Gels play an important role in biotechnological separations and in particular, the differential mobility of biopolymers driven by electric fields through gels has played a significant role in the development of genomics. While gels are already well established in capillary electrophoresis applications, where the entire capillary or microstructure can be filled with gel material, the producing localized gel plugs at specific locations in a microfluidic system is more involved. One configuration that is particularly attractive is to create a gel plug between two channels, which could then allow differential mobility (possibly driven by electric fields, see below). Although photosensitive gel materials are available, the creation of sharp gel boundaries in closed channel networks is problematic. One technique is to use the low Reynolds number highly laminar flow to sandwich a flow of gel material between two flows of aqueous buffer. Such an arrangement can be supported by differential depth of channels as shown in the left figure and realized by multistep anisotropic wet etching of silicon, followed by anodic bonding with PYREX glass substrates. Photopolymerization of specific plugs, using an appropriate mask, can be achieved at geometry supported locations without the gel material tearing upon gelation.
The process starts with filling the middle channel with a mixture of a photopolymerisable monomer PEG-DA (Poly-(ethyleneglycol)-diacrylate), cross linking agent (Methylene-bisacrylamide) and the photoinitiator HMPP (2-Hydroxy-2-methylpropiophenone) in TAE buffer solution (pH: 8,0; 40 mM Tris (hydroxy-methyl) aminomethan; 1mM EDTA). In the following step the microstructures were irradiated through a photomask (10s, UV-source: 250 W).