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Dual-mode Electronic Skin with Integrated Tactile Sensing and Visualized Injury Warning Yanli Zhang, Yunsheng Fang, Jia Li, Qihao Zhou, Yongjun Xiao, Kui Zhang, Beibei Luo, Jun Zhou, and Bin Hu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b13016 • Publication Date (Web): 04 Oct 2017 Downloaded from http://pubs.acs.org on October 5, 2017

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ACS Applied Materials & Interfaces

Dual-mode Electronic Skin with Integrated Tactile Sensing and Visualized Injury Warning Yanli Zhang,† Yunsheng Fang,† Jia Li,† Qihao Zhou,† Yongjun Xiao,†, ‡ Kui Zhang,† Beibei Luo,† Jun Zhou,† and Bin Hu†,*

†

Wuhan National Laboratory for Optoelectronics and School of Optical and Electronic

Information, Huazhong University of Science and Technology, Wuhan 430074, China Dr. Y. J. Xiao ‡

School of Physics and Electronic-Information Engineering, Hubei Engineering University,

Xiaogan 432000, Hubei, P. R. China

ABSTRACT: Mimicking pressure sensing behavior of biological skins using electronic devices has profound implications for prosthetics and medicine. The developed electronic skins based on single response mode for pressure sensing suffer from the rapid decrease in sensitivity with the increase of pressure. Their highly sensitive range covers a narrow part of tolerable pressure range of human skin and have weak response to the injurious high pressures. Herein, inspired by a bioluminescent jellyfish, we develop an electronic skin with dual-mode response characteristic, which is able to quantify and map the static and dynamic pressures by combining electrical and optical responses. The electronic skin shows notable changes in capacitance in low pressure regime and can emit bright luminescence in high pressure regime, which respectively imitates the functions of the mechanoreceptors and nociceptors in biological skin, enabling it to sense gentle tactile and injurious pressure with the sensitivity up to 0.66 kPa-1 and 0.044 kPa-1, respectively. The complementary highly sensitive sensing

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ranges of the electronic skin realize a reliable perception to different levels of pressure, and its mechanical robust and stretchable properties may find a wide range of applications in intelligent robots.

KEYWORDS: electronic skin, transparent electrode, pressure sensor, electroluminescence, dual-mode sensing

1. INTRODUCTION Skin as a vital interface between organisms and outside world plays a vital role in distinguishing different pressure levels from tens of Pa to hundreds of kPa.1-2 The tactile receptors in biological skins include four-type mechanoreceptors and one-type nociceptor. Their collaboration enables soft skin to sense innocuous physical contacts in daily life and respond invasive pressures to avoid body injury.3-4 Implementing these functions in electronic skins (e-skins) has attracted profound interest for human-machine interfaces and artificial intelligence. Significant achievements have been demonstrated currently to emulate the biological sensors by transducing external pressures into quantifiable electrical signals based on resistive5-8, capacitive9-11, piezoelectric12-17, and triboelectric18-19 mechanisms.20-21 However, unlike the biological skin using specific receptors to sense different levels of pressure, most artificial pressure sensors rely on single sensing mode, resulting in their sensitivity decrease rapidly with the increase of pressure. Meanwhile, highly sensitive measurement range of these devices only covers a narrow part of tolerable pressure range for human, and show weak response to the injurious high pressure. These issues in single sensing mode devices significantly restrict their applications in biomimetic robots or amputees. 2 ACS Paragon Plus Environment

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Integrating dual or multiple pressure sensing modes in a single device to cover the entire pressure range in daily life with complementary high sensitive ranges is an effective approach to address this issue. This function has been demonstrated by a type of jellyfish named as Atolla wyvillei, which can sense slight environmental pressures and can produce bioluminescence when suffered from nociceptive stimuli.22 This behavior inspired us to introduce optical response as a complementary mode to the single electrical sensing under high

pressure.

