Microfluidic Generation Research Articles

Microfluidic Production of Spherical and Nonspherical Fat Particles by Thermal Quenching of Crystallizable Oils

We report the microfluidic production of spherical and nonspherical fat particles from crystallizable oils. The method is based on microfluidic generation of oil droplets at a cross-junction followed by thermal solidification downstream in a microcapillary. We vary the drop production conditions and the device temperature and demonstrate that the size, shape, and crystallinity can be controlled. By measuring thermal gradients in the microcapillary, we show that crystalline fat particles are best produced when the device temperature is below the onset temperature of bulk fat crystallization. To produce monodisperse nonspherical fat particles, we find that the carrier fluid flow rate needs to be sufficiently high to provide strong hydrodynamic forces to transport the confined rod-like particles. We identify the scaling relationship between geometric confinement and particle elasticity necessary to maintain the nonspherical shape. Thus, our study provides guidelines for the production of spherical and nonspherical fat particles that can be potentially used for controlling microstructure, rheology, and drug encapsulation in foods, cosmetics, and pharmaceutical creams that employ crystallizable oils.

Microfluidic flow-free generation of chemical concentration gradients

This paper presents a class of novel microfluidic concentration gradient generation (CGG) devices that create temporally stable chemical concentration gradients with complex shapes in a flow-free environment. The devices feature a two-layer channel design and the incorporation of a semipermeable membrane, which effectively segregates the concentration gradient region in the lower layer from the flow of reagent sample (simply “sample” onward) and buffer in the upper layer. In the mean time, free diffusion across the membrane constantly replenishes sample and buffer to maintain a stable concentration. The shapes of the concentration gradients are controlled by the geometries of the micro-channels and chambers. Concentration gradients with complex shapes can be achieved by piecewise combining constituent gradients with elementary shapes. Capable of generating concentration gradients in a flow-free environment, our devices eliminate undesirable flow stimulation on biological cells under investigation, while maintaining a stable chemical environment for cell studies.

Engineering microfluidic concentration gradient generators for biological applications

This paper reviews the latest developments in the design and fabrication of concentration gradient generators for microfluidics-based biological applications. New gradient generator designs and their underlying mass transport principles are discussed. The review provides a blueprint for design considerations of concentration gradients in different applications, specifically biological studies. The paper discusses the basic phenomena associated with microfluidic gradient generation and the different gradient generation modes used in static and dynamic biological assays. Finally, the paper summarizes all factors to consider when using concentration gradient generators and puts forward perspectives on the future development of these devices.

Circuit model for microfluidic bubble generation under controlled pressure

We explore the microfluidic generation of bubbles in a flow-focusing junction using a pressure-controlled device rather than the more common flow rate-controlled devices. This device is a prototype for extending microfluidic drop generation methods to molten polymers. We show that the bubble generation process is highly sensitive to pressure: small changes in pressure induce large changes in bubble size and bubble formation frequency. A simple resistance circuit model can explain this pressure dependence. Briefly, we show that bubble generation is possible only within a finite pressure range. Near the ends of this pressure range, the ratio of the flow rates of the dispersed to continuous phase is highly sensitive to pressure, and therefore so also is the bubble generation process. The circuit model offers a way to use existing models of drop generation (which are based on flow rate-controlled operation) to predict pressure-controlled operation. We also examine drop formation using a highly viscous polymer as the dispersed phase. Drops are formed far downstream of the flow-focusing junction, and they are far smaller than the microfluidic channel dimensions. These results suggest that existing microfluidic drop generation methods may be exploited to make complex particles from thermoplastic polymers.

Electrical power generation by mechanically modulating electrical double layers

Since Michael Faraday and Joseph Henry made their great discovery of electromagnetic induction, there have been continuous developments in electrical power generation. Most people today get electricity from thermal, hydroelectric, or nuclear power generation systems, which use this electromagnetic induction phenomenon. Here we propose a new method for electrical power generation, without using electromagnetic induction, by mechanically modulating the electrical double layers at the interfacial areas of a water bridge between two conducting plates. We find that when the height of the water bridge is mechanically modulated, the electrical double layer capacitors formed on the two interfacial areas are continuously charged and discharged at different phases from each other, thus generating an AC electric current across the plates. We use a resistor-capacitor circuit model to explain the results of this experiment. This observation could be useful for constructing a micro-fluidic power generation system in the near future.

