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A Flexible Dual-Mode Tactile Sensor Derived from Three-Dimensional Porous Carbon Architecture Zifeng Wang, Ruijuan Jiang, Guangming Li, Yiyan Chen, Zijie Tang, Yukun Wang, Zhuoxin Liu, Hongbo Jiang, and Chunyi Zhi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 20 Jun 2017 Downloaded from http://pubs.acs.org on June 20, 2017

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ACS Applied Materials & Interfaces

A Flexible Dual-Mode Tactile Sensor Derived from ThreeDimensional Porous Carbon Architecture Zifeng Wang1, Ruijuan Jiang2, Guangming Li3*, Yiyan Chen2, Zijie Tang1, Yukun Wang1, Zhuoxin Liu1, Hongbo Jiang1 and Chunyi Zhi1* 1

Department of Physics and Materials Science, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong Special Administrative Region, China

2

Shenzhen Municipal Engineering Design & Research Institute Co., Ltd, Shenzhen, China

3

School of Mechanical, Electrical & Information Engineering, Shandong University, Weihai, China

Abstract Detecting and monitoring varieties of human activities is one of the most essential functions and design purposes of different kinds of wearable sensors. Apart from excellent sensitivity and durability, limited by the materials, most of the sensors reported in the literatures are only capable of detecting signals based on sole mechanism. In this work, a dual-mode flexible sensor derived from high temperature pyrolysized 3D carbon sponge (C-Sponge) was proposed as peculiar sensor materials that are able to detect human activities based on fundamentally different mechanisms, either by triboelectric effect or by piezoresistive effect. The sensor generated average open circuit voltage up to ～2 V and short circuit current up to ～70 nA when being used as self-powered triboelectric sensor, which was sufficiently sensitive for detecting finger touching and plantar pressure distribution of human feet. On the other hand, by incorporating MWCNT into the 3D structure, the sensor at piezoresistive mode exhibited a sensitivity improvement of nearly 20-fold, from less than 40% to more than 800%, and a durability improvement of more than 22fold (240,000 cycles) compared with those of original C-Sponge fabricated at 1000 oC (10,800 cycles). All the experimental results indicated that the proposed flexible dual-mode sensor is potentially applicable as wearable sensors for human activity monitoring. Keywords: dual-mode, piezoresistive, triboelectric self-power, tactile sensor, 3D architecture. Introduction Flexible and wearable electronics nowadays are receiving significant attention from both academic and industrial societies due to their potentials of realizing human-machine integration as well as offering fascinating functions that are able to fundamentally reshape our lifestyle. Even in recent years, a wide variety of flexible electronics spring out including electronic skins (e-skin) and wearable sensors1-4, wearable energy storage and generation devices5-8, soft robotics9-11 and flexible LEDs12-14. Among those, flexible and wearable sensors with abilities to detect chemical15, heat1, strain16 and electricity17-18 have been the most intensively reported ones because the functions of sensor are closely related to human activities. By detecting specific signal from human activities, such as strain from movement of limb, body heat at the surface of the skin, moisture from respiration or even heartbeat, it is possible to acquire and record necessary information reflecting the real conditions of body for future analysis.19

