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Fabrication of Low-Cost and Highly Sensitive Graphene-Based Pressure Sensor by Direct Laser Scribing Polydimethylsiloxane Yunsong Zhu, Hongbing Cai, Huaiyi Ding, Nan Pan, and Xiaoping Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b17085 • Publication Date (Web): 22 Jan 2019 Downloaded from http://pubs.acs.org on January 24, 2019

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ACS Applied Materials & Interfaces

Fabrication of Low-Cost and Highly Sensitive Graphene-Based Pressure Sensor by Direct Laser Scribing Polydimethylsiloxane

Yunsong Zhu,† Hongbing Cai,‡ Huaiyi Ding,‡ Nan Pan,‡,§ and Xiaoping Wang*,†,‡,§

†Department of physics, University of Science and Technology of China, Hefei 230026, China ‡Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China §Key Laboratory of Strongly-Coupled Quantum Matter Physics, Chinese Academy of Sciences, School of Physical Sciences, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China *E-mail: [email protected]

Abstract：

The cost-effective production of flexible interconnects is a challenge in epidermal electronics. Here we report a low cost approach for producing and patterning graphene films from polydimethylsiloxane (PDMS) films by direct laser scribing in the ambient air. The produced graphene films exhibit high electrical conductivity and excellent mechanical properties, and can thus be used directly as a flexible conductive layer without the need for metals. The skin-like pressure sensor with these layers exhibits

ultrahigh

sensitivity

(∼480

kPa-1)

while

ACS Paragon Plus Environment

maintaining

the

fast

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response/relaxation time (2μs/3μs) and excellent cycle stability (>4000 repetitive cycles). Moreover, it can naturally attach to the skin to monitor the wrist pulse. In addition, a 77 sensor array has been fabricated, which possesses the capability to detect the spatial distribution of pressure. This device has great potential for application in epidermal electronics due to its low cost and high performance.

Keywords: pressure sensing, graphene, epidermal sensor, laser direct writing, health monitoring

1. INTRODUCTION Conformal integration between the biological organs and sensor is the prerequisite for realizing functional human-machine interface in medical and wearable applications. Due to its inherent good elasticity and biomedical compliance,1 PDMS is widely used in the field of epidermal electronics. However, owing to its non-conduction, PDMS should be imperatively mixed with conductive materials for electronic applications.2-4 Metals5-7 and carbon materials8-11 are widely used in this regard. Among these materials, graphene is attractive due to its excellent electrical and mechanical properties. However, on the one hand, chemical vapor deposition (CVD) of graphene requires high temperature processing,8 making it impossible for direct synthesis on the PDMS substrate. As a result, device fabrication based on CVD graphene requires transfer and patterning processes, leading to high cost. On the other hand, although the reduced graphene oxide is a more cost-effective alternative to CVD graphene,10, 12 the treatment of acidic waste water from graphene oxide production is another problem. Therefore, low-cost and eco-friend synthesis of graphene on PDMS substrates is still a challenge. Recently, graphene has been successfully produced from graphite oxide12 and various commercial polymers13 via laser treatment. However, to the best of our knowledge, producing and patterning graphene directly from PDMS films by laser scribing in the air has not been reported. More importantly, fabricating high performance pressure sensors on insulating PDMS films through this method has not been demonstrated.


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In this work, we demonstrate producing and patterning graphene films from PDMS films by direct laser scribing in the ambient air. The produced graphenes are well characterized by Raman spectroscopy and electrical measurement. It is found that the crystal quality and the conductivity of the graphenes can be regulated by controlling the laser power. These patterned conductive graphenes can be used as electrodes to fabricate the skin-like pressure sensor. The sensor exhibits ultrahigh sensitivity (∼480 kPa-1) while maintaining fast response/relaxation time (2μs/3μs) and excellent cycle stability (>4000 repetitive cycles). Moreover, it can be naturally attached onto the skin to monitor the wrist pulse. In addition, we also demonstrated a 77 pressure sensor array that can detect the spatial distribution of applied pressure.

