Generation of solution and surface gradients using microfluidic systems

This paper describes a simple, versatile method of generating gradients in composition in solution or on surfaces using microfluidic systems. This met hod is based on controlled diffusive mixing of species in solutions that ar e flowing laminarly, at low Reynolds number, inside a network of microchann els. We demonstrate the use of this procedure to generate (1) gradients in the compositions of solutions, measured directly by colorimetric assays and (2) gradients in topography of the surfaces produced by generating concent ration gradients of etching reagents, and then using these gradients to etc h profiles into the substrate. The lateral dimensions of the gradients exam ined here, which went from 350 to 900 mum, are determined by the width of t he microchannels. Gradients of different size, resolution, arid shape have been generated using this method. The shape of the gradients can be changed continuously (dynamic gradients) by varying the relative flow velocities o f the input streams of fluids. The method is experimentally simple and high ly adaptable, and requires no special equipment except for an elastomeric r elief structure that can be readily prepared by rapid prototyping. This tec hnique provides a new platform with which to study phenomena that depend on gradients in concentration, especially dynamic phenomena in cell biology ( chemotaxis and haptotaxis) and surface chemistry (nucleation and growth of crystals, etching, and Marangoni effects).