Structure and thermal stability of microencapsulated phase-change materials

A series of microcapsules containing n-octadecane with a urea-melamine-formaldehyde copolymer shell were synthesized by in-situ polymerization. The surface morphology, diameter, melting and crystallization properties, and thermal stability of the microcapsules were investigated by using FTIR, SEM, DSC, TGA and DTA. The diameters of the microcapsules are in the range of 0.2–5.6 m. The n-octadecane contents in the microcapsules are in the range of 65–78wt%. The mole ratio of urea-melamine has been found to have no effect on the melting temperature of the microcapsules. Two crystallization peaks on the DSC cooling curve have been observed. The thermal damage mechanisms are the liquefied n-octadecane leaking from the microcapsule and breakage of the shell due to the mismatch of thermal expansion of the core and shell materials at high temperatures. The thermal stability of materials can be enhanced up to 10 °C by the copolymerization of urea, melamine and formaldehyde in a mole ratio 0.2:0.8:3. The thermal stability of 160 °C heat-treated microcapsules containing 8.8% cyclohexane can be further enhanced up to approximately 37 °C.     

One‐Step Synthesis of Natural Silk Sericin‐Based Microcapsules with Bionic Structures

Different techniques are being developed for fabricating microcapsules; it is still a challenge to fabricate them in an efficient and environment‐friendly process. Here, a one‐step green route to synthesize silk protein sericin‐based microcapsules without any assistance of organic solvents is reported. By carefully changing the concentration of calcium ions accompanied with stirring, the morphology of the microcapsules can easily be regulated to form either discoidal, biconcave, cocoon‐like, or tubular structures. The chelation of Ca²⁺ and shearing force from agitation may induce the conformational transformation of sericin, which possibly results in the formation of microcapsules through the self‐assembly of the protein subsequently. The as‐prepared cocoon‐like microcapsules exhibit pH‐dependent stability. A potential application of microcapsules being fabricated from natural water‐soluble silk protein sericin for controlled bioactive molecules loading and release system by a pH‐triggered manner is quite feasible.

     

Polyphosphazene Microcapsules Fabricated through Covalent Assembly

Hollow polyphosphazene microcapsules have been fabricated by the covalent layer‐by‐layer assembly of polydichlorophosphazene (PDCP) and hexamethylenediamine (HDA) on aminosilanized silica particles, followed by core removal in a HF/NH₄F solution. The hollow and intact microcapsules in both wet and dry states have been characterized by transmission electron microscopy and confocal laser scanning microscopy. The chemical structure of the microcapsules has been verified by FT‐IR spectroscopy. The microcapsules could be hydrolytically degraded in a phosphate buffer at biological pH.

     

壳聚糖溶液pH对载细胞海藻酸钠-壳聚糖微胶囊性能的影响

以激光共聚焦扫描显微镜为研究手段, 原位直观地考察了在不同pH条件下聚电解质膜的络合程度和蛋白扩散情况. 通过分析pH值对微胶囊膜性能的影响规律, 并结合不同种类细胞对环境pH的敏感特性, 确定了制备细胞培养用海藻酸钠-壳聚糖微胶囊的最佳pH值. 结果表明, 当壳聚糖溶液的pH值由3.50增加到6.50, 微胶囊膜的络合深度呈现高-低-高的趋势, 而微胶囊膜的膨胀性能呈现低-高-低的趋势, 模型蛋白通过微囊膜的扩散呈现低-高-低的趋势, 拐点均出现在pH=4.00和5.50处. 结合动物细胞及微生物细胞对环境pH耐受能力的考察, 确定制备微囊化动物细胞时, 微胶囊成膜反应溶液的最佳pH值为5.50; 制备微囊化大肠杆菌时, 反应溶液的最佳pH值为5.00; 制备微囊化酵母菌时, 反应溶液的最佳pH值为4.50.     

Fork in the road: patterned surfaces direct microcapsules to make a decision

Using computational modeling, we simulate the motion of compliant microcapsules on patterned surfaces. The microcapsules, which consist of an elastic shell and an encapsulated fluid, model biological cells or polymeric particles. We focus on a surface that is decorated with a Y-shaped pattern. As compared to the stem of the Y, one branch is relatively soft, and the other branch is relatively sticky. The capsules are driven to move over this substrate by an imposed fluid flow. Upon reaching the junction point, we find that deformable capsules preferentially move onto the sticky branch and stiffer capsules move onto the soft branch. Thus, through their inherent interactions with the patterned domains, the microcapsules are driven to "make decisions" about their path along the surface. Such surface patterning provides a facile means of routing particular capsules to specified locations in microfluidic devices and can form a fundamental component in creating fluidic circuits where microcapsules carry out simple logic operations.     

