Human Skin-Inspired Electronic Sensor Skin with Electromagnetic

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Applications of Polymer, Composite, and Coating Materials

Human Skin-inspired Electronic Sensor Skin with Electromagnetic Interference Shielding for the Sensation and Protection of Wearable Electronics Junhong Pu, Xiangjun Zha, Lisheng Tang, Lu Bai, Rui-Ying Bao, Zhengying Liu, Ming-Bo Yang, and Wei Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b15809 • Publication Date (Web): 02 Nov 2018 Downloaded from http://pubs.acs.org on November 4, 2018

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ACS Applied Materials & Interfaces

Human Skin-inspired Electronic Sensor Skin with Electromagnetic Interference Shielding for the Sensation and Protection of Wearable Electronics Jun-Hong Pu, Xiang-Jun Zha, Li-Sheng Tang, Lu Bai, Rui-Ying Bao, Zheng-Ying Liu, Ming-Bo Yang, Wei Yang* College of Polymer Science and Engineering, Sichuan University, State Key Laboratory of Polymer Materials Engineering, Chengdu, 610065, Sichuan, People’s Republic of China.

Abstract Increasingly serious electromagnetic radiation pollution puts higher demands on wearable devices. Electronic sensor skin capable of shielding electromagnetic radiation can provide extra protection in emerging fields such as electronic skins, robotics and artificial intelligence, but combining the sensation and electromagnetic shielding performance together remains a great challenge. Here, inspired by the structure and functions of human skin, a multifunctional electronic skin (M-E-skin) with both tactile sensing and electromagnetic radiation shielding functions is proposed. The tactile sensing of human skin is mimicked with irregularly dermis-like rough surfaces, and the electromagnetic shielding performance not available on natural skin is introduced by mimicking the ultraviolet (UV) electromagnetic radiation absorption of melanin in epidermis. The M-Eskin shows superior sensitivity (9.8×104 kPa-1 for the pressure range 0-0.2 kPa and 3.5×103 kPa-1 within 0.2-20 kPa), broad operating range (0-20 kPa), fast response and relaxation times (< 62.5 ms), great pressuring-relaxing stability (10 kPa, 1000 cycles), low operating voltage (0.1 V), low power consumption (1.5 nW) and low detection limit (5 Pa). Besides, a broad range of electromagnetic wave (0.5 to 7.5 GHz) is shielded for more than 99.66 % by the M-E-skin. This work holds great potential to enlarge the application scope of current electronic skins.

*Corresponding

author. Tel.: + 86 28 8546 0130; fax: + 86 28 8546 0130.

E-mail addresses: [email protected] (W Yang)

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Keywords: multifunctional electronic skin, electromagnetic radiation shielding, tactile sensing, silver nanowire

Introduction Flexible pressure sensors generate signals under a certain pressure, which have gained tremendous interest due to their potential applications in prosthetics, health monitoring, electronic skin, robotics, artificial intelligence and human-machine interactions.1-11 In the past few decades, flexible pressure sensors have got rapid development. Various mechanisms, such as piezo-resistive, piezo-electric, capacitive and tribo-electric sensing, have been demonstrated and utilized in various sensor designs.12, 13 With the development of material science, a lot of advanced materials, such as carbon nanotubes, graphene, metallic nanowires and MXene, were introduced into the fields of flexible pressure sensors.14-22 Among these materials, silver nanowires (AgNWs) have received much attention and are considered to be very promising candidates in flexible electronics,12, 15, 23, 24 owing

to the excellent electrical conductivity and outstanding mechanical flexibility.

