Synthesis of Crystals and Particles by Crystallization and Polymerization in Droplet-based Microfluidic Devices

摘要

This review reports briefly the recent advances in crystallization and polymerization assisted by droplet-based microfluidics to synthesize micro-particles and micro-crystals. The features of this method are：to generate droplets of diverse materials including aqueous solution, gels and polymers，to produce droplets of precise controlled structure, to produce particles of monodispersity, and to control the shape of particles in a new level. Microfluidic channels are fabricated conventionally on silicon and glass by photolithography and etching, or recently on some polymer materials (PDMS) by soft lithography technology. The nonphotolithographic soft lithography is a reliable, reproducible and cost-effective fabrication strategy for replicating a pattern. Up to now, numerous microfluidic devices have been employed to generate emulsions of monodisperse droplets in the range from nm to μm, such as T junctions, flow-focusing device or co-flow capillary microfluidic device. Double emulsions, i.e. droplets containing smaller droplets, have been produced by employing two-step (by using the effect of micro-channels wettability on the formation of hydrophilic or hydrophobic droplets) or single-step (by using the capillary flow-focusing). These precisely controlled droplets are ideal template to be employed as microreactors to generate crystals and polymer particles or to produce capsules used in encapsulation and drug delivery. Microfluidic devices have also been promising tools in the synthesis of micron polymer particles that have diverse applications such as the photonic materials, ion-exchange, and field-responsive rheological fluids. Processes assisted by microfluidic devices are able to produce the polymer particles with precise control over their sizes, size distribution, morphology and compositions. The process includes two steps, the generation of monodisperse droplets and the emulsion polymerization induced by thermal or optical method. The particles of various morphologies (beads, disks, rods and other morphology) were produced by the adjustment of flow parameters and channel geometries to modify the shape and by simultaneous polymerization to cure the monomers. Employed in self-assembly, in stabilization of emulsions, and in the dual-functionalized optical-electronic devices, Janus particles (JPs), known as microparticles with biphasic geometry of distinct properties, have been synthesized by forming droplets from the parallel flow of two fluids of distinct properties in some microfluidic systems. It has been proved that the operation of more than two immiscible liquids in microfluidic channels may produce the polymer capsules containing single or multiple liquid cores. Based on this technique, microfluidic devices that may produce microcapsules containing magnetical or fluorescent nanoparticles and drugs have been designed for the application in drug delivery system. The shell of the microcapsules is composed of bio-compatible polymers such as PLGA, and the nanoparticles inside are employed to meet the need of targeting and tracing. Another desirable material to be used in drug delivery is microgel that consists of cross-linked polymer network and is a good absorbent to carry functional solutions. The synthesis of monodisperse microgels has also been done by gelation in microfluidic devices. Crystallization, a conventional batch process, has proceed continuously in the microfluidic channels. Some nanoparticles, such as BaSO4 and DPA, were continuously produced by microfluidic-based emulsion, mixing and crystallization. The nucleation kinetics of some materials like KNO3 and PNIPAM colloidal crystals were investigated in different microfluidic devices. Because of the elimination of the interactions among crystallites in bulk systems, using independent droplets may help to measure the nucleation rate more accurately. In the structural biology, the plugs produced in microfluidic devices provide ideal platforms for protein crystallization on the nanoliter scale. Therefore, they become one of the promising tools to screen the optimal conditions of protein crystallization. Meanwhile, they also have been employed to discover and develop new drugs due to their technological capability to increase the experimental throughput. Droplet-based microfluidic devices are powerful tools to execute some precise controls and operations on the flows inside the microchannels by adjusting some fluid dynamics parameters to produce monodisperse emulsions or multiple-emulsions of various materials. The discrete, independently controllable droplets are manipulated in the channels continuously to synthesize the crystals, polymer particles and composite particles with polymer cover and functional cores by emulsion crystallization and emulsion polymerization in situ. These monodisperse particles or crystals of diverse functions are generated to try to meet different technical demands in many fields, such as in crystal engineering and drug delivery systems.