Three-dimensional electrodes for dielectrophoretic applications

This thesis reports the use of metal-coated three-dimensional SU-8 electrodes for dielectrophoretic bio sensing and particle manipulation applications. Placing free standing three-dimensional electrodes in microfluidic channels, electric fields can be applied homogeneously over the complete height of the device. Due to this at any vertical position in the channel an equal dielectrophoretic force is created. These electrodes have been used to parallelize individual single-cell analysis by the mean of electrorotation and to tune the sorting size of deterministic lateral displacement devices by adding dielectrophoretic forces.

Electrorotation uses rotating electric fields to make cells spin and determine their dielectric properties from their speed of rotation at different electric excitation frequencies. Using a double array of individual connected three-dimensional electrodes placed in a wide microfluidic channel, individual cells can be selectively trapped and released in single-cell dielectrophoretic micro cages and analyzed in parallel by electrorotation. The parallel analysis of cells in individual electrorotation cages is demonstrated for the first time and the membrane capacitance of Henrietta Lacks, human embryonic kidney 293, and human T lymphocytes are found in agreement with literature. The membrane capacitance of M17 neuroblastoma is investigated and found to be 7.49 ± 0.39 mF/m2. Additionally membrane capacitance changes within a human embryonic kidney 293 cell population are observed with the system based on an osmolarity study and the viability is assured by monitoring the membrane functionality using an erythrosin B dye. Failure of the membrane sealing function is observed in real-time by a shift of the electrorotation spectrum and a drop in the membrane capacitance.

Deterministic lateral displacement uses shifted rows of posts in a microfluidic labyrinth to sort particles based on their size. The principle uses the deterministic behavior of particles in the device, particles below a certain size move straight through the device, while larger particles are displaced to the side. Replacing the passive posts by three-dimensional electrodes allows the creation of local dielectrophoretic forces. Applying voltages of up to 15 Vpp dynamically reduces the sorting size of the device from 6 µm to 250 nm, enabling a device tunable for a specific sorting size. Nanometer sized particles can be separated in within micrometer structures reducing the problem of clogging. Additionally, this device could enable sorting of equal sized nanometer particles based on their dielectric properties.

These two examples show the applicability of metal-coated three-dimensional SU-8 electrodes for dielectrophoretic applications. They first enable the design of new devices, which could not be realized previously, as shown by the electrorotation device, and second traditional passive structures can be replaced by electrodes, which can help to improve their performance or enable new functionalities.