On-chip preparation of calcium alginate particles based on droplet templates formed by using a centrifugal microfluidic technique

Method for Measuring the Three-Dimensional Morphology of Near-Wall Bubbles and Droplets Based on LED Digital Holography

Digital holography, recognized for its noncontact nature and high precision in three-dimensional imaging, is effectively employed to measure the morphology of bubbles and droplets. However, in terms of near-wall bubbles and droplets, such as confined bubbles in microfluidic chips, the measurement of the interface morphology of bubbles near the glass surface has not yet been resolved due to the coherent noise resulting from glass surface reflections in microfluidic chips. Accordingly, an off-axis digital holography system was devised by using Linnik interferometry. Measuring the confined bubble interface near the wall within a microfluidic chip and droplet evaporation on solid surfaces was studied. Partially coherent LED sources and reference light modulation techniques were employed in the optical setup to mitigate the coherent noise. Dual exposure and weighted least-squares unwrapping algorithms were introduced to correct phase distortions, enhancing image quality. Imaging two confined CO2 bubbles was done near the wall in silicon oil within a porous microfluidic chip, and contact angles of 4.7 and 4.5° were measured. Additionally, the measurement of the three-dimensional morphology of vertically evaporating deionized water droplets on a glass surface was done, due to which calculation of contact angles at various orientations was possible. This work offers a feasible new method for measuring the 3D interface morphology of bubbles and droplets, particularly in microfluidic visualization, addressing current measurement gaps.

Characterization of Fluidic-Barrier-Based Particle Generation in Centrifugal Microfluidics

The fluidic barrier in centrifugal microfluidic platforms is a newly introduced concept for making multiple emulsions and microparticles. In this study, we focused on particle generation application to better characterize this method. Because the phenomenon is too fast to be captured experimentally, we employ theoretical models to show how liquid polymeric droplets pass a fluidic barrier before crosslinking. We explain how secondary flows evolve and mix the fluids within the droplets. From an experimental point of view, the effect of different parameters, such as the barrier length, source channel width, and rotational speed, on the particles’ size and aspect ratio are investigated. It is demonstrated that the barrier length does not affect the particle’s ultimate velocity. Unlike conventional air gaps, the barrier length does not significantly affect the aspect ratio of the produced microparticles. Eventually, we broaden this concept to two source fluids and study the importance of source channel geometry, barrier length, and rotational speed in generating two-fluid droplets.

Research on the Centrifugal Driving of a Water-in-Oil Droplet in a Microfluidic Chip with Spiral Microchannel

Combining the advantages of droplet-based microfluidics and centrifugal driving, a method for centrifugally driving W/O droplets with spiral microchannel is proposed in this paper. A physical model of droplet flow was established to study the flow characteristics of the W/O droplet in the spiral microchannel driven by centrifugal force, and kinematic analysis was performed based on the rigid body assumption. Then, the theoretical formula of droplet flow rate was obtained. The theoretical value was compared with the actual value measured in the experiments. The result shows that the trend of the theoretical value is consistent with the measured value, and the theoretical value is slightly larger than the experimentally measured value caused by deformation. Moreover, it is found that the mode of centrifugal driving with spiral microchannel has better flow stability than the traditional centrifugal driving structure. A larger regulation speed range can be achieved by adjusting the motor speed without using expensive equipment or precise instruments. This study can provide a basis and theoretical reference for the development of droplet-based centrifugal microfluidic chips.

Droplet Microfluidics for Food and Nutrition Applications

Droplet microfluidics revolutionizes the way experiments and analyses are conducted in many fields of science, based on decades of basic research. Applied sciences are also impacted, opening new perspectives on how we look at complex matter. In particular, food and nutritional sciences still have many research questions unsolved, and conventional laboratory methods are not always suitable to answer them. In this review, we present how microfluidics have been used in these fields to produce and investigate various droplet-based systems, namely simple and double emulsions, microgels, microparticles, and microcapsules with food-grade compositions. We show that droplet microfluidic devices enable unprecedented control over their production and properties, and can be integrated in lab-on-chip platforms for in situ and time-resolved analyses. This approach is illustrated for on-chip measurements of droplet interfacial properties, droplet-droplet coalescence, phase behavior of biopolymer mixtures, and reaction kinetics related to food digestion and nutrient absorption. As a perspective, we present promising developments in the adjacent fields of biochemistry and microbiology, as well as advanced microfluidics-analytical instrument coupling, all of which could be applied to solve research questions at the interface of food and nutritional sciences.

