Designer polymer-based microcapsules made using microfluidics

Filled microcapsules made from double emulsion templates in microfluidic devices are attractive delivery systems for a variety of applications. The microfluidic approach allows facile tailoring of the microcapsules through a large number of variables, which in turn makes these systems more challenging to predict. To elucidate these dependencies, we start from earlier theoretical predictions for the size of double emulsions and present quantitative design maps that correlate parameters such as fluid flow rates and device geometry with the size and shell thickness of monodisperse polymer-based capsules produced in microcapillary devices. The microcapsules are obtained through in situ photopolymerization of the middle oil phase of water-in-oil-in-water double emulsions. Using polymers with selected glass transition temperatures as the shell material, we show through single capsule compression testing that hollow capsules can be prepared with tunable mechanical properties ranging from elastomeric to brittle. A quantitative statistical analysis of the load at rupture of brittle capsules is also provided to evaluate the variability of the microfluidic route and assist the design of capsules in applications involving mechanically triggered release. Finally, we demonstrate that the permeability and microstructure of the capsule shell can also be tailored through the addition of cross-linkers and silica nanoparticles in the middle phase of the double emulsion templates.     

Novel calcium-alginate capsules with aqueous core and thermo-responsive membrane

Novel calcium-alginate (Ca-alginate) capsules with aqueous core and thermo-responsive membrane are successfully prepared by introducing a co-extrusion minifluidic approach, and the thermo-responsive gating characteristics of Ca-alginate capsule membranes embedded with poly(N-isopropylacrylamide) (PNIPAM) microspheres are investigated systematically. The experimental results show that the prepared Ca-alginate capsules are highly monodisperse, and the average diameter and membrane thickness of Ca-alginate capsules are about 2.96 mm and 0.11 mm respectively. The Ca-alginate capsule membranes exhibit desired thermo-responsive gating property. With increasing the content of PNIPAM microspheres embedded in the Ca-alginate capsule membranes, the thermo-responsive gating coefficient of the capsule membranes increases simply. When solute molecules diffuse through the capsule membrane, the thermo-responsive gating coefficient is significantly affected by the molecular weight of solute molecules.     

Developing a thermomechanical and thermochemical model for investigating the cooling rate effects on the distortion of unsymmetrical viscoelastic polymeric composite laminates

An experimental and semi-analytical study of distortion of asymmetric composite laminates with different cooling rates and lay-ups has been presented. In this study, thermomechanical constitutive equations of thin composite laminates are developed using basic viscoelastic constitutive law considering chemical and thermal effects with time-temperature dependent material properties. To solve a fully scouple problem, both the thermochemical and thermomechanical constitutive equations are formulated. The general heat conduction equation known as the Fourier-Biot equation, viscoelastic laws, Boltzmann superposition principle and composite equations are utilized to formulate thin composite laminates. A static model with constant properties in ambient temperature and a transient model by obtaining constitutive equations are simulated. Results are compared with experimental data. Changing lay-up from cross-ply to angle-ply and then quasi isotropic will increase the value of maximum distortion. Results indicated that the increasing cooling rate will increase the value of the maximum distortion. The differences between FEM results with static analysis of different lay-ups and experimental specimens that cooled in the oven, environment and refrigerator is about 3%, 35% and 55% respectively. The differences between FEM simulation with transient analysis of different lay-ups and experimental specimens that cooled in the environment and the refrigerator is less than 9%.     

Dynamic interactions between approaching surfaces of biological interest

The dynamic interactions arising when two interfaces approach each other are reviewed. Experimental methods are summarized and briefly discussed. The basic physical laws describing the dynamic behavior of interfaces and the liquid film between them are presented and the thin film approximation is used in order to simplify them. It was shown that interface deformability decreases the approach rate (increases interaction) while surface tangential mobility and membrane permeability increase it. Formulae are also presented for the dynamic interactions between solid particles of different shapes. The influence of the rheological properties of the liquid film between approaching surfaces is considered using viscoelastic, liquid-crystalline and non-linear models. The interactions between surface shape perturbations which are important for liquid film stability are considered with special attention to the case of membranes. A hydrodynamic theory of bilayer membrane formation and some similar phenomena is described which coincides semi-quantitatively with available experimental data. It is pointed out that viscous interactions can significantly decrease the rate of aggregation and fusion in multiparticle systems. The influence of external fields is also briefly discussed. The general conclusion is that in any case of very close approach of two interfaces the dynamic interactions can significantly increase leading to a decrease in the rate of adhesion and fusion.     

