Gel microcapsules

Makino et al. [63] measured the electrophoretic mobility of four types M1-M4 of hydrophilic gel microcapsules containing water prepared by an interfacial polymerization method. Each type of microcapsules has membranes of different compositions. The results of the analysis of the measured mobilitiy values on the basis of Eqs. (21.128) are given in Figs 21.10 and 21.11. 

El-Gibaly I, Anwar MM. Development, characterization and in vivo evaluation of polyelectrolyte complex membrane gel microcapsules containing melatonin-resin complex for oral use. Bull Pharm Sci Assiut XJniv 1998 21(2) 117-139. 

D. Avnir et al./Sol-Gel Technologies Sunscreen composition containing sol-gel microcapsules—CA 2370364 A1... 

The sol-gel process to make doped silica-based materials has evolved from encapsulates in irregular Si02 xerogel particles, to sophisticate core-shell particles capable to encapsulate high amounts of functional organic species, and effectively release the entrapped species under small load. Sol-gel microcapsule and microparticle delivery systems will soon be introduced by numerous industries. 

Figure 15. ESEM image of Silica-gel microcapsules loading with TMPTA monomer and Darocur 1173 photoinitiator...

Figure 16. Infrared spectrum of (a) Empty silica-gel microcapsules (b) silica- gel microcapsules loaded with TMPTA monomer and (c) pure TMPTA...

For that we have used silica-gel iiticrocapsules (empty microcapsules artd microcapsules loaded with TMPTA) with TEOS precursor alone (not methacrylate chain in this compound). The comparison between Infrared spectra of the empty silica-gel microparticles with TEOS precursor alone - Figure 17 (a) and silica-gel microparticles loaded with TMPTA - Figure 17 (c), shows the presence of new bands, in particular at 1721, 983 and 807 cm b These bands, characteristic of the TMPTA monomer, confirm the presence of TMPTA encapsulated in silica-gel microcapsules. 

Figure 18. Thermogravimetric Analysis of Empty Silica-gel Microcapsules...

EDX of polyimide film does not show any sUicium peak - Figure 21 (a) but this peak is present in the analysis of the polyimide with siUca-gel microcapsules. 

Complex Coacervation. This process occurs ia aqueous media and is used primarily to encapsulate water-iminiscible Hquids or water-iasoluble soHds (7). In the complex coacervation of gelatin with gum arabic (Eig. 2), a water-iasoluble core material is dispersed to a desired drop size ia a warm gelatin solution. After gum arabic and water are added to this emulsion, pH of the aqueous phase is typically adjusted to pH 4.0—4.5. This causes a Hquid complex coacervate of gelatin, gum arabic, and water to form. When the coacervate adsorbs on the surface of the core material, a Hquid complex coacervate film surrounds the dispersed core material thereby forming embryo microcapsules. The system is cooled, often below 10°C, ia order to gel the Hquid coacervate sheU. Glutaraldehyde is added and allowed to chemically cross-link the capsule sheU. After treatment with glutaraldehyde, the capsules are either coated onto a substrate or dried to a free-flow powder. 

The formation of ordered two- and three-dimensional microstructuies in dispersions and in liquid systems has an influence on a broad range of products and processes. For example, microcapsules, vesicles, and liposomes can be used for controlled drug dehvery, for the contaimnent of inks and adhesives, and for the isolation of toxic wastes. In addition, surfactants continue to be important for enhanced oil recovery, ore beneficiation, and lubrication. Ceramic processing and sol-gel techniques for the fabrication of amorphous or ordered materials with special properties involve a rich variety of colloidal phenomena, ranging from the production of monodispersed particles with controlled surface chemistry to the thermodynamics and dynamics of formation of aggregates and microciystallites. 

Because most food matrices are water soluble, many efforts were directed to the formulation of lipophilic pigments (mainly carotenoids) into water-soluble formulations (powders or gels). For hydrophilic pigments like flavonoids, polar dried microcapsules are the most popular ways to stabilize their functionality. Extracts rich in P-carotene were encapsulated using three different encapsulation techniques (spray drying, drum drying, and freeze drying)." ... 

