Micro-thermal focusing field-flow fractionation

Field-flow fractionation and hydrodynamic chromatography on a microfluidic chip

We present gravitational field-flow fractionation and hydrodynamic chromatography of colloids eluting through 18 μm microchannels. Using video microscopy and mesoscopic simulations, we investigate the average retention ratio of colloids with both a large specific weight and neutral buoyancy. We consider the entire range of colloid sizes, including particles that barely fit in the microchannel and nanoscopic particles. Ideal theory predicts four operational modes, from hydrodynamic chromatography to Faxén-mode field-flow fractionation. We experimentally demonstrate, for the first time, the existence of the Faxén-mode field-flow fractionation and the transition from hydrodynamic chromatography to normal-mode field-flow fractionation. Furthermore, video microscopy and simulations show that the retention ratios are largely reduced above the steric-inversion point, causing the variation of the retention ratio in the steric- and Faxén-mode regimes to be suppressed due to increased drag. We demonstrate that theory can accurately predict retention ratios if hydrodynamic interactions with the microchannel walls (wall drag) are added to the ideal theory. Rather than limiting the applicability, these effects allow the microfluidic channel size to be tuned to ensure high selectivity. Our findings indicate that particle velocimetry methods must account for the wall-induced lag when determining flow rates in highly confining systems.

Inertial lift enhanced phase partitioning for continuous microfluidic surface energy based sorting of particles

A new microfluidics technique that exploits the selectivity of phase partitioning and high-speed focusing capabilities of the inertial effects in flow was developed for continuous label-free sorting of particles and cells. Separations were accomplished by introducing particles at the interface of polyethylene glycol (PEG) and dextran (DEX) phases in rectangular high aspect-ratio microfluidic channels and allowing them to partition to energetically favorable locations within the PEG phase, DEX phase or interface at the center of the microchannel. Separation of partitioned particles was further enhanced via inertial lift forces that develop in high aspect-ratio microchannels that move particles to equilibrium positions close to the outer wall. Combining phase partitioning with inertial focusing ensures selectivity is possible using phase partitioning with sufficient throughput (at least an order of magnitude greater than phase partitioning alone) for application in the clinical and research setting. Using this system we accomplished separation of 15 μm polystyrene (PS) particles from 1-20 μm polymethylmethacrylate (PMMA) particles. Results confirm the feasibility of separation based on phase partitioning and enhancement of separation via inertial focusing. Approximately 86% of PS particles were isolated within the PEG phase whereas 78% of PMMA particles were isolated within the DEX phase. When a binary mixture of PS and PMMA was introduced within the device, ~83% of PS particles were isolated in the PEG phase and ~74% of PMMA particles were isolated in the DEX phase. These results confirm the feasibility of this technique for rapid and reliable separation of particles and potentially cells.

Relaxation of microparticles exposed to hydrodynamic forces in microfluidic conduits

The behavior of microparticles exposed to gravitational and lift forces and to the velocity gradient in flow velocity profile formed in microfluidic conduits is studied from the viewpoint of the transient period (the relaxation) between the moment at which a particle starts to be transported by the hydrodynamic flow and the time at which it reaches an equilibrium position, characterized by a balance of all active forces. The theoretical model allowing the calculation of the relaxation time is proposed. The numerical calculus based on the proposed model is compared with the experimental data obtained under different experimental conditions, namely, for different lengths of microfluidic channels, different average linear velocities of the carrier liquid, and different sizes and densities of the particles used in the study. The results are important for the optimization of microfluidic separation units such as microthermal field-flow fractionation channels in which the separation or manipulation of the microparticles of various origin, synthetic, natural, biological, etc., is performed under similar experimental conditions but by applying an additional thermodynamic force.

Protein aggregation: pathways, induction factors and analysis

Control and analysis of protein aggregation is an increasing challenge to pharmaceutical research and development. Due to the nature of protein interactions, protein aggregation may occur at various points throughout the lifetime of a protein and may be of different quantity and quality such as size, shape, morphology. It is therefore important to understand the interactions, causes and analyses of such aggregates in order to control protein aggregation to enable successful products. This review gives a short outline of currently discussed pathways and induction methods for protein aggregation and describes currently employed set of analytical techniques and emerging technologies for aggregate detection, characterization and quantification. A major challenge for the analysis of protein aggregates is that no single analytical method exists to cover the entire size range or type of aggregates which may appear. Each analytical method not only shows its specific advantages but also has its limitations. The limits of detection and the possibility of creating artifacts through sample preparation by inducing or destroying aggregates need to be considered with each method used. Therefore, it may also be advisable to carefully compare analytical results of orthogonal methods for similar size ranges to evaluate method performance.

Micro-thermal field-flow fractionation of bacteria

The retention of Staphylococcus epidermidis bacteria cells, achieved with the use of micro-thermal field-flow fractionation and described in this paper, represents the first experimental proof that the separation and characterization of the bio-macromolecules and biological particles is possible by exploiting Ludwig-Soret effect of thermal diffusion. The experiments were carried out under gentle experimental conditions preventing the denaturation of the bacteria. Lift forces, appearing at high linear velocities of the carrier liquid, generated the focusing mechanism of the retention which resulted in high-speed and high-performance separation performed in less than 10 min.

Separation of bacteria in temperature gradient: Micro-Thermal Focusing Field-Flow Fractionation

The separation of Staphylococcus epidermidis and Rhodococcus erythropolis bacteria was achieved with the use of Micro-Thermal Focusing Field-Flow Fractionation. This is the first performance of separation exploiting the Ludwig-Soret effect (thermal diffusion) of living biological cells, combined with lift forces and resulting in the focusing mechanism of separation. The experiments were carried out under carefully chosen experimental conditions preventing the denaturation of the bacteria.

Continuous separation of particles using a microfluidic device equipped with flow rate control valves

We propose herein an improved microfluidic system for continuous and precise particle separation. We have previously proposed a method for particle separation called "pinched flow fractionation." Using the previously reported method, particles can be continuously separated according to differences in their diameters, simply by introducing liquid flows with and without particles into a specific microchannel structure. In this study, we incorporated PDMS membrane microvalves for flow rate control into the microfluidic device to improve the separation accuracy. By adjusting the flow rates distributed to each outlet, target particles could be precisely collected from the desired outlet. We succeeded in separating micron and submicron-size polymer particles. This method can be used widely for continuous and precise separation of various kinds of particles, and can function as an important part of microfluidic systems.

Elimination of edge effects in micro-thermal field-flow fractionation channel of low aspect ratio by splitting the carrier liquid flow into the main central stream and the thin stream layers at the side channel walls

An optimized construction of the separation channel for micro-thermal field-flow fractionation (FFF) was proposed and studied experimentally. The sample is injected in such a manner that its zone moves along the channel only in the main central stream where the flow velocity profile in the plane parallel to the main accumulation wall is practically flat. This central stream is separated from the contact with the side walls of the channel by thin flowing layers of the free carrier liquid. The retained species do not reach the thin liquid streams at the side walls where the flow rate decreases rapidly to achieve zero at the side wall according to the established 3D flow velocity profile. Such a construction of the channel allows one to reduce the aspect ratio (the ratio of the channel breadth b to its thickness w) without increasing the zone broadening. The hydrodynamic splitting of the outlet streams allows one not only to increase the concentration of the detected species but also the determination of the sign of Soret coefficient.