3D shape evolution of microparticles and 3D enabled applications using non-uniform UV flow lithography (NUFL)

Microfluidic Approaches for Microactuators: From Fabrication, Actuation, to Functionalization

Microactuators can autonomously convert external energy into specific mechanical motions. With the feature sizes varying from the micrometer to millimeter scale, microactuators offer many operation and control possibilities for miniaturized devices. In recent years, advanced microfluidic techniques have revolutionized the fabrication, actuation, and functionalization of microactuators. Microfluidics can not only facilitate fabrication with continuously changing materials but also deliver various signals to stimulate the microactuators as desired, and consequently improve microfluidic chips with multiple functions. Herein, this cross-field that systematically correlates microactuator properties and microfluidic functions is comprehensively reviewed. The fabrication strategies are classified into two types according to the flow state of the microfluids: stop-flow and continuous-flow prototyping. The working mechanism of microactuators in microfluidic chips is discussed in detail. Finally, the applications of microactuator-enriched functional chips, which include tunable imaging devices, micromanipulation tools, micromotors, and microsensors, are summarized. The existing challenges and future perspectives are also discussed. It is believed that with the rapid progress of this cutting-edge field, intelligent microsystems may realize high-throughput manipulation, characterization, and analysis of tiny objects and find broad applications in various fields, such as tissue engineering, micro/nanorobotics, and analytical devices.

Review on Bioinspired Design of ECM-Mimicking Scaffolds by Computer-Aided Assembly of Cell-Free and Cell Laden Micro-Modules

Tissue engineering needs bioactive drug delivery scaffolds capable of guiding cell biosynthesis and tissue morphogenesis in three dimensions. Several strategies have been developed to design and fabricate ECM-mimicking scaffolds suitable for directing in vitro cell/scaffold interaction, and controlling tissue morphogenesis in vivo. Among these strategies, emerging computer aided design and manufacturing processes, such as modular tissue unit patterning, promise to provide unprecedented control over the generation of biologically and biomechanically competent tissue analogues. This review discusses recent studies and highlights the role of scaffold microstructural properties and their drug release capability in cell fate control and tissue morphogenesis. Furthermore, the work highlights recent advances in the bottom-up fabrication of porous scaffolds and hybrid constructs through the computer-aided assembly of cell-free and/or cell-laden micro-modules. The advantages, current limitations, and future challenges of these strategies are described and discussed.

Flow lithography for structured microparticles: fundamentals, methods and applications

Structured microparticles, with unique shapes, customizable sizes, multiple materials, and spatially-defined chemistries, are leading the way for emerging 'lab on a particle' technologies. These microparticles with engineered designs find applications in multiplexed diagnostics, drug delivery, single-cell secretion assays, single-molecule detection assays, high throughput cytometry, micro-robotics, self-assembly, and tissue engineering. In this article we review state-of-the-art particle manufacturing technologies based on flow-assisted photolithography performed inside microfluidic channels. Important physicochemical concepts are discussed to provide a basis for understanding the fabrication technologies. These photolithography technologies are compared based on the structural as well as compositional complexity of the fabricated particles. Particles are categorized, from 1D to 3D particles, based on the number of dimensions that can be independently controlled during the fabrication process. After discussing the advantages of the individual techniques, important applications of the fabricated particles are reviewed. Lastly, a future perspective is provided with potential directions to improve the throughput of particle fabrication, realize new particle shapes, measure particles in an automated manner, and adopt the 'lab on a particle' technologies to other areas of research.

Hipster microcarriers: exploring geometrical and topographical cues of non-spherical microcarriers in biomedical applications

Structure and organisation are key aspects of the native tissue environment, which ultimately condition cell fate via a myriad of processes, including the activation of mechanotransduction pathways. By modulating the formation of integrin-mediated adhesions and consequently impacting cell contractility, engineered geometrical and topographical cues may be introduced to activate downstream signalling and ultimately control cell morphology, proliferation, and differentiation. Microcarriers appear as attractive vehicles for cell-based tissue engineering strategies aiming to modulate this 3D environment, but also as vehicles for cell-free applications, given the ease in tuning their chemical and physical properties. In this review, geometry and topography are highlighted as two preponderant features in actively regulating interactions between cells and the extracellular matrix. While most studies focus on the 2D environment, we focus on how the incorporation of these strategies in 3D systems could be beneficial. The techniques applied to design 3D microcarriers with unique geometries and surface topographical cues are covered, as well as specific tissue engineering approaches employing these microcarriers. In fact, successfully achieving a functional histoarchitecture may depend on a combination of fine-tuned geometrically shaped microcarriers presenting intricately tailored topographical cues. Lastly, we pinpoint microcarrier geometry as a key player in cell-free biomaterial-based strategies, and its impact on drug release kinetics, the production of steerable microcarriers to target tumour cells, and as protein or antibody biosensors.