A simple and versatile microfluidic cell density gradient generator for quantum dot cytotoxicity assay

In this work, a simple and versatile microfluidic cell density gradient generator was successfully developed for cytotoxicity of quantum dots (QDs) assay. The microfluidic cell density gradient generator is composed of eight parallel channels which are respectively surrounded by 1-8 microwells with optimized length and width. The cells fall into microwells by gravity and the cell densities are obviously dependent of microwell number. In a case study, HepG2 and MCF-7 cells were successfully utilized for generating cell density gradients on the microfluidic chip. The microfluidic cell density gradient generator was proved to be easily handled, cell-friendly and could be used to conduct the subsequent cell-based assay. As a proof-of-concept, QD cytotoxicity was evaluated and the results exhibited obvious cell density-dependence. For comparison, QD cytotoxicity was also investigated with a series of cell densities infused by pipette tips. Higher reproducibility was observed on the microfluidic cell density gradient generator and cell density was demonstrated to be a vital factor in cytotoxic study. With higher efficiency, controllability and reproducibility, the microfluidic cell density gradient generator could be integrated into microfluidic analysis systems to promote chip-based biological assay.

Cardiac-like flow generator for long-term imaging of endothelial cell responses to circulatory pulsatile flow at microscale

In vitro models of circulatory hemodynamics are required to mimic the microcirculation for study of endothelial cell responses to pulsatile shear stress by live cell imaging. This study reports the design, fabrication and characterisation of a microfluidic device that generates cardiac-like flow in a continuous culture system with a circulatory volume of only 2-3 μL. The device mimics a single chamber heart, with the following cardiac phases: (1) closure of the ventricle inlet valve, (2) contraction of the ventricle (systole) followed by opening of the outlet valve and (3) relaxation of the ventricle (diastole) with opening of the inlet valve whilst the outlet valve remains closed. Periodic valve states and ventricular contractions were actuated by microprocessor controlled pneumatics. The time-dependent velocity-field was characterised by micro-particle image velocimetry (μ-PIV). μ-PIV observations were used to help tune electronic timing of valve states and ventricular contractions for synthesis of an arterial pulse waveform to study the effect of pulsatile shear stress on bovine artery endothelial cells (BAECs). BAECs elongated and aligned with the direction of shear stress after 48 h of exposure to a pulsatile waveform with a maximum shear stress of 0.42 Pa. The threshold for BAECs alignment and elongation under steady (non-pulsatile) flow reported by Kadohama et al. (2006) is 0.7-1.4 Pa. These cells respond to transient shear stress because the time average shear stress of the pulse waveform to generate this morphological response was only 0.09 Pa, well below the steady flow threshold. The microfluidic pulse generator can simulate circulatory hemodynamics for live cell imaging of shear-induced signalling pathways.

A microfluidic concentration-gradient droplet array generator for the production of multi-color nanoparticles

A microfluidic concentration-gradient droplet array generator (CDrAG) with parallel multi-channels and multi-layers was developed with 64 outlet channels producing 33 droplet gradient concentrations. A droplet production rate of 5 × 10(4) min(-1) was obtained, and the RSD value of droplet diameters in 64 groups is 5.5% (n = 64). Using the concentration gradient droplet array as parallel microreactors, 33 Au/Ag ratio nanoparticles were synthesized. The absorption spectra of the Au/Ag nanoparticles shifted from the spectrum of pure gold to one of pure silver. This demonstrates the CDrAG platform's promising potential to produce specific nanoparticle barcodes for high-throughput screening in chemistry, biology and a broad range of life science applications.

