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A breath sensor using carbon nanotubes operated by field effects of polarization and ionization
Sensors and Actuators A 158 (2010) 328–334
Contents lists available at ScienceDirect
Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
A breath sensor using carbon nanotubes operated by field effects of polarization and ionization Xiaohang Chen, Yanyan Wang, Yuhua Wang, Zhongyu Hou, Dong Xu, Zhi Yang, Yafei Zhang ∗ National Key Laboratory of Nano/Micro Fabrication Technology, Key Laboratory for Thin Film and Microfabrication of the Ministry of Education, Institute of Micro/Nano Science and Technology, Shanghai Jiao Tong University, Shanghai 200240, PR China
a r t i c l e
i n f o
Article history: Received 5 July 2009 Received in revised form 19 November 2009 Accepted 11 January 2010 Available online 28 January 2010 Keywords: Microfabrication breath sensor Multi-walled carbon nanotubes Exhaled breath Response Sensitivity
a b s t r a c t A novel microfabricated breath sensor (MBS) based on multi-walled carbon nanotubes (MWNTs) has been presented and tested. It has a simple structure of two nickel beams incorporating with MWNTs. The responses of the MBS to the behavior of the breath dynamic characteristics are consistent with the exhalation pulse of the human-volunteers, e.g. the exhale flow strength and frequency. It has a rapid response and high sensitivity in detecting feeble breath, and no recovery issues such as the adsorption-type sensors are detected. Furthermore, strong anti-interference ability to external air flow and temperature shift is observed. These unique results ensure this MBS can be successfully employed to real-time monitoring of human breath. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The breath of human, a basic characteristic of life carrying various information, is significant to the comprehensive analysis of lung function [1,2], asthma detection [3], diabetes mellitus diagnosis [4], etc. Meanwhile, health condition can also be reflected by the physical characteristics of the exhaled breath, such as breath frequency and tidal volume. The breath measurement has been accomplished by different approaches, such as the pressure sensors [5], which examine the pressure change induced by the exhaled gas flow. A tube connecting mouth or nose and the windtight device are necessary while operation of such sensor; otherwise, its pressuresensitive element may be disturbed by the ambient flow. The shape modification of chest or abdomen will also reflect the breath condition indirectly via pressure sensor; however, it will cause the discomfort for tying on the body. Another type of breath sensor takes advantage of the temperature shift near the nose during the breath. However, the stochastic nature of ambient temperature may deteriorate its signal quality significantly. Besides these commercial breath sensors, short time response humidity sensors [6–10] focus an increasing interest in breath diagnosis applications. However, response cycle of subsecond is difficult to accomplish. There is another breath sensor [11] based on the fact that the
∗ Corresponding author. Tel.: +86 21 3420 5665; fax: +86 21 3420 5665. E-mail address: [email protected] (Y. Zhang). 0924-4247/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2010.01.021
velocity of sound is directly modulated by air flow. However, the anti-interference ability to ambient noise is a weakness. Due to the unique physical properties [12,13] and potential in diverse range of applications [14–18], carbon nanotubes have attracted considerable attentions. Its high aspect ratio, small tip ratios and high electrical conductivity result in strong field enhancement effect [19,20]. There is another microfabricated breath sensor (MBS) based on multi-walled carbon nanotubes (MWNTs) reported by our group, which takes advantage of the tip effect of MWNTs [21]. In this paper herein; however, microfabrication processes are improved to deal with its uncontrollability of MWNTs distribution and unsatisfied bonding of MWNTs film on microelectrodes caused by screen printing. The focus of the MBS is on the performance in monitoring human breath under alternative current (AC). The information that MBS can provide is the frequency and strength of the exhaled breath. It has a considerable high quality of response, sensitivity and recovery performance, with excellent ability of anti-interference to ambient air flow and temperature shift. 2. Experiment 2.1. Sensor fabrication process Traditional “lift-off” and novel electrophoresis processes were employed to generate the microstructure. Firstly, the LOR 3A (not photosensitive) and AZ3612 (photosensitive) were spun on the
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were electroplated on the Cr/Cu seed layer, which thickened the height of the electrodes to be 4–6 m, in order to increase the effective current generation areas of the electrodes, as shown in Fig. 1d. Finally, The Cr/Cu/Ni electrodes were coated by a film of MWNTs (>95% purity) using an electrophoresis process. It was accomplished in acetone with a MWNTs content of 5 mg/L, at a constant electric field of 10 V/cm, for 5 min. Mg(NO3 )2 was used as an additive (6 mg/L) and the MWNTs were homogeneously dispersed in acetone using ultrasonic machine at power of 50 W for 30 min before electrophoresis. Additionally, another much thinner nickel film was electroplated on the MWNTs for better bonding and consistence of MWNT–film on the nickel surface, as shown in Fig. 1e. After washing with deionized water and drying, the device was completed, as schematically illustrated in Fig. 1f.