Due to the necessary requirements for sensor skin in practical applications, there are many attempts which not only mimic human skin sensing functions, but also endow additional features. Interestingly, functions and capabilities beyond human skin are more challenging and attractive.12, 25 Recently, Li et al introduced superhydrophobicity to wearable sensing electronics by fabricating micrometer-sized pit-like surface.26 Rong and colleagues utilized the freezing tolerance of H2O/ethylene glycol binary solvent to endow the organohydrogels based strain sensor with anti-freezing property.27 Zhao and coworkers fabricated a flexible organic tribotronic transistor to develop sensing ability in magnetic detection.28 Hua et al presented a highly stretchable and conformable matrix network based electronic skin with sensing functionality including temperature, in-plane strain, humidity, light, magnetic field, pressure and proximity.29 All these attractive properties that natural skin does not possess greatly expand the application range of current electronic sensor skin systems. Nowadays, much severer electromagnetic radiation is being generated because of the rapid development in electronics, information technology and wearable devices over the past few years, which brings about harmful effects on human beings as well as highly sensitive


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precision electronic instruments.30-32 Also, as the fast development of aerospace technology, the range of human activities will be extended to outer space, which is filled with harmful electromagnetic radiation. The most common method to shield these harmful electromagnetic radiations is to cover a layer of metal where protection is needed, which cuts off the sensation of the outside world.33 In order to solve these problems, AgNWbased conductors have been applied in flexible sensors and electromagnetic interference (EMI) shielding materials. Cao et al fabricated a flexible and wearable reduced graphene oxide - Ag NW @ cotton fiber based piezoresistive sensor with the face-to-face orientation, which showed a high sensitivity, accompanied with fast response and relaxation properties.34 Zhong and colleagues designed and prepared a fiber-shaped strain sensor with great flexibility and knittability by interpenetrating AgNWs into polyolefin elastomer nanofibrous yarn, which provided the possibility of large-scale production and great potential for practical applications.35 Jia et al reported a highly stretchable AgNW/polydimethylsiloxane (PDMS) shielding film, showing an EMI shielding effectiveness (EMI SE) of 20 dB. A highly efficient and reliable EMI shielding film composed of calcium alginate (CA), AgNWs and polyurethane (PU) was also demonstrated, which showed an EMI SE of 20.7 dB with a high optical transmittance of 92%.36 In spite of the great promise these systems provide, how to further improve the application prospects of wearable flexible electronic devices is urgently need to be solved. Specifically, electronic sensor skins with EMI shielding function which can protect human and artificial intelligence instruments from harmful radiation while maintaining sensing ability is of great importance, because such EMI shielding sensor skins can be applied in and promote the further development of many urgent fields, such as aerospace, artificial intelligence, medical facilities and wearable electronics.37 To be specific, it can protect robots and humans from EMI and damage in out-space and other situations and guarantee accurate sensing ability at the same time. It can also protect pacemaker from interference and monitor heartbeats at the same time for pacemaker users. However, the electronic sensor skin with EMI shielding protecting function has not been touched till now. Here, inspired by human skin, we present a multifunctional electronic skin (M-E-skin) with pressure sensing and EMI shielding ability for the first time. By mimicking epidermis and dermis of human skin, we fabricate an electronic skin based on protecting and sensing



layers. To demonstrate the utility of this M-E-skin, a personalized intelligent glove (PIG) made of these M-E-skins is fabricated. Finally, the pressure distribution and EMI shielding measurements further proves its effectiveness in skin-like tactile sensation and EMI shielding that goes beyond the basic functions of human skin. This work may lead to ME-skin applications related to electromagnetic radiation pollution.

Experimental Section Preparation of conductive AgNW network embedded in PDMS with irregularly rough surface (sensing layer): A piece of emery paper (#800) is pretreated with a nitrogen gun to get a clean rough template. The PDMS mixture (Dow Corning Sylgard 184; the weight ratio of base to cross linker was 10:1) was stirred for 10 min, degassed in vacuum for 10 min to remove bubbles at room temperature. The emery paper was attached flatly on a clean glass. Then the PDMS mixture was cast onto the emery paper and degassed again, cured at 80 °C for 1 h, and carefully peeled off to get PDMS thin film with irregularly rough surface. AgNWs (XFJ03, XFNANO Inc, Nanjing, China.) with a diameter of 50nm and a length of 100-200 μm was dispersed in ethanol, followed by ultrasonic bath treatment (KQ-400KDB, 40 kHz, 400 W, Kun Shan Ultrasonic Instruments Co. Ltd, China) for 30s to prepare the stable AgNW suspension. AgNW suspensions with different AgNW densities (30, 62.5, 125 and 250 mg/m2) were spray-coated onto the PDMS rough surface, and then the samples were annealed at 150 ºC for 30min to embed AgNWs in PDMS. Preparation of highly conductive AgNW networks embedded in PDMS film (protective layer): Firstly, the as-prepared AgNW suspension with different AgNW densities (62.5, 125 and 250 mg/m2) was spray-coated onto a clean polyimide (PI) film which was attached flatly on a clean glass. Secondly, a high pressure 15MPa was applied for 15s on the spraycoated film with a protective cover of clean polytetrafluoroethylene (PTFE) film. Finally, the same PDMS mixture was cast onto the PI film and degassed again, cured at 80 °C for 1 h, and carefully peeled off from the PI film to get PDMS thin film embedded with highly conductive AgNW network. Preparation of M-E-skin: With spin coating PDMS between the as prepared sensing layer (density = 125mg/m2 ) and protective layer ( density = 250 mg/m2 ), a unit of the M-E-skin was fabricated after the curing of PDMS. Two units were brought into contact with rough surface to rough surface and one side of each unit was placed with Ag paste and Cu wire


