Microfluidic Templated Multicompartment Microgels for 3D Encapsulation and Pairing of Single Cells

Advances in tissue engineering utilizing microfluidic platforms and techniques

This study aims to review the advancements in tissue engineering using microfluidic systems and techniques. In this way, tissue engineering has undergone a significant transformation through the employing of microfluidics knowledge in different areas of tissue engineering. This integration has made it possible to create biomimetic environments that more precisely resemble the physiological conditions of the human body. By using microfluidic platforms, researchers can manipulate biomaterials accurately and create complex three-dimensional (3D) scaffolds with specific characteristics. This research is devoted to illuminate the intricate, fascinating and improved collaboration between microfluidic systems and techniques such as produce high-quality hydrogels, precise vessel networks, enhanced nutrient transfer techniques, controlled hydrogel's size have been studied. These compounds are key components in scaffold fabrication for tissue engineering. By elucidating the principles, techniques, applications and progress involved in this interface, the broadened potential of this synergistic approach in advancing the frontiers of regenerative, tissue engineering, medicine and biomedical research was uncovered.

Encapsulation of single vesicles and single cells in a crosslinked microgel cage

Cell encapsulation provides an efficient strategy to enhance cell durability against harsh external conditions, that offers new possibilities for single-cell applications, such as, tissue engineering and regenerative medicine. Cell encapsulations in hydrogels is developed through various approaches. Still, it remains challenging to achieve single-cell encapsulation where the individual cells are surrounded by a hydrogel layer of well-defined thickness. In this study, temperature-responsive poly(N-isopropylacrylamide)-co-allylamine microgel particles are first assembled into a monolayer at the surface of giant unilamellar lipid vesicles and then inter-microgel crosslinked leading to single-vesicle encapsulation with a pre-defined hydrogel thickness. The same strategy is then extended to yeast cells. The successful encapsulation process is evidenced by the response of the encapsulated lipid vesicles/cells to osmotic gradient, the addition of detergent or salt, as well as changes in temperature. Moreover, cell viability tests show that the hydrogel cage can efficiently protect the cell against external harsh conditions, including elevated temperature, ultraviolet irradiation and osmotic stress. Furthermore, it is demonstrated that the microgel adsorption and interfacial assembly are significantly affected by membrane charge and structural heterogeneity of cell membrane, providing insight into rational design of single-cell encapsulation approach by regulating microgel adsorption on cell membranes with complex composition.

Anisotropic Microcarriers: Fabrication Strategies and Biomedical Applications

Anisotropic microcarriers (AMs) have attracted increasing attention. Although significant efforts have been made to explore AMs with various morphologies, their full potential is yet to be realized, as most studies have primarily focused on materials or fabrication methods. A thorough analysis of the interactional and interdependent relationships between these factors is required, along with proposed countermeasures tailored for researchers from various backgrounds. These countermeasures include specific fabrication strategies for various morphologies and guidelines for selecting the most suitable AM for certain biomedical applications. In this review, a comprehensive summary of AMs, ranging from their fabrication methods to biomedical applications, based on the past two decades of research, is provided. The fabrication of various morphologies is investigated using different strategies and their corresponding biomedical applications. By systematically examining these morphology-dependent effects, a better utilization of AMs with diverse morphologies can be achieved and clear strategies for breakthroughs in the biomedical field are established. Additionally, certain challenges are identified, new frontiers are opened, and promising and exciting opportunities are provided for fabricating functional AMs with broad implications across various fields that must be addressed in biomaterials and biotechnology.

Injectable microgel and micro-granular hydrogels for bone tissue engineering

Injectable microgels, made from both natural and synthetic materials, are promising platforms for the encapsulation of cells or bioactive agents, such as drugs and growth factors, for delivery to injury sites. They can also serve as effective micro-scaffolds in bone tissue engineering (BTE), offering a supportive environment for cell proliferation or differentiation into osteoblasts. Microgels can be injected in the injury sites individually or in the form of aggregated/jammed ones named micro-granular hydrogels. This review focuses on common materials and fabrication techniques for preparing injectable microgels, as well as their characteristics and applications in BTE. These applications include their use as cell carriers, delivery systems for bioactive molecules, micro-granular hydrogels, bio-inks for bioprinting, three-dimensional microarrays, and the formation of microtissues. Furthermore, we discuss the current and potential future applications of microgels in bone tissue regeneration.

Rapid Fabrication of Diverse Hydrogel Microspheres for Drug Evaluation on a Rotating Microfluidic System

Hydrogel microspheres are considered ideal carriers with broad applications in 3D cell culture, drug delivery, and microtissue construction. Although multiple methods have been developed for generating hydrogel microspheres, there is still a lack of a universal approach that combines operability, stability, cost-effectiveness, and biocompatibility. In this work, a novel rotating microfluidic system (RMS) is proposed, which can rapidly fabricate diverse poly(ethylene glycol) diacrylate/sodium alginate (PEGDA/SA) hydrogel microspheres by motor-driven rotation of the oil phase to form a special T-shaped structure with the needle. The main part of the system consists of commercially available motors, a beaker, and needles that do not require precision machining and are user-friendly with low cost. Moreover, by adjusting system parameters such as the needle structure, flow rate, and rotational speed, the platform enables rapid fabrication of hydrogel microspheres with different sizes and diverse cores, including crescent, thick wavy, oval, and spherical. Furthermore, tumor cell-laden hyaluronic acid methacrylate/sodium alginate (HAMA/SA) hydrogel microspheres were fabricated by using this system, which demonstrated good cell viability and proliferation in the subsequent 3D culture. In vitro drug evaluation of tumor models using cisplatin revealed the potential of this system for drug evaluation. These results indicated that RMS has good potential in other 3D cell culture-based biomedical applications.

Ionic Strength-Induced Compartmentalization for Nanogel-in-Microgel Colloids

Compartmentalization is crucial for control over complex biological cascade reactions. In microgels, the formation of discrete compartments allows for simultaneous uptake and orthogonal release of physicochemically distinct drugs, among others. However, many state-of-the-art approaches yielding compartmentalized microgels require the use of specific, though not always biocompatible, components and temperatures well above the physiological range, which may damage possible biological cargo. Therefore, a novel technique to fabricate compartmentalized microgels by exploiting ionic strength-induced precipitation as a mechanism for compartmentalization is developed. For this, a droplet-based microfluidic approach in which preformed nanogels are incorporated into poly(N-isopropylacrylamide)- or poly(acrylamide)-based microgels is employed. Allowing contact between the nanogel-monomer mixture and a salt solution only at the cross junction inhibits premature precipitation of the nanogels and aggregates form on the chip. It is demonstrated that this method is applicable to a variety of nanogel species in both stimuli-responsive and non-stimuli-responsive microgel networks. For temperature-responsive nanogel compartments in non-responsive microgels, anisotropic shape change is investigated by adjusting temperature or salt concentration or changing the solvent. Lastly, an exemplary uptake and release experiment demonstrates highly selective drug absorption, paving the way for more advanced biomimetic polymer structures.