Modular and Integrated Systems for Nanoparticle and Microparticle Synthesis-A Review

Nanoparticles (NPs) and microparticles (MPs) have been widely used in different areas of research such as materials science, energy, and biotechnology. On-demand synthesis of NPs and MPs with desired chemical and physical properties is essential for different applications. However, most of the conventional methods for producing NPs/MPs require bulky and expensive equipment, which occupies large space and generally need complex operation with dedicated expertise and labour. These limitations hinder inexperienced researchers to harness the advantages of NPs and MPs in their fields of research. When problems individual researchers accumulate, the overall interdisciplinary innovations for unleashing a wider range of directions are undermined. In recent years, modular and integrated systems are developed for resolving the ongoing dilemma. In this review, we focus on the development of modular and integrated systems that assist the production of NPs and MPs. We categorise these systems into two major groups: systems for the synthesis of (1) NPs and (2) MPs; systems for producing NPs are further divided into two sections based on top-down and bottom-up approaches. The mechanisms of each synthesis method are explained, and the properties of produced NPs/MPs are compared. Finally, we discuss existing challenges and outline the potentials for the development of modular and integrated systems.

Droplet and Particle Generation on Centrifugal Microfluidic Platforms: A Review

The use of multiphase flows in microfluidics to carry dispersed phase material (droplets, particles, bubbles, or fibers) has many applications. In this review paper, we focus on such flows on centrifugal microfluidic platforms and present different methods of dispersed phase material generation. These methods are classified into three specific categories, i.e., step emulsification, crossflow, and dispenser nozzle. Previous works on these topics are discussed and related parameters and specifications, including the size, material, production rate, and rotational speed are explicitly mentioned. In addition, the associated theories and important dimensionless numbers are presented. Finally, we discuss the commercialization of these devices and show a comparison to unveil the pros and cons of the different methods so that researchers can select the centrifugal droplet/particle generation method which better suits their needs.

Controllable preparation of monodisperse alginate microcapsules with oil cores

Here we report a novel strategy for controllable preparation monodisperse alginate microcapsules with oil cores, where the thickness of the alginate shells, as well as the number and diversity of the oil cores can be tailored precisely. Monodisperse oil-in-water-in-oil (O/W/O) emulsions are generated in a microfluidic device as templates, which contain alginate molecules and a water-soluble calcium complex in the middle aqueous phase. Alginate microcapsules are produced by gelling O/W/O emulsions in oil solution with acetic acid, where the pH decreasing will trigger the calcium ions being released from calcium complex and cross-linking with alginate molecules. Increasing the alginate molecule concentration in emulsion templates affects little on the thickness of the microcapsules but improves their stability in DI water. The strength of alginate microcapsules can be reinforced by post cross-linking in calcium chloride, polyetherimide, or chitosan solution. Typical payloads, such as thyme essential oil, lavender essential oil and W/O emulsions are encapsulated in alginate microcapsules successfully. Furthermore, tailoring the thickness of the alginate shells, as well as the number and the diversity of the oil cores precisely by manipulation the emulsion templates with microfluidics is also demonstrated. The proposed method shows excellent controllability in designing alginate microcapsules with oil cores.

