Highly Sensitive, Durable, and Multifunctional Sensor Inspired by a

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Highly Sensitive, Durable, and Multifunctional Sensor Inspired by Spider Chengzhi Luo, Junji Jia, Youning Gong, Zhongchi Wang, Qiang Fu, and Chunxu Pan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 22 May 2017 Downloaded from http://pubs.acs.org on May 25, 2017

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ACS Applied Materials & Interfaces

Highly Sensitive, Durable, and Multifunctional Sensor Inspired by Spider Chengzhi Luo a,⊥, Junji Jia b,⊥, Youning Gong a, Zhongchi Wang a, Qiang Fu a,c, Chunxu Pan a,c*

a

School of Physics and Technology, and MOE Key Laboratory of Artificial Micro- and

Nano-structures, Wuhan University, Wuhan 430072, China. b

Center for Theoretical Physics, Wuhan University, Wuhan 430072, China.

c

Center for Electron Microscopy, Wuhan University, Wuhan 430072, China.

*Author to whom correspondence should be addressed. E-mail: [email protected] (C. Pan). Tel: +86-27-68752481 ext. 8168. ⊥

These authors contributed equally to this work.

Keywords: sensor; spider inspired; single-walled carbon nanotubes; Au film; human health monitoring

ACS Paragon Plus Environment


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Abstract: Sensitivity, durability, and multifunction are the essential requirements for a highperformance wearable sensor. Here, we report a novel multifunctional sensor with high sensitivity and durability by using a buckled spider silk-like single-walled carbon nanotubes (SSL-SWNTs) film as the conducting network and a crack-shaped Au film as the sensitive transducer. Its high sensitivity is inspired by the crack-shaped structure of the spider’s slit organs, while the high durability is inspired by the mechanical robustness of the spider silk. Similar to the spider’s slit organs that can detect slight vibrations, our sensor also exhibits a high sensitivity especially to tiny strain. The proposed quantum tunneling model is consistent with experimental data. In addition, this sensor also responds sensitively to temperature with the sensitivity of 1.2%/°C. Due to the hierarchical structure like spider silk, this sensor possesses combined superiority of fast response (10000 cycles). We also fabricate a wearable device for monitoring various human physiological signals. It is expect that this high-performance sensor will have wide potential applications in intelligent devices, fatigue detection, body monitoring, and human–machine interfacing.


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Introduction Wearable sensors, devised to capture and monitor various human activities, have risen exponentially to be the next-generation electronics due to their flexible, stretchable, and biocompatible features.1-4 However, challenges restricting their widespread applications still exist. For example, in monitoring human health, the signals to be detected are usually very small, such as pulse, breath, body temperature fluctuation, etc., which requires that the sensor has a high sensitivity in the premise of keeping stable performance.5 In addition, the sensor has to satisfy the versatile capabilities due to the various signals to be detected including strain, pressure, temperature, etc.6 At present, the promising materials for fabricating wearable sensors involve nanotubes,7,8 nanowires,9 nanoflakes,10 and nanoparticles (NPs),11,12 in which NPs have received a wide attention because of their high sensitivity. The working principle of the NPs based sensor is that the change of spacing among the NPs induces the change in electrical resistance during deformation of the substrate. However, when the substrate is stretched to the maximum deformation, the irreversible breakage among NPs greatly reduces their reliability and durability. On the other hand, carbon nanotubes (CNTs), graphene, and metallic nanowires have been frequently employed in the flexible wearable devices, due to their excellent flexibility and electrical properties.13,14 For instance, Z. Niu et al.15 prepared a highly stretchable buckled single-walled carbon nanotube (SWNT) film with the strain tolerance up to 140%, which exhibited ultra-high mechanical stability. However, these materials are insensitive to strain, which restricts their usage as sensors.5 In order to improve the performance of sensor on both sensitivity and flexibility, the NPs are embedded into CNTs or graphene matrix for obtaining a sensor that inherited the merits of NPs and CNTs or graphene. K. Takei et al.16 reported electronic whiskers based on CNTs/AgNPs composite films. The electronic whiskers exhibited high sensitivity (respond to pressures as low as 1 Pa) and super-mechanical reliability after 1000 bending cycles. A. V. Zaretski et al.17 fabricated bi-layer sensors by evaporating metal NPs (Au, Ni, Cu, Ag) on the surface of graphene. The sensitivity and durability of these structures permit measurement of



