(207d) Traveling Wave Particle Separation in Fluidic Cell

This paper describes a traveling wave method for particle transport and separation in a fluidic medium. Prior work [1, 2] has resulted in the design of a particle concentrator capable of in excess of 100X concentration factor. A design of a purification cell by cascaded functions of concentration, focusing, and separation is described. Embodiments of the purification cell include: constant volume; flow-through with higher volume; and constant volume with re-circulating transport for higher purity concentration. Potential applications include: pre-concentrator front-end to detection in bio defense; water supply monitoring for utilities; food toxicology; blood plasma separation; cell enrichment; and native protein purification.

Introduction

A recent publication [3] detailed a hybrid particle concentrator device based on the combined performance of a modified field flow fractionation (mFFF) system for particulate deposition and a traveling wave (TW) transport mechanism to deliver and concentrate particulates into a sample well. A vertical compression field to push charged particulates downward to the floor of a flow channel where they are further transported for concentration into a sample well using traveling wave TW arrays. Two contiguous sections of rectilinear and chevron grids comprising inter-digitated electrodes are driven in 4-phase (or n phases with n>2) with a train of low voltage, low frequency pulses. The rectilinear array moves the deposited particulates to an edge where the orthogonal chevron array collapses the edge into a spot in a sample well. The resulting concentration is achieved by collecting particles within a much smaller volume of fluid. This technique will work for all material with net charge or zeta potential. Lab experiments have demonstrated concentration factors in excess of 100X.

This paper describes the extension of this concentrator function to include an initial separation based on dielectrophoretic field extraction as the particle stream is moved around a geometric corner. An initial design of a purification cell is constructed to demonstrate this functionality by incorporating the cascaded functions of concentration, focusing, and separation. Device design is preceded by extensive simulation to optimize device performance prior to device build using established modeling tools [4, 5].

Particle Separation

Most particulates have a native charge dependent on pH which leads to a Coulomb force, but may also polarize in a non-uniform field. The induced dipole moment (Clausius-Mossotti) results in a dipole force that allows dielectrophoresis to influence the particles close to the surface of the TW array.

Experiments on both bacillus thurengiensis and polystyrene beads in the 200 nm to 10 um size range show that electro-kinetic transport is a delicate balance of electro-osmotic flow (EOF), electrophoresis, and dielectrophoresis effects. This balance is exploited to extract the particles and re-direct and compact them into a small sample well.

The mode of separation is to move the particle stream around a corner where the TW grids transition such that the fields also reflect a change in direction. When particles of various sizes concentrate into the sample well, they have different turning radius depending on their relative size. Experimental results for the device indicate that for a sample mixture of 3 and 6 um polystyrene beads, the 6 um beads take a tighter turn around the corner than the smaller 3 um beads. The reason is that the dielectrophoretic force scales with volume (r3) so larger beads see immediate effects of the turning field and are able to turn faster. Result of a concentration run will be shown where the concentrate mixture has 6 um white, 3 um purple, and 1 um blue polystyrene beads. After separation, the 1, 3, and 6 um beads are distributed over a 1 cm wide swath.

Purification Cell

The combination of new TW layouts and sample separation mechanism may be incorporated together with the concentration and focusing aspects of the device to design a purification cell consisting of a concentration chamber, a focusing channel, and a separation chamber. The top of the separation chamber may be separated into a row of compartments to collect an increasing range of particle sizes proceeding from the side closest to the focusing channel. The TW arrays in the separation chamber may be a contiguous layout of chevrons to focus particulates in the different size ranges into the designated collection compartments at the top. The focusing section forms a narrow stream which will result in improved separation performance.

Another embodiment will be shown where a connecting bridge has been added to the top to close the loop on the cell. The notion is to allow the contents of one of the collected compartments to be re-circulated to result in increased purification. For large sample volumes, the purification cell may be incorporated into the mFFF cell geometry.

Conclusion

This separation mechanism has potentially several useful applications; including: a pre-concentrator front-end to detection in bio defense; water supply monitoring for utilities; food toxicology; blood plasma separation; cell enrichment; and native protein purification.

References [1] M.H. Lean, A.R. Völkel, H. Ben Hsieh, J.P. Lu, J.H. Daniel, B.T. Preas, and S.J. Limb, Proc. IEEE Conference on Microtechnologies in medicine & Biology, Hawaii (2005). [2] M.H. Lean, H. Ben Hsieh, A.R. Völkel, and J.L. Jensen, Proc. 2nd Joint Conference on Point Detection for Chemical and Biological Defense, Williamsburg, VA (2004). [3] M.H. Lean, H. Ben Hsieh, A.R. Völkel, Proc. HPCE 2003, 16th International Symposium on Microscale Separations and Analysis, San Diego, CA (2003). [4] M.H. Lean, IEEE Trans. Mag., 34, 3122-3125 (1998). [5] M.H. Lean, J.F. O'Brien, K. Pietrowski, H. Okuda, Proc.NIP-15: International Conference on Digital Printing Technologies, pp. 513-516, Orlando, FL, October (1999).