Flexible and Highly Sensitive Pressure Sensor Based on Microdome

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A Flexible and Highly Sensitive Pressure Sensor Based on Microdome-Patterned PDMS Forming with Assistance of Colloid Self-Assembly and Replica Technique for Wearable Electronics Yuan Zhang, Yougen Hu, Pengli Zhu, Fei Han, Yu Zhu, Rong Sun, and Ching-Ping Wong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b09617 • Publication Date (Web): 27 Sep 2017 Downloaded from http://pubs.acs.org on September 28, 2017

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ACS Applied Materials & Interfaces

A Flexible and Highly Sensitive Pressure Sensor Based on Microdome-Patterned PDMS Forming with Assistance of Colloid Self-Assembly and Replica Technique for Wearable Electronics Yuan Zhang†,‡, ║, Yougen Hu†, ║, Pengli Zhu†,*, Fei Han†,‡, Yu Zhu†, Rong Sun†,*, and Ching-Ping Wong⊥,# †

Shenzhen Institutes of Advanced Technology, Chinese Academy of Sciences, Shenzhen 518055,

China. ‡

Department of Nano Science and Technology Institute, University of Science and Technology of

China, Suzhou 215123, China. ⊥

Department of Electronics Engineering, The Chinese University of Hong Kong, Hong Kong

999077, China. #

School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia

30332, USA. *

Address corresponding to: [email protected]; [email protected]

║

These authors contributed equally to this work.

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ABSTRACT Flexible pressure sensors are one of the vital component units in the next generation of wearable electronics for monitoring human physiological signals. In order to improve the sensing properties of the sensors, we demonstrate flexible, tunable resistive pressure sensors based on elastic microstructured polydimethylsiloxane (PDMS) film via a simple, low-cost colloid self-assembly technology, which using monodispersed polystyrene (PS) microspheres as monolayer and ordered sacrificial template. The sensors exhibit high sensitivity of -15 kPa-1 in the low pressure (< 100 Pa), fast response time (< 100 ms), high stability over 1,000 cycles of pressure loading/unloading, low pressure detection limit of 4 Pa, and wide working pressure regime ( 10 kPa, enabling object manipulation) have attracted extensive investigation and made great progress. However, the challenges of how to fabricate pressure sensors with low-cost, high sensitivity, rapid response, and high reliability are still exist. Introducing advanced sensing materials and constructing surface microstructures have been proved two effective ways to gain high sensitivity of the flexible pressure sensors by many reports. In particular, microstructured pressure sensors show promise in producing high sensitivity flexible devices due to their high flexibility and sensitivity. Polydimethylsiloxane (PDMS) has been overwhelmingly used for the fabrication of flexible devices owing to its commercial availability and unique mechanical, chemical and optical properties. For example, Bao et al.14 used a new type of pyramid-structure PDMS thin films as dielectric layers to fabricate flexible pressure sensors which exhibited high sensitivity of 0.55 kPa-1 in the < 2 kPa pressure range substantially larger than that of the unstructured film (0.02 kPa-1) and they could sense a very small pressure as low as 3 Pa. Chen et al.20 also manufactured a flexible tactile sensor with high sensitivity at 3



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low pressure range (-5.5 kPa−1, < 100 Pa) based on microstructured graphene film. In order to improve the sensitivity and decrease the viscoelasticity of the flexible matrix, introducing microstructure is crucial in design of the flexible devices. As recently studies reported, microstructured or patterned polydimethylsiloxane (PDMS) has been demonstrated the most effective routes to improve the sensitivity of the resistive-type sensors due to its excellent elasticity and biocompatibility. Pyramid,21-23 micropillar,24 nanofibre,1 microdome25,26 have been reported to endow the elastic materials with high sensitivity to serve as electronic skins. Generally, all of the fabricated elastic micro-structured films which have the inverse features of the mould microstructures are prepared by conventional template techniques based on artificial Si wafer mould14,20,27-29 or natural biomaterials.30-32 Si micro-structured arrays, such as the typical pyramidal microstructures can be easily designed and modulated by lithography method, however, the fabrication of Si micro-structured mould is complicated, high-dependency on equipment, multi-step and high-cost manufacturing processes, such as photoresist coating, soft bake, exposure, development, hard bake, etching and strip photoresist. Natural biomaterials, such as rose petals,30,33,34 lotus leaves,10,34 mimosa leaves31 and butterfly wings,34,35 possessed of microstructured surfaces can be directly used to replicate the micro-structured patterns. But it is difficult to adjust the geometrical parameters of the microstructures due to their intrinsic characteristics. Therefore, a low-cost and simple fabrication technology is still highly desirable to construct highly sensitive pressure sensors with tunable microstructured surfaces that can be used in wide pressure ranges especially in 4


