(297e) Combining Focused Ion Beam Milling and Optical Lithography to Fabricate Microfluidic Devices for DNA Dielectrophoresis

Dielectrophoresis (DEP) refers to the migration of particles in a non-uniform electric field due to the interaction of the particle’s dipole and spatial electric field gradient. DEP thus finds application in concentration and fractionation. In the case of DC conditions, DEP forces on biomolecules compete with electrokinesis resulting from the combination of electroosmotic flow (EOF) and electrophoresis (EP). In order to achieve trapping of biomolecules in a microfluidic device, the DEP force must overcome EOF and EP. Otherwise, when electrokinesis dominates over DEP, streaming behavior arises. Therefore, the design of the microfluidic device has to focus on the enhancement of the electric field gradient in order to minimize the additional forces. A novel way to create electric field gradients for DEP is using insulator posts inside the microfluidic device well known as i-DEP. This method generates homogenous electric field gradients over the entire depth of a microchannel. In addition, lithography steps are simplified since a single material is used in the fabrication process. Here, we tailored insulating post geometries inside microfluidic channels using focused ion beam milling (FIBM) in combination with optical lithography to improve i-DEP of lambda-DNA over other electrokinetic effects.

Micro-posts in the microfluidic channel were patterned by standard photolithography followed by FIBM of nano-posts in between the micro-posts. A PDMS mold was fabricated from this master and the resultant mold was bonded on a glass slide. The experiments were performed by recording the movements of particles with fluorescence microscopy while applying a DC potential in the channel. As a control experiment, we used a microfluidic device without nano-posts. Finally, numerical simulations using a convection-diffusion model were performed.

Experimental results indicate that only streaming behavior is obtained in the case of micro-posts. However, lambda-DNA was trapped by iDEP when posts fabricated with FIBM are present in the channel. According with numerical simulations, the electric field gradients are enhanced by 2.5 orders of magnitude when nano-posts are integrated between the micro-posts. This study indicates that the novel fabrication process has the potential to improve applications for dielectrophoretic separation, concentration and fractionation of biomolecules.