Generation of Janus alginate hydrogel particles with magnetic anisotropy for cell encapsulation

Controlling the size and elastic modulus of in-aqueous alginate micro-beads

The fabrication of microgels, particularly those ranging from tens to hundreds of micrometers in size, represents a thriving area of research, particularly for biologists seeking controlled and isotropic media for cell encapsulation. In this article, we present a novel and robust method for producing structurally homogeneous alginate beads with a reduced environmental footprint, employing a co-flow focusing microfluidic device. These beads can be easily recovered in an oil-free aqueous medium, making the fabrication method highly suitable for diverse applications. We demonstrate precise control over the production of perfectly spherical beads across a wide range of diameters, from about 30 to 300 μm. We then measure Young's moduli of the beads, revealing a wide accessible range from 90 Pa to 11 kPa, contingent upon controlling the type (e.g. chain length) and concentration of alginate.

Janus structure hydrogels: recent advances in synthetic strategies, biomedical microstructure and (bio)applications

Janus structure hydrogels (JSHs) are novel materials. Their primary fabrication methods and various applications have been widely reported. JSHs are primarily composed of Janus particles (JNPs) and polysaccharide components. They exhibit two distinct physical or chemical properties, generating intriguing characteristics due to their asymmetric structure. Normally, one side (adhesive interface) is predominantly constituted of polysaccharide components, primarily serving excellent adhesion. On the other side (functional surface), they integrate diverse functionalities, concurrently performing a plethora of synergistic functions. In the biomedical field, JSHs are widely applied in anti-adhesion, drug delivery, wound healing, and other areas. It also exhibits functions in seawater desalination and motion sensing. Thus, JSHs hold broad prospects for applications, and they possess significant research value in nanotechnology, environmental science, healthcare, and other fields. Additionally, this article proposes the challenges and future work facing these fields.

Synthesis of Anisotropic Magnetic Polymeric Janus Particles by In Situ Separation of Magnetic Nanoparticles in a Microfluidic Device

Magnetic Janus particles have been studied extensively for medical and biological applications owing to their controllable mobility in fluid media. In this work, we report a novel microfluidic device designed for the synthesis of magnetically anisotropic Janus particles made of poly(ethylene glycol) diacrylate and embedded with magnetic iron oxide nanoparticles. Our method consists of a droplet generation step followed by magnetic separation using an external magnetic field and ultraviolet polymerization. The synthesized particles exhibit a monodisperse size distribution with a standard deviation of less than 3.5%, which is among the best size distributions obtained in the literature for magnetic Janus particles. The anisotropic magnetic property of the particles enable them to rotate about their own axes in the presence of an external magnetic field, introducing another degree of freedom to their motion. This microfluidic technique is simple, one-step, and versatile, offering control over the size distribution to synthesize magnetically anisotropic Janus particles.

Multicompartment calcium alginate microreactors to reduce substrate inhibition in enzyme cascade reactions

The formation of macromolecularly enriched condensates through associative or segregative liquid-liquid phase separation phenomena is known to play a central role in controlling various cellular functions in nature. The potential to spatially and temporally modulate multistep chemical reactions and pathways has inspired the use of phase-separated systems for the development of various synthetic colloidal micro- and nanoreactor systems. Here, we report a rational and synthetically minimal design strategy to emulate intended spatiotemporal functions in morphologically intricate and structurally defined calcium alginate hydrogel microreactors possessing multicompartmentalized internal architectures. Specifically, we implement a thermal phase separation protocol to achieve fine-control over liquid-liquid phase separation inside complex aqueous emulsion droplet templates that are loaded with hydrophilic polymer mixtures. Subsequent gelation of alginate-containing droplet templates using a novel freeze-thaw approach that can be applied to both scalable batch production or more precise microfluidic methods yields particle replicas, in which subcompartmentalized architectures can be retained. Larger active components can be enriched in the internal compartments due to their preferential solubility, and we show that selective sequestration of enzymes serves to create desired microenvironments to control and tune the reaction kinetics of a multistep enzyme cascade by reducing their mutual interference. This demonstration of mitigating substrate inhibition that is based primarily on optimizing the multicompartmentalized hydrogel particle morphology offers new opportunities for the simple and synthetically-minimal batch generation of hydrogel-based synthesis microreactors.

Starch Janus Particles: Bulk Synthesis, Self-Assembly, Rheology, and Potential Food Applications

Although incredible progress in the field of Janus particles over the last three decades has delivered many promising smart-material prototypes, from cancer-targeting drug delivery vehicles to self-motile nanobots, their real-world applications have been somewhat tempered by concerns over scalability and sustainability. In this study, we adapt a simple, scalable 3D mask method to synthesize Janus particles in bulk using starch as the base material: a natural biopolymer that is safe, biocompatible, biodegradable, cheap, widely available, and versatile. Using this method, starch granules are first embedded on a wax droplet such that half of the starch is covered; then, the uncovered half is treated with octenyl succinic anhydride, after which the wax coating is removed. Janus particles with 49% Janus balance can be produced in this way and were observed to self-assemble into wormlike strings in water due to their hydrophobic/hydrophilic nature. Our Janus starch granules outperform the non-Janus controls as thickening and gelling agents: they exhibit a fourfold increase in water-holding capacity, a 30% lower critical caking concentration, and a viscosity greater by orders of magnitude. They also form gels that are much firmer and more stable. Starch Janus particles with these functional properties can be used as novel, lower-calorie, highly efficient, plant-based super-thickeners in the food industry, potentially reducing starch use in food by 55%.