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the contractions of cardiomyocytes noninvasively. Up to now, the performance of composite films has been greatly improved. However, for the purpose of human health monitoring, the development of a multifunctional sensor satisfying the requirements of high sensitivity, flexibility and durability remains a challenge. As is known, the spider silk, which constitutes the spider web, possesses high strength, elasticity, flexibility, elongation, and fracture resistance.18 The mechanical behavior of the spider silk is determined by the hierarchical structure. Thus, inspired by the hierarchical structure of the natural spider silk, we can directly prepare spider silk-like single-walled carbon nanotubes (SSL-SWNTs) film. This SSL-SWNTs film possesses an ultra-higher mechanical property and a superior electrical conductivity than regular SWNTs. It allows SSL-SWNTs films to be used as conductive materials in sensor with high durability. As for the demand of high-sensitivity, we can draw inspiration from the fact that the spider can sense extremely small variations using crack-shaped slit organs near its leg joints.1,19 The principle for the slit geometry to have ultrasensitive displacement detection is that by allowing mechanical compliance, which results in the deformation of the slit in response to small external force variations. Therefore, we can propose an ideology to design a crack-based sensor to detect multiple physiological signals (for example speech patterns and heart rates) and small external forces.1,7,20 In this paper, we take advantage of the high mechanical and electrical property of the SSL-SWNTs film and the high sensitivity mechanism of the crack-shaped slit organs, and fabricate a composite film sensor that can detect strain and temperature simultaneously with high sensitivity and durability. This sensor is composited of the buckled SSL-SWNTs film as conducting network and ion sputtered Au film as sensitive transducer. The working principles of this multifunctional sensor are that: 1) the strain sensitivity is obtained from the electrical resistance variation during the Au film deformation, which causes the opening/closure of the Au film micro-cracks, 2) the temperature sensitivity is caused by the expansion/shrinkage of the substrate due to the variation of temperature. 3) Furthermore, the buckled SSL-SWNTs film, which forms a stretchable conducting network with enhanced mechanical robustness, makes the composite film stretch 10000 cycles without the significant change of the


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structural integrity. These unique properties of the composite film provide the sensor a great potential in human health monitoring, including pulse, respiration, joint motion, and body temperature fluctuation.

Results and Discussion SSL-SWNTs/Au Composite Film Sensor Figure 1a schematically illustrates the key steps in fabricating the SSL-SWNTs/Au composite film sensor (The experimental details are shown in Supplementary Information). The SSL-SWNTs film, which act as conducting network, was prepared in a magnetic field-assisted floating catalyst chemical vapor deposition (CVD) system.18,21 The SSL-SWNTs film is of ultra-high strain tolerance because of its continuous reticulate structure and tough junction just like the hierarchical structure of spider silk (Figure S1). To further enhance the stretchability, the SSL-SWNTs film was transferred on a prestrained Polydimethylsiloxane (PDMS) film. After the PDMS substrate beneath the SSL-SWNTs film was released, the buckled SSL-SWNTs film was formed on the PDMS substrate (Figure 1b). When an increasing strain was applied to the buckled SSL-SWNTs/PDMS film, the amplitude of the buckled structure became flat until vanished, as shown in Figure 1b-1d. When the strain was released, the flat SSL-SWNTs film returned to the buckled structure (Figure 1d-1f). Therefore, the buckled SSL-SWNTs film plays an important role in the durability of the SSL-SWNTs/Au composite film sensor due to the buckled structure disappeared gradually with increased strain, while the integrity of the SSL-SWNTs film still remained. Subsequently, a layer of Au NPs film, which acted as sensitive transducer, was deposited on the surface of the PDMS supported SSL-SWNTs film by using ion sputtering, and the PDMS supported SSL-SWNTs/Au composite film was obtained. Before initial strain, the Au NPs film remained the integrated and buckled structure (Figure S2). After several strain-release cycles, irreversible fracturing on the entire film created many micro-cracks and islands (Figure 1g). With increasing strain, the numbers and widths of the micro-crack increased (Figure 1g-1i), which caused the electrical current path to become narrower and longer.7,17 As a result, the resistance of the film was increased rapidly under the stretching



