Droplet microfluidics offers exquisite control over the flows of multiple fluids in microscale, enabling fabrication of advanced microparticles with precisely tunable structures and compositions in a high throughput manner. The combination of these remarkable features with proper materials and fabrication methods has enabled high efficiency, direct encapsulation of actives in microparticles whose features and functionalities can be well controlled. These microparticles have great potential in a wide range of bio-related applications including drug delivery, cell-laden matrices, biosensors and even as artificial cells. In this review, we briefly summarize the materials, fabrication methods, and microparticle structures produced with droplet microfluidics. We also provide a comprehensive overview of their recent uses in biomedical applications. Finally, we discuss the existing challenges and perspectives to promote the future development of these engineered microparticles.

This article presents a one-step method for generating complex emulsions that exploits the phase separation of the emulsion droplet generated in the microchannel. This approach easily produces double, triple, quadruple, and Janus emulsions with monodisperse size. These emulsions can be used as useful templates for the synthesis of new functional materials, such as microcapsules, hemispheres, Janus particles and microcarriers that are capable of simultaneously encapsulating hydrophilic and hydrophobic compounds with selective compartmentalization in a one-step process.

A microfluidic method for the in situ production of monodispersed alginate hydrogels using chaotic mixing is described. Aqueous droplets comprising of alginate and calcium as a cross-linking agent were formed as an immiscible continuous phase, and then the alginate and calcium in the droplet came into contact and were rapidly mixed. Gelation of the hydrogel was achieved in situ by the chaotic mixing of the droplets in the microfluidic device. Important operating parameters included: the capillary number (Ca) and the flow rate of the continuous phase, which mainly influenced the formation of three distinctive flow regimes, such as fluctuation, stable droplets, and laminar flow. Under the stable formation of droplets regime, monodispersed alginate microbeads having a narrow size distribution (below 3% of CV) were produced in the microfluidic device and the size of the microbeads, ranging from 60 to 95 microm, could be easily modulated by varying the flow rate, viscosity, and interfacial tension. In addition, this approach can be applied to the encapsulation of yeast cells in alginate hydrogels with a high monodispersity. This simple microfluidic technique for the production of monodispersed hydrogels and encapsulation of biomolecules shows strong potential for use in biosensors, cell sensors, drug delivery systems, and cell transplantation applications.

Controlled encapsulation and pairing of single cells within a confined 3D matrix can enable the replication of the highly ordered cellular structure of human tissues. Microgels with independently controlled compartments that can encapsulate cells within separately confined hydrogel matrices would provide precise control over the route of pairing single cells. Here, a one-step microfluidic method is presented to generate monodisperse multicompartment microgels that can be used as a 3D matrix to pair single cells in a highly biocompatible manner. A method is presented to induce microgels formation on chip, followed by direct extraction of the microgels from oil phase, thereby avoiding prolonged exposure of the microgels to the oil. It is further demonstrated that by entrapping stem cells with niche cells within separate but adjacent compartments of the microgels, it can create complex stem cell niche microenvironments in a controlled manner, which can serve as a useful tool for the study of cell-cell interactions. This microfluidic technique represents a significant step toward high-throughput single cells encapsulation and pairing for the study of intercellular communications at single cell level, which is of significant importance for cell biology, stem cell therapy, and tissue engineering.