Microfluidic Formulation of Topological Hydrogels for Microtissue Engineering

Microfluidics has recently emerged as a powerful tool in generation of submillimeter-sized cell aggregates capable of performing tissue-specific functions, so-called microtissues, for applications in drug testing, regenerative medicine, and cell therapies. In this work, we review the most recent advances in the field, with particular focus on the formulation of cell-encapsulating microgels of small "dimensionalities": "0D" (particles), "1D" (fibers), "2D" (sheets), etc., and with nontrivial internal topologies, typically consisting of multiple compartments loaded with different types of cells and/or biopolymers. Such structures, which we refer to as topological hydrogels or topological microgels (examples including core-shell or Janus microbeads and microfibers, hollow or porous microstructures, or granular hydrogels) can be precisely tailored with high reproducibility and throughput by using microfluidics and used to provide controlled "initial conditions" for cell proliferation and maturation into functional tissue-like microstructures. Microfluidic methods of formulation of topological biomaterials have enabled significant progress in engineering of miniature tissues and organs, such as pancreas, liver, muscle, bone, heart, neural tissue, or vasculature, as well as in fabrication of tailored microenvironments for stem-cell expansion and differentiation, or in cancer modeling, including generation of vascularized tumors for personalized drug testing. We review the available microfluidic fabrication methods by exploiting various cross-linking mechanisms and various routes toward compartmentalization and critically discuss the available tissue-specific applications. Finally, we list the remaining challenges such as simplification of the microfluidic workflow for its widespread use in biomedical research, bench-to-bedside transition including production upscaling, further in vivo validation, generation of more precise organ-like models, as well as incorporation of induced pluripotent stem cells as a step toward clinical applications.

Microfluidic synthesis of Janus-structured QD-encoded magnetic microbeads for multiplex immunoassay

Uniform and monodisperse quantum dot (QD)-encoded magnetic microbeads with Janus structure were produced in a microfluidic device via photopolymerization. UV light through a microscope objective was used to solidify the microbeads which showed sharp interfaces and excellent magnetic responses. QDs with different emission peaks (450 nm for blue and 640 nm for red) were mixed at different ratios to provide three spectral codes. The QD-encoded microbeads can be distinguished by analyzing their fluorescent images in HSV color space. After hydrolysis of the anhydride group in alkaline solution, protein was immobilized on microbeads via activation of carboxyl groups using (1-ethyl-3(3-dimethylaminoprophyl) carbodiimide/N-hydroxysuccinimide (EDC/NHS). A microhole array in polydimethylsiloxane (PDMS) substrates with a specific size was fabricated to trap individual microbeads in a single microhole. The combination of Janus-structured QD-encoded magnetic microbeads and microhole arrays facilitates both flexibility, binding kinetics, sensitivity for suspension assay, and fluorescence mapping analysis for conventional biochips, thus providing a novel platform for multiplex bioanalysis. The capability of this integration for multiplex immunoassays was verified using three kinds of IgG and their corresponding anti-IgG. A detection limit of 0.07 ng/mL was achieved for human IgG, indicating practical applications in various fields.

Microfluidic Methods in Janus Particle Synthesis

Janus particles have been at the center of attention over the years due to their asymmetric nature that makes them superior in many ways to conventional monophase particles. Several techniques have been reported for the synthesis of Janus particles; however, microfluidic-based techniques are by far the most popular due to their versatility, rapid prototyping, low reagent consumption and superior control over reaction conditions. In this review, we will go through microfluidic-based Janus particle synthesis techniques and highlight how recent advances have led to complex functionalities being imparted to the Janus particles.

Electrosprayed Regeneration-Enhancer-Element Microspheres Power Osteogenesis and Angiogenesis Coupling

Electrosprayed microspheres for bone regeneration are conventionally restricted by the lack of osteogenic modulation for both encapsulated stem cells and surrounding cells at the defect site. Here, sodium alginate microspheres encapsulating L-arginine doped hydroxyapatite nanoparticles (Arg/HA NPs) and bone mesenchymal stem cells (BMSCs) as regeneration-enhancer-element reservoirs (Arg/HA-SA@BMSC) for bone healing are electrosprayed. The Arg/HA NPs serve as a container of L-arginine and Ca²⁺ and the BMSCs inside the microspheres metabolize the released L-arginine into bioactive gas nitric oxide (NO) in the presence of Ca²⁺ to activate the nitric oxide (NO)/cyclic guanosine monophosphate (cGMP) signaling pathway. Meanwhile, the generated NO diffuses out of the microspheres together with the Ca²⁺ and L-arginine as exterior enhancers to promote the osteogenesis-angiogenesis coupling of surrounding BMSCs and endothelial cells (ECs) at the bone defect site, generating an internal/external modulation loop between the encapsulated cells and surrounding native cells. It is demonstrated that such regeneration-enhancer-element reservoirs could effectively increase the bone tissue formation and neovasculature using rat calvarial defect models. It is envisioned that the microsphere system could streamline vascularized bone regeneration therapy as a high throughput, minimally invasive yet highly effective strategy to accelerate bone healing.

