Yien Chian Kwok

Low-pressure, high-temperature thermal bonding of polymeric microfluidic devices and their applications for electrophoretic separation

A new method for thermally bonding poly(methyl methacrylate) (PMMA) substrates has been demonstrated. PMMA substrates are first engraved by CO2-laser micromachining to form microchannels. Both channel width and depth can be adjusted by varying the laser power and scanning speed. Channel depths from 50 ?m to 1500 ?m and widths from 150 ?m to 400 ?m are attained. CO2 laser is also used for drilling and dicing of the PMMA parts. Considering the thermal properties of PMMA, a novel thermal bonding process with high temperature and low bonding pressure has been developed for assembling PMMA sheets. A high bonding strength of 2.15 MPa is achieved. Subsequent inspection of the cross sections of several microdevices reveals that the dimensions of the channels are well preserved during the bonding process. Electroosmotic mobility of the ablated channel is measured to be 2.47 ? 10?4 cm2 V?1 s?1. The functionality of these thermally bonded microfluidic substrates is demonstrated by performing rapid and high-resolution electrophoretic separations of mixture of fluorescein and carboxyfluorescein as well as double-stranded DNA ladders (?X174-Hae III dsDNA digest). The performance of the CO2 laser ablated and thermally bonded PMMA devices compares favorably with those fabricated by other professional means.

A circular ferrofluid driven microchip for rapid polymerase chain reaction

In the past few years, much attention has been paid to the development of miniaturized polymerase chain reaction (PCR) devices. After a continuous flow (CF) PCR chip was introduced, several CFPCR systems employing various pumping mechanisms were reported. However, the use of pumps increases cost and imposes a high requirement on microchip bonding integrity due to the application of high pressure. Other significant limitations of CFPCR devices include the large footprint of the microchip and the fixed cycle number which is dictated by the channel layout. In this paper, we present a novel circular close-loop ferrofluid driven microchip for rapid PCR. A small ferrofluid plug, containing sub-domain magnetic particles in a liquid carrier, is driven by an external magnet along the circular microchannel, which in turn propels the PCR mixture through three temperature zones. Amplification of a 500 bp lambda DNA fragment has been demonstrated on the polymethyl methacrylate (PMMA) PCR microchip fabricated by CO(2) laser ablation and bonded by a low pressure, high temperature technique. Successful PCR was achieved in less than 4 min. Effects of cycle number and cycle time on PCR products were investigated. Using a magnet as the actuator eliminates the need for expensive pumps and provides advantages of low cost, small power consumption, low requirement on bonding strength and flexible number of PCR cycles. Furthermore, the microchip has a much simpler design and smaller footprint compared to the rectangular serpentine CFPCR devices. To demonstrate its application in forensics, a 16-loci short tandem repeat (STR) sample was successfully amplified using the PCR microchip.

High-Throughput Polymerase Chain Reaction in Parallel Circular Loops Using Magnetic Actuation

We report here a novel multichannel closed-loop magnetically actuated microchip for high-throughput polymerase chain reaction (PCR). This is achieved by designing a series of concentric circular channels on one microchip and exploiting a magnetic force to drive DNA samples flowing continuously through the closed loops. The magnetic force arises from an external permanent magnet through ferrofluid plugs inside the microchannels. The magnet enables simultaneous actuation of DNA samples in all the channels. As the samples go around the loops, they pass through three preset temperature zones. Parameters of PCR, such as incubation time, temperatures, and number of cycles, can be fully controlled and adjusted. High reproducibility was achieved for different channels in the same run and for the same channels in consecutive runs. Genetically modified organisms (GMOs) were amplified simultaneously using the developed device. This simple, reliable, and high-throughput PCR microchip would find wide applications in forensic, clinical, and biological fields.

Enhanced electrophoretic DNA separation in photonic crystal fiber

Joule heating generated by the electrical current in capillary electrophoresis leads to a temperature gradient along the separation channel and consequently affects the separation quality. We describe a method of reducing the Joule heating effect by incorporating photonic crystal fiber into a micro capillary electrophoresis chip. The photonic crystal fiber consists of a bundle of extremely narrow hollow channels, which ideally work as separation columns. Electrophoretic separation of DNA fragments was simultaneously but independently carried out in 54 narrow capillaries with a diameter of 3.7 μm each. The capillary bundle offers more efficient heat dissipation owing to the high surface-to-volume ratio. Under the same electrical field strength, notable improvement in resolution was obtained in the capillary bundle chip.