On the behaviour of asymmetric membranes in pervaporation

Symmetric and asymmetric membranes of the Loeb type are compared with respect to their performance in pervaporation. The experiments are carried out with water—isopropanol mixtures, employing cellulose acetate membranes of different structure, but of the same total thickness. These results are compared with calculations based on a 2-layer model for asymmetric membranes. Design criteria for optimal asymmetric membranes for pervaporation, as well as the performance characteristics for the two possible modes of installation — active layer facing the feed or facing the permeate — are discussed. Contrary to reverse osmosis, the installation of the membrane with the active layer facing the permeate proves to be superior — at least for low permeabilities of the membrane material. The interdependencies between thickness and permeability of the active layer, and porosity and thickness of the support layer are much stronger than in reverse osmosis.     

Large Ultrathin Shelled Drops Produced via Non‐Confined Microfluidics

We present a facile approach for producing large and monodisperse core–shell drops with ultrathin shells using a single‐step process. A biphasic compound jet is introduced into a quiescent third (outer) phase that ruptures to form core–shell drops. Ultrathin shelled drops could only be produced within a certain range of surfactant concentrations and flow rates, highlighting the effect of interfacial tension in engulfing the core in a thin shell. An increase in surfactant concentrations initially resulted in drops with thinner shells. However, the drops with thinnest shells were obtained at an optimum surfactant concentration, and a further increase in the surfactant concentrations increased the shell thickness. Highly monodisperse (coefficient of variation smaller than 3 %) core–shell drops with diameter of ～200 μm–2 mm with shell thickness as small as ～2 μm were produced. The resulting drops were stable enough to undergo polymerisation and produce ultrathin shelled capsules.     

Calculation of the Liquid and Gas Permeability of Hydrophobic Low-Porosity Membranes of an Arbitrary Thickness

A method for calculating the liquid and gas permeability of hydrophobic low-porosity membranes of an arbitrary thickness is described. The calculation is based on the solution of a problem on percolation—the procedure of finding the distribution of liquid and gas over the membrane thickness. The dependence of the permeability for liquid on the share of pores that are potentially accessible to being filled with liquid is obtained for both thin and thick membranes. This dependence is of a universal nature and can easily be recalculated into a dependence of permeability on the pressure drop for membranes with any distribution of pores by size. Numerical estimates of principal characteristics for a membrane that possesses pores of three types are performed. The characteristics in question include permeabilities for liquid and gas; fluxes of the liquid; critical pressures, at which the permeability for liquid turns other than zero; and the working range of pressures, in which the membrane is capable of working normally. All these data permit the optimization of the operation of similar membranes, in particular, gas-delivering membranes that are used in hydrogen–oxygen fuel cells with a solid polymer electrolyte.     

Hydrogen diffusion transfer through an asymmetric three-layer vanadium membrane

Hydrogen diffusion transfer through a three-layer membrane has been studied within the framework of the lattice model under the Bragg?Williams approximation. A set of equations describing hydrogen transfer through a vanadium membrane coated with thin palladium layers has been derived taking into account the interactions of hydrogen atoms in the membrane layers. The obtained equations have been solved using the Mathcad-14 software package. It has been shown that the interaction between hydrogen atoms has a significant influence on hydrogen permeability at near-atmospheric pressures. It has been found that the permeability of the vanadium membrane is markedly higher than that of a palladium one at the same thickness. The effect of asymmetric vanadium membrane embrittlement has been shown to depend on the location of palladium layers with different thicknesses. The embrittlement of the vanadium membrane begins at higher pressures, when a thicker palladium layer is located at the inlet. It has been revealed that, for asymmetric membranes, the value of the diffusion flux of hydrogen atoms may depend on the transfer direction. At the same membrane thickness, the permeability of the asymmetric membrane is actually equal to that of a symmetrical membrane, provided that a thicker palladium layer is located at the inlet. At the opposite orientation, of the permeability of the asymmetric membrane is lower than that of the symmetric one.     

