Mark A. Hayes

Discrete magnetic microfluidics

We present a method to move and control drops of water on superhydrophobic surfaces using magnetic fields. Small water drops (volume of 5–35μl) that contain fractions of paramagnetic particles as low as 0.1% in weight can be moved at relatively high speed (7cm∕s) by displacing a permanent magnet placed below the surface. Coalescence of two drops has been demonstrated by moving a drop that contains paramagnetic particles towards an aqueous drop that was previously pinned to a surface defect. This approach to microfluidics has the advantages of faster and more flexible control over drop movement.

Recent developments in electrophoretic separations on microfluidic devices.

Research combining the areas of separation science and microfluidics has gained popularity, driven by the increasing need to create portable, fast, and low analyte‐consumption devices. Much of this research has focused on the developments in electrophoretic separations, which use the electrokinetic properties of analytes to overcome many of the problems encountered during system scale‐down. In addition, new physical phenomenon can be exploited on the microscale not available in standard techniques. In this study, the innovative developments, including electrophoretic concentration, sample preparation/conditioning, and separation on‐chip are reviewed, along with some introductory discussions, from January 2008 to July 2010.

Bioanalytical separations using electric field gradient techniques

The field of separations science will be strongly impacted by new electric‐field‐gradient‐based strategies. Many new capabilities are being developed with analytical targets ranging from particles to small molecules, and soot to living cells. Here we review the emerging area of electric field gradient techniques, dividing the large variety of techniques by the target of separation. In doing so, we have contributions using dielectrophoresis, electric field gradient focusing (including dynamic, true moving bed, and pulsed field), electrocapture and electrophoretic focusing, temperature gradient focusing, and focusing with centrifugal force. We cover the literature from the start of 2007 to June 2008, along with some introductory discussions. Even with the relatively short time frame, this young and dynamic field of inquiry produced some 100 contributions describing new and unique techniques and several new applications.

Insulator-based dielectrophoretic separation of small particles in a sawtooth channel

Insulator‐based dielectrophoretic separation of small particles in a sawtooth channel is studied in the limit of dilute concentration. Pathlines for the movements of infinitesimal particles are constructed and the geometric changes of these pathlines are used to establish the criterion for blocking and trapping particles with different physical properties. The sharp corners of the sawtooth channel create much stronger dielectrophoretic force than channels with smooth corners for blocking particle movements. Particle blocking and trapping depend on particle properties and the geometry of the device. It is shown that once the channel geometric aspect ratios are specified, the blocking criterion depends on only a single dimensionless parameter C defined in terms of the particle mobility ratio (dielectrophoretic versus electrokinetic), the applied voltage and the spacing between the teeth. Selective blocking and trapping of particles can be realized by varying the geometry of the channel progressively. High‐resolution separation can be achieved by tuning the differential in the parameter C to a desired level.

Characterization of particle capture in a sawtooth patterned insulating electrokinetic microfluidic device

Here we present a scheme to separate particles according to their characteristic physical properties, including size, charge, polarizability, deformability, surface charge mobility, dielectric features, and local capacitance. Separation is accomplished using a microdevice based on direct current insulator gradient dielectrophoresis that can isolate and concentrate multiple analytes simultaneously at different positions. The device is dependent upon dielectrophoretic and electrokinetic forces incorporating a global longitudinal direct current field as well as using shaped insulating features within the channel to induce local gradients. This design allows for the production of strong local field gradients along a global field causing particles to enter, initially transported through the channel by electrophoresis and electroosmosis (electrokinetics), and to be isolated via repulsive dielectrophoretic forces that are proportional to an exponent of the field gradient. Sulfate‐capped polystyrene nano and microparticles (20, 200 nm, and 1 μm) were used as probes to demonstrate the influence of channel geometry and applied longitudinal field on separation behavior. These results are consistent with models using similar channel geometry and indicate that specific particulate species can be isolated within a distinct portion of the device, whereas concentrating particles by factors from 103 to 106.

