Using selective withdrawal to coat microparticles

Volatile Droplets on Water are Sculpted by Vigorous Marangoni-Driven Subphase Flow

The shapes of highly volatile oil-on-water droplets become strongly asymmetric when they are out of equilibrium. The unsaturated organic vapor atmosphere causes evaporation and leads to a strong Marangoni flow in the bath, unlike that previously seen in the literature. Inspecting these shapes experimentally on millisecond and submillimeter time and length scales and theoretically by scaling arguments, we confirm that Marangoni-driven convection in the subphase mechanically stresses the droplet edges to an extent that increases for organic droplets of smaller contact angle and accordingly smaller thickness. The viscous stress generated by the subphase overcomes the thermodynamic Laplace pressure. The oil droplets develop copious regularly spaced fingers, and these fingers develop spike-shaped and branched treelike structures. Unlike this behavior for single-component (surfactant-free) oil droplets, droplets composed of two miscible (surfactant-free) organic liquids develop a rim of the less volatile component along the droplet perimeter, from which jets of monodisperse smaller droplets eject periodically due to the Rayleigh-Plateau instability. When evaporation shrinks droplets to μm size, their shapes fluctuate chaotically, and ellipsoidal shapes rupture into smaller daughter droplets when subphase convection flow pulls them in opposite directions. The shape of the evaporating oil droplets is kneaded and sculpted by vigorous flow in the water subphase.

Biomedical Applications of Microfluidic Devices: A Review

Both passive and active microfluidic chips are used in many biomedical and chemical applications to support fluid mixing, particle manipulations, and signal detection. Passive microfluidic devices are geometry-dependent, and their uses are rather limited. Active microfluidic devices include sensors or detectors that transduce chemical, biological, and physical changes into electrical or optical signals. Also, they are transduction devices that detect biological and chemical changes in biomedical applications, and they are highly versatile microfluidic tools for disease diagnosis and organ modeling. This review provides a comprehensive overview of the significant advances that have been made in the development of microfluidics devices. We will discuss the function of microfluidic devices as micromixers or as sorters of cells and substances (e.g., microfiltration, flow or displacement, and trapping). Microfluidic devices are fabricated using a range of techniques, including molding, etching, three-dimensional printing, and nanofabrication. Their broad utility lies in the detection of diagnostic biomarkers and organ-on-chip approaches that permit disease modeling in cancer, as well as uses in neurological, cardiovascular, hepatic, and pulmonary diseases. Biosensor applications allow for point-of-care testing, using assays based on enzymes, nanozymes, antibodies, or nucleic acids (DNA or RNA). An anticipated development in the field includes the optimization of techniques for the fabrication of microfluidic devices using biocompatible materials. These developments will increase biomedical versatility, reduce diagnostic costs, and accelerate diagnosis time of microfluidics technology.

Massively Multiplexed Electrohydrodynamic Tip Streaming from a Thin Disc

We report an experimental study on the multiple tip streaming enabled by an externally wetted thin disc in electric fields. The electrohydrodynamic stress acting on the liquid-air interface triggers an interfacial instability that develops into multiple radial liquid ligaments at the rim of the disc. The scaling law suggests that the wave number is inversely proportional to the square of the peak electric field at the rim, which is determined by the combined thickness of the disc and the attached liquid layer. The thin disc edge effectively intensifies the electric field, which in turn leads to spacing between ligaments as short as 30  μm for ethanol, generating over 1000 cone jets for a 1 cm diam thin disc.

Regime Map and Triple Point in Selective Withdrawal

Entrainment in selective withdrawal occurs when both the top and bottom phases are withdrawn through a capillary tube oriented perpendicular to a flat gravitationally separated liquid-liquid interface. The tube introduces two distinct features to the conditions for fluid entrainment. First, the ratio of the two phases being withdrawn is affected by the region of influence of the flow upstream of the tube's orifice. Second, a minimum withdrawal flow rate must be reached for entrainment regardless of the distance between the interface and the tube. We show that these phenomena can be understood based on the Reynolds number that governs the external flow field around the capillary tube and the capillary number that regulates the effect of the viscosity and capillarity.

