Characterizing dispersion in microfluidic channels

Electroosmotic flow modulation and dispersion of uncharged solutes in soft nanochannel

We perform a systematic study on the modulation of electroosmotic flow (EOF), tuning the selectivity using electrolyte ions and hydrodynamic dispersion of the solute band across the soft nanochannel. The supporting walls of the channel are considered to be hydrophobic and bear non-zero surface charge. For such a channel, the inner side of the supporting rigid walls of the channel are coated with a soft polyelectrolyte layer (PEL). The inhomogeneous distribution of monomers and accompanying volume charge within the PEL is modelled via soft-step function. The dielectric permittivity of the PEL and electrolyte solution are in general different, which in turn leads to the ion partitioning effect. The impact of ion steric effects due to finite sized ions is further accounted through the modified ion activity coefficient. To model the EOF modulation considering the combined impact of the ion steric and ion partitioning effects as well as inhomogeneous distribution of monomers across the PEL, we adopt the modified Poisson-Boltzmann equation as the governing equation for electrostatic potential. The Debye-Bueche model is adopted to study the flow field across the PEL and the Stokes equation governs the EOF outside the PEL. In order to study the impact of the modulated EOF field on the dispersion of uncharged solution, we adopt three different models, i.e., a general 2D convective-diffusion model as well as cross-sectional averaged dispersion models due to Gill and late-time Taylor and Aris. Going beyond the widely employed Debye-Hückel approximation and uniform distribution of the monomer as well as accompanying volume charge, we find the results for the electric double layer (EDL) potential, EOF field and averaged throughput, by tuning the ion selectivity, etc., which is sufficient to analyze the transport of ionized liquid across the channel. The numerical results are supplemented with analytical results for the EDL potential as well as the EOF field under various limiting situations. Besides, we have further shown the impact of the modulated EOF field on the solute dispersion process. We have presented results that highlight the impact of parameters related to EOF field modulation, on solute dispersion governed by a convective-diffusive process, as well as obtaining the results for an effective dispersion coefficient. The dispersion models under the modulated EOF field adopted in the present study can thus be applied to study the dispersion process in engineered microdevices.

Numerical investigation of the solute dispersion in finite-length microchannels with the interphase transport

This paper utilizes a combined approach of the convection-diffusion theory and the moment analysis to conduct a comprehensive investigation of the solute dispersion under the influence of the interphase transport in finitely long inner coated microchannels. The present work has threefold novel contributions: (1) The 2D solute concentration contours in the stationary phase are calculated for the first time to facilitate the understanding the role of the interphase transport in the solute dispersion in the mobile phase. (2) The skewness of the elution curves is investigated to guide the control of solute band shape at the channel outlet. (3) The 2D diffusion-convection theory and zero-dimensional (0D) moment analysis complement each other to present a characterization of the solute dispersion behaviors more comprehensive than that by either of the two methods alone. Parametric studies are performed to clarify the effects of four major parameters related to the interphase transport (i.e., stationary phase Péclet number, interphase transport rate, partition coefficient, and stationary phase thickness) on the solute dispersion characteristics. The results from this study provide a straightforward understanding of the effects of interphase transport on the solute dispersion in finitely long microchannels and are of potential relevance to the design and operation of the microfluidics-based analytical devices.

3D Sugar Printing of Networks Mimicking the Vasculature

The vasculature plays a central role as the highway of the body, through which nutrients and oxygen as well as biochemical factors and signals are distributed by blood flow. Therefore, understanding the flow and distribution of particles inside the vasculature is valuable both in healthy and disease-associated networks. By creating models that mimic the microvasculature fundamental knowledge can be obtained about these parameters. However, microfabrication of such models remains a challenging goal. In this paper we demonstrate a promising 3D sugar printing method that is capable of recapitulating the vascular network geometry with a vessel diameter range of 1 mm down to 150 µm. For this work a dedicated 3D printing setup was built that is capable of accurately printing the sugar glass material with control over fibre diameter and shape. By casting of printed sugar glass networks in PDMS and dissolving the sugar glass, perfusable networks with circular cross-sectional channels are obtained. Using particle image velocimetry, analysis of the flow behaviour was conducted showing a Poisseuille flow profile inside the network and validating the quality of the printing process.

