This study presents a simple but highly versatile method of fabricating picoliter-volume hydrogel patterns on poly(dimethylsiloxane) (PDMS) substrates. Hydrophilic regions were prepared on hydrophobic PDMS plates by trapping and melting functional polymer particles and performing subsequent reactions with partially oxidized dextran. Small aliquots of a gelation solution were selectively trapped on the hydrophilic areas by a simple dipping process that was utilized to make thin hydrogel patterns by the in situ gelation of a sol solution. Using this process, we successfully formed calcium alginate, collagen I, and chitosan hydrogels with a thickness of several micrometers and shapes that followed the hydrophilized regions. In addition, alginate and collagen gel patterns were used to capture cells with different adhesion properties selectively on or off the hydrogel structures. The presented strategy could be applicable to the preparation of a variety of hydrogels for the development of functional biosensors, bioreactors, and cell cultivation platforms.

When analyzing the kinetics of liquid-liquid extraction (LLE), the change in the concentration of extracted target molecules over time should be monitored for a known interfacial area. Herein, we developed a microfluidic system for precisely analyzing the kinetics of LLE using droplets of a constant size even in the presence of additives. Extraction is initiated by exchanging the carrier fluid for the droplets with a target solution and then terminated by phase separation, based on the principle of hydrodynamic filtration. By using one out of several pairs of outlet/buffer inlet at a given time, the extraction time period is tuned stepwise without changing the flow rate condition. We successfully demonstrated droplet-based LLE by controlling the extraction period from ∼0.03 to ∼1.2 s and evaluated the extraction kinetics of rhodamine B from the continuous aqueous phase to droplets of 1-octanol with a diameter of ∼40 μm. In addition, the effect of additives on extraction efficiency was evaluated. The system presented in this study would be useful for determining rate constants for interfacial mass transfer in general LLE kinetic studies as well as for developing new extraction chemistries and optimizing microfluidic chemical/biochemical analysis systems that involve an LLE process.

Here, we present microfluidic methods to fabricate complex hydrogel structures for 3D tissue or organ-like cell structures in vitro. First, a microfluidic system to continuously synthesize chemically and physically anisotropic Ca–alginate hydrogel microfibers is proposed to enable the guidance of cell proliferation and differentiation. Next, the microfluidic preparation methods for yarn-ball-shape hydrogel particles and extremely-small hydrogel microspheres. Finally, a newly developed micro-molding and bonding method for hydrogel micro-patterned plates is reported.