Blanca H. Lapizco-Encinas

Dielectrophoretic monitoring of microorganisms in environmental applications

Dielectrophoresis (DEP) is the motion of polarizable particles in the presence of nonuniform electric fields. This novel electrokinetic technique has successfully been employed in many miniaturized systems for the manipulation and detection of microbes. This review article depicts the application of dielectrophoresis for the monitoring of microorganisms in microfluidic devices for environmental applications. The research studies described here are mainly conceived for water‐ and air‐monitoring assessments, and are classified considering the target aimed to detect, concentrate, and/or separate, including chemical and toxicant agents, and microorganisms ranging from virus to protozoa. Dielectrophoresis has also played an important role in biofilm formation studies. This review article comprises mainly studies published from 2000 to present. Even in this relatively short time frame, there have been many significant contributions of this powerful and nascent technique related to environmental monitoring; thus, unveiling its great potential for future research directions.

A continuous DC-insulator dielectrophoretic sorter of microparticles

A lab-on-a-chip device is described for continuous sorting of fluorescent polystyrene microparticles utilizing direct current insulating dielectrophoresis (DC-iDEP) at lower voltages than previously reported. Particles were sorted by combining electrokinetics and dielectrophoresis in a 250 μm wide PDMS microchannel containing a rectangular insulating obstacle and four outlet channels. The DC-iDEP particle flow behaviors were investigated with 3.18, 6.20 and 10 μm fluorescent polystyrene particles which experience negative DEP forces depending on particle size, DC electric field magnitude and medium conductivity. Due to negative DEP effects, particles are deflected into different outlet streams as they pass the region of high electric field density around the obstacle. Particles suspended in dextrose added phosphate buffer saline (PBS) at conductivities ranging from 0.50 to 8.50 mS/cm at pH 7.0 were compared at 6.85 and 17.1V/cm. Simulations of electrokinetic and dielectrophoretic forces were conducted with COMSOL Multiphysics® to predict particle pathlines. Experimental and simulation results show the effect of medium and voltage operating conditions on particle sorting. Further, smaller particles experience smaller iDEP forces and are more susceptible to competing nonlinear electrostatic effects, whereas larger particles experience greater iDEP forces and prefer channels 1 and 2. This work demonstrates that 6.20 and 10 μm particles can be independently sorted into specific outlet streams by tuning medium conductivity even at low operating voltages. This work is an essential step forward in employing DC-iDEP for multiparticle sorting in a continuous flow, multiple outlet lab-on-a-chip device.

Joule heating effects on particle immobilization in insulator‐based dielectrophoretic devices

In this work, the temperature effects due to Joule heating obtained by application of a direct current electric potential were investigated for a microchannel with cylindrical insulating posts employed for insulator‐based dielectrophoresis. The conductivity of the suspending medium, the local electric field, and the gradient of the squared electric field, which directly affect the magnitude of the dielectrophoretic force exerted on particles, were computationally simulated employing COMSOL Multiphysics. It was observed that a temperature gradient is formed along the microchannel, which redistributes the conductivity of the suspending medium leading to an increase of the dielectrophoretic force toward the inlet of the channel while decreasing toward the outlet. Experimental results are in good agreement with simulations on the particle‐trapping zones anticipated. This study demonstrates the importance of considering Joule heating effects when designing insulator‐based dielectrophoresis systems.

Insulator-based dielectrophoresis of microorganisms: Theoretical and experimental results

Dielectrophoresis (DEP) is the motion of particles due to polarization effects in nonuniform electric fields. DEP has great potential for handling cells and is a non‐destructive phenomenon. It has been utilized for different cell analysis, from viability assessments to concentration enrichment and separation. Insulator‐based DEP (iDEP) provides an attractive alternative to conventional electrode‐based systems; in iDEP, insulating structures are used to generate nonuniform electric fields, resulting in simpler and more robust devices. Despite the rapid development of iDEP microdevices for applications with cells, the fundamentals behind the dielectrophoretic behavior of cells has not been fully elucidated. Understanding the theory behind iDEP is necessary to continue the progress in this field. This work presents the manipulation and separation of bacterial and yeast cells with iDEP. A computational model in COMSOL Multiphysics was employed to predict the effect of direct current‐iDEP on cells suspended in a microchannel containing an array of insulating structures. The model allowed predicting particle behavior, pathlines and the regions where dielectrophoretic immobilization should occur. Experimental work was performed at the same operating conditions employed with the model and results were compared, obtaining good agreement. This is the first report on the mathematical modeling of the dielectrophoretic response of yeast and bacterial cells in a DC‐iDEP microdevice.