New regimes of dispersion in microfluidics as mediated by travelling temperature waves

We unveil new regimes of dispersion in miniaturized fluidic devices, by considering fluid flow triggered by a travelling temperature wave. When a temperature wave travels along a channel wall, it alters the density and viscosity of the adjacent fluid periodically. Successive expansion–contraction of the fluid volume through a spatio-temporally evolving viscosity field generates a net fluidic current. Based on the temporal evolution of the axial dispersion coefficient, new regimes of dispersion—such as a short-time ‘oscillating regime’ and a large-time ‘stable regime’—have been identified, which are absent in traditionally addressed flows through miniaturized fluidic devices. Our analysis reveals that the oscillation of axial dispersion persists until the variance of species concentration becomes equal to half of the square of the wavelength of the thermal wave. The time period of oscillation in the dispersion coefficient turns out to be a unique function of the thermal wavelength and net flow velocity induced by thermoviscous pumping. The results of this study are likely to contribute towards the improvement of microscale systems that are subjected to periodic temperature variations, including microreactors and DNA amplification devices.

Droplets for Sampling and Transport of Chemical Signals in Biosensing: A Review

The chemical, temporal, and spatial resolution of chemical signals that are sampled and transported with continuous flow is limited because of Taylor dispersion. Droplets have been used to solve this problem by digitizing chemical signals into discrete segments that can be transported for a long distance or a long time without loss of chemical, temporal or spatial precision. In this review, we describe Taylor dispersion, sampling theory, and Laplace pressure, and give examples of sampling probes that have used droplets to sample or/and transport fluid from a continuous medium, such as cell culture or nerve tissue, for external analysis. The examples are categorized, as follows: (1) Aqueous-phase sampling with downstream droplet formation; (2) preformed droplets for sampling; and (3) droplets formed near the analyte source. Finally, strategies for downstream sample recovery for conventional analysis are described.

Patterned surface charges coupled with thermal gradients may create giant augmentations of solute dispersion in electro-osmosis of viscoelastic fluids

Augmenting the dispersion of a solute species and fluidic mixing remains a challenging proposition in electrically actuated microfluidic devices, primarily due to an inherent plug-like nature of the velocity profile under uniform surface charge conditions. While a judicious patterning of surface charges may obviate some of the concerning challenges, the consequent improvement in solute dispersion may turn out to be marginal. Here, we show that by exploiting a unique coupling of patterned surface charges with intrinsically induced thermal gradients, it may be possible to realize giant augmentations in solute dispersion in electro-osmotic flows. This is effectively mediated by the phenomena of Joule heating and surface heat dissipation, so as to induce local variations in electrical properties. Combined with the rheological premises of a viscoelastic fluid that are typically reminiscent of common biofluids handled in lab-on-a-chip-based micro-devices, our results demonstrate that the consequent electro-hydrodynamic forcing may open up favourable windows for augmented hydrodynamic dispersion, which has not yet been unveiled.

Band Broadening Theories in Capillary Electrophoresis

In capillary electrophoresis (CE), analytes are separated along the axis of a single microcapillary by virtue of their differential migration in an applied electric field. CE can also be performed in channels etched on solid substrates such as glass or PDMS and can be integrated into a microfluidic chip with a complex network of electric and fluidic circuits. The measure of quality of a CE instrument is resolution which is limited fundamentally by mixing due to various physical processes. The theoretical limit on the best separation that can be achieved is set by molecular diffusion, which is inevitable. The goal is to eliminate or minimize the other sources of dispersion by design. This chapter provides an overview of the various mechanisms of band broadening and the mathematical results that make it possible to estimate their relative contributions.

Dispersion of charged solute in charged micro- and nanochannel with reversible sorption

We study dispersion of a charged solute in a charged micro- and nanochannel with reversible sorption and derive an analytical solution for mass fraction in the fluid, transport velocity and dispersion coefficient. Electrical double layer formed on the charged surface gives rise to a charge-dependent solute transport by modifying the transverse distribution of the solute. We discuss the effect of sorption and electrical double layer on solute transport and show that the coupling between sorption and electrical double layer gives rise to charge-dependent transport even for a thin double layer. However, in this case, it can be reduced to a simple non-charge-dependent case by introducing the intrinsic sorption equilibrium constant.