Extending these ideas to enzymatic catalysis, Jiang et al. reported the use of protamine-silica hybrid microcapsules in combination with a host gel-like bead structure to encapsulate several enzymes individually in the enzymatic conversion of C02 to methanol [20]. They used a layer-by-layer (LbL) method where alternately charged layers were deposited on an enzyme-containing CaC03 core. The layers, however, were not polyelectrolytes, but protamine and silica (Scheme 5.6). 

Scheme 5.7 Encapsulation of enzyme microcapsules into a gel-like structure (host gel bead) resulting in a capsules-in-bead microreactor. Reproduced from [20] by permission of The Royal Society of Chemistry.

It is enough to visit the clean, small production plant of Sol-Gel Technologies in Israel (Figure 8.3) to recognize that most of the value added to benzoyl peroxide entrapped in microcapsules comes from knowledge—and thus from human ingenuity—which originates the production of the microcapsules. The price at which the white water-based capsule formulation is sold to customers exceeds more than 1000 times the price of the raw materials used to prepare it. Put another way ... 

For example, Sol-Gel Technologies in Israel (Figure 2.2) manufactures on a large scale microcapsules made of sol-gel entrapped... 

Entrapping of bioactive ingredients by polymer matrix in gel or microgel particles heat-induced or cold-induced aggregation and gelation of globular proteins (microcapsules of 5-5000 pm)... 

An unusual type of derivative is the complex that forms between urease and bentonite in acid medium (61). The adsorbed form was found catalytically active. Similarly, urease immobilized in a polyacrylamide gel matrix has been used to prepare a urea-specific enzyme electrode (62). Yet another active water-insoluble derivative has been prepared (63) by allowing p-chloromercuribenzoate-treated urease to react with a diazotized copolymer of p-amino-D,L-Phe and L-Leu. Urease has been found to retain about 20% of its original activity when encapsulated in 100 n microcapsules of benzalkonium-heparin in collodion (64). 

The enzyme can be entrapped into the growing polymer or gel materials or microcapsules (microcncapsu-lation). The retention of enzymes in the reaction vessel can also be achieved by an ultrafiltration membrane. All these immobilization types have in common that the enzymes are not modified and are still acting in their soluble form, so that they should be freely mobile in their cage . An older but good review of these techniques is given by Tramper [72], Frequently used matrices for gel entrapment are polyacrylamide and collagen [41]. The membrane inclusion method is a broadly applicable one. 

These additives can also be placed into microcapsules with a thin polymer skin. For example, they are filled with natural aromas and applied to the textile from a water dispersion in combination with a polyurethane or silicone binder. The ratio of micro bubbles to binder determines the efficiency and permanence of the finish. Good results after 4-12 washing cycles are reported. The encapsulated materials are released during wearing as the micro bubbles burst from rubbing caused by body movement or by diffusion through the thin layer of the capsules. A market available encapsulation in micro bubbles, built from chitosan, is described by Hampe. The incorporation and controlled release of fragrance compounds is also provided by the sol-gel nano-technique described in Section 18.4. 

Design of delivery device Shape/geometry Cylinder or rod Microparticles (microsphere, microcapsule, nanoparticle) Film or sheet Viscous gel or liquid... 

Ichikawa, H. Fukumori, Y. A novel positively thermosensitive controlled-release microcapsules with membrane of nano-sized poly(V-isopropylacrylamide) gel dispersed in ethylcellulose matrix. J. Controlled Release 2000, 63, 107-119. 

The large number of variables involved in complex coacervation (pH, ionic strength, macromolecule concentration, macromolecule ratio, and macromolecular weight) affect microcapsule production, resulting in a large number of controllable parameters. These can be manipulated to produce microcapsules with specific properties. Complex coacervate microcapsules have been formulated as suspensions or gels, and have been compounded within suppositories and tablets.[ l... 

Dropping or spraying a sodium alginate solution into a calcium chloride solution produces microcapsules. The divalent calcium ions cross-link the alginate, forming gelled droplets. These gel droplets can be permanently cross-linked by addition to a polylysine solution. Lim and Sun developed this method for the encapsulation... 

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