Saurin Patel

Microfluidic separation of live and dead yeast cells using reservoir-based dielectrophoresis

Separating live and dead cells is critical to the diagnosis of early stage diseases and to the efficacy test of drug screening, etc. This work demonstrates a novel microfluidic approach to dielectrophoretic separation of yeast cells by viability. It exploits the cell dielectrophoresis that is induced by the inherent electric field gradient at the reservoir-microchannel junction to selectively trap dead yeast cells and continuously separate them from live ones right inside the reservoir. This approach is therefore termed reservoir-based dielectrophoresis (rDEP). It has unique advantages as compared to existing dielectrophoretic approaches such as the occupation of zero channel space and the elimination of any mechanical or electrical parts inside microchannels. Such an rDEP cell sorter can be readily integrated with other components into lab-on-a-chip devices for applications to biomedical diagnostics and therapeutics.

Numerical modeling of Joule heating effects in insulator-based dielectrophoresis microdevices

Insulator‐based DEP (iDEP) has been established as a powerful tool for manipulating particles in microfluidic devices. However, Joule heating may become an issue in iDEP microdevices due to the local amplification of electric field around the insulators. This results in an electrothermal force that can manifest itself in the flow field in the form of circulations, thus affecting the particle motion. We develop herein a transient, 3D, full‐scale numerical model to study Joule heating and its effects on the coupled transport of charge, heat, and fluid in an iDEP device with a rectangular constriction microchannel. This model is validated by comparing the simulation results with the experimentally obtained fluid flow patterns and particle images that were reported in our recent works. It identifies a significant difference in the time scales of the electric, temperature, and flow fields in iDEP microdevices. It also predicts the locations of electrothermal flow circulations in different halves of the channel at the upstream and downstream of the constriction.

Joule heating effects on reservoir-based dielectrophoresis

Reservoir‐based dielectrophoresis (rDEP) is a recently developed technique that exploits the inherent electric field gradients at a reservoir‐microchannel junction to focus, trap, and sort particles. However, the locally amplified electric field at the junction is likely to induce significant Joule heating effects that are not considered in previous studies. This work investigates experimentally and numerically these effects on particle transport and control in rDEP processes in PDMS/PDMS microchips. It is found that Joule heating effects can reduce rDEP focusing considerably and may even disable rDEP trapping. This is caused by the fluid temperature rise at the reservoir‐microchannel junction, which significantly increases the local particle velocity due to fluid flow and particle electrophoresis while has a weak impact on the particle velocity due to rDEP. The numerical predictions of particle stream width and electric current, which are the respective indicators of rDEP manipulation and fluid temperature, are demonstrated to both match the experimental measurements with a good accuracy.

An Unexpected Particle Oscillation for Electrophoresis in Viscoelastic Fluids Through a Microchannel Constriction

Electrophoresis plays an important role in many applications, which, however, has so far been extensively studied in Newtonian fluids only. This work presents the first experimental investigation of particle electrophoresis in viscoelastic polyethylene oxide (PEO) solutions through a microchannel constriction under pure DC electric fields. An oscillatory particle motion is observed in the constriction region, which is distinctly different from the particle behavior in a polymer-free Newtonian fluid. This stream-wise particle oscillation continues until a sufficient number of particles form a chain to pass through the constriction completely. It is speculated that such an unexpected particle oscillating phenomenon is a consequence of the competition between electrokinetic force and viscoelastic force induced in the constriction. The electric field magnitude, particle size, and PEO concentration are all found to positively affect this viscoelasticity-related particle oscillation due to their respective influences on the two forces.

Microfluidic electrical sorting of particles based on shape in a spiral microchannel.

Shape is an intrinsic marker of cell cycle, an important factor for identifying a bioparticle, and also a useful indicator of cell state for disease diagnostics. Therefore, shape can be a specific marker in label-free particle and cell separation for various chemical and biological applications. We demonstrate in this work a continuous-flow electrical sorting of spherical and peanut-shaped particles of similar volumes in an asymmetric double-spiral microchannel. It exploits curvature-induced dielectrophoresis to focus particles to a tight stream in the first spiral without any sheath flow and subsequently displace them to shape-dependent flow paths in the second spiral without any external force. We also develop a numerical model to simulate and understand this shape-based particle sorting in spiral microchannels. The predicted particle trajectories agree qualitatively with the experimental observation.

