Facile Preparation of Hybrid Structure Based on Mesodome and

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Functional Inorganic Materials and Devices

Facile Preparation of Hybrid Structure based on Mesodome and Micro-pillar Array as Flexible Electronic Skin with Tunable Sensitivity and Detection Range Bing Ji, Yongyun Mao, Qian Zhou, Jianhe Zhou, Ge Chen, Yibo Gao, Yanqing Tian, Weijia Wen, and Bingpu Zhou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b08419 • Publication Date (Web): 15 Jul 2019 Downloaded from pubs.acs.org on July 18, 2019

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ACS Applied Materials & Interfaces

Facile Preparation of Hybrid Structure based on Meso-dome and Micro-pillar Array as Flexible Electronic Skin with Tunable Sensitivity and Detection Range Bing Ji,a Yongyun Mao,a,b Qian Zhou,a Jianhe Zhou,c Ge Chen,a Yibo Gao,d Yanqing Tian,b Weijia Wen,d and Bingpu Zhoua,e,* a

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 b

Department of Materials Science and Engineering, Southern University of Science and Technology, No.

1088, Xueyuan Rd., Xili, Nanshan District, Shenzhen, Guangdong 518055, China c

Spinal Joint Surgery, Kanghua Hospital, Dongguan, Guangdong 523000, China

d

Department of Physics, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon,

Hong Kong 999077 e

Department of Physics and Chemistry, Faculty of Science and Technology, University of Macau, Avenida da

Universidade, Taipa, Macau 999078, China Keywords: Electronic skin, Pressure sensor, Hybrid structure, Silver nanowire, Polydimethylsiloxane.

Corresponding Author *

Email: [email protected]. Fax: +853-88222426. Tel: +853-88224196.

Abstract The development of flexible pressure sensor has attracted increasing research interest from wearable electronic skins to human healthcare monitoring. Herein, we demonstrated a piezoresistive pressure sensor based on AgNWs-coated hybrid architecture consisting of meso-scaled dome and micro-scaled pillar arrays. We experimentally showed that the key three-dimensional component for a pressure sensor can be conveniently acquired using the vacuum application during the spin-coating process instead of sophisticated and expensive approach. The demonstrated hybrid structure exhibits dramatically improved sensing capability when compared with the conventional onefold dome-based counterpart in terms of the sensitivity and detectable pressure range. The optimized sensing performance, by integrating D1000 dome and D50P100 MPA, reaches superior sensitivity of 128.29 kPa-1 (0-200 Pa), 1.28 kPa-1 (0.2-10 kPa), 0.26 kPa-1 (10-80 kPa), and the detection limit of 2.5 Pa with excellent durability. As a proof-of-concept, the pressure sensor based on the hybrid configuration was demonstrated as a versatile platform to accurately monitor kinds of physical signals or pressure sources, e.g. wrist pulse, voice vibration, finger bending/touching, gas flowing, as well as addressing spatial loading. We believe that the proposed architecture and developed methodology can be promising for future applications including flexible electronic devices, artificial skins, and interactive robotics. 1 ACS Paragon Plus Environment

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Introduction Recently, flexible and wearable devices have attracted remarkable research enthusiasm and been extensively studied for applications such as healthcare monitors, human-machine interfacing, soft robotics, and electronic skins.1-6 Existing methodologies for preparation of pressure sensor can generally be classified as piezoresistive,7-9 capacitive,10-14 piezoelectric,15-17 and triboelectric.18-20 Piezoresistive pressure sensors are attractive owing to the numerous superiorities such as facile readout mechanism, straightforward structure, and swift dynamic response.21-24 Traditionally, piezoresistive pressure sensors rely on the embedding of massive conductive fillers, e.g. graphene, carbon nanotube, to the elastomeric matrix which can response to the external pressure in terms of the variation of conductive passage numbers.25-27 The balance between the satisfactory conductivity and the deformation capability is thus one critical issue of the sensor because the filling ratio of conductive components beyond the percolation threshold would typically affect the mechanical flexibility of the matrix materials. Even though the sensor compressibility can be obviously improved via introducing specific porous or hollow structures, the complicated procedure and specific preparation requirement, e.g. high temperature, freeze drying, would somehow restrict the broad production and application of the demonstrated methodologies.28-32 In addition, conductive filler-based elastomers typically suffer from the poor sensitivity at low pressure range and significant response hysteresis that prevents the application as flexible electronic skins for tiny pressure sensing.21,33,34 Other than the delicate design of advanced conductive matrix, constructing artificial patterns on the elastomer surface is another effective approach to improve the sensing sensitivity.35-38 Taking advantage of the localized geometrical deformation under external pressure, the contact area between the facing conductive components can be effectively changed, leading to the instant reflection of the applied forces by monitoring the relative resistance variation. To date, various two-dimensional (2D) and three-dimensional (3D) architectures have been proposed and developed aiming to the sensitivity enhancement including pillars, wrinkles, pyramids, and domes, etc.39-44 Specifically, 3D patterns that enable larger localized geometrical variation and contact area change under external pressure, e.g. pyramid or dome-shaped structures, are more desirable towards the sensitivity requirement. For example, B. W. Zhu et al. presented the micro-pyramid structured graphene arrays translated from the silicon mold to reach the detection limit of 1.5 Pa with 0.2 ms response time.45 Via combining with the short-channel coplanar device, H. W. Li et al. demonstrated that the sharp pyramid-based microstructures can be used to improve the sensor performance with high sensitivity of 2000 kPa-1 and detection limit towards 0.075 Pa.46 Y. Zhang et al. also reported that the colloid self-assembly can be combined with the replica technique to form polydimethylsiloxane (PDMS) micro-dome arrays with tunable sensitivity by optimizing the colloid sizes.47 Recently, nano-imprinting technology based on intermediate polymer substrate has also been demonstrated to prepare micro-dome arrays with metal nanoparticle-polyurethane composite for pressure detection down to 4 Pa and sensitivity of 71.37 kPa1 48 . On one hand, serial sophisticated processes, e.g. dry oxidation, dry etching, and chemical etching, 2 ACS Paragon Plus Environment

