Robust and Wearable Pressure Sensor Assembled from AgNW

Apr 29, 2019 - ... pillar arrays were assembled face to face as the pressure sensing device. ... Sensitivity and Wide Detection Range. Yongyun ... Tec...
0 downloads 0 Views 10MB Size
Download PDF
Article Cite This: ACS Appl. Nano Mater. 2019, 2, 3196−3205

www.acsanm.org

Robust and Wearable Pressure Sensor Assembled from AgNWCoated PDMS Micropillar Sheets with High Sensitivity and Wide Detection Range Yongyun Mao,†,‡ Bing Ji,† Ge Chen,† Changxiang Hao,‡ Bingpu Zhou,*,† and Yanqing Tian*,‡

Downloaded via BUFFALO STATE on August 3, 2019 at 00:33:16 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

†

Joint Key Laboratory of the Ministry of Education, Institute of Applied Physics and Materials Engineering, University of Macau, Avenida da Universidade, Taipa, Macau 999078, China ‡ Department of Materials Science and Engineering, Southern University of Science and Technology, No. 1088, Xueyuan Road, Xili, Nanshan District, Shenzhen, Guangdong 518055, China S Supporting Information *

ABSTRACT: Polydimethysiloxane (PDMS)-based materials are emerging as an ideal category of flexible multifunctional membranes that are expected to be used for wearable and skinattachable pressure sensors. Here, we report a simple and effective fabrication strategy to construct a highly sensitive and robust wearable pressure sensor device based on interlocked connection structure (ILCS), which formed from silver nanowires (AgNW)-coated PDMS pillar arrays sheets. Two pieces of PDMS thin layers that manufactured with AgNWcoated pillar arrays were assembled face to face as the pressure sensing device. The interlocked structures enable a pressure sensitive variation in the contact area between AgNW-coated PDMS pillars under different pressure loading. Therefore, the electrical resistance changes according to the degree of interconnection and pillar deformation when different magnitudes of pressures were applied. The pressure sensor exhibits ultrahigh pressure sensitivity of ∼20.08 kPa−1 up to 0.8 kPa, and sensor response is highly reproducible and repeatable for more than 10000 cycles. Additionally, all the results demonstrate that the pressure sensor can be used as the device for the monitoring of the signals ranging from epidermis movements to air flows, such as mimic swallowing action, gently touching, bending, and torsion. We believe that the presented sensor can be used in many potential applications fields, such as flexible electronics, artificial e-skin, wearable sensor devices, and so on. KEYWORDS: pressure sensor, silver nanowires, PDMS pillar arrays, interlocked connection structure (ILCS), point-to-point connection structure (PPCS), wearable electronic devices ces.15−18 Typically, the pressure sensitivity of the microstructured films far surpassed the ones without microstructures, and the sensitivity can be flexibly regulated based on different microstructure design.14 Because of the utilization of microfabrication technique, many sensor devices were fabricated with various microstructures, such as pyramids,14,19 pillars,13,20 wrinkles,21−23 and hemispheres.24,25 Although many promising structural strategies toward the flexible PDMS-based electronic devices have been demonstrated, the aforementioned methods are still with complicated manufacturing process, have high-cost, and are minimally reproducible. Fortunately, because of the simple operations and low cost, silver nanowires (AgNW) have been widely used to construct flexible electronic devices, such as transparent fingerprint sensors,26 electronic skin,27 and flexible pressure sensors.28

1. INTRODUCTION Wearable and flexible pressure sensor devices have attracted a lot of attention for various future applications, such as flexible electronic skins,1 smart windows or displays,2 energy harvesting and biomedical devices.3,4 As everyone knows, the most intuitive approach for the wearable pressure sensor devices is to exploit intrinsically soft materials as the flexible and stretchable substrates, such as PDMS and PU.5−10 Apart from the extraordinarily high flexibility performance, one attractive feature of PDMS is the excellent biocompatibility, which is considered as the common prerequisite for the flexible electronic skin.1 Therefore, PDMS-based sensor devices/ electronic skins have been manufactured in various formats which are adaptable to their specific application fields and presented significantly different levels of sensitivity performance.1,11−14 With the development of the microfabrication technique, the microstructural design is a powerful approach for the fabrication of high performance wearable electronic devi© 2019 American Chemical Society

Received: March 18, 2019 Accepted: April 29, 2019 Published: April 29, 2019 3196

DOI: 10.1021/acsanm.9b00503 ACS Appl. Nano Mater. 2019, 2, 3196−3205

Article

ACS Applied Nano Materials Table 1. Comparable Results of the Pressure Sensors Reported in the Literature sensor AAO/WG Au-coated PDMS pillar Au-coated PDMS pillar rGO/polyaniline VACNT/PDMS NNMBA MP-ZnO/PS 3D graphene CNT thin film graphene/AgNW ILCS-pressure sensor PPCS-pressure sensor

sensitivity 1 (detection range)a

sensitivity 2 (detection range)b

−1

0.14 kPa−1 (1.5−4.5 kPa)

6.92 kPa (0.3−1.5 kPa) −1.8 kPa−1 (0.0−0.35 kPa) 2.0 kPa−1 (0−0.22 kPa) 0.152 kPa−1 (0−3.24 kPa) 0.3 kPa−1 (0−0.7 kPa) 6.21 kPa−1 (0.004−0.025 Pa) 10.3 kPa−1 (0−2 kPa) 110 kPa−1 (0−0.2 kPa) 278.5 kPa−1 (0−0.002 kPa) 0.29 kPa−1 (0−2.5 kPa) 20.08 kPa−1 (0.05−0.8 kPa) 8.32 kPa−1 (0.05−0.4 kPa)

0.87 kPa−1 (1.0−3.5 kPa) 0.0049 kPa−1 (12−27 kPa) 0.05 kPa−1 (0.7−2 kPa) 3.4 kPa−1 (7−11 kPa) 3.0 kPa−1 (0.2−15 kPa) 13.2 kPa−1 (0.002−0.5 kPa) 0.02 kPa−1 (3−10 kPa) 3.81 kPa−1 (0.8−2.1 kPa) 2.73 kPa−1 (0.4−12 kPa)

ref 11 13 20 32 33 34 35 36 37 38 this work this work

a

Sensitivity for the low pressure range. bSensitivity for the high pressure range.

