A Microfluidic Obstacle Course for Arraying and Detection of Microbeads, Lipobeads and Liposomes

Bead based microarrays are promising platforms for implementing the high throughput, multiplexed assaying of the binding interactions of biomolecules (for example the binding of antigens and antibodies, or the conjugation of membrane receptors with small molecule ligands). In these arrays, each bead contains a particular probe molecule on its surface, and a code to identify this probe. Particles with different probes are mixed and subsequently bound onto a surface in a regular array. The array is then incubated with a target, and the binding of the target to particular probe molecules is identified (usually by fluorescently labeling the targets and scanning to find luminescent beads). The bead code is then read to identify the probe and complete the multiplexed assay. The promise of this platform lies in the fact that by decreasing the size of the beads, ulra-mininaturized platforms capable of an increased number of binding assays can be constructed.

Bead arraying is essential to the formation of these bead based microarrays, and most research has focused on using the covalent binding of functional groups on the bead surface to functionalized sites on the platform surface. Arraying paradigms which sequester particles without reaction have the distinct advantage that they avoid chemical reaction conjugation. This presentation describes the design of a microfluidics cell for the placement, through flow, of micron-sized objects in a two dimensional array on a planar surface. The cell is fabricated out of poly(dimethylsiloxane) (PDMS) using soft lithography as a flow channel, with the dimensions of the channel approximately 50 microns in height and a few millimeters in length (i.e. along the flow direction) and width (perpendicular to the flow direction). Capture bodies project upwards from the floor of the channel and are approximately the same height as the channel. The capture bodies are arranged in rows perpendicular to the lengthwise (flow) direction of the channel; each capture body is separated from its neighbor in the row by 200 microns, and the rows are 200 microns apart. The capture bodies consist of three elements which are arranged to form a cup-shaped enclosure. The elements in each body are separated from one another by gaps of approximately 25 microns in width to allow fluid to flow through the cup along the channel direction. From row to row the bodies are staggered so that the gaps in each capture body arrange in approximately straight lines which are parallel to the lengthwise direction of the channel and insure approximately rectilinear streamlines through the obstacle course when the capture bodies are un-occupied. This obstacle course allows the enclosures of each capture body to retain objects that are suspended in an aqueous stream and allowed to flow from an inlet through the course. The elements of the capture body are curved to promote the deflection of particles not moving along the streamlines which intersect through the capture body. In this way the number of particles at each capture site is minimized.

The capture of spherical polymer beads, 40 microns in diameter, in this microfludic obstacle course is studied in order to construct a phase diagram of the statistics of bead occupancy as a function of the particle concentration, and the exposure time of the course to the flow. These studies are extended to the case of arraying liposomes and polymer beads with a lipid bilayer formed on the bead surface (lipobeads). Liposomes and lipobeads are more proper platforms for the display of membrane receptors for screening their interactions with target ligands. In this case, the capture bodies, floor and ceiling of the microfluidic cell are coated by a polyethylene glycol monolayer to prevent nonspecific adsorption of liposomes and lipobeads onto these surfaces. Retention of intact liposomes and lipobeads with intact bilayers are verified by confocal laser scanning confocal fluorescence microscopy of fluorescent labels sequestered in the bilayers.&'