2.2. Measurement set-up Fig. 1. Schematics of the fabrication processes: (a) spinning of photoresist; (b)lithograph and etching of photoresist; (c) deposition of Cr/Cu seed layer; (d) “liftoff” of resist; (e) electroplate of Ni beams; (f) electrophoresis of MWNTs film and electroplate of Ni coat; (g) schematic of MBS.
wafer in order. After spinning and baking, the AZ3612 was exposed and the developer (AZ400) was used to clear the exposed AZ3612 areas, but also etch away the LOR 3A. Since the LOR 3A had a higher dissolution rate in developer than the AZ3612, the resulting profile would have an overhang of photoresist, which prevented the sidewall deposition of film, as illustrated in Fig. 1a. Then, Cr/Cu seed layer (10 nm/140 nm) was deposited on the patterned resist, as shown in Fig. 1b. After deposition of Cr/Cu film, the resist was dissolved in dimethylsulfoxide and the patterns of microelectrodes were obtained, as shown in Fig. 1c. Subsequently, nickel beams
A platform of detecting MBS performance in monitoring human breath was presented, As shown in Fig. 2. One electrode of sensor was connected to the signal generator (NF 1946B Function Generator), which supplied an AC signal source. Another was linked to a lock-in amplifier, for signal amplification and amplitude demodulation. The MBS was fixed at a controlled distance (2 cm) to the exit of a microbridge mass flowmeter (MFM, AWM 5000, Honeywell) which was widely applied in the field of life-support machine. This MFM was introduced here to indicate the flow rate of various gases under test. During the test, the operating voltage was approximate (∼) 1 V and the frequency was chosen ranging from 1 to 10 kHz. All signals could be translated into voltage signals and recorded by a digital oscilloscope. The mathematical conversion equation of the voltage (Vo ) recorded by the oscilloscope and the response current (Io ) of MBS was: Vo = 4Io (V/A). The mathematical conversion
Fig. 2. The test platform: (a) the practical experimental configuration; (b) diagram of MBS and MFM; (c) the schematic of the platform (the distance between the sensitive section of MFM and MBS under test is ∼10 cm).
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Fig. 3. Surface characteristics of MBS: (a) the whole picture of the MBS; (b) the top view image of the electrode gap; (c) morphology of MWNTs layer on the sidewall; (c) schematic of MWNT-tips on the sidewall.
equation of the MFM output voltage (VMFM ) and its indicated flow rate (WD ) was: WD = 2.5(VMFM − 1) (slpm/V). The basic sensing responses to spontaneous breath had been measured through output voltage in atmospheric air at room temperature and 45–90% relative humidity. The experiments were carried out in the decontamination chamber in which the temperature and humidity could be controlled conveniently and exactly. It had been examined by more than 10 volunteers for the reliability. Unusual human exhaled breath was tested in order to observe performance of the sensor under the condition of varying breath frequency and strength. Buffered compressed air had been introduced through a tube to check the influence of ambient air flow. The device was placed in an oven to examine its performance under different temperatures ranging from 30 to 60 ◦ C. Volunteers’ breath flow rates were recorded by MFM consistently during all tests. 3. Result and discussion
tion of sidewall area. From the SEM images, L, W and N can be statistically calculated. As d is kept as a constant, the increment proportion of sidewalls area can be estimated as ∼40%. 3.2. Basic performance The current output modification of this MBS reflecting the spontaneous breath was collected by the lock-in amplifier. After processing, the signals were plotted by an oscilloscope to indicate the characteristics of the breath, including frequency, duration, and amplitude. Typical responses of the MFM and the MBS as well as the bald metal electrode structure (with the same size of MBS) are demonstrated in Fig. 4. There are few detectable responses of the metallic device without MWNTs. A signal delay of the MBS following that of MFM is observed (∼0.17 s for beginning of expiration and ∼1 s for end), which may be ascribed to the distance between MBS and the sensitive section of MFM.
3.1. Structure characteristics Fig. 3a–c shows a SEM image of the MBS, the top view images of the gap (∼8 m) between electrodes and morphology of MWNTs layer on the sidewall respectively. As shown in the images, nickel electrodes are coated by a film of MWNTs with diameters of about 30–50 nm, and tilted tips of MWNTs can be observed, especially at the sidewall of electrodes. As schematically shown in Fig. 3d, the equivalent area of this capacitor structure can be calculated as follows: S d × L × N = S h×W
(1)
where S and S denote the increment of relative area and initial relative area without MWNTs respectively. And d, L, W, N, h is the mean diameter of MWNTs, the mean length of titled tips, the length of electrode, the number of titled MWNTs in electrode length of W, and the thickness of electrodes, respectively. More than 50 images were captured by SEM in order to estimate the increment propor-
Fig. 4. The curves indicating responses of three devices to spontaneous breath: MBS (baseline 0 V); MFM (baseline 1 V); and metal electrodes device (baseline arbitrary).
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Fig. 5. The curves indicating MBS responses to unnatural breaths of different and changing frequency and strength. Experimental results of four volunteers are demonstrated.