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to form electrodes. Two pieces of paper were symmetrically used on the Ag paste layer to ensure a small air gap between the two rough surfaces. Then, PDMS permanently sealed both units to ensure stable flexibility. Control samples were fabricated in the same way with AgNW suspension of different AgNW densities in the sensing and protective layers. Characterization and measurements: The morphologies and structures of the materials were characterized by with a JEOL JSM-5900LV SEM. The electrical resistance of the ME-skin was measured with a four-point probe method using Keithley 2400 instrument. The EMI shielding properties of M-E-skins were measured in the frequency range of 0.5-7.5 GHz using an Agilent N5230A vector network analyzer. All the sensing characterizations were performed with a Keithley 6517B characterization system at 0.1 V. Pressure was applied and released by loading or unloading specific weight onto the device. In all measurements, a flat glass was adhered on the top of the device to uniformly load the applied pressure on the PDMS surface. For the force distribution monitoring and EMI shielding demonstration, 19 separate M-E-skins were assembled on a laboratory glove. When the grasping was in a steady state, the current of each M-E-skin was detected and recorded one by one using Keithley 6517B. The actual pressure was determined with the static results of the current response under various pressures. A homemade wireless power transmission equipment was fabricated, and images and video were taken to display the EMI shielding property.

Results and Discussion Structure design, working mechanism, fabrication process and application illustration. As epidermis is the interface of human with the outside world, it plays an important role in protecting human body against harmful influences from the environment.38 As shown in Figure 1a, melanin, widely distributed in epidermis, gives the barrier function to human skin by absorbing UV electromagnetic radiation, thus preventing cells from damage.39 Dermis provides sophisticated tactile sensing capabilities that contributes to many of our basic activities. Specialized corpuscular receptors, including Pacinian corpuscles, Meissner corpuscles, Ruffini’s corpuscles and Merkel’s disks, lie in the dermis allowing sensations of pressure and vibration. As the structure of epidermal-dermal junction is highly irregular



with many papillary dermis, there is a high local stress concentration at the tips around receptors, which improves the ability of pressure perception.40


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Figure 1 (a) Schematical illustration of the human skin structure and SEM images of a half of the M-E-skin consisting of a protective layer and a sensing layer. (b) Schematic diagram of the protective layer composed of highly conductive AgNW network in a PDMS film. Incident electromagnetic wave power (PI), reflected power (PR), transmitted power (PT), and magnified structure are illustrated. (c) Schematic illustration of the sensing mechanism and the current changes in response of loading and unloading (Ioff (open circuit): unloading; Ion: loading). (d) Fabrication process of the protective layer and sensing layer. (e) Illustration of the structure of ME-skin and the EMI shielding and pressure sensing functions when applied on a machine hand.

