Microprocessing of Liquid Plugs for Bio/chemical Analyses

Large-Scale Preparation of Hair Follicle Germs Using a Microfluidic Device

Hair follicle morphogenesis during embryonic development is driven by the formation of hair follicle germs (HFGs) via interactions between epithelial and mesenchymal cells. Bioengineered HFGs are potential tissue grafts for hair regenerative medicine because they can replicate interactions and hair follicle morphogenesis after transplantation. However, a mass preparation approach for HFGs is necessary for clinical applications, given that thousands of de novo hair follicles are required to improve the appearance of a single patient with alopecia. In this study, we developed a microfluidics-based approach for the large-scale preparation of HFGs. A simple flow-focusing microfluidic device allowed collagen solutions containing epithelial and mesenchymal cells to flow and generate collagen microbeads with distinct Janus structures. During the 3 days of culture, the collagen beads contracted owing to cellular traction forces, resulting in collagen- and cell-dense HFGs. The transplantation of HFGs into nude mice resulted in highly efficient de novo hair follicle regeneration. This method provides a scalable and robust tissue graft preparation approach for hair regeneration.

Simple manual roller pump-driven valve-free microfluidic solution exchange system for urgent bioassay

We introduce a simple-to-use manual roller pump (MRP)-driven and valve-free microfluidic system for sequential solution exchange, followed by a bioassay to detect protein. The polydimethylsiloxane (PDMS)/glass-based disposable device comprises a reaction chamber, multiple micro-flow channels (μFCs), and air vents. The practical solution exchange was realized by sequential injection and withdrawal of several solutions into and from the reaction chamber through constricted μFCs by utilizing changing air pressure of an MRP when a small cylindrical roller was pressed and rolled over a soft silicone tube using a finger. Furthermore, we investigated the effect of surface hydrophobicity on solution exchange. A sandwich fluorescence-based immunoassay to detect human interleukin 2 (IL-2) was performed using this simple microfluidic scheme to demonstrate its suitability for analytical bioassays. The system allowed quick IL-2 detection in 20 min in a pre-functionalized device with a detection limit of 80 pg mL⁻¹ and a range of 125 pg mL⁻¹ to 2.0 ng mL⁻¹. We have thus developed a microfluidic scheme that non-experts can efficiently perform and that can be the fundamental module for low-cost bioassays necessary for emergencies and situations where resources are constrained.

We report an easy microfluidic solution exchange system that employs a finger-driven manual roller pump (MRP) and valveless micro-flow structures to enable minimally trained personnel to execute instantaneous stepwise bioassays.

Generation of Concentration Gradients by a Outer-Circumference-Driven On-Chip Mixer

The concentration control of reagents is an important factor in microfluidic devices for cell cultivation and chemical mixing, but it is difficult to realize owing to the characteristics of microfluidic devices. We developed a microfluidic device that can generate concentration gradients among multiple main chambers. Multiple main chambers are connected in parallel to the body channel via the neck channel. The main chamber is subjected to a volume change through a driving chamber that surrounds the main chamber, and agitation is performed on the basis of the inequality of flow caused by expansion or contraction. The neck channel is connected tangentially to the main chamber. When the main chamber expands or contracts, the flow in the main chamber is unequal, and a net vortex is generated. The liquid moving back and forth in the neck channel gradually absorbs the liquid in the body channel into the main chamber. As the concentration in the main chamber changes depending on the pressure applied to the driving chamber, we generated a concentration gradient by arranging chambers along the pressure gradient. This allowed for us to create an environment with different concentrations on a single microchip, which is expected to improve observation efficiency and save space.

Shear-Induced Alignment of Anisotropic Nanoparticles in a Single-Droplet Oscillatory Microfluidic Platform

Flow-induced alignment of shape-anisotropic colloidal particles is of great importance in fundamental research and in the fabrication of structurally anisotropic materials; however, rheo-optical studies of shear-induced particle orientation are time- and labor-intensive and require complicated experimental setups. We report a single-droplet oscillatory microfluidic strategy integrated with in-line polarized light imaging as a strategy for studies of shear-induced alignment of rod-shape nanoparticles. Using an oscillating droplet of an aqueous isotropic suspension of cellulose nanocrystals (CNCs), we explore the effect of the shear rate and suspension viscosity on the flow-induced CNC alignment and subsequent relaxation to the isotropic state. The proposed microfluidic strategy enables high-throughput studies of shear-induced orientations in structured liquid under precisely controlled experimental conditions. The results of such studies can be used in the development of structure-anisotropic materials.

