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).