Designer polymer-based microcapsules made using microfluidics

Tunable mechanical properties of PDMS-TMPTMA microcapsules for controlled release in coatings

Within coating formulations, microcapsules serve as vehicles for delivering compounds like catalysts and self-healing agents. Designing microcapsules with precise mechanical characteristics is crucial to ensure their contents' timely release and minimize residual shell fragments, thereby avoiding adverse impacts on the coating quality. With these constraints in mind, we explored the use of 1 cSt PDMS oil as a diluent (porogen) in trimethylolpropane trimethacrylate (TMPTMA)-based to fabricate microcapsules with customized mechanical properties and submicrometer debris size after shell breakup that can encapsulate a wide range of compounds. Microcapsules were fabricated from double emulsion templates featuring an aqueous core and shells with different PDMS : TMPTMA volume percent ratios. Their mechanical properties under compression and their capacity for encapsulation were characterized. PDMS : TMPTMA ratios exceeding 20 : 80 caused phase separation during crosslinking, leading to a porous shell structure of TMPTMA clusters. The strength of the microcapsules decreased as the PDMS:TMPTMA ratio increased, with ratios above 30 : 70 resulting in mechanically fragile microcapsules fracturing into fragments <10 μm in size. Microcapsules produced with a 0 : 100 PDMS : TMPTMA ratio exhibited strong mechanical properties and were capable of encapsulating small volatile compounds like 1,4-diazabicyclo[2.2.2]octane (a catalyst), while those with ratios >30 : 70 were only suitable to encapsulate larger molecules. The combination of PDMS:TMPTMA chemistry and the precise control provided by the double emulsion generation process in microcapillary devices makes PDMS:TMPTMA a versatile system suitable for various pressure-sensitive encapsulation applications.

Syringable Microcapsules for Sustained, Localized, and Controllable siRNA Delivery

The clinical use of small interfering RNA (siRNA) and antisense oligonucleotides often requires invasive routes of administration, including intrathecal or intraocular injection. Additionally, these treatments often necessitate repeated injections. While nanoparticle formulation and chemical modifications have extended siRNA therapeutic durability, challenges persist, such as the side effects of bolus injections with high toxicity and maximum exposure in the acute phase. We present a microcapsule-based method to extend the activity of cholesterol-conjugated siRNA locally. Using microfluidics, microcapsules with well-defined size distribution and shell thickness are fabricated with poly(lactic-co-glycolic acid) (PLGA) with varying molecular weights and compositions. The microcapsules show a remarkably high drug encapsulation efficiency of nearly 100% and a high loading capacity (8900 μg siRNA/1 mg polymer). Additionally, microcapsules with an average diameter of 40 μm show superior syringeability when tested with needles ranging from gauge sizes of 27 to 32 G. This makes them suitable for various injection routes. Two sustained-release formulations were selected based on a 3-month in vitro release test. Subsequently, these formulations were injected subcutaneously into mice to verify their in vivo release profiles. The findings demonstrate that the microcapsules effectively shield the siRNAs from being cleared and enable them to be released constantly over 3 months. In contrast, unencapsulated siRNAs are rapidly cleared.

Thermoresponsive Phase Change Oleogel Microcapsules for Coencapsulation of Hydrophilic and Hydrophobic Actives

To address the concurrent needs of the personal care industry for simultaneous protection of active ingredients and enhancement of product functionality, this study employs a microfluidic technique to fabricate EstoGel M-based oleogel microcapsules capable of coencapsulating both hydrophilic and hydrophobic actives. The oleogels exhibit gel-like characteristics with a melting point of approximately 70 °C, ensuring high encapsulation efficiency for hydrophilic and hydrophobic actives within aqueous environments. The oleogel microspheres encapsulating hydrophobic actives are prepared using microfluidic technology with robust elasticity, which can be ruptured by a force of less than 15 mN, contributing to a favorable tactile sensation upon application. The structural integrity of these microspheres is preserved within a temperature range up to 70 °C, indicating their thermodynamic stability. In addition, oleogel microcapsules are prepared using microfluidic technology, and their effectiveness in coencapsulating hydrophilic and hydrophobic active ingredients is successfully demonstrated, along with excellent skin feel and temperature stability. The exceptional tactile properties of EstoGel M-based oleogel microcapsules offer a promising strategy for creating innovative personal care products that integrate high encapsulation efficiency with multifunctional attributes.