Recent Advances in Microgels: From Biomolecules to Functionality

The emerging applications of hydrogel materials at different length scales, in areas ranging from sustainability to health, have driven the progress in the design and manufacturing of microgels. Microgels can provide miniaturized, monodisperse, and regulatable compartments, which can be spatially separated or interconnected. These microscopic materials provide novel opportunities for generating biomimetic cell culture environments and are thus key to the advances of modern biomedical research. The evolution of the physical and chemical properties has, furthermore, highlighted the potentials of microgels in the context of materials science and bioengineering. This review describes the recent research progress in the fabrication, characterization, and applications of microgels generated from biomolecular building blocks. A key enabling technology allowing the tailoring of the properties of microgels is their synthesis through microfluidic technologies, and this paper highlights recent advances in these areas and their impact on expanding the physicochemical parameter space accessible using microgels. This review finally discusses the emerging roles that microgels play in liquid-liquid phase separation, micromechanics, biosensors, and regenerative medicine.

Microfluidic generation of monodispersed Janus alginate hydrogel microparticles using water-in-oil emulsion reactant

Microparticles with uniform anisotropic structures are widely used in physical, chemical, and biological fields owing to their ability to combine multiple functions on a micro-scale. Here, a microfluidic emulsion-based external gelation method was demonstrated for the first time to produce monodisperse Janus calcium alginate (Ca-alginate) hydrogel microparticles consisting of two compartments. This approach provided a fast reaction condition under which we could prepare magnetic Janus Ca-alginate microparticles with diameters ranging from 148 to 179 μm and a coefficient of variation (CV) less than 4%. Moreover, the boundaries between the two compartments were clear. In addition, the volume fraction of each compartment could be adjusted by varying the flow rate ratio between two dispersed phases. Next, we produced fluorescent Janus beads and magnetic-fluorescent Janus beads with an average diameter of ∼150 μm (CV < 4.0%). The magnetic Janus hydrogel microparticles we produced could be manipulated by applying a magnetic field to achieve self-assembly, rotation, and accumulation. Magnetic Janus hydrogel microparticles are also capable of mammalian cell encapsulation with good cell viability. This article presents a simple and stable approach for producing monodisperse bi-compartmental Janus hydrogel microparticles that could have great potential for application in physical, biochemical, and biomedical fields.

Janus particles and motors: unrivaled devices for mastering (bio)sensing

Janus particles are a unique type of materials combining two different functionalities in a single unit. This allows the combination of different analytical properties leading to new analytical capabilities, i.e., enhanced fluid mixing to increase sensitivity with targeting capturing abilities and unique advantages in terms of multi-functionality and versatility of modification, use, and operation both in static and dynamic modes. The aim of this conceptual review is to cover recent (over the last 5 years) advances in the use of Janus microparticles and micromotors in (bio)-sensing. First, the role of different materials and synthetic routes in the performance of Janus particles are described. In a second main section, electrochemical and optical biosensing based on Janus particles and motors are covered, including in vivo and in vitro methodologies as the next biosensing generation. Current challenges and future perspectives are provided in the conclusions section.

Microfluidic Preparation of Janus Microparticles With Temperature and pH Triggered Degradation Properties

Based on the phase separation phenomenon in micro-droplets, polymer-lipid Janus particles were prepared on a microfluidic flow focusing chip. Phase separation of droplets was caused by solvent volatilization and Janus morphology was formed under the action of interfacial tension. Because phase change from solid to liquid of the lipid hemisphere could be triggered by physiological temperature, the lipid hemisphere could be used for rapid release of drugs. While the polymer we selected was pH sensitive that the polymer hemisphere could degrade under acidic conditions, making it possible to release drugs in a specific pH environment, such as tumor tissues. Janus particles with different structures were obtained by changing the experimental conditions. To widen the application range of the particles, fatty alcohol and fatty acid-based phase change materials were also employed to prepare the particles, such as 1-tetradecanol, 1-hexadecanol and lauric acid. The melting points of these substances are higher than the physiological temperature, which can be applied in fever triggered drug release or in thermotherapy. The introduction of poly (lactic-co-glycolic acid) enabled the formation of multicompartment particles with three distinct materials. With different degradation properties of each compartment, the particles generated in this work may find applications in programmed and sequential drug release triggered by multiple stimuli.

