Wei-Heong Tan

A trap-and-release integrated microfluidic system for dynamic microarray applications

Dynamic microarrays hold great promise for advancing research in proteomics, diagnostics and drug discovery. However, this potential has yet to be fully realized due to the lack of reliable multifunctional platforms to transport and immobilize particles, infuse reagents, observe the reaction, and retrieve selected particles. We achieved all these functions in a single integrated device through the combination of hydrodynamic and optical approaches. Hydrodynamic forces allow simultaneous transportation and immobilization of large number of particles, whereas optical-based microbubble technique for bead retrieval gives dexterity in handling individual particles without complicated circuitry. Based on the criterion derived in this paper, the device was designed, and fabricated using standard photolithography and soft lithography methods. We examined the dynamics of bubble formation and dissipation in the device, and parametric studies revealed that higher power settings at short intervals were more efficient than low power settings at longer intervals for bead retrieval. We also demonstrated the capabilities of our device and its potential as a tool for screening methods such as the “one-bead-one-compound” (OBOC) combinatorial library method. Although both approaches, hydrodynamic confinement and optical-based microbubbles, are presented in one device, they can also be separately used for other applications in microchip devices.

A monolithically three-dimensional flow-focusing device for formation of single/double emulsions in closed/open microfluidic systems

This paper proposes a design concept and fabrication method of a planar three-dimensional (3D) microfluidic flow-focusing device (MFFD) that can produce monodisperse single/double emulsions in a closed/open microfluidic system. The device consists of three layers of SU-8 resist structures to form coaxial embedded orifices at the center of the microchannel with dimensions ranging from 50 µm to 200 µm by means of the black photoresist shadow method. Two or three immiscible fluids can be focused through the coaxial orifices, producing monodispersed droplets with a coefficient of variance (CV) of less than 4.1%. At the orifice, the inner liquid thread stays confined to the central axis of the microchannel, surrounded by the continuous phase. As the dispensed phase (inner fluid thread) does not wet channel walls, our proposed 3D MFFD can produce single emulsions for both water-in-oil (W/O) and oil-in-water (O/W) droplets utilizing the same device. The droplet diameter ranges from 50 µm to 300 µm. Also, double emulsions containing one to several internal droplets were successfully produced in the closed channel configuration. In addition, we demonstrated for the first time the feasibility of forming W/O droplets and polymer particles in an open channel configuration by withdrawing the fluid from the outlet channel. W/O droplets and polymer particles, smaller than 10 µm and 40 µm, respectively, were successfully produced. In contrast to the closed channel configuration where the droplet size decreases with an increasing flow rate, in an open channel configuration, the droplet size increases with an increasing withdrawal rate. The unique fabrication of the monolithic 3D MFFD device utilizing SU-8 resist overcomes problems regarding orifice sizes/shapes, alignment and assembly for current axisymmetric flow-focusing devices (AFFD) based on capillary microtubes, and provides flexibility for the future development of an integrated miniaturized lab-on-a-chip microsystem.

Microfluidic formation of lipid bilayer array for membrane transport analysis

We present a highly parallel and reproducible method for reconstituting an array of lipid bilayers to analyze membrane transport. We infuse buffer/lipid/buffer solutions sequentially into a microchannel with numerous microchambers in its walls and seal each chamber by a lipid bilayer containing membrane proteins. Due to the small volume of the chamber (2 pL), membrane transport of confined fluorescent molecules across the bilayer through the proteins is readily observed as changes in fluorescent intensity. We successfully perform quantitative measurement of the transport flux of fluorescent molecules (calcein) through alpha-hemolysin antibiotic pores.

“Housing” for cells in monodisperse microcages

We present a method to form monodisperse microcages encapsulating cells with poly-L-lysine (PLL) membrane. These microcages were prepared with a monolithic three-dimensional microfluidic axisymmetric flow-focusing device (3D AFFD) using internal gelation method. The production process of microcage is mild enough for cells. The microcages were sufficiently monodisperse and robust to be trapped in a beads-based microfluidic array system for easy observation. We also confirmed that (i) the PLL membrane is semi-permeable, (ii) cells are able to move freely inside the microcages, and (iii) cells can be successfully cultured inside these microcages.

A resettable dynamic microfluidic device

This paper describes a simple reusable device that hydrodynamically traps a large number of beads in an array. The device allows us to release the trapped beads by simply reversing a flow direction. The trap and reset operations are extremely simple, robust and highly efficient. With the device, we succeeded in arraying 1000 microbeads, and subsequently releasing them in a few minutes. As the demonstration of the resettability of the device, we performed two sequential assays and found that multiple experiments can be easily conducted, saving both cost and time.

Mass production of uniform alginate capsules for micro cell encapslation using micro chamber array

This work is motivated by the need for accurate positioning of both adherent and non-adherent cells into arrays that can be integrated with microfluidic channels in high-throughput screening (HTS) for drug discovery. We describe a microfluidic device that allows rapid production of uniform micro hydrogel capsules that can contain cells. Up to 1times104 hydrogel capsules can be simply formed and arrayed simultaneously using microchambers in a 10 mm long microchannel. In this work, alginate was used as the hydrogel capsules that can be dissolved and removed with EDTA; thus samples confined in the capsules are collectable after observation in the array. We demonstrated the microencapsulation of E. coli into alginate capsules, and dissolved the capsules with EDTA successfully.

Synthesis of bio-functionalized copolymer particles in 3D microfluidic devices

This paper proposes a design concept and fabrication method of a planar three-dimensional (3D) microfluidic flow-focusing device (MFFD) that can produce monodisperse copolymer (EGDMA/AA) microspheres carrying surface carboxyl in a closed/open microfluidic system. The devices were made up by PDMS or SU-8 resist structures to form coaxial embedded orifices for a closed/open microfluidic system, respectively. The copolymer microspheres produced in an open channel configuration can avoid the problem of clogging the microchannels during in-situ UV polymerization. By confining the comonomer liquid thread to the central axis of the microchannel, we avoid the wetting problem and successfully produced copolymer microspheres with diameters ranging from 95%m to 140%m and coefficient of variance (CV) below 4%. Besides, increasing the concentration of acrylic acid (AA) would decrease the particle size, but increase the distribution of carboxyl group on the particle surfaces. Bioconjugation of the carboxylated copolymer particles with the anti-rabbit IgG-Cy3 conjugates was successfully produced. The unique fabrication overcomes problems encountered for current 2D MFFD devices, and provides flexibility for lab-on-chip microsystems.

Arrayed Monodisperse Micro-Alginate Beads in μ-Fluidic Traps for Cell Assay

We present two achievements in this paper: (i) the preparation of monodisperse Micro-Alginate Beads (MABs) with coefficient of variation (C.V.) less than 3% by combining T-junction droplet formation with internal gelation method, and (ii) the manipulation of flow resistances in passive μ-Fluidic traps to achieve “ one bead-to-one trap” immobilization for cell array. In addition, we have succeeded in encapsulating Hep G2 cells in MABs. Based on these results, we can develop a gentle and easy to handle bead-based cell assay system.