Flow pattern maps of double emulsions transporting through bifurcation microchannels

The transportation behaviors of compound droplets in confined channels are widespread phenomena while the physical mechanisms are far from being completely unraveled. In this work, behaviors of double emulsions flowing through bifurcation microchannels are experimentally studied with the aim of building universal flow pattern maps. Three flow patterns are categorized according to different features of daughter droplets in terms of size, uniformity, and shell thickness. A detailed analysis of the dynamics of interfacial evolutions in different patterns is carried out and the coupling interaction between interfaces is found to affect the minimum tail distance during transportation. It is feasible to obtain the threshold of the occurrence of the coupling interaction, due to the different variation tendencies in the two states, which relies on three dimensionless parameters, i.e. droplet length, length ratio, and capillary number. Furthermore, a novel physical model is proposed to build the flow pattern map, with the two transition boundaries being expressed as different relationships in terms of the three identified parameters. The physical mechanisms are summarized with the aid of force analysis. An excellent agreement is shown between the model and experimental results in different liquid systems and bifurcation structures, indicating the generality of the proposed model.

Microcapsule Triggering Mechanics in Cementitious Materials: A Modelling and Machine Learning Approach

Self-healing cementitious materials containing microcapsules filled with healing agents can autonomously seal cracks and restore structural integrity. However, optimising the microcapsule mechanical properties to survive concrete mixing whilst still rupturing at the cracked interface to release the healing agent remains challenging. This study develops an integrated numerical modelling and machine learning approach for tailoring acrylate-based microcapsules for triggering within cementitious matrices. Microfluidics is first utilised to produce microcapsules with systematically varied shell thickness, strength, and cement compatibility. The capsules are characterised and simulated using a continuum damage mechanics model that is able to simulate cracking. A parametric study investigates the key microcapsule and interfacial properties governing shell rupture versus matrix failure. The simulation results are used to train an artificial neural network to rapidly predict the triggering behaviour based on capsule properties. The machine learning model produces design curves relating the microcapsule strength, toughness, and interfacial bond to its propensity for fracture. By combining advanced simulations and data science, the framework connects tailored microcapsule properties to their intended performance in complex cementitious environments for more robust self-healing concrete systems.

Tunable encapsulation of sessile droplets with solid and liquid shells

Droplet encapsulations using liquid or solid shells are of significant interest in microreactors, drug delivery, crystallization, and cell growth applications. Despite progress in droplet-related technologies, tuning micron-scale shell thickness over a large range of droplet sizes is still a major challenge. In this work, we report capillary force assisted cloaking using hydrophobic colloidal particles and liquid-infused surfaces. The technique produces uniform solid and liquid shell encapsulations over a broad range (5-200 μm shell thickness for droplet volume spanning over four orders of magnitude). Tunable liquid encapsulation is shown to reduce the evaporation rate of droplets by up to 200 times with a wide tunability in lifetime (1.5 h to 12 days). Further, we propose using the technique for single crystals and cell/spheroid culture platforms. Stimuli-responsive solid shells show hermetic encapsulation with tunable strength and dissolution time. Moreover, scalability, and versatility of the technique is demonstrated for on-chip applications.

Development of 3D-Printed Self-Healing Capsules with a Separate Membrane and Investigation of Mechanical Properties for Improving Fracture Strength

Studies on self-healing capsules embedded in cement composites to heal such cracks have recently been actively researched in order to improve the dimensional stability of concrete structures. In particular, capsule studies were mainly conducted to separately inject reactive healing solutions into different capsules. However, with this method, there is an important limitation in that the probability of self-healing is greatly reduced because the two healing solutions must meet and react. Therefore, we propose three-dimensional (3D) printer-based self-healing capsules with a membrane structure that allows two healing solutions to be injected into one capsule. Among many 3D printing methods, we used the fusion deposition modeling (FDM) to design, analyze, and produce new self-healing capsules, which are widely used due to their low cost, precise manufacturing, and high-speed. However, polylactic lactic acid (PLA) extruded in the FDM has low adhesion energy between stacked layers, which causes different fracture strengths depending on the direction of the applied load and the subsequent performance degradation of the capsule. Therefore, the isotropic fracture characteristics of the newly proposed four types of separated membrane capsules were analyzed using finite element method analysis. Additionally, capsules were produced using the FDM method, and the compression test was conducted by applying force in the x, y, and z directions. The isotropic fracture strength was also analyzed using the relative standard deviation (RSD) parameter. As a result, the proposed separated membrane capsule showed that the RSD of isotropic fracture strength over all directions fell to about 18% compared to other capsules.