So

far,

visualized

pressure

sensing

mechanisms

mainly

include

thermochromism23, electrochromism24-25, mechanochromism26, mechanoluminescence27-28, and electroluminescence29-32. However, few works can achieve wide measurement range with high sensitivity to quantify different levels of pressure. Very recently, Wang and Pan et al. demonstrated a self-powered optical and electrical dual-mode sensing by integrating triboelectric and mechanoluminescent sensor matrix, which can realize wide range dynamic pressure sensing but had limitation in static pressures detection.33 Moreover, other essential features of e-skin such as mechanical properties such as stretchability, toughness, durability are also required for practical applications.34-35 Motivated by these challenges, herein, we introduce a dual-mode pressure sensor as e-skin for distinguishing and mapping different levels of dynamic and static pressures. The e-skin integrated two sensing components with complementary pressure ranges. In low pressure regime (60 kPa), the phosphor particles embedded in the PDMS matrix can be triggered by enhanced electric field, the bright luminescence enables the pressure profile to be visualized instantly, 3 ACS Paragon Plus Environment


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and the luminescent intensity as a naked-eye-readable signal quantifies the magnitude of the pressures. These two different sensing modes via electrical and optical approaches respectively imitate the mechanoreceptors and nociceptors in biological skins, and the sensitivity under the gentle tactile and injurious pressure can be up to 0.66 kPa-1 and 0.044 kPa-1, respectively. In addition, stretchable elastomer matrix and transparent electrodes make the e-skin withstand large mechanical deformation, indicating its potential applications in whole-body covering for robots.

2. RESULTS AND DISCUSSION As schematically shown in Figure 1a, the e-skin can be simplified as a parallel-plate capacitor but the electrodes and dielectric were specifically designed for realizing dual-mode sensing. In this dual-mode system, the capacitance change was measured by LCR meter and the luminescence was recorded by an optical fiber spectrometer at the same time using time-division multiplexing (TDM). The schematic circuit layout is illustrated in Figure 1a, in which S1 and S2 indicate two channels derived by TDM system (Figure S1 and Supplementary note 1). In order to mimic the flexibility and stretchability of biological skin and simultaneously ensure effective light emission from the luminescent layer, the fabrication of high performance stretchable transparent electrode was important but challenging. Flexible silver nanowires (AgNWs) network embedded elastic PDMS film is an ideal candidate. Although thick and opaque AgNWs layer can be transferred into PDMS through in-situ polymerization36, it is still difficult to intactly transfer thin and transparent AgNWs due to the 4 ACS Paragon Plus Environment

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low modulus of PDMS.37 We developed a liquid nitrogen-assisted transfer method, which can freeze the PDMS thus transfer the ultrathin AgNWs network without any fracture. During this process, frozen PDMS below the glass transition temperature (150 K) treated by liquid-nitrogen (76 K) became stiff enough to firmly grasp the AgNWs and would not be stretched, thus the AgNWs network can be completely transferred into the PDMS as by rigid polymer substrates. The acquired AgNWs/PDMS film can restore to its original elastic state soon in room temperature and retained excellent electro-optical properties (Experimental Section and Figure S2a-b). The obtained AgNWs/PDMS electrode exhibited good tensile and bending stability with a transparency of 85% and a sheet resistance of ~10 Ω/sq (Figure S2c-f). Even if the electrode was stretched up to 80%, which exceeded the average tolerable strain of human skin (~75%)38, and the sheet resistance of the electrode remained within 100 Ω/sq and restored to the initial value as shown in Figure S2g. Although the stretchability of AgNWs/PDMS electrodes were lower than that of transparent hydrogel electrodes39-40, they were much stable for practical applications without the issue of dehydration. While in comparison with carbon materials based transparent electrodes41, high conductivity of AgNWs network can dramatically reduce the generated heat on the electrode during continuous operation. The intermediate layer of a PDMS matrix mixed with ZnS:Cu phosphor particles simultaneously served as the dielectric layer and luminescent layer for the capacitive and optical responses, respectively. Surface microstructure of PDMS was fabricated by a standard replica molding process using an etched silicon wafer as a mould (Figure S3a and S3b), to reduce the effective mechanical modulus of the elastomer and enhance the sensitivity in low 5 ACS Paragon Plus Environment