Oscillatory multiphase flow strategy for chemistry and biology

Continuous multiphase flow strategies are commonly employed for high-throughput parameter screening of physical, chemical, and biological processes as well as continuous preparation of a wide range of fine chemicals and micro/nano particles with processing times up to 10 min. The inter-dependency of mixing and residence times, and their direct correlation with reactor length have limited the adaptation of multiphase flow strategies for studies of processes with relatively long processing times (0.5-24 h). In this frontier article, we describe an oscillatory multiphase flow strategy to decouple mixing and residence times and enable investigation of longer timescale experiments than typically feasible with conventional continuous multiphase flow approaches. We review current oscillatory multiphase flow technologies, provide an overview of the advancements of this relatively new strategy in chemistry and biology, and close with a perspective on future opportunities.

On-demand generation and mixing of liquid-in-gas slugs with digitally-programmable composition and size

Microscopic droplets or slugs of mixed reagents provide a convenient platform for performing large numbers of isolated biochemical or chemical reactions for many screening and optimization applications. Myriad microfluidic approaches have emerged for creating droplets or slugs with controllable size and composition, generally using an immiscible carrier fluid to assist with the formation or merging processes. We report a novel device for generation of liquid slugs in air when the use of a carrier liquid is not compatible with the application. The slug generator contains two adjacent chambers, each of which has a volume that can be digitally adjusted by closing selected microvalves. Reagents are filled into the two chambers, merged together into a contiguous liquid slug, ejected at the desired time from the device using gas pressure, and mixed by flowing in a downstream channel. Programmable size and composition of slugs is achieved by dynamically adjusting the volume of each chamber prior to filling. Slug formation in this fashion is independent of fluid properties and can easily be scaled to mix larger numbers of reagents. This device has already been used to screen monomer ratios in supramolecular nanoparticle assembly and radiolabeling conditions of engineered antibodies, and here we provide a detailed description of the underlying device.

Micron scale directional coupler as a transducer for biochemical sensing

A microfabricated directional coupler (DC) was used for the detection of DNA conjugated with quantum dots. Output optical signals from DCs of a wide range of device lengths correspond well to theoretical and simulation results. Even 20 µm-long DC devices could detect changes in the output optical intensity by monitoring the near-field pattern using a CCD camera. The signal was enhanced 60 × using a 1500 µm-long DC device. For large cladding refractive-index changes between air and water, the normalized signal changed cyclically several times between 0 and 1. The results suggest that the DC can be the basis for miniaturized two-dimensionally integrated biochemical sensors.

Microdevice for on-site fish freshness checking based on K-value measurement

An electrochemical microfluidic device with two sensing sites in the upper and lower streams of a flow channel was fabricated to measure the K-value as a means of evaluating the freshness of fish. In this device, plugs of solutions were processed using mechanisms to place a plug at the sensing site and to merge and mix two plugs in a single flow channel. The sums of ATP-related compound concentrations used for the calculation of the K-value were measured at the first and second sensing sites. The ratio of the output currents agreed well with the K-value calculated from predetermined concentrations in standard solutions. The K-values of jack mackerel, yellow tail, and sea bream extracts were then obtained using the device and were found to agree well with those obtained by high-performance liquid chromatography (HPLC). In addition, the changes in the K-value with time were observed to depend strongly on the type of fish for these three fish extracts.

Processing of nanolitre liquid plugs for microfluidic cell-based assays

Plugs, i.e. droplets formed in a microchannel, may revolutionize microfluidic cell-based assays. This study describes a microdevice that handles nanolitre-scale liquid plugs for the preparation of various culture setups and subsequent cellular assays. An important feature of this mode of liquid operation is that the recirculation flow generated inside the plug promotes the rapid mixing of different solutions after plugs are merged, and it keeps cell suspensions homogeneous. Thus, serial dilutions of reagents and cell suspensions with different cell densities and cell types were rapidly performed using nanolitres of solution. Cells seeded through the plug processing grew well in the microdevice, and subsequent plug processing was used to detect the glucose consumption of cells and cellular responses to anticancer agents. The plug-based microdevice may provide a useful platform for cell-based assay systems in various fields, including fundamental cell biology and drug screening applications.