Establishing the Interface Layer on the Pentaerythritol Tetranitrate Surface via In Situ Reaction

Pentaerythritol tetranitrate (PETN) was coated by tannic acid (TA), polydopamine (PDA), and melamine-formaldehyde (MF) resins via in situ reaction to prepare PETN@TA, PETN@PDA, and PETN@MF microcapsules for reducing sensitivity and enhancing thermal stability of PETN. The coating effects of TA, PDA, and MF shells on PETN surfaces are characterized by scanning electron microscopy and atomic force microscopy. The structures of PETN@TA, PETN@PDA, and PETN@MF microcapsules are characterized by X-ray photoelectron spectroscopy, X-ray diffraction, and Fourier-transform infrared spectra. The performances of PETN@TA, PETN@PDA, and PETN@MF microcapsules are characterized by differential scanning calorimetry, accelerating rate calorimetry, explosion point, vacuum deflation volume, and mechanical sensitivity. The study results show that TA, PDA, and MF shells can coat the PETN surface well. Compared with pure PETN, the explosion point has an increase while the vacuum deflation volume and mechanical sensitivity have a decrease for PETN@TA, PETN@PDA, and PETN@MF microcapsules, illustrating that the safeties of PETN@TA, PETN@PDA, and PETN@MF microcapsules are enhanced. In addition, the initial decomposition temperature (T₀) and peak decomposition temperature (T_p) of PETN@TA, PETN@PDA, and PETN@MF microcapsules have a slight increase, demonstrating that the thermal stabilities of PETN@TA, PETN@PDA, and PETN@MF microcapsules are better than that of pure PETN. The obtained method can provide some guidance for the desensitizing of other energetic materials with high sensitivities.

High throughput production of microcapsules using microfluidics for self-healing of cementitious materials

Capsule-based self-healing of cementitious materials is an effective way of healing cracks, significantly extending the life of structures, without imposing changes due to the incorporation of capsules into products during mixing. The methodologies currently being used for the development of capsules with a liquid core as a healing agent yield a wide range of sizes and shell thicknesses for the microcapsules, preventing a detailed assessment and optimisation of the microcapsule size and its effects. Uniquely, microfluidic technology offers precise control over the size and shell thickness through the formation of double emulsions. The drawback is that only small quantities of material can be typically produced. Here, by using paralleled junctions in a microfluidic device, high throughput production of materials was achieved, focusing for the first time on self-healing of cementitious materials. A microfluidic chip was assembled with 4 channels in parallel and selected hydrophobicity for the formation of the double emulsions. A coefficient of variation below 2.5% was observed for the 4 junctions, demonstrating the formation of monodisperse capsules. The control over the size and shell thickness by adjusting the flow rates was demonstrated, yielding capsules with an outer diameter of 615-630 μm and a shell thickness varying between 50 and 127 μm. By using triethanolamine as a surfactant, capsules with an aqueous core were produced. Furthermore, by selecting PEA, an acrylate with low tensile strength, the capsules embedded in the cement paste were successfully triggered to release the healing agent by crack formation. Capsules were successfully produced continuously for 7 h, with inner and outer diameters of 500 ± 31 μm and 656 ± 9 μm at a production rate of ∼13 g h^-1 and a yield of around 80%. With these results and considering up to 6 chips in parallel, the production rate could be up to 1.5 kg per day. This demonstrates the huge potential of the microfluidic device with unique features to produce sufficiently large quantities of microcapsules for laboratory-scale assessment of self-healing performance.

Microfluidic encapsulation for controlled release and its potential for nanofertilisers

Nanotechnology is increasingly being utilized to create advanced materials with improved or new functional attributes. Converting fertilizers into a nanoparticle-form has been shown to improve their efficacy but the current procedures used to fabricate nanofertilisers often have poor reproducibility and flexibility. Microfluidic systems, on the other hand, have advantages over traditional nanoparticle fabrication methods in terms of energy and materials consumption, versatility, and controllability. The increased controllability can result in the formation of nanoparticles with precise and complex morphologies (e.g., tuneable sizes, low polydispersity, and multi-core structures). As a result, their functional performance can be tailored to specific applications. This paper reviews the principles, formation, and applications of nano-enabled delivery systems fabricated using microfluidic approaches for the encapsulation, protection, and release of fertilizers. Controlled release can be achieved using two main routes: (i) nutrients adsorbed on nanosupports and (ii) nutrients encapsulated inside nanostructures. We aim to highlight the opportunities for preparing a new generation of highly versatile nanofertilisers using microfluidic systems. We will explore several main characteristics of microfluidically prepared nanofertilisers, including droplet formation, shell fine-tuning, adsorbate fine-tuning, and sustained/triggered release behavior.

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.

Core–Shell Droplet Generation Device Using a Flexural Bolt-Clamped Langevin-Type Ultrasonic Transducer

Droplets with a core–shell structure formed from two immiscible liquids are used in various industrial field owing to their useful physical and chemical characteristics. Efficient generation of uniform core–shell droplets plays an important role in terms of productivity. In this study, monodisperse core-shell droplets were efficiently generated using a flexural bolt-clamped Langevin-type transducer and two micropore plates. Water and silicone oil were used as core and shell phases, respectively, to form core–shell droplets in air. When the applied pressure of the core phase, the applied pressure of the shell phase, and the vibration velocity in the micropore were 200 kPa, 150 kPa, and 8.2 mm/s, respectively, the average diameter and coefficient of variation of the droplets were 207.7 μm and 1.6%, respectively. A production rate of 29,000 core–shell droplets per second was achieved. This result shows that the developed device is effective for generating monodisperse core–shell droplets.

