There is a strong interest in the simultaneous detection of a number of different proteins in a single biological sample. Limitations on the size of the collected sample require that these measurements be done on as small a volume of fluid as possible. This interest has been one of the driving forces behind the development of microfluidic devices for biomedical applications. The move to these smaller-scale systems has a number of advantages. First, they are capable of analyzing smaller sample volumes. Second, in applications such as capillary electrophoresis, the microfluidic system can achieve the same separation resolution in much less time than a larger-scale system. Finally, the reduced size of the analysis setup raises the possibility of developing portable analytical devices. In collaboration with scientists at NIST, DBEPS is developing a microfluidic device for immunoaffinity electrophoresis, in which multiple proteins will be simultaneously isolated and detected. The immediate focus is on epidemiological studies, for which the simultaneous isolation and detection of multiple proteins from a large number of microliter samples is typically required; however, the device could ultimately be used for a variety of clinical and research applications. Using the microfabrication facilities at NIST, we are able to make micrometer-scale glass-encapsulated microfluidic systems with any desired two-dimensional configuration. The prototype device consists of twenty glass-encapsulated channels, each 50 micrometers x 15 micrometers x 1cm, connected in a serpentine pattern. Side ports at the ends of the microchannels in the array allow for independent electroosmotic loading and immobilization of antibody F(ab) fragments in each segment. The sample is loaded into the device using electroosmotic pumping, which permits adjustment of the sample residence time in each segment in order to optimize binding. This microfluidic device, including the immobilized antibodies, can be reused for multiple samples. After analysis of a sample, an acidic buffer gradient can be used to disrupt the antibody-antigen interaction, releasing the captured antigens without breaking the covalent attachment of the F(ab) fragments to the channel walls. The channel device architecture has several advantages over existing array technology: the proteins are detected by single-point capture, and much smaller sample volumes can be used. Recently, we have been investigating different chemistries for the covalent attachment of the reduced antibody fragments to the surfaces of the microchannels. The goals are to optimize the density of tethered antibody fragments, to increase the durability of the covalent linkage, and to minimize nonspecific adsorption of proteins to the channel walls.