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process. When the strain was released, the micro-cracks were closed and the resistance of the film is recovered (Figure 1i-1k). It means the opening/closure of the micro-cracks under mechanical deformation caused the increase/decrease of the electrical resistance.22 It is worth noting that the underlying SSL-SWNTs film was kept an integrated structure during stretching (Figure S3), and the micro-crack propagation through the Au NPs film was suppressed by the stiffness of the underlying SSL-SWNTs film (Figure S4), which guaranteed the structural integrity of the composite film. In order to investigate the strain sensing properties of the SSL-SWNTs/Au composite film, the resistance variation, ∆R/R0 (R0 and R are the resistance before and after stretching, respectively), was measured under different strain loading conditions by maintaining the constant voltage across the sensor device, as shown in Figure 2a. It can be seen that the resistance increased monotonically with tensile strain up to 50% but with a decreasing slope. The slope in Figure 2a reflected a representative parameter to assess sensitivity, the gauge factor (GF), which is defined as g(ε)=(dR/R)/(dL/L), where ε, R and L are strain, resistance, and length, respectively7. Apparently, the GF of the SSL-SWNTs/Au composite film was much higher than that of the pure SSL-SWNTs film (Figure S5a) and slightly lower than that of pure Au NPs film (Figure S5b), which demonstrated that the Au NPs film could significantly improve the sensitivity of the composite film. In addition, when the strain was reduced from 20% to 1%, the GF was increased from 10 to 70, which was 1 order of magnitude higher than the reported data,7,13,23,24 indicating its great potential in detection physiological signals as subtle as heart beat and respiratory. To avoid the breakage of the SSL-SWNTs film, the maximum strain in our experiment was set to 50%, which was still much higher than the 5% limit of conventional metal strain sensors11,25,26 and there was qualified to detect relatively drastic human motions. Owing to the ultra-high mechanical property of the SSL-SWNTs, the SSL-SWNTs/Au composite film exhibited excellent stability and durability, as shown in Figure 2b and Figure 2c. When a step strain of 4% was applied, the ∆R/R0 of the composite film kept almost constant in the platform and sharply increased in the step edge, when cyclic strains of 5% were applied, the output signal was highly reproducible, indicating the high stability of the


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composite film. In our experiment, the fabrication parameters are the same for different batches of sample. To validate the results for different batches of sample, we showed the statistical sensitivities of the 20 sensors at 5% strain in Figure S6. The sensitivities were concentrated on 2.8-3.3. These statistical sensitivities could give a faithful illustration about the controllability of the fabrication steps. In order to examine the mechanical durability, we measured the resistances of the composite film under multiple strain cycles using ~5% strain at 1.5 Hz, which is close to human heartbeat rate, as shown in Figure 2d. It was found that the composite film had no measurable change of the electrical properties up to 10000 strain cycles. Even at ~45% strain, the sensor also exhibited a superb mechanical durability after 10000 strain cycles (Figure S7). The cross-section SEM image of the SSL-SWNTs/Au composite film on PDMS (Figure S8a) revealed that there has no obvious delamination among the SSL-SWNTs, Au and PDMS layers. The fourier transform infrared (FTIR) spectroscopies showed that the FTIR peaks of the SSL-SWNTs/Au composite film on PDMS were nearly unchanged after 10000 strain cycles (Figure S8b). The thermogravimetric analysis (TGA) indicated that the composite film would be decomposed when the temperature was above 400 oC (Figure S8c). These tests proved the structural and thermal stability of the composite film. However, for the Au NPs film without the SSL-SWNTs support, the film would exhibit poor mechanical robustness (Figure S9) because of irreversible degradation of the film conductance caused by cracks and fractures.16 In addition to high sensitivity and durability, the SSL-SWNTs/Au composite film also exhibited a fast response, as shown in Figure 2e. The response time was measured by loading a quasi-transient strain of 1%. With the assistance of the real-time high-resolution response curve, the maximum response time was calculated to be less than 60 ms, i.e., the response frequency has exceeded 16 Hz, which is higher than that of other reported response frequency (~10 Hz).7,16,23 The fast response undoubtedly facilitates the real-time monitoring of human health indicators, such as pulse. Noticing that the SSL-SWNTs/Au composite film is sensitive to a small strain, we designed an experiment to measure the response to air flow. We fixed the composite film at two ends and exposed the rest part to a directional Ar gas flow with controlled velocity. The