Microfluidic Fabrication of Phase-Inverted Microcapsules with Asymmetric Shell Membranes with Graded Porosity

Microcapsules with liquid cores and solid shells are attractive as dispersible protective micron-sized containers. Applications that rely on molecular mass transport often require a combination of size selectivity, high permeability, and mechanical stability. Capsule architectures that combine all these features represent a material property, design, and fabrication challenge. In this work, the design of an asymmetric microcapsule shell architecture is reported to achieve a good combination of the desired features. Poly(methyl methacrylate) phase-inverted microcapsules featuring an asymmetric graded macroporous shell covered with a dense skin separation layer are obtained from water-in-oil-in-water double emulsion drops that are phase-inverted in a water-based coagulation bath. The phase-inverted microcapsules exhibit good mechanical stability and allow for high permeability of its shell membrane with molecular size dependence.

Encapsulating Well-Dispersed Carbon Nanoparticles for Applications in the Autonomous Restoration of Electronic Circuits

Two types of conductive microcapsules with a median size of less than 5 μm are proposed, and their high potential as a key functional material for self-restorable conductive pastes for applications in printed electronic circuits is verified. A well-dispersed suspension of carbon nanoparticles in toluene is prepared as the core material of the microcapsules. The restoration capabilities of the microcapsules for the physical structure and electrical conductivity of silver-based electronic circuit lines are compared. In the assessment of the microcapsule restoration efficiency, the two conductive microcapsules exhibit distinct capabilities for the restoration of damages caused by different mechanical fracturing. That is, the smaller microcapsule is more effective than the larger one to restore circuit lines from a tensile test, whereas the opposite result is obtained from a scratching test, demonstrating the significance of microcapsule size for the restoration of dissimilar fractures that may occur in various applications.

Fabrication of pH-responsive monodisperse microcapsules using interfacial tension of immiscible phases

Monodisperse, stimuli-responsive microcapsules are required for applications involving precise delivery of chemical payloads but are difficult to fabricate with high throughput and control over capsule geometry and shell wall properties, especially in the presence of organic solvents. In this paper, we adapt a facile technique based on the interfacial tension of immiscible phases for the generation of monodisperse emulsion templates and microcapsules. In this technique, either one (single emulsion) or two (double emulsion) dispersed phases are simultaneously delivered while reciprocating across the interface of a stationary immiscible continuous phase. The interfacial tension of the continuous phase results in the separation of a monodisperse droplet in every cycle. Monodisperse single emulsion-templated microcapsules based on cyclic poly(phthalaldehyde) (cPPA) and polymethacrylate (Eudragit E100) shell walls are formed with hydrophobic cores. The acid-triggered release of Eudragit and cPPA microcapsules containing an oil core is demonstrated in an acidic media. Tunable, monodisperse double emulsion templates with an aqueous core are formed with sizes ranging from 295 μm to 1200 μm and reciprocation frequencies of 1 Hz to 7 Hz. The double emulsion templates are converted to monodisperse, responsive microcapsules with a hydrophilic core through photocuring or selective solvent evaporation to form the polymer shell wall. Microcapsules with a variety of polymeric shell walls based on photocurable polyisocyanurate, cPPA and polylactide are fabricated. The acid-triggered release of cPPA microcapsules containing an aqueous core with a slower degradation rate is also demonstrated. We achieve excellent control over the emulsion templates and microcapsules, with polydispersity less than 2% and the ability to predict the size reliably based on process parameters. The cost-effectiveness, ease of fabrication and potential for scale-up make this technique very promising for fabrication of a diverse range of stimuli-responsive microcapsules.

Motion of an Elastic Capsule in a Trapezoidal Microchannel Under Stokes Flow Conditions

Even though the research interest in the last decades has been mainly focused on the capsule dynamics in cylindrical or rectangular ducts, channels with asymmetric cross-sections may also be desirable especially for capsule migration and sorting. Therefore, in the present study we investigate computationally the motion of an elastic spherical capsule in an isosceles trapezoidal microchannel at low and moderate flow rates under the Stokes regime. The steady-state capsule location is quite close to the location where the single-phase velocity of the surrounding fluid is maximized. Owing to the asymmetry of the trapezoidal channel, the capsule's steady-state shape is asymmetric while its membrane slowly tank-treads. In addition, our investigation reveals that tall trapezoidal channels with low base ratios produce significant off-center migration for large capsules compared to that for smaller capsules for a given channel length. Thus, we propose a microdevice for the sorting of artificial and physiological capsules based on their size, by utilizing tall trapezoidal microchannels with low base ratios. The proposed sorting microdevice can be readily produced via glass fabrication or as a microfluidic device via micromilling, while the required flow conditions do not cause membrane rupture.