Figure 1. Schematic illustration for the fabrication process of sandwiched pressure sensor devices: fabrication of PDMS pillar arrays (A−C); AgNW-coated PDMS pillar arrays sheets (D); ILCS-pressure sensor (E); PPCS-pressure sensor (F).

increase the contact area. Therefore, the large contact area was provided by the large deformation under large pressures, which result in a great change in contact resistance. Subsequently, the PPCS-pressure sensor exhibited a wide detectable range up to 12 kPa and high sensitivity before saturation (8.32 kPa−1 and 2.73 kPa−1 for the low and high pressure ranges). As a proof of concept, the ILCS-pressure sensor was used to monitor signals ranging from epidermis movements to air flows, such as gently touching, swallowing action, bending, and torsion. The results experimentally proved that the detection limit as well as the detectable range can be well controlled based on the different assembly approach. In terms of the sensor’s overall performance including the sensitivity and detectable pressure range, the sandwiched sensors based on AgNW-coated PDMS pillar arrays sheets developed in this work are more competitive compared with most reported counterpart, which were listed in Table 1.11,13,20,32−38 And the pressure detection limit of our sensor device was as low as 20 Pa. All results demonstrate the potential applications of our proposed AgNW-coated PDMS pillar arrays for wearable pressure sensors, artificial e-skin, biosensing, flexible electronic devices, and many other areas.

More importantly, 1D nanowires (NWs) with high aspect ratio have a much lower percolation threshold than the isotropic nanoparticles, which can significantly decrease the filler density compared with the 0D configurations.5 The low percolation of high aspect ratio nanowires facilitates the fabrication of lowcost flexible electronic devices.29−31 Therefore, structural PDMS substrates and 1D AgNW are regarded as the ideal components for the fabrication of flexible electronic devices with high sensory performance. To date, there are still no reports on AgNW-coated PDMS pillar array sheets assembled face to face as the pressure sensor devices with capabilities to improve the pressure sensors’ overall performance, e.g., sensitivity, highly repeatable, and wide detection range. Herein, we present a new type of interlocked structure based on two AgNW-coated PDMS pillar arrays sheets for highly sensitive and robust wearable pressure sensors. Two pieces of thin PDMS layers that are decorated with AgNW-coated pillar arrays were face to face assembled to form the pressure sensor devices. Interestingly, two different configurations were obtained via control the stack positions, named as “interlocked connection structure (ILCS)” and “point-to-point connection structure (PPCS)”, respectively. The interlocked structures enable a pressure sensitive variation in the contact area within the AgNW-coated PDMS pillar matrix. Compared to the PPCS geometry, the ILCS geometry possesses large contact area when the relatively small pressures were applied on it. The results demonstrated that the ILCS-pressure sensor based on interlocked geometry exhibited ultrahigh pressure sensitivity of ∼20.08 kPa−1 up to 0.8 kPa, and sensor response is highly reproducible and repeatable up to 10 000 cycles. However, almost no deformation or contact area changes occurred on the PPCS-pressure sensor under the same pressures, so it needs the larger pressure to provide the deformation and

2. EXPERIMENTAL DETAILS Synthesis of Silver Nanowires. AgNW with high aspect ratio was synthesized and purified as described in the literature.39−42 As atypical synthesis of AgNW, a one-pot reaction was employed to mix compounds and solvents. First, 1.5 g of polyvinylpyrrolidone (PVP) was dissolved in 250 mL of ethylene glycol (EG). Then, 1 mL of 10 mM FeCl3 was added into the PVP solution. Subsequently, 50 mL of AgNO3 (0.1 M) solution was added into the mixture and stirred for 40 min at 170 °C and then cooled down to room temperature. The process for the purification of AgNW is described in the literature.40 Subsequently, the AgNW were dispersed in ethyl alcohol and collected by centrifugation to remove the small amount of PVP 3197

