Eujin Um

Fabrication of a poly(dimethylsiloxane) membrane with well-defined through-holes for three-dimensional microfluidic networks

We report a simple method for the fabrication of a poly(dimethylsiloxane) (PDMS) membrane with through-holes by blowing a residual prepolymer away from a photoresist (PR)-patterned Si wafer. The fabrication method for the perforated polymer membrane is crucial to achieve both complicated three-dimensional microfluidic devices and polymer sieve sheets. This method has several advantages over the previous methods in that we can repeatedly make the well-defined holes on the PDMS membranes even if the excess prepolymer remains on the PR mold after spincoating at a relatively low rpm. In addition, the desired pattern can be selectively perforated from the whole wafer even if the mold is fabricated by single-step lithography.

One-step preparation of magnetic Janus particles using controlled phase separation of polymer blends and nanoparticles

We present a simple method with the aid of a microfluidic droplet-generation technique to fabricate magnetic Janus particles by utilizing a solvent evaporation-induced phase separation and preferential partitioning of magnetic nanoparticles in the polymer blends. Non-aqueous emulsion droplets of the polymer blends and nanoparticles solution are produced to become Janus particles after the evaporation of the solvent. The stabilizing polymer of the nanoparticles, which is compatible only with one of the polymer blends to be phase-separated, plays a key role in the anisotropic positioning of the nanoparticles in the Janus particles. Using this phase separation-based method and microfluidics, excellent control over the size, size distribution, and morphology of the particles is achieved. Especially, we could control the morphology of the Janus particles easily by varying the volume ratio of the polymers. However, with an analysis of the shapes of resulting Janus particles, we found that non-equilibrium aspects of the evaporation-induced phase separation play a major role in determining the particle morphology. We expect that the versatility of this method in the choice of polymer blends and functional nanoparticles will enable the fabrication of colloids with various functionality and desired morphology.

Random breakup of microdroplets for single-cell encapsulation

Microfluidic droplet-based technology enables encapsulation of cells in the isolated aqueous chambers surrounded by immiscible fluid but single-cell encapsulation efficiency is usually less than 30%. In this letter, we introduce a simple microgroove structure to break droplets into random sizes which further allows collecting of single-cell [Escherichia coli (E. coli)] containing droplets by their size differences. Pinched-flow separation method is integrated to sort out droplets of certain sizes which have high probability of containing one cell. Consequently, we were able to obtain more than 50% of droplets having single E. coli inside, keeping the proportion of multiple-cell containing droplets less than 16%.

Mesh-integrated microdroplet array for simultaneous merging and storage of single-cell droplets

We constructed a mesh-grid integrated microwell array which enables easy trapping and consistent addition of droplets. The grid acts as a microchannel structure to guide droplets into the microwells underneath, and also provides open access for additional manipulation in a high-throughput manner. Each droplet in the array forms a stable environment of pico-litre volume to implement a single-cell-based assay.

Microbridge structures for uniform interval control of flowing droplets in microfluidic networks

Precise temporal control of microfluidic droplets such as synchronization and combinatorial pairing of droplets is required to achieve a variety range of chemical and biochemical reactions inside microfluidic networks. Here, we present a facile and robust microfluidic platform enabling uniform interval control of flowing droplets for the precise temporal synchronization and pairing of picoliter droplets with a reagent. By incorporating microbridge structures interconnecting the droplet-carrying channel and the flow control channel, a fluidic pressure drop was derived between the two fluidic channels via the microbridge structures, reordering flowing droplets with a defined uniform interval. Through the adjustment of the control oil flow rate, the droplet intervals were flexibly and precisely adjustable. With this mechanism of droplet spacing, the gelation of the alginate droplets as well as control of the droplet interval was simultaneously achieved by additional control oil flow including calcified oleic acid. In addition, by parallel linking identical microfluidic modules with distinct sample inlet, controlled synchronization and pairing of two distinct droplets were demonstrated. This method is applicable to facilitate and develop many droplet-based microfluidic applications, including biological assay, combinatorial synthesis, and high-throughput screening.

On-chip gelation of temporally controlled alginate microdroplets

This paper presents a simple and robust microfluidic device that allows on-chip gelation and temporal control of alginate droplets in a continuous oil phase to enhance hydrogel recovery and collection rate. The additional control flow including calcified oleic acid leads to adjust droplet interval and gelation of the alginate droplet simultaneously through microtunnel structures. The gelated alginate microbeads were successfully gathered at the outlet with sufficient droplet interval. Furthermore, the droplet interval in the droplet-generating channel was flexibly modulated by adjusting the control flow rate under the fixed flow rate of droplet-generating flow. This approach is applicable to droplet synchronization for facilitating many droplet-based microfluidic applications.

On-chip generation of monodisperse giant unilamellar lipid vesicles containing quantum dots.

Monodispersed lipid vesicles have been used as a drug delivery vehicle and a biochemical reactor. To generate monodispersed lipid vesicles in the nano‐ to micrometer size range, an extrusion step should be included in conventional hand‐shaking method of lipid vesicle synthesis. In addition, lipid vesicles as a drug carrier still need to be improved to effectively encapsulate concentrated biomolecules such as cells, proteins, and target drugs. To overcome these limitations, this paper reports a new microfluidic platform for continuous synthesis of small‐sized (∼10 μm) giant unilamellar vesicles (GUVs) containing quantum dots (QDs) as a nanosized model drug. To generate GUVs, we introduced an additional cross‐flow to break vesicles into small size. 1,2 ‐ dimyristoyl‐sn‐glycero ‐ 3 ‐ phosphocholine (DMPC) in an octanol–chloroform mixture was used in the construction of self‐assembled membrane. Consequently, we have successfully demonstrated the fabrication of monodispersed GUVs with 7−12 μm diameter containing QDs. The proposed synthesis method of cell‐sized GUVs would be highly desirable for applications such as multipurpose drug encapsulation and delivery.