W.W.Y. Chow

The Chinese University of Hong Kong, Hong Kong, Hong Kong

Are you W.W.Y. Chow?

Claim your profile

Publications (12)8.42 Total impact

  • Source
    Winnie W. Y. Chow, Yanli Qu, Wen J. Li, Steve C. H. Tung
    [Show abstract] [Hide abstract]
    ABSTRACT: We have developed SWCNT sensors for air-flow shear-stress measurement inside a polymethylmethacrylate (PMMA) “micro-wind tunnel” chip. An array of sensors is fabricated by using dielectrophoretic (DEP) technique to manipulate bundled single-walled carbon nanotubes (SWCNTs) across the gold microelectrodes on a PMMA substrate. The sensors are then integrated in a PMMA micro-wind tunnel, which is fabricated by SU-8 molding/hot-embossing technique. Since the sensors detect air flow by thermal transfer principle, we have first examined the I–V characteristics of the sensors and confirmed that self-heating effect occurs when the input voltage is above ~1V. We then performed the flow sensing experiment on the sensors using constant temperature (CT) configuration with input power of ~230μW. The voltage output of the sensors increases with the increasing flow rate in the micro-wind tunnel and the detectable volumetric flow is in the order of 1×10−5m3/s. We also found that the activation power of the sensors has a linear relation with 1/3 exponential power of the shear stress which is similar to conventional hot-wire and polysilicon types of convection-based shear-stress sensors. Moreover, measurements of sensors with different overheat ratios were compared, and results showed that sensor is more sensitive to the flow with a higher overheat ratio. KeywordsCarbon nanotubes-CNT sensors-Micro-flow sensor-Micro shear-stress sensor
    Microfluidics and Nanofluidics 01/2010; 8(5):631-640. DOI:10.1007/s10404-009-0495-5 · 2.67 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Novel aqueous shear stress sensors based on bulk carbon nanotubes (CNTs) were developed by utilizing microelectrical mechanical system (MEMS) compatible fabrication technology. The sensors were fabricated on glass substrates by batch assembling electronics-grade CNTs (EG-CNTs) as sensing elements between microelectrode pairs using dielectrophoretic technique. Then, the CNT sensors were permanently integrated in glass-polydimethylsiloxane (PDMS) microfluidic channels by using standard glass-PDMS bonding process. Upon exposure to deionized (DI) water flow in the microchannel, the characteristics of the CNT sensors were investigated at room temperature under constant current (CC) mode. The specific electrical responses of the CNT sensors at different currents have been measured. It was found that the electrical resistance of the CNT sensors increased noticeably in response to the introduction of fluid shear stress when low activation current (Lt1 mA) was used, and unexpectedly decreased when the current exceeded 5 mA. We have shown that the sensor could be activated using input currents as low as 100 muA to measure the flow shear stress. The experimental results showed that the output resistance change could be plotted as a linear function of the shear stress to the one-third power. This result proved that the EG-CNT sensors can be operated as conventional thermal flow sensors but only require ultra-low activation power ( ~ 1 muW), which is ~ 1000 times lower than the conventional MEMS thermal flow sensors.
    IEEE Transactions on Nanotechnology 10/2008; DOI:10.1109/TNANO.2008.928572 · 1.62 Impact Factor
  • Source
    W.W.Y. Chow, W.J. Li, S.C.H. Tung
    [Show abstract] [Hide abstract]
    ABSTRACT: We have developed CNT sensors for gas-flow shear stress measurement inside a polymethylmethacrylate (PMMA) microchannel. An array of sensors is fabricated by using dielectrophoretic (DEP) technique to manipulate bundled single-walled carbon nanotubes (SWNTs) across the gold microelectrodes on a PMMA substrate. The sensors are then integrated in a PMMA microchannel, which is fabricated by SU-8 molding/hot-embossing technique. Since the sensors detect gas-flow by thermal transfer principle, we have first examined the I-V characteristics of the sensors and confirmed that self-heating effect occurs when the input voltage is above ~1V. We then performed the flow sensing experiment on the sensors using constant temperature (CT) configuration. The voltage output of the sensors increases with the increasing flow rate in the microchannel. We also found that the power of the sensors has a linear relation with 1/3 power of the shear stress. Moreover, measurements of sensors with different overheat ratios were compared and results showed that sensor is more sensitive to the flow with a higher overheat ratio.
    Nano/Micro Engineered and Molecular Systems, 2008. NEMS 2008. 3rd IEEE International Conference on; 02/2008