Passive micromixer for luminol-peroxide chemiluminescence detection

This paper reports a microchip with an integrated passive micromixer based on chaotic advection. The micromixer with staggered herringbone structures was used for luminol-peroxide chemiluminescence detection. The micromixer was examined to assess its suitability for chemiluminescence reaction. The relationship between the flow rate and the location of maximum chemiluminescence intensity was investigated. The light intensity was detected using an optical fiber attached along the mixing channel and a photon detector. A linear correlation between chemiluminescence intensity and the concentration of cobalt(ii) ions or hydrogen peroxide was observed. This microchip has a potential application in environmental monitoring for industries involved in heavy metals and in medical diagnostics.

Long path-length axial absorption detection in photonic crystal fiber.

In this paper, we present a long path-length axial absorption detection method in photonic crystal fibers (PCFs). A PCF, also called a holey fiber or microstructured fiber, is an optical fiber which consists of a periodic array of very tiny and closely spaced air holes on the scale of 1 microm running through the whole length of the fiber. Here, a PCF with porous microstructures was used as a sample container for absorption detection. Light was guided by total internal reflection and propagated axially in the air holes of PCFs that were filled with the solution of the absorbing species by vacuum pumping. Excellent linearity was obtained for different sample concentrations, and high sensitivity was achieved due to the long optical path length. In addition, as the dimension of the PCF is small, the sample volume is greatly reduced. Moreover, due to its robustness, the PCF can be coiled up to keep the footprint small, making it suitable for microchip absorption detection. It can be widely used for both off-chip and on-chip detection of absorbing species, such as ions, alkaloids, and biomolecules.

Sample loading and retrieval by centrifugation in a closed-loop PCR microchip

AbstractWe report on a novel concept of sample loading for microfluidic devices using a benchtop centrifuge and a magnetically actuated circular closed-loop PCR microchip as a model system. The PCR mixture and the ferrofluid were loaded into a specially designed microchip. The microchip was then placed in an off-the-shelf 50-mL tube and centrifuged. The strong centrifugal force drives the PCR mixture and the ferrofluid into the microchannels of the microchip, and simultaneously expels any trapped microbubbles. PCR was successfully carried out on single and parallel closed-loop PCR microchips. The addition of a few off-chip handling steps allows great simplification of the device design. This new loading concept may be useful for designing robust and low-cost lab-on-a-chip devices because benchtop centrifuges are quite common in most laboratories. FigureThis paper reports a novel concept of sample loading for microfluidic devices using a benchtop centrifuge. The addition of a few simple off-chip handling steps allows designing robust and low-cost lab-on-a-chip devices.

Evanescent field absorption detection in aqueous solutions using microstructured optical fibers

There is a growing interest in microstructured optical fibers (MOFs) for advanced sensors by infiltrating the air-holes with gas, liquid and polymers. The evanescent field overlap with infiltrations is enhanced compared with conventional fibers. The pure silica material is chemically and biologically inert, which well prevents the evaporation of water. Recently, liquid core waveguide cell has become widely used to minimize source light loss to the cell. However, the construction material Teflon® AF, is one of the most expensive commercial polymers. Moreover, it is highly gas permeable, causing problems of enhanced evaporation of water from the internal solution. In this paper, we demonstrate a novel and highly sensitive detection technique of Co2+ absorbance in aqueous solutions using two different structures of microstructured optical fibers (MOF). The evanescent field is numerically investigated in a systematic manner with the variation of structure parameters and wavelength. The effects of solution concentration, effective path-length, temperature stability and bending effect on the absorption sensitivity are explored and compared experimentally. The enhanced evanescent field from the second structure can achieve absorption sensitivity up to 1.6 Mol-1. The sensitivity using longitudinal detection method is at least sixty times higher compared with that using the perpendicular measurement technique