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are typically required to achieve the templates with such precise 3D structures apart from the conventional photo-patterning step.40,46 Furthermore, in order to realize the high sensitivity under ultralow pressure range, such patterns were usually designed with sharper and finer morphologies, and the dimension is commonly small.49,50 The structural deformation will thus quickly reach the saturation point even under the relatively smaller pressure loading. The rapid saturated deformation would thus lead to the decrease of sensing sensitivity within the higher pressure range, resulting in the limited detectable pressure ranging from hundreds of Pascal to several kilo-Pascal as previously reported.4548,51,52 Previous studies have shown continuous efforts paving the way for the practical application, however, great challenge of pressure sensors still exists regarding the high sensitivity while maintaining the wide pressure detection range.53 In principle, meso/macro-structures can maintain the detection capability towards broader pressure range, however, the competitiveness is usually restricted owing to the low sensitivity at low pressure and weak detection limit. To combine the finer and meso-structures as the hybrid architecture is thus one alternative solution to realize the high sensitivity while preserving the wide detection ranges simultaneously. In view of the above, herein for the first time we demonstrated the hierarchical structures, combined by the meso-scaled dome and micro-scaled pillar arrays (MPA), as the key component to realize high performance pressure sensor with excellent sensitivity and controllable detection ranges. The piezoresistive pressure sensor, based on AgNWs-coated hybrid architecture, can be conveniently achieved using the vacuum application during the spin-coating process instead of sophisticated and expensive approach. Dimensional properties, e.g. aspect ratio, curvature, can be flexibly tuned via regulating the experimental parameters without requirement of massive template productions. The micro-pillar arrays were replicated from the re-usable template obtained via conventional soft-lithography, without the serial fabrication procedures such as dry etching, wet etching, etc. We demonstrated that the combination of the micro-scaled pillars and meso-scaled domestructure can enable the sensitive monitoring of tiny pressure while preserving the pressure detection up to 80 kPa or above. The optimized sensing performance, by integrating D1000 dome (diameter of 1000 µm) and D50P100 MPA (diameter of 50 µm and pitch of 100 µm), reaches superior sensitivity of 128.29 kPa-1 (0-200 Pa), 1.28 kPa-1 (0.2-10 kPa), 0.26 kPa-1 (10-80 kPa), and the detection limit of 2.5 Pa with excellent durability. As a proof-of-concept, the pressure sensor based on the hybrid configuration was demonstrated here as a versatile platform to accurately monitor kinds of physical signals or pressure sources, e.g. wrist pulse, voice vibration, gas flowing, as well as addressing spatial loading with excellent stability. The results exhibit great promise of the proposed architecture and developed methodology in future wearable electronics applications.

Results and Discussion The fabrication procedure of onefold (dome array) or hybrid (hierarchical pillar-dome array) structure as sensing element is illustrated in Figure 1a. Originally, the PDMS membrane with pre-defined 3 ACS Paragon Plus Environment