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.

Microfluidic Platforms toward Rational Material Fabrication for Biomedical Applications

The emergence of micro/nanomaterials in recent decades has brought promising alternative approaches in various biomedicine-related fields such as pharmaceutics, diagnostics, and therapeutics. These micro/nanomaterials for specific biomedical applications shall possess tailored properties and functionalities that are closely correlated to their geometries, structures, and compositions, therefore placing extremely high demands for manufacturing techniques. Owing to the superior capabilities in manipulating fluids and droplets at microscale, microfluidics has offered robust and versatile platform technologies enabling rational design and fabrication of micro/nanomaterials with precisely controlled geometries, structures and compositions in high throughput manners, making them excellent candidates for a variety of biomedical applications. This review briefly summarizes the progress of microfluidics in the fabrication of various micro/nanomaterials ranging from 0D (particles), 1D (fibers) to 2D/3D (film and bulk materials) materials with controllable geometries, structures, and compositions. The applications of these microfluidic-based materials in the fields of diagnostics, drug delivery, organs-on-chips, tissue engineering, and stimuli-responsive biodevices are introduced. Finally, an outlook is discussed on the future direction of microfluidic platforms for generating materials with superior properties and on-demand functionalities. The integration of new materials and techniques with microfluidics will pave new avenues for preparing advanced micro/nanomaterials with enhanced performance for biomedical applications.

Capillary-Based Microfluidics-Coflow, Flow-Focusing, Electro-Coflow, Drops, Jets, and Instabilities

Capillary-based microfluidics is a great technique to produce monodisperse and complex emulsions and particulate suspensions. In this review, the current understanding of drop and jet formation in capillary-based microfluidic devices for two primary flow configurations, coflow and flow-focusing is summarized. The experimental and theoretical description of fluid instabilities is discussed and conditions for controlled drop breakup in different modes of drop generation are provided. Current challenges in drop breakup with low interfacial tension systems and recent progress in overcoming drop size limitations using electro-coflow are addressed. In each scenario, the physical mechanisms for drop breakup are revisited, and simple scaling arguments proposed in the literature are introduced.

Smoothed Particle Hydrodynamics multiphase modelling of an experimental microfluidic device for conformal coating of pancreatic islets

The paper discusses a Smoothed Particle Hydrodynamics (SPH) model for the analysis of the multiphase flow occurring in an experimental microfluidic device for conformal coating of pancreatic islets with a biocompatible and permeable polymer. The proposed numerical model, based on a weakly-compressible SPH approach, accurately mimics the encapsulation process while assuring phase conservation, thus overcoming potential limitations of grid-based models. The proposed SPH model is a triphasic multi-phase model that allows one: (i) to reproduce the physics of islet conformal coating, including the effects of surface tension at the interface of the involved fluids and of the islet diameter; and (ii) to evaluate how modulation of process parameters influences the fluid dynamics within the microfluidic device and the resulting coating characteristics. This model can represent a valuable, time- and cost-effective tool for the definition of optimized encapsulation conditions through in silico screening of novel combinations of conformal coating parameters, including polymeric coating blends, size range of insulin-secreting cell clusters, utilized chemical reagents, device geometry and scale.

Microfluidic bead encapsulation above 20 kHz with triggered drop formation

Microsphere beads are functionalized with oligonucleotides, antibodies, and other moieties to enable specific detection of analytes. Droplet microfluidics leverages this for single-molecule or -cell analysis by pairing beads and targets in water-in-oil droplets. Pairing is achieved with devices operating in the dripping regime, limiting throughput. Here, we describe a pairing method that uses beads to trigger the breakup of a jet into monodispersed droplets. We use the method to pair 105 Human T cells with polyacrylamide beads ten times faster than methods operating in the dripping regime. Our method improves the throughput of bead-based droplet workflows, enabling analysis of large populations and the detection of rare events.

