Wearable Resistive Pressure Sensor Based on Highly Flexible

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Wearable Resistive Pressure Sensor Based on Highly Flexible Carbon Composite Conductors with Irregular Surface Morphology Kang-Hyun Kim, Soon Kyu Hong, Nam-Su Jang, Sung-Hun Ha, Hyung-Woo Lee, and Jong-Man Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 04 May 2017 Downloaded from http://pubs.acs.org on May 5, 2017

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ACS Applied Materials & Interfaces

Wearable Resistive Pressure Sensor Based on Highly Flexible Carbon Composite Conductors with Irregular Surface Morphology Kang-Hyun Kim,1 Soon Kyu Hong,1 Nam-Su Jang,1 Sung-Hun Ha,1 Hyung Woo Lee,1,2,* and Jong-Man Kim1,2,* 1

Department of Nano Fusion Technology and BK21 Plus Nano Convergence Technology

Division, Pusan National University, Busan 46241, Republic of Korea 2

Department of Nanoenergy Engineering, Pusan National University, Busan 46241, Republic of

Korea

ACS Paragon Plus Environment

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ABSTRACT Wearable pressure sensors are crucial building blocks for potential applications in real-time health monitoring, artificial electronic skins, and human-to-machine interfaces. Here we present a highly sensitive, simple-architectured wearable resistive pressure sensor based on highly compliant yet robust carbon composite conductors made of a vertically aligned carbon nanotube (VACNT) forest embedded in a polydimethylsiloxane (PDMS) matrix with irregular surface morphology. A roughened surface of the VACNT/PDMS composite conductor is simply formed using a sandblasted silicon master in a low-cost and potentially scalable manner, and plays an important role in improving the sensitivity of resistive pressure sensor. After assembling two of the roughened composite conductors, our sensor shows considerable pressure sensitivity of ~0.3 kPa–1 up to 0.7 kPa, as well as stable steady-state responses under various pressures, a wide detectable range of up to 5 kPa before saturation, a relatively fast response time of ~162 ms, and good reproducibility over 5000 cycles of pressure loading/unloading. The fabricated pressure sensor can be used to detect a wide range of human motions ranging from subtle blood pulses to dynamic joint movements, and it can also be used to map spatial pressure distribution in a multipixel platform (in a 4 × 4 pixel array).

KEWORDS: wearable resistive pressure sensor, irregular surface morphology, vertically aligned carbon nanotube forest, carbon composite conductor, human motion detection


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1. INTRODUCTION There is continuously growing demand for real-time personal health monitoring in daily life, and considerable attention has recently been devoted to the development of highly flexible and stretchable pressure sensors for skin-mountable human motion detectors.1–21 Among a variety of transduction mechanisms of the pressure sensors, in particular, resistive sensing strategies have been the most widely applied these days mainly due to the easy signal collecting capability, simple sensor architecture, and facile fabrication.22 Resistive pressure sensors typically react to an applied pressure by a decrease in the contact resistance (or increase in the electrical conductivity) between the top and bottom sensing electrodes. In this point of view, over the past few years, various rational micro- or nano-structural designs for sensing electrodes have been extensively explored to achieve high pressure sensitivity and a broad range of usable pressures by gradually maximizing the total contact area between the two sensing electrodes upon pressing.1–21,23–25 Various elastomeric templates have been used to form functional structures on sensing electrodes, such as highly roughened tissue papers,12 polyurethane porous sponges,13,24 silk nanofiber

membranes,20

and

elastomeric

films

replicated

from

regulary14–17,21

and

irregularly18,19,23 patterned masters. To maintain high pressure sensitivity of the sensor when integrated on human body parts, in particular, the sensing electrodes should be electrically stable under the mechanical stresses that occur during human motions. This is because mechanical deformations such as bending, stretching, and twisting generally increase the electrical resistance of the sensing electrodes, resulting in degradation in the pressure sensitivity by lowering the rate of resistance decrease in response to applied pressure. Therefore, conductive nanomaterials are suitable materials for stable sensing electrodes, such as metallic nanowires,12 carbon black