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pressure regime.9 Scanning electron microscope (SEM) image in Figure 1b shows the regular pyramidal pits of PDMS pattern with height of 35 µm, width of 50 µm and periodic spacing of 50 µm on a 50 µm-thick flat base. The embedded ZnS:Cu particles are colored by green and their morphology was characterized in Figure S4a with corresponding wurtzite structure confirmed by X-ray diffraction (XRD) in Figure S4b. In order to better illustrate the concept of this dual-mode e-skin, a schematic diagram is given in Figure 1c. For a biological skin, the increased deformation can successively activate the mechanoreception under innocuous pressure and a nociception under excess pressure, which is executed by the complex sensor network in the somatosensory system.3 Our e-skin can simply mimic these functions from tactile sensing to pain warning by successively triggering the capacitance and luminescent responses with increasing deformation of the dielectric layer. Specifically, the applied stress distribution was initially concentrated at the pyramidal tips and caused a large deformation even under very low pressure, and the decrease separation (d) between top and bottom electrodes lead to the increase in capacitance according to the equation of C = εAd-1. In this step, the e-skin can sense a gentle touch with a high sensitivity. However, the stress distribution would gradually disperse onto the broad base of the pyramid structure with further increase of the pressure, the optical cross-section image in Figure 1d clearly shows the boundary of the compressed and pressure-free area. The dielectric became incompressible eventually due to the increasing elastic resistance of the PDMS, resulting in the notable decrease of the capacitive sensing sensitivity, and this nonlinear deformation process of the dielectric layer can be tuned by the geometrical shape and spacing of the pyramidal microstructures.42 On the other hand, the decrease of d enhanced the AC 6 ACS Paragon Plus Environment

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electrical field on the dielectric layer as well. The electrons in the ZnS:Cu phosphor particles would be accelerated into hot carriers under this AC field and caused excitation or ionization of the luminescent centers, then the generated excitons recombined within the phosphors and produced the luminescence (band diagram and luminescent principle are depicted in Figure S4c).43 Since the luminescent intensity is proportional to the electric field intensity, the bright light can be detected even visualized by naked eyes when the electric filed strength was high enough under high pressure. This conspicuous optical warning played the role of nociception in biological skin to avoid body injury. The capacitive and luminescent sensing behavior of the e-skin with the increase of pressures were investigated carefully as shown in Figure 2a. Within the pressure measurement range, the relative capacitance change (△C/C0) increased sharply in low pressure regime (< 10 kPa, gentle touch for human skin), then slowly increased in medium pressure regime (10-100 kPa, suitable for object manipulation) and eventually tended to be steady at more than 100 kPa (feel pain for human).9 While the luminescent sensing exhibited a complementary feature by selecting suitable AC voltage and frequency, and Figure 2a shows that the luminescent intensity can be neglected when the pressure was less than 10 kPa and increased very slowly from 10 kPa to 60 kPa. However, the intensity increased a rapidly when the pressure exceeded 60 kPa and approached saturation over 100 kPa. These two response curves experimentally demonstrated the proof-of-concept of dual-mode pressure sensing of our e-skin. Figure 2b depicts the derivatives of two curves to investigate their changes in sensitivity with the increase of the pressure. It is obvious that the e-skin had high sensitivity up to 0.66 kPa-1 under low pressure, which enabled it to reliably detect the weak airflow (Figure 2c) and an 7 ACS Paragon Plus Environment


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ultra-light weight, such as a bamboo leaf (Figure 2d). Due to the low viscoelasticity of the pyramidal structured PDMS42, the capacitive response times for the loading/unloading of a bamboo leaf were less than 20 ms as marked in Figure 2e, which are close to the response time of human skin (~15 ms).34 The e-skin was mechanically robust and exhibited stable capacitance change when subjected a periodic pressure of 10 kPa for 10000 cycles as shown in Figure 2f. On the other hand, the luminescent response of the e-skin also exhibited high sensitivity up to 0.044 kPa-1 in relatively high pressure regime as shown in Figure 2b, this value is much higher than the reported result of pressure visualization device (3.7×10-5 kPa-1).33 The corresponding optical photographs in Figure 2g clearly show the whole process of luminescent intensity from weak to bright by loading increasing pressures using a 10×10 mm2 glass fixed on the three-dimensional mobile platform (AC voltage of 175 V, frequency of 1 kHz). Since the response time of luminescence was less than 1 ms when applying 1 kHz AC voltage44, which is much fast than time resolution of human-eyes (20 ms) and the pressure loading time. Therefore, the change in luminescence intensity was only depended on the pressure change. The influence of voltage and frequency of AC signals on the luminescent characteristics were also investigated in Figure 3. Figure 3a shows the luminescent spectra under different AC voltages from 50 V to 200 V (fixed frequency of 1 kHz, pressure of 100 kPa). The relationship between maximum luminescent intensity and voltage is given in the inset of Figure 3a, which agrees well with the equation of I = I0exp (-β/ V1/2), where I is the intensity, V is the applied voltage, I0 and β are constants related to the device geometry. Similarly, the increase of the frequency from 50 Hz to 7 kHz enhanced the luminescent intensity as well as 8 ACS Paragon Plus Environment