Chitosan-coated hemoglobin microcapsules for use in an electrochemical sensor and as a carrier for oxygen

The article describes the preparation of chitosan-coated hemoglobin (Hb-CS) microcapsules by (a) preparing a CaCO₃ precipitate containing Hb, (b) crosslinking Hb with glutaraldehye, (c) coating the particles with chitosan, and (d) preparing Hb-CS microcapsules by removing the CaCO₃ template with a solution of disodium EDTA. The morphology and electrochemical properties of the Hb-CS microcapsules were investigated by scanning electron microscopy, cyclic voltammetry and electrochemical impedance spectroscopy. An oxygen sensor was obtained by immobilizing the Hb-CS microcapsules on the surface of a glassy carbon electrode (GCE) first modified with gold nanoparticles. The application of Hb-CS microcapsules facilitates electron transfer on the surface of GCE and warrants the integrity and biological activity of Hb. The oxygen sensor, operated best at a working voltage of ?0.335  V (vs. SCE), displays a low limit of detection (30 nM). The Hb-CS microcapsules also are shown to release loaded oxygen to an anaerobic aqueous environment within 300 min.

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Graphical abstract The hemoglobin-chitosan microcapsule shows an excellent ability of electrocatalysis and carrying of oxygen.

     

Influence of core materials on thermal properties of melamine-formaldehyde microcapsules

A series of melamine-formaldehyde microcapsules as an intrinsic intumescent system was prepared by an in situ polymerization. The structural and thermal properties of the resultant microcapsules were studied. The surface morphology and chemical structure of microcapsules were investigated using scanning electron microscope (SEM), and Fourier-transform infrared spectroscope (FT-IR), respectively. The thermal properties of samples were investigated by thermogravimetric analysis (TGA) and by differential scanning calorimetry (DSC). The results showed that the thermo-physical properties are strongly dependant on the nature core content and the synthesis conditions. From the thermal analysis, it was concluded that microcapsules containing di-ammonium hydrogen phosphate exhibits characteristics of an intumescent system during their thermal degradation and could be interpreted due to the interaction between phosphate and melamine.     

High-capacity functional protein encapsulation in nanoengineered polypeptide microcapsules

Addition of polyethylene glycol to aqueous assembly solutions of oppositely charged polypeptides enables high-capacity "loading" of functional protein in biocompatible microcapsules by template-supported layer-by-layer nanoassembly.     

Tramadol Release from a Delivery System Based on Alginate‐Chitosan Microcapsules

Alginate‐chitosan microcapsules to control the release of Tramadol‐HCl were prepared using two different methods. In the two‐stage procedure (Variant I) alginate was first pumped into a CaCl₂/NaCl solution and then transferred into a chitosan solution. In the one‐stage procedure (Variant II) alginate was directly pumped into a chitosan/CaCl₂ solution, and different behavior could be noted in each case. The microcapsules were spherical in both variants and they swelled to a greater extent in a basic medium as compared to an acid one. The drug release profile of Tramadol from microcapsules in simulated gastric fluid and simulated intestinal fluid was also studied. The maximum release of Tramadol at 24 h was 64% and 86% for Variant I and II, respectively, in simulated intestinal fluid. Release was adjusted using the power law of the semi‐empirical Peppas equation in order to gain information about the release mechanism. In both cases the values of the exponent were found to be between 0.53 and 0.84 for swellable microcapsules in simulated gastric and intestinal fluids, respectively, indicating anomalous drug transport for both variants. The good results obtained with alginate‐chitosan microcapsules are comparable to those of the best products so far described in the scientific bibliography and in addition, chitosan is useful in pharmacy.

Surface morphology of Tramadol‐loaded microcapsule.     

Designing patterned substrates to regulate the movement of capsules in microchannels

Using computational modeling, we simulate the fluid-driven motion of microcapsules on patterned surfaces to establish guidelines for creating simple microfluidic devices for bioassays and multistage chemical reactions. The microcapsules, which consist of an elastic shell and an encapsulated fluid, model biological cells or polymeric particles. We focus on patterned substrates that encompass chemically adhesive and mechanically compliant domains. By probing the interactions between the microcapsules and these patterned surfaces, we determine the factors that control the movement of the capsules along the substrates. Using this information, we optimize the arrangement of the adhesive and compliant surface domains to create robust systems that effectively discriminate between various soft particles moving through the microchannels and "autonomously" direct certain species to specific locations. These findings could facilitate the fabrication of low-cost, portable microfluidic devices for sorting cells or performing fundamental chemical studies.     