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.

Polymer Capsules with Tunable Shell Thickness Synthesized via Janus-to-core shell Transition of Biphasic Droplets Produced in a Microfluidic Flow-Focusing Device

Droplet microfluidics has enabled the synthesis of polymeric particles with controlled sizes, shell thickness, and morphologies. Here, we report the Janus to core-shell structural evolution of biphasic droplets formed in a microfluidic flow-focusing device (MFFD) for the synthesis of polymer microcapsules with oil core/thickness-tunable shell via off-chip photo- and thermally induced polymerization. First, nanoliter-sized biphasic Janus droplets comprising an acrylate monomer and silicone oil were generated in a co-flowing aqueous polyvinyl alcohol (PVA) solution in an MFFD on a glass chip. Immediately following their break-off, the produced Janus droplets started to change their geometry from Janus to core-shell structure comprising a single silicone-oil core and an acrylate-monomer shell by the minimization of interfacial energy. Thus, we could produce monodisperse core-shell drops with average diameters of 105-325 μm, coefficient of variation (CV) values of 1.0-4.5%, and shell thickness of 1-67 μm. Subsequently, these drops were synthesized to fabricate polymeric microcapsules with tunable shell thickness via photo- and thermally induced polymerization. By increasing the concentration of the photo- and thermal initiator, we successfully produced thinner and ultra-thin shell (800 nm thickness) microcapsules. The surface structure of resulting particles was smooth in photopolymerization and porous in thermal polymerization.

Production of monodisperse polyurea microcapsules using microfluidics

Methods to make microcapsules - used in a broad range of healthcare and energy applications - currently suffer from poor size control, limiting the establishment of size/property relationships. Here, we use microfluidics to produce monodisperse polyurea microcapsules (PUMC) with a limonene core. Using varied flow rates and a commercial glass chip, we produce capsules with mean diameters of 27, 30, 32, 34, and 35 µm, achieving narrow capsule size distributions of ±2 µm for each size. We describe an automated method of sizing droplets as they are produced using video recording and custom Python code. The sustainable generation of such size-controlled PUMCs, potential replacements for commercial encapsulated systems, will allow new insights into the effect of particle size on performance.

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.

Microcapsule-Containing Self-Reporting Polymers

Self-reporting polymers, which can indicate damage or exposure to excessive stress with a clearly perceptible optical signal, are potentially useful for several technological applications, including stress-sensitive sensors that enable in situ monitoring of mechanical events and structural health monitoring systems. A versatile and simple concept to realize this function is the exploitation of microcapsules that are filled with solutions of dyes that are released and chemically or physically activated when the protective shell is damaged. Such microcapsules can readily be incorporated into polymers and the composites thus made can be processed into films, coatings, or other objects. Mechanochromic effects can be realized with different types of dyes and activation schemes. In this concept article, a selection of recent key studies is presented to provide an overview of the state of the field. Different architectures and operating principles and their advantages and drawbacks are reviewed. The parameters that influence the design of microcapsule-based mechanochromic systems are considered and unexplored chromophore systems that might be useful to design future self-reporting polymers are discussed. Finally, specific aspects of capsule design, fabrication, and integration into polymers are presented. Throughout the article, challenges and opportunities of the concept are highlighted and possible future directions are discussed.

Microfluidic fabrication of polyethylene glycol microgel capsules with tailored properties for the delivery of biomolecules

Microfluidic encapsulation platforms have great potential not only in pharmaceutical applications but also in the consumer products industry. Droplet-based microfluidics is increasingly used for the production of monodisperse polymer microcapsules for biomedical applications. In this work, a microfluidic technique is developed for the fabrication of monodisperse double emulsion droplets, where the shell is crosslinked into microgel capsules. A six-armed acrylated star-shaped poly(ethylene oxide-stat-propylene oxide) pre-polymer is used to form the microgel shell after a photo-initiated crosslinking reaction. The synthesized microgel capsules are hollow, enabling direct encapsulation of large amounts of multiple biomolecules with the inner aqueous phase completely engulfed inside the double emulsion droplets. The shell thickness and overall microgel sizes can be controlled via the flow rates. The morphology and size of the shells are characterized by cryo-SEM. The encapsulation and retention of 10 kDa FITC-dextran and its microgel degradation mediated release are monitored by fluorescence microscopy.

