealing with additional PDMS prepolymer on four lateral edge surfaces of the sensor. Regarding the ultrathin sensor, PDMS thin films were prepared on PI and PVA sheets by spin-coating at speeds of 1,500 to 4,000 rpm. After the same process as those of thick sensors was performed for bonding, PI and PVA sheets were removed by simple detaching and rubbing with water, respectively. When applied to skin, the thin sensor was attached first before removing the PVA sheet. The sensors were evaluated by analyzing the output signals measured by a customized system, which was mainly composed of a function generator, shaker and signal amplifier. The shaker (Model 2075E, The Modal Shop, USA) equipped a jig and a punch for fixing and stimulating the developed sensor. To analyze the trend of sensor output according to inputs, the displacement generated from the shaker was quantified by a laser displacement sensor (LK-H027K, Keyence, Japan). To measure the reaction force caused by the pressing sensor, a load cell (UMMA 5 kgf, Dacell, Korea) was embedded between the punch and shaker tip. To obtain refined output signals, the sensor was connected to the custom-made amplifier (SM Information & Communication, Korea), including built-in high/low-pass


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filters (0.05–159 Hz passband) and a data acquisition system. The frequency cutoff allowed for the minimization of noise and recording of stable signals.

3. RESULTS AND DISCUSSION 3.1. Electrospun Nanofiber Mat as Active Layer of Sensor. In electrospinning, the polymer solution drop is processed into thin fibers down to the nanometer scale through a high-electric field (Figure 1a). As the electrospun fibers are further stretched, they achieve thinner dimensions and appear to retain stronger β-phase crystallinity and ferroelectric polarization compared with the typical aforementioned thin films.22, 25 Similarly, among the electrospun fibers, the further stretched thinner dimensions were hypothetically thought to exhibit a stronger β-phase crystalline form. In general, the polymer solution concentration for electrospinning is one of most crucial factors to determine the stretching compliance of solution jets and their final fiber diameters.45-46 As the concentration decreases, the solution becomes less viscous and can be more compliable and stretchable under the electric field due to the lower surface tension. However, the low solution concentration should be chosen carefully because an excessively low viscosity may generate sprayed beads or beadcontaining fibers, which are unfavorable defects in piezoelectric layers for sensor applications. We selected solution concentrations of 14, 18, and 22% (w/v) and successfully obtained continuous and defect-free fiber sheets at all conditions, as shown in Figures 1b, c, and d. The as-spun fiber mat from a lower concentration provided a thinner fiber diameter, down to 520±15 nm at a solution of 14%, compared with the solution of 22%, which provided a fiber diameter larger than 1.5 µm (Figure 1e). The Fourier transform infrared (FTIR) spectrum of each sample displayed the distinct peaks of β-phase crystalline structures at 840 and 1280 cm-



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1

, as shown in Figure 1f. As expected, the thinner fiber mat appeared to exhibit a stronger β-

phase crystalline signal, so we used the electrospun fiber mat of the smallest diameter for the subsequent fabrication of sensors. The schematic illustration in Figure 1g shows the simple integration of a flexible device sandwiched with an electrospun PVDF-TrFE fiber mat and two PDMS sheets with sputtered gold electrodes. The upper and lower PDMS sheets were prepared by volume-controlled casting in a petri dish, where the thickness could be controlled at approximately 500 µm. Aided by oxygen plasma-treated surfaces of both upper and lower PDMS sheets, the nanofiber layer was robustly packed and fixed within the sandwiched assembly. As shown in the photographs in Figure 1g, the integrated sensor with a total thickness of 1 mm could be stably manipulated and compliable under large deformations.