Inspired by the double layer structure of human skin, we designed a unit film composed of a protective layer which mimics epidermis, and a sensing layer which mimics dermis, as seen in Figure 1a, and the silver nanowires and rough surface can be seen in the enlarged figures. On the one hand, to mimic the UV protective function of melanin in epidermis, a highly conductive AgNW network film was fabricated to shield electromagnetic radiations. On the other hand, to mimic the tactile sensing function of receptors and epidermal-dermal junction, an irregularly rough surface with AgNWs was prepared to generate signals corresponding to applied pressure. The schematic illustration in Figure 1b shows the proposed protective layer, and a highly conductive AgNW network was embedded in PDMS. When the electromagnetic wave travels through the protective film, the mobile charge carriers in the conductive network will interact with the incoming electromagnetic wave. As a consequence, most of the electromagnetic wave will be reflected and absorbed, and only a small part of electromagnetic wave is transmitted. From literatures, high electrical conductivity is known to be the most beneficial for producing EMI shielding materials.41 Seen in Figure 1c, the pressure sensing function was accomplished with two rough surfaces. Without pressure loading, the conductive rough surfaces of two sensing layer are initially untouched, resulting an open circuit state. When force is applied, the electrical pathway was connected. As the pressure increases, the compressive deformation can enhance the contact area between the two rough sensing layers and reduce the contact electrical resistance. This causes an increase in the current when applying a fixed voltage. As the working principle of the pressure sensor is mainly based on contact resistance mechanism, a durable, irregular, rough and conductive surface is the core factor enabling sensitive pressure sensing. Owing to these considerations, the fabrication procedures were designed and shown in Figure 1d. On the one hand, AgNWs were firstly spray coated onto PI film to construct a


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highly conductive network. After a cold pressing process, the conductivity of the network was further improved for the junction welding of AgNWs. PDMS was then used to envelope the conductive network. On the other hand, an emery paper was used as a template to fabricate a PDMS film with irregularly rough surface. After spray coating AgNWs and annealing, a stable conductive layer was embedded on the functional rough surface. At last, seen in Figure 1e, the as-prepared protective layer and sensing layer were cured together. The final device was prepared by face to face package. For artificial intelligence robots, such an M-E-skin is potential to work as a flexible shield to protect from EMI and damage for key parts, and it can also provide tactile sensing ability at the same time, which is helpful to perceive and adapt to the outside world. It should be note that the size and shape of the M-E-skin can be easily adjusted according to practical requirements. Structure of protective layer and sensing layer. As shown in Figure 2a, when AgNWs were spray coated onto PI film, the AgNWs were loosely overlapped, and the small contact area results a big contact electrical resistance between two layers of overlapped AgNWs. Besides, the whole conductive network is too loose to undergo large deformation. To reduce the contact resistance and improve the stability of conductive network, a cold pressing treatment was performed. Seen in Figure 2b, the junctions between AgNWs were well fused while maintaining the diameter of AgNWs after the simple cold pressing process. As a consequence, all the sheet resistances decreased obviously (Supporting Information, Table S1), which is highly beneficial to improve EMI shielding properties. Finally, the uniform AgNW network was buried just under the surface of PDMS to further improve the mechanical robustness, as seen in Figure 2c. Figure 2d shows that an irregularly rough surface was successfully constructed on PDMS film by using an emery paper as template. Such a rich papillary structure is efficient to get deformed under outside pressure, which is the fundamental of electrical contact resistance based pressure sensor.42, 43 After a proper amount of AgNWs was spray coated onto the rough surface and a following annealing for 30 min at 150 °C, a conductive layer was stably embedded in PDMS without destroying the rough surface, as shown in Figure 2e and f. The fabricated flexible and stable conductive rough surface are hopeful to endow the device with a good pressure sensing ability.



It should be noted that with scanty AgNWs (62.5 mg/m2) on the rough surface, the conductivity of the AgNW layer is poor (Supporting Information, Table S2). High sheet resistance brings down the sensibility. In addition, superfluous AgNWs (250 mg/m2) on the rough surface would not only cover up the irregular papillary structure but also could not be totally embedded into PDMS, indicating that the movement ability of PDMS chains during annealing is not efficient enough to embed excess AgNWs (Supporting Information, Figure S1). Obviously, all these situations reduce pressure sensation properties, which demonstrated that rough surface with 125 mg/m2 AgNWs is suitable to be used as the sensing layer.