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thickness was gently attached to the silicon template that has been drilled with through-hole arrays via laser etching (Figure 1b). The coverage of the through-hole arrays is around 1.5 × 1.5 cm2. Once the membrane and the silicon template were placed onto the pallet of the spin-coater (KW-4A, Institute of Microelectronics, Chinese Academy of Sciences, Beijing, China) with underneath vacuum on, the pressure difference between the two surfaces of the membrane will automatically cause the curved depression. The through-holes could thus serve as the template to conveniently generate artificial patterns on the as-prepared PDMS membrane. The left panel of Figure 1c displays the cross-sectional profile, A-A’ (Figure 1a), of the membrane before PDMS gel application. For un-structured PDMS membrane, the pressure difference results in the curved surface constrained by the perimeter of the through-holes. We will also demonstrate that the curvature of the dome can be tuned by changing the thickness of the original PDMS membrane and the vacuum level. If the original PDMS membrane has been pre-decorated with structures, e.g. micro-pillar array, the curved surface can simply be considered as hybrid morphology consisted by curved morphology and micro-scaled pillar structures. The micropillars decorated PDMS membrane could be simply obtained via applying and curing the PDMS gel from micro-hole-patterned silicon wafer.54-56 We should note that to obtain the hierarchically structured PDMS, the side decorated with micro-arrays should be attached towards the silicon template (Figure 1d). After applying PDMS gel, the spin-coating process was introduced to ensure the back-side evenness of the substrate together with desired thickness (700 rpm for 20 s). The side view of the AA’ section after applying PDMS gel was described in the right panel of Figure 1c, where the hollows of the membrane can be easily filled by PDMS gel after the spin-coating of PDMS. An infrared lamp (power of 250 W, An Hong Da Optoelectronic Technology Co., Shenzhen, China) was used to in situ cure the PDMS gel, followed with peeling off from the silicon template to realize the final product. The distance between the lamp and PDMS membrane was fixed as 20 cm and the curing duration was set of 20 min to ensure the complete solidification of the PDMS gel. It should be noted that the vacuum was applied during the entire fabrication process of structured PDMS membrane. After forming of the conductive AgNWs film on top, the structured PDMS substrate was assembled face-to-face with AgNWs-coated PDMS electrode as the flexible pressure sensor (Figure 1e) for pressure monitoring. Applying the external pressure would hence cause the corresponding sensor deformation to increase the contact area between the top electrode and the bottom sensing architectures (Figure 1f). Therefore, the total electrical resistance can be reduced, resulting in increased electrical current under a given voltage enabling the pressure level sensing via monitoring the relative current variation. Note that to avoid the void space between top and bottom electrodes which may affect the accuracy and repeatability, the electrodes were closely interlocked by gluing the edge. The electric current was also carefully monitored during the assembly process of device to guarantee the stable contact of the electrodes. We firstly investigated the relationship amongst the height of the dome (H), the thickness of the original PDMS membrane (T), and the diameter of the through-holes (D) based on the proposed approach with a certain vacuum level of -0.06 MPa (Figure 2a). The through-holes was fabricated with diameters of 4 ACS Paragon Plus Environment

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500 µm (D500), 750 µm (D750), and 1000 µm (D1000), separated by 50 µm from edge to edge. Figure S2a depicts the design of the through-hole patterns, and Figure S2b-d provide the optical images of the silicon templates showing different diameters. For through-holes with given diameter, H decreased if T is increased owing to the level of deformation resistance that is proportional to the membrane thickness. T can be easily changed by tuning the spin-coating rate of the membrane preparation process, ranging from 55 µm to 280 µm (Figure S3). Similarly, for original membrane with specific thickness, H decreased if D is decreased due to the stronger constraint from the through-holes with smaller perimeter. For the smallest T of ~ 55 µm, corresponding H was 109 µm ± 6 µm for D500, 205 µm ± 12 µm for D750, and 334 µm ± 18 µm for D1000 (green dashed box in Figure 2a). We further calculated the deformation ratio, H/D, as this is one important criterion to evaluate the sensing capability of the structure in terms of detectable pressure range (Figure 2b). The highest value of H/D is 33.4 ± 1.9% for D1000 with T of ~ 55 µm. Under the same T, H/D decreased to 27.3 ± 0.8% for D750 and 21.8 ± 0.5% for D500. The influence of vacuum on H was then studied by regulating the vacuum level from -0.06 to -0.01 MPa, with a certain D of 1000 µm and T around 55 µm. The results indicate that H can also be well affected by vacuum level, with the H/D decreased from the highest value (33.4±1.9%) to 16.4±0.6% when the vacuum level was reduced to -0.01 MPa (Figure S4). In the following section, we will thus evaluate the onefold structures with highest H/D (i.e. T of about 55 µm, vacuum level of -0.06 MPa, red dashed box in Figure 2b) for sensing performance comparison. Figure 2c shows the typical cross-sectional profiles of the dome structures with different diameters when the original membrane thickness is ~ 55 µm, which clearly exhibits the curvature of the mesoscaled-dome-shaped structure. Other optical images showing the cross-sectional profiles prepared from various thickness of the original membranes can be found in Figure S5a. The top-view images of the PDMS dome structures prepared from various through-hole diameters clearly demonstrate the curved surface profile of the uniform onefold dome patterns (Figure 2d) and the overall coverage of the dome arrays can be referred to Figure S5b. Figure 2e shows the photograph of the as-prepared PDMS sample with hybrid structures (1.5 × 1.5 cm2) before application of AgNWs. The magnified optical image displays the regular dome array (D1000) decorated with uniform clusters of micro-pillars. The 3D optical image further confirms the curved surface of the individual dome that has been decorated with micro-scaled pillars (Figure 2f). The height of the dome from the 3D image is in line with the results from the cross-sectional image (Figure 2c) where we can observe that the height of the as-prepared structure is around 340 µm with parameter T of ~ 55 µm and D of 1000 µm. The optical images in Figure S6a-b further confirm the 3D structures via focusing the top and the bottom plane of the hierarchical structure. Figure 2g provides the typical scanning electron microscopy (SEM) images of the 2 × 2 hybrid structural array, where the D1000 dome structure was decorated with 50 µm micro-pillar arrays (MPA) and the center-tocenter pitch is 100 µm (D50P100). The height of MPA is set to be ~25 µm. To reveal the internal structures of the assembled pressure sensor, Figure S7a-c also provide the cross-sectional optical images of the assembled devices with different structures. Figure S7a is related to the device assembled 5 ACS Paragon Plus Environment