To investigate the ability of monitoring breath of high frequency and low strength, unnatural breath was tested, as indicated in Fig. 5a–d. The former two volunteers breathed with a constant frequency and strength, and the latter two volunteers breathed with an abruptly changing frequency and strength. The curves indicate stable operation of MBS in monitoring unnatural breath. This MBS can distinguish exhaled breath of a high frequency (100 times per minute), which is much higher than normal breath frequency (16–20 times per minute). Further, the delay time (t) of the exhaled gas acts on MBS and MFM can be denoted as Eq. (2): t =
D×A F
(2)
where D stands for the distance between the sensitive section of MFM and MBS, A for the cross-sectional area of MFM flow path, and F for the flow rate recorded by the MFM. When D is equal to 10 cm, the delay time (t) of the exhaled breath act on MBS and MFM can be calculated as ∼0.5 s. As indicated in the curves in Fig. 5, the response delay time (T) of MBS to MFM is averagely ∼0.15 s (T < t), indicating higher response speed of MBS than MFM. Meanwhile, exhalant air flow of 0.5 slpm (1.2 V voltage of MFM) is clearly reflected in Fig. 5, exhibiting unique sensitivity in detecting feeble breath.
from 35 to 60 ◦ C gradually. It is shown in Fig. 7a that the response signal amplitudes of the sensor to the breath in various temperatures are distributed in a very narrow range (3.3–4.3 V), which shows its thermal stability. Additionally, the higher temperatures enhance the removal of the exhaled flow and then shorten the recovery duration, indicating better performance in recovery process, as shown in Fig. 7b. In the tests, we also found that it could work steadily with various sampling distance of the spontaneous breath, ranging from 10 to 20 cm. 3.4. Modeling and theoretical analysis The differences in the performances of MBS and the bald metal electrode structure are inconsistent with the absorption or other related mechanism [22–24]. Furthermore, the increment propor-
3.3. Environmental influence In order to measure the ambient influence to MBS when monitoring breath, ambient air flow (buffered compressed air) was induced to MBS and the responses were recorded. As typically illustrated in Fig. 6, the responses of MBS are much less detectable than that of exhaled gas even the flow rate is much higher than spontaneous breath. It indicates its favorable anti-interference ability of surrounding flow. To examine the impact of temperature, tests of MBS at different temperatures are carried out and the outputs are collected. The ambient temperature was controlled by the oven, which varied
Fig. 6. Response spectra of MBS and MFM to the air flow pulse: MBS (baseline 0 V) and MFM (baseline 0 V).
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Fig. 7. The curve of MBS responses under different temperatures: (a) the response peak voltage distribution under temperature ranging from 30 to 60 ◦ C; (b) comparison of the MBS response cycle under 30, 45 and 60 ◦ C.
tion of sidewalls areas between the MBS and bald nickel beams can be estimated as ∼40% (Fig. 3), much smaller than the signal increment proportion (more than 10 as shown in Fig. 4). The remarkable performance of the MBS cannot be attributed absolutely to the increase of capacitance. A new model is proposed by us herein to explain the performance of MBS, which is detailed in [25]. As shown in Fig. 8a, the response process of MBS to exhaled breath can be divided into four stages. In stage I, the current is resulted from ionization current of initial environment (Ib ), which is composed of ambient atmosphere. In stage II, as exhaled gas is introduced to MBS, the background air is chemically changed because of numerous atomized water (∼6.2% fractional concentration) [26,27] and charged particles contained in breath gas [28,29]. Charges induced by nonuniform polarization of droplets result in polarization current (Jpd ) while droplets drifting across the gaps. Jpd is much greater than base cur-
Fig. 8. Schematic of proposed mechanism: (a) the response curve of MBS to exhaled breath according to the proposed model; (b) three stages of the actual response and recovery circle. The response voltages of MBS (baseline 0 V) and MFM (baseline 0 V) are denoted by left and right vertical axes, respectively; (c) schematic of MBS under human breath.
rent (Ib ) in atmosphere. Here, the sensitivity of MBS can be defined as:
∂Jpd ∂Wd
=−
q VE
E +
∂E Wd ∂Wd
៝ y, z)dV ε0 ∇ E(x,
(3)
VE
where Wd is the flow rate of exhaled breath, q is the electrical quantity of elementary charge, VE is the volume of current generation space, E is the averaged electric susceptibility of the medium in VE , ε0 is the dielectric constant in vacuum, E is the electric field vector, and dV is the differential unit of VE . Especially, E is related by inverse proportion of background air and the liquid droplets. Under the external AC source, electric field between the electrodes is greatly enhanced by the strong field convergence of the MWNTs tips [19]. As a result, the sensitivity of MBS to exhaled breath is greatly improved. When Wd is kept as a constant, the Jpd is only related to the exhaled flow composition, indicating stable response for every certain volunteer, as shown in Fig. 8a. When the exhaled breath is introduced with a fixed distance between exhaled breath and MBS, the curve of response current to time is analogical to the breath flow rate. Furthermore, as shown in Fig. 4, the third peak of MBS is huger than the former ones, which is caused by the reduction of distance between MBS and exhaled breath source. While the distance is shortened and the exhaled breath flow rate (WD ) is kept as a constant, the output voltage of MFM maintains unchanged, but less liquid droplets in breath gas will be lose during the diffusion process from exhaled breath source to MBS, leading to a higher proportion of liquid droplets in VE . As a result, the sensitivity of MBS is increased (as shown in Fig. 4). In this proposed model, the proportion of the liquid droplets contained in the exhaled gas is the key factor which affects the sensitivity of MBS according to the Eq. (3). When exhaled breath is introduced to MBS, the value of E is greatly increased as the background air is chemically changed by the exhaled gas. When ambient air flow pulse is introduced to MBS, the background gas remains unchanged, leading to low sensitivity of MBS. This is the reason why MBS is relatively insensitive to the ambient air. In stage III, the residual atomized droplets will be vaporized when the exhalation stops, which leads to sustaining decrease of response current. In this stage, current is composed of ionization current (Ii ) and another polarization current (Idp ) caused by the dielectrophoresis force to droplets. Taking Dl to denote the mean particle of liquid droplets in VE , Nl to denote the equivalent number of the droplets of Dl in diameter, and Wdp to denote the droplet drift