Proliferation and Insulin Secretion Function of Mouse Insulinoma Cells Encapsulated in Alginate/Sol-Gel Synthesized Aminopropyl-Silicate/Alginate Microcapsule

Alginate/aminopropyl-silicate/alginate microcapsules, ca. 15 m in membrane thickness and ca. 500 m in diameter, were prepared via sol-gel process. The aminopropyl-silicate membrane was derived from two silicone alkoxide precursors, tetramethoxysilane and 3-aminopropyl-trimethoxysilane on Ca-alginate micro gel beads. Pancreatic -cell line (MIN6) cells were encapsulated in the microcapsule. The encapsulated MIN6 cells proliferated and formed spheroidal tissues in vitro. The diameter of the MIN6 spheroids increased to approximately 250 m with an increase in the incubation period until the day 35. Storeptozotocin-induced diabetic mice became normoglycemia after implantation of the MIN6-enclosing microcapsules. The normoglycemic state remained until the retrieval of the implanted microcapsules for 1 month. These results indicate that the potential use of the alginate/aminopropyl-silicate/alginate microcapsule as a vehicle for a genetically engineered cell-enclosing therapeutic material delivery system.     

In situ Localization of Molecules in Crosslinked Particles during Electrohydrodynamic Process: Simple Route to Produce Microcapsules and Fibers with Controlled Release

This study reports a spontaneous selective localization of molecules in crosslinked particles during electrospraying and electrospinning polymer solutions containing the particles. It provides a facile way of preparing microcapsules and fibers with controlled release. The dye molecules were phase separated from the crystalline polymer matrix during the electrohydrodynamic process and moved to the solvent‐rich crosslinked particles. The position of the particles in the microcapsules and fibers could be controlled by adjusting compatibility of the particles with the matrix polymer. The microcapsules and fibers did not show the initial burst release of the molecules and gave considerably prolonged release behavior.

     

Mussel‐Inspired Photografting on Colloidal Spheres: A Generalized Self‐Template Route to Stimuli‐Responsive Hollow Spheres for Controlled Pesticide Release

A thermo‐controlled pesticide release system composed of poly(2‐(dimethylamino)ethyl methacrylate) (PDMAEMA) thin film grafted polydopamine (PDA) (PDMAEMA‐g‐PDA) microcapsules is reported. SiO₂ microparticles are used as a template to prepare PDA‐coated SiO₂ microparticles. The thermally‐responsive PDMAEMA thin films are grafted on PDA surfaces using a metal‐free surface‐initiated photopolymerization approach without adding any photo­initiator or photosensitizer under UV light irradiation. The subsequent acid etching yields PDMAEMA‐g‐PDA hollow microcapsules. PDMAEMA‐g‐PDA microcapsules exhibit well‐controlled release of avermectin (Av). The results show that the loading ability of PDMAEMA‐g‐PDA microcapsules of Av is up to 52.7% (w/w). The release kinetics of Av demonstrate that Av@PDMAEMA‐g‐PDA microcapsules exhibit temperature‐controlled release performance. This work is significant for controlled release systems. This simple design is expected to be used in various applications, such as in controlled drug release and agriculture‐related fields.

     

A novel phase-change composites based on silicone rubber containing energy-storage microcapsules

A novel phase-change composites based on silicone rubber (MVQ) containing n-octadecane/poly (styrene-methyl methacrylate) microcapsules were successfully obtained by mixing energy-storage microcapsules into MVQ matrix using three preparation methods. The effect of microcapsules content on thermal property of the composites was investigated by thermogravimetric analysis. The mechanical properties of the composites prepared by three methods were also investigated. The morphology and thermal properties of the composites were characterized by scanning electron microscopy (SEM), differential scanning calorimetry, and thermal response. Thermal and mechanical properties of the composites were excellent when the microcapsules were added into room temperature vulcanized silicone rubber with 2 phr (per hundred rubber) content and cured at room temperature. The composites were proved to have good energy-storage performance with 67.6 J g^?1 enthalpy value.     

Alginate Microspheres Loaded with Animal Cells and Coated by a Siliceous Layer

Animal cells or cell aggregates are suspended in Na-alginate solution and extruded under a flow of air saturated with silicon alcoxides. The immediate formation of a sol-gel siliceous membrane on the microdrop surface allows the reaction with Ca²⁺⁺, leading to solid Ca-alginate, and yields homogeneous, stable microcapsules 200 m in diameter. The cell function of the encapsulated biomass were tested in specific protein production, which is not affected by the siliceous membrane protection of Ca-alginate microcapsules. This fact, and the observed increase in mechanical stability and chemical durability, indicate that valuable features of silica encapsulation are added to those proper to Ca-alginate.     