2. RESULTS AND DISCUSSION As depicted in Fig. 1a, irradiation of a PDMS film by a continuous mode diode laser with wavelength of 405 nm in the air can convert the surface of PDMS into graphene. With the help of computer-controlled laser scribing, the device's graphics can be written directly on the PDMS film within 1 minutes with a laser power of 500 mW (Supporting Movie 1). The photograph shown in Fig. 1b demonstrates two distinguished regions: black one in the middle is corresponding to the area of PDMS exposed to the laser, and the transparent one is that unexposed. Fig. 1c shows the Raman spectra performed on above two regions. As seen, the peaks at ~490 cm−1, ~620 cm−1, ~690 cm−1, ~710 cm−1, ~790 cm−1, ~860 cm−1, ~1260 cm−1, ~1410 cm−1, ~2900 cm−1 and ~2960 cm−1 obtained from the transparent region are consistent with the data of PDMS.14 On the contrary, the result from black region shows three peaks: the D peak at ~1,350 cm−1, the G peak at ~1,580 cm−1 and the 2D peak at ~2,700 cm−1, implying that a few-layer graphene film has been generated on the surface of PDMS after the laser irradiation. The behavior is similar to those recent reports of fabrication of graphene by laser scribing of various materials. 13, 15 Here we show a direct laser



scribing PDMS films in the air to form graphene films.

Figure 1. Graphene produced by direct laser scribing PDMS films. (a) Schematic of the experiment. (b) Photograph of a PDMS film after laser scribing (scale bar, 5 mm). (c) Raman spectrum obtained from the exposed region (red line) and the unexposed region (blue line). (d) Raman spectra of graphene films obtained with different laser powers. (e) ID/IG and I2D/IG versus laser power. (f) Relationship between the sheet resistance of graphene film and laser power.

The plausible mechanism of direct laser scribing PDMS films to form graphene is considered as being a photothermal process.13, 15 Although the PDMS film appears transparent (with the transmittance of 90% at 405 nm, see Figure S1 in supporting information), it can still absorb 405 nm light and lead to increasing the local temperature of the irradiated surface area. The local temperature reached so much high that the laser-induced fluorescence can be observed in the experiment (Supplementary Movie 1). In principle, such high temperature could easily break the Si—C, Si—O and C—H bonds in PDMS. The silicon and hydrogen atoms can combine either the oxygen in the air or the oxygen atoms in PDMS to form silicon dioxide and gaseous water, while the left carbon atoms are arranged into graphitic structures.


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The above mechanism can be identified through the additional experiments. Firstly, we tried to focus the light in the inner of PDMS films to produce the conductive graphene inside. However, we have not found any graphene there, even with the maximum laser power. The result indicates that the oxygen is necessary for the reaction of converting PDMS to the graphene, which is lack in this case because the inner of PDMS films is isolated from the externally surrounding atmosphere. Secondly, we investigated the thickness dependence of reduction process of PDMS. To this end, three different thick PDMS films (17 m, 25 m and 50 m) have been laser scribed under 500 mW laser. The Raman spectra of produced graphene are shown as Figure S2 in supporting information. As seen from Fig. S2(a), with increasing the thickness of PDMS films, the D Raman peak of graphene decreases obviously while both G and 2D peaks become strong and sharp gradually. The behavior is observed more clearly in Fig. S2(b). The result can be understood from the fact that the thicker PDMS cannot conduct the heat effectively to the substrate and surrounding, leading to the higher temperature of the PDMS surface and consequently forming the graphene with high quality.

Graphene films obtained with various laser powers ranging from 100 mW to 700 mW were further characterized with Raman spectroscopy, and the results are shown in Fig 1d. As seen, all samples produced by different laser power show the specific D, G and 2D Raman peaks of graphene. Although the peak positions are nearly independent of the laser power, the relative intensities of Raman peaks change obviously, demonstrating the crystal quality of produced graphene can be influenced by the laser power. As shown in Figure 1e, ID/IG decreases obviously and I2D/IG increases continuously as the laser power rises from 100 mW to 500 mW. The results can be understood from the fact that the surface temperature of PDMS increases with increasing laser power, which is helpful to synthesize high quality few-layer graphene. However, ID/IG becomes large as further increasing laser power to 700 mW, indicating more defects occurr in the graphene. This phenomenon can be attributed to