Crosslinking Strategies for the Microfluidic Production of Microgels

This article provides a systematic review of the crosslinking strategies used to produce microgel particles in microfluidic chips. Various ionic crosslinking methods for the gelation of charged polymers are discussed, including external gelation via crosslinkers dissolved or dispersed in the oil phase; internal gelation methods using crosslinkers added to the dispersed phase in their non-active forms, such as chelating agents, photo-acid generators, sparingly soluble or slowly hydrolyzing compounds, and methods involving competitive ligand exchange; rapid mixing of polymer and crosslinking streams; and merging polymer and crosslinker droplets. Covalent crosslinking methods using enzymatic oxidation of modified biopolymers, photo-polymerization of crosslinkable monomers or polymers, and thiol-ene “click” reactions are also discussed, as well as methods based on the sol−gel transitions of stimuli responsive polymers triggered by pH or temperature change. In addition to homogeneous microgel particles, the production of structurally heterogeneous particles such as composite hydrogel particles entrapping droplet interface bilayers, core−shell particles, organoids, and Janus particles are also discussed. Microfluidics offers the ability to precisely tune the chemical composition, size, shape, surface morphology, and internal structure of microgels by bringing multiple fluid streams in contact in a highly controlled fashion using versatile channel geometries and flow configurations, and allowing for controlled crosslinking.

Engineering Hydrogel-Based Biomedical Photonics: Design, Fabrication, and Applications

Light guiding and manipulation in photonics have become ubiquitous in events ranging from everyday communications to complex robotics and nanomedicine. The speed and sensitivity of light-matter interactions offer unprecedented advantages in biomedical optics, data transmission, photomedicine, and detection of multi-scale phenomena. Recently, hydrogels have emerged as a promising candidate for interfacing photonics and bioengineering by combining their light-guiding properties with live tissue compatibility in optical, chemical, physiological, and mechanical dimensions. Herein, the latest progress over hydrogel photonics and its applications in guidance and manipulation of light is reviewed. Physics of guiding light through hydrogels and living tissues, and existing technical challenges in translating these tools into biomedical settings are discussed. A comprehensive and thorough overview of materials, fabrication protocols, and design architectures used in hydrogel photonics is provided. Finally, recent examples of applying structures such as hydrogel optical fibers, living photonic constructs, and their use as light-driven hydrogel robots, photomedicine tools, and organ-on-a-chip models are described. By providing a critical and selective evaluation of the field's status, this work sets a foundation for the next generation of hydrogel photonic research.

Generation and Dynamics of Janus Droplets in Shear-Thinning Fluid Flow in a Double Y-Type Microchannel

Droplets composed of two different materials, or Janus droplets, have diverse applications, including microfluidic digital laboratory systems, DNA chips, and self-assembly systems. A three-dimensional computational study of Janus droplet formation in a double Y-type microfluidic device filled with a shear-thinning fluid is performed by using the multiphaseInterDyMFoam solver of the OpenFOAM, based on a finite-volume method. The bi-phase volume-of-fluid method is adopted to track the interface with an adaptive dynamic mesh refinement for moving interfaces. The formation of Janus droplets in the shear-thinning fluid is characterized in five different states of tubbing, jetting, intermediate, dripping and unstable dripping in a multiphase microsystem under various flow conditions. The formation mechanism of Janus droplets is understood by analyzing the influencing factors, including the flow rates of the continuous phase and of the dispersed phase, surface tension, and non-Newtonian rheological parameters. Studies have found that the formation of the Janus droplets and their sizes are related to the flow rate at the inlet under low capillary numbers. The rheological parameters of shear-thinning fluid have a significant impact on the size of Janus droplets and their formation mechanism. As the apparent viscosity increases, the frequency of Janus droplet formation increases, while the droplet volume decreases. Compared with Newtonian fluid, the Janus droplet is more readily generated in shear-thinning fluid due to the interlay of diminishing viscous force, surface tension, and pressure drop.

Microfluidic-prepared, monodisperse, X-ray-visible, embolic microspheres for non-oncological embolization applications

Embolotherapy using particle embolics is normally performed with exogenous contrast to assist in visualization. However, the exact location of the embolics cannot be identified after contrast washout. We developed a novel, pseudo-check valve-integrated microfluidic device, that partitions barium- impregnated alginate from crosslinking solution, thereby preventing nozzle failure. This enables rapid and continuous generation of inherently X-ray-visible embolic microspheres (XEMs) with uniform size. The XEMs are visible under clinical X-ray and cone beam CT both in vitro and in vivo. In particular, we demonstrated the embolization properties of these XEMs in large animals, performing direct intra- and post-procedural assessment of embolic delivery. The persistent radiopacity of these XEMs enables real-time evaluation of embolization precision and offers great promise for non-invasive follow-up examination without exogenous contrast. We also demonstrated that bariatric arterial embolization with XEMs significantly suppresses weight gain in swine, as an example of a non-oncological application of embolotherapy.