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velocity due to dielectrophoresis forces, Jdp can be defined as:
Jdp (t) = K · Nl · VE
៝ Dl3 · Wdp · ∇ · EdV VE2
(4)
where K is only related to the electric susceptibility of liquid atomized droplets in breath gas. For the same volunteer, K and Dl are considered as a constant. The Idp is in direct proportion to Nl under steady electric field and decreases with vaporization of residual droplets. When the temperature increases, the evaporation rate of the liquid droplets in VE increases as a result of the more strenuous diffusion motion, leading to faster decrease of the equivalent number of the liquid droplets (Nl ). As demonstrated in Fig. 7b, higher temperature indicates higher speed decrease of response current in stage III and exhibits faster recovery. The elapsed time of vaporization is much longer than that of exhaled breath introduction, which is indicating that the recovery process is slower than the response process. In stage IV, when the environmental gas is restored, the current magnitude will return back to the initial value as shown in Fig. 8a. The curve of MBS response to exhaled breath is shown in Fig. 8a, according to this proposed model. In stage II, the response current is a constant while the exhaled flow rate (Wd ) is assumed as a constant in the model. Actually, Wd is always varying and there is a peak value during one breath, leading to a corresponding peak value of response current of MBS, as shown in Fig. 8b. After careful verification, this proposed model is validated to match the experimental results well. 4. Conclusion A novel MBS incorporating MWNTs has been fabricated and tested. The sensor has a simple capacitor structure, including a pair of metal electrodes, coated by a layer of MWNTs and followed by a nickel protection layer. The MWNT–film greatly improves the performance of the sensor in monitoring the exhaled breath. Meanwhile, this MBS can operate steadily with ambient interference of ambient air flow and temperature. Finally, the performances of response and recovery time are plausible, implying a congruous response to the characteristics of breath dynamics. The experimental results in this paper ensure that this MBS can be successfully employed to monitor human breath. This attractive feature will be exclusively examined in our future research. Acknowledgements This work is supported by Hi-Tech Research and Development Program of China No. 2007AA03Z328, Shanghai Natural Science Foundation (No. 09ZR1415000), National Natural Science Foundation of China (Nos. 60871032, 60906053 and 50902092). References [1] D.T.V. Anh, W. Olthuis, P. Bergveld, A hydrogen peroxide sensor for exhaled breath measurement, Sens. Actuators B: Chem. 111–112 (2005) 494–499. [2] C. Di Natale, A. Macagnano, E. Martinelli, R. Paolesse, G. D’Arcangelo, C. Roscioni, A. Finazzi Agrò, A. D’Amico, Lung cancer identification by the analysis of breath by means of an array of non-selective gas sensors, Biosens. Bioelectron. 18 (2003) 1209–1218. [3] M. Fleischer, E. Simon, E. Rumpel, H. Ulmer, M. Harbeck, M. Wandel, C. Fietzek, U. Weimar, H. Meixner, Detection of volatile compounds correlated to human diseases through breath analysis with chemical sensors, Sens. Actuators B: Chem. 83 (2002) 245–249. [4] J.B. Yu, H.G. Byun, M.S. So, J.S. Huh, Analysis of diabetic patient’s breath with conducting polymer sensor array, Sens. Actuators B: Chem. 108 (2005) 306–308.
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Biographies Xiaohang Chen received the B.S. degree in electronic science and technology from Shaanxi University of Science and Technology, China, in 2007. He is currently working toward the M.S. degree of microelectronics and solid-state electronics at Shanghai Jiaotong University, China. His research interests include digital integrated circuits, gas sensors, and humidity sensors. Yanyan Wang received the M.S. degree in condensed matter physics from Dalian University of Technology, China, in 2007. She is currently working toward the Ph.D. degree of microelectronics and solid-state electronics at Shanghai Jiaotong University, China. Her research interests include MEMS-based electronic devices, particularly micro-gas sensors. Yuhua Wang received the B.S. degree in Huazhong University of Science and Technology, China, in 2008. She is currently working toward the M.S. degree of microelectronics and solid-state electronics at Shanghai Jiaotong University, China. His research interests include gas sensors, and DBD structure sensors.
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Zhongyu Hou is currently an assistant professor at Shanghai Jiaotong University, China. His research interests include plasma science and technology in the context of low dimensional systems. Dong Xu is currently a professor at Shanghai Jiaotong University, China. Her research interests include micro/nano-electromechanical systems.
Zhi Yang is currently an assistant professor at Shanghai Jiaotong University, China. His research interests include nanoscale science and technology. Yafei Zhang is currently a professor at Shanghai Jiaotong University, China. His research interests include nanoscale science and technology.
