Steve Tung

A micromachined flow shear-stress sensor based on thermal transfer principles

Microhot-film shear-stress sensors have been developed by using surface micromachining techniques. The sensor consists of a suspended silicon-nitride diaphragm located on top of a vacuum-sealed cavity. A heating and heat-sensing element, made of polycrystalline silicon material, resides on top of the diaphragm. The underlying vacuum cavity greatly reduces conductive heat loss to the substrate and therefore increases the sensitivity of the sensor. Testing of the sensor has been conducted in a wind tunnel under three operation modes-constant current, constant voltage, and constant temperature. Under the constant-temperature mode, a typical shear-stress sensor exhibits a time constant of 72 /spl mu/s.

Interdigitated array microelectrode based impedance immunosensor for detection of avian influenza virus H5N1.

Continuous outbreaks of avian influenza (AI) in recent years with increasing threat to animals and human health have warranted the urgent need for rapid detection of pathogenic AI viruses. In this study, an impedance immunosensor based on an interdigitated array (IDA) microelectrode was developed as a new application for sensitive, specific and rapid detection of avian influenza virus H5N1. Polyclonal antibodies against AI virus H5N1 surface antigen HA (Hemagglutinin) were oriented on the gold microelectrode surface through protein A. Target H5N1 viruses were then captured by the immobilized antibody, resulting in a change in the impedance of the IDA microelectrode surface. Red blood cells (RBCs) were used as biolabels for further amplification of the binding reaction of the antibody-antigen (virus). The binding of target AI H5N1 onto the antibody-modified IDA microelectrode surface was further confirmed by atomic force microscopy. The impedance immunosensor could detect the target AI H5N1 virus at a titer higher than 10(3)EID(50)/ml (EID(50): 50% Egg Infective Dose) within 2h. The response of the antibody-antigen (virus) interaction was shown to be virus titer-dependent, and a linear range for the titer of H5N1 virus was found between 10(3) and 10(7)EID(50)/ml. Equivalent circuit analysis indicated that the electron transfer resistance of the redox probe [Fe(CN)(6)](3-/4-) and the double layer capacitance were responsible for the impedance change due to the protein A modification, antibody immobilization, BSA (bovine serum albumin) blocking, H5N1 viruses binding and RBCs amplification. No significant interference was observed from non-target RNA viruses such as Newcastle disease virus and Infectious Bronchitis disease virus. (The H5N1 used in the study was inactivated virus.).

A surface-micromachined shear stress imager

A new MEMS shear stress sensor imager has been developed and its capability of imaging surface shear stress distribution has been demonstrated. The imager consists of multi-rows of vacuum-insulated shear stress sensors with a 300 /spl mu/m pitch. This small spacing allows it to detect surface flow patterns that could not be directly measured before. The high frequency response (30 kHz) of the sensor under constant temperature bias mode also allows it to be used in high Reynolds number turbulent flow studies. The measurement results in a fully developed turbulent flow agree well with the numerical and experimental results previously published.

Improved micro thermal shear-stress sensor

Micro hot-film shear-stress sensors have been designed and fabricated by surface micromachining technology which is compatible with IC technology. A polysilicon strip, 2 /spl mu/m/spl times/80 /spl mu/m, is deposited on top of a thin silicon nitride film and functions as the sensor element. By using the sacrificial-layer technique, a cavity (a vacuum chamber of about 300 mtorr), 200/spl times/200/spl times/2 /spl mu/m, is placed between the silicon nitride film and the silicon substrate. This cavity significantly increases the sensitivity of the sensor by reducing the heat loss to the substrate. The frequency response of the sensor, however, is degraded by the cavity. For comparison purposes, a sensor structure without a cavity has also been designed and fabricated on the same chip. When operated in a constant temperature mode, the cutoff frequencies of the sensors with and without a cavity can reach 9 and 130 kHz, respectively. Wind tunnel calibration of the sensor with a cavity shows a sensitivity of about 10 mV/Pa, which is about two orders of magnitude higher than other micromachined shear stress sensors.

Rapid detection of avian influenza H5N1 virus using impedance measurement of immuno-reaction coupled with RBC amplification.

Avian influenza virus (AIV) subtype H5N1 was first discovered in the 1990 s and since then its emergence has become a likely source of a global pandemic and economic loss. Currently accepted gold standard methods of influenza detection, viral culture and rRT-PCR, are time consuming, expensive and require special training and laboratory facilities. A rapid, sensitive, and specific screening method is needed for in-field or bedside testing of AI virus to effectively implement quarantines and medications. Therefore, the objective of this study was to improve the specificity and sensitivity of an impedance biosensor that has been developed for the screening of AIV H5. Three major components of the developed biosensor are immunomagnetic nanoparticles for the separation of AI virus, a microfluidic chip for sample control and an interdigitated microelectrode for impedance measurement. In this study polyclonal antibody against N1 subtype was immobilized on the surface of the microelectrode to specifically bind AIV H5N1 to generate more specific impedance signal and chicken red blood cells (RBC) were used as biolabels to attach to AIV H5N1 captured on the microelectrode to amplify impedance signal. RBC amplification was shown to increase the impedance signal change by more than 100% compared to the protocol without RBC biolabels, and was necessary for forming a linear calibration curve for the biosensor. The use of a second antibody against N1 offered much greater specificity and reliability than the previous biosensor protocol. The biosensor was able to detect AIV H5N1 at concentrations down to 10(3) EID(50)ml(-1) in less than 2h.