Construction of Multiplexed Assays on Single Anisotropic Particles Using Microfluidics

Considerable efforts have been made to develop microscale multiplexing strategies. However, challenges remain due to the difficulty in deploying functional objects and decoding high-density signals on anisotropic microcarriers. Here, we report a microfluidic method to fabricate architecture-marked anisotropic particles for performing designable multiplexed assays in a label-free manner. By controlling fluid assembly and rapid in-air cross-linking, the particles are endowed with multiple functional regions and a unique architecture identifier. The marked architecture enables an addressing mechanism that allows the profiling of embedded label-free objects by mapping a well-defined reference architecture onto the target particle. By loading analytes of interest, such as molecular probes or cells, we showed the potential of these structurally flexible particles for detecting microRNAs and studying cell interactions. The architecture-marked particles represent a new approach for single-entity assays and can be the basis for exploring more advanced microscale multiplexed applications.

Versatile hydrogels prepared by microfluidics technology for bone tissue engineering applications

Bone defects are a prevalent issue resulting from various factors, such as trauma, degenerative diseases, congenital disabilities, and the surgical removal of tumors. Current methods for bone regeneration have limitations. In this context, the fusion of tissue engineering and microfluidics has emerged as a promising strategy in the field of bone regeneration. This study describes the classification of microfluidic devices based on the nature of flow and channel type, as well as the materials and techniques required. An overview of microfluidic methods used to prepare hydrogels and the advantages of using these hydrogels in bone tissue engineering (BTE) combining several basic elements of BTE to highlight its advantages is provided. Furthermore, this work emphasizes the benefits of using hydrogels prepared via microfluidics over conventional hydrogels in BTE because of their controlled release of cargo, they can be used for in situ injection, simplify the steps of single-cell encapsulation and have the advantages of high-throughput and precise preparation. Additionally, organ-on-a-chip models fabricated via microfluidics offer a platform for studying cell and tissue behaviors in an authentic and dynamic environment. Moreover, microfluidic devices can be utilized for noninvasive diagnosis and therapy. Finally, this paper summarizes the preclinical and clinical applications of hydrogels prepared via microfluidics for bone regeneration by focusing on their current developmental status, limitations associated with their application, and future challenges, which underscore their potential impacts on advancing regenerative medicine practices.

Material-driven immunomodulation and ECM remodeling reverse pulmonary fibrosis by local delivery of stem cell-laden microcapsules

Recent progress in stem cell therapy has demonstrated the therapeutic potential of intravenous stem cell infusions for treating the life-threatening lung disease of pulmonary fibrosis (PF). However, it is confronted with limitations, such as a lack of control over cellular function and rapid clearance by the host after implantation. In this study, we developed an innovative PF therapy through tracheal administration of microfluidic-templated stem cell-laden microcapsules, which effectively reversed the progression of inflammation and fibrotic injury. Our findings highlight that hydrogel microencapsulation can enhance the persistence of donor mesenchymal stem cells (MSCs) in the host while driving MSCs to substantially augment their therapeutic functions, including immunoregulation and matrix metalloproteinase (MMP)-mediated extracellular matrix (ECM) remodeling. We revealed that microencapsulation activates the MAPK signaling pathway in MSCs to increase MMP expression, thereby degrading overexpressed collagen accumulated in fibrotic lungs. Our research demonstrates the potential of hydrogel microcapsules to enhance the therapeutic efficacy of MSCs through cell-material interactions, presenting a promising yet straightforward strategy for designing advanced stem cell therapies for fibrotic diseases.

Profiling signaling mediators for cell-cell interactions and communications with microfluidics-based single-cell analysis tools

Cell-cell interactions and communication represent the fundamental cornerstone of cells' collaborative efforts in executing diverse biological processes. A profound understanding of how cells interface through various mediators is pivotal across a spectrum of biological systems. Recent strides in microfluidic technologies have significantly bolstered the precision and prowess in capturing and manipulating cells with exceptional spatial and temporal resolution. These advanced methodologies converge with multi-signal mediator detection systems, furnishing potent, high-throughput platforms for dissecting cell-cell interactions at the single-cell level. This approach empowers researchers to delve into intricate cellular dynamics with unprecedented accuracy and efficiency. Here, we present a critical evaluation of the latest advancements in microfluidics-driven techniques for detecting signal mediators involved in cell-cell interactions and communication at the single-cell level. We underscore notable biological applications that have benefited from these technologies and identify pressing challenges that must be addressed in future endeavors leveraging microfluidic tools for single-cell interaction studies.

Gelatin maleimide microgels for hematopoietic progenitor cell encapsulation

Hematopoietic stem cells (HSCs) are the apical cells of the hematopoietic system, giving rise to cells of the blood and lymph lineages. HSCs reside primarily within bone marrow niches that contain matrix and cell-derived signals that help inform stem cell fate. Aspects of the bone marrow microenvironment have been captured in vitro by encapsulating cells within hydrogel matrices that mimic native mechanical and biochemical properties. Hydrogel microparticles, or microgels, are increasingly being used to assemble granular biomaterials for cell culture and noninvasive delivery applications. Here, we report the optimization of a gelatin maleimide hydrogel system to create monodisperse gelatin microgels via a flow-focusing microfluidic process. We report characteristic hydrogel stiffness, stability, and swelling characteristics as well as encapsulation of murine hematopoietic stem and progenitor cells, and mesenchymal stem cells within microgels. Microgels support cell viability, confirming compatibility of the microfluidic encapsulation process with these sensitive bone marrow cell populations. Overall, this work presents a microgel-based gelatin maleimide hydrogel as a foundation for future development of a multicellular artificial bone marrow culture system.

Single-layer graphene oxide nanosheets induce proliferation and Osteogenesis of single-cell hBMSCs encapsulated in Alginate Microgels

Microfluidics cell encapsulation offers a way to mimic a 3D microenvironment that supports cell growth and proliferation, while also protecting cells from environmental stress. This technique has found extensive applications in tissue engineering and cell therapies. Several studies have demonstrated the advantages of graphene oxide (GO) as an osteogenic inducer; however, the significance of GO on stem cell fate in the single-cell state is still unclear. Here, a microfluidics-based approach is developed for continuous encapsulation of mesenchymal stem cells (MSCs) at the single-cell level using alginate microgels. So, single-layer graphene oxide (slGO) nanosheet is used to be encapsulated inside the alginate droplets. The results of AFM and SEM show that slGO can increase the roughness and reduce the stiffness of alginate hydrogels. The Young's modulus of the alginate and alginate-slGO was obtained as 1414 kPa and 985.9 kPa, respectively. Live/dead assay and fluorescence microscopy images illustrate that slGO can maintain the viability and proliferation of microencapsulated hBMSCs. The obtained results show that slGO increases the mineralization of the encapsulated hBMSCs, so that microgels containing hBMSCs gradually became opaque during 21 days of culture. RT-qPCR results indicate that the expression of OCN, Runx2, and ALP in the alginate-slGO microgels is significantly higher than in the alginate microgels. The expression of OCN and Runx2 in the alginate-slGO microgels is 4.27 and 5.87-fold higher than in the alginate microgels, respectively. It can be concluded that low doses of slGO nanosheets have the potential to be utilized in the development of tissue engineering and bone regeneration. This finding offers a new method for creating injectable tissue transplants that are minimally invasive.