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Ar gas was blown through a 0.6 cm diameter tube perpendicularly towards the sensor center that was fixed at 1 cm away from the tube mouth. Figure 2f illustrated the relationship between ∆R/R0 and the velocity of Ar gas flow. It could be seen that the composite film showed sensitive response to the Ar gas flow. The response limitation was as low as 1.2 cm/s (Figure S10a). The measured sensitivity was ~21%/(m/s), which was higher than that of the CNTs/Ag composite film based electronic whisker (13.3%/(m/s)).16 The cyclic on/off Ar gas flow measurements indicated that the composite film also retained high durability (Figure S10b). Taking advantage of the high sensitivity of the SSL-SWNTs/Au composite film to air flow velocity, we can directly monitor the gas exhaled by human, which undoubtedly provides a more effective way in monitoring human respiration than the indirect way by detecting the chest expansion and contraction.7,16,27 Modeling of the Mechanism in Strain Sensing With the above superb sensitivity and durability, and to find ways to further improve them, it is much desirable to understand the physical mechanism that is responsible for them in a more quantitative way. Here we constructed a quantum mechanical yet simple model to explain the observed GF and point out the way for further improvement. As shown in Figure 3a, the conducting part of sensor is modeled as a two-layer structure, with the lower layer made of SSL-SWNTs and upper layer made of Au film. The SSL-SWNTs layer is expected to have a constant resistance RS during stretching, as shown in Figure S5a. This is due to the fact that the buckled structures were just flattened under strain and the charge carriers transport mainly along the longitudinal direction of the SWNTs in this layer. The only one function of the SSL-SWNTs is to improve the mechanical durability. On the contrary, the resistance of Au layer shall increase very rapidly as strain is applied. It is generally known that the resistance of a thicker (yet small) Au film will scale linearly with its strain like (1 + ε ) 2

when ε is much smaller than 1.28,29 However, this relation is based on the

assumption that the Au film is continuously deformed without any cracks and its width and thickness become smaller while its volume is unchanged. In our case, it is obvious that the resistance of the crack part of the Au film shall increase much rapidly than (1 + ε ) 2 .


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Quantum mechanics has told us that for metal film with cracks, the conducting of current is mainly through the electron tunneling mechanism, which suggest that the crack resistance shall increase exponentially with the crack width,7,26 or equivalently with ( L − L0 ) . In the following part, we will show that a model based on this observation is indeed very adequate to explain the total resistance of the sensor. We first approximate the resistance of the entire Au film with crack, R A u , as the sum of constant resistance of the continuous part, R Au 0 ,

and that of the cracks,

R t [exp (α ( L − L 0 )) − 1] ,

which is zero when no crack develops and

increase exponentially as increases. Here Rt is the factor that measures the proportionality between the resistance and the exponential factor. Rt is expected to be inverse proportional to the cross-section area of the Au cracks, but independent of the elongation. The parameter α shall be proportional to

Φ

, i.e., α = c Φ , where Φ is the work function of the

metal Au and controls the tunneling process. The total resistance of the Au layer in Figure 3a is therefore given by

R Au = R Au 0 + Rt ( eα ( L − L0 ) − 1)

(1)

In general, there should also have a contact resistance R c between the SSL-SWNTs and Au layer. Since the electrodes during resistance measurement were directly contact with the Au film, this R c is serially connected with R S to become just one parameter

RS + c .