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by the flat PDMS electrode and the D1000 dome structure. Figure S7b is related to the device assembled by the flat PDMS electrode and the hybrid structure, while Figure S7c is related to the one assembled by two hybrid structures face-to-face. It can be clearly observed that the micro-scaled structures can be well deformed after applying a certain pressure, with the increase of the contact area for pressure sensing as will be discussed below (Figure S7d-f). The photography of the PDMS electrode before and after the AgNWs application can also be found in Figure S8a, where we can observe the uniform distribution of the AgNWs on the PDMS surface. The as-prepared AgNWs with average diameter of ~ 100 nm and average length of ~ 25 µm can be clearly observed from the optical image and SEM image in Figure S8b. It appears that the AgNWs uniformly cover the surface of the dome structure, the top and sidewall of the micro-pillars, and the interconnected network was formed as shown in Figure 2h. More information of the AgNWs distribution can be found in Figure S8c. We also measured the current-voltage characteristic curve of the AgNWs film on flat, dome and hybrid structure, as shown in Figure S8d. The straight lines reflect the excellent conductivity of AgNWs film, and the similar slope of the lines indicates that the conductive film can be well controlled even on different structures with certain parameters (AgNWs volume of 200 μl, spinning speed of 400 rpm and spin-coating time of 20 s). Figure S8e reveals that the AgNWs can still well adhere on the hybrid structured PDMS surface even after 10000 cycles of periodic pressing under 10 kPa. The electrical stability of the AgNWs-coated PDMS electrode was evaluated by periodic bending of the electrode and measuring the real-time relative current variation (Figure S9a-b). It can be observed that even with the bending ratio of 50%, the as-prepared electrode can maintain a relatively small electric current variation (ΔI/I0 less than 5%), which indicates the flexibility and stability of the electrical conductivity based on the AgNWs network towards the real applications. Furthermore, the electric current can restore to the value when un-bended even after 7000s’ bending and releasing manipulation. Figure S9d further confirms that the conductivity of AgNWs-coated electrode can maintain stable (ΔI/I0 less than 5%) even after more than 2000 cycles’ periodic pressuring with normal pressure of 1 kPa using the experimental setup (Figure S9c). The pressure sensing capabilities of the meso-scaled onefold structures, D500, D750, and D1000 dome-shaped arrays, were firstly evaluated by assembling with the flat AgNWs-coated PDMS electrode (Figure 3a). Sensitivity of the pressure sensor could be typically derived from the slope of the electric current response curve under a monotonously increased normal pressure, defined as:21, 42

S=

∆𝐼𝐼 𝛿𝛿( 𝐼𝐼 ) 0

𝛿𝛿𝛿𝛿

where P is the applied normal pressure, I is the electric current under typical pressure value, and I0 is the initial electric current when no pressure is applied onto the device. In the low-pressure range (0100 Pa), the sensitivity of the pressure sensor based on D1000 dome structure (28.89 kPa-1) is relatively higher than that of D750 (25.85 kPa-1) and D500 (11.53 kPa-1). In addition, owing to the smaller values 6 ACS Paragon Plus Environment

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of H and H/D, it was also observed that the sensing capabilities quickly became saturated when the pressure was further increased to 20 kPa, with sensitivity of 0.04 kPa-1 for D750 and 0.02 kPa-1 for D500. With the highest H and H/D from D1000 dome-shaped structure, the sensitivity maintained at 1.15 kPa-1 in the intermediate pressure range (0.1-4 kPa), and finally decreased to 0.06 kPa-1 with higher pressure range (4-60 kPa). The improved detection pressure range of D1000 dome-shaped structure is attributed to the higher extent of the deformation height as well as the deformation ratio. To systematically discuss the dimensional effect of the dome-shaped structures on the sensor performance, the sensitivity within low-pressure range can be approximately expressed as 𝑆𝑆𝑠𝑠𝑠𝑠𝑠𝑠 =