Temperature change rate actuated bubble mixing for homogeneous rehydration of dry pre-stored reagents in centrifugal microfluidics

In centrifugal microfluidics, dead volumes in valves downstream of mixing chambers can hardly be avoided. These dead volumes are excluded from mixing processes and hence cause a concentration gradient. Here we present a new bubble mixing concept which avoids such dead volumes. The mixing concept employs heating to create a temperature change rate (TCR) induced overpressure in the air volume downstream of mixing chambers. The main feature is an air vent with a high fluidic resistance, representing a low pass filter with respect to pressure changes. Fast temperature increase causes rapid pressure increase in downstream structures pushing the liquid from downstream channels into the mixing chamber. As air further penetrates into the mixing chamber, bubbles form, ascend due to buoyancy and mix the liquid. Slow temperature/pressure changes equilibrate through the high fluidic resistance air vent enabling sequential heating/cooling cycles to repeat the mixing process. After mixing, a complete transfer of the reaction volume into the downstream fluidic structure is possible by a rapid cooling step triggering TCR actuated valving. The new mixing concept is applied to rehydrate reagents for loop-mediated isothermal amplification (LAMP). After mixing, the reaction mix is aliquoted into several reaction chambers for geometric multiplexing. As a measure for mixing efficiency, the mean coefficient of variation (C[combining macron]V[combining macron], n = 4 LabDisks) of the time to positivity (t_p) of the LAMP reactions (n = 11 replicates per LabDisk) is taken. The C[combining macron]V[combining macron] of the t_p is reduced from 18.5% (when using standard shake mode mixing) to 3.3% (when applying TCR actuated bubble mixing). The bubble mixer has been implemented in a monolithic fashion without the need for any additional actuation besides rotation and temperature control, which are needed anyhow for the assay workflow.

Bubble-based acoustic micropropulsors: active surfaces and mixers

Acoustic micropropulsors present great potential for microfluidic applications. The propulsion is based on encapsulated 20 μm bubbles excited by a contacless ultrasonic transducer. The vibrating bubbles then generate a powerful streaming flow, with speeds 1-100 mm s^-1 in water, through the action of viscous stresses. In this paper we introduce a full toolbox of micropropulsors using a versatile three-dimensional (3D) microfabrication setup. Doublets and triplets of propulsors are introduced, and the flows they generate are predicted by a theoretical hydrodynamic model. We then introduce whole surfaces covered with propulsors, which we term active surfaces. These surfaces are excited by a single ultrasonic wave, can generate collective flows and may be harnessed for mixing purposes. Several patterns of propulsors are tested, and the flows produced by the two most efficient mixers are predicted by a simple theoretical model based on flow singularities. In particular, the vortices generated by the most efficient pattern, an L-shaped mixer, are analysed in detail.

Measurement of flow velocity and inference of liquid viscosity in a microfluidic channel by fluorescence photobleaching

We present a simple, noninvasive method for simultaneous measurement of flow velocity and inference of liquid viscosity in a microfluidic channel. We track the dynamics of a sharp front of photobleached fluorescent dye using a confocal microscope and measure the intensity at a single point downstream of the initial front position. We fit an exact solution of the advection diffusion equation to the fluorescence intensity recovery curve to determine the average flow velocity and the diffusion coefficient of the tracer dye. The dye diffusivity is correlated to solute concentration to infer rheological properties of the liquid. This technique provides a simple method for simultaneous elucidation of flow velocity and liquid viscosity in microchannels.

Dispersion of a Nanoliter Bolus in Microfluidic Co-Flow

Microfluidic systems enable reactions and assays on the scale of nanoliters. However, at this scale nonuniformities in sample delivery become significant. To determine the fundamental minimum sample volume required for a particular device, a detailed understanding of mass transport is required. Co-flowing laminar streams are widely used in many devices, but typically only in the steady-state. Because establishing the co-flow steady-state consumes excess sample volume and time, there is a benefit to operating devices in the transient state, which predominates as the volume of the co-flow reactor decreases. Analysis of the co-flow transient has been neglected thus far. In this work we describe the fabrication of a pneumatically controlled microfluidic injector constructed to inject a discrete 50nL bolus into one side of a two-stream co-flow reactor. Using dye for image analysis, injections were performed at a range of flow rates from 0.5-10μL/min, and for comparison we collected the co-flow steady-state data for this range. The results of the image analysis were also compared against theory and simulations for device validation. For evaluation, we established a metric that indicates how well the mass distribution in the bolus injection approximates steady-state co-flow. Using such analysis, transient-state injections can approximate steady-state conditions within predefined errors, allowing straight forward measurements to be performed with reduced reagent consumption.

