Separation of Bio-particles using Dielectrophoresis and Microfluidics
Date Issued
2010
Date
2010
Author(s)
Lin, Yu-Min
Abstract
Trapping and separation of particles entrained in a moving liquid stream were studied in this thesis via negative dielectrophoresis (nDEP). The nDEP force can be generated by applying ac voltage with 180 degree phase shift at two electrodes (separated by a 20 μm gap) built on the bottom wall of a straight micro channel. When a particle approaches the location of the electrode gap (called the DEP gate), it will be lifted by the negative DEP force, and held before the DEP gate if the negative DEP force is suitably designed such that it can resist the fluid drag. Much effort in previous literature aims to determine the critical condition (the maximum background flow rate at a given applied voltage, or the minimum applied voltage at a given flow rate) for a single particle (or rare particles) in a given microfluidic system. In practical applications, the flow through area decreases, and thus the fluid drag increases, as more particles are trapped at the DEP gate. Furthermore, the succeeding particles may impact on the previously trapped particles. Sooner or later, the trapped particles may gather enough momentum to cross the DEP gate, and thus the critical condition for single particle (or rare particles) fails to apply. The present thesis thus introduce the concept of capacity (define as the average number of particles trapped at the DEP gate when the system reaches its quasi steady state), and propose that the capacity as a suitable indicator for quantifying the performance of the related micro system with DEP gates.
The capacity of colorectal cancer cells (Colo205, 15~20 μm) was measured in a system with a straight electrode gap. It equals 15.7 cells per 200 μm channel width at 1 MHz frequency, 7 Vpp (peak-to-peak-voltage) and 156μm/s in flow rate. Separation between Colo205 cells and E. Coli was also performed using such a system: the Colo205 cells were blocked, with E. Coli past through the DEP gate. This suggests that the precision for the current Stool DNA test (less than 50% now) can be enhanced if the sample is pre-treated using the devices with DEP gate.
It was found that the device with curved electrode gap performs better for particle trapping over straight electrode gap, and up to 267% times when it is operated at 1MHz and 7Vpp with an average background flow speed at 156μm/s. The capacity can be further increased up to 367% for a two-step channel design. The negative DEP force possesses a larger component resisting the background flow for the curved electrodes, and thus the capacity is increased. The capacity of the two-step channel is enhanced because of the physical blockage as well as the additional DEP force associated with the intensified electric field generated at the corner of the flow geometry.
In literature, particle separation using DEP relies mainly on the particle size as the DEP force depends on the cubic power of its size. It is not easy to separate negative DEP particles with similar sizes. In this thesis, we have separated successfully the Colo205 cells and the polystyrene particles (at 15±0.15μm) based on their differences in Clausius-Mossotti factor using the straight electrode gap device. The operation parameters were 1MHz, 6V_pp and 468~624μm/s in flow speed.
Subjects
dielectrophoresis
cell trapping and separation
colorectal cancer cells
two-step channel
SDGs
Type
thesis
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