Hydrodynamic gating for sample introduction on a microfluidic chip.

We present a microfluidic sample introduction approach based on a novel flow control mechanism, hydrodynamic gated injection. It has the advantages of easy and flexible flow control, similar to its analog, electrokinetic gated injection, but exhibits the unique properties of strong driving force and good biocompatibility, ideal for applications involving live biological samples. Theories for hydrodynamic gating were proposed and validated by both numerical simulations and flow visualization experiments. An investigation with fluorescein revealed that pico-liter samples can be injected with high repeatability (RSD <1.9%). Selective injections of GFP-transfected nematode eggs were demonstrated with a survival rate of >95%. We expect the developed method to be potentially useful for microfluidic cell and organism analysis, either as a sample introduction module or a stand-alone analyzer.