DOI: 10.1021/acsanm.9b00503 ACS Appl. Nano Mater. 2019, 2, 3196−3205

Article

ACS Applied Nano Materials

Figure 2. Characterizations of AgNW and PDMS pillar arrays sheets. SEM images of AgNW (A, B), PDMS pillar arrays before (C) and after (D) coating AgNW; (E, F) SEM images of AgNW coated on the pillars’ surfaces (horizontal and lateral surfaces). was then put on the disk of a spin-coater, and 100 μL of AgNW ethanol solution with 5 mg/mL concentration was dropped and spincoated at 1000 rpm for 30 s in air. Subsequently, the AgNW-coated PDMS sheet was placed evenly on the hot plate at 125 °C for 20 min to evaporate the solvent and improve the adhesion of AgNW to the PDMS surface. Finally, the AgNW-coated PDMS pillar arrays sheets were obtained. After that, AgNW-coated PDMS pillar arrays sheets were face to face assembled into the ILCS-pressure sensor and PPCSpressure sensor as shown in Figure 1E and Figure 1F. During the entire assembly process, we observed the microstructures clearly under the assistance of the optical microscopy to ensure the precisely assembled architecture. In addition, the electrical conductance was monitored using the measurement system to maintain the initial insulation between the facing structures at the critical condition. The initial electrical current can thus be well maintained at zero after the sealing of the device, while exhibiting swift response to the external pressure in terms of the current increase due to the contact area change as will be described in the following section. Structure Characterization and Performance Measurement of the Pressure Sensors. The detailed morphologies of the AgNW, PDMS pillar arrays, and AgNW-coated pillars were investigated using an optical microscope (Carl Zeiss Microscopy, GmbH, 37081 Gottingen, Germany) with a CCD camera module and scanning electron microscopy (SEM, Sigma FE-SEM, Zeiss Corporation, Germany). A motion controller (Zolix,MC600, resolution: 2.5 μm) was employed to offer different forces on the pressure sensors, and the

residual. Finally, the AgNW were re-dispersed in ethyl alcohol with 5 mg/mL concentration and used in the following.39,40 Fabrication of AgNW-Coated PDMS Pillar Arrays Sheets and Pressure Sensors. Figure 1 illustrates the fabrication process of the sandwiched pressure sensor devices with ILCS geometry (Figure 1E) and PPCS geometry (Figure 1F), respectively. The fabrication of the PDMS pillar arrays sheets was conducted as follows:43,44 The PDMS base mixed with a curing agent at a 10:1 weight ratio was poured over the patterned region of the SU8 mold on silicon wafer (Figure 1A and Figure 1B) and placed in a vacuum oven for degasification to force the air trapped inside the microholes out so that the PDMS gel could completely fill the microholes.43 The SU8 hole arrays on silicon wafer were fabricated via conventional softlithography as described in our previous report.45 The optical images of the silicon mold and PDMS pillars were also presented in Figure S1. The diameter and depth of the SU8 microholes are 500 and 200 μm, respectively, while the distance between the microholes center was 1000 μm. The uncured PDMS was then placed evenly on the hot plate at 100 °C for 10 min. Before the curing process, the PDMS and the mold were put on the disk of the spin-coater. The uncured PDMS was spin-coated at 800 rpm for 10 s, with the consistent microstructured sheet thickness of approximately 500 μm. After curing, the PDMS pillar arrays sheets were gently peeled off from the mold, as shown in Figure 1C. The surface modification of the pillars was conducted by means of plasma treatment for 2 min, making AgNW absorb on pillars’ surface easily. The PDMS pillar arrays sheet 3198

DOI: 10.1021/acsanm.9b00503 ACS Appl. Nano Mater. 2019, 2, 3196−3205

Article

ACS Applied Nano Materials

Figure 3. Schematic illustration of the deformation under different applied pressures for ILCS-pressure sensor (A-1 to A-3) and PPCS-pressure sensor (B-1 to B-3), respectively.

Figure 4. (A) Sensor response to the different pressures loading and unloading. (B) Change in the electrical current of the sensor for10 000 cycles of pressure loading/unloading. (B-1, B-2, B-3) Magnified current curves extracted from B, which represent reliable response for repeated operations. Digital photographs of the basic experimental setup for characterizing the pressure sensor: software interface (C) and pressure measurement system (D). mass was displayed on the electronic scale and then was translated into pressure. The data of current tests were collected using a KEYSIGHT B2902A (Shanghai, China) operated at the voltage of 1 V.

AgNW and illustrates that the obtained product is mainly composed of nanowires with a mean length of 50 μm. Figure 2C and Figure 2D showed the SEM images of PDMS pillar arrays sheet and AgNW-coated PDMS pillar arrays sheet. From the top view of the PDMS pillar arrays, the surface of the pure PDMS pillar array is quite smooth (Figure 2C), which becomes rather more rough because of the existence of a layer of AgNW coated on the pillars’ surface (Figure 2D). As the vertical pressure was applied on the pressure sensor surface, the overall electrical resistance of the sensor decreased owing to the further interconnection of AgNW. Additionally,

3. RESULTS AND DISCUSSION Figure 2A and Figure 2B showed the typical SEM images of synthesized AgNW with diameter of 100 nm and length of 50 μm. Figure 2A is the high-magnification SEM image, which reveals that the NWs are uniform in diameter with a mean diameter of 100 nm. Figure 2B showed the SEM image of 3199

DOI: 10.1021/acsanm.9b00503 ACS Appl. Nano Mater. 2019, 2, 3196−3205

Article

ACS Applied Nano Materials

Figure 5. (A) Relative current−pressure plots for the pressure sensor based on ILCS, PPCS, and Pillar-flat pressure sensor. (B, C) Sensitivity plots for the two kinds of pressure sensor devices. (D) Steady-state sensor responses under various pressures for sensor based on the interlocked connection structure.

with continuously increased pressure, the compression of PDMS will result in further connections of the AgNW that has been preloaded to the sidewall of the pillars. The electrical resistance will thus be reduced, leading to obvious increase of the current as will be shown from the experimental results. It can be predicted that this assembly type of pressure sensor can be the prominent cause for wider detectable range. Figure 4A shows the current changes of the fabricated ILCSpressure sensor when gradually increasing the applied pressure up to 500 Pa. After releasing of the applied pressures, the connecting pillars are separated and the pressure sensor returns to its originally insulated state resulting from elastic characteristics of the PDMS pillars.11 In order to investigate repeatability of the obtained sensor, a constant pressure of 0.5 kPa was repeatedly applied on the sensor surface while monitoring the current change in real time, as shown in Figure 4B. Parts B-1, B-2 and B-3 of Figure 4 showed the magnified sensor responses curves which were randomly extracted from Figure 4B. The current−time curves reveal that there are no significant differences in the waveform of the responses during the 10 000 repeated pressure loading/unloading cycles. Moreover, SEM images of AgNW coated on the pillars’ surfaces after 10 000 test cycles were also presented in Figure S2, demonstrating that the morphology of AgNW on the pillar surface has no significant change after the cyclic test. All the results clearly suggest that the sensor device possesses better stability, excellent repeatability, and reliable response for the repeated detection operation. Figure 4C and Figure 4D presented the basic experimental setup for characterizing the pressure sensors, including the software and pressure measurement system.