"Hedgehog Ball"-Shaped Nanoprobes for Multimodal Detection and Imaging of Inflammatory Markers in Osteosarcoma Using Fluorescence and Electrochemiluminescence

Inflammation can affect the progression of cancer at tumor sites, such as in osteosarcoma, by intensifying metastasis and complicating outcomes. The current diagnostic methods lack the specificity and sensitivity required for early and accurate detection, particularly in differentiating between inflammation-induced changes and tumor activities. To address this, a novel "hedgehog ball"-shaped nanoprobe, Fe3O4@Au-pep-CQDs, was developed and designed to enhance the detection of caspase-1, a key marker of inflammation. This magnetic nanoprobe facilitates simultaneous fluorescence (FL) and electrochemiluminescence (ECL) detection. Magnetic separation minimizes the quenching of nanoparticles in solution and eliminates the need for frequent electrode replacement in ECL tests, thereby simplifying diagnostic procedures. The experimental results showed that in the detection of caspase-1, the nanoprobe had a detection limit of 0.029 U/mL (FL) and 0.033 U/mL (ECL) and had a dynamic range of 0.05 to 1.0 U/mL. Additionally, the nanoprobe achieved high recovery rates of 94.36 to 102.44% (FL) and 94.36-100.12% (ECL) in spiked biological samples. Furthermore, the nanoprobe's capabilities were extended to in vivo bioimaging to provide direct, intuitive visualization of biological processes. These novel nanoprobes were able to significantly enhance the accurate detection of inflammation at tumor sites, thereby optimizing both diagnostic and therapeutic strategies.

Droplet Microfluidics Powered Hydrogel Microparticles for Stem Cell-Mediated Biomedical Applications

Stem cell-related therapeutic technologies have garnered significant attention of the research community for their multi-faceted applications. To promote the therapeutic effects of stem cells, the strategies for cell microencapsulation in hydrogel microparticles have been widely explored, as the hydrogel microparticles have the potential to facilitate oxygen diffusion and nutrient transport alongside their ability to promote crucial cell-cell and cell-matrix interactions. Despite their significant promise, there is an acute shortage of automated, standardized, and reproducible platforms to further stem cell-related research. Microfluidics offers an intriguing platform to produce stem cell-laden hydrogel microparticles (SCHMs) owing to its ability to manipulate the fluids at the micrometer scale as well as precisely control the structure and composition of microparticles. In this review, the typical biomaterials and crosslinking methods for microfluidic encapsulation of stem cells as well as the progress in droplet-based microfluidics for the fabrication of SCHMs are outlined. Moreover, the important biomedical applications of SCHMs are highlighted, including regenerative medicine, tissue engineering, scale-up production of stem cells, and microenvironmental simulation for fundamental cell studies. Overall, microfluidics holds tremendous potential for enabling the production of diverse hydrogel microparticles and is worthy for various stem cell-related biomedical applications.

Advances in the Development of Granular Microporous Injectable Hydrogels with Non-spherical Microgels and Their Applications in Tissue Regeneration

Granular microporous hydrogels are emerging as effective biomaterial scaffolds for tissue engineering due to their improved characteristics compared to traditional nanoporous hydrogels, which better promote cell viability, cell migration, cellular/tissue infiltration, and tissue regeneration. Recent advances have resulted in the development of granular hydrogels made of non-spherical microgels, which compared to those made of spherical microgels have higher macroporosity, more stable mechanical properties, and better ability to guide the alignment and differentiation of cells in anisotropic tissue. The development of these hydrogels as an emerging research area is attracting increasing interest in regenerative medicine. This review first summarizes the fabrication techniques available for non-spherical microgels with different aspect-ratios. Then, it introduces the development of granular microporous hydrogels made of non-spherical microgels, their physicochemical characteristics, and their applications in tissue regeneration. The limitations and future outlook of research on microporous granular hydrogels are also critically discussed.

Microfluidic Engineering of Addressable Multicompartmental Microspheres for Multicellular Systems

With the advantages of high-throughput manufacturing and customizability, on-microsphere construction of in vitro multicellular analytical systems has garnered significant attention. However, achieving a precise, biocompatible cell arrangement and spatial signal analysis in hydrogel microspheres remains challenging. In this work, a microfluidic method is reported for the biocompatible generation of addressable supersegmented multicompartmental microspheres. Additionally, these microspheres are developed as novel label-free multicellular systems. In the microfluidic approach, controllable microfluidics is used to finely tune the internal microstructure of the microspheres, and the gas ejector ensures the biocompatibility of the preparation process. As a proof of concept, six- and twenty-compartment microspheres were obtained without the addition of any biohazardous reagents. For microsphere decoding, the visualization of two basic compartments can provide clues for identifying label-free cells due to the structural regularity of the microspheres. Finally, by encapsulating cells of different types, these microspheres as multicellular systems were successfully used for cell coculture and drug testing. These biocompatible, scalable, and analyzable microspheres will open up new prospects for biomedical analysis.

Microfluidics single-cell encapsulation reveals that poly-l-lysine-mediated stem cell adhesion to alginate microgels is crucial for cell-cell crosstalk and its self-renewal

Microfluidic cell encapsulation has provided a platform for studying the behavior of individual cells and has become a turning point in single-cell analysis during the last decade. The engineered microenvironment, along with protecting the immune response, has led to increasingly presenting the results of practical and pre-clinical studies with the goals of disease treatment, tissue engineering, intelligent control of stem cell differentiation, and regenerative medicine. However, the significance of cell-substrate interaction versus cell-cell communications in the microgel is still unclear. In this study, monodisperse alginate microgels were generated using a flow-focusing microfluidic device to determine how the cell microenvironment can control human bone marrow-derived mesenchymal stem cells (hBMSCs) viability, proliferation, and biomechanical features in single-cell droplets versus multi-cell droplets. Collected results show insufficient cell proliferation (234 % and 329 %) in both single- and multi-cell alginate microgels. Alginate hydrogels supplemented with poly-l-lysine (PLL) showed a better proliferation rate (514 % and 780 %) in a comparison of free alginate hydrogels. Cell stiffness data illustrate that hBMSCs cultured in alginate hydrogels have higher membrane flexibility and migration potency (Young's modulus equal to 1.06 kPa), whereas PLL introduces more binding sites for cell attachment and causes lower flexibility and migration potency (Young's modulus equal to 1.83 kPa). Considering that cell adhesion is the most important parameter in tissue engineering, in which cells do not run away from a 3D substrate, PLL enhances cell stiffness and guarantees cell attachments. In conclusion, cell attachment to PLL-mediated alginate hydrogels is crucial for cell viability and proliferation. It suggests that cell-cell signaling is good enough for stem cell viability, but cell-PLL attachment alongside cell-cell signaling is crucial for stem cell proliferation and self-renewal.