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We have developed carbon nanotubes (CNTs) based aqueous shear stress sensors integrated in microfluidic channels. The sensors utilized electronics-grade carbon nanotubes (EG- CNTs) as sensing elements, and were built by combining MEMS- compatible fabrication technology with AC dielectrophoretic (DEP) technique. The assembled sensing element has a room- temperature resistance of ~100 to 200 Omega by using the original concentration of 1:1 EG-CNTs in Dl-water. The I-V measurements of EG-CNTs show the heating effects of the sensors, and the current required to induce the nonlinearity of EG-CNTs is in the order of 100 mu Ultra-low-powered CNTs-based aqueous shear stress sensors integrated in microfluidic channelsA, which implies the operation power of the sensor is in the range of Ultra-low-powered CNTs-based aqueous shear stress sensors integrated in microfluidic channelsW. Upon exposure to DI- water flow, the characteristics of the sensor have been investigated at room temperature under constant current (CC) activation mode. It was found that the electrical resistance of the CNT sensors increased linearly with the introduction of constant fluidic shear stress. We have tested the response of the sensors with flow velocity from 0.3 to 3.4 m/s. The experimental results show that there is a linear relation between the output resistance change and the flow velocity to the one-third power. This result proved that the CNT sensors work with the same principle as conventional MEMS thermal shear stress sensors but only require ultra-low activation power (~1 muW), which is ~1000 times lower than that of conventional MEMS thermal shear stress sensor.
    Nano/Micro Engineered and Molecular Systems, 2008. NEMS 2008. 3rd IEEE International Conference on; 02/2008
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We present a biosensor design based on capturing the two-dimensional (2D) phase image of surface plasmon resonance (SPR). This 2D SPR imaging technique may enable parallel label-free detection of multiple analytes and is compatible with the microarray chip platform. This system uses our previously reported differential phase measurement approach, in which 2D phase maps obtained from the signal (P) and reference (S) polarizations are compared pixel by pixel. This technique greatly improves detection resolution as the subtraction step can eliminate measurement fluctuations caused by external disturbances as they essentially appear in both channels. Unlike conventional angular SPR systems, in which illumination from a range of angles must be used, phase measurement requires illumination from only one angle, thus making it well suited for 2D measurement. Also, phase-stepping introduced from a moving mirror provides the necessary modulation for accurate detection of the phase. In light of the rapidly increasing need for fast real-time detection, quantification, and identification of a range of proteins for various biomedical applications, our 2D SPR phase imaging technique should hold a promising future in the medical device market.
    Applied Optics 05/2007; 46(12):2325-32. DOI:10.1364/AO.46.002325 · 1.69 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We present a novel fabrication technique to fabricate functionalized CNT sensors rapidly using electrochemical deposition method. The basic fabrication process of this sensor includes fabrication of a gold (Au) microelectrode array by photolithography process, functionalization of multi-walled carbon nanotubes (MWNTs) with carboxylic acid groups (-COOH), and electro-chemical deposition of functionalized MWNTs (f-CNTs) on the Au microelectrode array. The adhesion between the f-CNTs and the Au microelectrodes could be enhanced if CNT sensors are built using the process described in this paper. The I-V characteristics of the sensors were investigated. Our experimental results show that CNT sensors fabricated by electro-chemical process have dramatically different I-V characteristics compared with sensors fabricated by DEP or AFM manipulation techniques. The power limit of the sensors ranges from 0.28mW to 6.21mW, which is much higher than most reported CNT sensors. Self-heating effect can be induced at input power of ~2.2mW. Based on these experimental results, we think that the novel f-CNT sensors should be investigated further for applications in thermal, mechanical, and biomedical systems.
    Nano/Micro Engineered and Molecular Systems, 2007. NEMS '07. 2nd IEEE International Conference on; 02/2007
  • [Show abstract] [Hide abstract]
    ABSTRACT: In this paper we present a biosensor design based on phase imaging of surface plasmon resonance (SPR). The system is adapted from our previously reported differential phase measurement scheme. We first conducted experiments on measuring the concentration of salt concentration in water in order to demonstrate the operation of this system. Biosensing experiments were performed to monitor the H3 influenza antigen-antibody binding interaction. In recent years, the needs for high-throughput biosensors in life sciences and biomedical areas have been increasing rapidly. Our phase-imaging SPR sensor is a non-labeling, real-time quantitative sensing approach compatible with the micro-array chip platform. It should therefore have a promising potential for various bio-related detection applications, such as clinical diagnostics.
    Biophotonics, Nanophotonics and Metamaterials, 2006. Metamaterials 2006. International Symposium on; 11/2006
  • Source
    Winnie Wing Yin Chow, Kin Fong Lei, Guangyi Shi, Wen Jung Li, Qiang Huang
    [Show abstract] [Hide abstract]
    ABSTRACT: A novel technique for bonding polymer substrates using PDMS-interface bonding is presented in this paper. This novel bonding technique holds promise for achieving precise, well-controlled, low temperature bonding of microfluidic channels. A thin (10–25 µm) poly(dimethylsiloxane) (PDMS) intermediate layer was used to bond two poly(methyl methacrylate) (PMMA) substrates without distorting them. Microchannel patterns were compressed on a PMMA substrate by a hot embossing technique first. Then, PDMS was spin-coated onto another PMMA bare substrate and cured in two stages. In the first stage, it was pre-cured at room temperature for 20 h to increase the viscosity. Subsequently, it was bonded to the hot embossed PMMA substrate. In the second stage, PDMS was completely cured at 90 °C for 3 h and the bonding was successfully achieved at this relatively low temperature. Tensile bonding tests showed that the bonding strength was about 0.015 MPa. Microfluidic channels with dimensions of 300 µm × 1.6 cm × 100 µm were successfully fabricated using this novel bonding method.
