silk-molded flexible, ultrasensitive, and highly stable ... · ﬂ exible electronic applications...

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ON Silk-Molded Flexible, Ultrasensitive, and Highly Stable

Electronic Skin for Monitoring Human Physiological Signals

Xuewen Wang , Yang Gu , Zuoping Xiong , Zheng Cui , and Ting Zhang *

X. W. Wang, Y. Gu, Z. P. Xiong, Prof. T. Zhang i-Lab, Suzhou Institute of Nano-Tech and Nano-Bionics Chinese Academy of Sciences Suzhou , 215123 , P. R. China E-mail: [email protected] Prof. Z. Cui Printable Electronics Research Center Suzhou Institute of Nano-Tech and Nano-Bionics Chinese Academy of Sciences Suzhou , 215123 , P. R. China

DOI: 10.1002/adma.201304248

Monitoring human physiological signals (PS) is considered to be an effective way for disease diagnosis and health assess-ment. Conventional hospital-centered healthcare sensing devices including infrared based photo-electric devices and rigid multi-electrode pressure sensors have been employed for PS detection, however the use is yet rather limited because of their poor portability and wearability. Recently, fl exible and stretchable artifi cial electronic skin (E-skin) has attracted increasing attention for its unique capability of detecting subtle pressure changes, which may open up its potential applica-tion in wearable individual-centered health monitoring, sensi-tive tactile information acquiring, minimally invasive surgery, and prosthetics. [ 1–3 ] Over the last few years, fl exible piezore-sistors, capacitors, and OFETs based on nanostructured mate-rials including single-walled carbon nanotubes (SWNTs), Ge/Si nanowires, and vertical ZnO nanowire arrays have demon-strated promising pressure sensing properties in low-pres-sure regimes (<10 kPa). [ 4–8 ] Compared to low-transparent and rigid metal/metal oxide nanowires, and conducting polymers with mechanical instability and low carrier mobilities, carbon nanotubes have been demonstrated as excellent candidates for fl exible electrodes and electronics, owing to their superior mechanical fl exibility and stability, great conductivity, and high transparency. [ 4,9 ] Despite great pressure sensing performance of these fl exible devices, it is still a challenge to form large-scale and uniform E-skin with cost-effective fabrication methods.

Polydimethylsiloxane (PDMS) fi lms are the most popular fl exible substrates to integrate sensitive nanomaterials for the fl exible electronic applications due to its excellent elasticity and biocompatibility. The microstructured PDMS fi lm is the key element of the E-skin devices, which has been proven to give higher sensitivity and faster response time than unstructured PDMS thin fi lm. [ 3,10,11 ] The PDMS thin fi lm with high-density micro-features greatly changes its mechanical properties. The applied external pressure will cause the microstructured PDMS features to elastically deform, which will store and release the

energy reversibly to minimize the problems due to visco-elastic behavior of PDMS. For unstructured PDMS thin fi lm, the com-pression causes increased relaxation time and lack of deform-able surface, which is problematic in response to a pressure load. For example, it has been reported that the pyramid-struc-tured PDMS fi lm gave 30-fold improvement in the pressure sensitivity compared to the unstructured fi lm, and the recovery time for microstructured PDMS fi lm was in millisecond range, while unstructured PDMS fi lm relaxed as long as 10 s. [ 10 ] Fabri-cation process for uniformly microstructured PDMS fi lm with regular patterns such as pyramid, cube, and line etc. is usually as follows: First, the patterned Si mould is made with photo-lithography, and followed by wet or dry etching process to form recess features. [ 10–12 ] Then PDMS elastomer and cross-linker mixture are prepared and casted on the Si mould. After degas-sing and annealing in vacuum, a PDMS thin fi lm with micro-structures is peeled off from Si mould. The geometry and shape of the micro-features are well-controlled by the patterns on Si mould. In the fi nal step, integration with nanomaterial assem-blies to form E-skin is normally achieved by spraying or spin-coating the nanomaterial dispersions onto the patterned PDMS thin fi lm. However, the fabrication of Si mould is complicated and expensive, requiring multi-steps (spin coating, lithography, and etching etc.), and large area integration with nanomaterial assemblies is less reproducible.

