Research status and application progress of microparticle preparation using microfluidic photocuring technology

In recent years, microparticles have attracted widespread interest due to their unique shapes, complex structures, and the ability to achieve multifunctional integration in individuals. They have broad application prospects in many fields such as bioanalysis and diagnosis, tissue engineering, anti-counterfeiting, engineering, and structural materials.

The material and shape of microparticles determine their functions and applications. For example, prismatic diamond microparticles with sharp cutting edges can be used for micro-parts processing, ceramic microspheres with high dielectric constants can be used as functional units of absorbing metamaterials, disc-shaped silicon micropillars can be used as dielectric units of terahertz magnetic mirrors, and UO₂ microspheres with uniform shapes can be used as fuel cores for high-temperature gas-cooled reactors. Multi-band component composite microparticles can be used in the coding field. Therefore, it is of great significance to study the preparation and application of microparticles with different shapes and structures of different materials.

Compared with traditional microparticle preparation methods such as spray drying, water/solvent thermal synthesis, anti-solvent precipitation, stirring emulsification, extrusion molding, microstereoscopic photocuring, laser polymerization, microwire electrospark machining, and microinjection molding, microfluidic photocuring technology has opened up a new way for the preparation of microparticles with advantages such as good monodispersity and quantity.

Microfluidic synthesis can be divided into two categories according to the forming mechanism - synthesis based on droplet templates and microfluidic photocuring. In recent years, a large number of review articles have introduced microfluidic forming based on droplet templates, including the preparation mechanism, methods and applications of microparticles of different shapes and materials. Microfluidic photocuring is an important part of microfluidic forming and has been used to prepare 2D stretched and 3D anisotropic microparticles with sharp edges that cannot be processed based on droplet templates.

The results show that sharp-edged microparticles can be used as building blocks for the construction of thin film stealth materials, structural materials, and microsystems, and bring about significant changes in performance. The research and development of microparticles prepared based on microfluidic photocuring has become increasingly important. It is necessary to comprehensively and systematically summarize the research progress in recent years from the shallow to the deep, point out the current limitations, and put forward constructive suggestions for the future development of this field.

According to MEMS Consulting, researchers from Tsinghua University and Nanjing University of Aeronautics and Astronautics recently published an article titled "Mropartes by microfluidic lithography" in the journal Materialstoday. Based on a comprehensive introduction to the basic elements of microfluidic photocuring technology (i.e., microfluidic devices, precursors, masks, and ultraviolet light), the article discussed the latest research progress in microfluidic photocuring technology and the diversity of microparticles produced. Post-processing technologies including self-assembly and sintering were proposed to potentially link laboratory preparation of microparticles with practical applications. The application prospects of microparticles were analyzed from three aspects: cell manipulation, biology, and anti-counterfeiting. Finally, the limitations of functional microparticles were summarized, and their future development was prospected, aiming to provide assistance for the controllable microfluidic preparation and application of functional microparticles.

Microfluidic photocuring and its basic elements

The photocurable precursor flows in the microchannel, and the ultraviolet light is projected into the microchannel through a mask with a light-transmitting hole of a specific shape. The precursor in the channel is instantly cured by ultraviolet light exposure to form microparticles. According to the continuity of the precursor flowing in the microchannel, it can be divided into continuous flow and intermittent flow microfluidic photocuring.

Figure 1 Continuous flow, intermittent flow photocuring and microparticle preparation: (a) Two-dimensional stretched columnar microparticles prepared based on continuous flow photocuring; (b) Comparison of the morphology of microparticles prepared by intermittent flow photocuring and the two photocuring processes

Figure 2 The development of research on the preparation of microparticles based on microfluidic photocuring technology. The structures of microfluidic channels are mainly divided into four types: rectangular straight channels, multi-entry channels, micro-pillar insertion channels and non-rectangular straight channels; the shapes of masks are divided into continuous 2D shapes, discontinuous 2D shapes and grayscale coded shapes; the morphology of microparticles ranges from simple two-dimensional stretched columns to stacked shapes and then to 3D anisotropic shapes.