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shown in Figure 3b (fixed AC voltage of 175 V, pressure of 100 kPa) and their relationship is inserted in Figure 3b. The observed slight blue shift of peak from 519 nm to 503 nm is probably due to the holes trapped by t2 state of Cu, that can recombine with the electrons in the conduction band (or shallow donor level), was gradually replaced by the holes in the valence band (or e state of Cu) as schematically shown in Figure S4c.45 The corresponding photographs in Figure 3c show the luminescent device under different voltages and frequencies (fixed pressure of 100 kPa). Since the luminescent properties can be modulated by the driver circuit, the corresponding luminescence-pressure behavior can be tuned for different scenarios as demonstrated in Figure 3d. Due to the same geometry of the device, all the curves showed similar trends with a rapid increase of luminescent intensity located between 60 kPa and 80 kPa, but the sensitivity, as well as maximum luminescent intensity, can be adjusted using different voltages and frequencies. Different mechanoreceptor densities distributed on a different part of skin determine the stress spatial resolution of the body, and this feature can be mimicked in the e-skin as well. For the capacitive sensing, the top and bottom transparent electrodes were patterned as parallel strips and placed into orthogonal arrays, each intersection can be considered as a single mechanoreceptor, and the density of electrode strip determined the resolution. The addressable e-skin of a matrix array of 6 × 6 is depicted in Figure 4a, and the inset gives the schematic of the orthogonal electrodes array. Patterned AgNWs transparent electrode ablated by laser with a strip width of 6 mm is shown in Figure 4b and 4c. By scanning mutual capacitance of 36 intersections as illustrated in Figure 4d, the position and loaded pressure can be mapped from the capacitance changes (△C) using interpolation method.46 Figure 4e 9 ACS Paragon Plus Environment


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shows the profile of capacitance change when placed 5 g and 10 g weights at coordinates (5, 2) and (2, 5) on the device as demonstrated in Figure 4a, the obtained clear contour image indicates the multi-point addressable sensing capability of the e-skin. Furthermore, different weights from 5 g to 50 g on a same place (Figure S5) produced increasing response peaks with fixed position as revealed in Figure 4f also proved the precise and reliable pressure sensing of the patterned e-skin. Compare to the capacitive addressing of which the resolution was dependent on the density of patterned electrode strips, the pressure sensing through luminescence showed much higher spatially resolution because each phosphor particle acted as an emitting pixel. This simple and efficient visualized approach provides supplementary information to capacitive addressing for more precise positioning of pressure. As demonstrated in Figure 5a, a transparent acrylics slab with “W” shape was pressed on the e-skin with the pressure of 100 kPa. After applying an AC voltage (175 V, 1 kHz), a clearly luminescent “W” can be recognized with a sharp boundary as well as other letters in Figure S6. The micrograph in Figure 5a shows that the pyramid areas exhibited much weak luminescent intensity compare to the spacing areas, which was a result of total internal reflection of emitted light at the interface between the side surface of pyramid and air, as schematically shown in Figure 5b and explained in Supplementary note 2. Therefore, reasonable pyramid geometry design or air-gap filling with refractive index matching materials is helpful to enhance the emitted luminescent intensity but probably has a trade-off to the capacitive sensitivity. In addition, to mimic the biological skin that can produce severe pain sensation when punched by sharp objects, the e-skin can generate bright spots luminescence when poked by the tweezer tips as 10 ACS Paragon Plus Environment

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shown in Figure 5c. Furthermore, the e-skin can maintain functionality under large mechanical deformations such as folding, twisting and stretching, as demonstrated in Figure 5d, and the stress concentrated region with brighter light can be distinguished with the increase of compressive stress as seen in Figure 5e, demonstrating the capability of the e-skin in pain sensing for injure warning.

