Isoelectric Split-Flow Thin (SPLITT) Fractionation of Proteins

Experiment, theory, and simulation of a flow-electrical-split flow thin particle separation device

Herein, we describe the simulation of a novel flow-electrical-split flow thin (Fl-El-SPLITT) separation device and validate it using existing theory and experimentation for the first time using polystyrene particles of 28 and 1000 nm diameters. The fraction of particles exiting selected ports with DC El-SPLITT is predicted with existing theory, but the theory does not include AC fields, nor does it incorporate the use of crossflows. Using DC fields the El-SPLITT simulation and theory calculated transition points result in the same values. These calculated values accurately predict the experimentally obtained transition point using a 50:50 outlet splitting plane (OSP). Relative to actual experimentally obtained transition points, the calculated values lag behind for a 90:10 OSP, and lead ahead for a 10:90 OSP. The simulation explains trends seen in AC testing, and reasonably predicts the fraction of particles exiting each port. As DC current increases, the amount of AC current required to scatter the particles away from the DC-intended port decreases. The simulation also models a crossflow in a SPLITT system with a DC current applied in a direction opposite the crossflow with some success. Long term steady-state testing without crossflows shows a DC voltage dependent loss of particles. At 8 V DC, total recovery of 28 and 1000 nm particles was 70% and 26%, respectively. This work effectively models a new Fl-El-SPLITT system via Matlab simulation by demonstrating key experimental results such as the influence of DC, AC, and crossflows on the SPLITT separation of polystyrene particles.

Development and Testing of a Continuous Flow-Electrical-Split-Flow Lateral Transport Thin Separation System (Fl-El-SPLITT)

In this work, a new high-volume, continuous particle separation device that separates based upon size and charge is described. Two continuous flow-electrical-split-flow lateral transport thin (Fl-El-SPLITT) device architectures (a platinum electrode on a porous membrane and a porous graphite electrode under a membrane) were developed and shown to improve particle separations over a purely electrical-SPLITT device. The graphite FL-El-SPLITT device architecture achieved the best separation of approximately 60% of small (28 nm) vs large (1000 nm) polystyrene particles. Fl-El-SPLITT (platinum) achieved a 75% separation on a single pass using these same particles. Fl-El-SPLITT (platinum) achieved a moderate 26% continuous separation of U87 glioma cell-derived small extracellular vesicles (EVs) from medium EVs. Control parameter testing showed that El-SPLITT continuously directed particle motility within a channel to exit a selected port based upon the applied voltage using either direct current or alternating current. The transition from one port to the other was dependent upon the voltage applied. Both large and small polystyrene particles transitioned together rather than separating at each of the applied voltages. These data present the first ever validation of El-SPLITT in continuous versus batch format. The Fl-El-SPLITT device architecture, monitoring, and electrical and fluid interfacing systems are described in detail for the first time. Capabilities afforded to the system by the flow addition include enhanced particle separation as well as the ability to filter out small particles or desalinate fluids. High-throughput continuous separations based upon electrophoretic mobility will be streamlined by this new technique that combines electrical and flow fields into a single device.

Particle separation in microfluidics using a switching ultrasonic field

We present a new method for separation of micro-sized constituents with positive acoustic contrast factors in a microfluidic channel using ultrasound. The ultrasound field is switched between the first and third resonant modes of the fluid channel, and the suspended constituents are separated onto the side and center pressure nodal lines according to their sizes or acoustic contrast factors. Initial hydrodynamic focusing of the constituents within a region of the channel near to the side nodal line is a crucial step in this separation method. This new method is shown to provide a novel "parallel-stream" separation of two species of particles with good robustness. Prior numerical simulations provide essential information on this operating region and also the voltage cycle to be applied to the ultrasonic actuators for optimal separation. Experiments were conducted using a prototype of the design with polystyrene microspheres of different sizes to demonstrate the efficiency and robustness of the separation process.

Characterization of nonspecific crossover in split-flow thin channel fractionation

Split-flow thin channel (SPLITT) fractionation is a technique for continuous separation of particles or macromolecules in a fluid stream into fractions according to the lateral migration induced by application of a field perpendicular to the direction of flow. Typical applications have involved isolation of different fractions from a polydisperse sample. Some specialized applications involve the separation of the fraction influenced by the transverse field from the fraction that is not. For example, immunomagnetically labeled biological cells may be separated from nonlabeled cells with the application of a transverse magnetic field gradient. In such cases, it may be critically important to minimize contamination of the labeled cells with nonlabeled cells while at the same time maximizing the throughput. Such contamination is known as nonspecific crossover (NSC) and refers to the real or apparent migration of nonmobile particles or cells across stream lines with the mobile material. The possible mechanisms for NSC are discussed, and experimental results interpreted in terms of shear-induced diffusion (SID) caused by viscous interactions between particles in a sheared flow. It is concluded that SID may contribute to NSC, but that further experiments and mathematical modeling are necessary to more fully explore the phenomenon.

Continuous flow separations in microfluidic devices

Biochemical sample mixtures are commonly separated in batch processes, such as filtration, centrifugation, chromatography or electrophoresis. In recent years, however, many research groups have demonstrated continuous flow separation methods in microfluidic devices. Such separation methods are characterised by continuous injection, real-time monitoring, as well as continuous collection, which makes them ideal for combination with upstream and downstream applications. Importantly, in continuous flow separation the sample components are deflected from the main direction of flow, either by means of a force field (electric, magnetic, acoustic, optical etc.), or by intelligent positioning of obstacles in combination with laminar flow profiles. Sample components susceptible to deflection can be spatially separated. A large variety of methods has been reported, some of these are miniaturised versions of larger scale methods, others are only possible in microfluidic regimes. Researchers now have a diverse toolbox to choose from and it is likely that continuous flow methods will play an important role in future point-of-care or in-the-field analysis devices.

