Application of generalized dispersion theory to vortex chromatography.

Acoustically induced secondary flows are applied to enhance lateral mass transfer beyond the relatively slow diffusion. This has the goal to reduce convective axial dispersion and the resulting band broadening which, in turn, limits the performance of column chromatography. Traditional approaches based on Taylor-Aris model are limited to one-dimensional rectilinear (unidirectional) tube- or channel-flows. We therefore apply the generalized dispersion theory (GDT) allowing for prediction of the dependence of potentially improved performance on the characteristics of the induced secondary flow, channel geometry and solute properties as well as providing qualitative physical insight into the role of lateral flows. Results corroborate agreement with our experimental observations (residual standard deviation, Sres = 3.88) and demonstrate the advantage of applying GDT relative to 3D time-dependent simulations.

Autonomous capillary microfluidic devices with constant flow rate and temperature-controlled valving.

In this paper, we report on a capillary microfluidic device with constant flow rate and temperature-triggered stop valve function. It contains a PDMS channel that was grafted by a thermo-responsive polymer poly(N-isopropylacrylamide) (PNIPAm). The channel exhibits a constant capillary filling speed. By locally increasing the temperature in the channel from 20 °C to 37 °C using a microfabricated heater, a change of the surface wettability from hydrophilic to hydrophobic is obtained creating a hydrophobic stop valve. The valve can be reopened by lowering the temperature. The device is simple to fabricate and can be used as an actuatable capillary pump operating around room temperature. To understand the constant capillary filling speed, we performed contact angle measurements, in which we found slow wetting kinetics of PNIPAm-g-PDMS surfaces at temperatures below the lower critical solution temperature (LCST) of PNIPAm and fast wetting kinetics above the LCST. We interpret this as the result of the diffusive hydration process of PNIPAm below the LCST and the absence of hydration on the hydrophobic PNIPAm thin layer above the LCST.

Inducing AC-electroosmotic flow using electric field manipulation with insulators.

Classically, the configuration of electrodes (conductors) is used as a means to determine AC-electroosmotic flow patterns. In this paper, we use the configuration of insulator materials to achieve AC-electroosmotic flow patterning in a novel approach. We apply AC electric fields between parallel electrodes situated on the top and bottom of a microfluidic channel and separated by an insulating material. Channels of various cross-sectional shapes (e.g. rectangular and parallelogram) were fabricated by shaping the insulating material between the electrodes. We found that vortex flow patterns are induced depending on the cross-sectional shape of the channel. A bell-shaped design with non-orthogonal corners gave rise to 2 vortices, whereas in a channel with a parallelogram shaped cross-section, only a single vortex was observed. The vortices were experimentally observed by analysing the 3D trajectories of fluorescent microparticles. From a theoretical analysis, we conclude that flow shaping is primarily caused by shaping the electrical field lines in the channel.

Reduction of Taylor-Aris dispersion by lateral mixing for chromatographic applications.

Chromatographic columns are suffering from Taylor-Aris dispersion, especially for slowly diffusing molecules such as proteins. Since downscaling the channel size to reduce Taylor-Aris dispersion meets fundamental pressure limitations, new strategies are needed to further improve chromatography beyond its current limits. In this work we demonstrate a method to reduce Taylor-Aris dispersion by lateral mixing in a newly designed silicon AC-electroosmotic flow mixer. We obtained a reduction in κaris by a factor of three in a 40 µm × 20 µm microchannel, corresponding to a plate height gain of 2 to 3 under unretained conditions at low to high Pe values. We also demonstrate an improvement of a reverse-phase chromatographic separation of coumarins.

Controlled pharmacokinetic anti-cancer drug concentration profiles lead to growth inhibition of colorectal cancer cells in a microfluidic device.

We present a microfluidic device to expose cancer cells to a dynamic, in vivo-like concentration profile of a drug, and quantify efficacy on-chip. About 30% of cancer patients receive drug therapy. In conventional cell culture experiments drug efficacy is tested under static concentrations, e.g. 1 µM for 48 hours, whereas in vivo, drug concentration follows a pharmacokinetic profile with an initial peak and a decline over time. With the rise of microfluidic cell culture models, including organs-on-chips, there are opportunities to more realistically mimic in vivo-like concentrations. Our microfluidic device contains a cell culture chamber and a drug-dosing channel separated by a transparent membrane, to allow for shear stress-free drug exposure and label-free growth quantification. Dynamic drug concentration profiles in the cell culture chamber were controlled by continuously flowing controlled concentrations of drug in the dosing channel. The control over drug concentrations in the cell culture chambers was validated with fluorescence experiments and numerical simulations. Exposure of HCT116 colorectal cancer cells to static concentrations of the clinically used drug oxaliplatin resulted in a sensible dose-effect curve. Dynamic, in vivo-like drug exposure also led to statistically significant lower growth compared to untreated control. Continuous exposure to the average concentration of the in vivo-like exposure seems more effective than exposure to the peak concentration (Cmax) only. We expect that our microfluidic system will improve efficacy prediction of in vitro models, including organs-on-chips, and may lead to future clinical optimization of drug administration schedules.

Mass Transport Determined Silica Nanowires Growth on Spherical Photonic Crystals with Nanostructure-Enabled Functionalities.

A robust and facile method has been developed to obtain directional growth of silica nanowires (SiO2 NWs) by regulating mass transport of silicon monoxide (SiO) vapor. SiO2 NWs are grown by vapor-liquid-solid (VLS) process on a surface of gold-covered spherical photonic crystals (SPCs) annealed at high temperature in an inert gas atmosphere in the vicinity of a SiO source. The SPCs are prepared from droplet confined colloidal self-assembly. SiO2 NW morphology is governed by diffusion-reaction process of SiO vapor, whereby directional growth of SiO2 NWs toward the low SiO concentration is obtained at locations with a high SiO concentration gradient, while random growth is observed at locations with a low SiO concentration gradient. Growth of NWs parallel to the supporting substrate surface is of great importance for various applications, and this is the first demonstration of surface-parallel growth by controlling mass transport. This controllable NW morphology enables production of SPCs covered with a large number of NWs, showing multilevel micro-nano feature and high specific surface area for potential applications in superwetting surfaces, oil/water separation, microreactors, and scaffolds. In addition, the controllable photonic stop band properties of this hybrid structure of SPCs enable the potential applications in photocatalysis, sensing, and light harvesting.

Gradient in the electric field for particle position detection in microfluidic channels.

In this work, a new method to track particles in microfluidic channels is presented. Particle position tracking in microfluidic systems is crucial to characterize sorting systems or to improve the analysis of cells in impedance flow cytometry studies. By developing an electric field gradient in a two parallel electrode array the position of the particles can be tracked in one axis by impedance analysis. This method can track the particle's position at lower frequencies and measure the conductivity of the system at higher frequencies. A 3-D simulation was performed showing particle position detection and conductivity analysis. To experimentally validate the technique, a microfluidic chip that develops a gradient in the electric field was fabricated and used to detect the position of polystyrene particles in one axis and measure their conductivity at low and high frequencies, respectively.