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nanoparticles,13 carbon nanotubes (CNTs),16,18 carbonized silk nanofibers,20 aligned CNT sheets,23 and graphene.21,24 The electrical properties of these materials are less sensitive to mechanical stress compared to continuous conductive films such as thin-film metals14,15 and conductive polymers.19,25 In addition to high sensitivity and stability of a wearable pressure sensor, simple and reproducible fabrication is also one of the most important requirements for developing practical devices. However, many of the template-assisted flexible and stretchable pressure sensors thus far involve complicated or cumbersome processes, such as multiple cycles of coating steps,12,13 photolithography or etching techniques,14–17,21,25 and high-vacuum metal deposition.14,15 In this work, we present a wearable resistive pressure sensor with a simple and stable architecture that is based on vertically aligned carbon nanotube (VACNT)-doped polydimethylsiloxane (PDMS) (VACNT/PDMS) sensing conductors with irregular surface morphology. The VACNT/PDMS composite conductors are prepared in a simple, cost-effective, and reproducible manner by filling PDMS into a VACNT forest that has been grown on a reusable sandblasted silicon master, followed by replication from the master. Threedimensionally (3D) interconnected networks of CNTs in the VACNT/PDMS conductor show sufficient electrical robustness against various mechanical deformations while retaining stable electrical performance. Based on a stable assembly process of the two composite conductors, the flexible pressure sensor shows high pressure sensitivity (~0.3 kPa–1 up to 0.7 kPa), stable steadystate responses at various pressures (20 Pa to 5 kPa), relatively fast response time (~162 ms) and relaxation time (~108 ms), and mechanical durability (5000 cycles) with reversible on/off behavior. We also confirm that our pressure sensor can clearly detect various human activities ranging from subtle (wrist and neck pulses) to large motions (dynamic flexion movements of the


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wrist and elbow). The concept of the sensor is also easily scaled up to a 4 × 4 multi-pixel sensor by combining a simple contact-transfer patterning of VACNT forest for potential artificial skin applications. The main achievement of this work is to improve the practical usability of such wearable pressure sensors by developing highly stable and sensitive sensor architecture based on the electrically robust VACNT/PDMS composite conductors with irregular surface morphology.

2. EXPERIMENTAL DETAILS Vertical Synthesis of Carbon Nanotube Forest. The CNT forest was vertically grown on a four-inch silicon master by thermal chemical vapor deposition (T-CVD) process.8,26 A ~10-nmthick alumina (Al2O3) layer and ~2-nm-thick iron (Fe) layer were sequentially deposited as a barrier layer and catalytic metal layer on a silicon master using electron-beam evaporation. Next, catalytic Fe islands that serve as a seed for CNT growth were formed by holding the Fe/Al2O3coated silicon master in a T-CVD chamber under a constant flow rate (700 sccm) of hydrogen (H2) gas for 30 s. The chamber pressure and temperature were kept at 80 mbar and 625 ºC, respectively. The VACNT forest was then grown by subsequently supplying 50 sccm of ethyne (C2H2) to the reaction chamber under a pressure of 80 mbar and a temperature of 675 ºC.

Fabrication of Wearable Resistive Pressure Sensor. To fabricate flexible VACNT/PDMS composite conductors with irregular surface morphology, a silicon master with a roughened surface was first prepared by a simple sandblasting technique with abrasive alumina particles under a spray pressure of 5 kg/cm2. The VACNT forest was then grown on the irregularly roughened silicon master by the T-CVD process. A mixture of PDMS prepolymer (Sylgard 184 A, Dow Corning) and a curing agent (Sylgard 184 B, Dow Corning) at a weight ratio of 10:1 was