Multifunctional Hierarchically-Assembled Hydrogel Particles with Pollen Grains via Pickering Suspension Polymerization

Hierarchical assembly of heterogeneous particles is of great importance to interface and colloid science. In this work, a facile but powerful approach for the large-scale production of multifunctional hydrogel particles armored with biological colloidal species is developed by combining Pickering stabilization and photopolymerization. Biocompatible hollow pollen grains extracted from naturally occurring pollen species with an average diameter of ∼32 μm serve as universal solid emulsifiers dispersed in an oil phase and are self-assembled at the interface between an oil phase and a photo-cross-linkable hydrogel to make water-in-oil (W/O) emulsion droplets. While droplets are solidified into hydrogel particles by UV-induced free-radical polymerization, self-assembled hollow pollen grains are transformed to a robust shell on hydrogel particles with supracolloidal structures. The physically adsorbed hollow pollen grains on the hydrogel core can be released by a hydration-induced swelling of hollow pollen grains, leading to a transient floating behavior of core-shell particles. The size of the resultant core-shell particles is easily controlled by tailoring the process parameters such as a liquid volume or a loading mass of hollow pollen grains. The incorporation of magnetic or upconverting luminescent nanoparticles into a hydrogel core successfully expands the functionality of core-shell particles that can provide new design opportunities for floating drug delivery or ecofriendly proppants.

Microfluidic diamagnetic water-in-water droplets: a biocompatible cell encapsulation and manipulation platform

Droplet microfluidics enables cellular encapsulation for biomedical applications such as single-cell analysis, which is an important tool used by biologists to study cells on a single-cell level, and understand cellular heterogeneity in cell populations. However, most cell encapsulation strategies in microfluidics rely on random encapsulation processes, resulting in large numbers of empty droplets. Therefore, post-sorting of droplets is necessary to obtain samples of purely cell-encapsulating droplets. With the recent advent of aqueous two-phase systems (ATPS) as a biocompatible alternative of the conventional water-in-oil droplet systems for cellular encapsulation, there has also been a focus on integrating ATPS with droplet microfluidics. In this paper, we describe a new technique that combines ATPS-based water-in-water droplets with diamagnetic manipulation to isolate single-cell encapsulating water-in-water droplets, and achieve a purity of 100% in a single pass. We exploit the selective partitioning of ferrofluid in an ATPS of polyethylene glycol-polypropylene glycol-polyethylene glycol triblock copolymer (PEG-PPG-PEG) and dextran (DEX), to achieve diamagnetic manipulation of water-in-water droplets. A cell-triggered Rayleigh-Plateau instability in the dispersed phase thread results in a size distinction between the cell-encapsulating and empty droplets, enabling diamagnetic separation and sorting of the cell-encapsulating droplets from empty droplets. This is a simple and biocompatible all-aqueous platform for single-cell encapsulation and droplet manipulation, with applications in single-cell analysis.

Novel strategies for the formulation and processing of poorly water-soluble drugs

Low aqueous solubility of active pharmaceutical ingredients presents a serious challenge in the development process of new drug products. This article provides an overview on some of the current approaches for the formulation of poorly water-soluble drugs with a special focus on strategies pursued at the Center of Pharmaceutical Engineering of the TU Braunschweig. These comprise formulation in lipid-based colloidal drug delivery systems and experimental as well as computational approaches towards the efficient identification of the most suitable carrier systems. For less lipophilic substances the preparation of drug nanoparticles by milling and precipitation is investigated for instance by means of microsystem-based manufacturing techniques and with special regard to the preparation of individualized dosage forms. Another option to overcome issues with poor drug solubility is the incorporation into nanospun fibers.