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low-pressure regimes. Herein, we demonstrate a novel and low-cost method, based on simple colloid self-assembly and PDMS-PDMS pattern transfer technology, to fabricate highly sensitive

pressure

sensors.

Specifically,

the

morphology

of

the

elastic

microdome-patterned PDMS films is tunable by tailoring the diameter of the monodispersed polystyrene (PS) microspheres. Therefore, the sensitivity of the pressure sensor can be modified by adjusting the feature size of the microstructure. The as-prepared flexible pressure sensors exhibit highly sensitive detection capability (-15 kPa−1 of sensitivity), fast response time (< 100 ms), high stability under repeated loading (1,000 cycles), a low limit of detection (4 Pa), and wide working pressure regime (< 5 kPa) by optimizing the size of PS microspheres. Furthermore, we also demonstrate that a fully functional wearable electronic skin with 5 × 5 sensor arrays can detect the spatial distribution in real-time. The flexible sensors are successfully applied for distinctively monitoring biological signals such as neck pulse, which exhibits great promise for innovative applications in wearable health monitoring devices. 2. EXPERIMENTAL SECTION 2.1 Materials: Poly(vinylpyrrolidone) (PVP, K-30), 2,2'-azobisisobutyronitrile (AIBN) were obtained from Shanghai Chemical Reagent Co. (China) and used as received. Styrene (St) was purchased from Shanghai Chemical Reagent Co. (China) and purified by treating with 5 wt% NaOH aqueous solution to remove the inhibitor. Acetone (≥99.5%), ammonia solution (NH3·H2O, 25-28%), hydrogen peroxide (H2O2, 5



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≥99.8%), tetrahydrofuran (THF, ≥99.0%) were all purchased from Shanghai Lingfeng Chemical Reagent Co., Ltd. Ethyl acetate was provided by Sinopharm Chemical Reagent Co., Ltd. Sulfuric acid (H2SO4, ≥95%), and absolute ethanol (EtOH) were provided by Dongguan Dongjiang Chemical Reagent Co., Ltd. PDMS prepolymer and the curing agent were supplied as two-part liquid component kits from Dow Corning (Sylgard 184). Deionized water with resistivity > 18.2 MΩ·cm was used for all experiments. 2.2 Preparation of the monodispersed polystyrene colloid spheres: Monodispersed polystyrene (PS) microspheres were synthesized via dispersion polymerization as described in our previous report with some modification.36 The typical recipe for PS microspheres with diameter of 2 µm was that 1.0 g PVP, 45 g EtOH, and 5.0 g H2O were simultaneously added into a 500 mL four-necked flask equipped with mechanical stirrer, thermometer with temperature controller, N2 inlet, a Graham condenser, and a heating mantle. After ultrasonically dissolved, a mixture of 15 g St and 1.0 g AIBN were charged into the flask. The mixture undergo deoxygenation by bubbling nitrogen gas at room temperature for about 30 min and then heated to 70 °C with the agitation rate of 200 rpm for 2 h, followed by addition of a solution containing 15 g of St and 45 g of EtOH. The reaction was carried out for 24 h and then cooled to room temperature. The obtained PS beads were separated from the reaction medium by successive centrifugation and redispersion with ethanol. The PS spheres with diameter of 5.6 µm were also prepared by the dispersion polymerization method as described in our previous work.37 6