Figure 2 (a, b) SEM images of AgNW junctions before and after cold pressing. (c) Highly conductive AgNW network (density = 250 mg/m2) embedded just under the PDMS film. (d) Irregularly rough surface of PDMS fabricated with an emery template. (e, f) Irregularly rough surface of PDMS with AgNWs (density = 125 mg/m2)

Sensing performance of M-E-skins. The pressure sensing performance of M-E-skins composed with the same protective layers were characterized under static and dynamic forces. The linear I-V curves indicate that the M-E-skin obeys the Ohm's law, and the current increases as external pressure increases (Supporting Information, Figure S2). Figure 3a shows the change in the current flowing through the fabricated sensing layer with


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125 mg/m2 AgNW when gradually increasing the applied pressure up to 20 kPa. There are two distinguishable sections with different pressure sensitivities, nonetheless, △ I/I monotonically increases as the pressure increases in the whole workable range, in other words, one stimulus (pressure) only corresponds to one signal (△I/I), which guarantee the responses of pressure sensors accurate and reliable in the operating range. Here the pressure sensitivity is defined as eq1:44 𝑆 = 𝛿(∆𝐼/𝐼0)/𝛿𝑃

(1)

where ΔI is the change in current, I0 is the initial current without applying pressure, and P refers to the applied pressure.

Figure 3. Sensing performance of M-E-skins. (a) Static results of the current response under various pressures. (b) Dynamic results of the current responses under repetitive pressure loading and unloading cycles. (c) Magnified sensing responses extracted from (b), the second cycle of 250 Pa to show response and relaxation times. (d) Current of M-E-skin under loading/unloading a pressure of 10 kPa for 1000 cycles. (e) Current–time curve for the detection of very small pressure (5 Pa) according to the loading and unloading of a mass of (100 mg). (f) Performance comparison with this work and previously reported piezo-resistive sensors.

With applied pressure in the range of 0-0.2 kPa, the M-E-skin shows a extraordinary high sensitivity of S = 9.8×104 kPa−1, which can be mainly attributed to two reasons. One is an ultra-low initial current I0 (open circuit current = 15 nA) achieved by introducing an air gap to separate two rough surfaces. The other one is that contact junctions and area between the sensing surfaces increases expeditiously as pressure increases. In the pressure range of 0.2-20 kPa, as the pressure keeps rising, the current changes mainly depending on the



further increase in contact area rather than the creation of new contact junctions. As a result, the pressure sensitivity decreased to S = 3.5×103 kPa−1. Eventually, the contact junctions and area become almost stable and saturated, which represents the resolvable pressure range of our M-E-skin. In addition, our M-E-skin shows an extremely low power consumption of 1.5 nW, which is a good candidate for developing low-power-consumption sensing devices.45 M-E-skin prepared with different AgNW densities show different performances (Supporting Information, Figure S3). In the low pressure range (0 - 2.5 kPa), the rate of current change would rise as the AgNW density increases, because a denser conductive network is more efficient to build up new contact junctions and improve the contact area. In the high pressure range (2.5 - 20 kPa), the current changes of higher and lower AgNW density are easier to get saturated. For high AgNW density (250 mg/m2) case, the resistance of dense conducive network is very small (Supporting Information, Table S2) as the resistance change from increasing contact area is quite limited. For low AgNW densities (30 mg/m2 and 62.5 mg/m2) case, the actual contact area of conductive network is only a small portion of the whole contact area, because the conductive network is very sparse for the density of 62.5 mg/m2 (Supporting Information, Figure S1). M-E-skin with AgNW densities of 125 mg/m2 on sensing layer is a proper choice for both high sensitivity and wide working range. Figure 3b presents comparisons of the current response of the M-E-skin over three on/off cycles under a series of pressures (50 Pa, 250 Pa, 500 Pa, 2500 Pa and 10000 Pa). All the current changes fast and obviously as soon as the pressures are applied and removed, and the currents keep almost constant in the process of pressure holding, indicating the excellent reliability of the M-E-skin in detecting and discerning different pressures. Magnified sensing responses at 250 Pa pressure are extracted from the second cycle of Figure 3b and shown in Figure 3c, and a fast response to loading and unloading with both the response time and relaxation time less than 62.5 ms is observed. Here, the 62.5 ms is the limitation of the testing equipment. It is interesting to note that as the applied pressure increases, the relaxation time increases at high pressure (Supporting Information, Table S3), which is owing to the viscoelasticity of PDMS.46 The reason why the response time