Quantification of ovarian cancer markers with integrated microfluidic concentration gradient and imaging nanohole surface plasmon resonance

Nanohole array-based biosensors integrated with a microfluidic concentration gradient generator were used for imaging detection and quantification of ovarian cancer markers. Calibration curves based on controlled concentrations of the analyte were created using a microfluidic stepped diffusive mixing scheme. Quantification of samples with unknown concentration of analyte was achieved by image-intensity comparison with the calibration curves. The biosensors were first used to detect the immobilization of ovarian cancer marker antibodies, and subsequently applied for the quantification of the ovarian cancer marker r-PAX8 (with a limit of detection of about 5 nM and a dynamic range from 0.25 to 9.0 μg.mL(-1)). The proposed biosensor demonstrated the ability of self-generating calibration curves on-chip in an integrated microfluidic platform, representing a further step towards the development of comprehensive lab-on-chip biomedical diagnostics based on nanohole array technology.

Microfluidic generation of magnetoresponsive Janus photonic crystal particles

Janus particles with features of an anisotropic photonic band gap (PBG) structure and magnetic property have been achieved by phase separation and self-assembly of nanoparticles in microfluidic droplets. The resultant particles enable optical encoding and magnetically controllable motion, making them excellent functional encoded particles in biomedical applications.

One-step fabrication of titania hollow spheres by controlled interfacial reaction in a droplet-based microfluidic system

We present a facile ethanol-in-oil droplet-based microfluidic approach for one-step fabrication of titania hollow spheres through controlled interfacial reaction. The method combines microfluidic generation of uniform ethanol-in-oil droplets and subsequent in situ controlled interfacial reaction within the microfluidic channel. Ethanol-based droplets are suspended in an oil continuous phase containing titanium tertabutoxide. The small amount of water in the droplet phase diffuses to the interface leading to hydrolysis and condensation, and titania solidifies around the droplet forming titania microcapsules. The vigorous reaction between titanium tetrabutoxide and water is controlled by analyzing a mass transfer model, and then by selecting suitable continuous and dispersed phases. Highly viscous paraffin oil in combination with a low-viscosity ethanol-based droplet phase facilitates the successful formation of titania at the interface rather than in the continuous phase. This research provides a new approach for the controlled fabrication of titania microcapsules having uniform particle size and unique folded and crumpled structure.

Microfluidic Investigation of BDNF‐Enhanced Neural Stem Cell Chemotaxis in CXCL12 Gradients

In vivo studies have suggested that gradients of CXCL12 (aka stromal cell-derived factor 1α) may be critical for neural stem cell (NSC) migration during brain development and neural tissue regeneration. However, traditional in vitro chemotaxis tools are limited by unstable concentration gradients and the inability to decouple cell migration directionality and speed. These limitations have restricted the reproducible and quantitative analysis of neuronal migration, which is required for mechanism-based studies. Using a microfluidic gradient generator, nestin and Sox-2 positive human embryonic NSC chemotaxis is quantified within a linear and stable CXCL12 gradient. While untreated NSCs are not able to chemotax within CXCL12 gradients, pre-treatment of the cells with brain-derived neurotrophic factor (BDNF) results in significant chemotactic, directional migration. BDNF pre-treatment has no effect on cell migration speed, which averages about 1 μm min(-1). Quantitative analysis determines that CXCL12 concentrations above 9.0 nM are above the minimum activation threshold, while concentrations below 14.7 nM are below the saturation threshold. Interestingly, although inhibitor studies with AMD 3100 revealed that CXCL12 chemotaxis requires receptor CXCR4 activation, BDNF pre-treatment is found to have no profound effects on the mRNA levels or surface presentation of CXCR4 or the putative CXCR7 scavenger receptor. The microfluidic study of NSC migration within stable chemokine concentration profiles provides quantitative analysis as well as new insight into the migratory mechanism underlying BDNF-induced chemotaxis towards CXCL12.

Surface tension effects on submerged electrosprays.