Microfluidic Devices for Drug Delivery Systems and Drug Screening

Microfluidic devices present unique advantages for the development of efficient drug carrier particles, cell-free protein synthesis systems, and rapid techniques for direct drug screening. Compared to bulk methods, by efficiently controlling the geometries of the fabricated chip and the flow rates of multiphase fluids, microfluidic technology enables the generation of highly stable, uniform, monodispersed particles with higher encapsulation efficiency. Since the existing preclinical models are inefficient drug screens for predicting clinical outcomes, microfluidic platforms might offer a more rapid and cost-effective alternative. Compared to 2D cell culture systems and in vivo animal models, microfluidic 3D platforms mimic the in vivo cell systems in a simple, inexpensive manner, which allows high throughput and multiplexed drug screening at the cell, organ, and whole-body levels. In this review, the generation of appropriate drug or gene carriers including different particle types using different configurations of microfluidic devices is highlighted. Additionally, this paper discusses the emergence of fabricated microfluidic cell-free protein synthesis systems for potential use at point of care as well as cell-, organ-, and human-on-a-chip models as smart, sensitive, and reproducible platforms, allowing the investigation of the effects of drugs under conditions imitating the biological system.

Strong Dual-Compartment Microcapsules Loaded with High Cargo Contents

Compartmentalized microcapsules are useful for the release of multiple cargos in medicine, agriculture, and advanced responsive materials. Although several encapsulation strategies that involve more than one cargo have been proposed, dual- or multicompartment capsules with high cargo loadings and sufficient mechanical stability are rarely reported. Here, we propose a single-step emulsification route for the preparation of strong dual-compartment capsules that can host the main cargo in their core in combination with another liquid cargo stored within their thick shell. Capsules are produced through the polymerization of the middle oil phase of water-oil-water double emulsions made by microfluidics. Compartmentalization results from the phase separation of monomers within the middle phase of the double emulsion. We investigate the effect of such phase separation process on the microstructure and mechanical properties of the capsules and eventually illustrate the potential of this approach by creating thermosensitive capsules with programmable bursting temperature. The large variety of possible mixtures of monomers and cargos that can be added in the oil and aqueous phases of the double emulsion templates makes this encapsulation approach a promising route for the fabrication of robust microcapsules for on-demand release of multiple cargos.

Direct Observation of Splitting in Oil-In-Water-In-Oil Emulsion Droplets via a Microchannel Mimicking Membrane Pores

Direct observation of double emulsion droplet permeation through a microchannel that mimicked 100 μm membrane pores with a porosity of 66.7% provided insights regarding splitting mechanisms in porous membranes. The microchannel was fabricated by standard soft lithography, and the oil-in-water-in-oil double emulsion droplets were prepared with a glass capillary device. By changing the flow rate from 0.5 to 5.0 × 10^-2 m s^-1, three characteristic behaviors were observed: (a) passage into one channel without splitting; (b) division into two smaller components; and (c) stripping of the middle water phase of the double emulsion droplets into a smaller double emulsion droplet and a smaller water-in-oil single emulsion droplet. The mechanisms are discussed with respect to the balance of viscous forces and interfacial tension, the contact point with the tip of the channel, and the relative position of the innermost droplet within the middle droplet.

One-Step Bulk Fabrication of Polymer-Based Microcapsules with Hard-Soft Bilayer Thick Shells

Microcapsules are important for the protection, transport, and delivery of cargo in a variety of fields but are often too weak to withstand the high mechanical stresses that arise during the preparation and formulation of products. Although thick-shell strong capsules have been developed to circumvent this issue, the microfluidic or multistep methods utilized thus far limit the ease of fabrication and encapsulation throughput. Here, we exploit the phase separation of ternary liquid mixtures to achieve a high-throughput fabrication of strong bilayer microcapsules using a one-step bulk emulsification process. Phase separation is induced by the diffusion of water from the continuous phase into droplets that initially contain a mixture of monomers, cross-linkers, an initiator, and cosolvent γ-butyrolactone. The double emulsions generated via such a phase separation are converted into microcapsules through a polymerization reaction triggered by UV illumination. Surprisingly, the shells of the consolidated capsules exhibit a hard-soft bilayer structure that can be designed to show a resilient eggshell-like fracture behavior. Our method allows for the production of large volumes of microcapsules with such a strong bilayer shell within a time scale of only a few minutes, thus offering an enticing pathway toward the high-throughput fabrication of mechanically robust encapsulation systems.