SERS monitoring of the Fenton degradation reaction based on microfluidic droplets and alginate microparticles

Surface-enhanced Raman scattering (SERS) spectroscopy as a powerful tool has been used to explore different catalysis degradation reactions, whereas some drawbacks caused by ferric ions still exist in the current SERS monitoring of the Fenton reaction process. In this work, microfluidic droplet- and alginate microparticle-based methods were, respectively, applied to realize SERS monitoring of the Fenton degradation process in a relatively stable environment, which benefited from reduction of the loss of ferrous ions and the aggregation of the SERS substrate. As expected, the spectroscopic evidence at the molecular level directly revealed the degradation mechanism of rhodamine dyes, showing that the chemical bonds between xanthene and carboxybenzene broke continuously during the reaction. Afterward, the degradation mechanism determined by SERS was verified via mass spectrometry detection, which confirmed the validity of the SERS-based method. More broadly, the microfluidic droplet- and microparticle-based methods are potentially applicable for SERS monitoring of more Fenton degradation reactions.

On-Chip Preparation of Amphiphilic Nanomicelles-in-Sodium Alginate Spheroids as a Novel Platform Against Triple-Negative Human Breast Cancer Cells: Fabrication, Study of Microfluidics Flow Hydrodynamics and Proof of Concept for Anticancer and Drug Delivery Applications

Spheroidal microparticles versatility as a drug carrier makes it a real workhorse in drug delivery applications. Despite of their long history, few research publications emphasize on how to improve their potential targeting ability, production rate, and dissolution characteristics. The current research presents an example of the combined state of the art of nano- and microparticles development technologies. Here in a novel on-chip, microfluidics approach is developed for encapsulating amphiphilic nanomicelles-in-sodium alginate spheroid. The designed nano-in-micro drug delivery system revealed a superior cytotoxicity against triple-negative human breast cancer cell line (MDA-MB-231), besides, a more sustained release of the drug. Hydrodynamics of the designed microchip was also investigated as a function of different flow rates with an insight on the dimensionless numbers; capillary number and Weber number throughout the microchannels. Our study confirmed the efficient encapsulation of nanomicelles within the alginate shell. The current microfluidics approach can be efficiently applied for uniform production of nano-in-microparticles with potential anticancer capability.

Gas-Shearing Fabrication of Multicompartmental Microspheres: A One-Step and Oil-Free Approach

Multicompartmental microparticles (MCMs) have attracted considerable attention in biomedical engineering and materials sciences, as they can carry multiple materials in the separated phases of a single particle. However, the robust fabrication of monodisperse, highly compartmental MCMs at the micro- and nanoscales remains challenging. Here, a simple one-step and oil-free process, based on the gas-flow-assisted formation of microdroplets ("gas-shearing"), is established for the scalable production of monodisperse MCMs. By changing the configuration of the needle system and gas flow in the spray ejector device, the oil-free gas-shearing process easily allows the design of microparticles consisting of two, four, six, and even eight compartments with a precise control over the properties of each compartment. As oils and surfactants are not used, the gas-shearing method is highly cytocompatible. The versatile applications of such MCMs are demonstrated by producing a magnetic microrobot and a biocompatible carrier for the coculturing of cells. This research suggests that the oil-free gas-shearing strategy is a reliable, scalable, and biofriendly process for producing MCMs that may become attractive materials for biomedical applications.

SERS-Fluorescence Dual-Mode pH-Sensing Method Based on Janus Microparticles

A surface-enhanced Raman scattering (SERS)-fluorescence dual-mode pH-sensing method based on Janus microgels was developed, which combined the advantages of high specificity offered by SERS and fast imaging afforded by fluorescence. Dual-mode probes, pH-dependent 4-mercaptobenzoic acid, and carbon dots were individually encapsulated in the independent hemispheres of Janus microparticles fabricated via a centrifugal microfluidic chip. On the basis of the obvious volumetric change of hydrogels in different pHs, the Janus microparticles were successfully applied for sensitive and reliable pH measurement from 1.0 to 8.0, and the two hemispheres showed no obvious interference. The proposed method addressed the limitation that sole use of the SERS-based pH sensing usually failed in strong acidic media. The gastric juice pH and extracellular pH change were measured separately in vitro using the Janus microparticles, which confirmed the validity of microgels for pH sensing. The microparticles exhibited good stability, reversibility, biocompatibility, and ideal semipermeability for avoiding protein contamination, and they have the potential to be implantable sensors to continuously monitor pH in vivo.