Dielectrophoretic mobility determination in DC insulator-based dielectrophoresis

Insulator‐based dielectrophoresis (iDEP) is a powerful tool for separating and characterizing particles, yet it is limited by a lack of quantitative characterizations. Here, this limitation is addressed by employing a method capable of quantifying the DEP mobility of particles. Using streak‐based velocimetry the particle properties are deduced from their motion in a microfluidic channel with a constant electric field gradient. From this approach, the DEP mobility of 1 μm polystyrene particles was found to be −2±0.4 10−8 cm4/(V2 s). In the future, such quantitative treatment will allow for the elucidation of unique insights and rational design of devices.

Extension of External Voltage Control of Electroosmosis to High-pH Buffers

Control of electroosmosis by an applied external voltage field in capillary electrophoresis has been limited to buffer pH approximately 5 or below. This poor control at higher pH is caused by a high density of surface charge induced by chemical equilibrium overwhelming the influence of the external voltage-induced charges within the electric double layer. A tert-butyldiphenylchlorosilane treatment was used on fused-silica capillaries to minimize chemically generated ζ-potential where this treatment allowed for control of electroosmosis over a large pH range (2-10). Blocking the surface with traditional polymer-based surface treatments does not work in this application since the polymers increase the viscosity within the electric double layer and impede electroosmosis. The surface created by this reaction is demonstrated in extremely narrow capillaries, down to 2-μm internal diameter. The treated surface is sterically hindered against acid- and base-catalyzed degradation reactions typically associated with organosilanes. This results in a surface that was stable to experimental buffer pH extremes, from pH 3 to pH 10, and was stable for at least 8 weeks exposed to both solution and air.

Manipulation and capture of Aβ amyloid fibrils and monomers by DC insulator gradient dielectrophoresis (DC-iGDEP)

Here we report a novel method for the manipulation and concentration of Aβ amyloid fibrils, implicated in Alzheimers disease, using DC insulating gradient dielectrophoresis (DC-iGDEP). Fibril enrichment was found to be ∼400%. Simulations suggest that capture of the full range of amyloid protein aggregates is possible with optimized device design.

Pressure-assisted electrokinetic supercharging for the enhancement of non-steroidal anti-inflammatory drugs.

Electrokinetic supercharging (EKS) combines field-amplified sample injection with transient isotachophoresis (tITP) to create a powerful on-line preconcentration technique for capillary electrophoresis. In this work, EKS is enhanced with a positive pressure (pressure-assisted EKS, or PA-EKS) during injection to improve stacking of non-steroidal anti-inflammatory drugs (NSAIDs). Several parameters, including buffer composition and concentration, terminating electrolyte, organic modifier, and injection voltage and injection time of both terminating electrolyte and sample were optimized. Detection limits for seven NSAIDs were determined and an enhancement in sensitivity of almost 50,000-fold was obtained. The PA-EKS method has the potential to be a simple MS compatible preconcentration method to improve the sensitivity of CE.

Phase sensitive enhancement for biochemical detection using rotating paramagnetic particle chains

Paramagnetic particle suspensions placed in a rotating unidirectional magnetic field form magnetic chains that rotate with the same frequency as the field. The motion of the fluid and particles surrounding the chain differs in phase and frequency from the chain rotation, a phenomenon that forms the basis of a sensitive detection scheme. Fluorescent particles that bind to the paramagnetic particles through their surface chemistry are used to demonstrate the concept. Epifluorescence video microscopy is used to capture images of the rotating chains. View windows placed over sequential images of rotating chains allows for measurement of the fluorescence brightness in the window, which is composed of periodic signal from the steady rotation of the chain plus the background. A lock-in reference synchronized to the chain rotation is used to enhance the fluorescence signal from chain and improve signal to noise. Two different modes of chain rotation and signal collection are demonstrated. This technique can be us...