3.2. Sensitivity and Durability of Flexible Sensor. To evaluate the piezoelectric performance of the developed device, it was mounted and tested within a sophisticated measurement system, which consisted of a function generator, an electrodynamic shaker equipped with a load cell, a laser displacement sensor, and a signal processing module, as shown in Figure 2a and Supporting Information, Figure S1. As the electrodynamic shaker in the system generated periodic waves, a punch pushed the sensor surface according to the waveforms while the displacement of the punch and the reaction force were measured by the laser displacement sensor and embedded load cell, respectively. The displacement of the punch, which indicated the deformation depth of the sensor surface, appeared to be linearly proportional to the input voltage of the function generator in the range of 0.02 to 5.4 V, which corresponded to deformation amplitudes of 1.2 µm to 400 µm (Supporting Information, Figures S2a and b). Figure 2b shows the resulting waveforms of output voltage signals measured from the piezoelectric sensor. The periodicity of the output voltage was retained in a regular manner, and the clearly discernable waveforms were shown even under a small


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deformation of 1.2 µm. The high sensitivity was also verified by dropping several light objects, such as a piece of paper (2 mg), a rice grain (24 mg), and a water droplet (30 µL), onto the sensor surface from a distance of 10 mm (Figure 2c). For quantitative analysis, we averaged the ten positive peak values of output voltage signals for each stepped input deformation and analyzed the relationship between the input and output signals. The result displayed a well-fitted linear distribution with a high slope of 114.2 mV/µm, as shown in Figure 2d. Because the sensor was composed of elastic materials and well-packed with the components, the relationship between the surface deformation and reaction force appeared to exhibit an exactly linear tendency (Supporting Information, Figure S2c). Based on the linear correlations among all inputs and outputs, the linear performance was demonstrated as a highly precise mechanical sensor for pressures below 1 kPa and deformations below 1 µm (Figure 2d). In addition to the high linearity and sensitivity, the output signals showed small standard deviations distributed from 10 mV to 40 mV throughout the overall working input deformations. In terms of the coefficient of variation (Cv), which is defined as the ratio of the standard deviation σ to the mean µ (σ/µ × 100%), the output peaks were reproduced within a Cv of 5% for deformations exceeding 4 µm, as shown in Supporting Information, Figure S3. This level of variation would be acceptable and reliable when considering a mechanical sensor able to perceive micrometer-scale stimuli. Because the total dipole moment in the device was proportional to the quantity of piezoelectric nanofibers, the thickness of the electrospun mat that could be modulated by the spinning time was a crucial factor for determining the piezoelectric performance. When the spinning time was reduced from 10 min to 5 min and 1 min, the thickness of the as-spun fiber mat decreased from 71.2±9.2 µm to 44.7±4.9 µm and 34.9±8.7 µm, respectively. As shown in Figure 2e, an excessively thin fiber mat, such as the 1 min spinning sample, provided unstable outputs of distracted linearity and large deviations. Although the sample spun for 5 min showed a decrease in the output signal



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and slope per displacement compared with the sample spun for 10 min, the tendency of linearity and deviation was sufficient for a viable sensor. Considering the output signal stability, the feasible input ranges of as-spun samples with 10 and 5 min of spinning were assessed to be 1.2 to 16.8 µm and 26.5 to 144 µm, respectively (0.81 to 6.77 kPa and 10.48 to 55.38 kPa in terms of the pressure in Figure 2e). The sample spun for 5 min exhibited a wider range of feasible input compared with the sample spun for 10 min, suggesting that the developed sensor could be properly designed by modulating the fiber mat thickness according to the range of sensing deformations or pressures needed. The all-flexible material system was expected to be durable and workable under repetitive external stimuli, particularly when employing an elastomer and polymeric piezolayer. To confirm the durability of the device, we evaluated the sensor outputs after exertion with large deformations, such as cyclic stretching and folding, as shown in Figure 3. We first prepared a sensor with a sensitivity of approximately 120 mV/µm and exerted a 30% strain of uniaxial stretching over 1,000 cycles. The amplitudes of the output signals for the same input deformation appeared to decrease to 70–80% after the repetitive stretching, thereby causing the overall sensitivity to decrease to approximately 90 mV/µm as well (Figure 3a). Next, the outputs were measured by repetitive folding (1,000 cycles) with respect to the midline of a sensor with a bending diameter of 5 mm. Similarly but more so than in the stretching test, decreased amplitudes were observed down to approximately 40%; thus, the sensitivity was decreased to 55 mV/µm. Interestingly, despite a certain amount of output loss throughout the durability tests of stretching or folding, the linear relationships between the inputs and outputs were not impaired. Furthermore, the coefficients of variation (Cv) for both cases of stretching and folding did not exceed 10% for input displacements of more than 2 µm, which were comparable with those of intact sensors (Supporting Information, Figure S3). These results implied that the integrated device in such a harsh condition as in these experiments