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spin-coated on the silicon master at 200 rpm for 40 s, which was then held in a vacuum desiccator to facilitate smooth penetration of the PDMS into the empty spaces between the VACNTs. After thermal curing in a convention oven at 70 ºC for 1 h, the VACNT/PDMS composite was peeled off from the silicon master. Through this process, the irregular surface morphology of the silicon master is reflected in reverse on the top surface of the VACNT/PDMS composite with electrical conductivity at the surface. The size of the fabricated composite conductor was 2.5 × 2 cm2. Next, the fabricated VACNT/PDMS composite conductor was fully embedded in the partially cured (70 ºC for 15 min) PDMS layer on a supporting polyethylene terephthalate (PET) film while exposing the roughened morphology of the composite conductor at the surface. In this case, a bonding area was also defined simultaneously. Two of the prepared substrates were assembled to each other by bonding them with the roughened surfaces facing each other (overlapped area: 2 × 2 cm2). Prior to bonding, electrical wires were connected separately to the ends of each conductor (electrode area for electrical wiring: 0.5 × 2 cm2) using a silver paste. The bonded substrate was then thermally treated at 70 ºC for 1 h under a slight pressure to ensure strong and stable bonding interface by fully solidifying the partially cured PDMS parts. Finally, the resistive pressure sensor was prepared by peeling off the supporting PET substrates from the device. For comparison, the pressure sensors with flat composite conductors were prepared by the same process described above, except that VACNT forest grown on a flat silicon substrate was used. A 4 × 4 multi-pixel pressure sensor was fabricated through the same process described for the single pressure sensor except for the patterning of the VACNT strips. Prior to spin-coating the PDMS prepolymer, periodic VACNT strip patterns were formed on the roughened silicon master by selectively removing unnecessary parts of the VACNT forest using a PDMS stamp with a


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sticky surface, which is based on our previous work.8 Finally, upper and lower electrodes with four VACNT/PDMS composite strips were assembled by bonding the two electrodes with the conductive strip patterns facing each other.

Characterization. The detailed morphology of the fabricated resistive pressure sensor was investigated using an optical microscope (OM; BX60M, Olympus) with a CCD camera module, a field-emission scanning electron microscope (FE-SEM; S7400, Hitachi), and a non-contact laser interferometric profiler (NV-1000, Nanosystem). Various mechanical loads (pressing, bending, stretching, and twisting) were applied to the fabricated VACNT/PDMS composite conductors and pressure sensors using a custom-made mechanical jig and a commercially available computer-controlled stage (JSV-H100, JISC) equipped with a push-pull force gauge (HF-10, JISC). The electrical properties of the VACNT/PDMS composite conductor under various mechanical deformations and the sensing responses of the sensors were characterized by measuring the change in the electrical resistance (R) using a two-point probe (TPP) method. For the measurements, a digital multimeter (34465A, Keysight Technologies) that is connected to a computer with data acquisition software was used. The sheet resistance (Rs) distribution of the VACNT/PDMS composite conductor was calculated as Rs = R × (w/l), where R is the electrical resistance obtained from the TPP measurements, w and l are the width and length of the composite conductor, respectively.27,28 The pulse and motion measurements were conducted by observing the electrical output signals of the sensor in real time using the digital multimeter after firmly attaching it to parts of the body using adhesive tape.


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3. RESULTS AND DISCUSSION The proposed resistive pressure sensor was fabricated by simply assembling the two VACNT/PDMS composite conductors with irregular surface morphology, as illustrated in Figure 1(a). In addition, the detailed assembly process of the two VACNT/PDMS composite conductors is also illustrated in Figure S1 in the Supporting Information (SI). Highly flexible nature of the fabricated sensor (with a sensing area of 2 × 2 cm2) due to the compliant architecture of VACNT forest embedded entirely in a PDMS matrix is clearly shown in Figure 1(b). The cross-sectional SEM image in Figure 1(c) shows the initial contact state between the upper and lower VACNT/PDMS electrodes of the sensor. After assembly, the two electrodes were electrically connected to each other through contact between several protrusions on each electrode with irregular surface morphology, as shown in Figure 1(c). Figure 1(d) shows a schematic illustration of the basic working principle of our pressure sensor. The sensor operation depends on the change in the contact resistance between the upper and lower electrodes in response to applied pressure. In the initial state without applying pressure (P = P0 = 0), the sensor has the highest contact resistance (R = R0) because the contact area between the roughened surfaces of each electrode is smallest at this point. When pressure is applied (P = P1 > P0), the contact resistance is gradually reduced (R = R1 < R0), which allows more paths for current conduction to form. This is made possible by two processes: (1) the increased contact area between protrusions that had already been in contact, and (2) the creation of new contacts between protrusions that had initially been separated. When increasing the pressure (P = P2 > P1), these two processes continue, which leads to a further reduction in the contact resistance (R = R2 < R1).