Droplet Microfluidics-Assisted Fabrication of Shape Controllable Iron-Alginate Microgels with Fluorescent Property

Droplet-based microfluidics-assisted fabrication of alginate microgels has extensive applications in biomaterials, biomedicines, and related fields. This approach is typically achieved by crosslinking droplets of an aqueous solution of sodium alginate with various divalent and trivalent ions, such as Ca2+, Ba2+, Sr2+, etc. Despite the exceptional features exhibited by bulk alginate hydrogels when using iron ions as the crosslinking reagent, including stimulus responsiveness and complex chemistry, no attention has been given to studying the fabrication of Fe-alginate microgels through droplet microfluidics. In this work, a facile method is presented for fabricating Fe-alginate microgels using single emulsion droplets as templates and an off-chip crosslinking technique to solidify the droplets. The morphologies of the resulting microgels can be systematically adjusted by manipulating different parameters such as viscosities and ionic strength of the collecting solutions. It should be noted that these resulting microgels undergo a color change from light brown to dark brown due to presumed self-oxidation of iron ions within their skeleton structure. Furthermore, these Fe-alginate microgels are functionalized by decorating them with a positively charged linear polymer via electrostatic interactions to impart them with stable fluorescent property. These functionalized Fe-alginate microgels may find potential applications in drug delivery carriers and biomimetic structures.

Biocompatible Janus microparticle synthesis in a microfluidic device

Janus particles are popular in recent years due to their anisotropic physical and chemical properties. Even though there are several established synthesis methods for Janus particles, microfluidics-based methods are convenient and reliable due to low reagent consumption, monodispersity of the resultant particles and efficient control over reaction conditions. In this work a simple droplet-based microfluidic technique is utilized to synthesize magnetically anisotropic TiO2-Fe2O3 Janus microparticles. Two droplets containing reagents for Janus particle were merged by using an asymmetric device such that the resulting droplet contained the constituents within its two hemispheres distinct from each other. The synthesized Janus particles were observed under the optical microscope and the scanning electron microscope. Moreover, a detailed in vitro characterization of these particles was completed, and it was shown that these particles have a potential use for biomedical applications.

Fabrication of polymeric microspheres for biomedical applications

Polymeric microspheres (PMs) have attracted great attention in the field of biomedicine in the last several decades due to their small particle size, special functionalities shown on the surface and high surface-to-volume ratio. However, how to fabricate PMs which can meet the clinical needs and transform laboratory achievements to industrial scale-up still remains a challenge. Therefore, advanced fabrication technologies are pursued. In this review, we summarize the technologies used to fabricate PMs, including emulsion-based methods, microfluidics, spray drying, coacervation, supercritical fluid and superhydrophobic surface-mediated method and their advantages and disadvantages. We also review the different structures, properties and functions of the PMs and their applications in the fields of drug delivery, cell encapsulation and expansion, scaffolds in tissue engineering, transcatheter arterial embolization and artificial cells. Moreover, we discuss existing challenges and future perspectives for advancing fabrication technologies and biomedical applications of PMs.

Microgels for Cell Delivery in Tissue Engineering and Regenerative Medicine

Microgels prepared from natural or synthetic hydrogel materials have aroused extensive attention as multifunctional cells or drug carriers, that are promising for tissue engineering and regenerative medicine. Microgels can also be aggregated into microporous scaffolds, promoting cell infiltration and proliferation for tissue repair. This review gives an overview of recent developments in the fabrication techniques and applications of microgels. A series of conventional and novel strategies including emulsification, microfluidic, lithography, electrospray, centrifugation, gas-shearing, three-dimensional bioprinting, etc. are discussed in depth. The characteristics and applications of microgels and microgel-based scaffolds for cell culture and delivery are elaborated with an emphasis on the advantages of these carriers in cell therapy. Additionally, we expound on the ongoing and foreseeable applications and current limitations of microgels and their aggregate in the field of biomedical engineering. Through stimulating innovative ideas, the present review paves new avenues for expanding the application of microgels in cell delivery techniques.

Developing organs-on-chips for biomedical applications

In recent years, organs-on-chips have been arousing great interest for their bionic and stable construction of crucial human organs in vitro. Compared with traditional animal models and two-dimensional cell models, organs-on-chips could not only overcome the limitations of species difference and poor predict ability but also be capable of reappearing the complex cell-cell interaction, tissue interface, biofluid and other physiological conditions of humans. Therefore, organs-on-chips have been regarded as promising and powerful tools in diverse fields such as biology, chemistry, medicine and so on. In this perspective, we present a review of organs-on-chips for biomedical applications. After introducing the key elements and manufacturing craft of organs-on-chips, we intend to review their cut-edging applications in biomedical fields, incorporating biological analysis, drug development, robotics and so on. Finally, the emphasis is focused on the perspectives of organs-on-chips.

Deterministic Single-Cell Encapsulation in PEG Norbornene Microgels for Promoting Anti-Inflammatory Response and Therapeutic Delivery of Mesenchymal Stromal Cells

Tissue engineering at single-cell resolution has enhanced therapeutic efficacy. Droplet microfluidics offers a powerful platform that allows deterministic single-cell encapsulation into aqueous droplets, yet the direct encapsulation of cells into microgels remains challenging. Here, the design of a microfluidic device that is capable of single-cell encapsulation within polyethylene glycol norbornene (PEGNB) hydrogels on-chip is reported. Cells are first ordered in media within a straight microchannel via inertial focusing, followed by the introduction of PEGNB solution from two separate, converging channels. Droplets are thoroughly mixed by passage through a serpentine channel, and microgels are formed by photo-photopolymerization. This platform uniquely enables both single-cell encapsulation and excellent cell viability post-photo-polymerization. More than 90% of singly encapsulated mesenchymal stromal cells (MSCs) remain alive for 7 days. Notably, singly encapsulated MSCs have elevated expression levels in genes that code anti-inflammatory cytokines, for example, IL-10 and TGF-β, thus enhancing the secretion of proteins of interest. Following injection into a mouse model with induced inflammation, singly encapsulated MSCs show a strong retention rate in vivo, reduce overall inflammation, and mitigate liver damage. These translational results indicate that deterministic single-cell encapsulation could find use in a broad spectrum of tissue engineering applications.