Biopolymer Microparticles Prepared by Microfluidics for Biomedical Applications

Biopolymers are macromolecules that are derived from natural sources and have attractive properties for a plethora of biomedical applications due to their biocompatibility, biodegradability, low antigenicity, and high bioactivity. Microfluidics has emerged as a powerful approach for fabricating polymeric microparticles (MPs) with designed structures and compositions through precise manipulation of multiphasic flows at the microscale. The synergistic combination of materials chemistry afforded by biopolymers and precision provided by microfluidic capabilities make it possible to design engineered biopolymer-based MPs with well-defined physicochemical properties that are capable of enabling an efficient delivery of therapeutics, 3D culture of cells, and sensing of biomolecules. Here, an overview of microfluidic approaches is provided for the design and fabrication of functional MPs from three classes of biopolymers including polysaccharides, proteins, and microbial polymers, and their advances for biomedical applications are highlighted. An outlook into the future research on microfluidically-produced biopolymer MPs for biomedical applications is also provided.

Experimental Studies of Microchannel Tapering on Droplet Forming Acceleration in Liquid Paraffin/Ethanol Coaxial Flows

The formations of micro-droplets are strongly influenced by the local geometries where they are generated. In this paper, through experimental research, we focus on the roles of microchannel tapering in the liquid paraffin/ethanol coaxial flows in their flow patterns, flow regimes, and droplet parameters, i.e., their sizes and forming frequencies. For validity, the non-tapering coaxial flows (the convergence angle α=0∘) are investigated, the experimental methods and experimental data are examined and analyzed by contrasting the details with previous works, and consistent results are obtained. We consider a slightly tapering microchannel (the convergence angle α=2.8∘) and by comparison, the experiments show that the tapering has significant effects on the flow patterns, droplet generation frequencies, and droplet sizes. The regimes of squeezing, dripping, jetting, tubing, and threading are differentiated to shrink toward the coordinate origin of the Cac–Wed space. The closer it is to the origin, the less variations will occur. For the adjacent regimes of the origin, i.e., dripping and squeezing, slight changes have occurred in both flow patterns, as well as the droplet characters. In the dripping and squeezing modes, the liquid droplets are generated near the orifice of the inner tube. Their forming positions (geometry) and flow conditions are almost the same. Therefore, the causes of minute changes in such regimes are physically understandable. While in the jetting regimes, the droplets shrink in size and their forming frequencies increase. The droplet sizes and the frequencies are both linearly related to those of the non-tapering cases with the corresponding relations derived. Furthermore, the threading and the tubing patterns almost did not emerged in the non-tapering data, as it seemed easier to form elongated jets, thinning or widening, in the tapered tubes. This can be explained by the stable analysis of the coaxial jets, which indicates that the reductions in the microchannel diameters can suppress the development of the interface disturbances.

Janus particles: from concepts to environmentally friendly materials and sustainable applications

Janus particles represent a unique group of patchy particles combining two or more different physical or chemical functionalities at their opposite sides. Especially, individual Janus particles (JPs) with both chemical and geometrical anisotropy as well as their assembled layers provide considerable advantages over the conventional monofunctional particles or surfactant molecules offering (a) a high surface-to-volume ratio; (b) high interfacial activity; (c) target controlling and manipulation of their interfacial activity by external signals such as temperature, light, pH, or ionic strength and achieving switching between stable emulsions and macro-phase separation; (d) recovery and recycling; (e) controlling the mass transport across the interface between the two phases; and finally (f) tunable several functionalities in one particle allowing their use either as carrier materials for immobilized catalytically active substances or, alternatively, their site-selective attachment to substrates keeping another functionality active for further reactions. All these advantages of JPs make them exclusive materials for application in (bio-)catalysis and (bio-)sensing. Considering “green chemistry” aspects covering biogenic materials based on either natural or fully synthetic biocompatible and biodegradable polymers for the design of JPs may solve the problem of toxicity of some existing materials and open new paths for the development of more environmentally friendly and sustainable materials in the very near future. Considering the number of contributions published each year on the topic of Janus particles in general, the number of contributions regarding their environmentally friendly and sustainable applications is by far smaller. This certainly pinpoints an important challenge and is addressed in this review article. The first part of the review focuses on the synthesis of sustainable biogenic or biocompatible Janus particles, as well as strategies for their recovery, recycling, and reusability. The second part addresses recent advances in applications of biogenic/biocompatible and non-biocompatible JPs in environmental and biotechnological fields such as sensing of hazardous pollutants, water decontamination, and hydrogen production. Finally, we provide implications for the rational design of environmentally friendly and sustainable materials based on Janus particles.

Microfluidics for Processing of Biomaterials

Microfluidics techniques can be used to process a wide range of biomaterials, from synthetic to natural origin ones. This chapter describes microfluidic processing of biomaterials, mainly polymeric materials of natural origin, focusing on water-soluble polymers that form non-flowing phases after crosslinking. Some polysaccharides and proteins, including agarose, alginate, chitosan, gellan gum, hyaluronic acid, collagen, gelatin, and silk fibroin are emphasized deu to their relevance in the field. The critical characteristics of these materials are discussed, giving particular consideration to those that directly impact its processability using microfluidics. Furthermore, some microfluidic-based processing techniques are presented, describing their suitability to process materials with different sol-gel transition mechanisms.

