Microfluidic melt emulsification for encapsulation and release of actives

One-step formation of polymorphous sperm-like microswimmers by vortex turbulence-assisted microfluidics

Microswimmers are considered promising candidates for active cargo delivery to benefit a wide spectrum of biomedical applications. Yet, big challenges still remain in designing the microswimmers with effective propelling, desirable loading and adaptive releasing abilities all in one. Inspired by the morphology and biofunction of spermatozoa, we report a one-step formation strategy of polymorphous sperm-like magnetic microswimmers (PSMs) by developing a vortex turbulence-assisted microfluidics (VTAM) platform. The fabricated PSM is biodegradable with a core-shell head and flexible tail, and their morphology can be adjusted by vortex flow rotation speed and calcium chloride solution concentration. Benefiting from the sperm-like design, our PSM exhibits both effective motion ability under remote mag/netic actuation and protective encapsulation ability for material loading. Further, it can also realize the stable sustain release after alginate-chitosan-alginate (ACA) layer coating modification. This research proposes and verifies a new strategy for the sperm-like microswimmer construction, offering an alternative solution for the target delivery of diverse drugs and biologics for future biomedical treatment. Moreover, the proposed VTAM could also be a general method for other sophisticated polymorphous structures fabrication that isn't achievable by conventional laminar flow.

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.

Revolutionizing targeting precision: microfluidics-enabled smart microcapsules for tailored delivery and controlled release

As promising delivery systems, smart microcapsules have garnered significant attention owing to their targeted delivery loaded with diverse active materials. By precisely manipulating fluids on the micrometer scale, microfluidic has emerged as a powerful tool for tailoring delivery systems based on potential applications. The desirable characteristics of smart microcapsules are associated with encapsulation capacity, targeted delivery capability, and controlled release of encapsulants. In this review, we briefly describe the principles of droplet-based microfluidics for smart microcapsules. Subsequently, we summarize smart microcapsules as delivery systems for efficient encapsulation and focus on target delivery patterns, including passive targets, active targets, and microfluidics-assisted targets. Additionally, based on release mechanisms, we review controlled release modes adjusted by smart membranes and on/off gates. Finally, we discuss existing challenges and potential implications associated with smart microcapsules.

Designable Micro-/Nano-Structured Smart Polymeric Materials

Smart polymeric materials with dynamically tunable physico-chemical characteristics in response to changes of environmental stimuli, have received considerable attention in myriad fields. The diverse combination of their micro-/nano-structural and molecular designs creates promising and exciting opportunities for exploiting advanced smart polymeric materials. Engineering micro-/nano-structures into smart polymeric materials with elaborate molecular design enables intricate coordination between their structures and molecular-level response to cooperatively realize smart functions for practical applications. In this review, recent progresses of smart polymeric materials that combine micro-/nano-structures and molecular design to achieve designed advanced functions are highlighted. Smart hydrogels, gating membranes, gratings, milli-particles, micro-particles and microvalves are employed as typical examples to introduce their design and fabrication strategies. Meanwhile, the key roles of interplay between their micro-/nano-structures and responsive properties to realize the desired functions for their applications are emphasized. Finally, perspectives on the current challenges and opportunities of micro-/nano-structured smart polymeric materials for their future development are presented.

Solvent-Free Fabrication of Biphasic Lipid-Based Microparticles with Tunable Structure

