Microfluidics for Biosynthesizing: from Droplets and Vesicles to Artificial Cells

A microfluidic platform for the synthesis of polymer and polymer-protein-based protocells

In this study, we demonstrate the fabrication of polymersomes, protein-blended polymersomes, and polymeric microcapsules using droplet microfluidics. Polymersomes with uniform, single bilayers and controlled diameters are assembled from water-in-oil-in-water double-emulsion droplets. This technique relies on adjusting the interfacial energies of the droplet to completely separate the polymer-stabilized inner core from the oil shell. Protein-blended polymersomes are prepared by dissolving protein in the inner and outer phases of polymer-stabilized droplets. Cell-sized polymeric microcapsules are assembled by size reduction in the inner core through osmosis followed by evaporation of the middle phase. All methods are developed and validated using the same glass-capillary microfluidic apparatus. This integrative approach not only demonstrates the versatility of our setup, but also holds significant promise for standardizing and customizing the production of polymer-based artificial cells.

Integrating microfluidics and synthetic biology: advancements and diverse applications across organisms

Synthetic biology is the design and modification of biological systems for specific functions, integrating several disciplines like engineering, genetics, and computer science. The field of synthetic biology is to understand biological processes within host organisms through the manipulation and regulation of their genetic pathways and the addition of biocontrol circuits to enhance their production capabilities. This pursuit serves to address global challenges spanning diverse domains that are difficult to tackle through conventional routes of production. Despite its impact, achieving precise, dynamic, and high-throughput manipulation of biological processes is still challenging. Microfluidics offers a solution to those challenges, enabling controlled fluid handling at the microscale, offering lower reagent consumption, faster analysis of biochemical reactions, automation, and high throughput screening. In this review, we diverge from conventional focus on automating the synthetic biology design-build-test-learn cycle, and instead, focus on microfluidic platforms and their role in advancing synthetic biology through its integration with host organisms - bacterial cells, yeast, fungi, animal cells - and cell-free systems. The review illustrates how microfluidic devices have been instrumental in understanding biological systems by showcasing microfluidics as an essential tool to create synthetic genetic circuits, pathways, and organisms within controlled environments. In conclusion, we show how microfluidics expedite synthetic biology applications across diverse domains including but not limited to personalized medicine, bioenergy, and agriculture.

Synthetic Cells from Droplet-Based Microfluidics for Biosensing and Biomedical Applications

Synthetic cells function as biological mimics of natural cells by mimicking salient features of cells such as metabolism, response to stimuli, gene expression, direct metabolism, and high stability. Droplet-based microfluidic technology presents the opportunity for encapsulating biological functional components in uni-lamellar liposome or polymer droplets. Verified by its success in the fabrication of synthetic cells, microfluidic technology is widely replacing conventional labor-intensive, expensive, and sophisticated techniques justified by its ability to miniaturize and perform batch production operations. In this review, an overview of recent research on the preparation of synthetic cells through droplet-based microfluidics is provided. Different synthetic cells including lipid vesicles (liposome), polymer vesicles (polymersome), coacervate microdroplets, and colloidosomes, are systematically discussed. Efforts are then made to discuss the design of a variety of microfluidic chips for synthetic cell preparation since the combination of microfluidics with bottom-up synthetic biology allows for reproductive and tunable construction of batches of synthetic cell models from simple structures to higher hierarchical structures. The recent advances aimed at exploiting them in biosensors and other biomedical applications are then discussed. Finally, some perspectives on the challenges and future developments of synthetic cell research with microfluidics for biomimetic science and biomedical applications are provided.

Recent advances of droplet-based microfluidics for engineering artificial cells

Artificial cells, synthetic cells, or minimal cells are microengineered cell-like structures that mimic the biological functions of cells. Artificial cells are typically biological or polymeric membranes where biologically active components, including proteins, genes, and enzymes, are encapsulated. The goal of engineering artificial cells is to build a living cell with the least amount of parts and complexity. Artificial cells hold great potential for several applications, including membrane protein interactions, gene expression, biomaterials, and drug development. It is critical to generate robust, stable artificial cells using high throughput, easy-to-control, and flexible techniques. Recently, droplet-based microfluidic techniques have shown great potential for the synthesis of vesicles and artificial cells. Here, we summarized the recent advances in droplet-based microfluidic techniques for the fabrication of vesicles and artificial cells. We first reviewed the different types of droplet-based microfluidic devices, including flow-focusing, T-junction, and coflowing. Next, we discussed the formation of multi-compartmental vesicles and artificial cells based on droplet-based microfluidics. The applications of artificial cells for studying gene expression dynamics, artificial cell-cell communications, and mechanobiology are highlighted and discussed. Finally, the current challenges and future outlook of droplet-based microfluidic methods for engineering artificial cells are discussed. This review will provide insights into scientific research in synthetic biology, microfluidic devices, membrane interactions, and mechanobiology.

Microfluidics-Based Nanobiosensors for Healthcare Monitoring

Efficient healthcare management demands prompt decision-making based on fast diagnostics tools, astute data analysis, and informatics analysis. The rapid detection of analytes at the point of care is ensured using microfluidics in synergy with nanotechnology and biotechnology. The nanobiosensors use nanotechnology for testing, rapid disease diagnosis, monitoring, and management. In essence, nanobiosensors detect biomolecules through bioreceptors by modulating the physicochemical signals generating an optical and electrical signal as an outcome of the binding of a biomolecule with the help of a transducer. The nanobiosensors are sensitive and selective and play a significant role in the early identification of diseases. This article reviews the detection method used with the microfluidics platform for nanobiosensors and illustrates the benefits of combining microfluidics and nanobiosensing techniques by various examples. The fundamental aspects, and their application are discussed to illustrate the advancement in the development of microfluidics-based nanobiosensors and the current trends of these nano-sized sensors for point-of-care diagnosis of various diseases and their function in healthcare monitoring.