3. CONCLUSIONS In summary, our work presents a dual-mode e-skin, which is able to quantify and map the static and dynamic pressures through electrical and optical sensing. By integrating the capacitive and luminescent sensing modes into a monolithic device via a simple and scalable fabrication process, the functions of the mechanoreceptors and nociceptors in biological skin can be mimicked. Surface-microstructured dielectric layer induces highly sensitive capacitance change (0.66 kPa-1) in low pressure regime, which is used to sense the gentle tactile as mechanoreceptor. While notable change (0.044 kPa-1) in visualized luminescent intensity in high pressure regime acts as nociceptor to warn the injurious mechanical stimuli. These two sensing modes with complementary sensitive ranges provide a reliable response to different levels of pressure, and mechanical robust and stretchable properties of the e-skins show their great potential towards the realistic applications in human-machine interfaces and intelligent robots.

4. EXPERIMENTAL SECTION 11 ACS Paragon Plus Environment


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4.1 Fabrication of transparent AgNWs/PDMS electrodes and strip pattern. AgNWs with the diameter of 90 nm and length of 125 µm were dispersed in ethanol and coated onto the plasma-treated polyimide (PI) film by blade-coating. A PDMS mixture (ratio of base: curing agent = 10:1) was coated onto the AgNWs film with the thickness of 0.5 mm, followed by curing at 80 oC for 1 h. Then, the PDMS/AgNWs/PI film was immersed in liquid nitrogen for a few seconds which can freeze the PDMS below its glass transition temperature (150 K) and make it stiff enough to grasp each. Then, the rigid PDMS could be easily peeled off from the PI film and the AgNWs network was completely transferred and embedded into the PDMS surface. The as prepared AgNWs/PDMS electrodes can be patterned into strip pattern by a Nd:YAG laser ablation controlled by a computer (laser power of 2 W, λ of 1064 nm, τ of 20 ns, repetition rate of 200 kHz, spot size of 30 µm, speed of 2000 mm/s) 4.2 Preparation of the e-skin device. A silicon wafer with 300 nm SiO2 was initially patterned by photolithography and then etched by buffered HF solution (mixed solution of 10 g NH4F, 15 ml H2O and 3 ml HF). Acquired SiO2-masked silicon wafer was etched by the mixed solution of isopropanol and 35 wt% KOH aqueous solution with a volume fraction ratio of 4:1 for 1 hour under 80 oC in oil bath to form pyramidal pits, and finally the residual oxide was removed by the buffered HF solution for 5 min at room temperature. A mixture of ZnS:Cu powder and PDMS with a concentration of 1 g/ml was spin-coated on the template (2000 r/min, 1 min). After heating at 70 oC for 10 min with the PDMS half-cured state, prepared AgNWs/PDMS electrode (or patterned electrode) was covered on the half-cured PDMS with AgNWs side facing down until the PDMS was fully cured and peeled off from the template. Then another 12 ACS Paragon Plus Environment

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AgNWs/PDMS electrode was laminated as the counter electrode. Finally, the conductive layer was bonded with copper wires and silver paste and encapsulated by PDMS for reliable electrical contacts. 4.3 Measurements. High-resolution field emission scanning electron microscope (SEM, FEI Nova Nano SEM 450) was used to characterize the morphologies of ZnS:Cu powder and PDMS pattern. X-ray diffraction analyzer (PANalytical) was used to characterize the crystalized structure of the ZnS:Cu powder. The transmittance of the transparent AgNW/PDMS electrodes was measured by SHIMADZU UV-2550. Sheet resistance (Rs) was measured using a four-point prober. The pressure was loaded by a 1× 1 cm2 glass fixed on the three-dimensional mobile platform and measured by a digital force gauge (IMADA, ZPS/Z2S-DPU-50). A function generator (SRS, DS345) connected with a high voltage amplifier (PINTEK, HA-400) was used to supply AC voltages for the luminescent devices. Capacitance was measured by an LCR meter (Agilent, E4980A) with AC voltage of 1 V and frequency of 1 kHz. Measurements of the luminescent spectra were carried out by an optical fiber spectrometer (AVANTES, AvaSpec-HS1024x58/122TEC). The mechanical stability testing was tested using a linear motor with straight reciprocating motions. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: XXXXXX. For the working principle and layout of the synchronous time-division multiplexing circuit; the explanation of total internal reflection of luminescent; the fabrication process of 13 ACS Paragon Plus Environment