Investigation of poly(vinylidene chloride) distribution in perfluorinated cation-exchange membranes MF-4SK upon UV- and γ-initiated graft polymerization

A new process for grafting poly(vinylidene chloride) (PVDC) to the membrane material MF-4SK by UV-initiated graft polymerization of the monomer from the gas phase has been developed. Modified membranes containing up to 20 wt % of UV-grafted PVDC have been obtained. Microphotographs of thin sections of the modified membranes have been investigated. It has been shown that the pretreatment of the membranes and variation of UV- or γ-grafting conditions make it possible to achieve an uniform distribution of grafted PVDC both along the thickness of the membrane and in a thin surface layer. The values of the parameters determining the character of the distribution have been estimated. Numerical simulation of the UV- and γ-initiated graft polymerization of VDC gave solutions for the grafted-PVDC distribution fitting with the experimental data.     

The effect of gel layer thickness on the salt rejection performance of polyelectrolyte gel-filled nanofiltration membranes

The effect of gel layer thickness on salt separation of positively charged pore-filled nanofiltration membranes has been examined both theoretically and experimentally. The extended Nernst-Planck (ENP) equation coupled with the Teorell-Meyer-Sievers (TMS) model were used to calculate the pressure-driven sodium chloride rejections for membranes having gel densities in the range typically used in nanofiltration applications. It was found that salt rejection was dependent on membrane (gel-layer) thickness with salt rejections increasing rapidly with thickness up to 50–75 μm. Further increases in thickness beyond this point had a much smaller effect on salt rejection. The theoretical predictions were examined experimentally by preparing a series of membranes with cross-linked poly(3-acrylamidopropyl)-trimethylammonium chloride (PAPTAC) gels with varying densities within the pores of a thin microporous polyethylene (PE) support. The membranes were characterized by their polymer volume fractions (gel concentration), thicknesses and effective charge densities. The effect of membrane thickness was examined by using single and stacks of two membranes. The pure water fluxes and salt rejections of the membranes and membrane stacks were determined in the pressure range 50–550 kPa. The single salt rejections of the membranes which were very dependent on the thickness of the membrane or membrane stack, were fully in accord with the calculated salt rejections of the membranes.     

Characterization of the mechanical properties of cross-linked serum albumin microcapsules: effect of size and protein concentration

A microfluidic technique is used to characterize the mechanical behavior of capsules that are produced in a two-step process: first, an emulsification step to form droplets, followed by a cross-linking step to encapsulate the droplets within a thin membrane composed of cross-linked proteins. The objective is to study the influence of the capsule size and protein concentration on the membrane mechanical properties. The microcapsules are fabricated by cross-linking of human serum albumin (HSA) with concentrations from 15 to 35 % (w/v). A wide range of capsule radii (～40–450 μm) is obtained by varying the stirring speed in the emulsification step. For each stirring speed, a low threshold value in protein concentration is found, below which no coherent capsules could be produced. The smaller the stirring speed, the lower the concentration can be. Increasing the concentration from the threshold value and considering capsules of a given size, we show that the surface shear modulus of the membrane increases with the concentration following a sigmoidal curve. The increase in mechanical resistance reveals a higher degree of cross-linking in the membrane. Varying the stirring speed, we find that the surface shear modulus strongly increases with the capsule radius: its increase is two orders of magnitude larger than the increase in size for the capsules under consideration. It demonstrates that the cross-linking reaction is a function of the emulsion size distribution and that capsules produced in batch through emulsification processes inherently have a distribution in mechanical resistance.     

Zwitterionic microgel based anti(-bio)fouling smart membranes for tunable water filtration and molecular separation