Lipid vesicle-based molecular robots

A molecular robot, which is a system comprised of one or more molecular machines and computers, can execute sophisticated tasks in many fields that span from nanomedicine to green nanotechnology. The core parts of molecular robots are fairly consistent from system to system and always include (i) a body to encapsulate molecular machines, (ii) sensors to capture signals, (iii) computers to make decisions, and (iv) actuators to perform tasks. This review aims to provide an overview of approaches and considerations to develop molecular robots. We first introduce the basic technologies required for constructing the core parts of molecular robots, describe the recent progress towards achieving higher functionality, and subsequently discuss the current challenges and outlook. We also highlight the applications of molecular robots in sensing biomarkers, signal communications with living cells, and conversion of energy. Although molecular robots are still in their infancy, they will unquestionably initiate massive change in biomedical and environmental technology in the not too distant future.

Light-Responsive Materials in Droplet Manipulation for Biochemical Applications

Miniaturized droplets, characterized by well-controlled microenvironments and capability for parallel processing, have significantly advanced the studies on enzymatic evolution, molecular diagnostics, and single-cell analysis. However, manipulation of small-sized droplets, including moving, merging, and trapping of the targeted droplets for complex biochemical assays and subsequent analysis, is not trivial and remains technically demanding. Among various techniques, light-driven methods stand out as a promising candidate for droplet manipulation in a facile and flexible manner, given the features of contactless interaction, high spatiotemporal resolution, and biocompatibility. This review therefore compiles an in-depth discussion of the governing mechanisms underpinning light-driven droplet manipulation. Besides, light-responsive materials, representing the core of light-matter interaction and the key character converting light into different forms of energy, are particularly assessed in this review. Recent advancements in light-responsive materials and the most notable applications are comprehensively archived and evaluated. Continuous innovations and rational engineering of light-responsive materials are expected to propel the development of light-driven droplet manipulation, equip droplets with enhanced functionality, and broaden the applications of droplets for biochemical studies and routine biochemical investigations.

Free-Boundary Microfluidic Platform for Advanced Materials Manufacturing and Applications

Microfluidics, with its remarkable capacity to manipulate fluids and droplets at the microscale, has emerged as a powerful platform in numerous fields. In contrast to conventional closed microchannel microfluidic systems, free-boundary microfluidic manufacturing (FBMM) processes continuous precursor fluids into jets or droplets in a relatively spacious environment. FBMM is highly regarded for its superior flexibility, stability, economy, usability, and versatility in the manufacturing of advanced materials and architectures. In this review, a comprehensive overview of recent advancements in FBMM is provided, encompassing technical principles, advanced material manufacturing, and their applications. FBMM is categorized based on the foundational mechanisms, primarily comprising hydrodynamics, interface effects, acoustics, and electrohydrodynamic. The processes and mechanisms of fluid manipulation are thoroughly discussed. Additionally, the manufacturing of advanced materials in various dimensions ranging from zero-dimensional to three-dimensional, as well as their diverse applications in material science, biomedical engineering, and engineering are presented. Finally, current progress is summarized and future challenges are prospected. Overall, this review highlights the significant potential of FBMM as a powerful tool for advanced materials manufacturing and its wide-ranging applications.

Advancing Biomimetic Functions of Synthetic Cells through Compartmentalized Cell-Free Protein Synthesis

Synthetic cells are artificial constructs that mimic the structures and functions of living cells. They are attractive for studying diverse biochemical processes and elucidating the origins of life. While creating a living synthetic cell remains a grand challenge, researchers have successfully synthesized hundreds of unique synthetic cell platforms. One promising approach to developing more sophisticated synthetic cells is to integrate cell-free protein synthesis (CFPS) mechanisms into vesicle platforms. This makes it possible to create synthetic cells with complex biomimetic functions such as genetic circuits, autonomous membrane modifications, sensing and communication, and artificial organelles. This Review explores recent advances in the use of CFPS to impart advanced biomimetic structures and functions to bottom-up synthetic cell platforms. We also discuss the potential applications of synthetic cells in biomedicine as well as the future directions of synthetic cell research.

Pinch-off droplet generator using microscale gigahertz acoustics

The generation and dispensing of microdroplets is a vital process in various fields such as biomedicine, medical diagnosis and chemistry. However, most methods still require the structures of nozzles or microchannels to assist droplet generation, which leads to limitations on system flexibility and restrictions on the size range of the generated droplets. In this paper, we propose a nozzle-free acoustic-based method for generating droplets using a gigahertz (GHz) bulk acoustic wave (BAW). Unlike most of the acoustofluidic approaches, the proposed method produces the droplet by pinching-off the liquid column generated by the acoustic body force at the oil-water interface. Benefitting from the focused acoustic energy and small footprint of the device, four orders of magnitude (ranging from 2 μm to 1800 μm) of droplet size could be produced by controlling the working time and power of the device. We also demonstrated cell encapsulation in the droplet and a high cell viability was achieved. The proposed acoustic-based droplet generation method exhibits capacity for generating droplets with a wide size range, versatility toward different viscosities, as well as biocompatibility for handling viable samples, which shows potential in miniaturization and scalability.

Single Nanovesicles Tracking Reveals Their Heterogeneous Extracellular Adsorptions

The vigorous nanomedicine offers significant possibilities for effective therapeutics of various diseases, and nanovesicles (NVs) represented by artificial liposomes and natural exosomes and cytomembranes especially show great potential. However, their complex interactions with cells, particularly the heterogeneous extracellular adsorptions, are difficult to analyze spatiotemporally due to the transient dynamics. In this study, by single NVs tracking, the extracellular NVs adsorptions are directly observed and their heterogeneous characteristics are revealed. Briefly, plenty of NVs adsorbed on HCT116 cells are tracked and classified, and it is discovered that they exhibit various diffusion properties from different extracellular regions: stable adsorptions on the rear surface and restricted adsorptions on the front protrusion. After the hydrolysis of hyaluronic acid in the extracellular matrix by hyaluronidase, the restricted adsorptions are further weakened and manifested as dissociative adsorptions, which demonstrated reduced total NVs adsorptions from a single-cell and single-particle perspective. Compared with traditional static analysis, the spatiotemporal tracking and heterogeneous results not only reveal the extracellular NVs-cell interactions but also inspire a wide variety of nanomedicine and their nano-investigations.