Gravity-Oriented Microfluidic Device for Biocompatible End-to-End Fabrication of Cell-Laden Microgels

Droplet microfluidics are extensively utilized to generate monodisperse cell-laden microgels in biomedical applications. However, maintaining cell viability is still challenging due to overexposure to harsh conditions in subsequent procedures that recover the microgels from the oil phase. Here, a gravity-oriented microfluidic device for end-to-end fabrication of cell-laden microgels is reported, which integrates dispersion, gelation, and extraction into a continuous workflow. This innovative on-chip extraction, driven by native buoyancy and kinetically facilitated by pseudosurfactant, exhibits 100% retrieval efficiency for microgels with a wide range of sizes and stiffnesses. The viability of encapsulated cells is perfectly maintained at ≈98% with minimal variations within and between batches. The end-to-end fabrication remarkably enhances the biocompatibility and practicality of microfluidics-based cell encapsulation and is promising to be compatible with various applications ranging from single-cell analysis to clinical therapy.

Complex Emulsions as an Innovative Pharmaceutical Dosage form in Addressing the Issues of Multi-Drug Therapy and Polypharmacy Challenges

This review explores the intersection of microfluidic technology and complex emulsion development as a promising solution to the challenges of formulations in multi-drug therapy (MDT) and polypharmacy. The convergence of microfluidic technology and complex emulsion fabrication could herald a transformative era in multi-drug delivery systems, directly confronting the prevalent challenges of polypharmacy. Microfluidics, with its unparalleled precision in droplet formation, empowers the encapsulation of multiple drugs within singular emulsion particles. The ability to engineer emulsions with tailored properties-such as size, composition, and release kinetics-enables the creation of highly efficient drug delivery vehicles. Thus, this innovative approach not only simplifies medication regimens by significantly reducing the number of necessary doses but also minimizes the pill burden and associated treatment termination-issues associated with polypharmacy. It is important to bring forth the opportunities and challenges of this synergy between microfluidic-driven complex emulsions and multi-drug therapy poses. Together, they not only offer a sophisticated method for addressing the intricacies of delivering multiple drugs but also align with broader healthcare objectives of enhancing treatment outcomes, patient safety, and quality of life, underscoring the importance of dosage form innovations in tackling the multifaceted challenges of modern pharmacotherapy.

Evaluation of aconitine cardiotoxicity with a heart-on-a-particle prepared by a microfluidic device

A heart-on-a-particle model based on multicompartmental microgel is proposed to simulate the heart microenvironment and study the cardiotoxicity of drugs. The relevant microgel was fabricated by a biocompatible microfluidic-based approach, where heart function-related HL-1 and HUVEC cells were arranged in separate compartments. Finally, the mechanism of aconitine-induced heart toxicity was elucidated using mass spectrometry and molecular biotechnology.

Injectable liposome-containing click hydrogel microparticles for release of macromolecular cargos

Hydrogel microparticles ranging from 0.1-100 μm, referred to as microgels, are attractive for biological applications afforded by their injectability and modularity, which allows facile delivery of mixed populations for tailored combinations of therapeutics. Significant efforts have been made to broaden methods for microgel production including via the materials and chemistries by which they are made. Via droplet-based-microfluidics we have established a method for producing click poly-(ethylene)-glycol (PEG)-based microgels with or without chemically crosslinked liposomes (lipo-microgels) through the Michael-type addition reaction between thiol and either vinyl-sulfone or maleimide groups. Unifom spherical microgels and lipo-microgels were generated with sizes of 74 ± 16 μm and 82 ± 25 μm, respectively, suggesting injectability that was further supported by rheological analyses. Super-resolution confocal microscopy was used to further verify the presence of liposomes within the lipo-microgels and determine their distribution. Atomic force microscopy (AFM) was conducted to compare the mechanical properties and network architecture of bulk hydrogels, microgels, and lipo-microgels. Further, encapsulation and release of model cargo (FITC-Dextran 5 kDa) and protein (equine myoglobin) showed sustained release for up to 3 weeks and retention of protein composition and secondary structure, indicating their ability to both protect and release cargos of interest.

A review of advanced hydrogels for cartilage tissue engineering

With the increase in weight and age of the population, the consumption of tobacco, inappropriate foods, and the reduction of sports activities in recent years, bone and joint diseases such as osteoarthritis (OA) have become more common in the world. From the past until now, various treatment strategies (e.g., microfracture treatment, Autologous Chondrocyte Implantation (ACI), and Mosaicplasty) have been investigated and studied for the prevention and treatment of this disease. However, these methods face problems such as being invasive, not fully repairing the tissue, and damaging the surrounding tissues. Tissue engineering, including cartilage tissue engineering, is one of the minimally invasive, innovative, and effective methods for the treatment and regeneration of damaged cartilage, which has attracted the attention of scientists in the fields of medicine and biomaterials engineering in the past several years. Hydrogels of different types with diverse properties have become desirable candidates for engineering and treating cartilage tissue. They can cover most of the shortcomings of other treatment methods and cause the least secondary damage to the patient. Besides using hydrogels as an ideal strategy, new drug delivery and treatment methods, such as targeted drug delivery and treatment through mechanical signaling, have been studied as interesting strategies. In this study, we review and discuss various types of hydrogels, biomaterials used for hydrogel manufacturing, cartilage-targeting drug delivery, and mechanosignaling as modern strategies for cartilage treatment.

Multichannel Multijunction Droplet Microfluidic Device to Synthesize Hydrogel Microcapsules with Different Core-Shell Structures and Adjustable Core Positions

Core-shell hydrogel microcapsules have sparked great interest due to their unique characteristics and prospective applications in the medical, pharmaceutical, and cosmetic fields. However, complex synthetic procedures and expensive costs have limited their practical application. Herein, we designed and prepared several multichannel and multijunctional droplet microfluidic devices based on soft lithography for the effective synthesis of core-shell hydrogel microcapsules for different purposes. Additionally, two different cross-linking processes (ultraviolet (UV) exposure and interfacial polymerization) were used to synthesize different types of core-shell structured hydrogel microcapsules. Hydrogel microcapsules with gelatin methacryloyl (GelMA) as the core and polyacrylamide (PAM) as the thin shell were synthesized using UV cross-linking. Using an interfacial polymerization process, another core-shell structured microcapsule with GelMA as the core and Ca2+ cross-linked alginate with polyethylenimine (PEI) as the shell was constructed, and the core diameter and total droplet diameter were flexibly controlled by carving. Noteworthy, these hydrogel microcapsules exhibit stimuli-responsiveness and controlled release ability. Overall, a novel technique was developed to successfully synthesize various hydrogel microcapsules with core-shell microstructures. The hydrogel microcapsules possess a multilayered structure that facilitates the coassembly of cells and drugs, as well as the layered assembly of multiple drugs, to develop synergistic therapeutic regimens. These adaptable and controllable hydrogel microdroplets shall held great promise for multicell or multidrug administration as well as for high-throughput drug screening.