Figure 2E and Figure 2F also demonstrated that both the upper surface and the sidewalls of the PDMS pillars were uniformly covered with AgNW. Finally, the proposed sandwiched pressure sensors were fabricated by simply assembling the two pieces AgNW-coated PDMS pillar arrays sheets face to face, as illustrated in Figure 1E and Figure 1F. In order to obtain the pressure sensors with high sensitivity, ILCS-pressure sensor (Figure 3A-1) and PPCS-pressure sensor (Figure 3B-1) based on AgNW-coated PDMS pillar arrays for wearable pressure sensors were designed and fabricated. Apart from the microstructures on the PDMS membrane surface, the assembly process is another critical parameter to determine the sensor performance. Figure 3 shows the schematic illustrations of the deformation and contact area changes based on two sensor architectures which can be realized under different assembly processes. Due to the different connection structures, the pressure sensors exhibited obviously different sensitivity and detectable pressure ranges. The continuous contact area changes and deformation of the connection joint with gradually increased pressure loading were demonstrated in detail as shown in Figure 3. For the sensor device based on PPCS geometry, relatively large forces are required to reach the saturation due to the particularity of structure and elastic characteristic of PDMS materials (Figure 3 B-1 to B-3). Therefore, almost no deformation or contact area changes occurred on the PPCS-pressure sensor under the same relatively small force, therefore larger pressure is needed to provide the deformation and increase the contact area. Meanwhile, a certain number of point-to-point pillars may be distorted under the large pressure loading, resulting in a degree of lateral deflection and pillar displacement. Consequently, 3200

DOI: 10.1021/acsanm.9b00503 ACS Appl. Nano Mater. 2019, 2, 3196−3205

Article

ACS Applied Nano Materials

Figure 6. (A) Current−pressure curves of the ILCS-pressure sensor obtained with cyclic loading−unloading of different pressures. (B) Current changes with the finger touching. (C) Current variations when air is blown toward the sensor surface. (D) Gently scratch the sensor surface.

pressure lower than 50 Pa was not used for the calculation of sensitivity.11 Subsequently, the sensor device based on the ILCS geometry possesses higher sensitivity (20.08 kPa−1, Figure 5B) compared that the sensor device with PPCS geometry (8.32 kPa−1, Figure 5C). Additionally, obvious step changes in the current−pressure plots for the two kinds of pressure sensors were demonstrated and there are two distinguishable sections and exhibited different sensory performance. According to the calculation results based on formula 1, the ILCS-pressure sensor exhibits outstanding sensitivity as high as 20.08 kPa−1 with linearity of R2 = 0.977 during the working pressure from 50 to 800 Pa. At the same time, high sensitivity performance (3.81 kPa−1, R2 = 0.966) was still maintained under the high pressure from 0.8 to 2.1 kPa. However, the relative current variation seems to be almost saturated because the AgNW-coated pillars reached the maximum contact area under the large applied pressure (above 2.1 kPa). All the results exhibited that the ILCS-pressure sensors presented the better sensing performance (sensitivity and wide detection range) than most of the previous reported results (Table 1). Additionally, both ILCS and PPCS pressure sensors exhibited better sensory performance than one with flat AgNW-coated PDMS electrode (pillar-flat pressure sensor) over all ranges of pressure, as shown in Figure 5A. When it comes to the PPCSpressure sensor, larger pressure was required to provide sufficient substrate deformation to increase the electrical contact area for electrical resistance detection. In summary, both ILCS- and PPCS-pressure sensors exhibit sensitive

Considering the two different connection geometry and structural effects, the current changes and sensitivity performance were analyzed under different pressures. We have tested the two types of pressure sensors under different pressures applied on the sensors’ surface. Figure 5A showed the electrical response to the pressures of ILCS-pressure sensor and PPCSpressure sensor, respectively. And the relative current variation ratio for both sensors were increased sharply within the low pressure range and became less sensitive under relatively higher applied pressure, demonstrating typical piezoresistive behavior.37,46 It is worth mentioning that the current response of ILCS-pressure sensor presented significantly increased during the low detectable range of 50−800 Pa compared that of PPCS-pressure sensor (50−400 Pa). This is mainly due to the fact that the ILCS geometry would achieve the large contact area easily compared to the PPCS geometry, when the relatively small pressure was applied on the sensor surface. In order to evaluate the operating performance of the sensors, the sensitivity S is essentially and commonly described by formula 1:11,13,20,46−48

( )

δ S=

ΔI I0

δP

(1)

where ΔI is the current change of the sensor under pressure, I0 is the initial current of the sensor, which is taken as response current under the pressure of 50 Pa for both ILCS-pressure sensor and PPCS-pressure sensor.11 In order to avoid overestimating the sensitivity, the response current under the 3201