Electrosprays are a powerful technique to generate charged micro/nanodroplets. In the last century, the technique has been extensively studied, developed, and recognized with a shared Nobel price in Chemistry in 2002 for its wide spread application in mass spectrometry. However, nowadays techniques based on microfluidic devices are competing to be the next generation in atomization techniques. Therefore, an interesting development would be to integrate the electrospray technique into a microfluidic liquid-liquid device. Several works in the literature have attempted to build a microfluidic electrospray with disputable results. The main problem for its integration is the lack of knowledge of the working parameters of the liquid-liquid electrospray. The "submerged electrosprays" share similar properties as their counterparts in air. However, in the microfluidic generation of micro/nanodroplets, the liquid-liquid interfaces are normally stabilized with surface active agents, which might have critical effects on the electrospray behavior. In this work, we review the main properties of the submerged electrosprays in liquid baths with no surfactant, and we methodically study the behavior of the system for increasing surfactant concentrations. The different regimes found are then analyzed and compared with both classical and more recent experimental, theoretical and numerical studies. A very rich phenomenology is found when the surface tension is allowed to vary in the system. More concretely, the lower states of electrification achieved with the reduced surface tension regimes might be of interest in biological or biomedical applications in which excessive electrification can be hazardous for the encapsulated entities.

Microfluidic Generation of Droplets with a High Loading of Nanoparticles

Microfluidic approaches for controlled generation of colloidal clusters, for example, via encapsulation of colloidal particles in droplets, have been used for the synthesis of functional materials including drug delivery carriers. Most of the studies, however, use a low concentration of an original colloidal suspension (<10 wt %). Here we demonstrate microfluidic approaches for directly making droplets with moderate (10-25 wt %) and high (>60 wt %) particle concentrations. Three types of microfluidic devices, PDMS flow-focusing, PDMS T-junction, and microcapillary devices, are investigated for direct encapsulation of a high concentration of polystyrene (PS) nanoparticles in droplets. In particular, it is shown that PDMS devices fabricated by soft lithography can generate droplets from a 25 wt % PS suspension, whereas microcapillary devices made from glass capillary tubes are able to produce droplets from a 67 wt % PS nanoparticle suspension. When the PS concentration is between 0.6 and 25 wt %, the size of the droplets is found to change with the oil-to-water flow rate ratio and is independent of the concentration of particles in the initial suspensions. Drop sizes from ~12 to 40 μm are made using flow rate ratios Q(oil)/Q(water) from 20 to 1, respectively, with either of the PDMS devices. However, clogging occurs in PDMS devices at high PS concentrations (>25 wt %) arising from interactions between the PS colloids and the surface of PDMS devices. Glass microcapillary devices, on the other hand, are resistant to clogging and can produce droplets continuously even when the concentration of PS nanoparticles reaches 67 wt %. We believe that our findings indicate useful approaches and guidelines for the controlled generation of emulsions filled with a high loading of nanoparticles, which are useful for drug delivery applications.

Droplet formation in microfluidic T-junction generators operating in the transitional regime. III. Dynamic surfactant effects

This study extends our previous work on droplet generation in microfluidic T-junction generators to include dynamic interfacial tension effects created by the presence of surfactants. In Paper I [T. Glawdel, C. Elbuken, and C. L. Ren, Phys. Rev. E 85, 016322 (2012)], we presented experimental findings regarding the formation process in the squeezing-to-transition regime, and in Paper II [T. Glawdel, C. Elbuken, and C. L. Ren, Phys. Rev. E 85, 016323 (2012)] we developed a theoretical model that describes the performance of T-junction generators without surfactants. Here we study dynamic interfacial tension effects for two surfactants, one with a small molecular weight that adsorbs quickly, and the other with a large molecular weight that adsorbs slowly. Using the force balance developed in Paper II we extract the dynamic interfacial tension from high speed videos obtained during experiments. We then develop a theoretical model to predict the dynamic interfacial tension in microfluidic T-junction generators as a function of the surfactant properties, flow conditions, and generator design. This model is then incorporated into the overall model for generator performance to effectively predict the size of droplets produced when surfactants are present.

Bubble Production Mechanism in a Microfluidic Foam Generator

We present the design and characterization of a microfluidic bubble generator that has the potential of producing monodisperse bubbles in 256 production channels that can operate in parallel. For a single production channel we demonstrate a production rate of up to 4kHz with a coefficient of variation of less than 1%. We observe a two-stage bubble production mechanism: initially the gas spreads onto a shallow terrace, and then overflows into a larger foam collection channel; pinning of the liquid-gas meniscus is observed at the terrace edge, the result being an asymmetric pinch-off. A semiempirical physical model predicts the scaling of bubble size with fluid viscosity and gas pressure from measurements of the pinned meniscus width.