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2.3 Preparation of the monolayer PS spheres array: A glass substrate (5 cm × 5 cm) was cleaned with acetone, ethanol, and deionized water in an ultrasonic bath, and then soaked in piranha solution (H2O2:H2SO4=3:7, volume ratio) for 30 min and subsequently rinsed with deionized water. Thereafter, the cleaned glass was sonicated in solution (H2O:H2O2:NH3·H2O=5:1:1, volume ratio) for 30 min to increase the hydrophilicity of the surface and followed by rinsing repeatedly with ethanol, and deionized water again and finally dried with nitrogen stream. Above all, the glass substrate was treated with oxygen plasma for 3 min. After this thorough cleaning, the surface of the glass was sufficiently hydrophilic. Then the close-packed monolayer PS array was prepared as follows. Monodispersed PS spheres suspensions (6 wt% in ethanol) were sonicated for 10 min. A cleaned glass substrate was flatwise placed and deionized water was dropped on it to form a water film covering the whole glass. The PS spheres suspension was then injected into the water surface, and PS microspheres were self-assembled at the air/water interface to obtain a large-area monolayer PS array after the water was evaporated. 2.4 Fabrication of the PDMS film with convex microdome: Liquid PDMS monomer was mixed with the curing agent (10:1, weight ratio) under mechanical stirring and then the mixture was degassed in a vacuum chamber to remove the trapped bubbles. PDMS mixture was slowly poured onto the top of the PS array and then placed inside the vacuum chamber for degassing again followed by heat cured at 90 °C for 2 h, forming a ~2 mm thick elastomer film. The cured PDMS film was peeled off from the glass substrate and immersed in THF solvent to dissolve the PS 7



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spheres. The prepared PDMS template with concave microhole was then treated with plasma for 4 min to get a rough surface. The plasma-treated PDMS template was covered with 80 wt% PDMS diluted with ethyl acetate and placed in a vacuum chamber for 5 h to ensure that the PDMS patterns were fully filled with the PDMS slurry. The desired PDMS replica was cured at 60 °C for 2 h and then peeled off from the PDMS template to obtain a PDMS film with convex microdome.38 2.5 Fabrication of the flexible pressure sensor: The PDMS film (1.5 cm × 1.5 cm) with convex microdome was coated with nanogold as a thin electrically conductive layer by magnetron sputtering. The surface resistance of the microdome PDMS film with nanogold is about 8-10 Ω. Then the pressure sensor was configured by interlocking two microdome PDMS films with the microdome surfaces facing each other. Copper tapes as test electrodes were bonded with the microdome PDMS film by commercial conductive adhesive. A pressure sensor array of 5 × 5 pixels was fabricated by using two face-to-face microstructured PDMS films forming a conventional crossbar configuration. Each of the microstructured PDMS films (5.5 cm × 5.5 cm) was selectively deposited with gold layer by a metal shadow mask to define the desired size. Then the two films were combined to form 25 cross-link areas (6 mm × 6 mm for each area). 2.6 Characterization: The morphologies and microstructures of the micropatterned PDMS films were characterized by field emission scanning electron microscope (FE-SEM, FEI Nova Nano SEM 450), Dimension Icon atom force microscopy (AFM, Bruker) and a three-dimension (3D) surface profiler (NanoFocus). The resistance was 8


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measured by an Agilent 34401A multimeter. The sensitivity of the sensors was measured by applying an external load using an electronic universal testing machine (Shimadzu AG-X plus, 100 N) in combination with GWINSTEK LCR-6002 meter. Response time was recorded by the LCR-6002 meter. The current-voltage (I-V) characteristics of the sensors were determined using a semiconductor parameter analyzer (Keithley, 4200-SCS).

3. RESULTS AND DISCUSSION A schematic illustration of the entire fabrication of the microdome structured pressure sensor is presented in Figure1a. The fabrication of the microstructure was a whole chemical preparation process and independent of the traditional lithography technique, realizing the tunability of the microstructure. The PS microspheres were prepared by the dispersion polymerization method. Figure Sl in the Supporting Information shows the SEM images of PS beads with different diameters. It can be seen that both of them are highly uniform spherical particles with average diameter of 2 µm (Figure S1a) and 5.6 µm (Figure S1b) measured from at least one hundred PS spheres, respectively. The uniform as-prepared PS microspheres were used to fabricate the monolayer PS microspheres array via colloid self-assembly technology. As shown in Figure1b, the monolayer 2 µm-PS microspheres show a large area periodic and close-packed array on glass substrate. From the magnified image inserted in Figure 1b, it can be seen that the close-packed array exhibits an orthohexagonal structure, which further confirms the existence of the excellent self-assembly PS microspheres array. The PS array was 9