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Figure 2(a) shows digital and cross-sectional SEM images of the sandblasted silicon master, VACNT forest synthesized on the roughened master, VACNT/PDMS composite replicated from the master, and silicon master after replication. Each step for fabricating the VACNT/PDMS composite conductors was conducted at a four-inch wafer scale in a simple and reproducible manner, which represents the great possibility for large-area fabrication of any size. The simple and low-cost sandblasting technique makes it possible to easily form functional, irregular surface structures on the silicon master, even on a large-area without any complicated and timeconsuming lithographic processes. The VACNT forest grown on the silicon master could be entirely transferred to the PDMS after being embedded in the matrix thanks to the relatively weak adhesion to the silicon master. The total thickness of the fabricated conductor was ~600 µm, including a ~200-µm-thick VACNT/PDMS composite layer. The average sheet resistance of the four-inch sized composite conductor was measured as ~140.7 Ω/sq. Moreover, the sheet resistance was found to be quite uniform regardless of the location, which offers sufficient potential for large-area device fabrication, as shown in Figure S2 in the SI. This also implies that the sensor can be easily scaled up to over four-inch in size through further optimization of the sandblasting and VACNT synthesis processes. The surface morphology of the as-prepared silicon master, VACNT/PDMS composite conductor, and the master after replication process was investigated further through magnified SEM images and cross-sectional profiles, as shown in Figure 2(b), 2(c), and 2(d), respectively. The reverse of the irregularly roughened surface profile on the silicon master was transferred to the surface of the VACNT/PDMS composite conductor, while revealing several conductive protrusions. The surface morphology of the silicon master remained similar to that of the as-


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prepared master after the replication process. Furthermore, the surface roughness of the master was stable even after several replication processes, as shown in Figure S3 in the SI. This means that the sandblasted silicon master can be reused repeatedly without any structural deformations due to the excellent mechanical properties (e.g., yield strength of ~7 GPa) and high melting point (> 1400 ºC). This clearly suggests that the fabrication process of the sensor can be cost-effective and highly reproducible. For reliable pressure measurements with a wearable resistive pressure sensor, high electrical robustness of the sensing electrodes upon various mechanical deformations is very important. Typically, the sensing electrode responds to mechanical loads induced by various human motions in a way that increases the electrical resistance. This can counteract the reduction in the overall resistance between the input and output terminals of the sensor when pressure is applied, which would lead to a considerable degradation in the pressure sensitivity. Figure 2(e), 2(f), and 2(g) show the change in the electrical resistance (∆R/R0) of the VACNT/PDMS composite conductor in response to bending, stretching, and twisting deformations, respectively. It was found that the electrical performance of the composite conductor is quite stable without significant degradation even under large and diverse mechanical loads (∆R/R0 = ~1.5% at a bending radius of up to ~1 mm, ∆R/R0 = ~5.8% at a tensile strain of up to ~20%, and ∆R/R0 = ~8% at a twisting angle of up to ~180 º). This probably originates from the ability of the contact points for electron conduction in the VACNT forest to sufficiently accommodate various mechanical loads without significant breaks after being embedded in the highly elastic PDMS matrix. Figure 3(a) shows the change in the electrical resistance of the fabricated resistive pressure sensor when gradually increasing the applied pressure up to 10 kPa. The resistance changes in


