Particle separation using Flow-induced electrokinetic trapping

Laurens-Jan Jellema, Sander Koster, Elisabeth Verpoorte

Pharmaceutical Analysis

Department of Pharmacy

University of Groningen

The area of microfluidics is rapidly growing, with a broad range of applications including clinical diagnostics, separations sciences, environmental sciences and analysis where living cells are cultivated in microfluidic channels for i.e. screening of drugs.¹ Separation techniques like chromatography and electrophoresis are often used on microfluidic chips.¹ However, there is less knowledge about capturing and manipulating particles in microfluidic channels.

The aim of this research is to generate a microfluidic system based on recirculating flows for capturing particles and manipulating them. Recirculating flows can be generated in channels which are typically 4 cm long, 50 µm deep and varying in width using converging and diverging elements (see Fig. 2). Channels are made with standard photolithography processes in glass using our cleanroom facilities.

Flows in microchannels can be generated using pressure flow (PF) with a parabolic flow profile, and electro-osmosic (EOF) with a flat flow profile. When opposing the EOF and PF in a microchannel a bidirectional flow is obtained. Application of this bidirectional flow in a diverging and converging microchannel results in a recirculating flow, shown in figure 1. The recirculating flows are exploited to trap particles and separate them, based on differences in charge or size.^2-4

Separation of particles having similar size but different electrokinetic properties, expressed as zeta-potential ( z ) was studied. Both straight channels of uniform cross-section and channels incorporating diverging and converging elements were used. When using recirculating flows in a non-uniform channel, particles can be separated with the additional advantage of trapping. One particle type can be trapped, while particles with lower z _p and higher z _p are removed from the trapping region by EOF and PF, respectively. A simple theoretical model is proposed to better understand the separation mechanism at work and predict under what conditions particles could be separated with this system.³

In collaboration with the Aero- and Hydrodynamics group of Delft University of Technology separation of particles with different size and the same charge was studied. Once trapped in the recirculating flow in the narrow channel, both small and large particles will recirculate. However, larger particles occupy a slightly smaller region of the channel, which results in a larger net drift of larger particles towards the low-pressure side of the channel. Depending on the magnitude of the EOF relative to the PF, smaller particles will be carried in a direction opposite to the PF component and can thus escape the trapping channel with the EOF. An increase of the electric field flips the sign of the net drift velocity, allowing larger particles to be transported by the EOF and hence escape the trapping channel.⁴

Currently the trapping and separation of biological particles, like cells and DNA, are under investigation.

References

[1] T. Vilkner, D. Janasek, A. Manz, “Micro total analysis system. Recent developments”, Anal. Chem., 2004, 76, 12, pp. 3373-3385.

[2] G. L. Lettieri, A. Dodge, G. Boer, N. F. De Rooij and E. M. J. Verpoorte, “A novel microfluidic concept for bioanalysis using freely moving beads trapped in recirculating flows”, Lab Chip, 2003, 3, 34-39.

[3] L. C. Jellema, T. Mey, S. Koster and E. Verpoorte, Charge-based particle separation in microfluidic devices using combined hydrodynamic and electrokinetic effects Lab Chip, 2009, 9, DOI: 10.1039/B819054B

[4] L. C. Jellema, A. P. Markesteijn, J. Westerweel and E. Verpoorte, Size separation of particles using flow-induced electrokinetic trapping, MicroTAS conference 2008, San Diego, USA