Electroemulsification in a Uniform Electric Field

Emulsification using electric fields is an easy alternative to flow-induced drop breakup, and the former is reported to be more effective and economical than the latter, especially when the medium phase is poorly conducting and highly viscous. The emulsification of a coarse water-in-oil emulsion in a uniform electric field is studied. We perform a detailed experimental analysis of the effect of applied electric field strength and the duration of applied electric field on the drop size distribution. The average diameter as well as the time for emulsification decreases with an increase in the intensity of the electric field. Moreover, a narrow size distribution is observed. An average size of a few microns of the dispersed phase could be achieved. New breakup mechanisms at play in the emulsification process are discussed. Identified mechanisms involve charged lobe disintegration, charged drop breakup, chain formation in which several water droplets are interconnected by thin water bridges, electrospraying and charge transfer, and coalescence. The study shows that charged drop disintegration could be the key mechanism of fine emulsification of an initially electrically neutral coarse emulsion.

Slit-surface electrospinning: a novel process developed for high-throughput fabrication of core-sheath fibers

In this work, we report on the development of slit-surface electrospinning – a process that co-localizes two solutions along a slit surface to spontaneously emit multiple core-sheath cone-jets at rates of up to 1 L/h. To the best of our knowledge, this is the first time that production of electrospun core-sheath fibers has been scaled to this magnitude. Fibers produced in this study were defect-free (i.e. non-beaded) and core-sheath geometry was visually confirmed under scanning electron microscopy. The versatility of our system was demonstrated by fabrication of (1) fibers encapsulating a drug, (2) bicomponent fibers, (3) hollow fibers, and (4) fibers from a polymer that is not normally electrospinnable. Additionally, we demonstrate control of the process by modulating parameters such as flow rate, solution viscosity, and fixture design. The technological achievements demonstrated in this work significantly advance core-sheath electrospinning towards commercial and manufacturing viability.

Microfluidic conformal coating of non-spherical magnetic particles

We present the conformal coating of non-spherical magnetic particles in a co-laminar flow microfluidic system. Whereas in the previous reports spherical particles had been coated with thin films that formed spheres around the particles; in this article, we show the coating of non-spherical particles with coating layers that are approximately uniform in thickness. The novelty of our work is that while liquid-liquid interfacial tension tends to minimize the surface area of interfaces-for example, to form spherical droplets that encapsulate spherical particles-in our experiments, the thin film that coats non-spherical particles has a non-minimal interfacial area. We first make bullet-shaped magnetic microparticles using a stop-flow lithography method that was previously demonstrated. We then suspend the bullet-shaped microparticles in an aqueous solution and flow the particle suspension with a co-flow of a non-aqueous mixture. A magnetic field gradient from a permanent magnet pulls the microparticles in the transverse direction to the fluid flow, until the particles reach the interface between the immiscible fluids. We observe that upon crossing the oil-water interface, the microparticles become coated by a thin film of the aqueous fluid. When we increase the two-fluid interfacial tension by reducing surfactant concentration, we observe that the particles become trapped at the interface, and we use this observation to extract an approximate magnetic susceptibility of the manufactured non-spherical microparticles. Finally, using fluorescence imaging, we confirm the uniformity of the thin film coating along the entire curved surface of the bullet-shaped particles. To the best of our knowledge, this is the first demonstration of conformal coating of non-spherical particles using microfluidics.

Preparation of cell-encapsulation devices in confined microenvironment

The entrapment of cells into hydrogel microdevice in form of microparticles or microfibers is one of the most appealing and useful tools for cell-based therapy and tissue engineering. Cell encapsulation procedures allow the immunoisolation of cells from the surrounding environment, after their transplantation and the maintenance of the normal cellular physiology. Factors affecting the efficacy of microdevices, which include size, size distribution, morphology, and porosity are all highly dependent on the method of preparation. In this respect, microfluidic based methods offer a promising strategy to fabricate highly uniform and morphologically controlled microdevices with tunable chemical and mechanical properties. In the current review, various cell microencapsulation procedures, based on a microfluidics, are critically analyzed with a special focus on the effect of the procedure on the morphology, viability and functions of the embedded cells. Moreover, a brief introduction about the optimal characteristics of microdevice intended for cell encapsulation, together with the currently used materials for the production is reported. A further challenging application of microfluidics for the development of "living microchip" is also presented. Finally, the limitations, challenging and future work on the microfluidic approach are also discussed.