Herein, we present a simple and low-cost method for fab-rication of large-area patterned PDMS conducting thin fi lms with uniformly microstructured patterns. We fi nd the high-quality textile made of silk which possesses microstructured surface intertextures are effective moulds for construction of patterned fl exible PDMS thin fi lms. By integrating the uniform free-standing ultrathin fi lm of SWNTs, which is fabricated by the reproducible layer-by-layer exfoliation method developed in our previous work, [ 13 ] the large-area, uniform, and fl exible pres-sure sensors (E-skins) can be constructed. The E-skin sensing devices demonstrated superior sensitivity (1.80 kPa −1 ), very low detectable pressure limit (0.6 Pa), fast response time (<10 ms), and high stability (>67 500 cycles) for detection of feather-light pressures. In addition, the fl exible E-skins demonstrated the capability of monitoring human PS such as wrist pulse and muscle movement when a person is speaking, which may broaden their potential applications for disease diagnosis and voice recognition.

Silk is a delicate textile with a history for thousands years, and provide mechanical strength, rich surface textures in microscale (Figure S1, Supporting Information), and ease of scalable tailoring. As illustrated in Figure 1 a, a piece of delicate and clean silk scarf (10 × 10 cm 2 ) was used as the mould for

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fabrication of micropatterned PDMS thin fi lm. After the mix-ture of PDMS base and cross-linker was coated onto the silk mould, solidifi ed and then peeled off from the silk mould, a reversed pattern of textile surface was easily molded onto the fl exible PDMS surface. The thickness of the PDMS thin fi lm was controlled in the range of 200 ± 15 μ m. The morphology and microstructure of patterned PDMS fi lm were character-ized by scanning electron microscopy (SEM) with different angle and magnifi cation, (Figure 1 b-f) which show that parallel

concave lines ( ∼ 11 μ m wide, hundreds of micrometer long) interweaves with each other to form a very uniform pattern. The size, microstructure unit density, and geometric aspect of the PDMS fi lms are tunable by choosing different types of silk scarves, and various morphologies of patterned PDMS fi lms can also be selected from the silk moulds with different knit-ting patterns. In this work, the textile mould we chose is the crisscross pattern, which is the most popular knitting pattern used for textiles. Three different types of silk based textiles are

Figure 1. (a) Schematic of the fabrication process of fl exible patterned PDMS fi lms. (b,c) SEM images of patterned L-PDMS and H-PDMS fi lms, respectively. (d–f) 45 o view, top view, and side view of H-PDMS (c). (f) High magnifi cation SEM image from the box (e). (g,h) The typical SEM images of patterned PDMS fi lms covered with SWNTs ultrathin fi lm at different magnifi cations.(i) Photographs of patterned PDMS fi lm with and without SWNT ultrathin fi lm.

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ON The SEM images and sheet resistances indicate that the SWNTs

are “partially embedded” in the patterned PDMS surface after thermal treatment, which gives high stability to avoid contact-induced degradation and good conductivity due to the exposed SWNTs surface.

The confi guration of the fl exible E-skin is illustrated sche-matically in Figure 2 a. Briefl y, the E-skin was constructed by two layers of SWNTs/PDMS fi lms with the patterned surfaces placed face-to-face, and Ag paste was placed on the edge of both fi lms as source-drain electrodes. The resistance of the device is the average value of every contact resistance of sensitive sites between top and bottom conducting SWNTs networks. To study the effect of microstructure density on the sensing performance, the sensitivity of the E-skin devices based on L-PDMS and H-PDMS were compared. Similar to the method reported by Bao’s group, [ 10 ] during the pressure sensing tests, a thin glass slide with the area of 170 mm 2 (17 mm × 10 mm) was placed over the entire E-skin device to improve its stability. The pressure from the glass slide (1.04 g) is defi ned as ‘base pressure’ for the devices, and additional pressure is defi ned as ‘applied pressure’. As shown in Figure 2 b, when applied pressure increased from 60 (1.04 g) to 1150 Pa on the device, the conductance of E-skin dramatically increased due to the increased contacting sites of top and bottom SWNTs/PDMS fi lms. The sensitivity S can be defi ned as:

S =*(

�II0

)

*p (1)

�I = I − I0 (2)