From shaping to functionalization of micro-droplets and particles

The structure of microdroplet and microparticle is a critical factor in their functionality, which determines the distribution and sequence of physicochemical reactions. Therefore, the technology of precisely tailoring their shape is requisite for implementing the user demand functions in various applications. This review highlights various methodologies for droplet shaping, classified into passive and active approaches based on whether additional body forces are applied to droplets to manipulate their functions and fabricate them into microparticles. The passive approaches cover batch emulsification, solvent evaporation and diffusion, micromolding, and microfluidic methods. In active approaches, the external forces, such as electrical and magnetic fields or optical lithography, are applied to microdroplets. Special attention is also given to latest technologies using microdroplets and microparticles, especially in the fields of biological, optical, robotic, and environmental applications. Finally, this review aims to address the advantages and disadvantages of the introduced approaches and suggests the direction for further development in this field.

Three-dimensional independent control of multiple magnetic microrobots via inter-agent forces

This article presents a method to independently control the position of multiple microscale magnetic robots in three dimensions, operating in close proximity to each other. Having multiple magnetic microrobots work together in close proximity is difficult due to magnetic interactions between the robots, and here we aim to control those interactions for the creation of desired multi-agent formations in three dimensions. Based on the fact that all magnetic agents orient to the global input magnetic field, the local attraction–repulsion forces between nearby agents can be regulated. For the first time, 3D manipulation of two microgripping magnetic microrobots is demonstrated. We also mathematically and experimentally prove that the center-of-mass external magnetic pulling of the multi-agent system is possible in three dimensions with an underactuated magnetic field generator. Here we utilize the controlled interaction magnetic forces between two spherical agents to steer them along two prescribed paths. We apply our method to independently control the motion of a pair of magnetic microgrippers as functional microrobot candidates each equipped with a five-degree-of-freedom motion mechanism and a grasp–release mechanism for targeted cargo delivery. A proportional controller and an optimization-based controller are introduced and compared, with potential to control more than two magnetic agents in three dimensions. Average tracking errors of less than 141 and 165 micrometers are accomplished for the regulation of agents’ positions using optimization-based and proportional controllers, respectively, for spherical agents with approximate nominal radius of 500 micrometers operating within several body-lengths of each other.

Degassed micromolding lithography for rapid fabrication of anisotropic hydrogel microparticles with high-resolution and high uniformity

Replica molding techniques, which are used to synthesize microparticles inside anisotropic micromolds, have been developed to enable the mass production of hydrogel particles. However, these techniques are limited in their ability to synthesize only a narrow range of particle compositions and shapes because of the difficulty in loading precursors into the micromolds as well as the low particle homogeneity due to the uneven evaporation of the precursors. Herein, we describe a simple yet powerful technique, called degassed micromolding lithography, which can load precursors within 1 min regardless of the wettability. This technique is based on the gas-solubility of a degassed micromold that acts as a suction pump to completely fill the mold by drawing precursor liquids in. The semi-closed system within the micromold prevents the uneven evaporation of the precursor, which is essential for the production of homogeneous particles. Furthermore, controlled uniformity of the hydrogel microparticles (C.V. < 2%) can be achieved by engineering the design of the micromold array.

Asymmetric Morphology Transformation of Azo Molecular Glass Microspheres Induced by Polarized Light

In this work, photoinduced asymmetric morphology transformation of a type of azo molecular glass microspheres was thoroughly investigated to understand the effects of controlling factors on the process, related mechanism and unique functions. The monodispersed microspheres with their sizes over ten microns were fabricated from an isosorbide-based azo compound (IAC-4) by microfluidics. Under irradiation with linearly polarized light, the ten-micron-scale microspheres were transformed into three-dimensional (3D) asymmetric particles through directional mass transfer. Microscopic observations and optics simulation were employed to investigate the morphology transformations. The results show that the penetration depth of light at different wavelengths plays an extremely important role to affect the asymmetric deformation behavior of the IAC-4 microspheres, which determines deformation region, deformation degree and final shapes of the particles. The light intensity (50-200 mW/cm²) is a less important factor, while the deformation rate of the light-penetrated part linearly increases with the intensity. When the light intensity varies in this range, the deformation degree and the final asymmetric morphology are determined by exposure energy (light intensity × irradiation time). The IAC-4 microspheres with different sizes show distinct morphology transformation behavior and the deformed particles possess different shapes, caused by the variation of volume fraction of the light-penetrated part in the microspheres. The increase in the ratio of the light-penetrated part to the total volume of the microspheres results in larger scale deformations. Based on the above understanding, asymmetric particles with various morphologies can be fabricated through a precisely controllable way. The asymmetric particles loaded on various surfaces show ability to render remarkable wetting anisotropy of water droplets on the substrates.