Building functional materials for health care and pharmacy from microfluidic principles and Flow Focusing

In this review, we aim at establishing a relationship between the fundamentals of the microfluidics technologies used in the Pharmacy field, and the achievements accomplished by those technologies. We describe the main methods for manufacturing micrometer drops, bubbles, and capsules, as well as the corresponding underlying physical mechanisms. In this regard, the review is intended to show non-specialist readers the dynamical processes which determine the success of microfluidics techniques. Flow focusing (FF) is a droplet-based method widely used to produce different types of fluid entities on a continuous basis by applying an extensional co-flow. We take this technique as an example to illustrate how microfluidics technologies for drug delivery are progressing from a deep understanding of the physics of fluids involved. Specifically, we describe the limitations of FF, and review novel methods which enhance its stability and robustness. In the last part of this paper, we review some of the accomplishments of microfluidics when it comes to drug manufacturing and delivery. Special attention is paid to the production of the microencapsulated form because this fluidic structure gathers the main functionalities sought for in Pharmacy. We also show how FF has been adapted to satisfy an ample variety of pharmaceutical requirements to date.

Excitation of fountain and entrainment instabilities at the interface between two viscous fluids using a beam of laser light

We report on two instabilities, called viscous fountain and viscous entrainment, triggered at the interface between two liquids by the action of bulk flows driven by a laser beam. These streaming flows are due to light scattering losses in turbid liquids, and can be directed either toward or forward the interface. We experimentally and numerically investigate these interface instabilities and show that the height and curvature of the interface deformation at the threshold and the jet radius after interface destabilization mainly depend on the waist of the laser beam. Analogies and differences between these two instabilities are characterized.

Future of the particle replication in nonwetting templates (PRINT) technology

Particle replication in nonwetting templates (PRINT) is a continuous, roll-to-roll, high-resolution molding technology which allows the design and synthesis of precisely defined micro- and nanoparticles. This technology adapts the lithographic techniques from the microelectronics industry and marries these with the roll-to-roll processes from the photographic film industry to enable researchers to have unprecedented control over particle size, shape, chemical composition, cargo, modulus, and surface properties. In addition, PRINT is a GMP-compliant (GMP=good manufacturing practice) platform amenable for particle fabrication on a large scale. Herein, we describe some of our most recent work involving the PRINT technology for application in the biomedical and material sciences.

Formation of Core-Shell Particles by Interfacial Radical Polymerization Initiated by a Glucose Oxidase-Mediated Redox System

A unique design paradigm to form core-shell particles based on interfacial radical polymerization is described. The interfacial initiation system is comprised of an enzymatic reaction between glucose and glucose oxidase (GOx) to generate hydrogen peroxide, which, in the presence of iron (Fe2+), generates hydroxyl radicals that initiate polymerization. Shell formation on prefabricated polymeric cores is achieved by localizing the initiation reaction to the interface of the core and a surrounding aqueous monomer formulation into which it is immersed. The interfacially confined initiation reaction is accomplished by incorporating one or more of the initiating species in the particle core and the remainder of the complementary initiating components in the surrounding media such that interactions and the resulting initiation reaction occur at the interface. This work is focused on engineering the reaction behavior and mass transport processes to promote interfacially confined polymerization, controlling the rate of shell formation, and manipulating the structure of the core-shell particle. Specifically, incorporating GOx in the precursor solution used to fabricate cores ranging from 100 to 200 μm, and the remainder of the complementary initiating components and monomer in the bulk solution prior to interfacial polymerization yielded shells whose average thickness was 20 μm after 4 min of immersion and at a bulk iron concentration of 12.5 mM. When the locations of glucose and GOx are interchanged, the average thickness of the shell was 15 or 100 μm for bulk iron concentrations of 45 and 12.5 mM, respectively. The initial locations of glucose and GOx also determine the degree of interpenetration of the core and the shell. Specifically, for a bulk iron concentration of 45 mM, the thickness of the interpenetrating layer averaged 12 μm when GOx was initially within the core, whereas no interpenetrating layer was observed when glucose was incorporated in the core. The polymeric shell formed by this technique is also demonstrated to be self-supporting following core degradation. This behavior is accomplished by fabricating the particle core hydrogel from monomers possessing degradable groups that can be irreversibly cleaved by light exposure following shell formation. When the coated particle was exposed to light, the shell remained intact while the core degraded as evidenced by a dramatic change in diffusion coefficient of fluorescent beads immobilized within the core.