the partial oxidation of graphene in high temperature. Fig. 1(f) shows the electrical characteristic of graphene films generated by different laser powers. As the power increases from 100 mW to 400 mW, the sheet resistance (Rs) decreases dramatically from 39 kΩ/ per square to 10 kΩ per square. However, it decreases gradually and become saturation at about 5 kΩ per square when the laser power is beyond 500 mW. Both results of Raman and electrical characterizations suggest that increasing laser power helps few-layer graphene to grow, but also makes oxidation worse. Under the competition and coexistence of these two mechanisms, we consider the laser power of 500 mW being an optimal choice. Therefore, all graphenes used for pressure sensors in the following paragraphs were fabricated with a laser power of 500 mW.

Considering the high electrical conductivity and good mechanical properties of produced graphene, it can thus be used directly as conductive channel to construct the flexible pressure sensor. The process for the fabrication of graphene-based pressure sensors is schematically illustrated in Fig. 2. Briefly, with computer-controlled laser scribing, the pattern of the device was directly written on the PDMS films. Then, two same conductive graphene films were wired out using silver paint and copper wires. Eventually, they were assembled and overlapped face-to-face to form the sensor. The photography of the sensor attached on the skin is shown in Fig. 2f. The detailed layout and fabrication of the sensor are shown in the Figures S3 and S4 in the Supporting Information.


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Figure 2. Skin-like graphene pressure sensor by direct laser scribing PDMS films. (a-e) Schematic illustration of the fabrication process. (f) Photograph of a sensor attached on the skin (scale bar, 5 mm)

The detail morphology of the conductive graphene layer to construct the device was characterized by the SEM and AFM. Fig. S5(a) shows the SEM top view of the laser scribing PDMS, Fig. S5(b) and Fig. S5(c) are the enlarged top and side images marked in Fig. S5(a), respectively, indicating the protrusion structure formed on the PDMS surface after laser scribing. Figure S6 show several AFM morphological images of the produced protrusion structure on the PDMS surface, and the height of protrusion is found to increase with the laser power. Combined AFM results and the enlarged image of SEM shown in Fig.S5(d), we infer that the micro-structure of laser scribing PDMS has the uniquely porous-like feature. This micro-structure is important to demonstrate the high sensitivity of the device, which will be interpreted in the following paragraph.

Static and dynamic pressure response measurements were then performed to investigate the performance of the pressure sensor. Fig. 3a shows the current–voltage (I-V) curves of the device under different pressures. The linear characters of the I-V curves in the voltage between -5 and 5 V demonstrate that the device obeys Ohm’s



law, indicating that the conductance (or the sensitivity) of the device is independent of the working voltage. Moreover. it can be found that the conductance exhibits an obvious increase as the external pressure. Fig. 3b shows the variation of the relative conductance ((G-G0)/G0) of the device with the pressure, where G and G0 denote the conductance with and without applied pressure, respectively. The sensitivity of the device, defined as S=δ(△G/G0)/δP, can be fitted and calculated from the trace in Fig. 3b. It is shown that the sensitivity is as high as ~480 kPa-1 in the low pressure range ( 400 Pa), the conductance change of the sensor is dominantly determined by the variation of the internal conductance of double layers through compressing the porous graphene foam structure, resulting in the similar response to that observed from the single layer sensor. We notice that Wang, X. et. al. have recently established and reported a model to reveal the rapid response and recovery time of graphene foam.16 In this context, we can contribute the underlying cause for highly fast and sensitive performance of our device to the uniquely porous-like micro-structure of graphene produced by laser scribing PDMS and the special two layers design.

It is worth mentioning that, as summarized in Table 1 , even our device is composed by PDMS films without deliberately designed microstructures, its sensitivity is much higher than those of sensors constructed by either carbon-based materials10,

17-24

or

metal materials25 and comparable to the microstructure enhanced sensors.26-27 The response time and relaxation time are two to five orders of magnitude faster than those of previous reports.10, 17-27 Here, the high performance of our proposed pressure sensor, including ultrahigh sensitivity and much fast response, is considered to be strongly related to the unique morphology in combination with the excellent mechanical and electrical properties of the produced porous graphene foam. Table 1. Performance comparison of our sensor with prior devices Materials

Sensitivity

Range

Response/relaxation

Sizes

(kPa-1)

(Pa)

time (ms)

(cm2)

CL/ PDMS

57

0-3k

60/40

rGO/ Patterned PDMS

1.71

0–225

6/6


Applications

Ref.