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Despite being powerful and versatile, there are certain requirements for wearable sensors that facilitate their integrations on human beings, such as flexibility that helps accommodate the complex shape of human body, durability that ensures long-term workability, lightweight for wearing, low power consumption and relative low cost.20 To meet those requirements, one of the possible approaches is to fabricate sensors using flexible carbon based materials, such as graphene21, carbon nanotubes (CNTs)22, carbon fibers23-24 and carbon sponges (C-Sponge)25. Compared with semiconductor and polymeric materials, carbon-based materials are intrinsically conductive that greatly benefits the devices fabrication and testing, especially for strain sensor that relies heavily on electric signals, while compared with metallic materials they are much lightweight, which is suitable for wearable sensors. However, while the graphene26-28 and CNTs-based sensors17, 29-30 have received tremendous interest of research, there are only few reports on C-Sponge based sensors25. Owing to the three-dimensional porous structure, C-Sponge has been reported as novel electrode materials31, scaffold for catalysts32 and also carbon aerogels33. However, by itself, carbon is relatively inert that responses to external stimuli passively, such as electrical resistance change owing to the strain exerted under external power source, which was the working principle for piezoresistive strain sensors.1718, 23, 27 What’s more, although being intrinsically conductive, the conductivity of C-sponge is relatively insufficient, leading to poor sensitivity. As a result, pure C-Sponge can hardly meet the demand where either active response or good sensitivity is needed. In this work, a novel C-Sponge based dual-mode sensor is proposed as wearable sensor. By infiltration of polydimethylsiloxane (PDMS) into the high temperature fabricated flexible C-Sponge, the material is able to generate electrical signal by triboelectric effect that exactly reflects the frequency and amplitude of the stimuli without the use of external power source. On the other hand, it is found that the incorporation of multi-walled CNTs (MWCNTs) greatly enhances the sensitivity of original C-Sponge when being used as piezoresistive strain sensor. In both cases, successful integration of the sensor on human body is achieved, which demonstrates the potential application of this dual-mode sensor as flexible and wearable sensor in the future. Experimental Section Materials Pristine melamine sponge was purchased from Chengdu Jiasideng Technology Co., LTD. MWCNTs were purchased from Beijing BoYu GaoKe New Material & Technology Co., LTD. Sylgard-184 polydimethylsiloxane (PDMS) and curing agent were purchased from Dow Corning Co. All the chemicals were used as received without further purifications. Fabrication of C-Sponge As shown schematically in Figure 1, in a typical fabrication, pristine melamine sponge was rinsed by acetone, ethanol and distilled water, respectively for 10 min with the assistance of ultrasonication and then dried at 60 oC in air prior to use. The dried melamine sponge was cut into a size of 4.0×4.0×1.5 cm3 sizes and then loaded into a quartz tube for high temperature pyrolysis at 800-1000 oC for 2 h at 2 oC/min ramping rate under the flow of Ar. Significant shrinkage in size was observed for the as-fabricated black C-Sponge (about 1.8×1.8×1.0 cm3 remained).


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Fabrication of the TENG Sensor In a typical fabrication process, Dow Corning 184 PDMS and curing agent with weight ratio of 10:1 were weighed and mixed thoroughly by Thinky Mixer for 10 min mixing and 5 min defoaming and then filled into C-Sponge by vacuum infiltration prior to 60 min curing process at 80 oC. After curing, excess part of the PDMS was removed and the upper part of the C-Sponge was exposed. Fabrication of the C-Sponge Piezoresistive Sensor As for the piezoresistive sensor, high temperature thermal pyrolysized C-Sponge was directly used as reference sensor for comparison. Besides, 3.0 g MWCNTs and 1.5 g polymeric dispersive additives were added into 100 ml DI water and mixed by ultrasonication in an ultrasonic bath for 4 h. C-Sponge fabricated at different temperatures were cut into 1.0×1.0×1.0 cm3 bricks and soaked in the black CNT ink for 10 min and dried at 60 oC in air prior to test, denoted as C-Sponge/CNT. Characterization and Sensor Performance Test The morphology of the pristine melamine sponge and C-Sponge as well as the C-Sponge/CNT was characterized by JEOL JSM 820 scanning electron microscope (SEM) and JEOL JSM 6335F fieldemission SEM. Mechanical compressional test was conducted on 'MTS, USA' Alliance RT 30kN ElectroMechanical Materials Tester. RTS-8 four-probe electrical resistivity measurement system (PROBES TECH) was applied to measure the square resistance of the samples. The sensor tests were performed on Stanford Research System and viewed via LabVIEWTM system. Open circuit voltage and short circuit current signal were measured by MODEL SR560 Low-Noise Preamplifier and MODEL SR570 Low-Noise Current Preamplifier, respectively. As for the piezoresistive sensor, UTP3705S power supply was used to provide constant DC voltage. The sensor was fixed onto vibrational exciter (JZK-2 Sinocera Piezotronics, Inc.). During the test, the amplitude and frequency of the vibration of the exciter were controlled by YE1311 Signal Generator and YE 5871A Power Amplifier, respectively. Current signal was recorded by MODEL SR570 Low-Noise Current Preamplifier and viewed via LabVIEW system.