Design of novel microfluidic concentration gradient generators suitable for linear and exponential concentration ranges

A novel microfluidic concentration gradient generator is designed where secondary flow, induced via a surface groove, is used to yield a concentration gradient across the output of the microfluidic device. The concentration gradient generator design consists of a single microfluidic channel with two inputs and a single obliquely angled surface groove within the base of the channel to induce the secondary flow and create the concentration gradient. The design allows a concentration gradient to be chosen, either linear or exponential, at the exit of the microfluidic channel with the shape and dimensions of the surface groove within the channel obtained by numerical optimisation. The designed device has a small footprint, suitable for integration within lab-on-a-chip structures for the delivery of a series of an agent to either (i) a single channel and with a concentration gradient or to (ii) a series of reactors with concentrations across a defined range, for bioscience or pharmaceutical screening applications or for chemical reactions.

Microfluidic Bubble Generation by Acoustic Field for Mixing Enhancement

We demonstrate the bubble generation in a microfluidic channel by both experimental observation and numerical simulations. The microfluidic channel contains a nozzle-shaped actuation chamber with an acoustic resonator profile. The actuation is generated by a piezoelectric disk below the chamber. It was observed that for a steady deionized (DI) water flow driven through the channel, bubbles occurred in the channel when the piezoelectric disk was actuated at frequencies between 1 kHz and 5 kHz. Outside this actuation frequency range, no bubble generation was observed in the channel. The experiment showed that the presence of bubbles in this frequency range could significantly enhance the fluid mixing in the microfluidic channel, which otherwise would not happen at all without the bubbles. To further understand the bubble generation, the flow field in the microchannel was numerically simulated by a two-dimensional model. The numerical results show that there is a low pressure region inside the actuation chamber where water pressure is below the corresponding vapor pressure and thus bubbles can be generated. The bubble generation was also experimentally observed in the microchannel by using a high speed camera.

Microfluidic Generation and Selective Degradation of Biopolymer-Based Janus Microbeads

We describe a microfluidic approach for generating Janus microbeads from biopolymer hydrogels. A flow-focusing device was used to emulsify the coflow of aqueous solutions of one or two different biopolymers in an organic phase to synthesize homo or hetero Janus microbeads. Biopolymer gelation was initiated, in the chip, by diffusion-controlled ionic cross-linking of the biopolymers. Pectin-pectin (homo Janus) and, for the first time, pectin-alginate (hetero Janus) microbeads were produced. The efficiency of separation of the two hemispheres, which reflected mixing and convection phenomena, was investigated by confocal scanning laser microscopy (CSLM) of previously labeled biopolymers. The interface of the hetero Janus structure was clearly defined, whereas that of the homo Janus microbeads was poorly defined. The Janus structure was confirmed by subjecting each microbead hemisphere to specific enzymatic degradation. These new and original microbeads from renewable resources will open up opportunities for studying relationships between combined enzymatic hydrolysis and active compound release.

Arp2/3 Is Critical for Lamellipodia and Response to Extracellular Matrix Cues but Is Dispensable for Chemotaxis

Lamellipodia are sheet-like, leading edge protrusions in firmly adherent cells that contain Arp2/3-generated dendritic actin networks. Although lamellipodia are widely believed to be critical for directional cell motility, this notion has not been rigorously tested. Using fibroblasts derived from Ink4a/Arf-deficient mice, we generated a stable line depleted of Arp2/3 complex that lacks lamellipodia. This line shows defective random cell motility and relies on a filopodia-based protrusion system. Utilizing a microfluidic gradient generation system, we tested the role of Arp2/3 complex and lamellipodia in directional cell migration. Surprisingly, Arp2/3-depleted cells respond normally to shallow gradients of PDGF, indicating that lamellipodia are not required for fibroblast chemotaxis. Conversely, these cells cannot respond to a surface-bound gradient of extracellular matrix (haptotaxis). Consistent with this finding, cells depleted of Arp2/3 fail to globally align focal adhesions, suggesting that one principle function of lamellipodia is to organize cell-matrix adhesions in a spatially coherent manner.