Contents lists available at ScienceDirect
Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
A breath sensor using carbon nanotubes operated by field effects of polarization and ionization Xiaohang Chen, Yanyan Wang, Yuhua Wang, Zhongyu Hou, Dong Xu, Zhi Yang, Yafei Zhang ∗ National Key Laboratory of Nano/Micro Fabrication Technology, Key Laboratory for Thin Film and Microfabrication of the Ministry of Education, Institute of Micro/Nano Science and Technology, Shanghai Jiao Tong University, Shanghai 200240, PR China
a r t i c l e
i n f o
Article history: Received 5 July 2009 Received in revised form 19 November 2009 Accepted 11 January 2010 Available online 28 January 2010 Keywords: Microfabrication breath sensor Multi-walled carbon nanotubes Exhaled breath Response Sensitivity
a b s t r a c t A novel microfabricated breath sensor (MBS) based on multi-walled carbon nanotubes (MWNTs) has been presented and tested. It has a simple structure of two nickel beams incorporating with MWNTs. The responses of the MBS to the behavior of the breath dynamic characteristics are consistent with the exhalation pulse of the human-volunteers, e.g. the exhale flow strength and frequency. It has a rapid response and high sensitivity in detecting feeble breath, and no recovery issues such as the adsorption-type sensors are detected. Furthermore, strong anti-interference ability to external air flow and temperature shift is observed. These unique results ensure this MBS can be successfully employed to real-time monitoring of human breath. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The breath of human, a basic characteristic of life carrying various information, is significant to the comprehensive analysis of lung function [1,2], asthma detection [3], diabetes mellitus diagnosis [4], etc. Meanwhile, health condition can also be reflected by the physical characteristics of the exhaled breath, such as breath frequency and tidal volume. The breath measurement has been accomplished by different approaches, such as the pressure sensors [5], which examine the pressure change induced by the exhaled gas flow. A tube connecting mouth or nose and the windtight device are necessary while operation of such sensor; otherwise, its pressuresensitive element may be disturbed by the ambient flow. The shape modification of chest or abdomen will also reflect the breath condition indirectly via pressure sensor; however, it will cause the discomfort for tying on the body. Another type of breath sensor takes advantage of the temperature shift near the nose during the breath. However, the stochastic nature of ambient temperature may deteriorate its signal quality significantly. Besides these commercial breath sensors, short time response humidity sensors [6–10] focus an increasing interest in breath diagnosis applications. However, response cycle of subsecond is difficult to accomplish. There is another breath sensor [11] based on the fact that the
∗ Corresponding author. Tel.: +86 21 3420 5665; fax: +86 21 3420 5665. E-mail address: [email protected] (Y. Zhang). 0924-4247/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2010.01.021
velocity of sound is directly modulated by air flow. However, the anti-interference ability to ambient noise is a weakness. Due to the unique physical properties [12,13] and potential in diverse range of applications [14–18], carbon nanotubes have attracted considerable attentions. Its high aspect ratio, small tip ratios and high electrical conductivity result in strong field enhancement effect [19,20]. There is another microfabricated breath sensor (MBS) based on multi-walled carbon nanotubes (MWNTs) reported by our group, which takes advantage of the tip effect of MWNTs [21]. In this paper herein; however, microfabrication processes are improved to deal with its uncontrollability of MWNTs distribution and unsatisfied bonding of MWNTs film on microelectrodes caused by screen printing. The focus of the MBS is on the performance in monitoring human breath under alternative current (AC). The information that MBS can provide is the frequency and strength of the exhaled breath. It has a considerable high quality of response, sensitivity and recovery performance, with excellent ability of anti-interference to ambient air flow and temperature shift. 2. Experiment 2.1. Sensor fabrication process Traditional “lift-off” and novel electrophoresis processes were employed to generate the microstructure. Firstly, the LOR 3A (not photosensitive) and AZ3612 (photosensitive) were spun on the
X. Chen et al. / Sensors and Actuators A 158 (2010) 328–334
329
were electroplated on the Cr/Cu seed layer, which thickened the height of the electrodes to be 4–6 m, in order to increase the effective current generation areas of the electrodes, as shown in Fig. 1d. Finally, The Cr/Cu/Ni electrodes were coated by a film of MWNTs (>95% purity) using an electrophoresis process. It was accomplished in acetone with a MWNTs content of 5 mg/L, at a constant electric field of 10 V/cm, for 5 min. Mg(NO3 )2 was used as an additive (6 mg/L) and the MWNTs were homogeneously dispersed in acetone using ultrasonic machine at power of 50 W for 30 min before electrophoresis. Additionally, another much thinner nickel film was electroplated on the MWNTs for better bonding and consistence of MWNT–film on the nickel surface, as shown in Fig. 1e. After washing with deionized water and drying, the device was completed, as schematically illustrated in Fig. 1f.
