Bioanalysis: Microbeads are easily fixed

Biological assays are an integral part of the researcher's toolkit in the fields of biomolecular chemistry and genomics. Microfluidic microbead systems, which consist of arrays of beads coated with an assay-specific reagent, have revolutionized biological assay technology by allowing the high-throughput detection of target molecules from small sample volumes. Fabrication of the microbead systems, however, requires great care and various ancillary devices.

Chee Chung Wong and colleagues from the A*STAR Institute of Microelectronics have now developed a passive and robust method for manufacturing sorted arrays of multiple microbead types.

The preparation of microbead systems conventionally involves the use of a pump to introduce a bead-carrying fluid into a microfluidic circuit. The beads then adsorb to the walls of the microchannels with little control over position or sorting. The resultant microbead-coated channels can be used for targeted molecule detection, but the beads can be easily dislodged by flow.

Recognizing the limitations of conventional systems, Wong and his colleagues set out to develop a passive, pumpless method for preparing more robust microbead arrays. "There are no pumpless bead sorting strategies currently available," notes Wong. As a result, "we had to research and study three-dimensional trap architectures that could efficiently perform size-based bead sorting."

The researchers used semiconductor fabrication technologies to create a trap architecture consisting of a top surface with larger micrometer-sized holes and an underlying diffusion gap. When a drop of fluid containing microbeads is placed on the top surface, the beads become trapped in the micrometer-sized holes while the fluid is free to flow through the diffusion layer and out of the array. This structure has the advantage of allowing beads of different sizes to be trapped and permanently fixed in different parts of the device as the fluid evaporates (see image).

"We studied how a droplet of liquid evaporates and how this affects the flow field," says Wong. "Based on simulations and experiments, we were able to optimize our microtrap architecture for efficient size-based sorting of a range of different bead sizes."

The researchers expect their fabrication method to alleviate ease-of-use issues associated with current bead sorting assays, but also to significantly speed-up throughput by allowing multiple molecular targets to be detected in one device. "The additional dimension of bead size would directly increase the number of analytes that can be detected," says Wong. "They could increase from two, for a conventional two-color system, to six for a system with three bead sizes in different trap regions."