Microfluidic Production of Biodegradable Microcapsules for Sustained Release of Hydrophilic Actives

Biodegradable microcapsules with a large aqueous lumen and ultrathin membrane are microfluidically designed for sustained release of hydrophilic bioactives using water-in-oil-in-water double-emulsion drops as a template. As a shell phase, an organic solution of poly(lactic-co-glycolic acid) is used, which is consolidated to form a biodegradable membrane. The encapsulants stored in the lumen are released over a long period of time as the membranes degrade. The period can be controlled in a range of -three to five months at neutral pH condition by adjusting membrane thickness, providing highly sustained release and potentially enabling the programed release of multiple drugs. At acidic or basic condition, the degradation is accelerated, leading to the release in the period of approximately two months. As the membrane is semipermeable, the microcapsules respond to the osmotic pressure difference across the membrane. The microcapsules are inflated in hypotonic condition and deflated in hypertonic condition. Both conditions cause cracks on the membrane, resulting in the fast release of encapsulants in a day. The microcapsules implanted in mice also show sustained release, despite the period is decreased to a month. It is believed that the microcapsules are promising for the in vivo sustained release of drugs for high and long-term efficacy.

Facile microfluidic production of composite polymer core-shell microcapsules and crescent-shaped microparticles

HYPOTHESIS

Core-shell microcapsules and crescent-shaped microparticles can be used as picolitre bioreactors for cell culture and microwells for cell trapping/immobilisation, respectively.

RESULTS

Monodisperse polylactic acid (PLA) core-shell microcapsules with a diameter above 200μm, a shell thickness of 10μm, and 96% water entrapment efficiency were produced by solvent evaporation from microfluidically generated W/O/W emulsion drops with core-shell structure, and used to encapsulate Saccharomyces cerevisiae yeast cells in their aqueous cores. The morphological changes of the capsules stained with Nile red were studied over 14days under different osmotic pressure and pH gradients.

FINDINGS

The shell retained its integrity under isotonic conditions, but buckling and particle crumbling occurred in a hypertonic solution. When the capsules containing 5wt% aqueous Eudragit® S 100 solution in the core were incubated in 10^-4M HCl solution, H⁺ diffused through the PLA film into the core causing an ionic gelation of the inner phase and its phase separation into polymer-rich and water-rich regions, due to the transition of Eudragit from a hydrophilic to hydrophobic state. Crescent-shaped composite microparticles with Eudragit cores and PLA shells were fabricated by drying core-shell microcapsules with gelled cores, due to the collapse of PLA shells encompassing water-rich crescent regions.

Strong Microcapsules with Permeable Porous Shells Made through Phase Separation in Double Emulsions

Microcapsules for controlled chemical release and uptake are important in many industrial applications but are often difficult to produce with the desired combination of high mechanical strength and high shell permeability. Using water-oil-water double emulsions made in microfluidic devices as templates, we developed a processing route to obtain mechanically robust microcapsules exhibiting a porous shell structure with controlled permeability. The porous shell consists of a network of interconnected polymer particles that are formed upon phase separation within the oil phase of the double emulsion. Porosity is generated by an inert diluent incorporated in the oil phase. The use of undecanol and butanol as inert diluents allows for the preparation of microcapsules covering a wide range of shell-porosity and force-at-break values. We found that the amount and chemical nature of the diluent influence the shell porous structure by changing the mechanism of phase separation that occurs during polymerization. In a proof-of-concept experiment, we demonstrate that the mechanically robust microcapsules prepared through this simple approach can be utilized for the on-demand release of small molecules using a pH change as exemplary chemical trigger.

Liquid-capped encoded microcapsules for multiplex assays

Although droplet microfludics is a promising technology for handling a number of liquids of a single type of analyte, it has limitations in handling thousands of different types of analytes for multiplex assay. Here, we present a novel "liquid-capped encoded microcapsule", which is applicable to various liquid format assays. Various liquid drops can be graphically encoded and arrayed without repeated dispensing processes, evaporation, and the risk of cross-contamination. Millions of nanoliter-scale liquids are encapsulated within encoded microcapsules and self-assembled in microwells in a single dispensing process. The graphical code on the microcapsule enables identification of randomly assembled microcapsules in each microwell. We conducted various liquid phase assays including enzyme inhibitor screening, virus transduction, and drug-induced apoptosis tests. The results showed that our liquid handling technology can be utilized widely for various solution phase assays.

Formaldehyde-free melamine microcapsules as core/shell delivery systems for encapsulation of volatile active ingredients

The release of volatile bioactive molecules, such as fragrances, can be controlled by microencapsulation in core–shell polymeric delivery systems.