Bidirectional Droplet Manipulation on Magnetically Actuated Superhydrophobic Ratchet Surfaces

Droplet manipulation has garnered significant attention in various fields due to its wide range of applications. Among many different methods, magnetic actuation has emerged as a promising approach for remote and instantaneous droplet manipulation. In this study, we present the bidirectional droplet manipulation on a magnetically actuated superhydrophobic ratchet surface. The surface consists of silicon strips anchored on elastomer ridges with superhydrophobic black silicon structures on the top side and magnetic layers on the bottom side. The soft magnetic properties of the strips enable their bidirectional tilting to form a ratchet surface and thus bidirectional droplet manipulation upon varying external magnetic field location and strength. Computational multiphysics models were developed to predict the tilting of the strips, demonstrating the concept of bidirectional tilting along with a tilting angle hysteresis theory. Experimental results confirmed the soft magnetic hysteresis and consequential bidirectional tilting of the strips. The superhydrophobic ratchet surface formed by the tilting strips induced the bidirectional self-propulsion of dispensed droplets through the Laplace pressure gradient, and the horizontal acceleration of the droplets was found to be positively correlated with the tilting angle of the strips. Additionally, a finite element analysis was conducted to identify the critical conditions for dispensed droplet penetration through the gaps between the strips, which hinder the droplet's self-propulsion. The models and findings here provide substantial insights into the design and optimization of magnetically actuated superhydrophobic ratchet surfaces to manipulate droplets in the context of digital microfluidic applications.

High-Throughput Microfluidic Production of Droplets and Hydrogel Microspheres through Monolithically Integrated Microchannel Plates

In this paper, we present a highly effective microfluidic emulsion system using an integrated microchannel plate (MCP), a porous glass membrane that is readily available and densely packs millions of through-microchannels, for high-throughput production of monodisperse droplets. The physical controls of droplet formation, including viscosity, flow rate, and pore size, have been extensively explored for optimum emulsification conditions. The performance of the device has been validated where monodisperse droplets with a narrow coefficient of variance (<5%) can be achieved at a dispersed phase flux of 3 mL h-1 from a piece of 4 × 4 mm2 MCP. The average droplet size is two times the nominal membrane pore diameter and thus can be easily controlled by choosing the appropriate membrane type. The preparation of hydrogel microspheres has also been demonstrated with a high throughput of 1.5 × 106 particles min-1. These microspheres with a uniform size range and rough surface morphology provide suitable bioenvironments and serve as ideal carriers for cell culture. Mouse fibroblasts are shown to be cultured on these 3D scaffolds with an average cell viability of over 96%. The cell attachment rate can reach up to 112 ± 7% in 24 h and the proliferation ability increases with the number of culture days. Furthermore, the device has been applied in the droplet digital polymerase chain reaction for absolute quantification of lung cancer-related PLAU genes. The detection limit achieved was noted to be 0.5 copies/μL with a dynamic range of 105 ranging from 1 × 102 to 1 × 106 copies/μL. Given the easy fabrication, robust performance, and simple operation, the emulsion system sets the stage for the laboratory's droplet-based assays and applications in tissue engineering.

Single-cell microfluidics enabled dynamic evaluation of drug combinations on antibiotic resistance bacteria

The rapid spread of antibiotic resistance has become a significant threat to global health, yet the development of new antibiotics is outpaced by emerging new resistance. To treat multidrug-resistant bacteria and prolong the lifetime of existing antibiotics, a productive strategy is to use combinations of antibiotics and/or adjuvants. However, evaluating drug combinations is primarily based on end-point checkerboard measurements, which provide limited information to study the mechanism of action and the discrepancies in the clinical outcomes. Here, single-cell microfluidics is used for rapid evaluation of the efficacy and mode of action of antibiotic combinations within 3 h. Focusing on multidrug-resistant Acinetobacter baumannii, the combination between berberine hydrochloride (BBH, as an adjuvant) and carbapenems (meropenem, MEM) or β-lactam antibiotic is evaluated. Real-time tracking of individual cells to programmable delivered antibiotics reveals multiple phenotypes (i.e., susceptible, resistant, and persistent cells) with fidelity. Our study discovers that BBH facilitates the accumulation of antibiotics within cells, indicating synergistic effects (FICI = 0.5). For example, the combination of 256 mg/L BBH and 16 mg/L MEM has a similar killing effect (i.e., the inhibition rates >90%) as the MIC of MEM (64 mg/L). Importantly, the synergistic effect of a combination can diminish if the bacteria are pre-stressed with any single drug. Such information is vital for understanding the underlying mechanisms of combinational treatments. Overall, our platform provides a promising approach to evaluate the dynamic and heterogenous response of a bacterial population to antibiotics, which will facilitate new drug discovery and reduce emerging antibiotic resistance.