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the low pressure regime (0–2 kPa) are shown in the inset in Figure 3(a). A steep change in the electrical resistance of the sensor occurred up to 2 kPa, and there are two distinguishable sections with different pressure sensitivity. Here the pressure sensitivity of our device is defined as S = (∆R/R0)/∆P, where ∆P is the change in applied pressure. With applied pressure in the range of 0– 0.7 kPa, the sensor had a high sensitivity of S = ~0.3 kPa–1, which means that many new contact junctions between the conductive protrusions on the upper and lower electrodes were developed in addition to the gradual increase in contact area between the initially contacting protrusions in this pressure regime. In the pressure range of 0.7–2 kPa, the pressure sensitivity decreased to S = ~0.05 kPa–1. This suggests that the resistance change of the sensor mainly depends on the increase in the contact area between the conductive protrusions rather than the creation of new contact junctions in this pressure regime. Eventually, the pressure response of the sensor was almost saturated at a pressure higher than 5 kPa, which represents the resolvable pressure range of our sensor. Moreover, the pressure sensor made using irregularly roughened VACNT/PDMS electrodes (i.e. sanding-sanding) showed higher pressure sensitivity than one with flat electrodes (i.e. flatflat) over all ranges of pressure, as shown in Figure 3(a). This occurs because the resistance change of the flat-flat sensor is based on only the increase in the contact area between the surfaces of the flat VACNT/PDMS composite conductors, in contrast to the sanding-sanding sensor. This is clear evidence that irregular surface structures of the composite conductors play an important role in improving the pressure sensitivity of the resulting pressure sensor. This also suggests that the sensitivity and detectable pressure range of the device can be further enhanced by simply optimizing the sandblasting conditions to fabricate a roughened silicon master.


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Figure 3(b) shows the steady-state responses of the pressure sensor under various applied pressures. Our sensor can stably hold the response characteristics (for a holding time of 15 s) in a wide range of applied pressures in a distinguishable manner, even under a slight pressure of 20 Pa. It is important to note that the minimum detection pressure of our sensor was actually lower than 20 Pa, but was limited by the resolution limit of a force gauge used in the experiment. The operational stability and reproducibility of the sensor were confirmed by experimentally observing that the sensor stably responds to repetitive input pressures that are continuously varying with different magnitudes, as shown in Figure 3(c). Figure 3(d) shows the step response of the sensor with an applied pressure of 1 kPa at constant loading and unloading speeds of 10 mm/s. Magnified sensor responses were extracted from Figure 3(d) and are shown in Figure 3(e), which indicates a relatively fast response time of ~162 ms and relaxation time of ~108 ms upon loading and unloading the input pressure, respectively. Furthermore, the fabricated sensor also showed stable responses with respect to applied pressure regardless of the loading and unloading speeds (10, 1.7, and 0.2 mm/s), as shown in Figure 3(f). To investigate reliability of the fabricated pressure sensor, a pressure of 1.25 kPa was repeatedly applied to the sensor while monitoring the corresponding change in electrical resistance of the device in real time, as shown in Figure 3(g). Figure 3(h) shows the magnified sensor responses extracted from Figure 3(g) at several time intervals. The results indicate that there are no remarkable discrepancies in the magnitude and waveform of the responses during 5000 repeated pressure loading and unloading cycles. This clearly suggests that the pressure sensor would be highly reliable in repeated operations. To investigate the possibility for use in a skin-mountable human motion detector, the fabricated pressure sensor was attached to various human parts of the human body. Figure 4(a)


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shows the time-dependent responses of the sensor in response to periodic pulses of the radial artery after attaching the sensor to the wrist of one of the authors. Two systolic blood pressure peaks (SBP1 and SBP2) with respect to the diastolic blood pressure (DBP) were clearly detected in a distinguishable manner, as shown in the magnified waveform in Figure 4(b). The precise pressure measurements for subtle stimuli such as the wrist pulses are probably contributed to excellent electrical robustness of the VACNT/PDMS electrodes of the sensor attached to the curved surface of the wrist as well as the irregular surface roughness, as confirmed in Figure 2(e). The traced arterial pressure waveform could provide reliable health information such as the radial augmentation index (AIr) to estimate arterial stiffness, which is one of the most important precursors of cardiovascular disease. From the result in Figure 4(b), the AIr value was determined to be ~68% by calculating using AIr = ((SBP2–DBP) / (SBP1–DBP)) × 100 (%).1,19,29,30 This suggests that the proposed pressure sensor can be used as a non-invasive monitoring tool for characterizing cardiovascular disease. In the similar way, the proposed pressure sensor could also precisely detect periodic neck pulses, as shown in Figure 4(c). In addition to the subtle stimuli such as the wrist and neck pulses, large motions of various body parts can also be recognized with our sensor. Figure 4(d) shows the change in the electrical resistance of the sensor attached to the wrist during a sequential increase in the flexion angle. Here the black arrows indicate moments when the wrist begins to bend at different angles. The greatest resistance change was observed at the first moment when the wrist was bent since the pressure sensitivity of the sensor is highest in the low pressure regime. However, the sensor could sufficiently distinguish different flexion angles of the wrist in real time, as shown in Figure 4(d). Our sensor also responded well to repeated dynamic flexion and straightening motions of the wrist and elbow while showing reproducible response and relaxation behaviors at each cycle,