Where I is the current when applied pressure on the devices, and I 0 is the current of device with only base pressure; p is the applied pressure. It was found when the applied pressure was less than 300 Pa, the sensitivity (S) of H-PDMS based device is 1.80 kPa −1 , which is much higher than those reported in most of previous literatures (see Table 1 ). The sensitivity of the sensor constructed with H-PDMS is ∼ 2.3 times of that constructed with L-PDMS patterns (0.79 kPa −1 ). For patterned PDMS thin fi lm, the microstructure density and surface geom-etry signifi cantly affects the performance of the E-skin devices. For H-PDMS based fl exible sensors, the surface with more microstructures per unit area has more effective contact sites, which will lead to larger charge transfer when the external pres-sure is applied (Figure 2 b). [ 11 ] Also for surface geometric aspect, it should be noted that the microstructures of H-PDMS fi lms provide much sharper contact edges than those of L-PDMS, (Figure S5) which will give higher pressure sensitivity with the same applied force. [ 11 ] In the following experiments, small insects, such as an ant (10 mg) or a bee (40 mg), were placed onto the H-PDMS based sensing device covered with very thin glass slides (17 mm × 10 mm, 71 mg). Figure 2 c shows the response of the device upon loading/unloading the two small insects, and the high sensitivity and fast response time indicate that the device is very suitable for detecting minute pressure. The corresponding pressure of the ant and bee were 0.6 Pa and 2.3 Pa, respectively. The detection limits of present fl exible pressure sensors and those reported in the previous literatures are summarized in Table 1 , which shows the E-skin device with H-PDMS has very low detection limit (0.6 Pa).

schematically listed in Figure S2, which provides various lux-uriant micro-structures. It is amazing that the large-area and uniform microstructures can be easily made by versatile low-cost silk based textiles with the manufacturing scalability.

Figure 1 b and c show two piece of patterned PDMS thin fi lms fabricated with different line-density of silk moulds, which are defi ned here as low-density structured PDMS pat-tern (L-PDMS, with the density of 27 × 38 lines per 1 cm −2 ) and high-density structured PDMS pattern (H-PDMS, with the density of 44 × 77 lines per 1 cm −2 ), respectively. Figure 1 d, e, and f are SEM images (45 o view, top view, and side view, respec-tively) showing the detailed microstructures of H-PDMS con-structed with plenty of sharp peaks and parallel concave lines. The distance between each adjacent peak is ∼ 11 μ m, and the height of single peak is ∼ 3 μ m. These patterned microstruc-tures with rich sharp peaks provide lots of effective contact sites for the following pressure sensing devices. In order to make micro-patterned PDMS surface conductive, the free-standing SWNTs ultrathin fi lm ( ∼ 30 nm thick) was transferred onto its patterned surface, and then annealed in air at 200 °C for 30 min. The detailed process for preparation of free-standing SWNTs ultrathin fi lm is described in our previous work. [ 13 ] Typical SEM images of patterned PDMS fi lms covered with SWNTs ultrathin fi lms after thermal annealing at different magnifi cations are shown in Figure 1 g and h, which indicate that SWNTs network is tightly attached on the PDMS surface to form a continuous conducting network (SWNTs/PDMS). The photo graphs in Figure 1 i shows patterned PDMS fi lms with and without SWNTs, and SWNTs/PDMS shows high transmit-tance with light-gray color.

The interface between the PDMS and the SWNTs plays a key role in related to the stability of SWNTs/PDMS conducting fi lm, and the thermal treatment (at 200 °C in air for 30 min) is an important process which affects its morphologies and the electrical properties. As shown in the typical SEM images with top view and side view in Figure S3, small bundles of SWNTs were tightly adhered to patterned PDMS substrate after thermal treatment. (Figure S3a and b) Because of strong van der Waals force between SWNTs and PDMS substrate after thermal treat-ment, it is energy favorable for SWNTs tightly adhering to the substrate instead of uplifting from the substrate. Figure S3c and d show SEM images of SWNTs/PDMS fi lms without thermal treatment, and there are the protrusive structures of SWNTs which are quite different with the tightly attached SWNTs after thermal treatment. The surface of SWNTs/PDMS fi lms is conducting, and the sheet resistances of SWNTs/PDMS conducting fi lms with and without thermal treatment were compared. (Figure S4) When 1 kPa pressure was applied on the SWNTs/PDMS conducting fi lms for 600 cycles (4 s for each cycle), the SWNTs/PDMS conducting fi lm with thermal treat-ment shows great stability with minimal changed sheet resist-ance (average ∼ 3.5 × 10 4 Ω sq −1 by four probe measurements), while the other conducting fi lm without thermal treatment pre-sents instability with degraded conductivity. Its sheet resistance increases from 4.0 × 10 4 Ω sq −1 to 9.5 ×10 4 Ω sq −1 gradually. The thermal annealing process improves the conductivity and stability of SWNTs/PDMS conducting fi lms by enhancing the contact junctions of SWNTs bundles and adhesion between SWNT ultrathin fi lm and micro-patterned PDMS fi lm. [ 14,15 ]