Synthesis and characterization of novel thermoresponsive suspensions via physical adsorption of poly[di(ethylene glycol) methyl methacrylate] onto polystyrene microparticles

Thermoreversible colloidal suspensions/gels have attracted recent research attention in the field of biomedical applications. In this study, a novel thermoresponsive particle suspension with thermoreversible gelation properties has been prepared for biomedical application. First, polystyrene (PS) microspheres were synthesized by dispersion polymerization and poly diethyleneglycolmethylmethacrylate (PDEGMA) polymer were synthesized via free radical polymerisation. Then, the new developed thermoresponsive suspensions were prepared via physical adsorption of a thermoresponsive polymer, poly[di (ethylene glycol) methyl methacrylate] (PDEGMA), onto the surface of polystyrene microspheres. PDEGMA acts as a steric stabilizer and induces thermoreversible gelation via chain extending and collapsing below and above its lower critical solution temperature (LCST), respectively. Scanning electron microscopy (SEM), 1H NMR spectroscopy, Gel permeation chromatography (GPC), UV-vis spectroscopy, Rheometric measurement were conducted to characterize the prepared particles, polymers and suspensions. SEM images show that monodisperse microspheres with the sizes range 1.5-3.5 μm were prepared. UV-vis measurements demonstrate thermoresponsive properties of PDEGMA. 1H NMR and GPC analysis confirms structural properties of prepared PDEGMA. Tube inversion tests demonstrated that the aqueous suspensions of the particles and polymer exhibited thermoreversible fluid-to-gel transitions. Rheological characterization revealed that the viscoelastic properties of the prepared suspension/gels can be fine tuned. This enables applications of the prepared gels as scaffolds for three-dimensional (3D) cell cultures.

A Practical Guide to Preparation and Applications of Giant Unilamellar Vesicles Formed via Centrifugation of Water-in-Oil Emulsion Droplets

Giant vesicles (GVs), which are closed lipid bilayer membranes with a diameter of more than 1 μm, have attracted attention not only as model cell membranes but also for the construction of artificial cells. For encapsulating water-soluble materials and/or water-dispersible particles or functionalizing membrane proteins and/or other synthesized amphiphiles, giant unilamellar vesicles (GUVs) have been applied in various fields, such as supramolecular chemistry, soft matter physics, life sciences, and bioengineering. In this review, we focus on a preparation technique for GUVs that encapsulate water-soluble materials and/or water-dispersible particles. It is based on the centrifugation of a water-in-oil emulsion layered on water and does not require special equipment other than a centrifuge, which makes it the first choice for laboratory use. Furthermore, we review recent studies on GUV-based artificial cells prepared using this technique and discuss their future applications.

Light energy transduction in liposome-based artificial cells

In this work we review the latest strategies for the bottom-up assembly of energetically autonomous artificial cells capable of transducing light energy into chemical energy and support internalized metabolic pathways. Such entities are built by taking inspiration from the photosynthetic machineries found in nature which are purified and reconstituted directly in the membrane of artificial compartments or encapsulated in form of organelle-like structures. Specifically, we report and discuss recent examples based on liposome-technology and multi-compartment (nested) architectures pointing out the importance of this matter for the artificial cell synthesis research field and some limitations and perspectives of the bottom-up approach.

Organoids/organs-on-a-chip: new frontiers of intestinal pathophysiological models

Organoids/organs-on-a-chip open up new frontiers for basic and clinical research of intestinal diseases. Species-specific differences hinder research on animal models, while organoids are emerging as powerful tools due to self-organization from stem cells and the reproduction of the functional properties in vivo. Organs-on-a-chip is also accelerating the process of faithfully mimicking the intestinal microenvironment. And by combining organoids and organ-on-a-chip technologies, they further are expected to serve as innovative preclinical tools and could outperform traditional cell culture models or animal models in the future. Above all, organoids/organs-on-a-chip with other strategies like genome editing, 3D printing, and organoid biobanks contribute to modeling intestinal homeostasis and disease. Here, the current challenges and future trends in intestinal pathophysiological models will be summarized.

Viscoelastic Particle Encapsulation Using a Hyaluronic Acid Solution in a T-Junction Microfluidic Device

The encapsulation of particles and cells in droplets is highly relevant in biomedical engineering as well as in material science. So far, however, the majority of the studies in this area have focused on the encapsulation of particles or cells suspended in Newtonian liquids. We here studied the particle encapsulation phenomenon in a T-junction microfluidic device, using a non-Newtonian viscoelastic hyaluronic acid solution in phosphate buffer saline as suspending liquid for the particles. We first studied the non-Newtonian droplet formation mechanism, finding that the data for the normalised droplet length scaled as the Newtonian ones. We then performed viscoelastic encapsulation experiments, where we exploited the fact that particles self-assembled in equally-spaced structures before approaching the encapsulation area, to then identify some experimental conditions for which the single encapsulation efficiency was larger than the stochastic limit predicted by the Poisson statistics.

A Guide to Your Desired Lipid-Asymmetric Vesicles

Liposomes are prevalent model systems for studies on biological membranes. Recently, increasing attention has been paid to models also representing the lipid asymmetry of biological membranes. Here, we review in-vitro methods that have been established to prepare free-floating vesicles containing different compositions of the classic two-chain glycero- or sphingolipids in their outer and inner leaflet. In total, 72 reports are listed and assigned to four general strategies that are (A) enzymatic conversion of outer leaflet lipids, (B) re-sorting of lipids between leaflets, (C) assembly from different monolayers and (D) exchange of outer leaflet lipids. To guide the reader through this broad field of available techniques, we attempt to draw a road map that leads to the lipid-asymmetric vesicles that suit a given purpose. Of each method, we discuss advantages and limitations. In addition, various verification strategies of asymmetry as well as the role of cholesterol are briefly discussed. The ability to specifically induce lipid asymmetry in model membranes offers insights into the biological functions of asymmetry and may also benefit the technical applications of liposomes.