𝐻𝐻 𝜋𝜋 ∙ 2𝜋𝜋𝜋𝜋 = ∙ (𝐷𝐷2 + 4𝐻𝐻 2 ) 𝑆𝑆0 𝐸𝐸 4𝑆𝑆0 𝐸𝐸

where S0 is the initial contact area between the apex and the flat electrode, D is the bottom diameter of the dome, H is the height of the dome, and E is the elastic modulus of PDMS. The detailed analysis on the relationship between the dome structure and the sensitivity can be referred to the supplementary information (Section S1). The formula clearly presents that both the diameter and the height of the dome act positively to the sensitivity at the low-pressure range as shown in Figure 3a. Additionally, herein we also assume that the flat PDMS electrode is in point contact with the bottom dome-shaped structure at the initial stage, from which we can consider S0 as approximately constant and much smaller when compared with other dimensional parameters, e.g. D, and H. To further optimize the pressure sensitivity under ultra-low pressure range, we proposed and fabricated the hybrid structure that could not only precisely distinguish the signal at low pressure but also maintain the detection capability with wide pressure range. Compared with the onefold dome-shaped structure as mentioned above, Figure 3b presents the improved sensitivity of the hybrid structure to 128.29 kPa-1 under the ultra-low pressure (0-200 Pa), 1.28 kPa-1 within the intermediate range (0.2-10 kPa), and 0.26 kPa-1 for the higher-pressure range (10-80 kPa). Even under the ultra-low pressure region, the hybrid structure can be more sensitive to provide the information of the applied tiny pressure. Figure 3c provides the schematic showing the hybrid structure with different colors to distinguish and classify the micro-pillars on different vertical planes on the dome structure. Initially, the top flat electrode was attached with the micro-scaled pillars on the apex of the curvature which ensures the relatively smaller contact area and the electric current (Figure 3d). To start with the ultra-low pressure loading, the tiny pressure is insufficient to induce effective deformation of the meso-scaled dome structure as the plots in Figure 3a shows. However, the flat top electrode initiated to contact with the micro-pillars close to the apex, which could easily increase the contact area between the facing conducting component for electric current variation at this stage. As observed from the SEM images (Figure 2h), the coating of AgNWs on the top and sidewall of the MPA can ensure the sufficient contact area for current change to reflect the tiny pressure at this moment. With further increased pressure, the meso-dome arrays started to be compressed together with the MPA. In one way, the continuous compression of the dome structure will lead to the more resistance to the compression with corresponding sensitivity decrease. 7 ACS Paragon Plus Environment


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However, the continuously increased number of the MPA under lower planes can compensate the reduced sensitivity by providing more conductive surface and contact areas at the same time. Continuously increased pressure will finally lead to the saturated compression, where the flat electrode can be considered to fully contact with the bottom of the highly deformed dome-structures, resulting in the capabilities to extend the detection range of the applied pressure when compared with the onefold structure. The hybrid structure can thus ensure the capability to detect tiny pressures while preserving the possibility to monitor the high pressure with broad range and improved sensitivity. Considering the pressure-sensing capabilities of single-sided hybrid structure, interlocked hybrid structures having double deformable surfaces are expected to further improve the sensing capabilities. In one way, the interlocked assembly can lead to the larger pressure-induced deformation of the interlocked micropillars and meso-domes. Also, the interlocked assembly brings more surface contact area owing to the increased numbers of micro-pillars. As presented by the red curve in Figure 3b, with the interlocked hybrid structures, the pressure sensing sensitivity has increased obviously to 374.50 kPa-1 (0-300 Pa), 3.86 kPa-1 (0.3-20 kPa), and 0.63 kPa-1 (20-80 kPa). To fully evaluate the sensing capabilities of the presented hybrid structure, we integrated the hybrid architecture with the flat PDMS electrode as the assembled sensor for the following systematical evaluation and demonstration. Furthermore, by designing MPA with different dimensional parameters, we also examined the influence of the pitches between the adjacent micro-pillars on the sensor sensitivity (Figure S10). Figure S10b exhibits the relative current variation based on the D1000 dome decorated with D50P150 MPA (Figure S10a) under the pressure ranging from 0 to 80 kPa. It was also found that the sensitivity of the pressure sensor improved to 8.18 kPa-1 (0-3 kPa), 0.22 kPa-1 (3-20 kPa), and 0.02 kPa-1 (20-80 kPa) when compared with the onefold structural counterpart. The sensitivity difference between D50P100 and D50P150 MPA decorated dome structures under low-pressure range was depicted in Figure S10c. Typically, for a larger gap between the adjacent micro-pillars, it can be observed that the compression of the uppermost micro-pillar should be larger to allow contacting the pillars at the relatively lower planes. Owing to the different value of Δh, a relatively higher pressure is required to induce the effective contact area variation as well as the electric current, which finally leads to the reduced sensitivity when compared with that of the D50P100 hybrid structure. Apart from the relatively larger gaps between neighboring pillars, the reduced number of micro-pillars on the same plane might also affect the sensitivity owing to the limited extension of contact areas. The detailed numerical analysis focusing on comparing the sensitivity based on the pillar arrangement can be found in the supplementary information (Section S2). To further evaluate the performance of the presented hybrid structure, we herein selected the D50P100 micro-pillars decorated D1000 dome array to assemble with the flat electrode as the pressure sensor for measurement. The device was firstly applied with periodical loading and unloading of different pressures (97 Pa, 453 Pa, and 1022 Pa) for 1000 s to evaluate the dynamic response as shown in Figure 4a. The inset clearly delivers the repeatedly relative changes in current (∆I/I0) when the external pressure is applied to or released from the sensor. Furthermore, it is obvious that the changes 8 ACS Paragon Plus Environment