Lipid-based biphasic microparticles are generally produced by long and complex techniques based on double emulsions. In this study, spray congealing was used as a solvent-free fabrication method with improved processability to transform water-in-oil non-aqueous emulsions into spherical solid lipid-based particles with a biphasic structure (b-MPs). Emulsions were prepared by melt emulsification using different compositions of lipids (Dynasan^®118 and Compritol^®888 ATO), surfactants (Cetylstearyl alcohol and Span^®60) and hydrophilic carriers (PEGs, Gelucire^®48/16 and Poloxamer 188). First, pseudo-ternary phase diagrams were constructed to identify the area corresponding to each emulsion type (coarse emulsion or microemulsion). The hydrophobicity of the lipid mostly affected the interfacial tension, and thus the microstructure of the emulsion. Emulsions were then processed by spray congealing and the obtained b-MPs were characterized in terms of thermal and chemical properties (by DSC and FT-IR), external and internal morphology (by SEM, CLSM and Raman mapping). Solid free-flowing spherical particles (main size range 200–355 µm) with different architectures were successfully produced: microemulsions led to the formation of particles with a homogeneous internal structure, while coarse emulsions generated “multicores-shell” particles consisting of variable size hydrophilic cores evenly distributed within the crystalline lipid phase. Depending on their composition and structure, b-MPs could achieve various release profiles, representing a more versatile system than microparticles based on a single lipid phase. The formulation and technological strategy proposed, provides a feasible and cost-effective way of fabricating b-MPs with tunable internal structure and release behavior.

Unimodal sized silica nanocapsules produced through water-in-oil emulsions prepared by sequential irradiation of kilo- and submega-hertz ultrasounds

This study investigates the regulation of the size of 100 nm hollow-sphere silica particles using surfactant-free water-in-oil (W/O) emulsion. First, water droplets were dispersed in soybean oil via sequential ultrasound irradiation (28 kHz → 200 kHz → 950 kHz). A precursor of hollow silica particles was prepared using hydrolysis and polymerization of methylsilyl trichloride into a stable W/O emulsion. The final structure/morphology of the silica particles was influenced by the volume ratio of water/soybean oil, the cycle number of the sequential ultrasound irradiation, and the amount of organosilane added to the emulsion. The emulsion was stabilized by Ostwald ripening, as the size distribution at 5/10³ (water/oil = v/v) was a bimodal split between a water droplet size of a few μm and some with a size of a few tens of nm. The most appropriate cycle number was 3 in this system. Further cycling to 5 resulted in a broad and bimodal size distribution of the final particles due to rapid coalescence of water droplets. Subsequent hydrolysis of methylsilyl trichloride consumed water with diminishing large droplets, forming fine and unimodal (0.12 ± 0.02 μm) hollow silica particles. Very fine and uniform-sized hollow particles (0.08 ± 0.01 μm) were successfully produced by decreasing the volume ratio to 1/10³ (water/oil) because of a transparent stable emulsion as a homogeneous template of the hollow structures.

Silica nanocapsules were prepared using water droplets dispersed in soybean oil via sequential ultrasound irradiation (28 kHz → 200 kHz → 950 kHz).

Deformation and rupture of microcapsules flowing through constricted capillary

The dynamics of deformable microcapsules flowing through constricted channels is relevant in target delivery of chemicals in physiological systems, porous media, microfluidic medical diagnostic devices and many other applications. In some situations, the microcapsules need to sustain the stress they are subjected to as they flow through constricted channels and in others, the stress may be the rupture trigger used to release the internal content. We experimentally investigate the flow of monodispersed gellan gum microcapsules through a constricted capillary tube by measuring the evolution of the pressure difference and flow visualization. The maximum pressure difference and capsule deformation is obtained for capsules with different diameter and shell thickness. We map the conditions, e.g. diameter and shell thickness, at which the capsule membrane ruptures during the flow, releasing its internal phase.

Interfacial behavior of surfactant-covered double emulsion in extensional flow

We analyze the interface-interface interactions of a surfactant-covered double emulsion using the lattice Boltzmann method and study the interaction of the inner and outer interfaces and the local surfactant distribution under a uniaxial extensional flow. First, the capillary effects are analyzed. Upon surfactant application, the outer droplet deformation increases and the inner droplet deformation decreases. The concentrated surfactants on the outer interface increase deformation, and the inner droplet is affected by the inner flow. At a fixed Péclet number (Pe), the surfactant concentration at the outer interface increases with an increase in capillary number (Ca); however, such a tendency is difficult to identify at the inner interface. Next, the Pe effects are analyzed. With an increase in Pe, the deformation of the inner droplet decreases significantly. The local distribution of the surfactant considerably affects the double emulsion stabilization, which is analyzed in terms of internal flow. The interfacial tension gradient induced by the surfactant generates vortices internally, which is verified by applying the surfactant to each interface independently. The radius ratio affects droplet deformation and surfactant transport. The compression of the inner flow region increases the viscous force and decreases the interface velocity. Therefore, with an increase in radius ratio, the deformation increases, and the surfactant transport becomes slow.