Bent-Capillary-Centrifugal-Driven Monodisperse Droplet Generator with Its Application for Digital LAMP Assay

We developed a bent-capillary-centrifugal-driven (BCCD) monodisperse droplet generator, which could achieve a perfect combination of driving and segmentation for the dispersed phase only using a rotating bent capillary immersed in the continuous phase (mineral oil). The sample could flow continuously to the bent-capillary outlet to form the droplet precursors, which were segmented into homogeneous droplets in the continuous phase. Through the investigation of influence factors on droplet size and stability, we found that the droplet size could be conveniently controlled by the rotational speed of the bent capillary. The droplet volumes could be adjusted with the range from 34 pL to 1 μL, and the coefficient variations (CVs) were less than 3%. Meanwhile, the BCCD droplet generator could realize the controllable droplet output with a high-efficiency sample utilization of 99.75 ± 1.15%, which offered a significant advantage in reducing the waste of precious samples in the droplet generation process. We validated this system with a digital loop-mediated isothermal amplification (dLAMP) assay for the absolute quantification of Mycobacterium tuberculosis complex nucleic acids. The results demonstrated that the BCCD droplet generator was easy to build, was of low cost, and was convenient to operate, as well as avoided sample loss and cross-contamination by coupling with a 96-well plate. Overall, the present platform, as a simple chip-free droplet generator, will provide an especially valuable droplet generation solution for biochemical applications based on droplets.

Sequentially Selective Coalescence of Binary Self-Propelled Droplets upon Collective Motion

Subsequent synthesis and detection using droplets as microreactors have shown promise in the development of novel materials and drugs because microreactors enable small-scale synthesis and detection of covalent/non-covalent intermolecular interactions. Self-organization exhibited by autonomous droplets under non-equilibrium conditions is beneficial for manipulating the sequentiality and selectivity of droplet coalescence because expensive equipment or elaborate techniques are not required with the autonomy of droplets. However, to our knowledge, selective coalescence caused by the collective motion of self-propelled droplets has not been demonstrated in inanimate systems. Here, we report sequentially selective coalescence based on the dynamic collective pattern of self-propelled droplets composed of ethyl salicylate (ES) or butyl salicylate (BS). When ES and BS droplets were placed on an aqueous sodium dodecyl sulfate (SDS) solution, the collective motion of droplets resulted in three stages of selective coalescence on the time development. Initially, coalescence was observed only between different types of self-propelled droplets. Subsequently, the formed droplets selectively coalesced with ES droplets. Finally, mature droplets merged with BS droplets. The sequentially selective coalescence was discussed from the dynamic pattern formation of swarming droplets and the collapse of the SDS monolayer at the o/w interface caused by the difference in Laplace pressure and the interfacial instability at the contact point between droplets. Thus, this study formulates a strategy of sequentially selective coalescence of droplets via the collective motion of non-identical self-propelled droplets, promoting a new type of powerful and efficient automation technology based on an autonomous inanimate manner of spatiotemporal pattern formation under non-equilibrium conditions for the droplet manipulation.

Toxicity of transition metal nanoparticles: A review of different experimental models in the gastrointestinal tract

The development of nanotechnology is becoming a major trend nowadays. Nanoparticles (NPs) have been widely used in fields including food, biomedicine, and cosmetics, endowing NPs more opportunities to enter the human body. It is well-known that the gut microbiome plays a key role in human health, and the exposure of intestines to NPs is unavoidable. Accordingly, the toxicity of NPs has attracted more attention than before. This review mainly highlights recent advances in the evaluation of NPs' toxicity in the gastrointestinal system from the existing cell-based experimental models, such as the original mono-culture models, co-culture models, three-dimensional (3D) culture models, and the models established on microfluidic chips, to those in vivo experiments, such as mice models, Caenorhabditis elegans models, zebrafish models, human volunteers, as well as computer-simulated toxicity models. Owing to these models, especially those more biomimetic models, the outcome of the toxicity of NPs acting in the gastrointestinal tract can get results closer to what happened inside the real human microenvironment.

Bottom-Up Synthetic Biology Using Cell-Free Protein Synthesis

Technical advances in biotechnology have greatly accelerated the development of bottom-up synthetic biology. Unlike top-down approaches, bottom-up synthetic biology focuses on the construction of a minimal cell from scratch and the application of these principles to solve challenges. Cell-free protein synthesis (CFPS) systems provide minimal machinery for transcription and translation, from either a fractionated cell lysate or individual purified protein elements, thus speeding up the development of synthetic cell projects. In this review, we trace the history of the cell-free technique back to the first in vitro fermentation experiment using yeast cell lysate. Furthermore, we summarized progresses of individual cell mimicry modules, such as compartmentalization, gene expression regulation, energy regeneration and metabolism, growth and division, communication, and motility. Finally, current challenges and future perspectives on the field are outlined.

DisGUVery: A Versatile Open-Source Software for High-Throughput Image Analysis of Giant Unilamellar Vesicles

Giant unilamellar vesicles (GUVs) are cell-sized aqueous compartments enclosed by a phospholipid bilayer. Due to their cell-mimicking properties, GUVs have become a widespread experimental tool in synthetic biology to study membrane properties and cellular processes. In stark contrast to the experimental progress, quantitative analysis of GUV microscopy images has received much less attention. Currently, most analysis is performed either manually or with custom-made scripts, which makes analysis time-consuming and results difficult to compare across studies. To make quantitative GUV analysis accessible and fast, we present DisGUVery, an open-source, versatile software that encapsulates multiple algorithms for automated detection and analysis of GUVs in microscopy images. With a performance analysis, we demonstrate that DisGUVery's three vesicle detection modules successfully identify GUVs in images obtained with a wide range of imaging sources, in various typical GUV experiments. Multiple predefined analysis modules allow the user to extract properties such as membrane fluorescence, vesicle shape, and internal fluorescence from large populations. A new membrane segmentation algorithm facilitates spatial fluorescence analysis of nonspherical vesicles. Altogether, DisGUVery provides an accessible tool to enable high-throughput automated analysis of GUVs, and thereby to promote quantitative data analysis in synthetic cell research.

