John D. Mai

Micro heat exchanger by using MEMS impinging jets

A micro impinging-jet heat exchanger is presented. Heat transfer is studied for single jet, slot arrays and jet arrays. In order to facilitate micro heat transfer measurements with these devices, a MEMS sensor chip, which has an 8/spl times/8 temperature-sensor array on one side, and an integrated heater on the other side has been designed and fabricated. This sensor chip allows 2-D surface temperature measurement with various jets impinging on it. It is found that micro impinging jets can be highly efficient when compared to existing macro impinging-jet microelectronics packages such as IBM 4381. For example, using a single nozzle jet (500-/spl mu/m diameter driven by 5 psig pressure), the sensor chip (2/spl times/2 cm/sup 2/) temperature can be cooled down from 70 to 33/spl deg/C. The cooling becomes more efficient when nozzle arrays (4/spl times/5 over 1 cm/sup 2/ area) are used under the same driving pressure. Interestingly, although higher driving pressure gives better cooling (lower surface temperature), the cooling efficiency, defined as h/0.5 /spl rho//spl nu//sup 2/, is actually higher for lower driving pressure.

Nucleic acid amplification using microfluidic systems

In the post-human-genome-project era, the development of molecular diagnostic techniques has advanced the frontiers of biomedical research. Nucleic-acid-based technology (NAT) plays an especially important role in molecular diagnosis. However, most research and clinical protocols still rely on the manual analysis of individual samples by skilled technicians which is a time-consuming and labor-intensive process. Recently, with advances in microfluidic designs, integrated micro total-analysis-systems have emerged to overcome the limitations of traditional detection assays. These microfluidic systems have the capability to rapidly perform experiments in parallel and with a high-throughput which allows a NAT analysis to be completed in a few hours or even a few minutes. These features have a significant beneficial influence on many aspects of traditional biological or biochemical research and this new technology is promising for improving molecular diagnosis. Thus, in the foreseeable future, microfluidic systems developed for molecular diagnosis using NAT will become an important tool in clinical diagnosis. One of the critical issues for NAT is nucleic acid amplification. In this review article, recent advances in nucleic acid amplification techniques using microfluidic systems will be reviewed. Different approaches for fast amplification of nucleic acids for molecular diagnosis will be highlighted.

Sensors and actuators on non-planar substrates

The integration of MEMS sensors and actuators onto macro mechanical parts is a critical technology necessary for the potential realization of intelligent mechanical structures. The current planar fabrication methods offered by the MEMS/IC industry restrict the possibility of integrating micro devices onto contoured (non-planar) mechanical structures. We have developed a lithographic technique to directly fabricate micron-sized sensing and actuation devices onto a cylindrical surface. This novel technology encompasses the development of flexible masks, photoresist spraying technique, and customized alignment systems. Results indicate that line resolution of <5 μm is possible for structures on the surface of a 2″ (5.08 cm) long cylinder with a diameter of 1.25″ (3.175 cm). This paper describes the procedures developed to fabricate sacrificially release micro structures onto a cylindrical surface. The performance of micro thermal actuators and shear stress sensors on a quartz cylindrical substrate are also presented.

2D Human Gesture Tracking and Recognition by the Fusion of MEMS Inertial and Vision Sensors

In this paper, we present an algorithm for hand gesture tracking and recognition based on the integration of a custom-built microelectromechanical systems (MEMS)-based inertial sensor (or measurement unit) and a low resolution imaging (i.e., vision) sensor. We discuss the 2-D gesture recognition and tracking results here, but the algorithm can be extended to 3-D motion tracking and gesture recognition in the future. Essentially, this paper shows that inertial data sampled at 100 Hz and vision data at 5 frames/s could be fused by an extended Kalman filter, and used for accurate human hand gesture recognition and tracking. Since an inertial sensor is better at tracking rapid movements, while a vision sensor is more stable and accurate for tracking slow movements, a novel adaptive algorithm has been developed to adjust measurement noise covariance according to the measured accelerations and the angular rotation rates. The experimental results verify that the proposed method is capable of reducing the velocity error and position drift in an MEMS-based inertial sensor when aided by the vision sensor. Compensating for the time delay due to the visual data processing cycles, a moving average filter is applied to remove the high frequency noise and propagate the inertial signals. The reconstructed trajectories of the first 10 Arabic numerals are further recognized using dynamic time warping with a direct cosine transform for feature extraction, resulting in an accuracy of 92.3% and individual numeral recognition within 100 ms.