An

effective circuit is illustrated in Figure 3b. Finally the total resistance of the two-layer structure is given by

R=

1 1 1 + RS + c RAu

=

1 1 1 + RS + c RAu 0 + Rt (eα ( L − L0 ) − 1)

Changing variables from

( L − L0 )

(2)

to ε L 0 and R to ∆R / R = ( R − R0 ) / R , the function (2)

become



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1 1 + R R ∆R S +c Au 0 = −1 1 1 R + RS + c RAu 0 + Rt (eαε L0 − 1)

(3)

This function can be directly fitted to our experimental data to a surprisingly high accuracy, as shown by the black curve of Figure 2a. In drawing this, Rt = 3.36×104 Ω, α =7.34 / cm , RS +c = 5.16×104 Ω and RAu 0 = 6.94 ×103 Ω were used. The good agreement between model and experimental data allows us to gauge the resistance to absolute strain. To further reveal the importance of the work function, we expand function (3) to the small ε case by using the relation

RS + c = L0 / (σ S + c AS ) and RAu = L0 / (σAu0 AAu ) , where

σ

is the (effective)

conductivity of the corresponding material and A is the cross-section area, to find

g(ε ) =

In above, the

Rt

AAuσ Au Rtα 1 1 AS σ S ( + ) AS σ S AAuσ Au

(4)

can be approximated as Rt = ∆ t / (σ Au A Auη ) where ∆t might be related

to the thickness of the thin layer of Au on two sides of the crack between which the tunneling happens, and η is the factor between the crack cross-section area and that of the Au film. Moreover, since in our case

R S + c >> R Au 0 ,

the above function can be further simplified to

obtain

g(ε ) ≈

∆tc Φ

(5)

η

Clearly, this simple relation suggests that in order to improve the GF of the sensor, the easiest way is to use coating materials with larger work function. At last, we point out that if the coating film had a larger resistance (for example by using a metal with smaller conductivity or coating thickness) comparing to the SWNT film, then one can show that the GF will never exceed the value given by (5).


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Application in Human Health Monitoring In order to demonstrate the potential applications of the SSL-SWNTs/Au composite film in human health monitoring, we fabricated a wearable sensor by connecting copper wires to the films to form the conducting pathway, as shown in Figure 4a. The wearable sensors could be attached conformably to different positions of human body by using medical tapes, and thus the various physiological signals could be obtained. For example, when it was fixed to the wrist, the human pulse pressure wave in the radial artery was monitored, as shown in Figure 4b and Figure 4c. Wrist pulse is a key physiological signal for determining arterial blood pressure and heart rate. The sensor clearly resolved the systole, diastole, dicrotic notch, and the amplitude and frequency of pulse could be read out readily in real time. The height and frequency of the peaks represented the pulse pressure and heart rate, respectively. By comparing with the pulse monitored before and after the exercise, it could be found that the pulse pressure and heart rate were significantly increased after exercise. When it was attached to upper lip, the sensor could monitor respiration by detecting the exhaled gas flow, as shown in Figure 4d and Figure 4e. The height and frequency of the peaks represented the respiration depth and rate, respectively. After exercise, the height and frequency of the peaks increased due to breathlessness situation. Furthermore, the present sensor was directly exposed in the exhaled gas, and thus the relative resistance variation also reflected the exhaled gas velocity. This sensor could be exploited as early warning systems for sudden infant death syndrome and sleep apnea in adults.30 As we all know that the joint motion is also vital to human health, and the flexible joints are the basic premise to ensure the smooth completion of various actions. When we attached our sensors on the human joints, the joint motion were also detected clearly and sensitively. In this work, we took the finger joints as an example to clarify the SSL-SWNTs/Au composite film sensor in detecting the joint motions, as shown in Figure 4f. During the bending motion of the finger, the ∆R/R0 of the sensor increased and each bending state corresponded to a curve platform. Therefore, according to these curves, we can judge a person’s maximum joint bending degree according to the different position of curve platform. And we can also record the shaking of the finger joint during bending from the vibration