Separation of mixtures of particles in a multipart microdevice employing insulator-based dielectrophoresis

Dielectrophoresis is the electrokinetic movement of particles due to polarization effects in the presence of non‐uniform electric fields. In insulator‐based dielectrophoresis (iDEP) regions of low and high electric field intensity, i.e. non‐uniformity of electric field, are produced when the cross‐sectional area of a microchannel is decreased by the presence of electrical insulating structures between two electrodes. This technique is increasingly being studied for the manipulation of a wide variety of particles, and novel designs are continuously developed. Despite significant advances in the area, complex mixture separation and sample fractionation continue to be the most important challenges. In this work, a microchannel design is presented for carrying out direct current (DC)‐iDEP for the separation of a mixture of particles. The device comprises a main channel, two side channels and two sections of cylindrical posts with different diameters, which will generate different non‐uniformities in the electric field on the main channel, designed for the discrimination and separation of particles of two different sizes. By applying an electric potential of 1000u2009V, a mixture of 1 and 4u2009μm polystyrene microspheres were dielectrophoretically separated and concentrated at the same time and then redirected to different outlets. The results obtained here demonstrate that, by carefully designing the device geometry and selecting operating conditions, effective sorting of particle mixtures can be achieved in this type of multi‐section DC‐iDEP devices.

Experimental and theoretical study of dielectrophoretic particle trapping in arrays of insulating structures: Effect of particle size and shape

Insulator‐based dielectrophoresis (iDEP) employs insulating structures embedded in a microchannel to produce electric field gradients. This contribution presents a detailed analysis of the regions within an iDEP system where particles are likely to be retained due to dielectrophoretic trapping in a microchannel with an array of cylindrical insulating structures. The effects of particle size and shape on dielectrophoretic trapping were analyzed by employing 1 and 2 μm polystyrene particles and Escherichia coli cells. This research aims to study the mechanism behind dielectrophoretic trapping and develop a deeper understanding of iDEP systems. Mathematical modeling with COMSOL Multiphysics was employed to assess electrokinetic and dielectrophoretic particle velocities. Experiments were carried out to determine the location of dielectrophoretic barriers that block particle motion within an iDEP microchannel; this supported the estimation of a correction factor to match experiments and simulations. Particle velocities were predicted with the model, demonstrating how the different forces acting on the particles are in equilibrium when particle trapping occurs. The results showed that particle size and shape have a significant effect on the magnitude, location, and shape of the regions of dielectrophoretic trapping of particles, which are defined by DEP isovelocity lines and EK isovelocity lines.

Effect of insulating posts geometry on particle manipulation in insulator based dielectrophoretic devices.

In this study, the effect of the geometry of insulating posts on microparticle trapping in insulator based dielectrophoresis (iDEP) was analyzed. The motivation for this research was to study how to improve particle trapping and enrichment by modifying the shape of insulating posts used in iDEP microdevices, while keeping post spacing constant. Mixtures of inert polystyrene particles were employed for demonstrating the effects of insulator shape on particle capture and enrichment. A series of experiments were carried out using an array of devices with different insulating post shapes. All the different post shapes employed had a width of 200 μm and were arranged in a square array of 250 μm center-to-center, thus, the spacing between posts was 50 μm in all cases. Mathematical modeling with COMSOL Multiphysics was employed to assess the magnitude of electric field gradients achieved with each one of the geometries tested. The results showed that the electric potential required to obtain effective particle trapping and enrichment can be significantly reduced by modifying the geometry of the insulating posts, without having to modify the separation distance between posts, thus, preserving the porosity of the microchannels. The separation of a mixture of 1-μm and 2-μm diameter particles was achieved in the form of dielectropherograms employing two different insulating post geometries (circle and diamond). Concentrated particles were released as peaks from the insulating post arrays where higher peak resolution separation was obtained with the sharper diamond geometry. Concentration enrichment above one order or magnitude was obtained for both particle types in both dielectropherograms. The results demonstrate that more efficient iDEP separations can be achieved at lower applied electric potentials by carefully selecting the geometry of the insulating structures.