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pressure sensor exhibits high repeatability, stability, and dura-bility with no hysteresis. The response time of this sensor is less than 10 ms. (Figure S7, the resolution limit of LabVIEW program controlled digital source meter (Kethley 2602) for real time I-t measurement is 10 ms) Theoretically, the response time will be further optimized when the thickness of PDMS/SWNTs conducting fi lm decreases, which will decrease the residual hysteresis effect from the compression and relaxation times of entangled PDMS polymer chain. [ 3,10 ] The patterned surface

To further investigate the stability of the pressure sensor, the conductance changes of the device were measured when repeatedly load/unload an applied pressure of 1 kPa for more than 67 500 cycles (3s for each cycle). The stability cycling test was set up by using the force gauge installed at the computer-controlled movable stepper motor. Figure S6 shows the setup of the measurement, in which the tip of force gauge did not touch the entire device, but hit the centered part of the E-skin device. The result is shown in Figure 2 d, which reveals that the

Figure 2. (a) Schematic of a typical E-skin. (b) Sensitivities of pressure sensors constructed with H-PDMS and L-PDMS. (c) Real-time I-t curve of the E-skin constructed with H-PDMS for detection of a bee (40 mg) and an ant (10 mg), respectively. (d) Real-time I-t curves of the E-skin constructed with H-PDMS for more than 67500 loading/unloading cycles, at 3 s for each cycle, with an applied pressure of 1 kPa.

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PDMS thin layers provide plenty of effective contact sites; and (ii) partially embedded SWNTs ultrathin fi lm onto patterned PDMS fi lms provides mechanic and electrical stability and make the conductive contact sites very sensitive upon the applied minute pressure. The fi rst parameter can be adjusted by choosing the silk textiles with different pattern densities and the thickness and conductivity of SWNTs ultrathin fi lm can be fi ne-tuned by

optimizing the fabrication method. [ 13 ] The size of as-prepared E-skin can be easily

tailored for different pressure sensing appli-cations. For example, an E-skin (2 × 2 cm 2 ) was attached onto a person’s neck to nonin-vasively monitor pressure difference of the muscle movement during speech ( Figure 3 a). As shown in Figure 3 b, the E-skin exhib-ited high sensitivity and distinct patterns when the speaker spoke different words and phrases such as “Hello”, “Nanotechnology”, “Inspire a generation”, and “One world one dream”, respectively. To further investigate its repeatability, the word “Hello” and the phrase “One world one dream” were recorded for three times (Figure 3 c and d). It is clear that the obtained I-t curves have similar character-istic peaks and valleys when the tester spoke the same words or phrases. The E-skin pro-vides an interesting and effective method for voice recognition through sensitive and fast pressure sensing, which is mainly caused by the deformation of epidermis and muscles around the throat during speech. It might be usefully for people with damaged vocal cords to recover their speech ability by training to control their throat muscle movement. The E-skin will also bring promise for remote control of human/machine interfaces.

morphologies of devices after cycling test were characterized by SEM with different magnifi cation and directions. (Figure S8). It is clear that the micro-patterns and the sharp peaks still remain intact after more than 67 500 cycling test.

The excellent sensing performance of the E-skin is mainly attributed to the following two factors: (i) large amount of sharp micro/nano-structured patterns on the interface of two SWNTs/

Figure 3. (a) Photograph showing E-skin directly attached to a tester’s neck for monitoring the muscle movement during speech. (b–d) Real-time I–V curves of the E-skin constructed with H-PDMS when monitoring muscle movement during the tester’s speech.

Table 1. Summary of performance of fl exible pressure sensors reported up to now.

