(225j) Enhanced Mass Transfer in the Capture of Targets to a Probe Library Consisting of Microbeads with Surface Capture Molecules and Confined in Wells at the Channel Floor of a Microfluidic Cell

Platforms which can screen a target biomolecule against a library of potential binding partners to determine which probes can bind the target are essential diagnostic tools in fundamental studies in molecular biology and drug discovery, and in the clinical identification of disease markers. These platforms are also the sensing elements of detectors for environmental surveillance and food monitoring.  The development of microfluidic screening platforms is under active research, as ultra miniaturized devices hold promise for high sensitivity and portability. Most importantly, lab on a chip screens require reduced volumes of assay reagents and of the target or sample. This feature is especially attractive, as target amounts can be very limited, especially in medical diagnostic assays.

Considerable research in microfluidic screening has  focused on a format in which the probes are hosted on microbeads which are arrayed in wells arranged at the bottom of a flow channel of a microfluidic cell to form the probe library. This arrangement allows microbeads to be individually addressed, and therefore different microbeads with different probes can be displayed. The depth of the wells are usually of the diameter of the microbeads, so only the top contacts the flow.  Target (usually fluorescently labeled) is streamed through the channel, and binding to the surface probes is identified by the fluorescence of the microbeads which have captured the target.

In prior research on this microbead platform for screening,  we have demonstrated through numerical simulation and experiments using the binding of Neutravidin as a target to biotin displayed on the microbead surface,  that sequestering the microbeads in wells can significantly limit the binding rate.  For large enough values of the convective flow of the target over the top of the microbead surface which contacts the flow,  a thin concentration boundary layer develops over the top surface of the microbead because of the convective flow, and target binds rapidly to that part of the microbead. However, the relatively stagnant layers of liquid in the well provide a diffusion barrier which slows the target transport, and for any flow parameters and surface binding,  the overall uptake of target to the microbead surface is slower than equivalent patches of probes arranged on the channel wall.

To enhance the mass transfer, we examine conditions in which the microbeads are localized in wells which are shorter in depth, so that the hemisphere of the microbead contacts the convective flow. This geometry significantly increases the overall transport of target to the microbead surface,  and consequently the dynamic response and sensitivity. Numerical simulations and accompanying experiments (again using Neutravidin-biotin) are presented which validate this approach, and the issue of displacement of microbeads only half confined in the wells is also discussed.