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as shown in Figure 4(e) and 4(f), respectively. This clearly suggests that the flexible pressure sensor can be used as a skin-mountable human motion detector with a wide detectable range. The proposed resistive pressure sensor can be expanded easily to a multi-pixel spatial pressure-sensing platform for potential artificial electronic skin applications. As a demonstration, a 4 × 4 pressure sensor array (16 pixels) was designed on a single device, as shown in Figure 5(a). A key step for fabricating the multi-pixel sensor is the contact-transfer patterning technique, which can produce VACNT strip patterns in a simple manner, as illustrated in Figure 5(b). Figure 5(c) shows a digital image of the fabricated multi-pixel pressure sensor. A total of 16 pressure-sensing pixels were defined after assembling two PDMS substrates with embedded VACNT strips so as to face the conductive surfaces, as shown in Figure 5(c). The contacttransfer patterning approach was highly effective for producing the VACNT strip patterns for the fabrication of the multi-pixel-arrayed sensor architecture. This also suggests that further design optimization of the PDMS stamp can potentially make it possible to fabricate a functional multiarrayed pressure sensor with high pixel density. To demonstrate the spatial pressure mapping capability of the fabricated multi-pixel sensor, input pressures were applied to the device while monitoring the change in electrical resistance of each pixel in real time. Figure 5(d) shows the resistance mapping result of pressure distribution on the device when a point load was applied to pixel (4, 3), which is marked with an “A” in Figure 5(c). The color contrast on the activated pixel was distinctly discriminated from the other pixels, which enables us to figure out the pressed position. The spatial detection mechanism relies on the largest reduction in the contact resistance between the upper and lower VACNT strips occurring at the pressed position, which allows for more electrons to conduct through the interface. As shown in Figure 5(e), the multi-pixel pressure sensor also responded well to a line


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load (marked with a “B” in Figure 5(c)) with a similar detection mechanism. These observations confirm that the proposed multi-pixel pressure sensor demonstrates the feasibility for being used as an artificial electronic skin in various robotics and health applications.

4. CONCLUSION In conclusion, we have developed a highly sensitive and wearable resistive pressure sensor that consists of robust carbon composite conductors with irregular surface morphology. The carbon composite conductors were prepared by embedding a 3D interconnected VACNT forest grown on a sandblasted silicon master into a PDMS matrix and subsequently replicating the VACNT/PDMS composite from the master. A synergetic combination of facile sandblasting and wafer-scale VACNT synthesis techniques make the device fabrication very simple and possibly scalable to large area. In addition, the entire fabrication is highly reproducible and cost-effective because the silicon master can be used repeatedly thanks to the desirable properties of silicon including superior mechanical properties and high melting temperature. The composite conductors were highly stable against various mechanical deformations while retaining their electrical properties without significant degradation due to the robust architecture. This highlights their potential for use as a reliable electrode in various skin-mountable devices. The irregularly structured conductive protrusions on the composite conductors were greatly helpful in achieving improved sensitivity of the resulting pressure sensor based on two key mechanisms: (1) an increase in contact area between initially contacting protrusions, and (2) the creation of new contacts between initially separated protrusions upon pressure application. The fabricated pressure sensor also exhibited several desirable features, including a wide detectable pressure range, stable steady-state responses under various pressures, relatively fast


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response/relaxation times, and high durability under multiple pressure loading/unloading cycles. The usability of our sensor as a skin-mountable human motion detector was verified by detecting various pressure levels induced by human activities such as blood pulses and joint motions. Finally, the concept of our sensor was extended to develop a multi-pixel device capable of mapping spatial pressure distribution for potential artificial skin applications. We believe that this simple, low-cost, and reproducible strategy for demonstrating wearable resistive pressure sensors offers sufficient feasibility for the development of various skin-like wearable electronics.

