A novel microfluidic concentration gradient generator is designed where secondary flow, induced via a surface groove, is used to yield a concentration gradient across the output of the microfluidic device. The concentration gradient generator design consists of a single microfluidic channel with two inputs and a single obliquely angled surface groove within the base of the channel to induce the secondary flow and create the concentration gradient. The design allows a concentration gradient to be chosen, either linear or exponential, at the exit of the microfluidic channel with the shape and dimensions of the surface groove within the channel obtained by numerical optimisation. The designed device has a small footprint, suitable for integration within lab-on-a-chip structures for the delivery of a series of an agent to either (i) a single channel and with a concentration gradient or to (ii) a series of reactors with concentrations across a defined range, for bioscience or pharmaceutical screening applications or for chemical reactions. (C) 2012 Elsevier B.V. All rights reserved.