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amplitude of the curve platform. These characters provide us a possibility to the early diagnosis of Parkinson’s disease.31 It was interested to note that the present SSL-SWNTs/Au composite film sensor also showed a functionality to monitor body temperature as a temperature sensor. Figure 5 illustrates the sensor’s responses to temperature fluctuations. Figure 5a and 5b showed the ∆R/R0 at 5-43°C, where R0 is the resistance at 37 °C (normal body temperature). It revealed that the ∆R/R0 exhibited a linear relationship with temperature, and the sensitivity extracted by the linear fit was 1.2%/°C, which is better than that of pure SWNTs film (0.4%/°C，Figure S11a) and the other temperature sensors (such as PEDOT:PSS/CNTs) fabricated on flexible substrates (~0.6%/°C).32,33,34 The cyclic tests of the temperature difference around body temperature (37-39 °C) indicated that this sensor could be performed consistently during different temperature cyclic tests, as shown in Figure 5c, which demonstrated its high reliability in response to temperature fluctuation. Obviously, this sensor could be used to diagnose the abnormal temperature of human body, such as the high fever. Our actual experiments revealed that this sensor also possessed an advantage to measure tiny temperature variations, even detect the variation of health person’s body temperature fluctuation (around 37 °C) at different times of a day, as shown in Figure 5d. The mechanism of the temperature sensor was proposed based upon the expansion/shrinkage of the PDMS substrate. The thermal expansion coefficient of PDMS is 0.03%/°C, which means that the expansion of PDMS film will lead to a change of resistance in 2.1%/°C in considering that the strain sensitivity of the sensor in small deformation is 70. It should be noted that the ∆R/R0 of the pure buckled SSL-SWNTs film on PDMS substrate was decreased with increasing of temperature (Figure S11a). In this case, the influence of the expansion of PDMS film on ∆R/R0 could be ignored because of the low strain sensitivity of buckled SSL-SWNTs film, as shown in Figure S5a. The decreased ∆R/R0 with the increasing of temperature was due to the electron hopping at the interface.32,33,34 In the actual situation, we should also take the temperature sensitivity without thermal expansion into consideration. We conducted the control experiment by using Si as substrates. The ∆R/R0 versus temperature for SSL-SWNTs/Au composite film on Si substrate was showed in Figure S11b. In this


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experiment, the thermal expansion of Si substrate could be ignored. It was revealed that the ∆R/R0 exhibited a linear relationship with temperature, and the sensitivity extracted by the linear fit was -0.66%/°C. The ∆R/R0 in this experiment was decreased with the increasing of temperature because of the temperature coefficient of Au and SSL-SWNTS and electron hopping at the interface. This decreased ∆R/R0 has been reported by many other researchers. 32,33,34

That is to say, the sensitivity of -0.6%/°C is always existed whether there is thermal

expansion.

Therefore,

the

real

sensitivity

can

be

roughly

calculated

as

2.1%/°C+(-0.66%/°C)=1.44%/°C. The measured temperature sensitivity (1.2%/°C) was closed to the calculated temperature sensitivity (1.44%/°C) at the order of magnitude, which indicates the correctness of the mechanism. Although our senor is sensitive to both strain and temperature, it only have one kind of output signal, ∆R/R0. Therefore, our sensor is mainly used to detect single signal. When detecting multiple signals, our sensor can distinguish some certain multiple signals, while the complex multiple signals are not easy to be resolved from the line-shape of ∆R/R0 curve. Nevertheless, we still take a big step when compared to other sensors. For example, the sensor was exposed to pulsed airflow at different temperature, and the obtained response was shown in Figure S12. It can be seem that the different position of curve platforms represented the change of temperature, while the peaks represented the pulsed airflow. The sensor can also clearly resolve the multiple signal of pulse pressure, such as systole, diastole, dicrotic notch (Figure 4c). Of course, there is still a long way for the practical application. In the future, we expect that this challenge can be overcome by introducing a waveform analysis system or changing the form of output signal.