Artificial Cells: Past, Present and Future

Artificial cells are constructed to imitate natural cells and allow researchers to explore biological process and the origin of life. The construction methods for artificial cells, through both top-down or bottom-up approaches, have achieved great progress over the past decades. Here we present a comprehensive overview on the development of artificial cells and their properties and applications. Artificial cells are derived from lipids, polymers, lipid/polymer hybrids, natural cell membranes, colloidosome, metal-organic frameworks and coacervates. They can be endowed with various functions through the incorporation of proteins and genes on the cell surface or encapsulated inside of the cells. These modulations determine the properties of artificial cells, including producing energy, cell growth, morphology change, division, transmembrane transport, environmental response, motility and chemotaxis. Multiple applications of these artificial cells are discussed here with a focus on therapeutic applications. Artificial cells are used as carriers for materials and information exchange and have been shown to function as targeted delivery systems of personalized drugs. Additionally, artificial cells can function to substitute for cells with impaired function. Enzyme therapy and immunotherapy using artificial cells have been an intense focus of research. Finally, prospects of future development of cell-mimic properties and broader applications are highlighted.

Cell-size space effects on phase separation of binary polymer blends

Within living cells, a diverse array of biomolecules is present at high concentrations. To better understand how molecular behavior differs under such conditions (collectively described as macromolecular crowding), the crowding environment has been reproduced inside artificial cells. We have previously shown that the combination of macromolecular crowding and microscale geometries imposed by the artificial cells can alter the molecular behaviors induced by macromolecular crowding in bulk solutions. We have named the effect that makes such a difference the cell-size space effect (CSE). Here, we review the underlying biophysics of CSE for phase separation of binary polymer blends. We discuss how the cell-size space can initiate phase separation, unlike nano-sized spaces, which are known to hinder nucleation and phase separation. Additionally, we discuss how the dimensions of the artificial cell and its membrane characteristics can significantly impact phase separation dynamics and equilibrium composition. Although these findings are, of themselves, very interesting, their real significance may lie in helping to clarify the functions of the cell membrane and space size in the regulation of intracellular phase separation.

Acoustics-Controlled Microdroplet and Microbubble Fusion and Its Application in the Synthesis of Hydrogel Microspheres

Droplet fusion technology is a key technology for many droplet-based biochemical medical applications. By integrating a symmetrical flow channel structure, we demonstrate an acoustics-controlled fusion method of microdroplets using surface acoustic waves. Different kinds of microdroplets can be staggered and ordered in the symmetrical flow channel, proving the good arrangement effect of the microfluidic chip. This method can realize not only the effective fusion of microbubbles but also the effective fusion of microdroplets of different sizes without any modification. Further, we investigate the influence of the input frequency and peak-to-peak value of the driving voltage on microdroplets fusion, giving the effective fusion parameter conditions of microdroplets. Finally, this method is successfully used in the preparation of hydrogel microspheres, offering a new platform for the synthesis of hydrogel microspheres.

Encapsulated Microstructures of Beneficial Functional Lipids and Their Applications in Foods and Biomedicines

Beneficial functional lipids are essential nutrients for the growth and development of humans and animals, which nevertheless possess poor chemical stability because of heat/light-sensitivity. Various encapsulation technologies have been developed to protect these nutrients against adverse factors. Different microstructures are exhibited through different encapsulation methods, which influence the encapsulation efficiency and release behavior at the same time. This review summarizes the effects of preparation methods and process parameters on the microstructures of capsules at first. The mechanisms of the different microstructures on encapsulation efficiency and controlled release behavior of core materials are analyzed. Next, a comprehensive overview on the beneficial functional lipids capsules in the latest food and biomedicine applications are provided as well as the matching relationship between the microstructures of the capsules and applications are discussed. Finally, the remaining challenges and future possible directions that have potential interest are outlined. The purpose of this review is to convey the construction of beneficial functional lipids capsules and the function mechanism, a critical analysis on its current status and challenges, and opinions on its future development. This review is believed to promote communication among the food, pharmacy, agronomy, engineering, and nutrition industries.

Microfluidic synthesis as a new route to produce novel functional materials

By geometrically constraining fluids into the sub-millimeter scale, microfluidics offers a physical environment largely different from the macroscopic world, as a result of the significantly enhanced surface effects. This environment is characterized by laminar flow and inertial particle behavior, short diffusion distance, and largely enhanced heat exchange. The recent two decades have witnessed the rapid advances of microfluidic technologies in various fields such as biotechnology; analytical science; and diagnostics; as well as physical, chemical, and biological research. On the other hand, one additional field is still emerging. With the advances in nanomaterial and soft matter research, there have been some reports of the advantages discovered during attempts to synthesize these materials on microfluidic chips. As the formation of nanomaterials and soft matters is sensitive to the environment where the building blocks are fed, the unique physical environment of microfluidics and the effectiveness in coupling with other force fields open up a lot of possibilities to form new products as compared to conventional bulk synthesis. This Perspective summarizes the recent progress in producing novel functional materials using microfluidics, such as generating particles with narrow and controlled size distribution, structured hybrid materials, and particles with new structures, completing reactions with a quicker rate and new reaction routes and enabling more effective and efficient control on reactions. Finally, the trend of future development in this field is also discussed.

One-Step Digital Droplet Auto-Catalytic Nucleic Acid Amplification with High-Throughput Fluorescence Imaging and Droplet Tracking Computation

Digital droplet technology has emerged as a powerful new tool for biomarker analysis. Temperature cycling, enzymes, and off-chip processes are, nevertheless, always required. Herein, we constructed a digital droplet auto-catalytic hairpin assembly (ddaCHA) microfluidic system to achieve digital quantification of single-molecule microRNA (miRNA). The designed continuous chip integrates droplet generation, incubation, and fluorescence imaging on the chip, avoiding the requirement for extra droplet re-collection and heating operations. Clearly, the digital readout was obtained by partitioning miRNA into many individual pL-sized small droplets in which the target molecule is either present ("positive") or absent ("negative"). Importantly, the suggested enzyme-free auto-catalytic hairpin assembly (aCHA) in droplets successfully mitigated the effects of the external environment and thermal cycling on droplets, and its reaction rate is significantly superior to that of traditional CHA. We got excellent sensitivity with a linear correlation from 1 pM to 10 nM and a detection limit of 0.34 pM in the fluorescence spectrum section, as well as high selectivity to other miRNAs. Furthermore, the minimum target concentration could be reduced to 10 fM based on the high-throughput tracking computation of fluorescent droplets with a self-developed Python script, and the fluorescence intensity distribution agreed well with the theoretical value, demonstrating that it is feasible to detect miRNA efficiently and accurately, which has great potential applications in clinical diagnostics and biochemical research.

