Bead-based assays on very large numbers of molecules in gene expression studies, drug screening and clinical diagnostics, require the encoding of each of the microspheres according to the particular ligand bound to its surface. This allows mixing the uniquely encoded microspheres and subjecting them to an assay simultaneously. When a particular microsphere gives a positive reaction, the substance on its surface can be identified by reading the code. Previously reported techniques for colour encoding polymer microspheres only allow for a limited number of unique codes. Graphical encoding methods use metallic particles, which are rather uncommon in screening applications. Here, we demonstrate a new approach to encode polymer microspheres that are commonly used in screening applications, such as polystyrene microspheres, with a method that provides a virtually unlimited number of unique codes. Patterns can be written in fluorescently dyed microspheres by 'spatial selective photobleaching' and can be identified by confocal microscopy. Such encoded microparticles can find broad application in the collection and analysis of genetic information, high-throughput screening, medical diagnostics and combinatorial chemistry, and can also be used for labelling of consumer goods or as security labels to prevent counterfeiting.

In the field of medical diagnostics there is a growing need for inexpensive, accurate, and quick highthroughput assays. On the one hand, recent progress in microfluidics technologies is expected to strongly support the development of miniaturized analytical devices, which will speed up (bio)analytical assays. On the other hand, a higher throughput can be obtained by the simultaneous screening of one sample for multiple targets (multiplexing) by means of encoded particle-based assays. Multiplexing at the macro level is now common in research labs and is expected to become part of clinical diagnostics. This review aims to debate on the "added value" we can expect from (bio)analysis with particles in microfluidic devices. Technologies to (a) decode, (b) analyze, and (c) manipulate the particles are described. Special emphasis is placed on the challenges of integrating currently existing detection platforms for encoded microparticles into microdevices and on promising microtechnologies that could be used to downscale the detection units in order to obtain compact miniaturized particle-based multiplexing platforms.

Ten different tests of blood neutrophil function were studied in 20 patients with primary myelodysplastic syndromes (PMDS). The patients were selected according to the new diagnostic criteria for PMDS of the FAB-cooperation group. Impairments of granulocyte functions were found in all patients. Moreover, several steps in the mobilization of granulocytes at the site of injury seemed to be affected: decreased adhesion (P less than 0.05), deficient chemotaxis (P less than 0.05), decreased enzyme content (P less than 0.001), 'slower' chemiluminescence (P less than 0.005), decreased phagocytosis (P less than 0.05) and impaired microbicidal capacity (P less than 0.025). No significant correlation between disease category and severity of granulocyte dysfunction was discerned, though an increasing number of blasts was associated with more severe granulocytic disability. Results in seven patients with abnormal karyotypes were not significantly different from 13 others with normal karyotypes. Our results indicate that defects in blood neutrophil function are a common feature in PMDS and might account for the increased frequency of infection in these patients.

The "layer-by-layer" (LbL) technology has been widely investigated for the coating of flat substrates and capsules with polyelectrolytes. In this study, LbL polyelectrolyte coatings applied at the surface of digitally encoded microcarriers were evaluated for the quantitative, sensitive, and simultaneous detection of proteins in complex biological samples like serum, plasma, and blood. LbL coated microcarriers were therefore coupled to capture antibodies, which were used as capture agents for the detection of tumor necrosis factor (TNF-alpha), P24, and follicle stimulating hormone (FSH). It was found that the LbL coatings did not disassemble upon incubating the microcarriers in serum and plasma. Also, nonspecific binding of target analytes to the LbL coating was not observed. We showed that the LbL coated microcarriers can reproducibly detect TNF-alpha, P24, and FSH down to the picogram per milliliter level, not only in buffer but also in serum and plasma samples. Microcarrier-to-microcarrier intratube variations were less then 30%, and interassay variations less than 8% were observed. This paper also shows evidence that the LbL coated digitally encoded microcarriers are ideally suited for assaying proteins in "whole" blood in microfluidic chips, which are of high interest for "point-of-care" diagnostics.

In the field of medical diagnostics there is a growing need for inexpensive, accurate, and quick "multiplexing" assays. By making use of encoded microparticles, such assays allow simultaneous determination of the presence of several analytes in a biological sample. The microparticles under investigation in this study are encoded by writing a digital dot or bar code in their central plane. This study evaluates to what extent a "multifunctional" coating can be applied around the digitally encoded microparticles by the layer-by-layer (LbL) technology. We show that a LbL coating containing CrO2 nanoparticles allows (a) an optimal (optical) readout of the dot and bar codes, (b) a perfect orientation of the microparticles, necessary to be able to read the code, and (c) an optimal coupling of capture probes to the surface of the microparticles.