Controllable microfluidic fabrication of microstructured functional materials

Microstructured functional materials such as microfibers and microparticles are widely used for a myriad of applications. Precise manipulation of the functional components and structure is important for the microstructured functional materials to achieve desired functions for advanced application. This review highlights the recent progress on the controllable microfluidic fabrication of microstructured functional materials from liquid templates. First, microfluidic strategies for controllable generation of liquid templates including laminar jets and emulsion droplets are introduced. Then, strategies for fabricating microfibers and microparticles with diverse structures and advanced functions from the liquid templates are highlighted. These strategies mainly focus on precisely engineering the functional components and microstructures of the microfibers and microparticles by tailoring those of their liquid templates to achieve desired advanced functions. Finally, future development of microfluidic techniques for industrial-scale production of the microstructured functional materials is discussed.

Dripping, jetting and tip streaming

Dripping, jetting and tip streaming have been studied up to a certain point separately by both fluid mechanics and microfluidics communities, the former focusing on fundamental aspects while the latter on applications. Here, we intend to review this field from a global perspective by considering and linking the two sides of the problem. First, we present the theoretical model used to study interfacial flows arising in droplet-based microfluidics, paying attention to three elements commonly present in applications: viscoelasticity, electric fields and surfactants. We review both classical and current results of the stability of jets affected by these elements. Mechanisms leading to the breakup of jets to produce drops are reviewed as well, including some recent advances in this field. We also consider the relatively scarce theoretical studies on the emergence and stability of tip streaming in open systems. Second, we focus on axisymmetric microfluidic configurations which can operate on the dripping and jetting modes either in a direct (standard) way or via tip streaming. We present the dimensionless parameters characterizing these configurations, the scaling laws which allow predicting the size of the resulting droplets and bubbles, as well as those delimiting the parameter windows where tip streaming can be found. Special attention is paid to electrospray and flow focusing, two of the techniques more frequently used in continuous drop production microfluidics. We aim to connect experimental observations described in this section of topics with fundamental and general aspects described in the first part of the review. This work closes with some prospects at both fundamental and practical levels.

Construction of core-shell microcapsules via focused surface acoustic wave microfluidics

The ability to construct core-shell microcapsules has the potential to shift the paradigm in the development of new delivery systems for nutrients, cosmetics, and drugs. In this work, we demonstrate an application of focused surface acoustic wave (FSAW) microfluidics to produce microcapsules with a core-shell structure using one or two focused interdigital transducers (FIDTs) on the microfluidic device. Solid particles or liquid microdroplets without any special modification in multiphase laminar flow are driven by the acoustic radiation force arising from the FSAW, and cross the oil/water interface back and forth, which is not only suitable for generation of core-shell microcapsules with solid cores but also used for coating an aqueous microdroplet core with an oil shell. On this basis, more FIDTs can be added to the device to manufacture more layers of microcapsules if needed. Single-layer, two-layer, or even multi-layer microcapsules can be selectively fabricated. This work provides a promising and robust platform to construct core-shell microcapsules via FSAW microfluidics, which are suitable for a wide range of applications.

Multiphase flow in microfluidics: From droplets and bubbles to the encapsulated structures

Microfluidic technologies have a unique ability to control more precisely and effectively on two-phase flow systems in comparison with macro systems. Controlling the size of the droplets and bubbles has led to an ever-increasing expansion of this technology in two-phase systems. Liquid-liquid and gas-liquid two-phase flows because of their numerous applications in different branches such as reactions, synthesis, emulsions, cosmetic, food, drug delivery, etc. have been the most critical two-phase flows in microfluidic systems. This review highlights recent progress in two-phase flows in microfluidic devices. The fundamentals of two-phase flows, including some essential dimensionless numbers, governing equations, and some most well-known numerical methods are firstly introduced, followed by a review of standard methods for producing segmented flows such as emulsions in microfluidic systems. Then various encapsulated structures, a common two-phase flow structure in microfluidic devices, and different methods of their production are reviewed. Finally, applications of two-phase microfluidic flows in drug-delivery, biotechnology, mixing, and microreactors are briefly discussed.