A Facile Strategy for Visualizing and Modulating Droplet-Based Microfluidics

In droplet-based microfluidics, visualizing and modulating of droplets is often prerequisite. In this paper, we report a facile strategy for visualizing and modulating high-throughput droplets in microfluidics. In the strategy, by modulating the sampling frequency of a flash light with the droplet frequency, we are able to map a real high frequency signal to a low frequency signal, which facilitates visualizing and feedback controlling. Meanwhile, because of not needing synchronization signals, the strategy can be directly implemented on any droplet-based microfluidic chips. The only cost of the strategy is an additional signal generator. Moreover, the strategy can catch droplets with frequency up to several kilohertz, which covers the range of most high-throughput droplet-based microfluidics. In this paper, the principle, setup and procedure were introduced. Finally, as a demonstration, the strategy was also implemented in a miniaturized picoinjector in order to monitor and control the injection dosage to droplets. We expect that this facile strategy supplies a low-cost yet effective imaging system that can be easily implemented in miniaturized microfluidic systems or general laboratories.

Design Principles and Multifunctionality in Cell Encapsulation Systems for Tissue Regeneration

Cell encapsulation systems are being increasingly applied as multifunctional strategies to regenerate tissues. Lessons afforded with encapsulation systems aiming to treat endocrine diseases seem to be highly valuable for the tissue engineering and regenerative medicine (TERM) systems of today, in which tissue regeneration and biomaterial integration are key components. Innumerous multifunctional systems for cell compartmentalization are being proposed to meet the specific needs required in the TERM field. Herein is reviewed the variable geometries proposed to produce cell encapsulation strategies toward tissue regeneration, including spherical and fiber-shaped systems, and other complex shapes and arrangements that better mimic the highly hierarchical organization of native tissues. The application of such principles in the TERM field brings new possibilities for the development of highly complex systems, which holds tremendous promise for tissue regeneration. The complex systems aim to recreate adequate environmental signals found in native tissue (in particular during the regenerative process) to control the cellular outcome, and conferring multifunctional properties, namely the incorporation of bioactive molecules and the ability to create smart and adaptative systems in response to different stimuli. The new multifunctional properties of such systems that are being employed to fulfill the requirements of the TERM field are also discussed.

Magnetic Janus particles synthesized using droplet micro-magnetofluidic techniques for protein detection

Magnetic droplets on a microfluidic platform can act as micro-robots, providing wireless, remote, and programmable control. This field of droplet micro-magnetofluidics (DMMF) is useful for droplet merging, mixing and synthesis of Janus structures. Specifically, magnetic Janus particles (MJP) are useful for protein and DNA detection as well as magnetically controlled bioprinting. However, synthesis of MJP with control of the functional phases is a challenge. Hence, we developed a high flow rate, surfactant-free, wash-less method to synthesize MJP by integration of DMMF with hybrid magnetic fields. The effects of the flow rate, flow rate ratio, and hybrid magnetic field on the magnetic component of the Janus droplets and the MJP were investigated. It was found that the magnetization, particle size, and phase distribution inside MJP could be readily tuned by the flow rates and the magnetic field. The magnetic component in the MJP could be concentrated after mixing at flow rate ratio values less than 7.5 and flow rates less than 3 ml h^-1. The experimental results and our simulations are in good agreement. The synthesized magnetic-fluorescent Janus particles were used for protein detection, with BSA as a model protein.

Multiplex coaxial flow focusing for producing multicompartment Janus microcapsules with tunable material compositions and structural characteristics

We propose a simple but efficient multiplex coaxial flow focusing (MCFF) process for single-step fabrication of multicompartment Janus microcapsules (MJMs) in a wide range of operating parameters. The produced MJMs consist of a multicompartmental core-shell structure with material compositions tunable in individual shell and core compartments. Potential applications of such a MJM agent are demonstrated in both benchtop and in vitro experiments. For the benchtop experiment, magnetic nanoparticles are loaded into one of the shell compartments and photopolymerized under ultraviolet light for controlled alignment and rotation of the microcapsules in a magnetic field. For the in vitro experiment, four different types of cells are encapsulated in the desired compartments of sodium alginate MJMs and co-cultured for seven days. By increasing the number of coaxial needles, we are also able to produce MJMs with three or more compartments. Our studies have shown that the proposed MCFF process is able to produce MJMs with desired material compositions and narrow size distribution. This process is inexpensive and scalable for mass production of various MJMs in its potential applications in biomedical imaging, drug delivery, and regenerative medicine.