Microparticles, Microspheres, and Microcapsules for Advanced Drug Delivery

Microparticles, microspheres, and microcapsules are widely used constituents of multiparticulate drug delivery systems, offering both therapeutic and technological advantages. Microparticles are generally in the 1–1000 µm size range, serve as multiunit drug delivery systems with well-defined physiological and pharmacokinetic benefits in order to improve the effectiveness, tolerability, and patient compliance. This paper reviews their evolution, significance, and formulation factors (excipients and procedures), as well as their most important practical applications (inhaled insulin, liposomal preparations). The article presents the most important structures of microparticles (microspheres, microcapsules, coated pellets, etc.), interpreted with microscopic images too. The most significant production processes (spray drying, extrusion, coacervation, freeze-drying, microfluidics), the drug release mechanisms, and the commonly used excipients, the characterization, and the novel drug delivery systems (microbubbles, microsponges), as well as the preparations used in therapy are discussed in detail.

Controlled generation of spiky microparticles by ionic cross-linking within an aqueous two-phase system

Microparticles are used in a variety of different fields, such as drug delivery. Recently, non-spherical microparticle generation has become desirable. The high surface-to-volume ratio of non-spherical microparticles allows for enhanced targeting, and attachment to cells and tissue. Current non-spherical microparticle generation techniques require complicated setup, and utilizing natural micrograins, such as pollen grains, as non-spherical delivery vehicles, requires extensive post-processing. Here, we describe a unique and facile chemical synthesis approach, for controlled generation of pollen-like microparticles, based on ionic cross-linking of alginate and calcium chloride (CaCl2), within an all-biocompatible aqueous two-phase system (ATPS) of dextran (DEX) and polyethylene glycol (PEG). Our technique controls the length of spikes that emerge on the surface of these microparticles. We anticipate that these pollen-like spiky microparticles may be used as drug delivery vehicles, and this new chemical synthesis approach may be used for generating other biomaterials.

Shape Coding Microhydrogel for a Real-Time Mycotoxin Detection System Based on Smartphones

How to create a portable and quick way to detect multiple coexisting toxins is closely related to everyone's health. In this paper, we have established a real-time mycotoxin detection system that combined shape-encoded hydrogel particle preparation technology and image processing technology with smartphone portable devices. First, hydrogel microparticles containing a specific recognition toxin aptamer were programmable synthesized by stop-flow lithography. The hydrogel particles prepared by us had clear, variable signals and high coding capacity. Then, the indirect competitive detection based on aptamers was simple and rapid; the total reaction time was no more than 1 h 45 min and the image processing process was no more than 10 s. Finally, images could be captured by cameras on portable devices and smartphones. The self-built Android app that used the image recognition program installed on the smartphone would analyze the image and return the results in real time. The results showed that the detection limit reached 0.1 ng/mL, which was lower than the standard. In summary, this platform provides a fast, portable, high-throughput detection solution for real-time detection of mycotoxins, with excellent application prospects.

Designable 3D Microshapes Fabricated at the Intersection of Structured Flow and Optical Fields

3D structures with complex geometric features at the microscale, such as microparticles and microfibers, have promising applications in biomedical engineering, self-assembly, and photonics. Fabrication of complex 3D microshapes at scale poses a unique challenge; high-resolution methods such as two-photon-polymerization have print speeds too low for high-throughput production, while top-down approaches for bulk processing using microfabricated template molds have limited control of microstructure geometries over multiple axes. Here, a method for microshape fabrication is presented that combines a thermally drawn transparent fiber template with a masked UV-photopolymerization approach to enable biaxial control of microshape fabrication. Using this approach, high-resolution production of complex microshapes not producible using alternative methods is demonstrated, such as octahedrons, dreidels, and axially asymmetric fibers, at throughputs as high as 825 structures/minute. Finally, the fiber template is functionalized with conductive electrodes to enable hierarchical subparticle localization using dielectrophoretic forces.

Shaping monodispersed azo molecular glass microspheres using polarized light

Azo molecular glass (IAC-4) microspheres with a monodispersed diameter over ten microns were fabricated by microfluidics and unique shape manipulation was achieved based on their fascinating photoinduced deformation behaviour. After irradiation with a polarized laser beam (λ = 488 nm), the IAC-4 microspheres were transformed into uniform mushroom-like particles, and their three-dimensional (3D) asymmetric shapes were precisely manipulated by adjusting the irradiation time and the polarization state of light. By observing the particle morphology in three orthogonal views (top view, front view and side view) by scanning electron microscopy (SEM), the photoinduced deformation behaviour of the ten-micron-sized particles was comprehensively revealed in the 3D space for the first time. It was observed that the photoinduced deformation asymmetrically occurred on the upper part of the microspheres due to the strong optical absorption of the azo chromophores. Besides, the deformation manner of the upper part was decided by the direction of the electric vibration of the refracted light. This work not only depicts a clear picture of the photoinduced deformation behaviour of the ten-micron-sized azo particles upon polarized light irradiation, but also provides a new method to controllably manipulate the particle shape from spheres to complex 3D architectures.