Generation of Ultra-Thin-Shell Microcapsules Using Osmolarity-Controlled Swelling Method

Microcapsules are attractive core-shell configurations for studies of controlled release, biomolecular sensing, artificial microbial environments, and spherical film buckling. However, the production of microcapsules with ultra-thin shells remains a challenge. Here we develop a simple and practical osmolarity-controlled swelling method for the mass production of monodisperse microcapsules with ultra-thin shells via water-in-oil-in-water (W/O/W) double-emulsion drops templating. The size and shell thickness of the double-emulsion drops are precisely tuned by changing the osmotic pressure between the inner cores and the suspending medium, indicating the practicability and effectiveness of this swelling method in tuning the shell thickness of double-emulsion drops and the resultant microcapsules. This method enables the production of microcapsules even with an ultra-thin shell less than hundreds of nanometers, which overcomes the difficulty in producing ultra-thin-shell microcapsules using the classic microfluidic emulsion technologies. In addition, the ultra-thin-shell microcapsules can maintain their intact spherical shape for up to 1 year without rupturing in our long-term observation. We believe that the osmolarity-controlled swelling method will be useful in generating ultra-thin-shell polydimethylsiloxane (PDMS) microcapsules for long-term encapsulation, and for thin film folding, buckling and rupturing investigation.

Compound-Droplet-Pairs-Filled Hydrogel Microfiber for Electric-Field-Induced Selective Release

The separate co-encapsulation and selective controlled release of multiple encapsulants in a predetermined sequence has potentially important applications for drug delivery and tissue engineering. However, the selective controlled release of distinct contents upon one triggering event for most existing microcarriers still remains challenging. Here, novel microfluidic fabrication of compound-droplet-pairs-filled hydrogel microfibers (C-Fibers) is presented for two-step selective controlled release under AC electric field. The parallel arranged compound droplets enable the separate co-encapsulation of distinct contents in a single microfiber, and the release sequence is guaranteed by the discrepancy of the shell thickness or core conductivity of the encapsulated droplets. This is demonstrated by using a high-frequency electric field to trigger the first burst release of droplets with higher conductivity or thinner shell, followed by the second release of the other droplets under low-frequency electric field. The reported C-Fibers provide novel multidelivery system for a wide range of applications that require controlled release of multiple ingredients in a prescribed sequence.

Microfluidic Systems for Droplet Generation in Aqueous Continuous Phases: A Focus Review

Microfluidics is one of the most fascinating fields that researchers have been trying to apply in a large number of scientific disciplines over the past two decades. Among them, the discipline of food and pharmaceutical formulation encountered several obstacles when combining microfluidics with aqueous media. Indeed, the physical properties of liquids at micrometric volumes being particular, the droplet generation within microfluidic devices is a big challenge to be met. This focus review is intended to be an initiation for those who would like to generate microdroplets in microfluidic systems involving aqueous continuous phases. It provides a state-of-the-art look at such systems while focusing on the microfluidic devices used, their applications to form a wide variety of emulsions and particles, and the key role held by the interface between the device channels and the emulsion. This review also leads to reflections on new materials that can be used in microfluidic systems with aqueous continuous phases.

Integration of Horizontal and Vertical Microfluidic Modules for Core-Shell Droplet Generation and Chemical Application

This paper presents a method for utilizing three-dimensional microfluidic channels fully to realize multiple functions in a single device. The final device structure was achieved by combining three independent modules that consisted of horizontal and vertical channels. The device allowed for the one-step generation of water-in-oil-in-water droplets without the need for partial treatment of the polydimethylsiloxane channel surface using separate modules for generating water-in-oil droplets on the horizontal plane and oil-in-water droplets on the vertical plane. The second vertically structured module provided an efficient flow for the generation of highly wettable liquid droplets, and tuning of the first horizontally structured module enabled different modes of inner-core encapsulation within the oil shell. The successful integration of the vertical and horizontal channels for core-shell droplet generation and the chemical synthesis of a metal complex within the droplets were evaluated. The proposed approach of integrating independent modules will expand and enhance the functions of microfluidic platforms.