Alginate gel particles-A review of production techniques and physical properties

The application of hydrocolloid gel particles is potentially useful in food, chemical, and pharmaceutical industries. Alginate gel particles are one of the more commonly used hydrocolloid gel particles due to them being biocompatible, nontoxic, biodegradable, cheap, and simple to produce. They are particularly valued for their application in encapsulation. Encapsulation in alginate gel particles confers protective benefits to cells, DNA, nutrients, and microbes. Slow release of flavors, minerals, and drugs can also be achieved by encapsulation in gel particles. The particle size and shape of the gel particles are crucial for specific applications. In this review, current methods of producing alginate gel particles will be discussed, taking into account their advantages, disadvantages, scalability, and impact on particle size. The physical properties of alginate gel particles will determine the effectiveness in different application conditions. This review will cover the current understanding of the alginate biopolymer, gelation mechanisms and factors affecting release properties, gel strength, and rheology of the alginate gel particle systems.

Liquid–liquid microflow reaction engineering

Engineering characteristics of liquid–liquid microflow and its advantages in chemical reactions.

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.

One-Step Generation of Cell-Encapsulating Compartments via Polyelectrolyte Complexation in an Aqueous Two Phase System

Diverse fields including drug and gene delivery and live cell encapsulation require biologically compatible encapsulation systems. One widely adopted means of forming capsules exploits cargo-filled microdroplets in an external, immiscible liquid phase that are encapsulated by a membrane that forms by trapping of molecules or particles at the drop surface, facilitated by the interfacial tension. To eliminate the potentially deleterious oil phase often present in such processes, we exploit the aqueous two phase system of poly(ethylene glycol) (PEG) and dextran. We form capsules by placing dextran-rich microdroplets in an external PEG-rich phase. Strong polyelectrolytes present in either phase form complexes at the drop interface, thereby forming a membrane encapsulating the fluid interior. This process requires considerable finesse as both polyelectrolytes are soluble in either the drop or external phase, and the extremely low interfacial tension is too weak to provide a strong adsorption site for these molecules. The key to obtaining microcapsules is to tune the relative fluxes of the two polyelectrolytes so that they meet and complex at the interface. We identify conditions for which complexation can occur inside or outside of the drop phase, resulting in microparticles or poor encapsulation, respectively, or when properly balanced, at the interface, resulting in microcapsules. The resulting microcapsules respond to the stimuli of added salts or changes in osmotic pressure, allowing perturbation of capsule permeability or triggered release of capsule contents. We demonstrate that living cells can be sequestered and interrogated by encapsulating Pseudomonas aeruginosa PAO1 and using a Live/Dead assay to assess their viability. This method paves the way to the formation of a broad variety of versatile functional membranes around all aqueous capsules; by tuning the fluxes of complexing species to interact at the interface, membranes comprising other complexing functional moieties can be formed.

Design Advances in Particulate Systems for Biomedical Applications

The search for more efficient therapeutic strategies and diagnosis tools is a continuous challenge. Advances in understanding the biological mechanisms behind diseases and tissues regeneration have widened the field of applications of particulate systems. Particles are no more just protective systems for the encapsulated drugs, but they play an active role in the success of the therapy. Moreover, particles have been explored for innovative purposes as templates for cells growth and as diagnostic tools. Until few years ago the most relevant parameters in particles formulation were the chemistry and the size. Currently, it is known that other physical characteristics can remarkably affect the performance of particulate systems. Particles with non-conventional shapes exhibit advantages due to the increasing circulation time in blood stream, less clearance by the immune system and more efficient cell internalization and trafficking. Creation of compartments has been found useful to control drug release, to tune the transport of substances across biological barriers, to supply the target with more than one bioactive agent or even to act as theranostic systems. It is expected that such complex shaped and compartmentalized systems improve the therapeutic outcomes and also the patient's compliance, acting as advanced devices that serve for simultaneous diagnosis and treatment of the disease, combining agents of very different features, at the same time. In this review, we overview and analyse the most recent advances in particle shape and compartmentalization and applications of newly designed particulate systems in the biomedical field.

Bistable self-assembly in homogeneous colloidal systems for flexible modular architectures

This paper presents a homogeneous system of magnetic colloidal particles that self-assembles via two structural patterns of different symmetry. Based on a qualitative comparison between a real magnetic particles system, analytical calculations and molecular dynamics simulations, it is shown that bistability can be achieved by a proper tailoring of an anisotropic magnetization distribution inside the particles. The presented bistability opens new possibilities to form two-dimensionally extended and flexible structures where the connectivity between the particles can be changed in vivo.

Strategies to Maximize the Potential of Marine Biomaterials as a Platform for Cell Therapy

Marine biopolymers have been explored as a promising cell therapy system for efficient cell delivery and tissue engineering. However, the marine biomaterial-based systems themselves have exhibited limited performance in terms of maintenance of cell viability and functions, promotion of cell proliferation and differentiation as well as cell delivery efficiency. Thus, numerous novel strategies have been devised to improve cell therapy outcomes. The strategies include optimization of physical and biochemical properties, provision of stimuli-responsive functions, and design of platforms for efficient cell delivery and tissue engineering. These approaches have demonstrated substantial improvement of therapeutic outcomes in a variety of research settings. In this review, therefore, research progress made with marine biomaterials as a platform for cell therapy is reported along with current research directions to further advance cell therapies as a tool to cure incurable diseases.