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would still be feasible as a mechanical sensor. The sustainable performance, good linearity and small variation of output were attributed to the highly elastic property that was given by the component characteristics of all flexible materials. Electrospun nanofiber mats generally show great resilience and compliance against external loads owing to their internal nonwoven networks. Under stretching or folding, the deformation of the overall structure would be imposed mainly on the transformation of the fiber network or slippage between the adjacent fibers rather than the longitudinal elongation or even fracture of individual fibers. Despite these flexible properties of components, the damage to the Au electrode was inevitable under the harsh condition deformations, as shown in Supporting Information, Figure S4. There was only a certain increase in resistance up to approximately 10 Ω at the two diagonal ends of the Au electrode (approximately 30 mm distance). However, no electrical disconnection was found throughout all regions of the electrode after the stretching and folding processes equivalent to the experiments above, which likely contributed to the sustained stability of the sensor performance, exhibiting only a non-random decrease in the output amplitude.

3.3. Thickness-Tunable and Skin-Attachable Sensor. It was highly advantageous to utilize PDMS for the structural components of the sensor given its shape variability in terms of thickness and flexibility. By employing the spin-coated thin PDMS films as the upper and lower supporting layers in our sensor, we developed a skin-attachable ultrathin device to monitor real-time pulse waves on skin. The ultrathin geometry was intended to provide sufficient flexural compliance to allow the sensor to be transferred onto the arbitrary and microscopic curvature of body skin and to work with the minute skin displacement generated by the arterial pulse. Figure 4a illustrates the fabrication procedures of the ultrathin sensor using the thin PDMS films with sputtered Au electrodes instead of the previous sheets with a thickness of approximately 500 µm. The spin-coated PDMS film should be manipulated



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while attached to a removable stiff sheet as a bilayer entity owing to the ease with which it flutters and collapses.47-49 We used two different removable sheets of easily detachable lowsurface-energy polyimide (PI) and water-washable polyvinyl alcohol (PVA) sheets for the underlying substrates of the lower and upper parts in a sandwich structure sensor. Using the two bilayer sheets containing spin-coated PDMS films, the ultrathin sensor could be easily fabricated at a total thickness of less than 100 µm, as shown in Figure 4b. This geometry was possible owing to the thin components of the PDMS layer and PVDF-TrFE nanofiber mat; the thicknesses of these components were less than 25 µm and 50 µm and were tuned by a spin-coating speed of 4,500 rpm and 5 min of electrospinning time, respectively (Figure 4c). There was no collapse or wrinkle in the completed sensor during the integration process with the thin layers. Even after peeling off the lower PI sheet for attachment to the skin, thanks to the supporting PVA sheet, the thin sensor could be easily manipulated without any impairment and stably transferred onto the skin (Figure 4d). As the upper PVA sheet was washed out by rubbing with water, the final sensor could provide sufficient compliance to work in accordance with the minute skin displacement caused by physiological movement, such as the heart pulse. Because the PDMS surface could provide adhesion via van der Waals forces, the adhesion energy for stable skin attachment was expected to be able to surpass the bending energy of the PDMS film. When a certain sheet with a specific thickness (t) and elastic modulus (E) is bent and adhered to a curved surface with a curvature radius (R), the bending energy (Ubend) per unit area (A) can be written as = 24 where Ec is the Young’s modulus of the PDMS–nanofiber layer composite, which is ideally under an isostrain condition, yielding