Automated detection and sorting of microencapsulation via machine learning

Microfluidic-based microencapsulation requires significant oversight to prevent material and quality loss due to sporadic disruptions in fluid flow that routinely arise. State-of-the-art microcapsule production is laborious and relies on experts to monitor the process, e.g. through a microscope. Unnoticed defects diminish the quality of collected material and/or may cause irreversible clogging. To address these issues, we developed an automated monitoring and sorting system that operates on consumer-grade hardware in real-time. Using human-labeled microscope images acquired during typical operation, we train a convolutional neural network that assesses microencapsulation. Based on output from the machine learning algorithm, an integrated valving system collects desirable microcapsules or diverts waste material accordingly. Although the system notifies operators to make necessary adjustments to restore microencapsulation, we can extend the system to automate corrections. Since microfluidic-based production platforms customarily collect image and sensor data, machine learning can help to scale up and improve microfluidic techniques beyond microencapsulation.

Smart Microcapsules with Molecular Polarity- and Temperature-Dependent Permeability

Microcapsules with molecule-selective permeation are appealing as microreactors, capsule-type sensors, drug and cell carriers, and artificial cells. To accomplish molecular size- and charge-selective permeation, regular size of pores and surface charges have been formed in the membranes. However, it remains an important challenge to provide advanced regulation of transmembrane transport. Here, smart microcapsules are designed that provide molecular polarity- and temperature-dependent permeability. With capillary microfluidic devices, water-in-oil-in-water (W/O/W) double-emulsion drops are prepared, which serve as templates to produce microcapsules. The oil shell is composed of two monomers and dodecanol, which turns to a polymeric framework whose continuous voids are filled with dodecanol upon photopolymerization. One of the monomers provides mechanical stability of the framework, whereas the other serves as a compatibilizer between growing polymer and dodecanol, preventing macrophase separation. Above melting point of dodecanol, molecules that are soluble in the molten dodecanol are selectively allowed to diffuse across the shell, where the rate of transmembrane transport is strongly influenced by partition coefficient. The rate is drastically lowered for temperatures below the melting point. This molecular polarity- and temperature-dependent permeability renders the microcapsules potentially useful as drug carriers for triggered release and contamination-free microreactors and microsensors.

Droplet-based microfluidics systems in biomedical applications

Microfluidics has made a very impressive progress in the past decades due to its unique and instinctive advantages. Droplet-based microfluidic systems show excellent compatibility with many chemical and biological reagents and are capable of performing variety of operations that can implement microreactor, complex multiple core-shell structure, and many applications in biomedical research such as drug encapsulation, targeted drug delivery systems, and multifunctionalization on carriers. Droplet-based systems have been directly used to synthesize particles and encapsulate many biological entities for biomedicine applications due to their powerful encapsulation capability and facile versatility. In this paper, we review its origin, deviation, and evolution to draw a clear future, especially for droplet-based biomedical applications. This paper will focus on droplet generation, variations and complication as starter, and logistically lead to the numerous typical applications in biomedical research. Finally, we will summarize both its challenge and future prospects relevant to its droplet-based biomedical applications.