Microparticle preparation and morphology control

Figure 3 Particle morphology adjustment based on UV light control: (a) Particle morphology is adjusted by regulating UV light intensity distribution and exposure time; (b) Particle morphology is adjusted by UV exposure time and mask shape control; (c) Particle morphology is adjusted by UV focal plane position and mask shape control

Figure 4 Control of microparticle morphology based on microchannel structure adjustment: (a) Lock-release intermittent streamer curing to prepare two-layer microparticles; (b) Prepare height-adjustable multi-layer microparticles in the microchannel by adjusting the air pressure of the upper air chamber of the microchannel; (c) Prepare multi-layer microparticles by adjusting the height of the microchannel using a pressure head; (d) Prepare 3D-shaped microparticles through non-rectangular microchannels made by thermal stretching; (e) Prepare polyhedral microparticles through non-rectangular microchannels made by folding

Figure 5 Control of microparticle morphology based on precursor composition: (a) Preparation of two-component microparticles with both hydrophilic and hydrophobic properties; (b) Preparation of microparticles with curved surfaces by utilizing the surface energy difference between photocurable and non-photocurable precursor phases; (c) Preparation of step-shaped microparticles by controlling the concentration of opaque additives in different laminar phases; (d) Preparation of microparticles with superparamagnetic colloids embedded in specific locations; (e) Preparation of bullet-shaped microparticles by utilizing magnetic additives with special UV light properties

Figure 6 Control of microparticle morphology based on multi-factor regulation: (a) Microparticle formation based on microchannel structure, precursor composition and UV light control; (b) Microparticles obtained based on (a) plus additional time control factors; (c) Control of the shape and size of microparticles by setting a tapered neck in the microchannel

Microparticle post-processing

Figure 7 Self-assembly of microparticles: (a) Self-assembly of rectangular microparticles in droplets; (b) 2D stretched shape microparticle assembly structure; (c) Self-assembly of hydrophobic-hydrophilic two-phase microparticles at the interface of water-in-oil emulsion; (d) Assembly of three-layer hexagonal columnar hydrogel microparticles; (e) Assembly of Archimedean (truncated) tetrahedral microparticles; (f) Layer-by-layer assembly process of spherical structure; (g) Microfluidic assembly of microparticles based on "track-fin" structure; (h) Microfluidic assembly of microparticles based on microchannel cross-sectional geometric constraints.

Figure 8 Microparticle sintering: (a) low solid content SiO₂ microparticles; (b) low solid content Al₂O₃ microparticles; (c) denser SiO₂ microparticles; (d) high solid content SiO₂ microgears; (e) two-component magnetic microgears

Microparticle Application

Figure 9 Applications of hydrogel microparticles in cell manipulation: (a) 2D stretched hydrogel microparticles for cell culture; (b) disc-shaped and octopus-shaped microparticles for cell delivery; (c) multi-component microparticles for cell adhesion; (d) hydrogel microparticle assemblies for mouse fibroblast culture

Figure 10 Applications of microparticles in biological detection: (a) Multi-probe encoded microparticles and their applications in biological detection; (b) Color site encoded magnetic microparticles and their applications in DNA detection and analysis; (c) Shape-encoded hydrogel microparticles for simultaneous detection of miRNA 21 and miRNA let-7a

Figure 11 Application of microparticles in anti-counterfeiting: (a) Two-dimensional code microparticles and their application in capsule drug anti-counterfeiting; (b) Microparticles used for drug and food labeling; (c) Imaging of coded microparticles using a portable decoder in different challenging environments

In general, this study reviews the current status of microfluidic photocuring preparation and application of anisotropic microparticles in recent years. Starting from the four basic elements of microfluidic photocuring - microfluidic devices, precursors, masks and ultraviolet light, the latest progress in the preparation and post-processing technology of new microparticles is introduced. The ever-expanding shapes and unique structures make microparticles ideal carriers for various applications such as cell manipulation, biological detection and anti-counterfeiting. However, although microfluidic photocuring technology has made many inspiring and significant progress in the controllable synthesis of microparticles of different morphologies in recent years, there is still much room for improvement.