Continuous separation of microparticles in aqueous medium by means of dielectrophoresis

Abstract

There is a widespread need to separate microparticles suspended in liquid media. Dielectrophoresis (DEP), a technique for manipulating the motion trajectories of suspended particles, has enormous potential for solving difficult particle-particle separation problems. Nevertheless, the great majority of DEP applications have been limited so far to microchannels and lab-on-a-chip devices, with throughput typically in the A LA min-1 range. A promising, alternative solution to this problem is anticipated by upscaling DEP systems to enable high-throughput DEP separation on a clinical or industrial scale. To achieve this, a novel interdigitated electrode (IDE) design is proposed to meet the need for a high electric field when upscaling a DEP system. Numerical simulation using OpenFOAM demonstrated that, when replacing conventional plate IDE by cylindrical IDE (cIDE) in microchannel systems, the dielectrophoretic force field, represented by the gradient of the squared electric field, becomes stronger and more homogeneously distributed along the electrode array. The resulting particle DEP velocities were also higher for the cIDE. Simulations confirmed by experiments allow further predictions of particle motion in enlarged cIDE-DEP systems. Understanding how the interplay of channel geometry and electrode concept affects induced particle velocity is crucial when designing DEP separators having sufficiently high throughput to reach preparative scale. The objective of tailored design is to control particle motion trajectories predominantly by DEP while avoiding electrothermal interference in the form of fluid convection induced by a temperature gradient in the liquid phase due to Joule heating. One solution to this Joule heating problem in large-scale DEP systems is to tailor the ratios of electrode diameter, electrode distance and channel height. Based on model calculations, the influence on particle trajectories of both DEP force and drag force due to thermal convection was predicted for a case study involving a channel with rectangular cross section and an array of cIDEs at the bottom. The models were successfully verified by experimentally measuring and quantitatively analysing velocities of polyelectrolytic resin microparticles located at the subsurface of demineralized water. This allowed a qualitative sensitivity analysis of the impact of voltage input, particle size and medium properties on critical design parameters. From this, design criteria were deduced for the cIDE-DEP system that allow the influence of Joule heating to be minimised. There is still a need for continuous, contact-free fractionation of microparticles at high throughput. To achieve this, a sheath-flow-assisted dielectrophoretic continuous field-flow separator with a tailored arrangement of cIDE was developed, and size-dependent trajectories of dispersed particles were observed. Using a voltage input of 200 Veff at a frequency of 200 kHz, polystyrene particles (45, 25, and 11 Amicrometre in diameter) were levitated to different heights due to a negative DEP force. Experimental observations agree well with simulated particle trajectories that were obtained from by a modified Lagrangian particle tracking model in combination with Laplace's and Navier-Stokes equations. A theoretically calculated system throughput of up to 47 mLA min-1 was found to be possible by trading off design and operation parameters, enabling contact-free fractionation of sensitive microparticles with negligible shear stress. For further upscaling of the cIDE-DEP separation system, a new separation device with concentrically arranged cIDE configuration was proposed. Proof-of-concept is demonstrated by numerically predicting microparticle motion trajectories within the separator. Simulations show that a remarkable increment of suspension throughput can be achieved by the concentric cIDE separator compared to the cIDE separator under the same circumstances. From an evaluation of the impact of operating parameters on particle displacement, it can be deduced that continuous fractionation is possible even at system throughputs in the of hundreds of mLA min-1 range by using the concentric cIDE separator. These theoretical findings lay the foundation for continuous DEP-based microparticle separation on an industrial scale.