Analytical and preparative applications of magnetic split-flow thin fractionation on several ion-labeled red blood cells

Background

Magnetic Split-flow thin (SPLITT) fractionation is a newly developed technique for separating magnetically susceptible particles. Particles with different field-induced velocities can be separated into two fractions by adjusting applied magnetic forces and flow-rates at inlets and outlets.

Methods

Magnetic particles, Dynabeads, were used to test this new approach of field-induced velocity for susceptibility determination using magnetic SF at different magnetic field intensities. Reference measurements of magnetic susceptibility were made using a superconducting quantum interference device (SQUID) magnetometer. Various ion-labeled red blood cells (RBC) were used to study susceptibility determination and throughput parameters for analytical and preparative applications of magnetic SPLITT fractionation (SF), respectively. Throughputs were studied at different sample concentrations, magnetic field intensities, and channel flow-rates.

Results

The susceptibilities of Dynabeads determined by SPLITT fractionation (SF) were consistent with those of reference measurement using a superconducting quantum interference device (SQUID) magnetometer. Determined susceptibilities of ion-labeled RBC were consistent within 9.6% variations at two magnetic intensities and different flow-rates. The determined susceptibilities differed by 10% from referenced measurements. The minimum difference in magnetic susceptibility required for complete separation was about 5.0 × 10^-6 [cgs]. Sample recoveries were higher than 92%. The throughput of magnetic SF was approximately 1.8 g/h using our experimental setup.

Conclusion

Magnetic SF can provide simple and economical determination of particle susceptibility. This technique also has great potential for cell separation and related analysis. Continuous separations of ion-labeled RBC using magnetic SF were successful over 4 hours. The throughput was increased by 18 folds versus early study. Sample recoveries were 93.1 ± 1.8% in triplicate experiments.

Study of continuous two-dimensional thermal field-flow fractionation of polymers

Two-dimensional thermal field-flow fractionation (2D-ThFFF) is a new instrumental technique devised for continuous fractionation of soluble macromolecules and particles. The sample mixture is introduced into a disc-shaped channel and the separated sample components are collected continuously from the channel outlets. The method is based on a two-dimensional fractionation mechanism with radial and tangential flow components in the channel. The effects of flow components and thermal gradient on the fractionation were studied in the separation of polystyrene samples of different molecular masses using cyclohexane or a binary solvent consisting of 25% ethylbenzene and 75% cyclohexane as carrier. The continuous separation of polystyrene samples was improved with increasing thermal gradient and with the use of slow radial and tangential flow rates. The technique can be applied to preparative continuous separation of macromolecules.

Protein purification by Off-Gel electrophoresis

A novel free-flow protein purification technique based on isoelectric electrophoresis is presented, where the proteins are purified in solution without the need of carrier ampholytes. The gist of the method is to flow protein solutions under an immobilised pH gradient gel (IPG) through which an electric field is applied perpendicular to the direction of the flow. Due to the buffering capacity of the IPG gel, proteins with an isoelectric point (pI) close to pH of the gel in contact with the flow chamber stay in solution because they are neutral and therefore not extracted by the electric field. Other proteins will be charged when approaching the IPG gel and are extracted into the gel by the electric field. Both a demonstration experiment with pI markers and a simulation of the electric field distribution are presented to highlight the principle of the system. In addition, an isoelectric fractionation of an Escherichia coli extract is shown to illustrate the possible applications.

Particle magnetic susceptibility determination using analytical split-flow thin fractionation

Magnetic split-flow thin (SPLITT) fractionation is a newly developed S

Formation of natural pH gradients in a microfluidic device under flow conditions: model and experimental validation

A new isoelectric focusing technique has been developed that incorporates natural pH gradient formation in microfluidic channels under flowing conditions. In conjunction, a one-dimensional finite difference model has been developed that solves a system of algebraic-ordinary differential equations that describe the phenomena occurring in the system, including hydrolysis at the electrodes, buffering effects of weak acids and bases, and mass transport due to both diffusion and electrophoresis. A quantitative, noninvasive, optically based method of monitoring pH gradient formation is presented, and the experimental data generated by this method are found to be in good agreement with model predictions. In addition, the model provides a theoretical explanation for initially unexpected experimental results. Model predictions are also shown to match well with experimental results of microfluidic isoelectric focusing of a single protein species. Accounting for the nonuniform velocity profile, characteristic of pressure-driven flow in microfluidic channels, is found to improve predictions of dynamic pH changes close to the electrodes and overall time required to reach steady state, but to reduce the accuracy of dynamic pH change predictions in other regions of the channel.

Magnetic split-flow thin fractionation of magnetically susceptible particles

We recently built a magnetic separation system to extend the applications of split-flow thin (SPLITT) fractionation to magnetically susceptible particles. Here, we characterize the magnetic SPLITT system using magnetically susceptible particles and ion-labeled particles. The flow axis of separation channel was orientated parallel and perpendicular to gravitational forces to exclude and include, respectively, gravitational effects on separation. Both operating modes were used to test the theory experimentally, with emphasis on the parallel mode. The magnetic susceptibilities of carrier and ion-labeled particles were varied, and various ion-labeled and unlabeled particles were studied experimentally, resulting in successful separation of labeled particles, yeasts, and cells from unlabeled ones. The minimal difference in magnetic susceptibility (delta(chi)) required for complete particle separation was about 1.75 x 10(-5) [cgs], corresponding to about 10(9) labeling ions per particle in this study. The throughput was around 7.2 x 10(8) particles/h using the present setup. Magnetic SPLITT fractionation shows good potential for use in obtaining particles magnetic susceptibilities from a simple theoretical treatment.