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Figure 1. Schematics of the fabrication and testing processes. The C-Sponge fabrication fabricated by high temperature pyrolysis (e.g. 1000 oC) was then either incorporated with CNT or infiltrated with PDMS for strain sensor or self-powered TENG sensor testing, respectively. Results and Discussions Originally, the pristine melamine sponge is white in color constructed by interconnected microfibers, forming three-dimensional continuous network, as shown in Figure 2 (a). Particularly, the length of individual microfiber that forms the whole porous architecture ranges from tens of micrometers to several hundred micrometers while the diameters of the fibers are generally less than 10 µm, resulting in very high aspect ratio. It is worth mentioning that the long-range 3D ordered porous structure consists of repeated unit of near hexagonal ring structure resembling that of benzene ring, as pointed out by the red hexagonal dot line in the inset figure. The melamine sponge used in the study is composed of formaldehyde and melamine resin with sodium bisulfite as an additive so that the scaffold of the sponge will not collapse as usual during the high temperature pyrolysis.34 Interestingly, as shown in Figure 2 (b)(d), although volumetric contraction of the product C-Sponge is observed, the 3D porous architecture of initial melamine sponge maintained intact after carbonization at 800-1000 oC, which is one of the predominant factors that give rise to the peculiar sensing performance as it provides three-dimensional electrically conductive network. Figure 2 (e) and (f) show the microstructure of the C-Sponge dip-coated with CNT, which exhibits different morphologies compared with that of pure C-Sponge. Apparently, the surface of the sponge fibers is covered by thin layer of CNT network while the porous structure is still preserved. It can be clearly seen from the localized SEM image in Figure 2 (f) that individual fibers are wrapped by CNTs. As a result, the introduction of CNT enhances the connection between sponge fibers and forms additional conductive network, which not only improves the mechanical strength but also improves the sensing sensitivity of C-Sponge.


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Figure 2. (a) Typical SEM image of pristine melamine sponge, inset figure shows the magnified 3D architecture of the sponge, indicating the sponge is constructed by plenty of near hexagonal units; (b)(d) Typical SEM images of the C-Sponge fabricated at 800 oC, 900 oC and 1000 oC, respectively, showing the 3D porous structure is well maintained after high temperature decomposition; (e) Typical SEM image of the CNT incorporated C-Sponge fabricated at 1000 oC; (f) Localized SEM image of the CNT incorporated C-Sponge, showing the sponge scaffold is wrapped by CNT network. Self-Powered Triboelectric Nanogenerator Sensor (TENG sensor) First, the TENG sensor was demonstrated. As illustrated in Figure 3 inset, the PDMS infiltrated C-Sponge is flexible and can be bended to some extent. The size of the TENG sensor is about 1.8×1.8×0.5 cm3 with the upper and bottom parts covered by copper tape electrodes and connected to the cables, as schematically shown in Figure 1. The self-powered sensing process can be understood as follows. TENGs are able to harvest mechanical energy and generate triboelectricity, which has been demonstrated intensively by Wang et al.35-36 Basically, as illustrated in Figure 3, upon finger touching that creates compressional force to the TENG sensor, original negatively charged C-Sponge fibers are deformed leading to more uneven distribution of charges so that more negative charges (electrons) are prone to be attracted by PDMS, which has strong affinity to capture electrons.37 As a result, C-Sponge fibers near the interface are left positively charged and this actually builds up electrostatic potential at the interface between PDMS and fibers of C-Sponge, causing transient flow of electrons through the 3D conductive architecture. Upon releasing the compressional force, the uneven distribution of charges as well as the deformed sponge fibers recovers to the original state, leading to instantaneous current flow in the opposite direction. Fortunately, both the potential that builds up into triboelectric open circuit voltage and the short circuit current are strong enough to be recognized as signals.