Controlled production of size-tunable Janus droplets for submicron particle synthesis using an electrospray microfluidic chip

Controllable fabrication of Janus droplets and submicron Janus particles using an electrospray microfluidic approach has been developed.

Composites of Polymer Hydrogels and Nanoparticulate Systems for Biomedical and Pharmaceutical Applications

Due to their unique structures and properties, three-dimensional hydrogels and nanostructured particles have been widely studied and shown a very high potential for medical, therapeutic and diagnostic applications. However, hydrogels and nanoparticulate systems have respective disadvantages that limit their widespread applications. Recently, the incorporation of nanostructured fillers into hydrogels has been developed as an innovative means for the creation of novel materials with diverse functionality in order to meet new challenges. In this review, the fundamentals of hydrogels and nanoparticles (NPs) were briefly discussed, and then we comprehensively summarized recent advances in the design, synthesis, functionalization and application of nanocomposite hydrogels with enhanced mechanical, biological and physicochemical properties. Moreover, the current challenges and future opportunities for the use of these promising materials in the biomedical sector, especially the nanocomposite hydrogels produced from hydrogels and polymeric NPs, are discussed.

Formation of Cucurbit[8]uril-Based Supramolecular Hydrogel Beads Using Droplet-Based Microfluidics

Herein we describe the use of microdroplets as templates for the fabrication of uniform-sized supramolecular hydrogel beads, assembled by supramolecular cross-linking of functional biopolymers with the macrocyclic host molecule, cucurbit[8]uril (CB[8]). The microdroplets were formed containing diluted hydrogel precursors in solution, including the functional polymers and CB[8], in a microfluidic device. Subsequent evaporation of water from collected microdroplets concentrated the contents, driving the formation of the CB[8]-mediated host-guest ternary complex interactions and leading to the assembly of condensed three-dimensional polymeric scaffolds. Rehydration of the dried particles gave monodisperse hydrogel beads. Their equilibrium size was shown to be dependent on both the quantity of material loaded and the dimensions of the microfluidic flow focus. Fluorescein-labeled dextran was used to evaluate the efficacy of the hydrogel beads as a vector for controlled cargo release. Both passive, sustained release (hours) and triggered, fast release (minutes) of the FITC-dextran was observed, with the rate of sustained release dependent on the formulation. The kinetics of release was fitted to the Ritger-Peppas controlled release equation and shown to follow an anomalous (non-Fickian) transport mechanism.

Microfluidic Production of Alginate Hydrogel Particles for Antibody Encapsulation and Release

Owing to their biocompatibility and reduced side effects, natural polymers represent an attractive choice for producing drug delivery systems. Despite few successful examples, however, the production of monodisperse biopolymer-based particles is often hindered by high viscosity of polymer fluids. In this work, we present a microfluidic approach for production of alginate-based particles carrying encapsulated antibodies. We use a triple-flow micro-device to induce hydrogel formation inside droplets before their collection off-chip. The fast mixing and gelation process produced alginate particles with a unique biconcave shape and dimensions of the mammalian cells. We show slow and fast dissolution of particles in different buffers and evaluate antibody release over time.

Fabrication of shape controllable Janus alginate/pNIPAAm microgels via microfluidics technique and off-chip ionic cross-linking

A novel method to fabricate shape controllable alginate/pNIPAAm complex microgels is reported. Monodisperse alginate/pNIPAAm droplets are created via microfluidics and cross-linked in different concentrations of hot glycerol/barium acetate water solutions. By changing the initial droplet size and glycerol concentration of the collecting solution, the resultant microgel shape and surface details can be systematically tuned. High-speed imaging is used to visualize and explain the microgel formation process.

Microfluidics-assisted diffusion self-assembly: toward the control of the shape and size of pectin hydrogel microparticles

We demonstrated the generation of pectin hydrogel microparticles having complex shapes either by combining the phenomenon of gelation and water diffusion-induced self-assembly in microfluidic channels (on-chip) or by the deformation of the pregelled droplets outside the channels (off-chip) at a fluid-fluid interface. We proved that by tuning the mode of pectin cross-linking (CaCl2 vs CaCO3) and the degree of shrinking (water content in the dimethyl carbonate (DMC) organic continuous phase) we can control the shape of the final particle. Sphere, doughnut, oblate ellipsoid, or mushroom-type morphologies were thus produced, demonstrating the ability to control the formation of anisotropic biopolymer-based hydrogel microparticles using microfluidics. Shape changes were explained by the redistribution of calcium ions in combination with the local Peclet number experienced by the microdroplets during the on-chip process. Moreover, during the off-chip process, the interplay between elastic and viscous forces for microdroplets entering the CaCl2-DMC interface caused deformation of the pregelled droplets to occur and therefore resulted in the formation of microparticles with a mushroom-like morphology.