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keep unchanged is that the response limitation of the equipment is always larger than the real response time of the M-E-skin. To further illustrate the reliability, a high pressure of 10 kPa was alternately loaded and unloaded onto the M-E-skin for 1000 cycles (Figure 3d). It still shows quite limited variation responding to the loading, indicating a good stability and its potential for practical applications. To understand the reasons for the good stability, the structures and currentcycle properties of the devices with different AgNWs densities were studied (Supporting Information, Figure S4 and S5). With sparse (62.5 mg/m2) and proper (125 mg/m2) AgNWs, the conductive networks were well embedded in PDMS matrix, which played an important role in protecting the conductive networks from damage. The current changes almost maintained at the same level without any noticeable degradation. However, with superfluous AgNWs (250 mg/m2), exposed AgNWs would be destroyed after a long time cycling test. The current of the senor under pressure dropped to 75% of the original one. So, the durability can be attributed to the embedding process during annealing which protects the conductive networks. The limit of detection is shown in Figure 3e. The M-E-skin immediately responded to a subtle pressure of 5Pa, which corresponds to the pressure of a mass of 100 mg on an area of 2 cm2. The sensitivity and pressure sensing range are two most important key parameters to evaluate the performance of pressure sensors. As shown in Figure 3f, our M-E-skin displays an excellent pressure sensing performance with both high sensitivity and wide sensing range over reported pressure sensors.20,

42, 44, 47, 48

These outstanding features

confirm that the pressure sensing of our M-E-skin can meet various kinds of applications where skin-like tactile sensing is desired. EMI performance of M-E-skins. In general, when an electromagnetic wave strikes an EMI shielding material, a reflected wave will be created at the external surface. As the transmitted wave from the external surface travels into the shielding material, the strength of the wave decreases due to absorption. At last, the remaining electromagnetic wave transmitted throughout the shielding material. According to the law of energy conservation, the relationship among different parts of power can be described as eq2:33 𝑃𝐼 = 𝑃𝑅 + 𝑃𝐴 + 𝑃𝑇

(2)



where PI, PR, PA and PT are the incident, reflected, absorbed, and transmitted powers of an electromagnetic wave, respectively. The EMI shielding effectiveness of a material can be defined using the ratio of the incident power (PI) to the transmission power (PT) as shown in eq 3:11 𝑃𝑇

𝑆𝐸𝑇 = ―10𝑙𝑜𝑔 𝑃𝐼

(3)

Generally, EMI SE of a material is expressed in decibel (dB). A SE value of 20 dB, corresponding to 99% attenuation of an EM radiation, is efficient to be applied in practical fields. Figure 4a shows the EMI SE performance of M-E-skins (125 mg/m2 AgNW in sensing layers) with various AgNW densities in the protective layers over 0.5 to 7.5 GHz (the frequency range of typical electromagnetic pollution sources in daily life, Supporting Information, Table S4). PDMS is transparent to electromagnetic waves and exhibits almost no shielding ability, which means that only sensing layers without protective layers almost have no EMI shielding ability. This is caused by the low density and overlap construction of AgNWs. The protective layers show strong EMI shielding ability, which increases with AgNW density. This should be attributed to the reduced sheet resistance, originating from the compacting of the conductive AgNW network with increasing AgNW density. It is well-established that the EMI SE of a conductive material is related to its electrical performance. The sheet resistance of AgNW network decreases as AgNW density increases (Supporting Information, Table S1). The conductivity of AgNW network is further reduced with cold pressing, resulting a higher utilization of free electrons in the AgNW network that can interact with the incoming electromagnetic waves. It should be note that even though the sheet resistance obviously rises after AgNW network embedded just under the surface of insulated PDMS, the EMI shielding ability is not influenced because the functional layer is the internal AgNW network.


