Concentration gradients have important applications in chemical and biological studies. Here we have achieved rapid generation of spatially and temporally controllable concentration gradients of diffusible molecules (i.e. proteins or toxins) in a portable microfluidic device. The formation of the concentration gradients was initiated by a passive-pump-induced forward flow and further optimized during an evaporation-induced backward flow. We were able to stabilize the gradient for up to 12 hours by stopping the flow. The computational simulations illustrated the combined effects of convection and diffusion on the gradient generation, and fit well with the experimental data. HL-1 cardiac cells were cultured in the device, and were shown to be responsive to the stabilized concentration gradient of a cardiac toxin, alpha-cypermethrin. The approach presented here may be useful for many biological and chemical processes that require rapid generation of long-range gradients in a portable microfluidic device.
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