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increase when the applied pressure was tuned from 97 Pa to 1022 Pa, indicating the reliable and stable sensing performance of the device under different pressure values. The stabilities of the sensor exposed to constant pressure loadings were measured with broad range from ultra-low pressure (5 Pa) to relatively higher pressure (10020 Pa). Figure 4b demonstrates the current variation of the pressure sensor under the constant pressure loading for ~ 1 min, and typically a higher-pressure input induced the larger current change. The experimental data exhibit that the pressure sensor can be highly reliable to reflect the applied signal under ultra-low pressure or high-pressure range. To further evaluate the long-term stability of the pressure sensor, cyclic pressure was applied to the sensor for 80000 s under the pressure value of 10 kPa (Figure 4c) and frequency of 8 s. The insets further confirm the swift response of the sensor to the external pressure with high stability and reproducibility at different times. It can be observed that after 10000 cycles’ loading and unloading test, the relative current changes remain almost the same value, which indicates the high durability and precision of the pressure sensor. We attribute the excellent sensing durability and reproducibility to the outstanding elastic property of the PDMS hybrid structure which can endure mechanical deformation under pressure and swiftly recover once the pressure is released. The assembled sensor has been applied to monitor different pressure sources (Figure 5). Realtime response performance of the pressure sensor to the gas flowing was firstly investigated. Without direct contact, the air serves as the medium to transmit the pressure from the rubber suction bulb to the surface of the pressure sensor, resulting in the prompt deformation under various air pressure intensities (Figure 5a). The sharp current variation also confirms the capability of the sensor that can quickly respond to external loading and return to the original status once the loading was withdrawn. Figure 5b presents the response of the sensor to the repeated finger touches. It can be observed that not only the normal pressure strengths can be monitored, but also the pressing duration can be distinguished as shown in dynamic current variations. We further introduced kinds of lightweight materials to evaluate the detection capacity of the pressure sensor towards ultra-low-pressure limit (Figure 5c). Repeated loading/unloading process of the tiny substances, e.g. paper, rice, could be well recorded by the sensor with obvious current variation for five cycles. The weights of the rice were 15 mg and 26 mg, respectively, leading to distinct variation as shown in the plot. The 25 mg piece of paper, with area of 1 cm2, can also be precisely detected with the equal pressure of 2.5 Pa. Even under such ultra-low pressure range, the presented sensor also maintains the capability to reflect the pressure information stably and precisely. The inset in Figure 5c demonstrates the rapid response of the sensor even under such low pressure, which can return to original current status once the lightweight materials were removed from the sensor. The experimental results indicate that the as-prepared sensor might be potentially applicable for various areas such as pressure monitoring in aerodynamics, electronic devices with touch screen, tiny pressure measurement, etc. Subsequently, the assembled sensor was adhered to the neck of a volunteer to monitor the voice vibrations during speech to investigate the feasibility of the flexible pressure sensor as wearable 9 ACS Paragon Plus Environment


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electronic skin (Figure 5d). Distinct one peak was observed when the volunteer pronounced a monosyllabic word as ‘Good’, and the voice volume can also be recognized in terms of variation of the relative current change. The disyllabic ‘Good luck’ and sentence ‘May the force be with you’ can also be well distinguished with double and multiple peaks, respectively. The interval of the peaks reveals the obvious voice pause, and the intensity variation is corresponding to the voice volume from the volunteer speaking. The repeated peak profiles reflect that the flexible pressure sensor possesses the excellent capability and reliability to voice sources. Later, the flexible sensor was attached to the knuckle of the index finger for human motion record. The relative current variation-time curve in Figure 5e presents the motion of the finger with periodic bending and relaxation, owing to the resistance variation of the sensor when exposed to the joint bending and relaxation. The different extent of bending can also be well recognized from the relative current variation extent as the demonstrated peaks in the plot. The assembled sensor was finally attached to the bandage and stuck to the wrist for physical force monitoring (Figure 5f). An artery pulse waveform containing three distinct peaks, P1 for the main pulse pressure and P2, P3 for the reflected wave pressures can be well observed from the enlarged curve, respectively.57,58 We also note that further optimization of the sensor is required to obtain more stable and reliable physical information towards the real application in the future. To spatially acquire the pressure information that is applied to the device is another important criterion to evaluate the capability and broaden the application of the pressure sensor. As a proof-ofconcept, the AgNWs-coated hybrid structure (1.5×1.5 cm2) was herein cut into individual functional unit and integrated with top electrode as pixel-addressable matrix (Figure 6a). Figure 6b presents the assembled 4×4 sensing arrays where each pixel dimension is ~ 3×3 mm2 and connected with series of conductive wires for input/output records. When the pixel array was loaded with bended stainless-steel rod, the mapping of the output current variation from the 16 individual sensing unit could well match the touching contour of the rod (Figure 6c). Only the pixels, A2, B3, C4, and D4, which have been compressed by the external pressure input can generate the current variation for perception. Furthermore, the sensing matrix can detect the various weights of the steel balls that have been simultaneously placed on different elements of the device (Figure 6d). When the four weights (0.01 g, 0.03 g, 0.26 g, and 0.50 g) were applied to different pixels of the matrix, the device can recognize the pressure distribution and the weight level was also precisely reflected from the extent of the relative current variation. The stainless-steel rods that cover two or three pixels of the array was demonstrated as shown in Figure S11. The relative current changes can well match the external pressure input in terms of spatial distributions and intensities. The overall tests in this work were carried out with a typical operating voltage of 1 V, which renders the relatively lower power consumption during the operation.59-60 Also, it should be noted that the preparation process of the pressure sensor in this work is facile and highly feasible to scale up for more functional applications with capability of overall pressure mapping.