Types of devices Sensitivity Detection limit Working voltage Reference

OFET 0.05 kPa −1 - V ds = 20 V; V gs = 20 V; [ 7 ]

10 −4 kPa −1 - V ds = 20 V; V gs = 10 V [ 16 ]

8.4 kPa −1 - V ds = 100 V; V gs = 100 V [ 3 ]

10 −5 kPa −1 2.5 kPa V ds = 50 V; V gs = 80 V [ 17 ]

- - V ds = 3 V; V gs = 3 V [ 18 ]

Ferroelectret transistor 10 −3 kPa −1 2 Pa V ds = 8 V; V gs = 8 V [ 19 ]

6.7 × 10 −4 kPa −1 2 MPa V ds = 15 V; V gs = 10 V [ 20 ]

Piezoelectric 0.02 kPa −1 - V ds = 10 V; [ 21 ]

0.131 kPa −1 3.5 kPa V ds = 1 V; [ 6 ]

Capacitance 5 × 10 −3 kPa −1 - - [ 22 ]

5 × 10 −4 kPa −1 980 Pa - [ 23 ]

2.3 × 10 −4 kPa −1 - - [ 4 ]

0.55 kPa −1 3 Pa - [ 9 ]

Resistance-type - 3 Pa - [ 12 ]

SWNTs/PDMS based resistance-type 1.80 kPa −1 0.6 Pa V ds = 2 V This work

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the initial systolic portion of the pressure pulse. The rapid and strong P-wave of the healthy person is associated with a faster and greater momentum of blood ejection compared to that of the pregnant woman. Comparative analyses of adjacent P-wave intervals based on nonlinear chaotic theories for the healthy person and the pregnant woman are shown in Figure 4 d, [ 28 ] in which x axis (PP(n)) and y axis (PP(n+1)) are time intervals between the two continuous P-wave peaks, and z axis is the value of half-band-width for each P-wave peak. This analytical method effectively provides the pattern recognition for the healthy and pregnant persons with signifi cant wrist pulse dif-ference. [ 28 ] In addition, the pulse signals of our E-skin devices were compared with those of high-accuracy capacitive tactile pressure sensors. (PPS’s pulse Tact System from Pressure Pro-fi le Systems, Inc.) A few hundreds of WPC cycles of the healthy person were collected by the E-skin and PPS sensors, and the average WPCs were calculated by one-dimensional linear inter-polation in Matlab. As shown in Figure 4 c, the average WPCs of E-skin and the PPS system are well-matched. The similarity calculated by Pearson correlation coeffi cient is 0.9901, which clearly indicates the fl exible E-skin device present an accurate measurement for monitoring wrist pulse.

In summary, we have developed a novel method for the fab-rication of ultrasensitive E-skin devices by combining uniform micro-patterned PDMS fi lms with SWNT ultrathin fi lms. For the fi rst time, the delicate silk-based textiles have been used as moulds to replicate their reversed micropatterns onto the PDMS surface. The fl exible E-skin sensing device constructed

In modern medical practice, wrist pulse is key indicator of arterial blood pressure and heart rate, and provides lots of useful information for non-invasive medical diagnosis. For example, cardiovascular disease such as atherosclerosis is asymptomatic in the very early stage, but it leads arterial pulse pathologic and affects arterial blood pressure, hence continuous monitoring arterial blood pressure by wrist pulse will provide a rapid and noninvasive way for diagnosing cardiovascular dis-eases. [ 24 ] In this work, the fl exible E-skin devices can be placed over the radial artery of wrist to differentiate the subtle pres-sure difference between people with different body conditions ( Figure 4 a and Supporting Information Movie 1). Figure 4 b pre-sents real-time I-t curves of the E-skin device over the artery of wrist with a healthy person and a pregnant woman (21 weeks). The fast response time (<10 ms) of the E-skin provides high resolution to obtain the detailed clinical information of wrist pulse. It shows clearly that the pulse frequency of the healthy person is 75 beats/min with regular and repeatable pulse shape, and the pulse frequency of pregnant person is 91 beats/min with irregular shape and intensity. The wrist pulse con-tours (WPCs) contain important clinical messages which can be derived from certain characteristic points through mathe-matical analysis. As shown in Figure 4 c, the typical characteris-tics of wrist pulses were collected clearly containing percussion wave (P-wave), tidal wave (T-wave), Valley, and diastolic wave (D-wave), which are related to the systolic and diastolic blood pressure, the ventricular pressure, and the heart rate. [ 25–27 ] For example, the P-wave is the dominant peak of a WPC, which is