Conclusions In summary, we reported a wearable sensor by using a buckled SSL-SWNTs film as conducting network and crack-shaped Au NPs film as sensitive transducer. Its high sensitivity and durability are obtained by taking inspiration from the crack-shaped structure of the spider’s slit organs and the hierarchical structure of the spider silk, respectively. When stretched, the Au-NPs film developed micro-cracks that made the resistance of the composite film increased exponentially, while the buckled SSL-SWNTs film with mechanical robustness kept the composite film integrated. A quantum tunneling model was built and the



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experimental data on the resistance was well explained. Using this model, we also point out that the large gauge factor at small strain is proportional to the square root of the work function of the conducting metal film. Therefore, the sensor shows combined superiority of high sensitivity, fast response, and high durability. In addition to strain sensor, the sensor also showed fast and sensitive response to temperature fluctuation as a temperature sensor. This wearable sensor can monitor various human health indicators, including pulse, respiration, joint motion, and body temperature fluctuation. The ultrahigh sensitivity and multifunction of the demonstrated sensors may enable a wide range of applications in intelligent devices, fatigue detection, body monitoring, and human–machine interfacing.

Methods section Preparation of Au/ SSL-SWNT/PDMS Composite Films The SSL-SWNTs were prepared in a self-made magnetic field-assisted floating catalyst chemical vapor deposition (FCCVD) system. Different from the normal FCCVD method, our preparation procedure include three steps, i.e. synthesis of SWNTs on Al foil by using normal FCCVD; magnetic field induced deposition of Fe particles on the surface of SWNTs; coating an amorphous carbon layer on the surface of Fe particles and SWNTs. The experimental details are shown in Supporting Information. The thickness of the SSL-SWNT film is about 100 nm. Then, SSL-SWNT film was removed from the Al foil by etching in a 1 M HCl aqueous solution. After the Al foil was dissolved, the SSL-SWNT film would be floated on the HCl solution. Then, a PDMS substrate with ~1 mm thick was fixed on a glass slide. Then a force was applied to the other end of the PDMS along its length to stretch it. When the PDMS was stretched to the expected strain, the end was fixed and the prestrained PDMS was obtained. The prestrained PDMS was brought into contact with the floated SSL-SWNT film and it was “pulled” from the solution. Finally, the prestrained PDMS substrate beneath the


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SSL-SWNT film was released and buckled SSL-SWNT film was formed on the PDMS substrate. The Au nanofilms were deposited upon the buckled SSL-SWNT/PDMS film via ion sputtering (SBC-12, KYKY, China). The distance between the sample and the target was 5 cm. The sputtering current was controlled in 6 mA. In order to obtain different Au nanofilms depositions, the sputtering times were adjusted in a range of 60 - 300 seconds. In our experiment, the thickness of the Au film is about 200 nm. For comparision, the buckled SSL-SWNT/PDMS film and Au/PDMS film have also prepared in the same condition.

Fabrication of Wearable Sensors The obtained Au/SSL-SWNTs/PDMS composite films were tailored with the width of 10 mm and an overall length of 30 mm. Copper wires were connected using silver paste to form the conducting pathway. PDMS glue and conductive adhesive was used for covering the electrodes to protect them from exfoliating and then the wearable sensors were fabricated. Lastly, the sensors were glued to the target place of human body.

Characterizations

The morphologies and microstructures of the samples were characterized using scanning electron microscopy (SEM) (S-4800, HITACHI, Japan) and high resolution transmission electron microscopy (HRTEM) (JEM 2010FEFHRTEM, JEOL, Japan). For the HRTEM observations, the SSL-SWNTs were directly deposited upon the ϕ3 mm TEM Cu grids. To obtain the resistance variation, a constant voltage (2 V) was loaded on the sensors to acquire a real-time current signal, using an electrochemical workstation (CHI 660D, Chenhua, China).



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ASSOCIATED CONTENT

Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. Experimental details; SEM and HRTEM microstructures of the SSL-SWNTs; SEM morphologies of the SSL-SWNTs/Au composite film before initial stretch; SEM images of the integrated SSL-SWNT film that is underlying the crack of Au nanofilm; SEM images show the crack propagation through the Au nanofilm is suppressed by the stiffness of the underlying SWNTs; Relative resistance variation versus strain for pure buckled SSL-SWNT film and Au film; Statistical relative resistance variations of the 20 SSL-SWNTs/Au composite films at 5 % strain; Relative resistance variation of the SSL-SWNTs/Au composite film at 45 % strain as a function of strain cycle; The structural stability characterization of the SSL-SWNTs/Au composite film on PDMS; Relative resistance variation of the Au film at 5 % strain as a function of strain cycle; Gas flow sensing properties of the SSL-SWNTs/Au composite films; Relative resistance variation versus temperature for pure buckled SSL-SWNT film on PDMS substrate and SSL-SWNTs/Au composite film on Si substrate; Relative resistance variation of SSL-SWNTs/Au composite film for the detection of pulsed airflow at different temperature. AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected], Tel: +86-27-68752481 ext. 8168