ASSOCIATED CONTENT Supporting Information. Schematic illustration of assembly process of two VACNT/PDMS composite conductors (Figure S1), sheet resistance distribution measured on the VACNT/PDMS composite conductor prepared at four-inch wafer scale (Figure S2), cross-sectional surface morphologies of VACNT/PDMS composite conductor replicated several times from the same silicon master (Figure S3). This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (H. W. Lee), [email protected] (J. -M. Kim) ACKNOWLEDGEMENT This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning


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(NRF-2015R1A2A2A01004038)

and

the

Ministry

of

Education

(NRF-

2015R1D1A3A01019420).

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Figure 1. Resistive pressure sensor based on flexible VACNT/PDMS composite conductors with irregular surface morphology. (a) schematic illustration of the fabrication process of the flexible pressure sensor, (b) digital image of the fabricated sensor (scale bar: 20 mm), (c) cross-sectional SEM image of the sensor (scale bar: 10 µm), and (d) schematic illustration of the basic working principle of the sensor.


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Figure 2. Flexible VACNT/PDMS composite conductor. (a) digital and cross-sectional SEM images of the sandblasted silicon master, VACNT forest synthesized on the roughened silicon master, VACNT/PDMS composite replicated from the master, and silicon master after replication (scale bars: 20 mm (black), 50 µm (white)). Cross-sectional surface morphologies of (b) the sandblasted silicon master, (c) VACNT/PDMS composite conductor, and (d) silicon master after replication (inset: corresponding SEM images (scale bars: 20 µm) and 3D profiles).


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Change in the electrical resistance of the VACNT/PDMS composite conductor under (e) bending, (f) stretching, and (g) twisting deformations (inset: digital images of the composite conductor under the tests).


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Figure 3. Pressure sensing performance of a flexible pressure sensor based on irregularly roughened VACNT/PDMS composite conductors. (a) change in the electrical resistance of the sensors as a function of applied pressure (inset: magnified sensor responses representing pressure-sensitivities), (b) steady-state sensor responses under various different pressures, (c) sensor responses under repetitive pressure loading and unloading cycles with different values, (d) step response of the sensor with an applied pressure of 1 kPa at a constant loading and unloading speeds of 10 mm/s, (e) magnified sensor responses extracted from (d) to show response and relaxation times, (f) cyclic responses of the sensor under different pressure loading and


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unloading speeds (inset: magnified sensor responses), (g) change in the electrical resistance of the sensor under 5000 loading and unloading cycles with a maximum pressure of 1.25 kPa, and (h) magnified waveforms extracted from (g) at several time intervals, representing reliable sensor response for repeated operations.


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Figure 4. Flexible pressure sensor as a skin-mountable human motion detector. (a) measurement of wrist pulse (inset: digital image of device attached on the wrist for measurements (scale bar: 20 mm)), (b) magnified waveform extracted from (a) of a period of the wrist pulse, (c) measurement of neck pulse (inset: digital image of device attached on the neck for measurements (scale bar: 20 mm)), (d) sensor response to sequential flexion motions of the wrist, and sensor responses under repeated dynamic flexion and straightening motions of (e) the wrist and (f) the elbow (inset: digital images of the sensor under the tests (scale bars: 20 mm)).


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Figure 5. Multi-pixel resistive pressure sensor based on strip-patterned VACNT/PDMS composite conductor array. Schematic illustrations of (a) structural design (4 × 4 pixel array) and (b) key fabrication sequence of the multi-pixel pressure sensor, (c) digital image of the fabricated device (scale bars: 10 mm), and resistance mapping of pressure distribution on the device in response to (d) point load (“A”) and (e) line load (“B”) applied to the positions marked in (c).


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TABLE OF CONTENTS GRAPHIC


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Wearable Resistive Pressure Sensor Based on Highly Flexible

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