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stretchable transparent electrodes and sandwich-structured e-skin; the characterizations of phosphor particles; photographs of the addressable e-skin and different luminescent shapes.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

ORCID Bin Hu: 0000-0003-3143-746X Jun Zhou: 0000-0003-4799-8165

Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (61674064, 61434001), Frontier and Key Technological Innovation Special Foundation of Guangdong Province (2014B090915001), and the Fundamental Research Funds for the Central Universities of China, HUST (2016YXMS030). We wish to thank the support facilities at the Center for Nanoscale Characterization & Devices, WNLO of Huazhong University of Science and Technology (HUST) and the Analytical and Testing Center of HUST.


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Figure 1. Conceptual illustration of dual-mode e-skin. (a) Structure schematic and circuit layout of the e-skin. An electroluminescent layer sandwiched between two stretchable transparent AgNWs/PDMS electrodes. S1 and S2 represent two channel switches in the synchronous time-division multiplexing system. (b) Cross-section SEM image of the pyramidal microstructure of the PDMS dielectric layer with embedded phosphor particles. (c) Illustration of the nonlinear deformation of the e-skin with the increase of pressure and insets schematically show different sensing modes in different pressure ranges. (d) Cross-section micrograph of the PDMS pyramids at the pressure boundary. Dashed line indicates the boundary of the pressing area. The inset shows the high-magnification optical image of the dashed square.



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Figure 2. Capacitive and luminescent responses of the e-skin to different pressures. (a) Dual-mode sensing behavior. (b) Sensitivity of two sensing modes in Figure 1a. Capacitive responses to (c) wind blow and (d) 30 mg bamboo leaf, respectively. Insets show the experimental photos. (e) Capacitive response time to the loading/unloading of the bamboo leaf in Figure 2d. (f) The mechanical stability of capacitive sensing of the e-skin by applying a periodic 10 kPa pressure for 10000 cycles. (g) Photographs of instantaneous luminescent response with the increased pressures (AC voltage of 175 V with frequency of 1 kHz, luminescent area of 10×10 mm2).


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Figure 3. Relationship between electrical parameters and luminescent properties. (a) Luminescent spectra with the increase of AC voltages from 50 V to 200 V (1 kHz, 100 kPa); Inset: the relationship of maximum luminescent intensity (505 nm) and voltage. (b) Luminescent spectra with the increase of frequencies from 50 Hz to 7 kHz (175 V, 100 kPa). Inset: the relationship of the maximum luminescent intensity (505 nm) and frequency. (c) Photographs at different AC voltages and frequencies (100 kPa, luminescent area: 10×10 mm2). (d) Luminescent intensity as a function of pressure turned by AC voltages and frequencies.



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Figure 4. Multi-point perception of the capacitive pressure sensor. (a) Photograph of the 6 × 6 pixels e-skin with a 10 g and a 5 g weight placed in coordinate (2, 5) and (5, 2), respectively; Inset: schematic of the sandwiched structure with 6 × 6 orthogonal electrode arrays. (b) Optical photograph of the patterned AgNWs/PDMS transparent electrodes. (c) Optical micrograph of the laser-ablated AgNWs electrode strip. (d) Schematic of mutual capacitance at an intersection. (e) The contour image of loaded pressure on the e-skin as experimentally shown in (a). (f) Capacitance changes of the 6 × 6 pixels with the increase of weight at the same coordinate (3, 5).


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Figure 5. Visualized pressure mapping of the e-skin. (a) Optical photographs and microscope images of the e-skin pressed by “W” shaped slab under 100 kPa before (upper) and after (bottom) applying voltage (175 V, 1 kHz). (b) Schematic light-emitting path from the patterned pyramidal layer. (c) Intense luminescence of the e-skin poked by tweezers tips. (d) Luminescent pictures of folded, twisted and stretched e-skin. (e) The increased brightness at stress concentration regions with the compression of the e-skin.



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TOC Figure


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Dual-Mode Electronic Skin with Integrated Tactile ... - ACS Publications

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