Controlled agitation during hybridization: surface acoustic waves are shaking up microarray technology

Microarray hybridization experiments are mostly based on quite small sample volumes being confined between the microarray itself and a cover slip or lifter slip on top of the narrow fluid layer. Under such conditions, the system is governed by the rules of microfluidics, i.e., by the regime of small Reynold's numbers. Here, diffusion is the only source for moving sample molecules toward their target spots. However, for a typical macromolecule such as that used in microarray hybridization experiments, the diffusion constant is very small. Hence, because they are driven by diffusion only, traveling over typical distances on a microarray may take them a very long time. Additionally, the slow time constants associated with the diffusion limit lead to pronounced depletion effects, which strongly influence the dynamics of a hybridization assay. In this report, we describe a novel technique to overcome the diffusion limit in microarray hybridization experiments. Surface acoustic waves on a piezoelectric substrate are coupled with the sample fluid on a microarray, where they act as a highly efficient agitation source. We demonstrate that the diffusion limit can be overcome in this fashion, leading to a remarkable increase in signal intensity and homogeneity in fluorescence-labeled microarray assays.