  Spatial gradients of diffusible signalling molecules play crucial roles in
controlling diverse cellular behaviour such as cell differentiation, tissue
patterning and chemotaxis. In this paper, we report the design and testing of a
microfluidic device for diffusion-based gradient generation for cellular
assays. A unique channel design of the device eliminates cross-flow between the
source and sink channels, thereby stabilising gradients by passive diffusion.
The platform also enables quick and flexible control of chemical concentration
that makes highly dynamic gradients in diffusion chambers. A model with the
first approximation of diffusion and surface adsorption of molecules
recapitulates the experimentally observed gradients. Budding yeast cells
cultured in a gradient of a chemical inducer expressed a reporter fluorescence
protein in a concentration-dependent manner. This microfluidic platform serves
as a versatile prototype applicable to a broad range of biomedical
investigations.
