“Digital” Microfluidics based on Isotachophoresis (ITP)
“Digital” microfluidics means that small portions of liquid stay in a confined space when being processed without suffering from hydrodynamic dispersion or other detrimental effects that dilute the sample. The most popular realization of digital microfluidics relies on small droplets that are transported and processed using electrowetting-on-dielectric or surface acoustic waves. However, handling droplets comes along with certain disadvantages such as evaporation or difficulties with the merging and splitting of small sample amounts.
We propose an alternative route towards digital microfluidics which is based on isotachophoretic sample transport. In isotachophoresis (ITP) samples are separated and stacked according to their electrophoretic moblities by sandwiching them between two buffers of high and low mobilities and applying an electric field, typically of the order of several hundred volts per centimeter. This allows us to create and to transport small sample plugs (10µm in length) over long distances (some ten millimeters) without dispersion. Our aim is to establish the usual operations of digital microfluidics for this system, e.g. merging, splitting, mixing etc.
ITP Zone Merging and Contacting
ITP zone synchronization and merging of two fluorescent sample plugs is shown in the figures below. The first two figures show how the two ITP zones coming from the two legs of a T-crossing reach a common exit channel.
The sample coming from the top (sample 1) reaches the intersection first where it suffers a large distortion as it travels around the corner. Soon after that it refocuses into a sharp sample zone.
The sample subsequently coming from below (sample 2) shows a somewhat different behavior. After it has reached the exit channel it becomes smeared out and travels with an increased speed.
This is due to the fact that at this stage there is a mixture of high and low mobility buffer between the two sample zones, giving rise to a large electric field strength accelerating the ions. This effect synchronizes the two ITP zones and finally lets them merge as shown in the left figure.
Below we sketch the contacting of two different samples in a microchannel structure. This forms the basis of mixing and reaction between two plugs as well as subsequent separation of the products and remaining reactants.
Another line of research investigates the interplay between electroosmotic flow (EOF) and isotachophoresis (ITP). When transporting an ITP zone through a channel, the EOF in the two buffers sandwiching the zone are usually different. Since the ITP zone moves at a certain average speed, this leads to the buildup of pressure in the fluid which in turn leads to a parabolic flow profile superimposed on the plug flow. This parabolic flow profile leads to a distortion of the ITP zone. If this distortion becomes large, the sample gets smeared out significantly, therefore potentially invalidating the “digital” sample transport. Below we show experiments and results from numerical simulations showing this distortion at different positions in the channel.
1. G.Goet , T.Baier, S,Hardt, “Micro contactor based on isotachophoretic sample transport”, Lab Chip, 9, 3586 (2009).
2. F.Schönfeld, G.Goet, T.Baier, S.Hardt, “Transition Zone Dynamics in Combined Isotachophoretic and Electroosmotic Transport”, Phys. Fluids, 21, 092002 (2009).
This project is conducted as part of the DFG research group “Micro and Nanochemistry