Tunable gating polymeric nanostructured membrane with excellent water permeability and precise molecular separation is highly advantageous for smart nanofiltration application. Polymeric nanostructures such as microgels with functionalizable cross-linkable moieties can be an excellent choice to construct membranes with a thin separation layer, functionality, and tunable transport properties. In the present work, we prepared switchable anti(bio)fouling membranes using zwitterionically functionalized antibacterial thermoresponsive aqueous core-shell microgels with a thin separation layer for controlled filtration and separation applications. The microgels were synthesized using a one-step graft copolymerization of poly(N-isopropylacrylamide) and polyethyleneimine (PEI) followed by zwitterionization of free amine groups of PEI chains with 1,3-propane sultone. Microgel synthesis and zwitterionization were confirmed by spectroscopic and elemntal analysis. The obtained microgels were thoroughly characterized to analyze their thermoresponsive behavior, morphology, charge, and antibacterial properties. After that, characterizations were performed to elucidate the surface properties, water permeation, rejection, and flux recovery of the microgel membranes prepared by suction filtration over a track-etched support. It was observed that zwitterionic membrane provides better hydrophilicity, lower bovine serum albumin (BSA) adsorption, and desirable antimicrobial activity. The pure water permeability was directly related to the microgel layer thickness, applied pressure, and temperature of the feed solution. The novel nanostructured membrane leads to an excellent water permeance with a high gating ratio, high flux recovery rate with low irreversible fouling, better rejection for various dyes, and foulant. Most importantly, the long-term performance of the membrane is appreciable as the microgel layer remains intact and provides excellent separation up to a longer period. Owing to easy preparation and well control over thickness, the zwitterionic microgel membranes constitute unique and interactive membranes for various pressure-driven separation and purification applications.     

Discrete resistance changes in thin lipid membranes

Semispherical membranes were formed using a mixed lipid preparation extracted from beef brain gray matter. The membrane obtained had a thickness between several hundred angstroms and about one thousand angstroms as determined by the interference color of reflected light. The initial thick membrane and newly formed primary black membrane had a resistance greater than 10¹⁰ ohms, similar to that of the bulk lipid phase. However, when a small (< 15 mV) transmembrane potential was applied, the resistance decreased in a stepwise fashion to a final resistance of about 10⁵10⁶ ohms. The resultant thin membrane exhibited stepwise temperature-dependent fluctuations in conductance similar to those obtained with membranes to which substances such as the “excitability-inducing material” of Mueller and Rudin and certain ionophores have been added. Further, the membrane showed rectification when calcium was present on only one side of the membrane.     

The role of molecular interactions and interfaces in diffusion: transport diffusivity and evaluation of the Darken approximation

Kinetic Monte Carlo (KMC) simulations are carried out to directly study diffusion of benzene through thin (37-100 nm) NaX zeolite membranes under a gradient in chemical potential. Nonlinearities in adsorbate loading near the membrane boundaries are shown to arise from the difference in adsorbate density between the zeolite and adjacent fluid phase. Direct extraction of the transport diffusivity from gradient KMC simulations enables testing of the Darken approximation. This rigorous approach reveals limitations of the Darken approximation and, for the first time, the potentially complex nonunique functionality and multiplicity of the transport diffusivity for strongly interacting adsorbates. In the companion paper we explore these nonlinear interfacial effects in the context of permeation through both single-crystal and polycrystalline membranes.     

On the rheology of cold drawing. I. Elastic materials

In this, the first paper in a series on neck formation and steady-state drawing of polymeric fibers and films under uniaxial tension, the emphasis is laid on those aspects of the mechanics of cold drawing that are not sensitive to viscoelastic effects and, therefore, can be treated by use of constitutive assumptions appropriate to elastic materials. It is here shown that a unidimensional theory which has been employed to model the mechanics of slender bars in tension₆ can be derived as an approximation for three-dimensional bars and, in a sense which can be made precise, is valid to within an error of the order of the fourth power of the thickness. A particular constitutive equation for incompressible, three-dimensional, elastic materials is explored in detail and is found to yield, for such slender bars as thin fibers and wide (but thin) strips of film, equations of equilibrium whose solutions are in good qualitative accordance with the necks and drawing configurations observed in practice.     

Deformation and bursting of nonspherical polysiloxane microcapsules in a spinning-drop apparatus

We analyze the deformation and bursting process of nonspherical organosiloxane capsules in centrifugal fields. Measurements were performed in a commercial spinning-drop tensiometer at different values of tube rotation. A theoretical analysis of the mechanics of initially ellipsoidal elastic shells subjected to centrifugal forces is developed where the deformation of the capsule is predicted as a function of the initial geometry and membrane elastic properties. For different types of organosiloxane membranes the Poisson number varies between 0 and 0.9. This phenomenon points to a considerable reduction of the membrane thickness at the onset of mechanical stress. Membrane-breaking processes always initiated at one of the pole ends of the capsules. Such rupture processes can be interpreted in terms of the derived theoretical model.     