Microfluidic device for robust generation of two-component liquid-in-air slugs with individually controlled composition

Using liquid slugs as microreactors and microvessels enable precise control over the conditions of their contents on short-time scales for a wide variety of applications. Particularly for screening applications, there is a need for control of slug parameters such as size and composition. We describe a new microfluidic approach for creating slugs in air, each comprising a size and composition that can be selected individually for each slug. Two-component slugs are formed by first metering the desired volume of each reagent, merging the two volumes into an end-to-end slug, and propelling the slug to induce mixing. Volume control is achieved by a novel mechanism: two closed chambers on the chip are initially filled with air, and a valve in each is briefly opened to admit one of the reagents. The pressure of each reagent can be individually selected and determines the amount of air compression, and thus the amount of liquid that is admitted into each chamber. We describe the theory of operation, characterize the slug generation chip, and demonstrate the creation of slugs of different compositions. The use of microvalves in this approach enables robust operation with different liquids, and also enables one to work with extremely small samples, even down to a few slug volumes. The latter is important for applications involving precious reagents such as optimizing the reaction conditions for radiolabeling biological molecules as tracers for positron emission tomography. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1007/s10404-010-0617-0) contains supplementary material, which is available to authorized users.

On-chip manipulation of continuous picoliter-volume superparamagnetic droplets using a magnetic force

A microfluidic device for generating monodisperse superparamagnetic droplets and rapidly manipulating desired droplets into designated sub-microchannels by an external magnetic force is described. Superparamagnetic magnetite (Fe3O4) nanoparticles are synthesized by a chemical co-precipitation method. They are well dispersed in the water-phase to form a superparamagnetic fluid that is sheared into picoliter-volume monodisperse superparamagnetic droplets by the oil-phase in a T-junction PDMS microchannel. Superparamagnetic droplets always flow into sub-microchannel 1 due only to laminar flow without a magnetic field. But they are deflected from the direction of laminar flow by a perpendicular magnetic field. The results show that the deflection is proportional to the magnetic field gradient and magnetic nanoparticle concentration, and it is closely related to the magnet position. The observed experimental results make a good match with theoretical analysis. Single or bulk superparamagnetic droplets are successfully manipulated into the designated sub-microchannels 2 and 3 respectively, only by changing the positions of the magnet. Relatively high efficiency is obtained with more than 10 superparamagnetic droplets precisely manipulated per second. This simple and robust apparatus has wide applications in high throughput drug delivery/screening, immunoassay, cell research and synthesis of magnetic microparticles due to good biological compatibility and monodispersity of superparamagnetic droplets.

A fast and efficient microfluidic system for highly selective one-to-one droplet fusion

Microdroplets in microfluidic systems can be used as independent microreactors to perform a range of chemical and biological reactions. However, in order to add new reagents to pre-formed droplets at defined times, to start, modify, or terminate a reaction, it is necessary to perform a controlled fusion with a second droplet. We describe and characterize a simple and extremely reliable technique for the one-to-one fusion of droplet pairs in a microfluidic system at kHz frequencies. The technique does not require special channel treatment, electrical fields or lasers to induce droplet fusion. Instead, we make use of transient states in the stabilization of the droplet interface by surfactant, coupled to a proper geometrical design of a coalescence module, to induce the selective fusion of a droplet stabilized by surfactant (re-injected) with a droplet which is not fully stabilized (generated on-chip). Using a 1.2-fold excess of the surfactant-stabilized droplets approximately 99% of the partially stabilized droplets were fused one-to-one with surfactant-stabilized droplets. Even when the surfactant-stablized droplets were in 5-fold excess, over 96% of the partially stabilized droplets were fused one-to-one. The fused droplet contains enough surfactant to inhibit further fusion events. After fusion, the droplets were fully stabilized by additional surfactant provided in the carrier oil, which allowed the fused droplets to be collected, incubated off-chip and re-injected onto a microfluidic device without any undesired coalescence.