Abstract
We present the development of a microfluidic device that is able to selectively and nondisturbingly remove or add components to liquid samples, which allows control and conditioning of the samples for biomedical tests. The device consists of a series of chambers for sample retention and a through channel. Because smaller particles diffuse faster, small particles in the sample such as salt ions rapidly escape the chamber by diffusion and are subsequently removed by a carrier flow in the channel, leaving macromolecules of interest in the "desalted" solution. Conversely, components lacking in the sample can be diffused in by reversing the concentration gradient between the flow and the sample chamber. The ability to control the ionic strength of a sample offers many advantages in biological sample preparation as most biofluids contain high salt contents, making them unsuitable for downstream molecular analyses without additional sample treatments which could cause sample loss, contamination, and cost increase. Making use of the nature of laminar flow in a microfluidic device and mass transport by diffusion, we have developed an analytical model to calculate concentration profiles for different particles. Excellent agreements were found between the theory and the experiment, making the results highly reliable and predictable. Since the device and the principle is applicable to a wide range of biological samples, it can be incorporated into the workflow of various applications for research and in vitro diagnosis such as ion exchange, DNA sequencing, immuno assay, vesicle, cell secretion analysis, etc.