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Conclusion In summary, we have presented a piezoresistive pressure sensor based on the AgNWs-coated hybrid architecture through a facile and scalable strategy. For the first time, we report that the key 3D component for a pressure sensor can be conveniently acquired taking advantage of the spin-coating vacuum without assistance from sophisticated and expensive approach. Additionally, we also experimentally proved that the hybrid structure, consisting of meso-scaled dome and micro-scaled pillar array, can be introduced to obviously improve sensing performance when compared with the conventional onefold dome-based counterpart in terms of low-pressure sensitivity and wide detection range. Via combination of D1000 dome and D50P100 MPA, the sensitivity of the pressure sensor has been optimized to 128.29 kPa-1 within 0-200 Pa, 1.28 kPa-1 (0.2-10 kPa), 0.26 kPa-1 (10-80 kPa), and the detection limit of 2.5 Pa with competitive stability. As a practical flexible demonstration, the assembled pressure sensor was used as a versatile platform to accurately monitor various pressure sources, e.g. gas flowing, finger bending/touching, wrist pulse, as well as addressing the spatial loading. We attribute the sensing capability of the pressure sensor, with high sensitivity at ultra-low pressure and wide detection range, to the proposed novel hybrid structure consisting of meso-scaled and microscaled structures. We believe that the superior sensing sensitivity, along with the simple process of the hybrid structure, is promising for a broad spectrum of sensing applications in the near future.

Experimental section Materials: The silicon templates with through-hole arrays were fabricated using commercial laser cutting technology by Suzhou Delphi Laser Co., Ltd, China. PDMS (Sylgard 184 silicone elastomer kit) was purchased from Dow Corning, USA, and prepared by mixing the base and curing agent with typical weight ratio of 10:1. To prepare PDMS membrane with pre-defined micro-pillar arrays, silicon wafer with hole arrays or SU8 photoresist hole patterns were used, which can be conveniently achieved by the well-developed photolithography or soft-lithography technique.54,56 A brief fabrication process flow of the silicon mold with micro-hole arrays were provided in Figure S12. Prior to the application of PDMS, the silicon mold was treated with 1H,1H,2H,2H-Perfluorooctyltriethoxysilane (Sigma Aldrich, USA) to reduce the adhesion between the mold and the PDMS gel during the peeling-off process. Synthesis of high aspect ratio silver nanowires (AgNWs): AgNWs with high aspect ratio was synthesized and purified as described in the literatures.61-64 Firstly, 1.5 g polyvinylpyrrolidone (PVP, Sigma Aldrich, USA) was dissolved in 250 mL of ethylene glycol (EG, J&K Scientific Ltd.). Then, 1 mL 10 mM FeCl3 was directly added into to the PVP solution. Subsequently, 50 mL of AgNO3 (0.1 M) solution was added to the mixture and uniformly stirred for 40 min at 170 ºC, followed with cooling down to the room temperature. The process for the purification of AgNWs was described as the literatures.62 The AgNWs were subsequently dispersed in ethyl alcohol and collected by centrifugation to remove the small amount of PVP residual. Finally, the AgNWs were re-dispersed in ethyl alcohol 11 ACS Paragon Plus Environment


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with 5 mg/mL concentration and used in the dip-coating process for conductive film formation. Fabrication of flat electrode and AgNWs-coated structured PDMS substrate: The flat electrode substrate was fabricated by spin-coating PDMS gel on a clean glass substrate at 700 rpm for 20 s and then placed horizontally on the hotplate at 150 ºC for 20 min. Prior to the application of AgNWs for conductive film formation, the AgNWs suspension was under ultrasonic treatment for 20 s to ensure the uniform distribution of the nanowires in the solvent. Also, prior to the application of AgNWs, the PDMS surface was treated with plasma cleaning (Harrick Plasma, New York, USA) for 2 min to improve the surface adhesion capability of the AgNWs. A dip-coating process (with spinning speed 400 rpm, spinning time 20 s and 200 µl AgNWs) was then carried out to form uniform AgNWs film on the surface of the flat PDMS membrane. After application of the AgNWs, the sample was moved to the hotplate with 120 ºC for 15 minutes for complete solvent evaporation and adhesion improvement. The flat electrode was then cut to 4 cm × 1.5 cm for sensor assembly and performance evaluation. Such process was also applied to form the flexible and conductive AgNWs film on the onefold and hybrid structured substrate. Characterization and Measurements: High-resolution captures of the as-prepared structures and AgNWs were obtained via Scanning Electron Microscopy (SEM, Sigma FE-SEM, Zeiss Corporation, Germany). Optical images were captured via the Carl Zeiss Digital Microscopy and Olympus Optical Microscopy. The loading and withdrawing of specific pressure were controlled by a motorized motion platform (Zolix, MAR 100-90, China) with a motion controller (Zolix, MC600, China). The real-time monitoring of the electric current was carried out by Keysight B2902A meter with voltage supply of 1 V. The values of the loading were recorded using a precise balance (Sartorius Lab Instruments GmbH & Co.KG, 37070 Goettingen, Germany). Pressure was calculated using the force value from the balance and the contact area between the pressure load and the sensing window (1 cm2). The data of sensor measurement were finally analyzed and formatted by OriginLab.