1

HMI

[20]

0.0001

HMI,ES

[18]

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PS ball@rGO/ PDMS

50.9

0-1k

50/50

1.5

HM,HMI

[22]

CVD graphene/ Patterned PDMS

110

0-200

30/-

5

HM,HMI

[26]

MOF-derived PC/ PDMS

15.63

0–2k

60/75

3

HM

[17]

Au/ Patterned PDMS

15

0-100

100/100

2.25

HM, ES

[25]

CSilkNM/ PDMS

34.47

0-400

16.6/-

2

HM, HMI

[19]

ACNTs/G/ Patterned PDMS

19.8

0-300

16.7/-

2.25

HMI

[21]

PEDOT:PSS/ Patterned PDMS

851

0-3k

0.15 /-

0.25

HM, HMI

[27]

Graphene foam

22.8

0-10

100/-

16

HMI

[23]

Laser scribing GO

0.96

0-50k

0.4/212

1

HMI

[24]

LSG/PDMS

2

0-200

0.15/0.3

1.6

HMI,HM

[10]

LIG/PDMS

480

0-100

0.002/0.003

0.36

HM, ES

this work

Note: CL, carbonized lignin; PS, polystyrene; MOF, metal–organic frameworks; PC, porous carbon; Au, gold; CSilkNM, aarbonization of silk nanofiber membrane; ACNT/G, aligned carbon nanotubes/graphene; PEDOT:PSS, Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate); GO, graphite oxide; LSG, laser-scribed graphene; LIG, laser-induced graphene; HMI, human-machine interfaces; HM, health monitor; ES, electronic skin.

Since the device is fabricated directly on conformable PDMS films, it can be naturally attached to the skin of the wrist (Fig. 4a) to monitor the sphygmus (Supporting Movie 2). The response to the sphygmus was monitored and recorded using an oscilloscope. As shown in Fig. 4b, a heart rate of ~75 beats per minute is deduced, corresponding to the value for healthy adults. Moreover, the repeated beatings from the sum of the incident wave and reflected wave from the hand (P1) and the reflected wave from the lower body minus end-diastolic pressure (P2) can be clearly distinguished in the radial artery pulse waves of the sphygmus. In addition, the radial augmentation index (AIr =P2/P1) and the digital volume pulse time (ΔTDVP= tP2-tP1), representing a diagnosis of arterial stiffness of a volunteer,28 are found to be about 0.45 and 0.3 s, which is in the acceptable ranges for a healthy adult aged mid-twenties.

We also design a 7×7 pressure sensor array (each size is 2 mm×2 mm) to detect the



spatial distribution of pressure, as shown in Fig. 4c. Three screws, each weighing 0.75 g, were placed on the sensor array, and the corresponding output conductance at each pixel was measured and recorded in grayscale map, in which the brighter the pixel, the larger the pressure. As seen in Fig. 4d, the local pressure distribution deduced from the grayscale mapping is well agreed with the screw positions shown in Fig. 4c, indicating the good capability of the pressure sensor array. Consequentially, the ultra-high sensitivity, fast response/relaxation time and simple fabrication of these pressure sensors, based on graphene from direct laser scribing PDMS, facilitate its great potential in electronic-skin applications.

Figure 4. Applications of the skin-like pressure sensor and sensor array. (a) Photograph of a skin-attachable sensor directly above the artery of the wrist (scale bar, 5mm). (b) Real-time responses of the sensor to sphygmus. (c) Three screws were placed on the as-fabricated 7 × 7 pressure sensor array (scale bar is 5 mm). (d) Relative conductance mapping of pressure distributions for the screws in (c).