Reservoir-based dielectrophoresis for microfluidic particle separation by charge

The separation of particles from a complex mixture is important to a wide range of applications in industry, biology, medicine etc. This work demonstrates a microfluidic approach to separate similar‐sized fluorescent and nonfluorescent particles based upon the difference in their surface charges inside a reservoir. Such a separation exploits the reservoir‐based dielectrophoresis, which is induced by the inherent electric field gradient formed at the reservoir–microchannel junction, to isolate the trapped fluorescent particles within the reservoir from the streaming nonfluorescent particles. The effects of the DC field magnitude (or equivalently the electrokinetic flow magnitude) and the AC field frequency of DC‐biased AC electric fields are investigated on particle separation. A numerical model is also developed to simulate the electrokinetic transport behaviors of the two types of particles. This demonstrated reservoir‐based dielectrophoresis particle sorter can operate in parallel to increase the flow throughput. It is suitable for integration with other functional parts into lab‐on‐a‐chip devices for diverse particle handling.

Joule heating effects on electroosmotic entry flow

Electroosmotic flow is the transport method of choice in microfluidic devices over traditional pressure‐driven flow. To date, however, studies on electroosmotic flow have been almost entirely limited to inside microchannels. This work presents the first experimental study of Joule heating effects on electroosmotic fluid entry from the inlet reservoir (i.e., the well that supplies fluids and samples) to the microchannel in a polymer‐based microfluidic chip. Electrothermal fluid circulations are observed at the reservoir‐microchannel junction, which grow in size and strength with the increasing alternating current to direct current voltage ratio. Moreover, a 2D depth‐averaged numerical model is developed to understand the effects of Joule heating on fluid temperature and flow fields in electrokinetic microfluidic chips. This model overcomes the problems encountered in previous unrealistic 2D and costly 3D models, and is able to predict the observed electroosmotic entry flow patterns with a good agreement.

Continuous-flow dielectrophoretic trapping and patterning of colloidal particles in a ratchet microchannel

Trapping and concentrating particles in a continuous flow is critical for their detection and analysis as well as removal in many fields. A variety of electrical and non-electrical forces have been demonstrated to continuously capture and enrich particles in microfluidic devices. This work presents an experimental study of the development of particle trapping in an asymmetric ratchet microchannel under dc-biased ac electric fields. The dc/ac dielectrophoretic accumulation of particles in the first pair of ratchets and the dc electrokinetic shifting of particles into the second and subsequent ratchets are studied, which are found to depend on the particle moving direction with respect to the asymmetric ratchets. The dielectrophoretically trapped particles are eventually patterned into triangular zones in all but the first pair of ratchets for both the forward and backward motions. This developing process of particle trapping can be qualitatively simulated by modifying the channel geometry in the computational domain to mimic the particle chains/clusters formed in the ratchets.

Electrokinetic particle separation in a single-spiral microchannel

The efficient separation of discrete particle species is a topic of interest in numerous research fields for its practical application to problems encountered in both academia and industry. We have recently developed an electrokinetic technique that exploits the curvature-induced dielectrophoresis (C-iDEP) to continuously sort particles by inherent properties in asymmetric double-spiral microchannels. Herein we demonstrate that a single-spiral microchannel is also sufficient for a continuous-flow sheathless electrokinetic particle separation. This method relies on C-iDEP to focus particles to a tight stream and the wall-induced electric lift to manipulate the aligned particles to size-dependent equilibrium positions, both of which happen simultaneously inside the spiral. A theoretical model is developed to understand this size-based separation, which has been implemented for both a binary mixture and a ternary mixture of colloidal particles. The obtained analytical formulae predict with a close agreement both the experimentally measured particle center–wall distance and the necessary electric field for a complete particle focusing in the spiral.

Three-Dimensional Reservoir-Based Dielectrophoresis (rDEP) for Enhanced Particle Enrichment

Selective enrichment of target species is crucial for a wide variety of engineering systems for improved performance of subsequent processes. Dielectrophoresis (DEP) is a powerful electrokinetic method that can be used to focus, trap, concentrate, and separate a variety of species in a label-free manner. The commonly employed methods for DEP suffer from limitations such as electrode fouling and high susceptibility to Joule heating effects. Recently, our group has demonstrated DEP-based manipulations of particles and cells using a novel method of reservoir-based dielectrophoresis (rDEP) which exploits the naturally produced electric field gradients at the reservoir-microchannel junction. Although this method reasonably addresses the limitations mentioned above while maintaining a high simplicity of fabrication, all of our demonstrations so far have used a two-dimensional rDEP, which limits the performance of the devices. This work aims to improve their performance further by making the DEP three-dimensional. Through detailed experimental and numerical analysis, we demonstrate a six-fold increase in the enrichment performance of latex beads and a significant reduction in the power consumption for the new devices, which would allow a more reliable integration of the same into micro-total analysis systems.