Straight-through Microchannel Array Research Articles

Asymmetric straight-through microchannel arrays made of aluminum for producing monodisperse O/W emulsions

The aim of this study was to stably produce monodisperse food-grade oil-in-water (O/W) emulsions using an asymmetric straight-through microchannel (MC) array that is microfabricated on an aluminum chip. Circular MCs of the inlet side were fabricated using a microdrilling process, while microslots of the outlet side were fabricated by electric discharge machining. We used refined soybean oil as the dispersed phase and a Milli-Q water solution containing 1 wt% Tween 20 as the continuous phase. An aluminum MC array with a circular MC diameter (dMC) of 116.9 μm produced monodisperse O/W emulsion droplets with an average diameter (dav) of about 300 μm and coefficients of variation (CVs) below 4%, whereas an aluminum MC array with 82.1 μm diameter stably generated monodisperse O/W emulsion droplets with dav of about 150 μm and CVs below 4%. The dav of the produced oil droplets was affected by the dimensions of asymmetric straight-through MCs. Monodisperse O/W emulsions were successfully generated below the critical dispersed-phase flow rate, with little variation in dav and CV values. The force balance during droplet generation was also analyzed for discussing the results for the effect of the dispersed-phase flow rate.

Monodisperse W/O/W emulsions encapsulating l-ascorbic acid: Insights on their formulation using microchannel emulsification and stability studies

Stabilizing l-ascorbic acid is a challenge for food industries. The present study aimed to formulate monodisperse food-grade water-in-oil-in-water (W/O/W) emulsions containing a high concentration of l-ascorbic acid in an inner aqueous phase using homogenization and subsequent microchannel emulsification (MCE). The microchannel (MC) array plate used here was a silicon asymmetric straight-through MC array that consists of numerous 10μm×100μm microslots with a 30μm depth, each connected to a 10μm-diameter circular MC with a 70μm depth. Water-in-oil (W/O) emulsions contained a soybean oil solution with 4–8% (w/w) tetraglycerin condensed ricinoleic acid ester as a continuous phase and an aqueous solution with 10–30% (w/v) l-ascorbic acid, 1% (w/w) magnesium sulfate, and 1% (w/v) gelatin as an inner aqueous phase. The W/O emulsion droplets formulated using a rotor-starter homogenizer had average droplet diameters of 2.6–2.9μm and coefficients of variation (CVs) of 13–17%. MCE was performed using a dispersed W/O emulsion phase and a 5mM phosphate buffer containing 1% (w/w) decaglycerol monolaurate and 10–30% (w/v) D(+)-glucose as an outer aqueous phase. Monodisperse W/O/W emulsions containing W/O droplets with average diameters of 26.0–31.5μm and CVs below 10% were successfully formulated via an asymmetric straight-through MC array at a low hydrophobic emulsifier concentration, regardless of l-ascorbic acid concentration. The W/O droplets dispersed in these monodisperse W/O/W emulsions were physically stable in variation of average diameter and CV for more than 10d of storage at 4°C. The monodisperse W/O/W emulsions also exhibited l-ascorbic acid retention exceeding 80% during storage.

Effects of Channel and Operation Parameters on Emulsion Production Using Oblong Straight-Through Microchannels

Straight-through microchannel (MC) arrays consisting of numerous oblong straight-through holes are promising MC emulsification devices for producing monodisperse emulsions in large droplet-production scales. We investigated the effects of MC dimensions and velocity of a crossflowing continuous phase on the production of soybean oil-in-water emulsions using straight-through MC arrays made of single-crystal silicon. The aspect ratio of oblong MCs with equivalent diameters of 21μm to 22μm significantly af fected droplet generation and the sensitivity of droplet size to continuous-phase velocity. Uniform oil droplets with diameters of about 100μm and coefficients of variation of less than 3% were generated using large oblong MCs with equivalent diameters of 54μm to 55μm. Data analysis demonstrated that oblong MCs with aspect ratios exceeding a threshold of 3 were necessar y to generate uniform oil droplets in a wide range of MC cross-sectional sizes. Moreover, the cross-flow of the continuous phase did not affect the size of the droplets generated using oblong MCs with a large aspect ratio.

Controlled preparation of giant vesicles from uniform water droplets obtained by microchannel emulsification with bilayer-forming lipids as emulsifiers

We investigated a preparation method of giant vesicles using monodisperse water-in-oil (W/O) emulsions stabilized by bilayer-forming emulsifiers. A mixture of phosphatidylcholine, cholesterol and stearylamine was used both to stabilize the water droplets formed in the emulsion and to form the vesicles. Using this lipid mixture, we obtained monodisperse W/O emulsions with mean droplet diameters of 10–40 μm and coefficients of variation as small as ca 5% by means of the microchannel (MC) emulsification technique. Utilization of an asymmetric straight-through MC array device enabled a monodisperse droplet productivity of up to 80 ml/h. The obtained water droplets were converted to giant vesicles via evaporative removal of the continuous-phase solvent followed by addition of an aqueous buffer solution. The resulting vesicles were similar in size to their starting water droplets, and a hydrophilic fluorescent marker was entrapped inside the vesicles.

High-aspect-ratio through-hole array microfabricated in a PMMA plate for monodisperse emulsion production

Microchannel (MC) emulsification is a promising technique for producing monodisperse emulsions consisting of highly uniform droplets. The authors developed a high-aspect-ratio microstructure (HARMST) made of poly(methyl methacrylate) (PMMA) as a new MC emulsification device. A PMMA straight-through MC array plate consisting of 31,250 through-holes with a 7.3 × 22.9-μm oblong section and a 200-μm depth was fabricated by a process of synchrotron radiation (SR) lithography and etching. Oblong MCs fabricated in a PMMA straight-through MC array were highly uniform with a coefficient of variation of less than 2%. The fabricated PMMA straight-through MC array plate was used to produce water-in-oil (W/O) emulsions. Monodisperse W/O emulsions with average droplet diameters of approximately 25 μm and a minimum coefficient of variation of 3.2% were produced via a hydrophobic PMMA straight-through MC array. The PMMA straight-through MC array plate also produced monodisperse W/O emulsions at droplet production rates of up to 1.7 × 104/s. The PMMA straight-through MC array plates developed in this work are expected to expand the application field of emulsification using straight-through MC array plates, which have previously been made of single-crystal silicon.