A tuneable array of unique steady-state microfluidic gradients.

We report an on-chip gradient generator that has been designed, modelled, fabricated, and characterized to facilitate temporal tuning of several unique gradients in parallel for multiple applications. This design allows for steady state programming of the intensities across multiple orders of magnitude while producing exponential, linear, and logarithmic gradient profiles. The magnitude of the gradients is controlled through regulating the ratio of the two on-chip flow inlets without the need for valves or other active mixers. On-chip binding of biotin by a fluorescent streptavidin complex creates a diffusive barrier that regulates access to the gradient inlets, providing a second orthogonal mechanism for regulating the microgradient intensities. The device is also characterized using an on-chip enzymatic reaction to produce an array of tuneable product concentrations within the various microchannels.