Magnetic switch of permeability for polyelectrolyte microcapsules embedded with Co@Au nanoparticles

We explored using a magnetic field to modulate the permeability of polyelectrolyte microcapsules prepared by layer-by-layer self-assembly. Ferromagnetic gold-coated cobalt (Co@Au) nanoparticles (3 nm diameter) were embedded inside the capsule walls. The final 5 mum diameter microcapsules had wall structures consisting of 4 bilayers of poly(sodium styrene sulfonate)/poly(allylamine hydrochloride) (PSS/PAH), 1 layer of Co@Au, and 5 bilayers of PSS/PAH. External alternating magnetic fields of 100-300 Hz and 1200 Oe were applied to rotate the embedded Co@Au nanoparticles, which subsequently disturbed and distorted the capsule wall and drastically increased its permeability to macromolecules like FITC-labeled dextran. The capsule permeability change was estimated by taking the capsule interior and exterior fluorescent intensity ratio using confocal laser scanning microscopy. Capsules with 1 layer of Co@Au nanoparticles and 10 polyelectrolyte bilayers are optimal for magnetically controlling permeability. A theoretical explanation was proposed for the permeability control mechanisms. "Switching on" of these microcapsules using a magnetic field makes this method a good candidate for controlled drug delivery in biomedical applications.     

Cellular Uptake of Covalent Poly(allylamine hydrochloride) Microcapsules and Its Influences on Cell Functions

The mechanism of the cellular uptake of polyelectrolyte microcapsules and its influences on the functions and toxicity of human SMCs are explored. The covalently assembled poly(allylamine hydrochloride)/glutaraldehyde microcapsules are easily ingested by SMCs mainly through macropinosis and caveolae‐mediated endocytosis pathways. The capsules mainly disperse in cytoplasm without colocalization in early endosomes and cell nucleus. The results of gene chips reveal substantial and profound alternation of cell phenotypes and functions. Uptake of the microcapsules cause a slight decrease of cell viability, but leads to significant changes in cytoskeleton organization, cell cycle, as well as cell adhesion and migration ability.

     

Crystallization behaviors of n-nonadecane in confined space: observation of metastable phase induced by surface freezing

Crystallization and phase transition behaviors of n-nonadecane in microcapsules with a diameter of about 5 mum were studied with the combination of differential scanning calorimetry (DSC) and synchrotron radiation X-ray diffraction (XRD). As evident from the DSC measurement, a surface freezing monolayer, which is formed in the microcapsules before the bulk crystallization, induces a novel metastable rotator phase (R(II)), which has not been reported anywhere else. We argue that the existence of the surface freezing monolayer decreases the nucleating potential barrier of the R(II) phase and induces its appearance, while the lower free energy in the confined geometry turns the transient R(II) phase to a "long-lived" metastable phase.     

A facile pathway to prepare enzymatically degradable microcapsules with tunable capsule shell properties

Dextran sulfate (DS)/poly-l-lysine (PLL) microcapsules are fabricated by an in situ coacervation method using DS-doped CaCO₃ microparticles as templates. Twinned superstructures or spherical CaCO₃ microparticles are produced depending on DS concentration in the starting solution. DS/PLL microcapsules with ellipsoidal or spherical outline are obtained after removal of templates in disodium ethylene diamine tetraacetate dehydrate (EDTA) without PLL. Their shell thickness and negative surface charges increase with the DS weight percentage in the templates. The surface potential of DS/PLL microcapsules, fabricated by core removal in an EDTA/PLL solution, can be easily tuned by altering PLL concentration in template removal solution. DS/PLL microcapsules fabricated by template removal in solution with or without PLL are both degraded by α-chymotrypsin, and different degradation profiles are observed because of shell thickness differences. DS/PLL may be used as transport vehicles for various compounds regardless of their charge sign in biomedical fields.     

Fabrication of Diverse Microcapsule Arrays of High Density and Good Stability

Microcapsule arrays attract a lot of interest due to their potential applications in sensing technology. A strategy for fabricating diverse microcapsule arrays through covalent linking is reported here. The self‐assembly of microcapsules was directed by using a poly(allylamine hydrochloride) (PAH)‐patterned template, which was created via microcontact printing. The microcapsules with PAH as the outermost layer were treated with glutaraldehyde and then covalently immobilized on the PAH regions, resulting in ordered microcapsule arrays. The arrays had a high density of capsules and the aggregate number in a pattern could be well controlled by adjusting the area of the PAH pattern. A single microcapsule array could be obtained if the diameter of the PAH region was smaller than that of the microcapsules. These covalently assembled arrays could survive through successive incubation in solutions of high ionic strength and extreme pHs. Such good stability ensures further treatments, such as chemical reactions and loading of functional substances.