An automated process for layer-by-layer assembly of polyelectrolyte multilayer thin films on viable cell aggregates

An automated process for modifying the surface of pancreatic islets grows uniform polyelectrolyte multilayer thin films, eliminating user variability associated with previous manual methods. Machine vision feedback allows for tight control of small fluid volumes, maintaining an islet microenvironment. This process is adaptable to other fragile micrometer-scale particle systems.

Pancreatic islet cell transplantation: an update

Transplantation of pancreatic islets, as a therapeutic modality for type 1 diabetes mellitus (T1DM), at this stage of its development, is reserved for patients with severe glycemic variability, progressive diabetic complications, and life threatening hypoglycemia unawareness, regardless of intensive insulin management. It has not succeeded to become the method of choice for treating T1DM because of limited supply and suboptimal yields of procurement and isolation of islets, graft failure, and relatively high requirements, i.e., at least 10,000 functional Islet Equivalents per kg of patient weight, to achieve prolonged insulin independence and glucose stability. Efforts aimed at making islet transplantation a competitive alternative to exogenous insulin injections for treating T1DM have focused on improving the longevity and functionality of islet cells. In order to succeed, these efforts need to be complemented by others to optimize the rate and efficiency of encapsulation.

Permeability and micromechanical properties of silk ionomer microcapsules

We studied the pH-responsive behavior of layer-by-layer (LbL) microcapsules fabricated from silk fibroin chemically modified with different poly amino acid side chains: cationic (silk-poly L-lysine, SF-PL) or anionic (silk-poly-L-glutamic acid, SF-PG). We observed that stable ultrathin shell microcapsules can be assembled with a dramatic increase in swelling, thickness, and microroughness at extremely acidic (pH < 2.5) and basic (pH > 11.0) conditions without noticeable disintegration. These changes are accompanied by dramatic changes in shell permeability with a 2 orders of magnitude increase in the diffusion coefficient. Moreover, the silk ionomer shells undergo remarkable softening with a drop in Young's modulus by more than 1 order of magnitude due to the swelling, stretching, and increase in material porosity. The ability to control permeability and mechanical properties over a wide range for the silk-based microcapsules, with distinguishing stability under harsh environmental conditions, provides an important system for controlled loading and release and applications in bioengineering.

Free surface electrospinning of fibers containing microparticles

Many materials have been fabricated using electrospinning, including pharmaceutical formulations, superhydrophobic surfaces, catalysis supports, filters, and tissue engineering scaffolds. Often these materials can benefit from microparticles included within the electrospun fibers. In this work, we evaluate a high-throughput free surface electrospinning technique to prepare fibers containing microparticles. We investigate the spinnability of polyvinylpyrrolidone (PVP) solutions containing suspended polystyrene (PS) beads of 1, 3, 5, and 10 μm diameter in order to better understand free surface electrospinning of particle suspensions. PS bead suspensions with both 55 kDa PVP and 1.3 MDa PVP were spinnable at 1:10, 1:5, and 1:2 PS:PVP mass loadings for all particle sizes studied. The final average fiber diameters ranged from 0.47 to 1.2 μm and were independent of the particle size and particle loading, indicating that the fiber diameter can be smaller than the particles entrained and can furthermore be adjusted based on solution properties and electrospinning parameters, as is the case for electrospinning of solutions without particles.