Statistical analysis on wall shear stress of turbulent boundary layer in a channel flow using micro-shear stress imager

Abstract Measurements of wall shear stress of turbulent boundary layers in the channel flow were carried out using a micro-electro-mechanical-system (MEMS)-based micro-shear stress imaging chip. The study was carried out in a turbulent channel flow facility. One array of 25 micro-shear stress sensors in the chip that covers a length of 7.5 mm is used to measure the instantaneous span-wise distribution of the surface shear stress. The characteristics of high shear stress area (streaks) were described with statistics. Based on the measurement, the physical quantities associated with the high shear stress streaks, such as their length, width with the high shear stress level, were obtained. To further explore the relationship between the shear stress slope and the peak shear stress, the probability density function (PDF) of the ratio of peak shear stress to shear stress slope at different Reynolds number Re is examined. As for the distribution of PDF, it was found that the distribution concentrated towards a certain value in each Re . This result is extremely important because it points to the possibility of predicting the peak shear stress level based on the shear stress distribution at the leading edge of the streaks.

Development and Applications of Portable Biosensors

The significance of microfluidics-based and microelectromechanical systems–based biosensors has been widely acknowledged, and many reviews have explored their potential applications in clinical diagnostics, personalized medicine, global health, drug discovery, food safety, and forensics. Because health care costs are increasing, there is an increasing need to remotely monitor the health condition of patients by point-of-care-testing. The demand for biosensors for detection of biological warfare agents has increased, and research is focused on ways of producing small portable devices that would allow fast, accurate, and on-site detection. In the past decade, the demand for rapid and accurate on-site detection of plant disease diagnosis has increased due to emerging pathogens with resistance to pesticides, increased human mobility, and regulations limiting the application of toxic chemicals to prevent spread of diseases. The portability of biosensors for on-site diagnosis is limited due to various issues, including sample preparation techniques, fluid-handling techniques, the limited lifetime of biological reagents, device packaging, integrating electronics for data collection/analysis, and the requirement of external accessories and power. Many microfluidic, electronic, and biological design strategies, such as handling liquids in biosensors without pumps/valves, the application of droplet-based microfluidics, paper-based microfluidic devices, and wireless networking capabilities for data transmission, are being explored.

Evaluation study of a portable impedance biosensor for detection of avian influenza virus.

Current methods for detection of avian influenza virus (AIV) based on virus culture and RT-PCR are well established, but they are either time consuming or require specialized laboratory facilities and highly trained technicians. A simple, rapid, robust, and reliable test, suitable for use in the field or at the patients bedside, is urgently needed. In this study, the performance of a newly developed portable impedance biosensor was evaluated by comparison with real-time reverse transcriptase PCR (rRT-PCR) and virus culture for detection of AIV in tracheal and cloacal swab samples collected from experimentally H5N2 AIV infected chickens. The impedance biosensor system was based on a combination of magnetic nanobeads, which were coated with AIV subtype-specific antibody for capture (separation and concentration) of a target virus, and a microfluidic chip with an interdigitated array microelectrode for transfer and detection of target virus, and impedance measurement of the bio-nanobeads and AI virus complexes in a buffer solution. A comparison of results obtained from 59 swab samples using virus culture, impedance biosensor and rRT-PCR methods showed that the impedance biosensor technique was comparable in sensitivity and specificity to rRT-PCR. Detection time for the impedance biosensor is less than 1h.

Measurements of wall shear stress of a turbulent boundary layer using a micro-shear-stress imaging chip

Measurements of wall shear-stress streaks of a turbulent boundary layer in the channel flow were carried out using a MEMS-based micro-shear-stress imaging chip, which contains about 100 sensors. The chip is designed and fabricated by surface micromachining technology. One arrray of 25 micro-shear-stress sensors in the chip that covers a length of 7.5 mm is used to measure the instantaneous spanwise distribution of the surface shear stress. The statistics of high shear-stress streaks were established. Based on the measurement, the physical quantities associated with the high shear-stress streaks, such as their length, width and peak shear-stress level, were obtained. We found out that a high correlation exists between the peak shear-stress level and front-end shear-stress slope of a high shear-stress streak. This important property is currently being applied to the deisgn of a real-time flow control logic.

Micromachined thermal shear-stress sensor for underwater applications

This paper reports the development of micromachined thermal shear-stress sensors for underwater applications. The thermal shear-stress sensor is a polysilicon resistor sitting atop a vacuum-insulated nitride diaphragm. Special challenges for underwater measurements, such as the waterproof coating and minimization of pressure crosstalk, have been addressed. More rigid diaphragms than the aerial sensors are implemented to increase the operating range and reduce pressure crosstalk, with the cost of larger power consumption and lower sensitivity. Sensors with different diaphragm dimensions and resistor lengths have been fabricated and tested. Nearly zero pressure sensitivity has been achieved by either reducing the diaphragm width or adjusting the sensing element length. The effects of overheat ratio and operating mode on the sensors pressure crosstalk have been discussed. Parylene C is chosen as the waterproof material for the underwater shear-stress sensors. The primary failure mode is identified as the corrosion of the soldering pads.