Nicholas Mavrogiannis

Fluidic dielectrophoresis: The polarization and displacement of electrical liquid interfaces

Traditional particle‐based dielectrophoresis has been exploited to manipulate bubbles, particles, biomolecules, and cells. In this work, we investigate analytically and experimentally how to utilize Maxwell–Wagner polarization to initiate fluidic dielectrophoresis (fDEP) at electrically polarizable aqueous liquid–liquid interfaces. In fDEP, an AC electric field is applied across a liquid electrical interface created between two coflowing fluid streams with different electrical properties. When potentials as low as 2 volts are applied, we observe a frequency‐dependent interfacial displacement that is dependent on the relative differences in the electrical conductivity ( Δσ) and dielectric constant ( Δɛ) between the two liquids. At low frequency this deflection is independent of dielectric constant, while at high frequency it is independent of electrical conductivity. At intermediate frequencies, we observe an fDEP cross‐over frequency that is independent of applied voltage, sensitive to both fluid electrical properties, and where no displacement is observed. An analytical fDEP polarization model is presented that accurately predicts the liquid interfacial cross‐over frequency, the dependence of interfacial displacement on liquid electrical conductivity and dielectric constant, and accurately scales the measured fDEP displacement data. The results show that miscible aqueous liquid interfaces are capable of polarizing under AC electric fields, and being precisely deflected in a direction and magnitude that is dependent on the applied electric field frequency.

Label-free biomolecular detection at electrically displaced liquid interfaces using interfacial electrokinetic transduction (IET).

Biosensors require a biorecognition element that specifically binds to a target analyte, and a signal transducer, which converts this targeted binding event into a measurable signal. While current biosensing methods are capable of sensitively detecting a variety of target analytes in a laboratory setting, there are inherent difficulties in developing low-cost portable biosensors for point-of-care diagnostics using traditional optical, mass, or electroanalytical-based signal transducers. It is therefore important to develop new biosensing transducer elements for recognizing binding events at low cost and in portable environments. Here, we demonstrate a novel electrokinetic liquid biosensing method for the sensitive label-free detection of a model biomolecule against a background of serum protein. The biosensor is based on the motion of a microfluidic-generated electrical liquid interface when subjected to an external alternating current electrical field. We demonstrate that the electric field-induced motion of the interface can be used as a sensitive and specific transducer for the detection of avidin at femtomolar concentrations in solution. This new detection strategy does not require surface functionalization or fluorescent labels, and has the potential to serve as a sensitive low-cost method for portable biomarker detection.

Microfluidics made easy: A robust low-cost constant pressure flow controller for engineers and cell biologists

Over the last decade, microfluidics has become increasingly popular in biology and bioengineering. While lab-on-a-chip fabrication costs have continued to decrease, the hardware required for delivering controllable fluid flows to the microfluidic devices themselves remains expensive and often cost prohibitive for researchers interested in starting a microfluidics project. Typically, microfluidic experiments require precise and tunable flow rates from a system that is simple to operate. While many labs use commercial platforms or syringe pumps, these solutions can cost thousands of dollars and can be cost prohibitive. Here, we present an inexpensive and easy-to-use constant pressure system for delivering flows to microfluidic devices. The controller costs less than half the price of a single syringe pump but can independently switch and deliver fluid through up to four separate fluidic inlets at known flow rates with significantly faster fluid response times. It is constructed of readily available pressure regulators, gauges, plastic connectors and adapters, and tubing. Flow rate is easily predicted and calibrated using hydraulic circuit analysis and capillary tubing resistors. Finally, we demonstrate the capabilities of the flow system by performing well-known microfluidic experiments for chemical gradient generation and emulsion droplet production.

Microfluidic Mixing and Analog On-Chip Concentration Control Using Fluidic Dielectrophoresis

Microfluidic platforms capable of complex on-chip processing and liquid handling enable a wide variety of sensing, cellular, and material-related applications across a spectrum of disciplines in engineering and biology. However, there is a general lack of available active microscale mixing methods capable of dynamically controlling on-chip solute concentrations in real-time. Hence, multiple microfluidic fluid handling steps are often needed for applications that require buffers at varying on-chip concentrations. Here, we present a novel electrokinetic method for actively mixing laminar fluids and controlling on-chip concentrations in microfluidic channels using fluidic dielectrophoresis. Using a microfluidic channel junction, we co-flow three electrolyte streams side-by-side so that two outer conductive streams enclose a low conductive central stream. The tri-laminar flow is driven through an array of electrodes where the outer streams are electrokinetically deflected and forced to mix with the central flow field. This newly mixed central flow is then sent continuously downstream to serve as a concentration boundary condition for a microfluidic gradient chamber. We demonstrate that by actively mixing the upstream fluids, a variable concentration gradient can be formed dynamically downstream with single a fixed inlet concentration. This novel mixing approach offers a useful method for producing variable on-chip concentrations from a single inlet source.

Microfluidic platform for the real time measurement and observation of endothelial barrier function under shear stress

Electric cell-substrate impedance sensing (ECIS) is a quickly advancing field to measure the barrier function of endothelial cells. Most ECIS systems that are commercially available use gold electrodes, which are opaque and do not allow for real-time imaging of cellular responses. In addition, most ECIS systems have a traditional tissue culture Petri-dish set up. This conventional set-up does not allow the introduction of physiologically relevant shear stress, which is crucial for the endothelial cell barrier function. Here, we created a new ECIS micro-bioreactor (MBR) that incorporates a clear electrode made of indium tin oxide in a microfluidic device. Using this device, we demonstrate the ability to monitor the barrier function along culture of cells under varying flow rates. We show that while two cell types align in the direction of flow in responses to high shear stress, they differ in the barrier function. Additionally, we observe a change in the barrier function in response to chemical perturbation. Following exposure to EDTA that disrupts cell-to-cell junctions, we could not observe distinct morphological changes but measured a loss of impedance that could be recovered with EDTA washout. High magnification imaging further demonstrates the loss and recovery of the barrier structure. Overall, we establish an ECIS MBR capable of real-time monitoring of the barrier function and cell morphology under shear stress and allowing high-resolution analysis of the barrier structure.

Microfluidic free-flow zone electrophoresis and isotachophoresis using carbon black nano-composite PDMS sidewall membranes.

We present a new type of free‐flow electrophoresis (FFE) device for performing on‐chip microfluidic isotachophoresis and zone electrophoresis. FFE is performed using metal gallium electrodes, which are isolated from a main microfluidic flow channel using thin micron‐scale polydimethylsiloxane/carbon black (PDMS/CB) composite membranes integrated directly into the sidewalls of the microfluidic channel. The thin membrane allows for field penetration and effective electrophoresis, but serves to prevent bubble generation at the electrodes from electrolysis. We experimentally demonstrate the ability to use this platform to perform on‐chip free‐flow electrophoretic separation and isotachophoretic concentration. Due to the small size and simple fabrication procedure, this PDMS/CB platform could be used as a part of an on‐chip upstream sample preparation toolkit for portable microfluidic diagnostic applications.