Electrochemically created highly surface roughened Ag nanoplate arrays for SERS biosensing applications

Highly surface-roughened Ag nanoplate arrays are fabricated using a simple electrodeposition and in situ electrocorrosion method with inorganic borate ions as capping agent. The electrocorrosion process is induced by a change in the local pH value during the electrochemical growth, which is used to intentionally carve the electrodeposited structures. The three dimensionally arranged Ag nanoplates are integrated with substantial surface-enhanced Raman scattering (SERS) hot spots and are free of organic contaminations widely used as shaping agents in previous works, making them excellent candidate substrates for SERS biosensing applications. The SERS enhancement factor of the rough Ag nanoplates is estimated to be > 109. These Ag nanoplate arrays are used for SERS-based analysis of DNA hybridization monitoring, protein detection, and virus differentiation without any additional surface modifications or labelling. They all exhibit an extremely high detection sensitivity, reliability, and reproducibility.

Microwave bonding of polymer-based substrates for micro-nano fluidic applications

Microwave-based bonding of polymer substrates is presented in this paper to illustrate a promising technique for achieving precise, well-controlled, low temperature bonding. Microwave power is absorbed by a very thin film metal layer already deposited on the polymer (PMMA) substrate surface. The intense thin-film volumetric heating promotes localized melting of refractory metals such as gold. One of the advantages of the process is that PMMA is relatively transparent to microwave energy in the 2.4 GHz regime. This makes it an excellent substrate material for microwave bonding. Selective heating and melting of the thin layers of metal also causes localized melting of the PMMA substrates and improves adhesion at the interface. We have shown that /spl sim/1 /spl mu/m of interfacial layer can be generated which composed of the melted gold and PMMA, and which can hold the substrates together under applied tension greater than 1001b/in/sup 2/. We also used lithographically patterned metal lines on the PMMA substrate to demonstrate that the PMMA remains optically transparent after microwave processing.

Exploring bubble oscillation and mass transfer enhancement in acoustic-assisted liquid-liquid extraction with a microfluidic device.

We investigated bubble oscillation and its induced enhancement of mass transfer in a liquid-liquid extraction process with an acoustically-driven, bubble-based microfluidic device. The oscillation of individually trapped bubbles, of known sizes, in microchannels was studied at both a fixed frequency, and over a range of frequencies. Resonant frequencies were analytically identified and were found to be in agreement with the experimental observations. The acoustic streaming induced by the bubble oscillation was identified as the cause of this enhanced extraction. Experiments extracting Rhodanmine B from an aqueous phase (DI water) to an organic phase (1-octanol) were performed to determine the relationship between extraction efficiency and applied acoustic power. The enhanced efficiency in mass transport via these acoustic-energy-assisted processes was confirmed by comparisons against a pure diffusion-based process.

Non-ultraviolet-based patterning of polymer structures by optically induced electrohydrodynamic instability

We report here an approach to rapidly construct organized formations of micron-scale pillars from a thin polydimethylsiloxane (PDMS) film by optically induced electrohydrodynamic instability (OEHI). In OEHI, a heterogeneous electric field is induced across two thin fluidic layers by stimulating a photoconductive thin film in a parallel-plate capacitor configuration with visible light. We demonstrated that this OEHI method could control nucleation sites of pillars formed by electrohydrodynamic instability. To investigate this phenomenon, a tangential electric force component is assumed to have arisen from the surface polarization charge and is introduced into the traditional perfect dielectric model for PDMS films. Numerical simulation results showed that this tangential electric force played an important role in OEHI.

A foundry fabricated high-speed rotation sensor using off-chip RF wireless signal transmission

A novel MEMS surface-micromachined non-contact high-speed rotation sensor with total surface area under 4 mm/sup 2/ was developed using the MCNC Multi-User MEMS Processes (MUMPs). This paper reports the initial characterization of the sensor, including rotation and vibration tests. Initial results indicate that this piezoresistive sensor is capable of wirelessly measuring rotation speeds at /spl sim/2Hz/rpm/V with 5V input in the 100 to 6000 rpm rotation range. We believe our groundwork will allow the MEMS community to use the MUMPs foundry service to design simple and reliable high-speed rotation sensors that can be interfaced with commercial wireless chips for signal transmission.

Distinguishing cells by their first-order transient motion response under an optically induced dielectrophoretic force field

This letter reports our characterization of the transient motion of cells under an optically induced dielectrophoresis (ODEP) force field. Different types of human cells repeatably undergo a first-order transient motion response when subjected to a specific ODEP force field. A kernel function is derived to describe this transient motion. This function can be generally matched to experimental data for Raji cells and red blood cells by measuring two parameters: the initial velocity and the transient time-constant. They are uniquely different for Raji cells and RBCs. Support vector machine is used to distinguish between them based on their transient response characteristics.