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Figure 3. Schematic showing the working mechanism of the TENG sensor, inset figure indicates the flexible nature of the as-fabricated TENG sensor. In order to evaluate the sensory performance, a piece of TENG sensor with the size of 1.8×1.8×0.5 cm3 was fixed and a home-made impact tester was programmed to generate periodic impact onto the TENG sensor. Figure 4 (b) and (c) show the open circuit voltage and short circuit current generated. A rectifier circuit shown in Figure 4 (a) was used to convert the AC signal to DC signal so that the magnitude of the signal can be measured easily (otherwise the rectifier was not used unless being mentioned). It can be seen clearly that both of the voltage and current signals show excellent consistency with the periodic impact generated at different frequencies, from 0.2-4 Hz. The magnitude of the voltage and current signals varied from 1.5 to 2 V and 50-70 nA, respectively. A slightly increase of signal magnitude was observed due to increased frequency, which has been reported previously by other group.38 As a result, it is demonstrated that the TENG sensor generates highly detectable signals with good signal-to-noise ratio, which can be used as sensor for detecting human activities. As shown in Figure 4 (d), when being applied as tactile sensor, the TENG sensor is able to generate voltage signals response to finger touch. It can be roughly estimated that the frequency of the finger touch is about 1 Hz so that there are two consecutive signals within 2 seconds. Moreover, upon touching, the sensor generates a positive voltage signal up to 1.2 V while opposite voltage signal of almost the same magnitude appears during releasing, showing agreement with the aforementioned working mechanism. In addition, as shown in Figure 4 (e) and (f), the open circuit voltage and short circuit current signal were recorded as well at different touching frequencies, from 0.2 Hz to 4 Hz, respectively. Obviously, apart from being AC signal, the measured open circuit voltage and short circuit current signals show consistency with those generated by impact tester. Yet, the signal at 4 Hz frequency seems to be more irregular than those measured at lower frequencies, probably resulting from the difficult for finger to complete high frequency motion.18, 39 This confirms that the TENG sensor proposed is able to provide


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sufficient sensitivity to detect human motion as well as the ability to resolve high frequency motions, of which the performance can be seen from the videos in the supplementary information.

Figure 4. Voltage and current signal generated from TENG sensor with an area of 3.24 cm2 at different frequencies. (a) Schematic diagram of the rectifier circuit for recording the sensory signal; (b) Voltage generated at 0.2-4 Hz frequencies; (c) Current signal generated at 0.2-4 Hz frequencies; (d) Voltage signal generated through finger press (e) Voltage generated at 0.2 Hz to 4 Hz with 3.24 cm2 TENG sensor; (f) Current generated at 0.2 Hz to 4 Hz with 3.24 cm2 TENG sensor. In addition to finger touch, as-fabricated TENG sensors were attached on the insole, as shown in Figure 5 (a)-(d), to monitor transient plantar pressure signal during exercise. As is known to all, human plantar pressure differs at different regions of the feet.40 As shown in Figure 5 (a), there are generally four different regions that define the distribution of pressure under the feet, which have been marked with pressure bars. At region I and IV, which are regions under relative high pressure, both voltage and current signal recorded from the sensors attached at corresponding positions are apparently higher than other two regions while region III where the pressure seems to be the least generates signals with the smallest amplitude, as seen from Figure 5 (b) and (c). This result indicates that the amplitude of signals, voltage or current, measured by TENG sensor attached at different positions of the insole can directly reflect the transient plantar pressure at the corresponding regions so that it can be used to monitor the plantar pressure during walking or running.



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Figure 5. Demonstration of the self-powered C-Sponge TENG sensor. (a) Schematic diagram showing the plantar pressure distribution of human during walking; inset is the insole with TENG sensor attached; (b) and (c) Voltage and current signal generated from TENG insole attached at different position (I-IV) of human foot; (d) Monitoring of human activity by TENG insole. Figure 5 (d) shows the sensory signals monitored during normal speed walking, slow walking and fast walking, respectively. Apparently, the signals generated reflect the frequency and scale of the motion. During normal walking, the frequencies of the signals generated are obviously higher than that recorded from slow walking and lower than that recorded from fast walking, respectively, which exactly represents the pace speed. Moreover, the signals generated during fast walking obtain higher amplitude compared with those from slow walking, which can be related to the higher impact pressure subjected as it positively correlated to the magnitude of triboelectricity generated.37 As it can be seen, the results clearly demonstrated that the TENG sensor proposed exhibits excellent performance in terms of monitoring human activities and can be used practically as self-powered wearable sensors.