Effects of dielectric gradients-mediated ions partitioning on the electrophoresis of composite soft particles: An analytical theory

In this work, we report original analytical expressions defining the electrophoretic mobility of composite soft particles comprising an inner core and a surrounding polymer shell with differentiated permeabilities to ions from aqueous background electrolyte and to fluid flow developed under applied DC field conditions. The existence of dielectric permittivity gradients operational at the core/shell and shell/solution interfaces is accounted for within the Debye–Hückel approximation and flat plate configuration valid in the thin double layer regime. The proposed electrophoretic mobility expressions, applicable to weakly to moderately charged particles with size well exceeding the Debye layer thickness, involve the relevant parameters describing the particle core/shell structure and the electrohydrodynamic features of the core and shell particle components. It is shown that the analytical expressions reported so far in literature for the mobility of hard (impermeable) or porous particles correspond to asymptotic limits of the more generic results detailed here. The impacts of dielectric-mediated effects of ions partitioning between bulk solution and particle body on the electrophoretic response are further discussed. The obtained expressions pave the way for a refined quantitative, analytical interpretation of electrophoretic mobility data collected on soft (nano)particles (e.g., functionalized dendrimers and multilayered polyelectrolytic particles) or biological cells (e.g., viruses) for which the classical hard core-soft shell representation is not appropriate.     

Microcapsule mechanics: From stability to function

Microcapsules are reviewed with special emphasis on the relevance of controlled mechanical properties for functional aspects. At first, assembly strategies are presented that allow control over the decisive geometrical parameters, diameter and wall thickness, which both influence the capsule's mechanical performance. As one of the most powerful approaches the layer-by-layer technique is identified. Subsequently, ensemble and, in particular, single-capsule deformation techniques are discussed. The latter generally provide more in-depth information and cover the complete range of applicable forces from smaller than pN to N. In a theory chapter, we illustrate the physics of capsule deformation. The main focus is on thin shell theory, which provides a useful approximation for many deformation scenarios. Finally, we give an overview of applications and future perspectives where the specific design of mechanical properties turns microcapsules into (multi-)functional devices, enriching especially life sciences and material sciences.     

Electrochemical characterization of a bioceramic material: The shell of the Eastern oyster Crassostrea virginica

The shell of the Eastern oyster (Crassostrea virginica) is composed of multiple incongruent mineralized layers. This bioceramic composite material was investigated to determine the effects of shell thickness, orientation and layer composition on its electrochemical behavior using electrochemical impedance spectroscopy, potentiodynamic polarization and scanning electron microscopy-energy dispersive spectroscopy. SEM-EDS analysis of the oyster shell revealed that the multilayered biocomposite material is composed of calcium carbonate (CaCO(3)). EIS measurements in 3.5wt.% NaCl indicated that the impedance of the whole oyster shell in the low frequency region exhibited high impedance values which exhibited a decreasing trend with increasing immersion time. In terms of overall shell thickness, limiting currents measured by potentiodynamic techniques through the shell were observed to increase when the outer layers of the shell were sequentially removed by grinding, thus decreasing the shell thickness. These limiting current values remained relatively constant when the inner layers of the shell were removed. The impedance values of the oyster shell material as measured by EIS were shown to decrease with decreasing shell thickness. These findings suggest that the prismatic (outermost) shell layer in combination with the soluble organic matrix between all shell layers may influence the ionic conductivity through the oyster shell.     

Template-free uniform-sized hollow hydrogel capsules with controlled shell permeation and optical responsiveness

This study has established a robust and straightforward method for the fabrication of uniform poly(vinylamine) hydrogel capsules without using templates that combines the dispersion polymerization and the sequential hydrolysis/cross-linking. The particle sizes are determined by the degree of cross-linking as well as by the cross-linking reaction time, while the shell thickness is independent of these variables. Diffusion-limited reactions occur at the periphery of the particles, leading to the formation of hydrogel shells with a constant thickness. The treatment of the surfaces of hollow hydrogel capsules with oppositely charged biopolymers limits the permeability through the shell of species even with low molecular weights less than 400 g/mol. Furthermore, we demonstrated that the hydrogel shell phase decorated with Au nanoparticles can be optically ruptured by exposure to laser pulse, a feature that has potential uses in optically responsive drug delivery.