Droplet Microfluidics

Microfluidics is a technology that allows the processing and manipulation of small amounts of liquid (10^-9-10^-18 L) systems at the micro- and nano-scale.Microfluidics-based devices called microfluidic chips are miniaturized and integrated.

Droplet microfluidics refers to the generation of discrete droplets and their manipulation by mutually incompatible multiphase fluids in microchannels.

Droplets generated based on microfluidic technology have the advantages of good monodispersity, no cross-contamination, high reproducibility and so on, which are widely used in many disciplines, such as biology, chemistry, physics and so on, and have become an important branch in the field of microfluidics.

Droplet generation method

There are various methods and techniques for generating droplets, which can be categorized into passive and active methods depending on whether external energy is applied during droplet generation.

The passive method of droplet generation refers to the use of different microchannel structures to enable immiscible dispersed-phase and continuous-phase fluids to meet at the channel junction, and droplets of controllable size can be generated downstream of the channel junction by adjusting the channel structure, the size of the flow rate of the two phases, and the flow rate ratio.

Depending on the channel geometry, the passive method of generating droplets can be further categorized into the T-channel method (T-junction), the flow-focusing method (flow-focusing) and the coaxial flow method (co-flow).

Compared with the T-type structure and coaxial structure, the flow-focused structure restricts the droplets to the center of the channel, avoiding the droplets from shearing with the wall, and the morphology of the droplet generation process is more stable, so it is most widely used in the study of droplet microfluidics.

Active method of droplet generation refers to the control of droplet generation by locally applying external forces such as electric field force, magnetic field force and centrifugal force during droplet generation.

A brief overview of the development of droplet microfluidics

In chip fabrication, microfluidic chips have undergone changes from silicon, glass, polydimethylsiloxane (PDMS) to paper-based materials due to the continuous introduction of new materials and technological advances;

Research on theoretical principles, the application of droplet microfluidic technology has received attention, and more research on the dynamics of droplet generation in this direction has ensued, with more in-depth research and development on the principles of droplet generation, which has contributed to the development of the emerging design of microfluidic systems.

Applications, with the advancement of theory and technology, droplet microfluidics has occupied an important place in many fields, such as microreactors, tissue engineering and regenerative medicine, drug delivery, artificial cells, tumor immunotherapy and single-cell research.

Applied research based on droplet microfluidic technology

Cytogel microspheres

The research on cytogel microspheres is more extensive and applied in more fields, and this paper focuses on this aspect.

Preparation of Gel Microspheres

Material

Droplet microfluidics enables the high-throughput preparation of monodisperse droplets of controllable size, composition, and functionality that can encapsulate one or more types of cells and serve as templates for the preparation of cellular gel microspheres with specific physicochemical properties and capable of resisting shear stress.

There is a wide variety of hydrogel materials used for encapsulating cells, which can be categorized into two main groups:

1. Natural polymers, such as fibrinogen and peptides, have good biocompatibility and degradability;
2. Synthetic polymers, including polyethylene glycol (PEG), polyacrylic acid (PAA), and polyvinyl alcohol (PVA), have advantages in terms of mechanical properties and controllability of biochemical signals.

Hybrid hydrogels are commonly used in practical applications, combining the advantages of both to be able to provide cells with a controlled microenvironment and the necessary anchoring sites, and to regulate the cells' life activities.

Processing Methods:

In a typical cell encapsulation process, the cell-containing hydrogel precursor solution undergoes continuous-phase shearing to form a single monodisperse droplet, which, by triggering cross-linking, forms cell-gel microspheres.

Depending on the hydrogel material used to encapsulate the cells, different cross-linking methods were used to prepare the cell gel microspheres, which mainly included photocross-linking, ionic cross-linking and temperature-induced cross-linking.

Background on the application of cytogel microspheres

• human cell environment

Cells in the human body exist in a complex three-dimensional microenvironment-ECM composed of proteins and polysaccharides, and most of the existing cell cultures are based on two-dimensional petri dish cultures, which are difficult to realistically reflect the in vivo microenvironment of the cells.

• Characteristics of hydrogel

With unique properties such as high water content, biocompatibility, degradability, and porosity that are similar to natural ECM, hydrogels are widely used in biomedical fields.

Adding suitable growth factors into the hydrogel to construct an artificial ECM capable of controlling cell growth, proliferation and differentiation, the cells grown therein can well interact with the hydrogel wrapped around it, and it has an excellent prospect to use this as an in vitro three-dimensional cell culture platform.

Examples of cytogel microsphere applications

• organ model

Human organs are composed of a variety of tissues, and in order to simulate human organs, it is necessary to construct three-dimensional models in which different types of cells are spatially distributed in an artificial ECM. Using droplet microfluidics-based gel microspheres to encapsulate different types of cells, it is expected to construct in vitro tissue or organ models. Examples include lungs, liver microarrays and kidney microarrays.

• cell culture

Cell-cell interactions significantly affect cell behavior, so constructing single-cell co-culture systems is important for studying cell-cell interactions between individual cells, and droplet microfluidics is one of the methods to effectively construct single-cell co-culture systems. For example, 3D Cell Culture Chip.

protein crystallization

Crystallization is a widely used separation and purification technique in the pharmaceutical industry. With the advancement of biofermentation technology, protein drugs have been rapidly developed.

With good biocompatibility and high efficiency compared to traditional small molecule drugs, protein drugs show great potential in the treatment of cancer, metabolic diseases and autoimmune diseases.

In order to be able to further reduce the production cost of protein drugs, crystallization has received much attention as a successor to chromatographic purification.

Protein crystallization is a complex physicochemical process compared to small molecules or ions.

Protein molecules are large in size, with low diffusion rates and weak aggregation tendencies, and the crystallization process is much slower than that of small molecule compounds in general.

In addition, many external factors such as protein concentration, temperature, pH, precipitants, buffers and other environmental factors complicate the process of protein crystallization.

Therefore, how to effectively increase the yield and shorten the induction time of protein crystallization is a challenge in protein crystallization research.

Microbial Geotechnical Engineering

Biomineralization is a common natural phenomenon of biologically mediated generation of minerals, and some microorganisms can participate in the biogeochemical cycles of the Earth system by interacting with the environment to deposit minerals.

The application of such microorganisms to the soil, by changing the surrounding physicochemical conditions to catalyze the precipitation of minerals in the soil particles, can fill the pores of the soil and “bridge” role of the loose soil particles together, to achieve the effect of “curing agent”, to achieve a good The effect of “curing agent” can be realized to achieve good engineering purpose.

Compared with traditional civil materials, microbial minerals have obvious application advantages due to their low environmental impact, milder reaction process, low energy consumption, and low engineering disturbance. Therefore, the droplet microfluidic technology can be utilized in soil reinforcement and restoration projects.