Figure 4. (a) Photograph of an E-skin device for detection of wrist pulses. (b) Original signals of I-t curves for monitoring wrist pulses of a healthy person and a pregnant woman. (c) The average WPCs of E-skin and the PPS system of a healthy person, calculated from a few hundred of WPC cycles. (d) Comparative analyses of adjacent P-wave intervals for the healthy person and the pregnant woman, x axis (PP(n)) and y axis (PP(n+1)) are time intervals between the two continuous P-wave peaks, and z axis is the value of half-band-width for each P-wave peak.

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ON and Taiwan Science & Technology Cooperation Program of China

(2012DFH50120).

Received: August 23, 2013 Revised: September 17, 2013

Published online: December 17, 2013

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with two layers of patterned SWNTs/PDMS fi lms demonstrates ultrahigh sensitivity for the detection of minute forces with fast response time, great stability and repeatability, and with a detection limit of the device as low as 0.6 Pa. Moreover, the applications of E-skin devices in monitoring human physiolog-ical signals, such as voice (pressure) recognition and real-time wrist pulse detection, have been shown with excellent sensing performance, which may broaden their application in cost-effective wearable electronics for the prevention of illnesses and the prediction of early diseases.

Experimental Section Preparation of Flexible Micro-Patterned SWNTs/PDMS Conducting

Films : The pretreatment of silk based textiles is as follows: fi rst, a piece of tailored silk based textile was washed with soap and rinsed in deionized water for 10 min, followed by sonication in deionized water for 20 min. Then, it was blown dry with a nitrogen gun to get a piece of clean textile. The fabrication of micro-patterned PDMS fi lm is described below: The PDMS mixture of base and cross-linker (Dow Corning Sylgard 184; the weight ratio of base to cross linker was 10:1) was stirred at least for 20 min, degased in vacuum for 10 min to get rid of bubbles at room temperature. The silk mould was attached very fl at on a clean glass. Then the PDMS mixture was spin-coated onto the silk mould at 600 rpm, solidifi ed at 70 °C for 2 h, and carefully peeled off from the silk mould to get micro-patterned PDMS thin fi lm. The fabrication of SWNTs/PDMS conducting fi lm is as follows: First, free-standing SWNTs ultrathin fi lm ( ∼ 30 nm) was prepared according to the method we developed in our previous work. [ 13 ] Then, the free-standing SWNTs ultrathin fi lm was transferred onto the patterned surface of PDMS fi lm. In the transferring process, the micro-patterned PDMS fi lm was attached to a glass substrate, and was gradually inserted into deionized water with free-standing SWNTs ultrathin fi lm fl oating on water surface, then the glass was lifted gradually to pick up SWNTs ultrathin fi lm, and to attach it over the PDMS fi lm, the angle of the glass was kept 45 o with the water surface to make sure that the SWNTs ultrathin fi lm intact during lifting. Finally, the SWNTs/PDMS fi lm was annealed in air at 200 °C for 30 min to improve its stability.

Flexible E-skin Fabrication and Measurements : The fl exible E-skin was constructed with two layers of as-prepared micro-patterned SWNTs/PDMS conducting fi lms. One side of a conducting fi lm was placed with Ag paste to form an electrode. After Ag paste dried in oven at 100 °C for 1 h, two SWNTs/PDMS conducting fi lms were overlapped together with patterned surfaces touching each other (Ag electrode not touching the surface of the other fi lm). Finally the edges of the device were bonded with Kapton tape to form the fl exible E-skin device. The computer controlled movable stage (Beiguang SC movable stage) and force gauge (Handpi Digital force gauge, HP2) were used to apply the external pressure. The electrochemical workstation (CH Instruments 660D) and LabVIEW controlled digital source meter (Kethley 2602) were used to measure I-t curves in real-time, and the Source-Drain voltage was 2 VDC.

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements We acknowledge the funding support from the National Natural Science Foundation of China (91123034, 21107132), and the Hong Kong, Macao

Adv. Mater. 2014, 26, 1336–1342

silk-molded flexible, ultrasensitive, and highly stable ... · ﬂ exible electronic applications...

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