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Figure 4. EMI shielding properties of M-E-skins. (a, b) Shielding effectiveness and shielding efficiency of the PDMS and M-E-skins with various AgNW densities in the protective layers over 0.5 – 7.5 GHz. (c, d) Shielding effectiveness and average shielding effectiveness before and after 1000 times pressing of M-E-skins with various AgNW densities in the protective layers.

As shown in Figure 4b, the EMI shielding efficiency of the protective layers is getting higher as the AgNW density increases. With the AgNW density of 250 mg/m2, the shielding efficiency of the protective layers is above 99%, which means more than 99 % of the electromagnetic waves ranging from 0.5-7.5 GHz can be shielded. Therefore, the protective layers with such an AgNW density were chosen to be a part of M-E-skin. The final M-E-skin was made up of sensing layer and protective layer with AgNW densities of 125 mg/m2 and 250 mg/m2, respectively. In real applications, the EMI shielding durability is also a key requirement. Considering that the EMI shielding layers are used as a part of M-E-skin, the same pressure with 10kPa is loaded and unloaded for 1000 times on the protective layers. As Figure 4c and d shows,



after the long-time cycling test, the EMI SEs over the whole frequency range keep almost unchanged and all the average EMI SEs remains at high levels of around 97 %, 95 % and 97 %, from 6.56, 15.2 and 25.4 to 6.36, 14.5 and 24.71 dB, respectively. The excellent EMI shielding robustness of the protective layers are mainly attributed to two reasons: (1) the welding points prevent the separation of AgNWs from each other (seen in Figure 2b) and (2) the perfect encapsulation of AgNW network just under PDMS hinders AgNWs from being pulled off from PDMS matrix (seen in Figure 2c). After 1000 cycles pressure test, the morphology of the protective layer showed almost no difference from the beginning (Supporting Information, Figure S6). Application of the M-E-skin for tactile sensation and EMI shielding. To be closer to the cases in actual applications, we tested the M-E-skin in scenarios such as real-time monitoring, pressure switch and human motion monitoring. We touched the M-E-skin with a finger once in a while, and recorded the changes in the output current of the sensor. The result showed that sensitive and fast responses were generated by the M-E-skin (Supporting Information, Figure S7 and Movie S1). In addition, seen in Figure 5a, the M-E-skin was placed in series with a battery and a light emitting diode (LED) to form a simple circuit, it is obvious that the LED can be controlled by pressing the M-E-skin, which acted like a switch (Supporting Information, Movie S2). Shown in Figure 5b, the M-E-skin was attached on the wrist to detect large-range motion of human. The current signals were collected during the bending-extension sequence (Supporting Information, Movie S3). The wrist, inclined at different angular positions, i.e., different bending degrees such as 0°, 35°, 50°and 55°, can be precisely and rapidly tracked by the M-E-skin. The electrical current raised up steeply when the bending angle of the wrist gradually increased, therefore, the larger the degree of bending, the higher the current intensity. What’s more, the current jumped back as the wrist returned to the original state, which shows a good stability. To further demonstrate the

utility of this M-E-skin, a PIG designed and constructed by overlaying M-E-skins on a thin glove is fabricated. Hands are primarily responsible for the sophisticated activities and work, their degree of flexibility, movement and perceived accuracy are closely related to human activities of daily living. Furthermore, as the fast development of robotics, artificial intelligence and human-machine interactions, a “hand” for robotic is more and more important. Here, such a PIG is able to precisely locate and measure the force distribution when contacting and communicating with outside world. In addition, the efficient EMI