On-Chip Fabrication of Colloidal Suprastructures by Assembly and Supramolecular Interlinking of Microgels

In this report, a versatile method is demonstrated to create colloidal suprastructures by assembly and supramolecular interlinking of microgels using droplet-based microfluidics. The behavior of the microgels is systematically investigated to evaluate the influence of their concentration on their distribution between the continuous, the droplet phase, and the interface. At low concentrations, microgels are mainly localized at the water-oil interface whereas an excess of microgels results, following the complete coverage of the water-oil interface, in their distribution in the continuous phase. To stabilize the colloidal suprastructure, on-chip gelation is introduced by adding natural polyphenol tannic acid (TA) in the water phase. TA forms interparticle linking between the poly(N-vinylcaprolactam) (PVCL) microgels by supramolecular interactions. The combination of supramolecular interlinking with the variation of the microgel concentration in microfluidic droplets enables on-chip fabrication of defined colloidal suprastructures with morphologies ranging from colloidosomes to colloidal supraballs. The obtained supracolloidal structures exhibit a pH-responsive behavior with a disintegration at alkaline conditions within a scale of seconds. The destabilization process results from the deprotonation of phenolic groups and destruction of hydrogen bonds with PVCL chains at higher pH.

Fabrication and Performance of Bubble-Containing Multicompartmental Particles: Novel Self-Orienting Carriers

In this work, a class of bubble-containing multicompartmental particles with self-orienting capability is developed, where a single bubble is enclosed at the top of the super-segmented architecture. Such bubbles, driven by potential energy minimization, cause the particles to have a bubble-upward preferred orientation in liquid, enabling efficient decoding of their high-density signals in an interference-resistant manner. The particle preparation involves bubble encapsulation via the impact of a multicompartmental droplet on the liquid surface and overall stabilization via rational crosslinking. The conditions for obtaining these particles are systematically investigated. Methodological compatibility with materials is demonstrated by different hydrogel particles. Finally, by encapsulating cargoes of interest, these particles have found broad applications in actuators, multiplexed detection, barcodes, and multicellular systems.

Encapsulation of Caenorhabditis elegans in Water-in-Water Microdroplets to Study the Worm Viability: Alternative Avenue to Manipulate Microdroplet Environment

Due to the great biocompatibility of the aqueous two phase system (ATPS), biological cells have been widely encapsulated in ATPS microdroplets (diameter < 50 μm). However, the immobilization of relatively large multicellular organisms such as Caenorhabditis elegans in ATPS droplets remains challenging as the spontaneous generation of droplets greater than 200 μm is difficult without external perturbations. In this study, we utilize a microneedle-assisted coflow microfludic channel to passively form ATPS microdroplets larger than 200 μm and successfully entrap C. elegans in the microdroplets. We monitor the worm viability and its temporal stroke frequency up to 6 h. We study the effects of dextran (DEX)-to-polyethylene glycol (PEG) flow ratios and worm concentration on the droplet diameter, worm encapsulation efficiency, and the number of droplets containing individual worms. Larger ATPS microdroplets (>200 μm) form in the ranges of capillary number (Ca) between 0.020 to 0.20 and Weber number (We) between 10-5 and 10-3. An ATPS with the encapsulation ability and biocompatibility can offer an alternative immobilization tool for multicellular organisms to existing platforms such as water/oil droplets.

Microfluidic-based platforms for cell-to-cell communication studies

Intercellular communication is critical to the understanding of human health and disease progression. However, compared to traditional methods with inefficient analysis, microfluidic co-culture technologies developed for cell-cell communication research can reliably analyze crucial biological processes, such as cell signaling, and monitor dynamic intercellular interactions under reproducible physiological cell co-culture conditions. Moreover, microfluidic-based technologies can achieve precise spatial control of two cell types at the single-cell level with high throughput. Herein, this review focuses on recent advances in microfluidic-based 2D and 3D devices developed to confine two or more heterogeneous cells in the study of intercellular communication and decipher the advantages and limitations of these models in specific cellular research scenarios. This review will stimulate the development of more functionalized microfluidic platforms for biomedical research, inspiring broader interests across various disciplines to better comprehend cell-cell communication and other fields, such as tumor heterogeneity and drug screening.

Culture-Delivery Live Probiotics Dressing for Accelerated Infected Wound Healing

Probiotic therapy in infected wound healing is hindered by its low viability and colonization efficiency during treatments. Developing dressings that maintain metabolic activity and prevent the potential leakage of probiotics is imperative. Herein, a culture-delivery live probiotics hydrogel dressing is designed and synthesized, formed by gelatin modified with norbornene (GelNB) and sulfhydryl (GelSH), distributing Lactobacillus reuteri (L. reuteri)-laden alginate microspheres (AlgMPs). GelNB-GelSH hydrogel (GelNBSH) incorporating AlgMPs embedding L. reuteri (GelNBSH-L) possesses bioprintability and efficient polymerization that can maintain the activity of L. reuteri in situ, promote its proliferation, and limit its leakage. Thereby, GelNBSH-L achieved a sustainable antimicrobial effect against both S. aureus and E. coli (>90%). Above all, the results show that GelNBSH-L could ensure propitious viability and efficient antibacterial properties of probiotics, effectively inhibit the further development of bacterial infectious wounds and shorten the repair cycle, aiding in ameliorating future clinical probiotic biotherapy.

Photosynthetic Living Fiber Fabrication from Algal-Bacterial Consortia with Controlled Spatial Distribution

Living materials that combine living cells and synthetic matrix materials have become promising research fields in recent years. While multicellular systems present exclusive benefits in developing living materials over single-cell systems, creating artificial multicellular systems can be challenging due to the difficulty in controlling the multicellular assemblies and the complexity of cell-to-cell interactions. Here, we propose a coculture platform capable of isolating and controlling the spatial distribution of algal-bacterial consortia, which can be utilized to construct photosynthetic living fibers. Through coaxial extrusion-based 3D printing, hydrogel fibers containing bacteria or algae can be deposited into designated structures and further processed into materials with precise geometries. In addition, the photosynthetic living fibers demonstrate a significant synergistic catalytic effect resulting from the immobilization of both bacteria and algae, which effectively optimizes sewage treatment for bioremediation purposes. The integration of microbial consortia and 3D printing yields functional living materials with promising applications in biocatalysis, biosensing, and biomedicine. Our approach provides an optimized solution for constructing efficient multicellular systems and opens a new avenue for the development of advanced materials.