DOI: 10.1021/acsanm.9b00503 ACS Appl. Nano Mater. 2019, 2, 3196−3205

Article

ACS Applied Nano Materials

Figure 7. (A) Real time current−time response of the sensor for monitoring finger bending motions. (B) Current changes under different bending radius. (C) The current changes under torsion and release for several cycles. (D) Measurement of the movement of Adam’s apple (inset: digital image of the sensor device attached on the protrusion for the measurement).

detection of applied pressure due to the elastic properties of the as-prepared microstructural sensors. The differences of the sensitivity and the detection range are mainly contributed to the assembly methodology of the sensors, where the relative position of the micropillar arrays from two identical sensor sheets played an important role. Even though the PPCS type sensor demonstrated the relatively lower sensitivity under low pressure range, the PPCS-pressure sensor obviously exhibited a wide detectable range up to 12 kPa before saturation (Figure 5C). And Figure 5C also reveals that PPCS-pressure sensor possesses the sensitivity of 8.32 kPa−1 with linearity of R2 = 0.995 during the working pressure from 50 to 400 Pa. Additionally, high sensitivity performance (2.73 kPa−1, R2 = 0.996) was still maintained under the high pressure between 0.4 and 12 kPa. The results demonstrated that the PPCSpressure sensor not only has a wide detection range but also possesses the higher sensitivity performance, compared with many previous reported results.11,13,20,32−38 Combined with the previous research data, results show that our pressure sensors not only have better sensitivity performance but also presented a wide detection range that can be controlled by the different assembling configuration which can be potentially applied for various fields. Figure 5D revealed the steady-state response of the ILCS-pressure sensor under various applied pressures. The results demonstrated that the ILCS-pressure sensor can hold the response characteristics in a wide range of applied pressures in a distinguishable manner and the detection limit of the pressure sensor device was 20 Pa.32

Figure 6A exhibits the current−pressure cyclic response of the ILCS-pressure sensor device under a wide range of different applied pressures of 300, 800, and 1500 Pa, indicating the excellent stability of the pressure sensor for monitoring the wide pressure range. Additionally, the current−pressure cyclic response of the PPCS-pressure sensor device under large compressions of 400, 5300, and 14500 Pa was also presented in Figure S3. Both the sensor devices exhibit swift current responses under the higher compression and restored to their initial isolated state after removing the applied pressure (Figure 6A and Figure S3). However, for the PPCS-pressure sensor device, high pressure is required to produce the high current response compared to that of the ILCS-pressure sensor device. This different current response between the two kinds of pressure sensors were mainly attributed to the variations in microstructures of ILCS geometry and PPCS geometry. Additionally, we monitored the current changes arising from a variety of pressures applied on the sensor surface, including slightly touching with finger (Figure 6B), gas blowing (Figure 6C), and gently scratching (Figure 6D). All the results demonstrated that the ILCS-pressure sensor was capable of transmitting the response signals quickly and accurately according to different applied pressures. Interestingly, all the response signals from slightly touching, gas blowing, and gently scratching sensor surface have a sharp peak, which illustrates a fast response, no hysteresis, and restoration to their initial isolated state with the swift switching (parts B, C, and D of Figure 6). 3202

DOI: 10.1021/acsanm.9b00503 ACS Appl. Nano Mater. 2019, 2, 3196−3205

ACS Applied Nano Materials

■

In order to explore the potential applications in wearable electronic devices, ILCS-pressure sensor was used to monitor human body activities. Figure 7A demonstrated that the robust ILCS-pressure sensor can be attached to the index finger and monitor the finger joint motions, and obvious current variations under different bending radius were recorded. It is evident from Figure 7A that the subtle bending motion could also be promptly and accurately detected by the ILCS-pressure sensor. To achieve the continual current change curves, the index finger was downward with different bending angles and subsequently restored to the original position for several cycles. The results demonstrated that as the finger gradually bent with small bending angle, the current increased with gradual increase of the bending angles of the index finger via a step by step manner. Meanwhile, the current returns to its originally insulated state after the index finger was restored to the original position (photographs in Figure 7A). Similarly, Figure 7B exhibited the current changes in response to the freely bent sensor film under different pressures applied. It demonstrated the reduction of electrical resistance, with increase of the current owing to the increase of pressure and bend extent. Figure 7C revealed the ILCS-pressure sensor was sensitive to torsion stimulus and exhibited relatively stable current changes under the torsion loading and unloading.33 The ILCS-pressure sensor device could also be attached onto the neck to monitor epidermal movement as shown in Figure 7D. It demonstrated the detected current change signals during the mimicking movement of Adam’s apple and displaying repeatable signal patterns corresponding to the motion. Finally, the outstanding sensing performance in bending, torsion, and mimicking movement of human body enabled the pressure sensor to monitor the human body real-time signal in smart wearable or electronic skin devices.

Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsanm.9b00503.

■

Fabrication process of silicon model, SEM images and optical microscopy imaging, and current−pressure plots of the PPCS-pressure sensor obtained with cyclic loading−unloading of different pressures (PDF)

AUTHOR INFORMATION

Corresponding Authors

*B.Z.: e-mail, [email protected]. *Y.T.: e-mail, [email protected]. ORCID

Yongyun Mao: 0000-0001-9538-5772 Bingpu Zhou: 0000-0003-4866-3105 Yanqing Tian: 0000-0002-1441-2431 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors are thankful for the support from National Science Foundation of China (Grants 21574061, 21774054), Multi Year Research Grant MYRG2017-00089-FST from the Research & Development Administration Office at the University of Macau, the Science and Technology Development Fund from Macau SAR (Grants FDCT-073/2016/A2, FDCT-0037/2018/A1), Shenzhen Fundamental Research Programs (Grant JCYJ20170412152922553), and the startup funds of SUSTech (Grants FRG-SUSTC1501A-01, Y01256009).