An investigation into dispersion upon switching between solvents within a microfluidic system using a chemically resistant integrated optical refractive index sensor

A planar Bragg grating device has been developed that is capable of detecting changes in the refractive index of a wide range of fluids including solvents, acids and bases. The integration of this high precision refractive index sensor within a chemically resistant microfluidic flow system has enabled the investigation of diverse fluid interactions. By cycling between different solvents, both miscible and immiscible, within the microfluidic system it is shown that the previous solvent determines the nature of the refractive index profile across the transition in composition. This solvent dispersion effect is investigated with particular attention to the methanol-water transition, where transients in refractive index are observed that are an order of magnitude larger in amplitude than the difference between the bulk fluids. The potential complications of such phenomenon are discussed together with an example of a device that exploits this effect for the unambiguous composition measurement of a binary solvent system.

Computational study of velocity profile obtained in microfluidic channel bearing a fluidic transistor: toward highly resolved electrophoretic separation

The present work is a computational study of velocity profiles in microfluidic channels bearing field flow effect transistors (FFET). In particular, this work investigates perturbations and distortions of the sample band during electrophoretic transport in a rectangular separation channel. The EOF heterogeneity and its induced pressure render the predictions of the analytical performances rather complex. In this context, we propose a systematic numerical inquiry that focuses on the distribution of the velocities for several geometries and EOF modulations. We compare the calculated parabolic velocity profiles to the bare glass microchips. Here, the reported parabolic velocity profiles are coherent with recent experimental results that have been published elsewhere. From the presented equations, in such active hybrid microfluidic chip that integrates a FFET gate layer, separation can be optimized by playing on the gate coverage ratio. The flow fields obtained from analytical models allow further investigations about the efficiency and resolution during electrophoresis. The resulting induced pressure gradient and the associated band broadening underline the need to optimize the resolution in the detriment of the efficiency in such active microfluidic chips.

Electromigration dispersion in a capillary in the presence of electro-osmotic flow

The differential migration of ions in an applied electric field is the basis for separation of chemical species by capillary electrophoresis. Axial diffusion of the concentration peak limits the separation efficiency. Electromigration dispersion is observed when the concentration of sample ions is comparable to that of the background ions. Under such conditions, the local electrical conductivity is significantly altered in the sample zone making the electric field, and therefore, the ion migration velocity concentration dependent. The resulting nonlinear wave exhibits shock like features, and, under certain simplifying assumptions, is described by Burgers' equation (S. Ghosal and Z. Chen Bull. Math. Biol. 201072, pg. 2047). In this paper, we consider the more general situation where the walls of the separation channel may have a non-zero zeta potential and are therefore able to sustain an electro-osmotic bulk flow. The main result is a one dimensional nonlinear advection diffusion equation for the area averaged concentration. This homogenized equation accounts for the Taylor-Aris dispersion resulting from the variation in the electro-osmotic slip velocity along the wall. It is shown that in a certain range of parameters, the electro-osmotic flow can actually reduce the total dispersion by delaying the formation of a concentration shock. However, if the electro-osmotic flow is sufficiently high, the total dispersion is increased because of the Taylor-Aris contribution.

Analytical and biological methods for probing the blood-brain barrier

The blood-brain barrier (BBB) is an important interface between the peripheral and central nervous systems. It protects the brain against the infiltration of harmful substances and regulates the permeation of beneficial endogenous substances from the blood into the extracellular fluid of the brain. It can also present a major obstacle in the development of drugs that are targeted for the central nervous system. Several methods have been developed to investigate the transport and metabolism of drugs, peptides, and endogenous compounds at the BBB. In vivo methods include intravenous injection, brain perfusion, positron emission tomography, and microdialysis sampling. Researchers have also developed in vitro cell-culture models that can be employed to investigate transport and metabolism at the BBB without the complication of systemic involvement. All these methods require sensitive and selective analytical methods to monitor the transport and metabolism of the compounds of interest at the BBB.