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= + where EPDMS is assumed to be a representatively known value of 2 MPa.50 ENF layer has typically been measured at less than 200 MPa.19 Thus, the maximum elastic modulus of the composite would be approximately 100 MPa according to the relationship given above. When considering the outer radius of the radial artery, which typically exceeds 1.5 mm,51 and a skin thickness exceeding 1 mm,52 the skin curvature would range over that summation. Assuming a minimum curvature radius (R) of 2.5 mm, the aforementioned maximum bending energy of our 100- µm-thick sensor is calculated as approximately 0.67 J m-2. This value suggests that the sensor can be stably attached to skin fluctuating by a pulse owing to its sufficiently lower bending energy compared with the previously measured adhesion energy (1.2 J m-2).53 The conformal fluctuation of the thin sensor surface could be observed with attachment onto the skin, as shown in Supporting Information, Video 1. Figure 5a schematically illustrates the skin displacement generated by the pulse, by which the skin-attached sensor is deflecting accordingly. As reported previously, the volume expansion of the arterial blood vessel caused by the heart pulse produces a microscale skin displacement of approximately 50–60 µm of the carotid artery and 30 µm of the radial artery.54-55 To confirm the feasibility of the thin sensor in the microscale displacement, we carried out experiments using a punctured jig in the vibrating system, in which the thin film sensor could be freely deflected according to the punch vibration (left inset schematic image in Figure 5b). This bendable mode was expected to simulate the pulse-based deflection of the skin-attached sensor, as shown in Figure 5a. Compared with the unstable output distribution in compression mode (data shown by red diamonds), the results of bending mode appeared to provide a more stably linear output trend in a wide range of inputs from several micrometers of skin displacement to tens of micrometers of sensor thickness (data shown by blue diamonds). This good linear performance implied that the thin constituent layers yielded



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harmonious dynamic deformation with good resilience. Although the thin sensor had limited feasibility for compression owing to its low compressibility by the thin geometry, in the bent state, the functionalities of the thin sensor surpassed those of the thick sensor using 500 µm PDMS sheets (data shown by grey circles) in terms of output amplitude and sensitivity (Figure 5b). Regarding the validity of the input range in the bending mode, whereas the thick sensor produced unstable output signals below input displacements of 70 µm, the outputs of the thin sensor were feasible down to 10 µm. The fine sensitivity of the thin sensor in bending mode was expected to be effective to measure the microscale skin displacement caused by the heart pulse. Figure 5c and Supporting Information, Video 2 show the pulse waveform measured from each thin sensor attached to the skin region of the radial artery and carotid artery. The output peak values for radial and carotid pulse waves were approximately 200 mV and 600 mV, respectively. Thus, approximately 20 µm and 60 µm of skin displacement for radial and carotid pulses, respectively, could be inversely estimated from the trend line of the thin sensor in Figure 5b. The measurement would be quite reliable in that the resulting value was comparable to the previously known values of skin displacement mentioned above.54-55 Figure 5d shows the real-time signal waves of the radial pulse recorded from the thin sensors transferred onto wrist skin. As observed in every period of pulse data, the waveform appeared to show multi-peak shapes, which were attributed to the characteristic superposition of the primary systolic wave from the central aorta and the reflected waves at the peripheral arteries.56 Because a piezoelectric device generally provides a sufficiently rapid response to measure high-frequency signals, it could resolve multi-peak waveforms consisting of small features, such as percussion, tidal, and dicrotic waves, as shown in the inset image of Figure 5c. This suggests the potential to apply our device to medical diagnostics based on the physiology of the pulse waveform. In addition, the pulse measurements were performed


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serially at three time points distinguished by body conditions: normal condition, immediately after exercise (rushing up and down 300 stairs), and released after 3 min. As expected, there were notable differences in the peak intervals and peak heights among the three types of pulse waves. The average time intervals over 10 s changed throughout the three time points: 0.85 s – 0.56 s – 0.67 s, corresponding to 71 bpm – 107 bpm – 90 bpm (beats per minute). Owing to the stable attachment to skin, even after an intense exercise, the pulse wave signals could be secured consecutively during the measuring periods from the normal to the recovered conditions after exercise. Collectively, with further sensor optimization and a technical measurement algorithm, the developed sensor is expected to be used in real-time health monitoring devices for medical purposes based on heart rate variability (HRV).

4. CONCLUSION This study demonstrated the feasibility of using a polymer-based piezoelectric nanofiber mat conjugated with Au-deposited elastomer sheets as a mechanical sensor with ultrahigh sensitivity, flexibility and durability. The highly polarized PVDF-TrFE electrospun nanofibers could provide great input–output linearity and the sensing ability to detect minute stimuli, including deformations as small as 1 µm. The device also showed great durability against multiple stretching or folding (

Flexible and Stretchable Piezoelectric Sensor with Thickness-Tunable

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