Spatial confinement of multi-enzyme for cascade catalysis in cell-inspired all-aqueous multicompartmental microcapsules

Biocatalytic reaction networks in eukaryotic cells is realized by the immobilized and compartmental multi-enzymatic system. Inspired by the spatial localization of natural cells, multiple enzymes were confined within the multicompartmental microcapsules, which were created using a gas-shearing method coupled with surface-triggered in situ gelation strategy. Heterogeneous multicompartmental (two-, three-, four-, six-, or eight-faced) core particles, due to their capacity for positional assembly, were encapsuled in alginate hydrogel shells. The generated microcapsules integrate logic network to access complex digital design through a three-step convergent enzymatic cascade reaction as a model, and the capsules with high stability, recyclability and cytocompatibility are ideal enzymatic reactor systems to be used for biomimetic biocatalysis process.

Microfluidic-assisted synthesis and modeling of stimuli-responsive monodispersed chitosan microgels for drug delivery applications

Droplet microfluidic has been established to synthesize and functionalize micro/nanoparticles for drug delivery and screening, biosensing, cell/tissue engineering, lab-on-a-chip, and organ-on-a-chip have attracted much attention in chemical and biomedical engineering. Chitosan (CS) has been suggested for different biomedical applications due to its unique characteristics, such as antibacterial bioactivities, immune-enhancing influences, and anticancer bioactivities. The simulation results exhibited an alternative for attaining visions in this complex method. In this regard, the role of the flow rate ratio on the CS droplet features, including the generation rate and droplet size, were thoroughly described. Based on the results, an appropriate protocol was advanced for controlling the CS droplet properties for comparing their properties, such as the rate and size of the CS droplets in the microchip. Also, a level set (LS) laminar two-phase flow system was utilized to study the CS droplet-breaking process in the Flow Focused-based microchip. The outcomes demonstrated that different sizes and geometries of CS droplets could be established via varying the several parameters that validated addressing the different challenges for several purposes like drug delivery (the droplets with smaller sizes), tissue engineering, and cell encapsulation (the droplets with larger sizes), lab-on-a-chip, organ-on-a-chip, biosensing and bioimaging (the droplets with different sizes). An experimental study was added to confirm the simulation results. A drug delivery application was established to verify the claim.

Spatial confinement toward creating artificial living systems

Lifeforms are regulated by many physicochemical factors, and these factors could be controlled to play a role in the construction of artificial living systems. Among these factors, spatial confinement is an important one, which mediates biological behaviors at multiscale levels and participates in the biomanufacturing processes accordingly. This review describes how spatial confinement, as a fundamental biological phenomenon, provides cues for the construction of artificial living systems. Current knowledge about the role of spatial confinement in mediating individual cell behavior, collective cellular behavior, and tissue-level behavior are categorized. Endeavors on the synthesis of biomacromolecules, artificial cells, engineered tissues, and organoids in spatially confined bioreactors are then emphasized. After that, we discuss the cutting-edge applications of spatially confined artificial living systems in biomedical fields. Finally, we conclude by assessing the remaining challenges and future trends in the context of fundamental science, technical improvement, and practical applications.

Protocells: Milestones and Recent Advances

The origin of life is still one of humankind's great mysteries. At the transition between nonliving and living matter, protocells, initially featureless aggregates of abiotic matter, gain the structure and functions necessary to fulfill the criteria of life. Research addressing protocells as a central element in this transition is diverse and increasingly interdisciplinary. The authors review current protocell concepts and research directions, address milestones, challenges and existing hypotheses in the context of conditions on the early Earth, and provide a concise overview of current protocell research methods.

Mussel Inspired Trigger-Detachable Adhesive Hydrogel

Adhesion to many kinds of surfaces, including biological tissues, is important in many fields but has been proved to be extremely challenging. Furthermore, peeling from strong adhesion is needed in many conditions, but is sometimes painful. Herein, a mussel inspired hydrogel is developed to achieve both strong adhesion and trigger-detachment. The former is actualized by electrostatic interactions, covalent bonds, and physical interpenetration, while the latter is triggered, on-demand, through combining a thixotropic supramolecular network and polymer double network. The results of the experiments show that the hydrogel can adhere to various material surfaces and tissues. Moreover, triggered by shear force, non-covalent interactions of the supramolecular network are destroyed. This adhesion can be peeled easily. The possible mechanism involved is discussed and proved. This work will bring new insight into electronic engineering and tissue repair like skin care for premature infants and burn victims.

Fabrication of Biomaterials and Biostructures Based On Microfluidic Manipulation

Biofabrication technologies are of importance for the construction of organ models and functional tissue replacements. Microfluidic manipulation, a promising biofabrication technique with micro-scale resolution, can not only help to realize the fabrication of specific microsized structures but also build biomimetic microenvironments for biofabricated tissues. Therefore, microfluidic manipulation has attracted attention from researchers in the manipulation of particles and cells, biochemical analysis, tissue engineering, disease diagnostics, and drug discovery. Herein, biofabrication based on microfluidic manipulation technology is reviewed. The application of microfluidic manipulation technology in the manufacturing of biomaterials and biostructures with different dimensions and the control of the microenvironment is summarized. Finally, current challenges are discussed and a prospect of microfluidic manipulation technology is given. The authors hope this review can provide an overview of microfluidic manipulation technologies used in biofabrication and thus steer the current efforts in this field.