2.2. Measurement set-up Fig. 1. Schematics of the fabrication processes: (a) spinning of photoresist; (b)lithograph and etching of photoresist; (c) deposition of Cr/Cu seed layer; (d) “liftoff” of resist; (e) electroplate of Ni beams; (f) electrophoresis of MWNTs film and electroplate of Ni coat; (g) schematic of MBS.
wafer in order. After spinning and baking, the AZ3612 was exposed and the developer (AZ400) was used to clear the exposed AZ3612 areas, but also etch away the LOR 3A. Since the LOR 3A had a higher dissolution rate in developer than the AZ3612, the resulting profile would have an overhang of photoresist, which prevented the sidewall deposition of film, as illustrated in Fig. 1a. Then, Cr/Cu seed layer (10 nm/140 nm) was deposited on the patterned resist, as shown in Fig. 1b. After deposition of Cr/Cu film, the resist was dissolved in dimethylsulfoxide and the patterns of microelectrodes were obtained, as shown in Fig. 1c. Subsequently, nickel beams
A platform of detecting MBS performance in monitoring human breath was presented, As shown in Fig. 2. One electrode of sensor was connected to the signal generator (NF 1946B Function Generator), which supplied an AC signal source. Another was linked to a lock-in amplifier, for signal amplification and amplitude demodulation. The MBS was fixed at a controlled distance (2 cm) to the exit of a microbridge mass flowmeter (MFM, AWM 5000, Honeywell) which was widely applied in the field of life-support machine. This MFM was introduced here to indicate the flow rate of various gases under test. During the test, the operating voltage was approximate (∼) 1 V and the frequency was chosen ranging from 1 to 10 kHz. All signals could be translated into voltage signals and recorded by a digital oscilloscope. The mathematical conversion equation of the voltage (Vo ) recorded by the oscilloscope and the response current (Io ) of MBS was: Vo = 4Io (V/A). The mathematical conversion
Fig. 2. The test platform: (a) the practical experimental configuration; (b) diagram of MBS and MFM; (c) the schematic of the platform (the distance between the sensitive section of MFM and MBS under test is ∼10 cm).
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Fig. 3. Surface characteristics of MBS: (a) the whole picture of the MBS; (b) the top view image of the electrode gap; (c) morphology of MWNTs layer on the sidewall; (c) schematic of MWNT-tips on the sidewall.
equation of the MFM output voltage (VMFM ) and its indicated flow rate (WD ) was: WD = 2.5(VMFM − 1) (slpm/V). The basic sensing responses to spontaneous breath had been measured through output voltage in atmospheric air at room temperature and 45–90% relative humidity. The experiments were carried out in the decontamination chamber in which the temperature and humidity could be controlled conveniently and exactly. It had been examined by more than 10 volunteers for the reliability. Unusual human exhaled breath was tested in order to observe performance of the sensor under the condition of varying breath frequency and strength. Buffered compressed air had been introduced through a tube to check the influence of ambient air flow. The device was placed in an oven to examine its performance under different temperatures ranging from 30 to 60 ◦ C. Volunteers’ breath flow rates were recorded by MFM consistently during all tests. 3. Result and discussion
tion of sidewall area. From the SEM images, L, W and N can be statistically calculated. As d is kept as a constant, the increment proportion of sidewalls area can be estimated as ∼40%. 3.2. Basic performance The current output modification of this MBS reflecting the spontaneous breath was collected by the lock-in amplifier. After processing, the signals were plotted by an oscilloscope to indicate the characteristics of the breath, including frequency, duration, and amplitude. Typical responses of the MFM and the MBS as well as the bald metal electrode structure (with the same size of MBS) are demonstrated in Fig. 4. There are few detectable responses of the metallic device without MWNTs. A signal delay of the MBS following that of MFM is observed (∼0.17 s for beginning of expiration and ∼1 s for end), which may be ascribed to the distance between MBS and the sensitive section of MFM.
3.1. Structure characteristics Fig. 3a–c shows a SEM image of the MBS, the top view images of the gap (∼8 m) between electrodes and morphology of MWNTs layer on the sidewall respectively. As shown in the images, nickel electrodes are coated by a film of MWNTs with diameters of about 30–50 nm, and tilted tips of MWNTs can be observed, especially at the sidewall of electrodes. As schematically shown in Fig. 3d, the equivalent area of this capacitor structure can be calculated as follows: S d × L × N = S h×W
(1)
where S and S denote the increment of relative area and initial relative area without MWNTs respectively. And d, L, W, N, h is the mean diameter of MWNTs, the mean length of titled tips, the length of electrode, the number of titled MWNTs in electrode length of W, and the thickness of electrodes, respectively. More than 50 images were captured by SEM in order to estimate the increment propor-
Fig. 4. The curves indicating responses of three devices to spontaneous breath: MBS (baseline 0 V); MFM (baseline 1 V); and metal electrodes device (baseline arbitrary).
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Fig. 5. The curves indicating MBS responses to unnatural breaths of different and changing frequency and strength. Experimental results of four volunteers are demonstrated.