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3. CONCLUSIONS In summary, we report a novel approach for directly producing and patterning graphene films from PDMS films by laser scribing in the ambient air. The produced graphene films exhibit high electrical conductivity and excellent mechanical properties. The graphene-based pressure sensor fabricated by direct laser scribing PDMS is demonstrated. The sensor exhibits ultrahigh sensitivity (∼480 kPa-1) while maintaining the fast response/relaxation time (2 μs/3 μs) and excellent cycle stability (>4000 repetitive cycles). Moreover, it can monitor wrist pulse attached on the skin. In addition, we also demonstrate a 7x7 sensor array that can detect the spatial distribution of pressure. This low-cost and eco-friendly technology, which is easy to implement mass production and integration, not only has greatly potential applications for high performance epidermal electronics, but also expend new routes to fabricate the graphene device directly on the surface of polymer. 4. EXPERIMENTAL SECTION 4.1．Preparation of PDMS film. A 10:1 mixture of PDMS elastomer (Sylgard 184, Dow Corning) to cross-linker was prepared and stirred evenly. Subsequently, about 5 ml of the solution was transferred onto a poly(ethyleneterephthalate) (PET) sheet and cured at room temperature to form the PDMS film. The sheet was then cut into small pieces (20 mm20 mm). After that, the PDMS film was peeled off from the PET sheet, and transferred onto a glass slide. The thickness of the PDMS film is about 50 m.

4.2．Device fabrication. A home-made laser engraving system was used in this work. The system was comprised of a diode laser, a home-made XY stage, a focal lens (N.A.=0.25), an air pump and a height-adjustable table. The focal length of the lens was fixed at 10 mm throughout the experiment. In the experiment, we used a commercially continuous mode diode laser with wavelength of 405 nm and maximum output power of 800 mW. The reasons why a blue diode laser chosen are based on



following two points: one is that the PDMS has absorption between 350-800 nm (Fig. S1), and the other is to achieve the high power density by small focus spot using short wavelength laser. The G-codes used for the laser scribing process is produced by a Matlab program. Various laser power was attempted with a fixed scanning speed (about 10 mm/s) with one pass. All the electrodes were made using silver paste and copper wires.

4.3 ． Pressure response measurement. The measurement method and process are similar to our previous reported work with slight modification.10 Briefly, to measure the responses of the sensor to low pressures, a system containing an electromagnetic balance and a source-meter was designed. A small plate (18mm×18mm) between the device and load determined the sensitive area to be 324 mm2. The salt was used as the load in our experiment because it can be varied readily little by little. To investigate the responses of the sensor to dynamic pressures, a system containing a computer-controlled stepping motor and a source-meter was designed. The weight moves up and down under the control of a stepping motor to apply and remove external pressures. To measure the response of the sensor to the pulse pressure or sphygmus, a homemade bleeder circuit (Figure S10 in Supporting Information) was used to convert the resistance change into the voltage change.

4.4 ． Characterization. The loading weight was measured by an electromagnetic balance (Hang-ping FA1204B). Scanning electron microscopy (SEM) images were performed by Zeiss Sigma 300. Atomic force microscopy (AFM) images were carried out with Bruker Dimension Icon. Raman spectra were measured by LabRam HR 800. Absorption spectra were obtained by HITACHI U-4100 spectrophotometer. The conductivity measurements of graphene films as well as the pressure sensor were carried out using a Keithley 2450 with a two-probe mode at a voltage scan from -5 to 5 V. The conductance was calculated from the linear fit of the current versus voltage curve. 16 samples were measured for average to improve accuracy. The response and relaxation time of the pressure sensor were measured using an oscilloscope (Tektronix


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TDS2012). Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ORCID

Hongbing Cai: 0000-0003-3186-1041 Huaiyi Ding: 0000-0002-2512-4013 Xiaoping Wang: 0000-0002-8296-385X

Notes The authors declare no competing financial interest.

Acknowledgements We acknowledge the financial supports from Ministry of Science and Technology of China (2016YFA0200602), National Natural Science Foundation of China (21421063, 11404314, 11474260, 11504364, 11504359).

