Generating fixed concentration arrays in a microfluidic device

We have designed and built a laminar microfluidic diffusion diluter (μDD) to obtain fixed concentration gradients inside lithographically patterned lab-on-a-chip architectures. The driving force for this investigation was the desire to minimize the amount of precious analyte consumed in high throughput measurements performed as a function of concentration. This was achieved by engineering a microfluidic system capable of delivering minute volumes of analyte by very slow pressure-driven flow. The μDD consists of a Y-junction that allows inflow of two different streams into a main channel, which eventually splits into a linear array of independent microchannels. The arraying technique is based on convective/diffusive transport of nanoliter quantities of an analyte from one fluid stream into the other. The μDD design allows output channels to exhibit predetermined analyte concentration values, which can be controlled by regulating the flow rate. Experiments were performed for flow rates ranging from 500 to 50 nl/min. Theoretical studies of convective/diffusive transport in the main channel have been performed as a function of the Peclet number and the normalized channel dimensions. These results were validated using fluorescence microscopy experiments as well as two- and three-dimensional numerical simulations. The computational results compared well with the experimental measurements, validating the μDD design.