The size, shape and material of the BEADS are critical properties that must be met for BI
applications. The particles must be spherical, the size in the range 20 to 150 μm. Smaller beads are difficult to capture and perfuse, while larger beads may become trapped in flow channels, creating “bead jams.” Soft polymer beads are preferable, as rigid beads (glass, silica) may abrade valves. For BI-Spectroscopy, optical transparency is an additional requirement. Sephadex and Sepharose beads are ideal for BI applications as they fulfill all criteria, as they are transparent and can be can be monitored by UV-VIS and by fluorescence. Sephadex and Sepharose beads are available with a wide variety of functional groups. Cation and anion exchange, C-18, Protein A, were used for assays of inorganic species, for trace element preconcentration and for selective capture of biomolecules, such as antibodies and antigens. Cytodex beads were used as carriers for live cells probed by fluorescence microscopy and even graphite beads were used as renewable electrode material for voltammetry of enzyme-based assays. A typical amount of packed beads per assay by BI Spectroscopy is 2 to 10 mcrL, corresponding approximately to 2,000 to 10,000 beads. Since bead count is large and beads are metered from a large reservoir, the bead size does not need to be uniform, especially if the optics of the flow cell are designed to integrate a response from a large number of beads. Finally, mechanical properties of the beads must be considered. Sepharose 4B beads are more elastic than 6B, which are more crosslinked. Cytodex beads are very soft and fragile. Polysorb and graphite beads are sturdy.