Supporting Information Available: Supporting Information: Absorption spectrum of the PDMS film, Raman spectra of graphene films produced by irradiating different thickness PDMS films under 500 mW laser, the layout of laser scribing PDMS process, schematic illustration of device assembly, the SEM and AFM images of laser scribing PDMS, variation of relative conductance with the pressure for five devices, conductance of single and double PDMS-rGO sheets varies with pressure, schematic illustration of the sensing mechanism, and the experimental setup used to measure response/relaxation time (PDF)



Supporting Movie 1: This movie shows the fabrication process of LIG pressure sensor. It took ~1 min to fabricate these devices with ~6 mm × ~6 mm area (AVI) Supporting Movie 2: Real-time monitoring of radial pulse waves using a LIG pressure sensor (AVI) This material is available free of charge via the Internet at http://pubs.acs.org.

References 1. Mi, Y.; Chan, Y.; Trau, D.; Huang, P.; Chen, E., Micromolding of PDMS Scaffolds and Microwells for Tissue Culture and Cell Patterning: A New Method of Microfabrication by the Self-Assembled Micropatterns of Diblock Copolymer Micelles. Polymer 2006, 47 (14), 5124-5130. 2. Chen, Z.; Ren, W.; Gao, L.; Liu, B.; Pei, S.; Cheng, H.-M., Three-Dimensional Flexible and Conductive Interconnected Graphene Networks Grown by Chemical Vapour Deposition. Nature Materials 2011, 10, 424-428. 3. Niu, X.; Peng, S.; Liu, L.; Wen, W.; Sheng, P., Characterizing and Patterning of PDMS ‐ Based Conducting Composites. Advanced Materials 2007, 19 (18), 2682-2686. 4. Kim, K. H.; Vural, M.; Islam, M. F., Single‐Walled Carbon Nanotube Aerogel ‐Based Elastic Conductors. Advanced Materials 2011, 23 (25), 2865-2869. 5. Kim, D.-H.; Lu, N.; Ma, R.; Kim, Y.-S.; Kim, R.-H.; Wang, S.; Wu, J.; Won, S. M.; Tao, H.; Islam, A.; Yu, K. J.; Kim, T.-i.; Chowdhury, R.; Ying, M.; Xu, L.; Li, M.; Chung, H.-J.; Keum, H.; McCormick, M.; Liu, P.; Zhang, Y.-W.; Omenetto, F. G.; Huang, Y.; Coleman, T.; Rogers, J. A., Epidermal Electronics. Science 2011, 333 (6044), 838-843. 6. Jeong, J. W.; Kim, M. K.; Cheng, H.; Yeo, W. H.; Huang, X.; Liu, Y.; Zhang, Y.; Huang, Y.; Rogers, J. A., Capacitive Epidermal Electronics for Electrically Safe, Long ‐ Term Electrophysiological Measurements. Advanced Healthcare Materials 2014, 3 (5), 642-648. 7. Joo, Y.; Byun, J.; Seong, N.; Ha, J.; Kim, H.; Kim, S.; Kim, T.; Im, H.; Kim, D.; Hong, Y., Silver Nanowire-Embedded PDMS with a Multiscale Structure for a Highly Sensitive and Robust Flexible Pressure Sensor. Nanoscale 2015, 7 (14), 6208-6215. 8. Kim, K. S.; Zhao, Y.; Jang, H.; Lee, S. Y.; Kim, J. M.; Kim, K. S.; Ahn, J.-H.; Kim, P.; Choi, J.-Y.; Hong, B. H., Large-Scale Pattern Growth of Graphene Films for Stretchable Transparent Electrodes. Nature 2009, 457, 706-710. 9. Jung, H.-C.; Moon, J.-H.; Baek, D.-H.; Lee, J.-H.; Choi, Y.-Y.; Hong, J.-S.; Lee, S.-H., CNT/PDMS Composite Flexible Dry Electrodes for Long-Term ECG Monitoring. IEEE Transactions on Biomedical Engineering 2012, 59 (5), 1472-1479. 10. Zhu, Y.; Li, J.; Cai, H.; Wu, Y.; Ding, H.; Pan, N.; Wang, X., Highly Sensitive and Skin-Like Pressure Sensor Based on Asymmetric Double-Layered Structures of


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Fabrication of Low-Cost and Highly Sensitive ... - ACS Publications

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