To investigate the ability of monitoring breath of high frequency and low strength, unnatural breath was tested, as indicated in Fig. 5a–d. The former two volunteers breathed with a constant frequency and strength, and the latter two volunteers breathed with an abruptly changing frequency and strength. The curves indicate stable operation of MBS in monitoring unnatural breath. This MBS can distinguish exhaled breath of a high frequency (100 times per minute), which is much higher than normal breath frequency (16–20 times per minute). Further, the delay time (t) of the exhaled gas acts on MBS and MFM can be denoted as Eq. (2): t =
D×A F
(2)
where D stands for the distance between the sensitive section of MFM and MBS, A for the cross-sectional area of MFM flow path, and F for the flow rate recorded by the MFM. When D is equal to 10 cm, the delay time (t) of the exhaled breath act on MBS and MFM can be calculated as ∼0.5 s. As indicated in the curves in Fig. 5, the response delay time (T) of MBS to MFM is averagely ∼0.15 s (T < t), indicating higher response speed of MBS than MFM. Meanwhile, exhalant air flow of 0.5 slpm (1.2 V voltage of MFM) is clearly reflected in Fig. 5, exhibiting unique sensitivity in detecting feeble breath.
from 35 to 60 ◦ C gradually. It is shown in Fig. 7a that the response signal amplitudes of the sensor to the breath in various temperatures are distributed in a very narrow range (3.3–4.3 V), which shows its thermal stability. Additionally, the higher temperatures enhance the removal of the exhaled flow and then shorten the recovery duration, indicating better performance in recovery process, as shown in Fig. 7b. In the tests, we also found that it could work steadily with various sampling distance of the spontaneous breath, ranging from 10 to 20 cm. 3.4. Modeling and theoretical analysis The differences in the performances of MBS and the bald metal electrode structure are inconsistent with the absorption or other related mechanism [22–24]. Furthermore, the increment propor-
3.3. Environmental influence In order to measure the ambient influence to MBS when monitoring breath, ambient air flow (buffered compressed air) was induced to MBS and the responses were recorded. As typically illustrated in Fig. 6, the responses of MBS are much less detectable than that of exhaled gas even the flow rate is much higher than spontaneous breath. It indicates its favorable anti-interference ability of surrounding flow. To examine the impact of temperature, tests of MBS at different temperatures are carried out and the outputs are collected. The ambient temperature was controlled by the oven, which varied
Fig. 6. Response spectra of MBS and MFM to the air flow pulse: MBS (baseline 0 V) and MFM (baseline 0 V).
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Fig. 7. The curve of MBS responses under different temperatures: (a) the response peak voltage distribution under temperature ranging from 30 to 60 ◦ C; (b) comparison of the MBS response cycle under 30, 45 and 60 ◦ C.
tion of sidewalls areas between the MBS and bald nickel beams can be estimated as ∼40% (Fig. 3), much smaller than the signal increment proportion (more than 10 as shown in Fig. 4). The remarkable performance of the MBS cannot be attributed absolutely to the increase of capacitance. A new model is proposed by us herein to explain the performance of MBS, which is detailed in [25]. As shown in Fig. 8a, the response process of MBS to exhaled breath can be divided into four stages. In stage I, the current is resulted from ionization current of initial environment (Ib ), which is composed of ambient atmosphere. In stage II, as exhaled gas is introduced to MBS, the background air is chemically changed because of numerous atomized water (∼6.2% fractional concentration) [26,27] and charged particles contained in breath gas [28,29]. Charges induced by nonuniform polarization of droplets result in polarization current (Jpd ) while droplets drifting across the gaps. Jpd is much greater than base cur-
Fig. 8. Schematic of proposed mechanism: (a) the response curve of MBS to exhaled breath according to the proposed model; (b) three stages of the actual response and recovery circle. The response voltages of MBS (baseline 0 V) and MFM (baseline 0 V) are denoted by left and right vertical axes, respectively; (c) schematic of MBS under human breath.
rent (Ib ) in atmosphere. Here, the sensitivity of MBS can be defined as:
∂Jpd ∂Wd
=−
q VE
E +
∂E Wd ∂Wd
៝ y, z)dV ε0 ∇ E(x,
(3)
VE
where Wd is the flow rate of exhaled breath, q is the electrical quantity of elementary charge, VE is the volume of current generation space, E is the averaged electric susceptibility of the medium in VE , ε0 is the dielectric constant in vacuum, E is the electric field vector, and dV is the differential unit of VE . Especially, E is related by inverse proportion of background air and the liquid droplets. Under the external AC source, electric field between the electrodes is greatly enhanced by the strong field convergence of the MWNTs tips [19]. As a result, the sensitivity of MBS to exhaled breath is greatly improved. When Wd is kept as a constant, the Jpd is only related to the exhaled flow composition, indicating stable response for every certain volunteer, as shown in Fig. 8a. When the exhaled breath is introduced with a fixed distance between exhaled breath and MBS, the curve of response current to time is analogical to the breath flow rate. Furthermore, as shown in Fig. 4, the third peak of MBS is huger than the former ones, which is caused by the reduction of distance between MBS and exhaled breath source. While the distance is shortened and the exhaled breath flow rate (WD ) is kept as a constant, the output voltage of MFM maintains unchanged, but less liquid droplets in breath gas will be lose during the diffusion process from exhaled breath source to MBS, leading to a higher proportion of liquid droplets in VE . As a result, the sensitivity of MBS is increased (as shown in Fig. 4). In this proposed model, the proportion of the liquid droplets contained in the exhaled gas is the key factor which affects the sensitivity of MBS according to the Eq. (3). When exhaled breath is introduced to MBS, the value of E is greatly increased as the background air is chemically changed by the exhaled gas. When ambient air flow pulse is introduced to MBS, the background gas remains unchanged, leading to low sensitivity of MBS. This is the reason why MBS is relatively insensitive to the ambient air. In stage III, the residual atomized droplets will be vaporized when the exhalation stops, which leads to sustaining decrease of response current. In this stage, current is composed of ionization current (Ii ) and another polarization current (Idp ) caused by the dielectrophoresis force to droplets. Taking Dl to denote the mean particle of liquid droplets in VE , Nl to denote the equivalent number of the droplets of Dl in diameter, and Wdp to denote the droplet drift