A General Route for Nanoemulsion Synthesis Using Low-Energy Methods at Constant Temperature

The central dogma of nanoemulsion formation using low-energy methods at constant temperature-popularly known as the emulsion inversion point (EIP) method-is that to create O/W nanoemulsions, water should be added to a mixture of an oil and surfactant. Here, we demonstrate that the above order of mixing is not universal and a reverse order of mixing could be superior, depending on the choice of surfactant and liquid phases. We propose a more general methodology to make O/W as well as W/O nanoemulsions by studying the variation of droplet size with the surfactant hydrophilic-lypophilic balance for several model systems. Our analysis shows that surfactant migration from the initial phase to the interface is the critical step for successful nanoemulsion synthesis of both O/W and W/O nanoemulsions. On the basis of our understanding and experimental results, we utilize the reverse order of mixing for two applications: (1) crystallization and formulation of pharmaceutical drugs with faster dissolution rates and (2) synthesis of alginate-based nanogels. The general route provides insights into nanoemulsion formation through low-energy methods and also opens up possibilities that were previously overlooked in the field.

Mass Production of Cell-Laden Calcium Alginate Particles with Centrifugal Force

This paper describes a centrifuge-based device for oil-free and mass production of calcium-alginate (Ca-alginate) particles. The device is composed of four components: a tank with a glass capillary for forming sodium alginate droplets, a collecting bath with calcium chloride (CaCl₂ ) solution, a waste liquid box, and a bypass channel bridged between the collecting bath and the waste liquid box. When the device is centrifuged, extra CaCl₂ solution in the collecting bath is delivered to the waste liquid box to maintain the appropriate liquid level of CaCl₂ solution for the production of monodisperse Ca-alginate particles. The proposed device enables oil-free production of over 45 000 uniformly sized Ca-alginate particles in a single 240 s process, whereas using the conventional method with only a glass capillary, ≈1000 particles are formed within the same processing time. Because of the high biocompatibility of the oil-free process, the device is applicable to cell encapsulation in Ca-alginate particles with high cell viability, as well as the formation of a macroscopic 3D cellular structure using Ca-alginate particles covered with cells as assembly modules. These results suggest that the device can be a useful tool for preparing experimental platforms in biomedical and tissue engineering fields.

Recent advances in engineering microparticles and their nascent utilization in biomedical delivery and diagnostic applications

Complex microparticles (MPs) bearing unique characteristics such as well-tailored sizes, various morphologies, and multi-compartments have been attempted to be produced by many researchers in the past decades. However, a conventionally used method of fabricating MPs, emulsion polymerization, has a limitation in achieving the aforementioned characteristics and several approaches such as the microfluidics-assisted (droplet-based microfluidics and flow lithography-based microfluidics), electrohydrodynamics (EHD)-based, centrifugation-based, and template-based methods have been recently suggested to overcome this limitation. The outstanding features of complex MPs engineered through these suggested methods have provided new opportunities for MPs to be applied in a wider range of applications including cell carriers, drug delivery agents, active pigments for display, microsensors, interface stabilizers, and catalyst substrates. Overall, the engineered MPs expose their potential particularly in the field of biomedical engineering as the increased complexity in the engineered MPs fulfills well the requirements of the high-end applications. This review outlines the current trends of newly developed techniques used for engineered MPs fabrication and focuses on the current state of engineered MPs in biomedical applications.

Droplet-microfluidics towards the assembly of advanced building blocks in cell mimicry

Therapeutic cell mimicry is an approach in nanomedicine aiming at substituting for missing or lost cellular functions employing nature-inspired concepts. Pioneered decades ago, only now is this technology empowered with the arsenal of nanotechnological tools and ready to provide radically new solutions such as assembling synthetic organelles and artificial cells. One of these tools is droplet microfluidics (D-μF), which provides the flexibility to generate cargo-loaded particles with tunable size and shape in a fast and reliable manner, an essential requirement in cell mimicry. This minireview aims at outlining the developments in D-μF from the past four years focusing on the assembly of nanoparticles, Janus-shaped and other non-spherical particles as well as their loading with biological payloads.