3D Printing of Hierarchical Silk Fibroin Structures

Like many other natural materials, silk is hierarchically structured from the amino acid level up to the cocoon or spider web macroscopic structures. Despite being used industrially in a number of applications, hierarchically structured silk fibroin objects with a similar degree of architectural control as in natural structures have not been produced yet due to limitations in fabrication processes. In a combined top-down and bottom-up approach, we exploit the freedom in macroscopic design offered by 3D printing and the template-guided assembly of ink building blocks at the meso- and nanolevel to fabricate hierarchical silk porous materials with unprecedented structural control. Pores with tunable sizes in the range 40-350 μm are generated by adding sacrificial organic microparticles as templates to a silk fibroin-based ink. Commercially available wax particles or monodisperse polycaprolactone made by microfluidics can be used as microparticle templates. Since closed pores are generated after template removal, an ultrasonication treatment can optionally be used to achieve open porosity. Such pore templating particles can be further modified with nanoparticles to create a hierarchical template that results in porous structures with a defined nanotopography on the pore walls. The hierarchically porous silk structures obtained with this processing technique can potentially be utilized in various application fields from structural materials to thermal insulation to tissue engineering scaffolds.

Building the design, translation and development principles of polymeric nanomedicines using the case of clinically advanced poly(lactide(glycolide))-poly(ethylene glycol) nanotechnology as a model: An industrial viewpoint

The design of the first polymeric nanoparticles could be traced back to the 1970s, and has thereafter received considerable attention, as evidenced by the significant increase of the number of articles and patents in this area. This review article is an attempt to take advantage of the existing literature on the clinically tested and commercialized biodegradable PLA(G)A-PEG nanotechnology as a model to propose quality building and outline translation and development principles for polymeric nano-medicines. We built such an approach from various building blocks including material design, nano-assembly - i.e. physicochemistry of drug/nano-object association in the pharmaceutical process, and release in relevant biological environment - characterization and identification of the quality attributes related to the biopharmaceutical properties. More specifically, as envisaged in a translational approach, the reported data on PLA(G)A-PEG nanotechnology have been structured into packages to evidence the links between the structure, physicochemical properties, and the in vitro and in vivo performances of the nanoparticles. The integration of these bodies of knowledge to build the CMC (Chemistry Manufacturing and Controls) quality management strategy and finally support the translation to proof of concept in human, and anticipation of the industrialization takes into account the specific requirements and biopharmaceutical features attached to the administration route. From this approach, some gaps are identified for the industrial development of such nanotechnology-based products, and the expected improvements are discussed. The viewpoint provided in this article is expected to shed light on design, translation and pharmaceutical development to realize their full potential for future clinical applications.

Oil-in-Water-in-Oil Multinanoemulsions for Templating Complex Nanoparticles

Complex nanoemulsions involving nanodroplets with a defined inner structure have great potential for encapsulation and templating applications. We report a method to form novel complex oil-in-water-in-oil nanoemulsions using a combination of high-energy processing with mixed nonionic surfactants that simultaneously achieve ultralow interfacial tension and frustrated curvature of the water-oil interface. The method produces multinanoemulsions possessing morphologies resembling water-swollen reverse vesicles with core-shell and multicore-shell morphologies of water in cyclohexane. A combination of macroscopic and microscopic characterization conclusively verifies and quantifies the complex morphologies, which vary systematically and reproducibly with water content for water volume fractions between 0.01 and 0.10. The complex morphologies are stable tens of hours, providing a route for their use as liquid templates for internally structured nanoparticles. As a demonstration, we test the complex nanoemulsions' ability to template complex polymer nanogels.

Application of Microfluidic Technique in Drug Delivery

Drug delivery as a strategy to improve the effect of therapeutic treatment is gaining tremendous interest in biomedical research. The recent advancement in microfluidic technique designed to precisely control the liquid at micro or nano liter level has shed some new lights on reshaping the ongoing drug delivery research. In this aspect, this present mini-review gives an overview on the potential applications of microfluidic technique in the area of drug delivery, which basically covers the fabrication of drug delivery carriers and the design of microfluidic-based smart systems for localized in vivo drug delivery.

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.

Microfluidic Droplet-Facilitated Hierarchical Assembly for Dual Cargo Loading and Synergistic Delivery

Bottom-up hierarchical assembly has emerged as an elaborate and energy-efficient strategy for the fabrication of smart materials. Herein, we present a hierarchical assembly process, whereby linear amphiphilic block copolymers are self-assembled into micelles, which in turn are accommodated at the interface of microfluidic droplets via cucurbit[8]uril-mediated host-guest chemistry to form supramolecular microcapsules. The monodisperse microcapsules can be used for simultaneous carriage of both organic (Nile Red) and aqueous-soluble (fluorescein isothiocyanate-dextran) cargo. Furthermore, the well-defined compartmentalized structure benefits from the dynamic nature of the supramolecular interaction and offers synergistic delivery of cargos with triggered release or through photocontrolled porosity. This demonstration of premeditated hierarchical assembly, where interactions from the molecular to microscale are designed, illustrates the power of this route toward accessing the next generation of functional materials and encapsulation strategies.