Bead Based Assays

A broad variety of particle sizes are available, from microspheres with less than 5-pm particle size up to resins of 700-pm particle size. The large beads can be used as polymeric microreactors.P l As the separation between resin and liquid is still performed by filtration, different glass filters have to be used the porosity and nominal pore size of these filters are listed in Table 5. In those applications where single beads are used (split/mix, single-bead analysis, bead-based assays) it is essential to know the capacity of the single beads. For practical purposes a correlation between bead size and capacity per bead is reported in Table 6. 

Massively parallel analysis or high throughput screening allows the parallel handling of several hundred to up to billions of assays or samples within one run, and performs an according readout for each assay in parallel. Main application examples are microarrays, bead based assays and picowell-plates. 

In bead based assays [281, 290] small solid phase spheres (Fig. 18b) or particles are used. Each bead is bearing one unique chemical compound. Such a bead library can consist of billions of different beads. For screening, the beads are mixed and incubated with the sample and consecutively with the assay buffers, performing one assay at each bead in parallel. The readout is commonly fluorescence based and the positive beads are sorted out and analysed one by one in series. T q)ically this technique is used for binding assays or DNA-analysis. 

Here, the microfluidic actuation principles that are utiziled in massively parallel analysis are outlined briefly. This is followed by some commercial application examples. Due to the similar principle, microarrays and picowell plates are presented together, followed by bead based assays. 

Today, many manual steps and skilled personnel are required for the described systems and a real microfluidic platform is still not reached. However, microarrays, picowell plates and bead based assays are a very useful combination of solid phase and liquid handling for massively parallel assays in the number of millions. The material consumption per assay is quite low and the reaction time quite fast. The time-to-result is longer compared to a single assay, but several magnitudes faster compared to serially performing the same number of assays. 

Bead-based assays Cytometric bead immunoassay Particle manipulation... 

ECL is often combined with other assay techniques to detect the concentration of a coreactant or an emitter more precisely. For example, in order to determine the precise coreactant concentration, ECL assays are used with the combination of some analytical methods for the separation of molecules such as high-performance liquid chromatography (HPLC) and capillary electrophoresis (CE) [9,10]. On the other hand, the detection of the ECL label as the emitter is often combined with magnetic bead-based assays (Fig. 3) that enables to... 

Rothacker, J., Ramsay, R.G., Ciznadija, D., Gras, E., Neylon, C.B., Elwood, N.J., Bouchier-Hayes, D., Gibbs, P., Rosenthal, M.A., Nice, E.C., 2007. A novel magnetic bead-based assay with high sensitivity and selectivity for analysis of telomerase in exfoliated cells from patients with bladder and colon cancer. Electrophoresis 28,4435 1446. 

---