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shielding function can block harmful electromagnetic wave out, which not only protects human health but also prevents interference for the nonhuman field like robots. Figure 5a shows that PIG is constituted of 19 pieces of M-E-skins on the glove. As grasping motion is one of the basic movements of robot and human hands, a beaker with different weights 73 g (empty) and 184 g (with water) were grasped steadily with a volunteer’s gloved hand and the pressure of each M-E-skin was recorded at the same time. The pressure distribution on the hand was illustrated in Figure 5b and c, respectively. As the beaker became heavier, bigger forces were needed to grasp it. The highest value among the pressures was always on the thumb, which means that the highest pressure was applied by the thumb when grabbing a beaker. To demonstrate the EMI shielding function of the M-E-skins, a homemade wireless power transmission equipment was fabricated and Figure 5d shows the photo and the schematic diagram. The energy is transmitted in the form of electromagnetic waves from the left coil to the right coil, which turns on the LED, seen in Figure 5e. When a finger with M-E-skin is inserted between two coils, as Figure 5f shows, the electromagnetic wave energy transmission is efficiently stopped and the LED is turned off. A simple demonstration (Supporting Information, Movie S4) shows the potential application of the M-E-skins for efficient EMI shielding in wearable electronics components. Briefly, our PIG could be used not only in fields such as health monitoring, robotics, artificial intelligence and human-machine interactions. What’s more, it can guarantee body function and robots running in complex situations, such as aerospace, pacemaker, large scale electronic equipment and military.



Figure 5. Application of M-E-skins. (a) The working state of the M-E-skin as a pressure switch. (b) The current response of the M-E-skin monitoring wrist with different bending angles. (c) A digital image of a personalized intelligent glove. (d, e) Pressure distribution contour on hand when a left hand wearing personalized intelligent glove is grabbing a beaker without and with water. (f) Digital image and schematic diagram of a homemade wireless power transmission equipment. (g) Digital image and schematic diagram of a homemade wireless power transmission equipment without personalized intelligent glove inside coils, LED on. (h) Digital image and schematic diagram of a homemade wireless power transmission equipment with personalized intelligent glove inside coils, LED off.

Conclusions In summary, we presented M-E-skin with functions of pressure sensing and electromagnetic shielding, which was inspired by the structure and functions of human skin. By using an emery paper as a template and the annealing processing after spray coating AgNWs at the density of 125 mg/m2, an irregular conductive rough surface based on PDMS


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was successfully prepared. With an initial open circuit state by introducing an air gap, the M-E-skin showed an extremely high sensitivity (9.8×104 kPa-1 for 0-0.2 kPa and 3.5×103 kPa-1 for 0.2-20 kPa) and a broad operating range (20 kPa). Our M-E-skin also exhibited fast response and relaxation time less than 62.5 ms, great stability over 1000 cycles (10 kPa), low operating voltage of 0.1 V, low power consumption of 1.5 nW and a detection limit as low as 5Pa. By constructing a highly conductive and robustness AgNW network via spray coating, cold pressing and PDMS casting, the M-E-skin exhibits an EMI SE of 24.7 dB, meaning to shield more than 99.66 % of the electromagnetic radiation, ranging from 0.5 - 7.5 GHz at an AgNW density of 250mg/m2. An outstanding EMI shielding durability with a negligible EMI SE change is observed, even under a high pressure loading and unloading cycle test with 10kPa for 1000 times. A personalized intelligent glove (PIG) was fabricated to demonstrate that our M-E-skin can accurately measure the magnitude and distribution of force on a working hand. At the same time, the M-E-skin can efficiently shield electromagnetic radiation. The M-E-skin with these outstanding performances makes it promising to be used in advanced wearable devices, human-machine interactions and robots, which not only provides basic sensing function but also endows extra electromagnetic radiation protection that skin doesn’t have.

Supporting Information Sheet resistances of AgNW networks for protective layers and sensing layers, SEM images of irregular rough surface of PDMS with AgNWs, I-V curves of the M-E-skin, current responses under pressures of different M-E-skins, response and relaxation times for M-Eskin under different pressure, SEM images of different sensing AgNW networks before and after long-cycle test, the currents responses of different sensor layers during 1000 pressure cycles, frequency ranges of the typical electromagnetic pollution sources in daily life, SEM image of protective layer after 1000 pressure cycles, the current response with and without finger pressure.

Acknowledgments This work was supported by the National Natural Science Foundation of China (NNSFC Grants 51422305 and 51721091).

References



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Table of Contents

A multifunctional electronic skin (M-E-skin) is fabricated by mimicking the structure and functions of human skin. The M-E-skin is capable of electromagnetic radiation shielding and tactile sensing.


Human Skin-Inspired Electronic Sensor Skin with Electromagnetic

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