Shaping Synthetic Multicellular and Complex Multimaterial Tissues via Embedded Extrusion-Volumetric Printing of Microgels

In living tissues, cells express their functions following complex signals from their surrounding microenvironment. Capturing both hierarchical architectures at the micro- and macroscale, and anisotropic cell patterning remains a major challenge in bioprinting, and a bottleneck toward creating physiologically-relevant models. Addressing this limitation, a novel technique is introduced, termed Embedded Extrusion-Volumetric Printing (EmVP), converging extrusion-bioprinting and layer-less, ultra-fast volumetric bioprinting, allowing spatially pattern multiple inks/cell types. Light-responsive microgels are developed for the first time as bioresins (µResins) for light-based volumetric bioprinting, providing a microporous environment permissive for cell homing and self-organization. Tuning the mechanical and optical properties of gelatin-based microparticles enables their use as support bath for suspended extrusion printing, in which features containing high cell densities can be easily introduced. µResins can be sculpted within seconds with tomographic light projections into centimeter-scale, granular hydrogel-based, convoluted constructs. Interstitial microvoids enhanced differentiation of multiple stem/progenitor cells (vascular, mesenchymal, neural), otherwise not possible with conventional bulk hydrogels. As proof-of-concept, EmVP is applied to create complex synthetic biology-inspired intercellular communication models, where adipocyte differentiation is regulated by optogenetic-engineered pancreatic cells. Overall, EmVP offers new avenues for producing regenerative grafts with biological functionality, and for developing engineered living systems and (metabolic) disease models.

Digital light processing 3D printing for microfluidic chips with enhanced resolution via dosing- and zoning-controlled vat photopolymerization

Conventional manufacturing techniques to fabricate microfluidic chips, such as soft lithography and hot embossing process, have limitations that include difficulty in preparing multiple-layered structures, cost- and labor-consuming fabrication process, and low productivity. Digital light processing (DLP) technology has recently emerged as a cost-efficient microfabrication approach for the 3D printing of microfluidic chips; however, the fabrication resolution for microchannels is still limited to sub-100 microns at best. Here, we developed an innovative DLP printing strategy for high resolution and scalable microchannel fabrication by dosing- and zoning-controlled vat photopolymerization (DZC-VPP). Specifically, we proposed a modified mathematical model to precisely predict the accumulated UV irradiance for resin photopolymerization, thereby providing guidance for the fabrication of microchannels with enhanced resolution. By fine-tuning the printing parameters, including optical irradiance, exposure time, projection region, and step distance, we can precisely tailor the penetration irradiance stemming from the photopolymerization of the neighboring resin layers, thereby preventing channel blockage due to UV overexposure or compromised bonding stability owing to insufficient resin curing. Remarkably, this strategy can allow the preparation of microchannels with cross-sectional dimensions of 20 μm × 20 μm using a commercial printer with a pixel size of 10 μm × 10 μm; this is significantly higher resolution than previous reports. In addition, this method can enable the scalable and biocompatible fabrication of microfluidic drop-maker units that can be used for cell encapsulation. In general, the current DZC-VPP method can enable major advances in precise and scalable microchannel fabrication and represents a significant step forward for widespread applications of microfluidics-based techniques in biomedical fields.

Hydrogel Drug Delivery Systems for Bone Regeneration

With the in-depth understanding of bone regeneration mechanisms and the development of bone tissue engineering, a variety of scaffold carrier materials with desirable physicochemical properties and biological functions have recently emerged in the field of bone regeneration. Hydrogels are being increasingly used in the field of bone regeneration and tissue engineering because of their biocompatibility, unique swelling properties, and relative ease of fabrication. Hydrogel drug delivery systems comprise cells, cytokines, an extracellular matrix, and small molecule nucleotides, which have different properties depending on their chemical or physical cross-linking. Additionally, hydrogels can be designed for different types of drug delivery for specific applications. In this paper, we summarize recent research in the field of bone regeneration using hydrogels as delivery carriers, detail the application of hydrogels in bone defect diseases and their mechanisms, and discuss future research directions of hydrogel drug delivery systems in bone tissue engineering.

Microfluidic-templated cell-laden microgels fabricated using phototriggered imine-crosslinking as injectable and adaptable granular gels for bone regeneration

Injectable granular gels consisting of densely packed microgels serving as scaffolding biomaterial have recently shown great potential for applications in tissue regeneration, which allow administration via minimally invasive surgery, on-target cargo delivery, and high efficiency in nutrient/waste exchange. However, limitations such as insufficient mechanical strength, structural integrity, and uncontrollable differentiation of the encapsulated cells in the scaffolds hamper their further applications in the biomedical field. Herein, we developed a new class of granular gels via bottom-up assembly of cell-laden microgels via photo-triggered imine-crosslinking (PIC) chemistry based on the microfluidic technique. The particulate nature of the granular gels rendered them with shear-thinning and self-healing behavior, thereby functioning as an injectable and adaptable cellularized scaffold for bone tissue regeneration. Specifically, single cell-laden, monodisperse microgels composed of methacrylate- and o-nitrobenzene-functionalized hyaluronic acid and gelatin were prepared using a high-throughput microfluidic technique with a production rate up to 3.7 × 10⁸ microgels/hr, wherein the PIC chemistry alleviated the oxygen inhibition on free-radical polymerization and facilitated enhanced fabrication accuracy, accelerated gelation rate, and improved network strength. Further in vitro and in vivo studies demonstrated that the microgels can serve as carriers to support the activity of the encapsulated mesenchymal stem cells; these cell-laden microgels can also be used as cellularized bone fillers to induce the regeneration of bone tissues as evidenced by the in vivo experiment using the rat femoral condyle defect model. In general, these results represent a significant step toward the precise fabrication of engineered tissue mimics with single-cell resolution and high cell-density and can potentially offer a powerful tool for the design and applications of a next generation of tissue engineering strategy. STATEMENT OF SIGNIFICANCE: Using microfluidic droplet-based technology, we hereby developed a new class of injectable and moldable granular gels via bottom-up assembly of cell-laden microgels as a versatile platform for tissue regeneration. Phototriggered imine-crosslinking chemistry was introduced for microgel cross-linkage, which allowed for the fabrication of microgels with improved matrix homogeneity, accelerated gelation process, and enhanced mechanical strength. We demonstrated that the microgel building blocks within the granular gels facilitated the proliferation and differentiation of the encapsulated mesenchymal stem cells, which can further serve as a cellularized scaffold for the treatment of bone defects.