■

4. CONCLUSIONS We demonstrated a simple and effective fabrication strategy to construct the wearable pressure sensors with high sensitivity and wide detection range using two pieces of AgNW-coated PDMS pillar array sheets, which are face to face assembled into the pressure sensor devices with ILCS geometry and PPCS geometry, respectively. All the experimental results demonstrated that the ILCS-pressure sensor exhibited ultrahigh pressure sensitivity of ∼20.08 kPa−1 up to 0.8 kPa, and sensor response is highly reproducible and repeatable up to 10 000 cycles, performances which are fairly superior to those of existing PDMS pillar-based pressure sensors. Compared with ILCS-pressure sensor, PPCS-pressure sensor exhibited a wide detectable range up to 12 kPa and high sensitivity before saturation, which result from the special interconnection structure. The pressure sensor was used to monitor signals ranging from epidermis movements to air flows, such as gently touching, swallowing action, bending, and torsion. Results showed that the pressure sensors possess high sensitivity, excellent repeatability and reproducibility with superior on/off switching behavior. Due to the excellent performance, simple manufacturing process, and low cost, the proposed pressure sensors constructed with AgNW-coated PDMS pillar arrays sheets have the potential to be used for wearable pressure sensors, artificial e-skin, biosensing, flexible electronic devices, and so forth.

REFERENCES

(1) Schwartz, G.; Tee, B. C. K.; Mei, J.; Appleton, A. L.; Kim, D. H.; Wang, H.; Bao, Z. Flexible polymer transistors with high pressure sensitivity for application in electronic skin and health monitoring. Nat. Commun. 2013, 4, 1859. (2) Wang, C.; Hwang, D.; Yu, Z.; Takei, K.; Park, J.; Chen, T.; Ma, B.; Javey, A. User-interactive electronic skin for instantaneous pressure visualization. Nat. Mater. 2013, 12, 899. (3) Hammock, M. L.; Chortos, A.; Tee, B. C.-K.; Tok, J. B.-H.; Bao, Z. 25th Anniversary Article: The Evolution of Electronic Skin (ESkin): A Brief History, Design Considerations, and Recent Progress. Adv. Mater. 2013, 25, 5997−6038. (4) Yang, Y.; Zhang, H.; Lin, Z.-H.; Zhou, Y. S.; Jing, Q.; Su, Y.; Yang, J.; Chen, J.; Hu, C.; Wang, Z. L. Human Skin Based Triboelectric Nanogenerators for Harvesting Biomechanical Energy and as Self-Powered Active Tactile Sensor System. ACS Nano 2013, 7, 9213−9222. (5) Wang, C.; Wang, C.; Huang, Z.; Xu, S. Materials and Structures toward Soft Electronics. Adv. Mater. 2018, 30, 1801368. (6) Tewari, A.; Gandla, S.; Bohm, S.; McNeill, C. R.; Gupta, D. Highly Exfoliated MWNT−rGO Ink-Wrapped Polyurethane Foam for Piezoresistive Pressure Sensor Applications. ACS Appl. Mater. Interfaces 2018, 10, 5185−5195. (7) Mitrakos, V.; Macintyre, L.; Denison, F. C.; Hands, P. J. W.; Desmulliez, M. P. Y. Design, Manufacture and Testing of Capacitive Pressure Sensors for Low-Pressure Measurement Ranges. Micromachines 2017, 8, 41. (8) Someya, T.; Sekitani, T.; Iba, S.; Kato, Y.; Kawaguchi, H.; Sakurai, T. A large-area, flexible pressure sensor matrix with organic field-effect transistors for artificial skin applications. Proc. Natl. Acad. Sci. U. S. A. 2004, 101, 9966.