Formation of multilayered biopolymer microcapsules and microparticles in a multiphase microfluidic flow

This paper reports the development of a scalable continuous microfluidic-based method for the preparation of multilayered biopolymer microcapsules and microparticles, with a size range of 1 to 100 μm, in a single-layered polydimethylsiloxane-based device. This new approach has been utilised to produce polyethylene oxide (PEO)-based microparticles, layered with subsequent stage wise coatings of polylactide-based block copolymers and polyvinylpyrrolidone. The production process was shown to allow for on-chip encapsulation of protein and vitamin molecules in the biopolymer micro particles, without any further handling after collection from the device. We have studied the release profiles in the case of model molecules of distinctive molecular weights, namely, vitronectin, horse radish peroxidase, and vitamin B(12). We compared the release properties of the microparticles to those from macro-gels of the same materials prepared off-chip. The results indicated that the microparticles have definitively different molecular weight cut-off characteristics, likely due to a denser microstructure within the microparticles compared to the bulk hydrogels. This difference suggests that significant benefits may exist in the use of this method to produce layered biopolymer microparticles in achieving improved controlled release and encapsulation.

Global stability of the focusing effect of fluid jet flows

The global stability of the steady jetting mode of liquid jets focused by coaxial gas streams is analyzed both theoretically and experimentally. Numerical simulations allow one to identify the physical mechanisms responsible for instability in the low viscosity and very viscous regimes of the focused liquid. The characteristic flow rates for which global instability takes place are estimated by a simple scaling analysis. These flow rates do not depend on the pressure drop (energy) applied to the system to produce the microjet. Their dependencies on the liquid viscosity are opposite for the two extremes studied: the characteristic flow rate increases (decreases) with viscosity for very low (high) viscosity liquids. Experiments confirmed the validity of these conclusions. The minimum flow rates below which the liquid meniscus becomes unstable are practically independent of the applied pressure drop for sufficiently large values of this quantity. For all the liquids analyzed, there exists an optimum value of the capillary-to-orifice distance for which the minimum flow rate attains a limiting value. That limiting value represents the lowest flow rate attainable with a given experimental configuration in the steady jetting regime. A two-dimensional stability map with a high degree of validity is plotted on the plane defined by the Reynolds and capillary numbers based on the limiting flow rate.

Functional multilayered capsules for targeting and local drug delivery

One of the key challenges in the field of bio-nanotechnology for drug delivery systems (DDS) is the development of nano- or micro-sized delivery carriers possessing both targeting functionalities for specific tissues or cells, and controlled release properties for encapsulated drug molecules, proteins and genes. Hollow capsules developed by layer-by-layer (LbL) assembly have attracted much attention over the past few years owing to their ability to be modified, their capacity to encapsulate a wide range of chemicals, and the variety of functionalities with which they can be enhanced. Current research on LbL capsules focuses on the development of functionalized capsules for specific targeting of cancer or immune cells, and on controlling their release properties by environmental stimuli. This review discusses recent advances in DDS using functional hollow capsules specific for the cellular and tissue-targeted delivery, as well as stimuli-responsive controlled release. DDS based on functional hollow capsules may contribute to the development of new nano-medicines.

Force balance at the transition from selective withdrawal to viscous entrainment

We simulate the evolution of the steady-state interface in the selective withdrawal regime. Selective withdrawal ends when the upward pull exerted by the viscous flow in the withdrawing liquid layer overcomes the downward force due to surface tension. The lower-layer dynamics are unimportant. The dominant contribution to the surface-tension force comes from the large area where the interface is weakly deflected, instead of the small area where the surface is most distorted. A scaling estimate based on this idea yields results that agree quantitatively with both simulations and previous experiments.