Vesicle-based artificial cells: materials, construction methods and applications

The construction of artificial cells with specific cell-mimicking functions helps to explore complex biological processes and cell functions in natural cell systems and provides an insight into the origins of life. Bottom-up methods are widely used for engineering artificial cells based on vesicles by the in vitro assembly of biomimetic materials. In this review, the design of artificial cells with a specific function is discussed, by considering the selection of synthetic materials and construction technologies. First, a range of biomimetic materials for artificial cells is reviewed, including lipid, polymeric and hybrid lipid/copolymer materials. Biomaterials extracted from natural cells are also covered in this part. Then, the formation of microscale, giant unilamellar vesicles (GUVs) is reviewed based on different technologies, including gentle hydration, electro-formation, phase transfer and microfluidic methods. Subsequently, applications of artificial cells based on single vesicles or vesicle networks are addressed for mimicking cell behaviors and signaling processes. Microreactors for synthetic biology and cell-cell communication are highlighted here as well. Finally, current challenges and future trends for the development and applications of artificial cells are described.

Pickering emulsion droplet-based biomimetic microreactors for continuous flow cascade reactions

A continuous flow cascade of multi-step catalytic reactions is a cutting-edge concept to revolutionize stepwise catalytic synthesis yet is still challenging in practical applications. Herein, a method for practical one-pot cascade catalysis is developed by combining Pickering emulsions with continuous flow. Our method involves co-localization of different catalytically active sub-compartments within droplets of a Pickering emulsion yielding cell-like microreactors, which can be packed in a column reactor for continuous flow cascade catalysis. As exemplified by two chemo-enzymatic cascade reactions for the synthesis of chiral cyanohydrins and chiral ester, 5 − 420 fold enhancement in the catalysis efficiency and as high as 99% enantioselectivity were obtained even over a period of 80 − 240 h. The compartmentalization effect and enriching-reactant properties arising from the biomimetic microreactor are theoretically and experimentally identified as the key factors for boosting the catalysis efficiency and for regulating the kinetics of cascade catalysis.

A continuous flow cascade of multi-step catalytic reactions would provide significant advantages in faster reaction times, waste reduction, and lowered step-count of syntheses, yet this ideal remains challenging in practical applications. Here the authors describe continuous flow cascade catalysis through co-localization of two catalytically active subcompartments within Pickering emulsion droplets.

Three-Dimensional Soft Material Micropatterning via Grayscale Photolithography for Improved Hydrophobicity of Polydimethylsiloxane (PDMS)

In this present work, we aim to improve the hydrophobicity of a polydimethylsiloxane (PDMS) surface. Various heights of 3D PDMS micropillars were fabricated via grayscale photolithography, and improved wettability was investigated. Two approaches of PDMS replication were demonstrated, both using a single master mold to obtain the micropillar arrays. The different heights of fabricated PDMS micropillars were characterized by scanning electron microscopy (SEM) and a surface profiler. The surface hydrophobicity was characterized by measuring the water contact angles. The fabrication of PDMS micropillar arrays was shown to be effective in modifying the contact angles of pure water droplets with the highest 157.3-degree water contact angle achieved by implementing a single mask grayscale lithography technique.

Biocatalytic self-assembled synthetic vesicles and coacervates: From single compartment to artificial cells

Compartmentalization is an intrinsic feature of living cells that allows spatiotemporal control over the biochemical pathways expressed in them. Over the years, a library of compartmentalized systems has been generated, which includes nano to micrometer sized biomimetic vesicles derived from lipids, amphiphilic block copolymers, peptides, and nanoparticles. Biocatalytic vesicles have been developed using a simple bag containing enzyme design of liposomes to multienzymes immobilized multi-vesicular compartments for artificial cell generation. Additionally, enzymes were also entrapped in membrane-less coacervate droplets to mimic the cytoplasmic macromolecular crowding mechanisms. Here, we have discussed different types of single and multicompartment systems, emphasizing their recent developments as biocatalytic self-assembled structures using recent examples. Importantly, we have summarized the strategies in the development of the self-assembled structure to improvise their adaptivity and flexibility for enzyme immobilization. Finally, we have presented the use of biocatalytic assemblies in mimicking different aspects of living cells, which further carves the path for the engineering of a minimal cell.

Core-shell microparticles: From rational engineering to diverse applications

Core-shell microparticles, composed of solid, liquid, or gas bubbles surrounded by a protective shell, are gaining considerable attention as intelligent and versatile carriers that show great potential in biomedical fields. In this review, an overview is given of recent developments in design and applications of biodegradable core-shell systems. Several emerging methodologies including self-assembly, gas-shearing, and coaxial electrospray are discussed and microfluidics technology is emphasized in detail. Furthermore, the characteristics of core-shell microparticles in artificial cells, drug release and cell culture applications are discussed and the superiority of these advanced multi-core microparticles for the generation of artificial cells is highlighted. Finally, the respective developing orientations and limitations inherent to these systems are addressed. It is hoped that this review can inspire researchers to propel the development of this field with new ideas.

Therapeutic Applications of Extracellular Vesicles for Myocardial Repair

Cardiovascular disease is the leading cause of human death worldwide. Drug thrombolysis, percutaneous coronary intervention, coronary artery bypass grafting and other methods are used to restore blood perfusion for coronary artery stenosis and blockage. The treatments listed prolong lifespan, however, rate of mortality ultimately remains the same. This is due to the irreversible damage sustained by myocardium, in which millions of heart cells are lost during myocardial infarction. The lack of pragmatic methods of myocardial restoration remains the greatest challenge for effective treatment. Exosomes are small extracellular vesicles (EVs) actively secreted by all cell types that act as effective transmitters of biological signals which contribute to both reparative and pathological processes within the heart. Exosomes have become the focus of many researchers as a novel drug delivery system due to the advantages of low toxicity, little immunogenicity and good permeability. In this review, we discuss the progress and challenges of EVs in myocardial repair, and review the recent development of extracellular vesicle-loading systems based on their unique nanostructures and physiological functions, as well as the application of engineering modifications in the diagnosis and treatment of myocardial repair.