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then covered by the PDMS layer and peeled off from the glass substrate. To obtain the concave PDMS layer, the PS array was effectively sacrificed by immersing the sample in THF solvent to remove PS microspheres embedded in the surface of the PDMS layer. Figure 1c shows the SEM image of the obtained inverse-opal-structure PDMS layer, which possesses a uniform bowl-like surface without any residue of PS spheres, indicating the successfully and completely removal of the PS spheres. Typically, the concave PDMS film shows iridescent color, as shown in Figure S2 (Supporting Information), indicating the periodic array due to the diffraction of the film. The AFM image of the concave PDMS film shown in Figure 1h further clearly demonstrates its surface morphology. Figure S3a and S3c (Supporting Information) show the 2D SPM images and line profile at the bottom of the concave, which represent that the periodic concave holes of the PDMS film are ~2 µm in diameter and ~900 nm in depth. These uniform hemispherical holes suggest that the PS microspheres array is fully embedded in PDMS film surface in plane direction and the PS microspheres with half of the height are embedded in vertical direction due to the close-packed structure. Figure 1d shows the SEM image of the corresponding PDMS film with convex microdome replicated from the concave PDMS. It can be seen that the microstructured PDMS array is regular and remarkably uniform. The successful fabrication of the micropatterned PDMS with microdome array is also reflected in the AFM image as shown in Figure 1i. As shown in Figure S3b and S3d (Supporting Information), the perfect hexagonally arranged dome is approximately 2 µm in diameter and 600 nm in height, which is basically consistent with the size of the 10


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corresponding concave PDMS, indicating a successful and complete replication of the surface microstructure from the concave hole array. The damage-free replica is mainly attributed to the plasma treatment of the concave PDMS films, which offers a rough surface to prevent from the strong sticking of the two PDMS films. The monolayer PS spheres array with diameter of 5.6 µm was also fabricated by the same self-assembly method. The SEM images of the self-assembly monolayer 5.6 µm-PS spheres, the corresponding concave PDMS and convex PDMS film are shown in Figure 1e, 1f and 1g, respectively. They all exhibit perfect microstructure with order arranged arrays, which are similar to that of the PS microspheres with diameter of 2 µm. It should be pointed out that the convex PDMS film fabricated by 5.6 µm-PS spheres possess order microdomes with diameter of ~5 µm in plane direction and pitch of ~2.5 µm in vertical direction as shown in the AFM image of Figure 1j, 1k and Figure S4a-d (Supporting Information), indicating the size of the surface microstructure can be easily tunable by using PS spheres with different diameters. Figure 1l (left) displays the photograph of the pressure sensor (3 cm × 3 cm). The pressure sensor was obtained by intergrating two nanogold-coated PDMS films with surface microstructured array. The surface topography of the single layer of PDMS microstructure array is investigated by Figure 1l (right), presenting the uniform micro-dome array with continuous gold layer film on PDMS surface, and the thickness of the nanogold layer is about 77 nm (Figure S5).

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Figure 1. Microdome-based flexible pressure sensors. (a) Schematic illustration of the fabrication process of flexible PDMS films with surface microdome array and the corresponding pressure sensors. SEM images of: (b-d) The monolayer PS spheres arrays fabricated by 2 µm-PS microspheres, the concave PDMS film, and the PDMS surface with micro-dome patterns. The insets show enlarged images of the corresponding orthohexagonal structures. (e-g) The PS spheres arrays fabricated by 5.6 µm-PS microspheres, the concave PDMS film, and the PDMS surface with micro-dome patterns. The inset images show magnified views of the corresponding orthohexagonal structures. 3D surface morphologies of: (h-i) The concave PDMS film, and the PDMS surface with micro-dome patterns with 1 µm depth and 2 µm diameter. (j-k) The concave PDMS film, and the PDMS surface with micro-dome patterns with 13



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2 µm depth and 5.6 µm diameter. (l) Digital photograph of the fabricated pressure sensor and the top view SEM image of the PDMS nanostructured layer with nanogold.