ORCID


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Chengzhi Luo: 0000-0001-8806-1139 Youning Gong: 0000-0002-8114-8095 Zhongchi Wang: 0000-0001-8527-6353 Chunxu Pan: 0000-0001-9007-8562

Author Contributions C.L. and C.P. conceived and designed the project. C.L., Y.G., and Z.W. optimized and fabricated the sensors. J.J. provided theoretical calculation. C.L. and Q.F. tested the sensors. C.L., J.J., and C.P. wrote the paper. All authors contributed to discussions of the results. All authors reviewed the manuscript. C.L. and J.J. contributed equally to this work.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work is supported by the National Nature Science Foundation of China (Nos. 11174227, 11504276 and 11547310), Specialized Research Fund for the Doctoral Program (No. 20130141120079), Ministry of Science and Technology of the People’s Republic of China (No. 2014GB109004), and National Science Foundation of Hubei Province, China (No. ZRY2014000988). REFERENCES (1) Kang, D.; Pikhitsa, P. V.; Choi, Y. W.; Lee, C.; Shin, S. S.; Piao, L.; Park, B.; Suh, K. Y.; Kim, T. I.; Choi, M. Ultrasensitive Mechanical Crack-Based Sensor Inspired by the Spider Sensory System. Nature 2014, 516, 222-226.



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Figures

Figure 1. SSL-SWNTs/Au composite film. (a) Key steps in fabricating the SSL-SWNTs/Au composite film. (b-f) SEM morphologies of the buckled SSL-SWNT film on strain and release. (g-k) SEM morphologies of the SSL-SWNTs/Au composite film on strain and release.


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Figure 2. Strain sensing properties of the SSL-SWNTs/Au composite film. (a) Relative resistance variation and gauge factor versus strain from 0 % to 50%. (b) Relative resistance variation under step strain from 0 % to 20 % strain. (c) Multicycle tests of relative resistance variation upon stretching to 5 % strain. (d) Relative resistance variation at 5 % strain as a function of strain cycle. Inset shows the relative resistance variation characteristics after 10 and 10000 cycles. (e) Response time of the sensor to a quasi-transient input step strain of 1 %. (f) The relative resistance variation as a function of gas flow velocity.



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Figure 3. Modeling of the mechanism in strain sensing. (a) The schematic illustration of the structure of SSL-SWNTs/Au composite film sensor under strain. (b) Model of the basic circuit of the composite film sensor.


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Figure 4. Applications as wearable strain sensor for human-health monitoring. (a) Photograph of a wearable sensor. (b) Wearable sensor attached to the wrist for detection of the pulse. (c) Responsive curves of wearable sensor on the wrist before and after exercise. Note the high resolution of the pulse pressure-waveform (in the blow-out) with distinguishable systolic and diastolic pressures, the dicrotic notch (aortic valve closure), and other cardiac cycle events. (d) Wearable sensor attached to the upper lip for detection of the respiration. (e) Responsive curves of wearable sensor on the upper lip under normal breath and breathlessness. (f) Response signal of wearable sensor in monitoring finger bending. Inset: Photographs of finger-bending to the corresponding positions.



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Figure 5. Temperature sensing properties of the SSL-SWNTs/Au composite film sensor and its applications as wearable sensor for human body-temperature monitoring. (a) Relative resistance variation versus temperature. (b) Relative resistance variation under step temperature from 5 to 43 oC. (c) Repeated cycle test of temperature between 37 and 39 oC. (d) Human body-temperature monitoring from 6 am to 10 pm.


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Highly Sensitive, Durable, and Multifunctional Sensor Inspired by a

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