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Figure 6. Sensory test results of sponge fabricated at 800 oC, 900 oC and 1000 oC, with and without the addition of CNT at different testing conditions. (a) shows the C-Sponge sensors fabricated at 1000 oC with or without the addition of CNT tested at 25% strain and 4 Hz frequency; (b) shows the relative change in resistance of C-Sponges (shown as the colored dot lines), fabricated at different temperatures, measured during compressional test with strain value up to 80% (shown as the black line); (c) Average square resistance of 1×1×1 cm3 C-Sponge with or without the addition of CNT fabricated at different temperatures; (d) C-Sponge incorporated with CNT tested at 25% to 90% strain values; (e) C-Sponge incorporated with CNT tested at 4 to 32 Hz frequencies. Compressible C-Sponge/CNT Strain Sensor Besides the application as self-powered TENG sensor, owing to the compressible and 3D conductive nature of the C-Sponge, it can also be used as compressible strain sensor. As mentioned above, the ∆ / , was found to be greatly sensitivity of the C-Sponge strain sensor, defined as / improved by adding CNT. It can be observed from Figure 6 (a) that the sensitivity of the sensors, reflected by relative change in current, improves from less than 30% to more than 200% for the C-Sponge fabricated at 1000 oC, after the incorporation of CNT when subjected to 25% strain periodically compared with pure C-Sponge without CNT. Moreover, not only the sensitivity improves but also the signal itself becomes better after the incorporation of CNT as the signals seems to be much smoother than those without CNT. This can be understood as the reinforcing effect offered by CNT network. As it can be seen from Figure 2 (e) and (f) as well as Figure 6 (c), the CNT dispersion in the polymeric dispersive agent forms networks wrapping on the sponge fibers, offering not only extra electrical conductive pathways, which help improve the sensitivity of the sponge, but also mechanical support to the original C-Sponge scaffold so that the whole sponge becomes more mechanically robust. In contrast, the initial C-Sponge without reinforcement is relatively fragile so that the breakage of sponge fibers happens easily, which



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leads to lose of conductive network, resulting in fluctuation of the current signals. The mechanical compressional tests of the sponges in Figure S4 also provide evidence of the reinforcement effect. In order to further verify the idea that the sensitivity and mechanical properties of the sponge can be improved by introducing CNT, mechanical compression test was conducted and the electrical resistance of the sensor was recorded simultaneously. Basically, C-Sponge with size of 1.0×1.0×1.0 cm3 was cut and fixed onto the stage of mechanical test machine and a constant voltage was applied to the sponge sensor while the current signal was recorded simultaneously. An 80% strain value was applied to each of the sample at 4 mm/min strain rate for five consecutive cycles and the resistance was calculated for comparison. Figure 6 (b) and Figure S2 (c) show the sensitivity of the sensors, represented by relative change in resistance, recorded from C-Sponge fabricated at different temperatures with or without the addition of CNT. As they shown clearly, the sensitivity of pure C-sponges fabricated at 800-1000 oC fall into the range between 39.5% and 132.3% while those for samples with CNT added fall between 49.7% and 808.2%. Interestingly, the 1000 oC series show the highest nearly 20-fold improvement from less than 40% to more than 800% while the sensitivity for 900 oC series, although obtain the highest sensitivity around 132.3% initially, reach as high as 516.5% after adding CNT, corresponding to 3.9-fold improvement. Although the improvement for the C-Sponge fabricated at 800 oC is not as effective as those fabricated at 900 oC and 1000 oC, a remarkable drop of average square resistance for the sample fabricated at 800 oC can be found in Figure 6 (c), from 22.38 to 7.06 kΩ/sq. By contrast, the average square resistances of 900 o C and 1000 oC series are much smaller than the 800 oC ones and so do the improvements. As a result, it is highly possible that the 900 oC and 1000 oC series reach percolative threshold much easier while the 800 oC series remain below the value even when being squeezed by 80%, probably resulting from the incomplete carbonization at 800 oC. Clearly, the improvement of sensitivity for the 1000 oC series is the most obvious among all the others so that the following tests were performed using the 1000oC series.

Figure 7. (a) Cyclic performance test of the C-Sponge incorporated with CNT. Inset shows the corresponding data obtained from C-Sponge without the addition of CNT; (b) SEM image of CSponge fabricated at 1000 oC after cyclic test; (c) SEM image of C-Sponge/CNT fabricated at 1000 o C after cyclic test.