The pressure-sensing capabilities of the fabricated pressure sensors based on 2 µm and 5.6 µm sized PS sphere are tested by measuring relative resistance changes. The pressure sensitivity (S) can be calculated by S = δ(∆R/R0)/δP, where p denotes the applied pressure, and ∆R and R0 denote the resistance change with applied pressure and initial resistance without pressure or load, respectively. Figure 2a provides the relative changes in resistance of the pressure sensors based on 2 µm and 5.6 µm sized PS sphere, which were measured against a linearly increasing pressure. And the corresponding sensitivity of the magnified low-pressure range is shown in Figure S6 (Supporting Information). It can be seen that the resistances of both sensors firstly decrease rapidly and then keep nearly constant with gradually increasing of the pressure, showing a typical two-range linear character. According to the response curves, in the low-pressure range, the sensitivity of the pressure sensor based on 2 µm sized PS sphere is -15 kPa−1 (< 100 Pa), which is higher than the sensitivity of the pressure sensor based on 5.6 µm sized PS sphere (-2 kPa−1) (< 400 Pa), indicating better sensing ability of the former than the latter and high sensitivity of the two sensors upon low-pressure. The reason for the difference between sensitivities of the pressure sensors based on different PS sphere sizes is related to the variation of surface area. The microstructured films with smaller feature sizes consist of a larger 14


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number of microdome patterns in the same area, as illustrated in Figure 2b, resulted in more severe deformation, greater change in contact area, and greater relative variation in resistance when the same pressure is applied to the sensor. In the high-pressure range of response curves, the sensitivities of the sensors based on 2-µm PS and 5.6-µm PS are only -0.003 kPa−1 (0.1~1.6 kPa) and -0.012 kPa−1 (0.4~1.6 kPa), respectively, indicating low sensitivity of the sensors upon high pressure. It also can be seen that the saturation pressure of the two sensors is about 0.1 kPa and 0.4 kPa, respectively. This result can be explained by the deformation behavior of the elastomeric PDMS microdomes under pressure. For small sized PDMS microdomes, they can endure lower pressure for their maximum mechanical deformations than that of the large sized PDMS microdomes when the pressure was applied. Therefore, the sensitivities and pressure ranges of sensors can be easily adjusted by regulating the feature sizes of microstructures, which can be realized by the self-assembly of different sized PS sphere.

Figure 2. (a) Sensitivity of different pressure sensors based on 2 µm and 5.6 µm sized PS sphere. (b) Schematic representation for the changes as the same external pressure is applied upon the pressure sensors based on 2 µm and 5.6 µm sized PS sphere. 15



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In order to evaluate the effect of the surface microstructure to the sensitivity of the sensors, three different pressure sensors based on 2 µm PS sphere of single planar type without surface microstructure, single microdome type and interlocking microdome type were fabricated (Figure 3a). Their electrical resistance changes under forcing external pressure were recorded and compared in Figure 3b. All of them exhibit a decrease in resistance as the pressure increase, the responses of the interlocked microdome sensors are distinctly different from that of the planar sensor. As shown in Figure S7 (Supporting Information), we can see the significant response for the microstructured sensors in response to the pressure, especially in the low-pressure range. When the applied pressure is less than 100 Pa, the sensitivity of the interlocked microdome sensor is as high as -15 kPa−1 (0-100 Pa), which is obviously higher than the sensitivity of the unstructured sensor of -0.01 kPa-1 (0-150 Pa) and single micro-structured sensor of -1.3 kPa-1 (0-230 Pa). The sensitivity of this interlocked pressure sensor with surface microdomes is significantly higher than that of previous reports such as a resistive pressure sensor with a micropapillae structure (1.35 kPa−1, 0-2 kPa)30 and a biodegradable pressure sensor with square pyramids structure (0.76 kPa−1, 0-2 kPa)39. When the applied pressure is higher than 100 Pa, the sensitivity of the interlocked microstructured sensor is about -0.002 kPa-1 and that of the unstructured sensor and single micro-structured sensor is -0.001 kPa-1 and -0.001 kPa-1, respectively. The observed distinct difference in sensitivity demonstrates that the interlocked microstructured sensor is very sensitive and much higher than that of 16