Deterministic Single Cell Encapsulation in Asymmetric Microenvironments to Direct Cell Polarity

Various signals in tissue microenvironments are often unevenly distributed around cells. Cellular responses to asymmetric cell-matrix adhesion in a 3D space remain generally unclear and are to be studied at the single-cell resolution. Here, the authors developed a droplet-based microfluidic approach to manufacture a pure population of single cells in a microscale layer of compartmentalized 3D hydrogel matrices with a tunable spatial presentation of ligands at the subcellular level. Cells elongate with an asymmetric presentation of the integrin adhesion ligand Arg-Gly-Asp (RGD), while cells expand isotropically with a symmetric presentation of RGD. Membrane tension is higher on the side of single cells interacting with RGD than on the side without RGD. Finite element analysis shows that a non-uniform isotropic cell volume expansion model is sufficient to recapitulate the experimental results. At a longer timescale, asymmetric ligand presentation commits mesenchymal stem cells to the osteogenic lineage. Cdc42 is an essential mediator of cell polarization and lineage specification in response to asymmetric cell-matrix adhesion. This study highlights the utility of precisely controlling 3D ligand presentation around single cells to direct cell polarity for regenerative engineering and medicine.

Water-in-water droplet microfluidics: A design manual

Droplet microfluidics is utilized in a wide range of applications in biomedicine and biology. Applications include rapid biochemical analysis, materials generation, biochemical assays, and point-of-care medicine. The integration of aqueous two-phase systems (ATPSs) into droplet microfluidic platforms has potential utility in oil-free biological and biomedical applications, namely, reducing cytotoxicity and preserving the native form and function of costly biomolecular reagents. In this review, we present a design manual for the chemist, biologist, and engineer to design experiments in the context of their biological applications using all-in-water droplet microfluidic systems. We describe the studies achievable using these systems and the corresponding fabrication and stabilization methods. With this information, readers may apply the fundamental principles and recent advancements in ATPS droplet microfluidics to their research. Finally, we propose a development roadmap of opportunities to utilize ATPS droplet microfluidics in applications that remain underexplored.

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 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.

Micro-ecology restoration of colonic inflammation by in-Situ oral delivery of antibody-laden hydrogel microcapsules

In-situ oral delivery of therapeutic antibodies, like monoclonal antibody, for chronic inflammation treatment is the most convenient approach compared with other administration routes. Moreover, the abundant links between the gut microbiota and colonic inflammation indicate that the synergistic or antagonistic effect of gut microbiota to colonic inflammation. However, the antibody activity would be significantly affected while transferring through the gastrointestinal tract due to hostile conditions. Moreover, these antibodies have short serum half-lives, thus, require to be frequently administered with high doses to be effective, leading to low patient tolerance. Here, we develop a strategy utilizing thin shell hydrogel microcapsule fabricated by microfluidic technique as the oral delivering carrier. By encapsulating antibodies in these microcapsules, antibodies survive in the hostile gastrointestinal environment and rapidly release into the small intestine through oral administration route, achieving the same therapeutic effect as the intravenous injection evaluated by a colonic inflammation disease model. Moreover, the abundance of some intestinal microorganisms as the indication of the improvement of inflammation has remarkably altered after in-situ antibody-laden microcapsules delivery, implying the restoration of micro-ecology of the intestine. These findings prove our microcapsules are exploited as an efficient oral delivery agent for antibodies with programmable function in clinical application.

Microfluidics Fabrication of Micrometer-Sized Hydrogels with Precisely Controlled Geometries for Biomedical Applications

Micrometer-sized hydrogels are cross-linked three-dimensional network matrices with high-water contents and dimensions ranging from several to hundreds of micrometers. Due to their excellent biocompatibility and capability to mimic physiological microenvironments in vivo, micrometer-sized hydrogels have attracted much attention in the biomedical engineering field. Their biological properties and applications are primarily influenced by their chemical compositions and geometries. However, inhomogeneous morphologies and uncontrollable geometries limit traditional micrometer-sized hydrogels obtained by bulk mixing. In contrast, microfluidic technology holds great potential for the fabrication of micrometer-sized hydrogels since their geometries, sizes, structures, compositions, and physicochemical properties can be precisely manipulated on demand based on the excellent control over fluids. Therefore, micrometer-sized hydrogels fabricated by microfluidic technology have been applied in the biomedical field, including drug encapsulation, cell encapsulation, and tissue engineering. This review introduces micrometer-sized hydrogels with various geometries synthesized by different microfluidic devices, highlighting their advantages in various biomedical applications over those from traditional approaches. Overall, emerging microfluidic technologies enrich the geometries and morphologies of hydrogels and accelerate translation for industrial production and clinical applications.

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.

Dual-sensitive fluorescent nanoprobes for detection of matrix metalloproteinases and low pH in a 3D tumor microenvironment

The overexpression of matrix metalloproteinases and low extracellular pH are two key physiological parameters involved in cancer initiation, progression, and metastasis. These have been the targets for several cancer detection and imaging modalities. Here, dual-sensitive nanoprobes have been fabricated from carbon nanoparticles decorated with a MMP-9 sensitive peptide sequence. Carbon nanoparticles are known for their intrinsic fluorescence properties and hence used as a pH-sensing moiety in the nanoprobes. In addition to this, selective-cleavage of the peptide sequence by MMP-9 results in the generation of a fluorescence signal due to separation of the quencher molecule from the fluorophore attached onto the MMP-9 sensitive peptide sequence, resulting in its detection. This protease-specific activation of the nanoprobes helps in precise tumor environment detection and imaging. The nanoprobes were thoroughly characterized for their chemical, physical and biological activities. The potential of these dual-sensitive nanoprobes to distinguish tumor-like microenvironments (low pH and elevated MMP-9 levels) from non-cancerous ones was evaluated in vitro in 2D cell culture as well as in 3D microscaffolds. The fluorescence microscopy images obtained in both in vitro systems revealed that low pH and high MMP-9 levels could be successfully visualised using these dual-sensitive nanoprobes. Therefore, these nanoprobes would find potential applications as a non-invasive imaging tool for visualising tumor margins in real-time.

Multi-compartment supracapsules made from nano-containers towards programmable release

The assembly of nanomaterials into suprastructures offers the possibility to fabricate larger scale functional materials, whose inner structure strongly influences their functionality for a vast range of applications. In spite of the many current strategies, achieving multi-compartment structures in a targeted and versatile way remains highly challenging. Here, we describe a controllable and straightforward route to create uniform suprastructured materials with a multi-compartmentalized architecture by confining primary nanocapsules into droplets using a cross-junction microfluidic device. Following solvent evaporation from the droplets, the nanocapsules spontaneously assemble into precisely sized multi-compartment particles, which we term supracapsules. Thanks to the process, each spatially separated nanocapsule unit retains its cargo and functionalities within the resulting supracapsules. However, new collective properties emerge, and, particularly, programmable release profiles that are distinct from those of single-compartment capsules. Finally, the suprastructures can be disassembled into single-compartment units by applying ultra-sonication, switching their release to a burst-release mode. These findings open up exciting opportunities to fabricate multi-compartment suprastructures incorporating diverse functionalities for materials with emerging properties.