3203

DOI: 10.1021/acsanm.9b00503 ACS Appl. Nano Mater. 2019, 2, 3196−3205

Article

ACS Applied Nano Materials (9) Li, T.; Chen, L.; Yang, X.; Chen, X.; Zhang, Z.; Zhao, T.; Li, X.; Zhang, J. A flexible pressure sensor based on an MXene−textile network structure. J. Mater. Chem. C 2019, 7, 1022−1027. (10) Xu, F.; Li, X.; Shi, Y.; Li, L.; Wang, W.; He, L.; Liu, R. Recent Developments for Flexible Pressure Sensors: A Review. Micromachines 2018, 9, 580. (11) Chen, W.; Gui, X.; Liang, B.; Yang, R.; Zheng, Y.; Zhao, C.; Li, X.; Zhu, H.; Tang, Z. Structural Engineering for High Sensitivity, Ultrathin Pressure Sensors Based on Wrinkled Graphene and Anodic Aluminum Oxide Membrane. ACS Appl. Mater. Interfaces 2017, 9, 24111−24117. (12) Ha, M.; Lim, S.; Park, J.; Um, D.-S.; Lee, Y.; Ko, H. Bioinspired Interlocked and Hierarchical Design of ZnO Nanowire Arrays for Static and Dynamic Pressure-Sensitive Electronic Skins. Adv. Funct. Mater. 2015, 25, 2841−2849. (13) Shao, Q.; Niu, Z.; Hirtz, M.; Jiang, L.; Liu, Y.; Wang, Z.; Chen, X. High-Performance and Tailorable Pressure Sensor Based on Ultrathin Conductive Polymer Film. Small 2014, 10, 1466−1472. (14) Mannsfeld, S. C. B.; Tee, B. C. K.; Stoltenberg, R. M.; Chen, C. V. H. H.; Barman, S.; Muir, B. V. O.; Sokolov, A. N.; Reese, C.; Bao, Z. Highly sensitive flexible pressure sensors with microstructured rubber dielectric layers. Nat. Mater. 2010, 9, 859−864. (15) Shuai, X.; Zhu, P.; Zeng, W.; Hu, Y.; Liang, X.; Zhang, Y.; Sun, R.; Wong, C.-p. Highly Sensitive Flexible Pressure Sensor Based on Silver Nanowires-Embedded Polydimethylsiloxane Electrode with Microarray Structure. ACS Appl. Mater. Interfaces 2017, 9, 26314− 26324. (16) Yoon, S. G.; Park, B. J.; Chang, S. T. Highly Sensitive Piezocapacitive Sensor for Detecting Static and Dynamic Pressure Using Ion-Gel Thin Films and Conductive Elastomeric Composites. ACS Appl. Mater. Interfaces 2017, 9, 36206−36219. (17) Lee, Y.; Park, J.; Cho, S.; Shin, Y.-E.; Lee, H.; Kim, J.; Myoung, J.; Cho, S.; Kang, S.; Baig, C.; Ko, H. Flexible Ferroelectric Sensors with Ultrahigh Pressure Sensitivity and Linear Response over Exceptionally Broad Pressure Range. ACS Nano 2018, 12, 4045− 4054. (18) Wei, Y.; Chen, S.; Lin, Y.; Yang, Z.; Liu, L. Cu−Ag core−shell nanowires for electronic skin with a petal molded microstructure. J. Mater. Chem. C 2015, 3, 9594−9602. (19) Pang, C.; Koo, J. H.; Nguyen, A.; Caves, J. M.; Kim, M.-G.; Chortos, A.; Kim, K.; Wang, P. J.; Tok, J. B.-H.; Bao, Z. Highly SkinConformal Microhairy Sensor for Pulse Signal Amplification. Adv. Mater. 2015, 27, 634−640. (20) Park, H.; Jeong, Y. R.; Yun, J.; Hong, S. Y.; Jin, S.; Lee, S.-J.; Zi, G.; Ha, J. S. Stretchable Array of Highly Sensitive Pressure Sensors Consisting of Polyaniline Nanofibers and Au-Coated Polydimethylsiloxane Micropillars. ACS Nano 2015, 9, 9974−9985. (21) Jiang, H.; Khang, D.-Y.; Song, J.; Sun, Y.; Huang, Y.; Rogers, J. A. Finite deformation mechanics in buckled thin films on compliant supports. Proc. Natl. Acad. Sci. U. S. A. 2007, 104, 15607. (22) Sun, Y.; Choi, W. M.; Jiang, H.; Huang, Y. Y.; Rogers, J. A. Controlled buckling of semiconductor nanoribbons for stretchable electronics. Nat. Nanotechnol. 2006, 1, 201. (23) Bae, H. J.; Bae, S.; Yoon, J.; Park, C.; Kim, K.; Kwon, S.; Park, W. Self-organization of maze-like structures via guided wrinkling. Sci. Adv. 2017, 3, e1700071. (24) Ko, H. C.; Stoykovich, M. P.; Song, J.; Malyarchuk, V.; Choi, W. M.; Yu, C.-J.; Geddes, J. B., III; Xiao, J.; Wang, S.; Huang, Y.; Rogers, J. A. A hemispherical electronic eye camera based on compressible silicon optoelectronics. Nature 2008, 454, 748. (25) Park, J.; Kim, M.; Lee, Y.; Lee, H. S.; Ko, H. Fingertip skin− inspired microstructured ferroelectric skins discriminate static/ dynamic pressure and temperature stimuli. Sci. Adv. 2015, 1, e1500661. (26) An, B. W.; Heo, S.; Ji, S.; Bien, F.; Park, J.-U. Transparent and flexible fingerprint sensor array with multiplexed detection of tactile pressure and skin temperature. Nat. Commun. 2018, 9, 2458. (27) Miyamoto, A.; Lee, S.; Cooray, N. F.; Lee, S.; Mori, M.; Matsuhisa, N.; Jin, H.; Yoda, L.; Yokota, T.; Itoh, A.; Sekino, M.;