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the sensor with a planar surface and can provide more contact regions when subjected to the same applied pressure. Therefore, the sensitivity of the structured sensor for external pressure can be effectively enhanced compared with the pressure sensor using unstructured PDMS. This result is mainly attributed to the decreased contact resistance between the two interlocked conductive films which generated by the increased contact area under external load. The remarkable sensing performance of our sensor is ascribed to the change of the contact area, which is caused by the deformation of the microstructure. The electrical characteristics of the pressure sensor were also investigated. The current-voltage (I-V) curves of the pressure sensor under different weights are presented in Figure 3c. The applied pressure is kept constant while sweeping the voltage from -0.1 V to 0.1 V. As the applied load increases, the slope of the I-V curves increases, indicating the resistance decrease as external pressure rises. The linear character of the I-V curves indicates that the device behavior obeys the Ohm's contact characteristics. To evaluate the reproducibility and stability properties of the pressure sensor, the as-fabricated sensor was applied with series of external pressures ranging from 11 Pa to 1250 Pa. The relative changes in resistance under the increasing loading are monitored and shown in Figure 3d, which presents comparisons of the static resistance real-time responses of the pressure sensor for 7 loading/unloading cycles under low pressure (11 Pa), medium pressure (50 Pa, 125 Pa and 150 Pa), and relatively high pressure (500 Pa and 1250 Pa), respectively. It is obvious that relative 17



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changes in resistance repeatedly change under various external pressures when they increase from 11 Pa to 1250 Pa, indicating stable sensing performance and great reproducibility of the pressure sensor upon various pressures regardless of low pressure, medium pressure, or high pressure. It also demonstrates that the sensor rapidly responds to the loading/unloading applied force without hysteresis. As shown in Figure 3e, the response and relaxation time upon the application of a repeated pressure of 100 Pa was detected. The relative changes in resistance for the 6 cycles are almost the same, demonstrating the cycling stability of the flexible sensor under the pressure of 100 Pa. The insets indicate the instant response times of our sensor are less than 100 ms both pressure loading and unloading, which is better than that of previously reported pressure sensors (< 150 ms)40. Furthermore, the cycling test of the pressure sensor was conducted under loading and unloading an applied pressure of 100 Pa for 1,000 cycles. As shown in Figure 3f, the relative resistance changes remain about the same value after 1,000 loading/unloading cycles, which demonstrates high durability of the pressure sensor. We attribute this high durability to the excellent elasticity of the PDMS microdomes which can endure many mechanical deformation cycles. According to the enlarged insert, the resistance changes with loading pressure of the pressure sensor are relatively constant after 1,000 loading–unloading cycles, implying good reliability and high stability of the pressure sensor.

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Figure 3. Pressure-sensing capabilities of the microstructured pressure sensors. (a) The diagram of three different sensor structures. (b)The comparison of pressure sensitivities of different sensor structures: the planar film (blue), single microdome film (black), and the interlocked microdome film (red). (c) Current-voltage (I-V) curves of the pressure sensor with different applied pressures. (d) Time-resolved static resistance change response of the sensor under repeated mechanical loads, with different pressures ranging from 11 Pa to 1250 Pa. (e) Response time of the pressure 19



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sensor with an applied pressure of 100 Pa. The insets show the section of the curve within the dashed lines which corresponds to the loading of pressure. (f) Relative resistance changes of the sensor with repetition of 1,000 loading/unloading cycles by 100 Pa. The inset shows the resistance change curves of the sensor extracted from the red part.

In order to demonstrate the detect capacity upon low pressure of the sensor, we monitored the resistance changes arising from a variety of pressure sources such as a lightweight flower and neck pulse. Figure 4a shows the real-time resistance changes of the sensor by the placement and removal of a dry rose flower (~17 mg) on the top of the sensor, which corresponds to a tiny pressure of only ~4 Pa. The dry rose was repeatedly placed on the sensor with a size of 1.5 cm × 1.5 cm and rapidly removed from the sensor, resulting in the resistance decreases and increases alternately. It demonstrates that the sensor is highly sensitive and capable of accurately detecting the subtle variations in pressure. This result is better than the report of the capacitive pressure sensor with the multiscale-structured electrode (15 Pa)41. Because of the high sensitivity and fast response, the pressure sensor can also be used to detect random finger touch. Figure 4b shows the dynamic resistance variation responding to random finger touch, demonstrating the pressure sensor can sense and distinguish different levels of pressure. We can see that the resistance sharply decreases on a finger press and timely returns to its initial resistance after withdraw the finger.