Supporting Information Detailed numerical analysis focusing on the sensitivity based on dome structure and pillar arrangement; photographs of silicon templates, meso-scaled dome structures and MPAs; relationship between spincoating rate and thickness of original PDMS membrane; photograph of cross-sectional profile of the combined structure of sensor before and after loading pressure; influence of vacuum on deformation height and deformation ratio of meso-scaled dome; optical and SEM images of AgNWs deposited on flat PDMS substrate; SEM images of AgNWs-coated hybrid structure before and after 10000 cyclic pressing test at 10 KPa; current-voltage characteristic curve of AgNWs film on flat, dome and hybrid structure; stability test of AgNWs-coated top electrode under cyclic bending, relaxation and pressing test; SEM images of 2 × 2 dome-shaped array with D50P150 MPA; relative current variations of hybrid structure (D1000 dome with D50P150) under pressure range from 0 to 80 KPa; mapping results of sensor matrix under pressure from iron rod coving two and three pixels; fabrication process flow of 12 ACS Paragon Plus Environment

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the silicon mold.

Acknowledgements The authors appreciate the support of the Science and Technology Development Fund from Macau SAR (FDCT-073/2016/A2, FDCT-0037/2018/A1) and Multi-Year Research Grant (MYRG201700089-FST, MYRG2018-00063-IAPME) from the Research & Development Administration Office at University of Macau.

Conflict of Interest The authors declare no conflict of interest.

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Figure 1. a) Schematic illustration of the fabrication process of onefold and hybrid structure. b) Photograph of the fabricated silicon template with through-hole arrays. c) Schematic of the crosssectional profile A-A’ before and after the application of PDMS gel during the spin-coating process. d) Schematic of the attaching of PDMS membrane that has been decorated with micro-pillar arrays to the silicon template. e) Schematic of the assembled pressure sensor consisting of the flat electrode and sensing architecture. f) Deformation capability of the pressure sensor under different levels of external pressure resulting in the conductive pass change for pressure sensing.



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Figure 2. a) Relationship between the thickness of the original PDMS membrane and the height of the obtained dome structure for different through-hole diameters. b) Relationship between the deformation ratio of the as-prepared dome structure and the original thickness of PDMS membrane for different through-hole diameters. c) Typical optical images of the cross-sectional profiles of the dome structure for original PDMS membrane of ~ 55 µm from different through-hole diameters. d) Photographs of the onefold dome-shaped arrays with diameter of 500 µm, 750 µm, and 1000 µm. e) Representative photographs of the hybrid structure from D1000 dome structure decorated with micro-pillar arrays. f) 3D optical image of an individual hybrid structure presenting the dimensional information. g) SEM images of the hybrid structures consisting of D1000 dome structure and D50P100 MPA. h) Top-view SEM images of the hybrid structures, D1000 dome with D50P100 MPA, coated with uniform high aspect ratio AgNWs film.


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Figure 3. a) Relative current variations of the onefold structure (dome array) under pressure range from 0 to 20 kPa (D750 and D500) and 60 kPa (D1000). b) Relative current variations of the hybrid structure (D1000 dome with D50P100) and the interlocked hybrid structure under pressure range from 0 to 80 kPa. c) Top view of the spatial arrangement of the MPA on the dome-shaped structure, where the color indicates the micro-pillars on different vertical planes. d) Schematic of the continuous structural variation of the hybrid structure with gradually increased pressure loadings.



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Figure 4. a) Dynamic pressure monitoring of the hybrid structure (D1000 dome with D50P100 MPA) under 97 Pa, 453 Pa, and 1022Pa for 1000 s. The inset shows the detailed signals from 600 s to 700 s. b) Signal stability of the pressure sensor under different pressures with 1 minute. c) Long-term stability of the sensor under cyclic 10 kPa loading/unloading at frequency of 8 s, with insets showing signals from 3500 s to 3640 s, 42000 s to 42140 s, and 70000 s to 70140 s.


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Figure 5. a) Real-time response measurement and the response signal of the pressure sensor to the air flow. b) Real-time response test and the response signal of the pressure sensor to finger touches. c) Repeated loading/unloading of lightweight materials, 2.5 Pa paper, 15 mg, and 26 mg rice, to the pressure sensor and the corresponding signal. d) Real-time monitoring of voice vibration in response to pronunciations of ‘Good’, ‘Good luck’, and ‘May the force be with you’. e) Real-time monitoring of flexible finger bending and relaxation with enlarged view of the signal. f) Real-time blood pressure monitoring test from the artery of the wrist with enlarged view of one typical waveform.



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Figure 6. a) Schematic of the pressure sensor matrix consisting of the bottom electrode, hybrid structure-based sensing unit, and the top electrode. b) Photograph of the assembled 4 × 4 sensing matrix, with nomination of A-D for the column and 1-4 for the row. c) Mapping result of the sensor matrix under the pressure from a bended iron rod. d) Mapping result of the sensor matrix under the pressures from the iron balls with various weights placed on different positions of the array.

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