Photonics of Template-Mediated Lattices of Colloidal Clusters

The recent progress in microfluidic microfabrication enables mass production of "colloidal molecules" with a preprogrammed geometry (e.g., dumbbells, tetrahedrons, etc.). Such colloids can be used as elementary building blocks in the fabrication of colloidal crystals with unique optical properties. Anisotropic clusters, however, cannot be readily assembled into regular lattices. In this paper, we study photonic properties of compact cubic templates of microdrops encapsulating complex "colloidal molecules". Because monodisperse droplets can be easily packed into dense cubic lattices and encapsulation techniques (e.g., using microfluidics) are well developed, such a material is experimentally feasible. The rationale behind such a methodology is that for a particular alignment of the encapsulated "colloidal molecules" (e.g., by applying an external magnetic or electric field), the resulting structures resemble a diamond lattice, which is known to exhibit a wide complete photonic band gap. The photonic properties of two cubic templates encapsulating dumbbells (symmetric and asymmetric) and tetrahedrons are investigated numerically. In particular, we show the emergence of the complete 3D band gap (∼8% wide for the dielectric contrast ε = 14) for symmetric dumbbells embedded within a face-centered cubic template and oriented along the space diagonal of the elementary cubic cell.

Batch-Scale Preparation of Reverse Janus Emulsions

A strategy is proposed to produce novel (W1 + W2)/O reverse Janus emulsions in batch scale simply by one-step vortex mixing. Aqueous two-phase systems (ATPSs), i.e., two immiscible aqueous phases dominated by sodium carbonate and ethanol, respectively, are employed as inner phases and vegetable oil (VO) as continuous phase. The geometry of the Janus droplets, although formed as a result of a kinetic process, is tunable and controllable easily by adjusting the composition of ATPSs based on three-phase diagram. Reducing the relatively higher water/oil interfacial tensions to a comparable value of water/water interface, which is extremely low in order of 0.1 mN/m, is achieved by employing a fluorocarbon surfactant. Moreover, the weak acid-induced deprotonation of the fatty acid in the VO phase due to the presence of sodium carbonate also contributes to the lower water/oil interfacial tension. The total free-energy values calculated verify the overwhelmingly favored Janus geometry, which indicates that this topology is heavily preformed as local equilibrium state. The approach proposed provides vehicle for the synthesis of aqueous-based materials with various advanced morphologies.

Mechano-responsive microcapsules with uniform thin shells

Capsules often prolong the shelf-life of active ingredients, such as many types of drugs, food additives, or cosmetic substances, because they delay oxidation of these substances or prevent their reactions with molecules contained in the surrounding. If capsules are appropriately designed, they can offer an additional benefit: they allow close control over the timing and location of the release of active ingredients. To take advantage of these features, capsules must possess shells whose thickness and composition are well-defined. However, the shell thickness of capsules often varies considerably even within a single capsule, thereby hampering good control over the release kinetics of encapsulants. These variations can be reduced, and hence the degree of control over the release kinetics increased, if shells are made thin. Unfortunately, the controlled fabrication of mechanically stable microcapsules with well-defined sub-μm thick shells is difficult. Here, we introduce a method to fabricate capsules with uniform semi-permeable shells with a thickness as low as 400 nm. This is achieved using water-oil-water double emulsions with 800 nm thick shells as templates to fabricate capsules with uniform 400 nm thin shells. These shells occupy less than 2% of the capsule volume, thereby minimizing their footprint. Despite their thin shells, these capsules are mechanically robust: they withstand pressures up to 1.3 MPa without deformation and remain intact if exposed to pressures up to 2.75 MPa. Moreover, while they are permeable towards water, they retain low molecular weight encapsulants even if dried and re-dispersed. The thin shells of the capsules open up new possibilities of their use to functionalize materials with at least one dimension that is small, such as coatings, where thick shells introduce defects, or as building blocks of new types of functional materials.

Structural Formation of Oil-in-Water (O/W) and Water-in-Oil-in-Water (W/O/W) Droplets in PDMS Device Using Protrusion Channel without Hydrophilic Surface Treatment

This paper presents a simple method of droplet formation using liquids that easily wet polydimethylsiloxane (PDMS) surfaces without any surface treatment. Using only structural features and uniform flow focusing, Oil-in-Water (O/W) and Water-in-Oil-in-Water (W/O/W) droplets were formed in the full PDMS structure. Extrusion channel and three-dimensional flow focusing resulted in effective fluidic conditions for droplet formation and the droplet size could be precisely controlled by controlling the flow rate of each phase. The proposed structure can be utilized as an important element for droplet based research, as well as a droplet generator.