Dynamic microparticle manipulation with an electroosmotic flow gradient in low-frequency alternating current dielectrophoresis.

In this study, the potential of low‐frequency AC insulator‐based DEP (iDEP) was explored for the separation of polystyrene microparticles and yeast cells. An EOF gradient was generated by employing an asymmetrical, 20 Hz AC electrical signal in an iDEP device consisting of a microchannel with diamond‐shaped insulating posts. Two types of samples were analyzed, the first sample contained three types of polystyrene particles with different diameters (0.5, 1.0, and 2.0 μm) and the second sample contained two types of polystyrene particles (1.0 and 2 μm) and yeast cells (6.3 μm). This particular scheme uses a tapered AC signal that allows for all particles to be trapped and concentrated at the insulating post array, as the signal becomes asymmetrical (more positive), particles are selectively released. The smallest particles in each sample were released first, since they require greater dielectrophoretic forces to remain trapped. The largest particles in each sample were released last, when the applied signal became cyclical. A dielectropherogram, which is analogous to a chromatogram, was obtained for each sample, demonstrating successful separation of the particles by showing “peaks” of the released particles. These separations were achieved at lower applied potentials than those reported in previous studies that used solely direct current electrical voltages. Additionally, mathematical modeling with COMSOL Multiphysics was carried out to estimate the magnitude of the dielectrophoretic and EOF forces acting on the particles considering the low‐frequency, asymmetrical AC signal used in the experiments. The results demonstrated the potential of low‐frequency AC‐iDEP systems for handling and separating complex mixtures of microparticles and biological cells.

Assessment of cell viability after manipulation with insulator‐based dielectrophoresis

The effects of insulator‐based DEP (iDEP) manipulation on cell viability were investigated by varying operating conditions and the shape of the insulating structures. Experiments were conducted with Escherichia coli, Bacillus subtilis, and Saccharomyces cerevisiae cells by varying the applied potential (300–1000 V), exposure time (1–4 min), and composition of the suspending medium (0–10% glucose); using devices made from polydimethylsiloxane. Cell viability was quantified employing Trypan blue staining protocols. The results illustrated a strong decrease in cell survival at higher applied electric potentials and exposure times; and an increase in cell viability obtained by increasing suspending medium osmolality. The composition and structure of the cell wall also played a major role on cell survival, where prokaryotic Gram‐positive B. subtilis was the most resilient cell strain, while eukaryotic S. cerevisiae had the lowest survival rate. Due to the popularity of iDEP in applications with biological cells, characterizing how iDEP operating conditions affect cell viability is essential.

Sperm cells manipulation employing dielectrophoresis

Infertility studies are an important growing field, where new methods for the manipulation, enrichment and selection of sperm cells are required. Microfluidic techniques offer attractive advantages such as requirement of low sample volume and short processing times in the range of second or minutes. Presented here is the application of insulator-based dielectrophoresis (iDEP) for the enrichment and separation of mature and spermatogenic cells by employing a microchannel with cylindrical insulating structures with DC electric potentials in the range of 200–1500xa0V. The results demonstrated that iDEP has the potential to concentrate sperm cells and distinguish between mature and spermatogenic cells by exploiting the differences in shape which lead to differences in electric polarization. Viability assessments revealed that a significant percentage of the cells are viable after the dielectrophoretic treatment, opening the possibility for iDEP to be developed as a tool in infertility studies.