Microfluidic devices: useful tools for bioprocess intensification

The dawn of the new millennium saw a trend towards the dedicated use of microfluidic devices for process intensification in biotechnology. As the last decade went by, it became evident that this pattern was not a short-lived fad, since the deliverables related to this field of research have been consistently piling-up. The application of process intensification in biotechnology is therefore seemingly catching up with the trend already observed in the chemical engineering area, where the use of microfluidic devices has already been upgraded to production scale. The goal of the present work is therefore to provide an updated overview of the developments centered on the use of microfluidic devices for process intensification in biotechnology. Within such scope, particular focus will be given to different designs, configurations and modes of operation of microreactors, but reference to similar features regarding microfluidic devices in downstream processing will not be overlooked. Engineering considerations and fluid dynamics issues, namely related to the characterization of flow in microchannels, promotion of micromixing and predictive tools, will also be addressed, as well as reflection on the analytics required to take full advantage of the possibilities provided by microfluidic devices in process intensification. Strategies developed to ease the implementation of experimental set-ups anchored in the use of microfluidic devices will be briefly tackled. Finally, realistic considerations on the current advantages and limitation on the use of microfluidic devices for process intensification, as well as prospective near future developments in the field, will be presented.

A chemical signal generator for resolving temporal dynamics of single cells

To investigate rapid cell signaling, analytical methods are required that can generate repeatable chemical signals for stimulating live cells with high temporal resolution. Here, we present a chemical signal generator based on hydrodynamic gating, permitting flexible stimulation of single adherent cells with a temporal resolution of 20 ms. Studies of adenosine triphosphate (ATP)-induced calcium signaling in HeLa cells were demonstrated using this developed method. Consecutive treatment of the cells with ATP pulses of 20 or 1 s led to an increase of latency, which might be another indicator of receptor desensitization in addition to the decrease in the amplitude of calcium spikes. With increasing duration of ATP pulses from milliseconds to a few seconds, the cellular responses transitioned from single calcium spikes to calcium oscillation gradually. We expected this method to open up a new avenue for potential investigation of rapid cell signaling.

Development of a microfluidic cell-based biosensor integrating a millisecond chemical pulse generator

The use of cell-based biosensors is usually limited by agonist-induced desensitization of cell-surface receptors. In this work, a microfluidic cell-based biosensor (μCBB) was developed for the detection of ATP in liquid environments. It consists of a millisecond chemical pulse generator for sample introduction in a pulsatile manner and a single NIH-3T3 cell expressing endogenous P2Y receptors as the sensing element. ATP solutions were used to simulate input signals for investigating the μCBB. By controlling negative pressures on two outlets of a cross-shaped microfluidic chip, pulses of ATP solutions were generated based on hydrodynamic gated injection. With ATP pulses of 100 ms every 50s, the amplitude of the resulting calcium spikes maintained at a similar level, suggesting that the receptor desensitization was minimized. Consequently, the developed μCBB could be used for detecting pulsatile samples with extended use times. The sensitivity of the μCBB for detecting ATP was further determined and the cellular responses to millisecond ATP pulses were investigated in comparison to long-term stimulations.

Convection-driven generation of long-range material gradients

Natural materials exhibit anisotropy with variations in soluble factors, cell distribution, and matrix properties. The ability to recreate the heterogeneity of the natural materials is a major challenge for investigating cell-material interactions and for developing biomimetic materials. Here we present a generic fluidic approach using convection and alternating flow to rapidly generate multi-centimeter gradients of biomolecules, polymers, beads and cells and cross-gradients of two species in a microchannel. Accompanying theoretical estimates and simulations of gradient growth provide design criteria over a range of material properties. A poly(ethylene-glycol) hydrogel gradient, a porous collagen gradient and a composite material with a hyaluronic acid/gelatin cross-gradient were generated with continuous variations in material properties and in their ability to regulate cellular response. This simple yet generic fluidic platform should prove useful for creating anisotropic biomimetic materials and high-throughput platforms for investigating cell-microenvironment interactions.