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Figure 6 (d) and (e) show the sensory performance tests of the 1000 oC series at different strain values and frequencies. By comparing Figure 6 (d) and Figure S3 (a) as well as Figure 6 (e) and Figure S3 (b), respectively, it is obvious that the C-Sponge/CNT samples fabricated at 1000 oC exhibit the best sensitivity response to the strain applied, from 25% to 90% strain value at 4 Hz frequency and the best frequency response to different frequencies applied, from 4 Hz to 32 Hz, respectively. When the strain value increases from 25% to 50%, the pure C-Sponge exhibit quite fluctuating signals, probably caused by breakage and disconnections of sponge fibers as discussed above, while the signals for C-Sponge/CNT keep smooth. Further increase of the strain value up to 90% leads to highly asymmetric characteristics of the signal, which could be ascribed to the loss of signal detected, while the C-Sponge/CNT still maintains the performance.

Figure 8. Demonstration of the C-sponge/CNT strain sensor. (a) Demonstration of the C-sponge/CNT sensor attached on glove for real-time monitoring of finger movements, showing the slow, normal and fast movement of human fingers recorded by the sensor attached; (b) Monitoring of the finger movement, showing the ability of the sensor to distinguish the amplitude as well as the speed of the finger movement. In order to demonstrate the durability of the sensor material, the cyclic stability of C-Sponge/CNT sensor fabricated at 1000 oC was tested at 25% strain at 4 Hz frequency while the C-Sponge without CNT addition was chosen as the reference. The results from Figure 7 clearly show that the cyclic stability of the CNT reinforced sensor is much better than the pure C-Sponge. The C-Sponge/CNT sensor works as many as 240,000 cycles and still maintains well-functioned while the C-Sponge only works slightly more than 10,000 cycles with quite fluctuating signals even from the beginning of the test. By contrast, the sensor reinforced by CNT, although decrease of sensitivity was still observed owing to aging effect, the



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signals exhibit much less fluctuation and gradually stabilize finally. The SEM images in Figure 7 (b) and (c) present the morphologies of corresponding sensors after cyclic tests and reveal the reason for that. It can be seen that the fibers of pure C-Sponge seem to be more susceptible to mechanical fracture under periodic strain as fiber breakage appears massively inside the sponge. On the contrary, by virtue of the reinforcement of the CNT network, the fibers are joint with each other becoming more mechanically robust. The existence of CNT also provides extra electrical conductivity even though the fibers fracture. Furthermore, to demonstrate the practical application, the C-Sponge/CNT sensor was assembled and fixed onto the fingers for monitoring finger movements. As shown in Figure 8, when being attached onto three chosen fingers, the sensor is able to detect signals generated by strain-induced piezoresistive effect. Interestingly, it is found that finger movement of different frequencies and amplitudes can be well resolved by the sensor. Figure 8 (a) show clearly the finger movement of different frequencies, either fast or slow, while Figure 8 (b) (b1 and b2) show the sensor’s ability to differentiate the amplitude of the movement, reflecting as particular intensity of the current signals. While Figure 8 (a) present the ability of the sensor to differentiate different frequencies of finger flexing, i.e., the finger being monitored flexes and returns quickly and after certain period of time the movement is repeated, Figure 8 (b) (b3 and b4) show the ability of the sensor to detect the speed of flexing, i.e., the finger being monitored flexes and returns slowly and after certain period of time the movement is repeated. This difference can be understood by the width of the signal peaks shown by the arrows. The signal of quick finger flexing and returning looks much sharper than that when flexed and returned slowly, indicating the sensor’s ability to monitor real-time body movement, which is one of the unique and intriguing characteristics of the sensor proposed in this study. Conclusions To conclude, in this work, a dual-mode sensor derived from high temperature carbonized melamine sponge was introduced and developed as either TENG sensor or piezoresistive sensor for monitoring human activities. In the self-powered TENG sensory mode, the sensor harvests mechanical energy from human movement and converts into detectable open circuit voltage (up to ~2 V) and short circuit current (up to ~70 nA) signals that directly reflects the amplitude and frequency of the movement. On the other hand, in the piezoresistive sensory mode, by incorporating CNT into the sponge, the sensitivity of the sensor greatly improved up to nearly 800%, reflected by the relative change in current or resistance. The cyclic stability of the sensor material has also been highly improved due to the mechanical reinforcement of the sponge fibers and extra conductive network provided by CNT network. As a result, both modes of the sensor show good performance and prove that the materials demonstrated in this work are highly promising for future wearable sensor applications. ASSOCIATED CONTENT Supporting Information. Scanning electron microscopic (SEM) images showing the morphologies of the sponge fabricated at 800oC, 900oC and 1000oC, respectively. Electromechanical sensory tests under different strain values and frequencies of C-Sponge with and without the addition of CNT fabricated at different temperatures. SEM images of cross-sectional images of PDMS/C-Sponge sensors. Optical images showing the compressibility of the C-Sponge material. Demonstration videos, denoted as ‘Finger Demonstration-1’ and ‘Finger Demonstration-2’, showing the electrical signals generated by human