Self-Assembly-Mediated Release of Peptide Nanoparticles through Jets Across Microdroplet Interfaces

The release of nanoscale structures from microcapsules, triggered by changes in the capsule in response to external stimuli, has significant potential for active component delivery. Here, we describe an orthogonal strategy for controlling molecular species' release across oil/water interfaces by modulating their intrinsic self-assembly state. We show that although the soluble peptide Boc-FF can be stably encapsulated for days, its self-assembly into nanostructures triggers jet-like release within seconds. Moreover, we exploit this self-assembly-mediated release to deliver other molecular species that are transported as cargo. These results demonstrate the role of self-assembly in modulating the transport of peptides across interfaces.

Droplet microfluidics for the construction of compartmentalised model membranes

The design of membrane-based constructs with multiple compartments is of increasing importance given their potential applications as microreactors, as artificial cells in synthetic-biology, as simplified cell models, and as drug delivery vehicles. The emergence of droplet microfluidics as a tool for their construction has allowed rapid scale-up in generation throughput, scale-down of size, and control over gross membrane architecture. This is true on several levels: size, level of compartmentalisation and connectivity of compartments can all be programmed to various degrees. This tutorial review explains and explores the reasons behind this. We discuss microfluidic strategies for the generation of a family of compartmentalised systems that have lipid membranes as the basic structural motifs, where droplets are either the fundamental building blocks, or are precursors to the membrane-bound compartments. We examine the key properties associated with these systems (including stability, yield, encapsulation efficiency), discuss relevant device fabrication technologies, and outline the technical challenges. In doing so, we critically review the state-of-play in this rapidly advancing field.

Uniform-Sized Silica Nanocapsules Produced by Addition of Salts to a Water-In-Oil Emulsion Template

Size control of silica hollow particles was achieved using a water-in-oil (W/O) emulsion system, where interfacial condensation of organosilanes (octyltrichlorosilane and methyltrichlorosilane) took place around the aqueous droplets. Good emulsion stability was obtained using soybean oil as the oil phase because of its high viscosity. This high stability led to bimodal distributions in the sizes of the aqueous droplets and final hollow particles, with particle sizes observed of a few tens of nanometers and a few micrometers. The presence of NaCl was found to be required in the water phase to afford uniform-sized silica hollow spheres. Hydrolysis of the organosilanes caused a supersaturation of the aqueous NaCl solution dispersing in the oil continuous phase, followed by crystallization from droplets. Nanosized aqueous droplets acted as a template to form uniform-sized nanospherical hollow silica particles as a result of the diminishing number of larger aqueous droplets.

Microfluidic fabrication of microparticles for biomedical applications

Droplet microfluidics offers exquisite control over the flows of multiple fluids in microscale, enabling fabrication of advanced microparticles with precisely tunable structures and compositions in a high throughput manner. The combination of these remarkable features with proper materials and fabrication methods has enabled high efficiency, direct encapsulation of actives in microparticles whose features and functionalities can be well controlled. These microparticles have great potential in a wide range of bio-related applications including drug delivery, cell-laden matrices, biosensors and even as artificial cells. In this review, we briefly summarize the materials, fabrication methods, and microparticle structures produced with droplet microfluidics. We also provide a comprehensive overview of their recent uses in biomedical applications. Finally, we discuss the existing challenges and perspectives to promote the future development of these engineered microparticles.

Trojan-Horse-Like Stimuli-Responsive Microcapsules

Multicompartment microcapsules, with each compartment protected by a distinct stimuli-responsive shell for versatile controlled release, are highly desired for developing new-generation microcarriers. Although many multicompartmental microcapsules have been created, most cannot combine different release styles to achieve flexible programmed sequential release. Here, one-step template synthesis of controllable Trojan-horse-like stimuli-responsive microcapsules is reported with capsule-in-capsule structures from microfluidic quadruple emulsions for diverse programmed sequential release. The nested inner and outer capsule compartments can separately encapsulate different contents, while their two stimuli-responsive hydrogel shells can individually control the content release from each capsule compartment for versatile sequential release. This is demonstrated by using three types of Trojan-horse-like stimuli-responsive microcapsules, with different combinations of release styles for flexible programmed sequential release. The proposed microcapsules provide novel advanced candidates for developing new-generation microcarriers for diverse, efficient applications.