Kawasaki, H.; Ebihara, T.; Amagai, M.; Someya, T. Inflammation-free, gas-permeable, lightweight, stretchable on-skin electronics with nanomeshes. Nat. Nanotechnol. 2017, 12, 907. (28) Wei, Y.; Chen, S.; Lin, Y.; Yuan, X.; Liu, L. Silver nanowires coated on cotton for flexible pressure sensors. J. Mater. Chem. C 2016, 4, 935−943. (29) Kim, K. K.; Hong, S.; Cho, H. M.; Lee, J.; Suh, Y. D.; Ham, J.; Ko, S. H. Highly Sensitive and Stretchable Multidimensional Strain Sensor with Prestrained Anisotropic Metal Nanowire Percolation Networks. Nano Lett. 2015, 15, 5240−5247. (30) Joo, Y.; Byun, J.; Seong, N.; Ha, J.; Kim, H.; Kim, S.; Kim, T.; Im, H.; Kim, D.; Hong, Y. Silver nanowire-embedded PDMS with a multiscale structure for a highly sensitive and robust flexible pressure sensor. Nanoscale 2015, 7, 6208−6215. (31) Woo, S.-J.; Kong, J.-H.; Kim, D.-G.; Kim, J.-M. A thin allelastomeric capacitive pressure sensor array based on micro-contact printed elastic conductors. J. Mater. Chem. C 2014, 2, 4415−4422. (32) Kim, K.-H.; Hong, S. K.; Jang, N.-S.; Ha, S.-H.; Lee, H. W.; Kim, J.-M. Wearable Resistive Pressure Sensor Based on Highly Flexible Carbon Composite Conductors with Irregular Surface Morphology. ACS Appl. Mater. Interfaces 2017, 9, 17499−17507. (33) Zhang, S.; Wang, F.; Peng, H.; Yan, J.; Pan, G. Flexible Highly Sensitive Pressure Sensor Based on Ionic Liquid Gel Film. ACS Omega 2018, 3, 3014−3021. (34) You, I.; Choi, S.-E.; Hwang, H.; Han, S. W.; Kim, J. W.; Jeong, U. E-Skin Tactile Sensor Matrix Pixelated by Position-Registered Conductive Microparticles Creating Pressure-Sensitive Selectors. Adv. Funct. Mater. 2018, 28, 1801858. (35) Xia, K.; Wang, C.; Jian, M.; Wang, Q.; Zhang, Y. CVD growth of fingerprint-like patterned 3D graphene film for an ultrasensitive pressure sensor. Nano Res. 2018, 11, 1124−1134. (36) Park, S.-J.; Kim, J.; Chu, M.; Khine, M. Flexible Piezoresistive Pressure Sensor Using Wrinkled Carbon Nanotube Thin Films for Human Physiological Signals. Adv. Mater. Technol. 2018, 3, 1700158. (37) Ge, G.; Cai, Y.; Dong, Q.; Zhang, Y.; Shao, J.; Huang, W.; Dong, X. A flexible pressure sensor based on rGO/polyaniline wrapped sponge with tunable sensitivity for human motion detection. Nanoscale 2018, 10, 10033−10040. (38) Dong, X.; Wei, Y.; Chen, S.; Lin, Y.; Liu, L.; Li, J. A linear and large-range pressure sensor based on a graphene/silver nanowires nanobiocomposites network and a hierarchical structural sponge. Compos. Sci. Technol. 2018, 155, 108−116. (39) Zhang, Y.; Guo, J.; Xu, D.; Sun, Y.; Yan, F. One-Pot Synthesis and Purification of Ultralong Silver Nanowires for Flexible Transparent Conductive Electrodes. ACS Appl. Mater. Interfaces 2017, 9, 25465−25473. (40) Li, B.; Ye, S.; Stewart, I. E.; Alvarez, S.; Wiley, B. J. Synthesis and Purification of Silver Nanowires To Make Conducting Films with a Transmittance of 99%. Nano Lett. 2015, 15, 6722−6726. (41) Mao, Y.; Liu, Z.; Liang, L.; Zhou, Y.; Qiao, Y.; Mei, Z.; Zhou, B.; Tian, Y. Silver Nanowire-Induced Sensitivity Enhancement of Optical Oxygen Sensors Based on AgNWs−Palladium Octaethylporphine−Poly(methyl methacrylate) Microfiber Mats Prepared by Electrospinning. ACS Omega 2018, 3, 5669−5677. (42) Mao, Y.; Yang, H.; Hu, C.; Guo, J.; Meng, X.; Yang, Y. Largescale synthesis of AgNWs with ultra-high aspect ratio above 4000 and their application in conductive thin film. J. Mater. Sci.: Mater. Electron. 2017, 28, 5308−5314. (43) Mao, Y.; Zhao, Q.; Pan, T.; Shi, J.; Jiang, S.; Chen, M.; Zhou, B.; Tian, Y. Platinum porphyrin/3-(trimethoxysily)propylmethacrylate functionalized flexible PDMS micropillar arrays as optical oxygen sensors. New J. Chem. 2017, 41, 5429−5435. (44) Mao, Y.; Gao, Y.; Wu, S.; Wu, S.; Shi, J.; Zhou, B.; Tian, Y. Highly enhanced sensitivity of optical oxygen sensors using microstructured PtTFPP/PDMS-pillar arrays sensing layer. Sens. Actuators, B 2017, 251, 495−502. (45) Zhou, B.; Gao, Y.; Mao, Y.; Wen, W. Facile preparation of superhydrophobic PDMS with patternable and controllable water adhesion characteristics. J. Mater. Sci. 2017, 52, 11428−11441. 3204

DOI: 10.1021/acsanm.9b00503 ACS Appl. Nano Mater. 2019, 2, 3196−3205

Article

ACS Applied Nano Materials (46) Su, B.; Gong, S.; Ma, Z.; Yap, L. W.; Cheng, W. MimosaInspired Design of a Flexible Pressure Sensor with Touch Sensitivity. Small 2015, 11, 1886−1891. (47) Bae, G. Y.; Pak, S. W.; Kim, D.; Lee, G.; Kim, D. H.; Chung, Y.; Cho, K. Linearly and Highly Pressure-Sensitive Electronic Skin Based on a Bioinspired Hierarchical Structural Array. Adv. Mater. 2016, 28, 5300−5306. (48) Lee, D.; Lee, H.; Jeong, Y.; Ahn, Y.; Nam, G.; Lee, Y. Highly Sensitive, Transparent, and Durable Pressure Sensors Based on SeaUrchin Shaped Metal Nanoparticles. Adv. Mater. 2016, 28, 9364− 9369.

3205

DOI: 10.1021/acsanm.9b00503 ACS Appl. Nano Mater. 2019, 2, 3196−3205