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finger press can also be found accordingly. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors * Email: [email protected], [email protected]. Author Contributions All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. Acknowledgement This research was supported by the NSFC/RGC Joint Research Scheme, under Project N_CityU123/15 and 5151101197, as well as a grant from City University of Hong Kong (PJ 7004645). The authors thank Z. G. Wang and T. F. Hung for experimental support. References 1. Zhang, F.; Zang, Y.; Huang, D.; Di, C.-a.; Zhu, D., Flexible and Self-Powered TemperaturePressure Dual-Parameter Sensors Using Microstructure-Frame-Supported Organic Thermoelectric Materials. Nat. Commun. 2015, 6, 8356. 2. Mannsfeld, S. C.; Tee, B. C.; Stoltenberg, R. M.; Chen, C. V. H.; Barman, S.; Muir, B. V.; Sokolov, A. N.; Reese, C.; Bao, Z., Highly Sensitive Flexible Pressure Sensors with Microstructured Rubber Dielectric Layers. Nat. Mater. 2010, 9 (10), 859-864. 3. Wang, C.; Hwang, D.; Yu, Z.; Takei, K.; Park, J.; Chen, T.; Ma, B.; Javey, A., User-Interactive Electronic Skin for Instantaneous Pressure Visualization. Nat. Mater. 2013, 12 (10), 899-904. 4. Schwartz, G.; Tee, B. C.-K.; Mei, J.; Appleton, A. L.; Kim, D. H.; Wang, H.; Bao, Z., Flexible Polymer Transistors with High Pressure Sensitivity for Application in Electronic Skin and Health Monitoring. Nat. Commun. 2013, 4, 1859. 5. Lee, M.; Chen, C. Y.; Wang, S.; Cha, S. N.; Park, Y. J.; Kim, J. M.; Chou, L. J.; Wang, Z. L., A Hybrid Piezoelectric Structure for Wearable Nanogenerators. Adv. Mater. 2012, 24 (13), 1759-1764. 6. Lee, Y.-H.; Kim, J.-S.; Noh, J.; Lee, I.; Kim, H. J.; Choi, S.; Seo, J.; Jeon, S.; Kim, T.-S.; Lee, J.Y., Wearable Textile Battery Rechargeable by Solar Energy. Nano Lett. 2013, 13 (11), 5753-5761. 7. Huang, Y.; Zhong, M.; Huang, Y.; Zhu, M.; Pei, Z.; Wang, Z.; Xue, Q.; Xie, X.; Zhi, C., A SelfHealable and Highly Stretchable Supercapacitor Based on a Dual Crosslinked Polyelectrolyte. Nat. Commun. 2015, 6, 10310. 8. Fu, Y.; Cai, X.; Wu, H.; Lv, Z.; Hou, S.; Peng, M.; Yu, X.; Zou, D., Fiber Supercapacitors Utilizing Pen Ink for Flexible/Wearable Energy Storage. Adv. Mater. 2012, 24 (42), 5713-5718. 9. Majidi, C., Soft Robotics: A Perspective—Current Trends and Prospects for the Future. Soft Robotics 2014, 1 (1), 5-11. 10. Rus, D.; Tolley, M. T., Design, Fabrication and Control of Soft Robots. Nature 2015, 521 (7553), 467-475.



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Flexible Dual-Mode Tactile Sensor Derived from ... - ACS Publications

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