(257a) Kinetic and Diffusive Mass Transfer in the Binding of Targets to Probes Displayed On the Bead Surface in a Microfluidic Cell

Platforms which can display biomolecules in a microarray format as probes for the multiplexed screening of the binding interactions against a target are important tools in biomedical research (drug discovery), clinical diagnostics (biomarker identification) and sensor development. Biomolecular microarrays which integrate the array and the binding detection in a single, microfluidic format have the advantage of reduced volumes of reagents and targets, high sensitivity, and increased throughput. Microbead-based microarrays are biomolecular displays in which the probe molecules are first linked to micron-sized beads which are arranged and fixed to a surface. These arrays can easily be constructed in a microfluidic platform by using a concourse of traps designed to hydrodynamically capture the beads as they stream through the course. The arrangement of the traps serves as the template for the microbead array. By trapping the beads in a sequence of stages, in which at each stage, beads displaying only one probe are trapped, with their entrapment locations mapped, a fully indexed array can be formed.  The array can then be screened against targets which are passed through the concourse.

The dynamic response of the array is determined by the rate of mass transfer of the target to the probe on the bead surface. The binding can be divided into two steps. After fluorescently labeled target is streamed into the cell, the target diffuses to the surface of the bead, and then binds to the receptor or probe on the surface. Because the beads are localized in the traps, most of the streamlines, instead of passing through the gap between the traps, tend to go around the trap. This reduced convection increases the size of diffusion boundary layers. For this reason, diffusion can play an important role on the dynamic response of the microbeads array. Numerical solutions are constructed for the surface density of bound targets as a function of time for different ratios of the surface reaction rate to the bulk diffusion (Damkohler number) and the convection of target to diffusion (Peclet number).  Values for these numbers are determined for which the dynamic response is either diffusion or kinetically limited. The mass transfer limitation, which arises for small Peclet number, is especially important when there is a limited amount of target, and flow rates through the device are small. The theory is compared to experiments in which the dynamic response of the microbead array is measured for the binding of the target Neutravidin to the probe biotin on the bead surface, and for this prototype and the flow rates used the binding is shown to be kinetically limited.