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velocity due to dielectrophoresis forces, Jdp can be defined as:
Jdp (t) = K · Nl · VE
៝ Dl3 · Wdp · ∇ · EdV VE2
(4)
where K is only related to the electric susceptibility of liquid atomized droplets in breath gas. For the same volunteer, K and Dl are considered as a constant. The Idp is in direct proportion to Nl under steady electric field and decreases with vaporization of residual droplets. When the temperature increases, the evaporation rate of the liquid droplets in VE increases as a result of the more strenuous diffusion motion, leading to faster decrease of the equivalent number of the liquid droplets (Nl ). As demonstrated in Fig. 7b, higher temperature indicates higher speed decrease of response current in stage III and exhibits faster recovery. The elapsed time of vaporization is much longer than that of exhaled breath introduction, which is indicating that the recovery process is slower than the response process. In stage IV, when the environmental gas is restored, the current magnitude will return back to the initial value as shown in Fig. 8a. The curve of MBS response to exhaled breath is shown in Fig. 8a, according to this proposed model. In stage II, the response current is a constant while the exhaled flow rate (Wd ) is assumed as a constant in the model. Actually, Wd is always varying and there is a peak value during one breath, leading to a corresponding peak value of response current of MBS, as shown in Fig. 8b. After careful verification, this proposed model is validated to match the experimental results well. 4. Conclusion A novel MBS incorporating MWNTs has been fabricated and tested. The sensor has a simple capacitor structure, including a pair of metal electrodes, coated by a layer of MWNTs and followed by a nickel protection layer. The MWNT–film greatly improves the performance of the sensor in monitoring the exhaled breath. Meanwhile, this MBS can operate steadily with ambient interference of ambient air flow and temperature. Finally, the performances of response and recovery time are plausible, implying a congruous response to the characteristics of breath dynamics. The experimental results in this paper ensure that this MBS can be successfully employed to monitor human breath. This attractive feature will be exclusively examined in our future research. Acknowledgements This work is supported by Hi-Tech Research and Development Program of China No. 2007AA03Z328, Shanghai Natural Science Foundation (No. 09ZR1415000), National Natural Science Foundation of China (Nos. 60871032, 60906053 and 50902092). References [1] D.T.V. Anh, W. Olthuis, P. Bergveld, A hydrogen peroxide sensor for exhaled breath measurement, Sens. Actuators B: Chem. 111–112 (2005) 494–499. [2] C. Di Natale, A. Macagnano, E. Martinelli, R. Paolesse, G. D’Arcangelo, C. Roscioni, A. Finazzi Agrò, A. D’Amico, Lung cancer identification by the analysis of breath by means of an array of non-selective gas sensors, Biosens. Bioelectron. 18 (2003) 1209–1218. [3] M. Fleischer, E. Simon, E. Rumpel, H. Ulmer, M. Harbeck, M. Wandel, C. Fietzek, U. Weimar, H. Meixner, Detection of volatile compounds correlated to human diseases through breath analysis with chemical sensors, Sens. Actuators B: Chem. 83 (2002) 245–249. [4] J.B. Yu, H.G. Byun, M.S. So, J.S. Huh, Analysis of diabetic patient’s breath with conducting polymer sensor array, Sens. Actuators B: Chem. 108 (2005) 306–308.
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Biographies Xiaohang Chen received the B.S. degree in electronic science and technology from Shaanxi University of Science and Technology, China, in 2007. He is currently working toward the M.S. degree of microelectronics and solid-state electronics at Shanghai Jiaotong University, China. His research interests include digital integrated circuits, gas sensors, and humidity sensors. Yanyan Wang received the M.S. degree in condensed matter physics from Dalian University of Technology, China, in 2007. She is currently working toward the Ph.D. degree of microelectronics and solid-state electronics at Shanghai Jiaotong University, China. Her research interests include MEMS-based electronic devices, particularly micro-gas sensors. Yuhua Wang received the B.S. degree in Huazhong University of Science and Technology, China, in 2008. She is currently working toward the M.S. degree of microelectronics and solid-state electronics at Shanghai Jiaotong University, China. His research interests include gas sensors, and DBD structure sensors.
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Zhongyu Hou is currently an assistant professor at Shanghai Jiaotong University, China. His research interests include plasma science and technology in the context of low dimensional systems. Dong Xu is currently a professor at Shanghai Jiaotong University, China. Her research interests include micro/nano-electromechanical systems.
Zhi Yang is currently an assistant professor at Shanghai Jiaotong University, China. His research interests include nanoscale science and technology. Yafei Zhang is currently a professor at Shanghai Jiaotong University, China. His research interests include nanoscale science and technology.