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For the biological monitoring and clinical diagnosis, wearable electronic has been paid much more attentions recently. To demonstrate the application possibility of our pressure sensor in monitoring physical signals such as the neck pulse, the sensor was fixed to the carotid artery by 3Mtegaderm absorbent and neck pulse is monitored (the resistance changes of the sensor were recorded in real-time). As shown in Figure 4cand 4d, three subtle peaks represented to early systolic peak pressure (P1), late systolic peak pressure (P2) and diastolic pulse waveform (P3) are observed, exhibiting that the sensor can accurately and clearly detect the pulses and can be used to assess physiological diagnosis. The relative resistance change shows the neck pulse frequency is about 66 beats per minute which consistent with an adult, proving that the sensor is potential to serve as a wearable device with high sensitivity and fast-response for monitoring very weak physiological signals in real time.

Figure 4. The corresponding resistance signals for several loading/unloading cycles. 21



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(a) Response and relaxation of the pressure sensor for loading and unloading of a dry rose flower (weight: 17 mg) with a pressure of 4 Pa. (b) The measurement of a finger random touching. (c) The measurement of neck pulse patterns by the sensor attached on the carotid artery. (d) The enlarged view of the single pulse of Figure (c).

To intuitively illustrate the sensing ability of space pressure distribution, the sensors were pixelated into a 5 × 5 array for collecting spatial pressure changes information. Figure 5 presents the pressure sensor array is successively placed four different paperboards with S, I, A and T shapes (about 2.3 g, 1.4 g, 2.2 g and 1.5 g, corresponding to ~26 Pa, 16 Pa, 24 Pa, and 17 Pa, respectively), and a bright black color represents a higher relative resistance change. It can be seen that the loaded areas show apparent darker color than other areas, showing large decrements of resistance, indicating the spatial pressure distribution of the slight weight object is clearly recognized. The demonstration simultaneously proves that the high sensitivity of our sensors. As shown in Figure S7 (Supporting Information), it can be found that the pressure sensor array has excellent flexibility and can be bent and fitted the human palm, which is expected to be applied to wearable sensing devices.

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Figure 5. Spatial pressure distribution capability test of the 5 × 5 sensor arrays using paperboards that are shaped as the letters “S”, “I”, “A” and “T”.

4. CONCLUSIONS In conclusion, we have developed a new method whereby the sensitivity of the pressure sensors can be changed by adjusting the feature size of microstructure. The flexible resistive pressure sensors based on the different size PS spheres and microstructured PDMS films are successfully prepared. The pressure sensors exhibit fast response, good durability, stability over 1,000 working cycles, and high sensitivity in the low-pressure range. This designed pressure sensors can real-time detect very weak pressure down to 4 Pa and clearly perform reliable finger-touch and neck pulse monitoring. A 5 × 5 array of the pressure sensor shows the high sensitivity and can accurately recognize spatial pressure distribution information on a plane. In addition, the size of the microdomes i.e. the performance of the pressure sensors can be easily tuned by adjusting PS microspheres diameter, and it does not involve complex process and equipment for fabricating the microstructure. The low-cost, 23



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simple fabrication process and excellent sensitivity demonstrate the possibility of our pressure sensor to use as future wearable electronics for human-health monitoring.

ASSOCIATED CONTENT

Supporting Information The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. SEM image of PS beads with 2 µm and 5.6 µm diameter (Figure S1), Photograph of the concave PDMS film (Figure S2), 2D SPM image and line profile at the bottom of the concave and at the top of the microdome based on 2 µm-PS microspheres (Figure S3), 2D SPM image and line profile at the bottom of the concave and at the top of the microdome based on 5.6 µm-PS microspheres (Figure S4).

AUTHOR INFORMATION Corresponding Authors *Address corresponding to: [email protected] and [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors are grateful for the ﬁnancial support from National Key R&D Project from Minister of Science and Technology of China (2016YFA0202702, 24


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2017YFB0406000), National Natural Science Foundation of China (61701488 and 21571186), Guangdong Provincial Key Laboratory (2014B030301014), Youth Innovation Promotion Association (2017411), Guangdong TeZhi plan youth talent of science

and

technology

(2014TQ01C102),

Shenzhen

basic

research

plan

(JCYJ20140610152828685 and JSGG20150512145714246) and SIAT Innovation Program for Excellent Young Researchers (2016005).

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Flexible and Highly Sensitive Pressure Sensor Based on Microdome

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