    Smart Materials and Structures 12/2005; 15(1):S112. DOI:10.1088/0964-1726/15/1/018 · 2.45 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: A microfluidic platform integrated with a high-sensitivity surface plasmon resonance (SPR) imaging sensor based on Mach-Zehnder interferometer design is presented. The disposable polydimethylsiloxane (PDMS)-based microfluidic platform consists of four independent flow chambers that can be detected four different concentrated solutions by SPR imaging biosensor simultaneously. The novel feature of the SPR imaging biosensor is the use of a Wollaston prism through which the phase quantities of the p and s polarizations are interrogated synchronously. Since SPR affects only the p polarization, the signal due to the s polarization can be used as the reference. Consequently, the differential phase between the two polarizations allows us to eliminate all common-path phase noise while keeping the phase change caused by the SPR effect. Two experiments were conducted: 1) detecting different concentrations of salt-water mixtures, and 2) monitoring the reaction between BSA-BSA antibody. This technique is shown here to have a sensitivity of 0.44mug/ml for salt-water mixture. Given that we have now demonstrated the possibility of SPR phase extraction from digitized images, direct application of this technique in two-dimensional (four independent flow chambers) sensor arrays with high measurement throughput is also suggested
    Sensors, 2005 IEEE; 01/2005
  • [Show abstract] [Hide abstract]
    ABSTRACT: We have demonstrated a carbon nanotube (CNT) based thermal flow sensor array capable of detecting air flow inside a polymethylmethacrylate (PMMA) micro fluidic chamber. The micro sensors are fabricated on a PMMA substrate using an AC electrophoretic technique to form bundled multi-walled carbon nanotube (MWNT) sensing elements between microelectrodes and then embedded inside the PMMA chamber which is fabricated using SU-8 molding/hot-embossing technique. We have tested the sensors using a hot-film anemometry constant current configuration for dynamic characterization. Preliminary results indicated that the MWNT sensors were capable of sensing input fluid flow variations inside the chamber. We have also measured the I-V characteristic of the resulting device and the results revealed that the sensor could be operated in muW range, which is three orders of magnitude lower than conventional MEMS polysilicon based shear stress sensors. Based on these experimental evidences, we propose that carbon nanotubes is a novel material for fabricating micro flow sensors on polymer substrates - polymer-based devices which may serve as alternative sensors for silicon based flow sensors when bio-compatibility and low-cost applications are required
    Sensors, 2005 IEEE; 01/2005
  • [Show abstract] [Hide abstract]
    ABSTRACT: This paper describes a polymer-based microfluidic mixing system developed by integrating two vortex micropumps and a micromixer. Fluid transmission with steady flow rate is driven by the circumrotating motion of the impellers in the micropumps. The mixing principle is based on the ultrasonic wave generated by the PZT substrate oscillation. We will illustrate the designs, the fabrication processes and the operation theories of both devices. Experiments will be conducted to investigate the performance of the mixing system.
    Intelligent Control and Automation, 2004. WCICA 2004. Fifth World Congress on; 07/2004
  • Source
    Yanli Qu, Mengxing Ouyang, Winnie W Y Chow, Wen J Li
    [Show abstract] [Hide abstract]
    ABSTRACT: In this paper, we present carbon nanotube (CNT) based thermal shear stress sensors integrated inside optically transparent Polymethylmethacrylate (PMMA) microfluidic systems. The sensors were fabricated on PMMA substrates by batch assembling multi-walled carbon nanotubes (MWNTs) as sensing elements between microelectrode pairs using AC dielectrophoretic (DEP) technique. PMMA chambers were fabricated using SU-8 molding/hot-embossing technique. Then, the PMMA substrate with a micro chamber and vortex micropump was bonded to the other PMMA substrate embedded with the MWNT sensor array to form a closed flow chamber. Experiments showed that the CNT sensors could detect volumetric air flow rate in the order of 10 -8 m 3 /s inside this microchannel system. We have also proved that upon exposure to constant liquid (DI-water) flow, the electrical resistance of the CNT sensor was found to increase linearly at low activation current of 100μA. And a linear relation between the change of output resistance and one-third power of flow rate was observed for flow rate from 0.3 to 2.3m/s. This result proved that the CNT sensors work with the same principle as conventional MEMS based thermal shear stress sensors, but only require ultra-low activation power (~μW) to achieve comparable sensitivity, which is three orders of magnitude lower than conventional MEMS polysilicon based flow sensors.