Electrocoalescence of paired droplets encapsulated in double-emulsion drops

We utilize an ac electric field to trigger the on-demand fusion of two aqueous cores inside water-in-oil-in-water (W/O/W) double-emulsion drops. We attribute the coalescence phenomenon to field-induced structural polarization and breakdown of the stress balance at interfaces. This method provides not only accurate control over the reaction time of coalescence but also protection of the reaction from cross contamination.

Microfluidic production of multiple emulsions and functional microcapsules

Recent advances in microfluidics have enabled the controlled production of multiple-emulsion drops with onion-like topology. The multiple-emulsion drops possess an intrinsic core-shell geometry, which makes them useful as templates to create microcapsules with a solid membrane. High flexibility in the selection of materials and hierarchical order, achieved by microfluidic technologies, has provided versatility in the membrane properties and microcapsule functions. The microcapsules are now designed not just for storage and release of encapsulants but for sensing microenvironments, developing structural colours, and many other uses. This article reviews the current state of the art in the microfluidic-based production of multiple-emulsion drops and functional microcapsules. The three main sections of this paper discuss distinct microfluidic techniques developed for the generation of multiple emulsions, four representative methods used for solid membrane formation, and various applications of functional microcapsules. Finally, we outline the current limitations and future perspectives of microfluidics and microcapsules.

Shrinking, growing, and bursting: microfluidic equilibrium control of water-in-water droplets

We demonstrate the dynamic control of aqueous two phase system (ATPS) droplets in shrinking, growing, and dissolving conditions. The ATPS droplets are formed passively in a flow focusing microfluidic channel, where the dextran-rich (DEX) and polyethylene glycol-rich (PEG) solutions are introduced as disperse and continuous phases, respectively. To vary the ATPS equilibrium condition, we infuse into a secondary inlet the PEG phase from a different polymer concentration ATPS. We find that the resulting alteration of the continuous PEG phase can cause droplets to shrink or grow by approximately 45 and 30%, respectively. This volume change is due to water exchange between the disperse DEX and continuous PEG phases, as the system tends towards new equilibria. We also develop a simple model, based on the ATPS binodal curve and tie lines, that predicts the amount of droplet shrinkage or growth, based on the change in the continuous phase PEG concentration. We observe a good agreement between our experimental results and the model. Additionally, we find that when the continuous phase PEG concentration is reduced such that PEG and DEX phases no longer phase separate, the ATPS droplets are dissolved into the continuous phase. We apply this method to controllably release encapsulated microparticles and cells, and we find that their release occurs within 10 seconds. Our approach uses the dynamic equilibrium of ATPS to control droplet size along the microfluidic channel. By modulating the ATPS equilibrium, we are able to shrink, grow, and dissolve ATPS droplets in situ. We anticipate that this approach may find utility in many biomedical settings, for example, in drug and cell delivery and release applications.

Water-oil Janus emulsions: microfluidic synthesis and morphology design

In this work we developed a facile method to prepare water-oil Janus emulsions in situ with tunable morphologies by using a double-bore capillary microfluidic device. In addition, by combining the theory model and our liquids' properties, we propose a method to design the morphology of water-oil Janus emulsions. To systematically research Janus morphologies we combined the theory model and the fluids' properties. Under the model guidance, we carefully selected the liquids system where only the interfacial tension between the water phase and the continuous phase changed while keeping the other two interfacial tensions unchanged. Thus we could adjust the Janus morphology by changing the surfactant mass fraction in the continuous phase. In addition, with the double-bore capillary, we prepared water-oil Janus emulsions with a large flow ratio range. By adjusting the flow ratio and the surfactant mass fraction, we successfully prepared Janus emulsions with gradual morphology changes, which would be meaningful in fields that have a high demand for morphology designing of amphiphilic Janus particles.