Transformation of Se@Ag2Se core--shell colloids and nanowires into trigonal se nanorods and uniform spherical Ag2Se colloids

Physical separation of Ag(2)Se colloids and trigonal (t) Se nanowires transformed from amorphous (a) Se@Ag(2)Se core-shell spherical colloids and from t-Se@Ag(2)Se core-shell nanowires was conducted above the softening temperature ( approximately 132 degrees C) of Ag(2)Se. The Ag(2)Se shells in a-Se@Ag(2)Se core-shell colloids were first collapsed and then transformed into isolated solid spheres as the a-Se cores were dissolved in solvent and transformed into separate t-Se nanowires. In t-Se@Ag(2)Se core-shell nanowires, the soft Ag(2)Se shells dewetted on the t-Se nanowires and transformed into small droplets. While the formation of the spherical Ag(2)Se was not affected by the presence of surfactants, the structure of t-Se was highly dependent on the chemical structure of the polymeric surfactants.

Islet transplantation: the quest for an ideal source

The progress of islet transplantation as a new therapy for patients with diabetes mellitus depends directly upon the development of efficient and practical immunoisolation methods for the supply of sufficient quantities of islet cells. Without these methods, large scale clinical application of this therapy would be impossible. Two eras of advances can be identified in the development of islet transplantation. The first was an era of experimental animal and human research that centered on islet isolation procedures and transplantation in different species as evidence that transplanted islets have the capability to reverse diabetes. The second was the era of the Edmonton protocol, when the focus became the standardization of isolation procedures and introduction of new immunosuppressive drugs to maintain human allograft transplantation. The quest for an alternative source for islets (xenographs, stem cells and cell cultures) to overcome the shortage of human islets was an important issue during these eras. This paper reviews the history of islet transplantation and the current procedures in human allotransplantation, as well as different types of immunoisolation methods. It explores novel approaches to enhancing transplantation site vascularity and islet cell function, whereby future immunoisolation technology could offer additional therapeutic advantages to human islet allotransplantation.

Biopolymer microparticle and nanoparticle formation within a microfluidic device

This paper reports a novel microfluidic method for the production of cross-linked alginate microparticles and nanoparticles. We describe a continuous process relying on both thermodynamic and hydrodynamic factors to form microdroplets. A rapid cross-linking reaction thereafter allows solidification of the polymer droplets either within the microfluidic device or "off-chip" to form alginate micro- and nanoparticles. Monodisperse droplets are generated by extruding an aqueous alginate solution using an axisymmetric flow-focusing design. As they flow downstream in the channel, due to water and the continuous phase being partially miscible, the water diffuses very slowly out of the polymeric droplets into the transport fluid, which causes the shrinkage of the drops and the condensation of the polymer phase. The resulting size of the solid particles depends on the polymer concentration and the ensuing balance between the kinetics of the cross-linking reaction and the volume loss due to solvent diffusion. This work details both a single-step microfluidic technique for the formation of alginate microparticles of sizes ranging from 1 to 50 microm via near-equilibrium solvent diffusion within a microfluidic device and thereafter a two-step method, which was shown to generate biopolymer nanoparticles of sizes ranging from 10 to 300 nm. These novel methodologies are extremely flexible and can be extended to the preparation of micro- and nanoparticles from a wide range of single or mixed synthetic and biologically derived polymers.

Effect of the gelation process on the production of alginate microbeads by microfluidic chip technology

The present paper reports the production of Ba-alginate microspheres by microfluidic chip technology. The general production strategy is based on the formation of an alginate multiphase flow by a 'Y' junction squeezing mechanism. Special emphasis is given to the relationship existing between the gelation process and the final morphological characteristics of the produced microbeads. A series of different gelation strategies, namely: 'external gelation', 'internal gelation' and 'partial gelation' were compared in terms of size, size distribution and morphology of the produced microbeads.