Microencapsulation

Microencapsulation cannot be defined as a product or as a component of a product Rather, it is described as a process of enclosing micron-sized particles of solids or droplets of liquids or gasses in an inert shell, which in turn isolates and protects them from the external environment The inertness is related to the reactivity of the shell with the core material. This technology is mainly used for the purpose of protection, controlled release, and compatibility of the core materials (see Chapter 7). 

Enzymes may be immobilized by encapsulation in nonpermanent (e.g., liposomes) or permanent (e.g., nylon) microcapsules. The enzyme is trapped inside by a semi-permeable membrane, where substrates and products are small enough to freely diffuse across the boundary. While nonpermanent microcapsules are useful in biochemical research, only permanent microencapsulations yield analytically useful systems, because of their mechanical stability. 

Nylon microcapsules are formed by the interfacial polymerization of hexa-methylenediamine with sebacoyl chloride.22 The hexamethylenediamine is initially present in an aqueous enzyme solution, while the sebacoyl chloride is present in an organic phase such as 4 1 hexane/chloroform. The two solutions are mixed to form an emulsion. The nylon-6,10 membrane forms around the emulsified microdroplets, at the organic-aqueous interface  

The microcapsules are then rinsed and transferred to an aqueous solution containing a water-soluble surfactant, to prevent aggregation. This immobilization method requires a high-protein concentration in an aqueous phase ( 10 mg/mL), resulting 

In this stepwise process, pyrite is first oxidized in the presence of hydrogen peroxide and releases Fe. The Fe ion then reacts with PO to form an acidic-resistant precipitate such as FePO or FePO 211 0 that coats the surface of pyrite. This process is shown schematically in Fig. 4. 

PyiHe Felilil Silica layer hydi oxide 

When compared to traditional and alkaline treatments, passivation is a low-cost technique (Kapadia et al. 1999). One of the problems associated with using passivation on weathered rock samples is the large amount of lime required to maintain a pH of 12 (De Vries 1996). Successful application of this technique may be difficult (Glover 2007) and the chemistry of the permanganate coating is still not clear (De Vries 1996 Felipe 2008) hence, more laboratory and long-term field studies are required to better understand the technique and its long term effectiveness. 

Recently, fly ash (from coal combustion) was identified as a suitable alternative alkaline material because of its low-cost, local availability, and self-healing capacity (Perez-Ldpez et al. 2007a). Fly ash prevents AMD generation in two ways (1) over short periods, fly ash increases pH, resulting in the precipitation of Fe oxyhydroxide 

Cementation of fly asfi by aragonite Cementation of interface by gypsum Jarosite at the interface Fe-oxides coating on pyrite grairts 

The chemistry of polymers for high-solids coatings has been extensively reviewed [4]. 

The process of electrodeposition has been developed to provide uniform, highly adhesive, corrosion-resistant primer coats, particularly for the automotive industry. (The ultimate solution to the corrosion problem is simply to replace metal with plastic, as is done for many applications.) In the most common form of the process, polymers that contain carboxylic acid functionalities are produced. These polymers are then solubilized in an aqueous medium by partial neutralization with a base to give macroanions (P represents a polymer backbone) the macroanions are beheved to exist as micelles. 

A cationic process can also be used. Theoretically, it offers the best potential for mst resistance. This process is based on amine-functional polymers treated with an acid  

Following electrodeposition, objects are usually rinsed to remove any non-adhering material and then baked to dry and fuse the film, and perhaps crosslink it. 

Microencapsulation is a way of coating materials while ending up with a powder or very fine particles. Polymers are particularly useful, as they can form continuous thin films while in the liquid state and then harden upon coohng or drying of a solvent. Microcapsules are 

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Ic/gc [ANALYTICAL TffiTHODS - HYPHENATED INSTRUTffiNTS] (Vol 2) -lime and limestone in prod n of piME AND LIMESTONE] (Vol 15) -microencapsulation of [MICROENCAPSULATION] (Vol 16)... 

The identity of the moiety (other than glycerol) esterified to the phosphoric group determines the specific phosphoHpid compound. The three most common phosphoHpids in commercial oils are phosphatidylcholine or lecithin [8002-45-5] (3a), phosphatidylethanolamine or cephalin [4537-76-2] (3b), and phosphatidjlinositol [28154-49-7] (3c). These materials are important constituents of plant and animal membranes. The phosphoHpid content of oils varies widely. Laurie oils, such as coconut and palm kernel, contain a few hundredths of a percent. Most oils contain 0.1 to 0.5%. Com and cottonseed oils contain almost 1% whereas soybean oil can vary from 1 to 3% phosphoHpid. Some phosphoHpids, such as dipaLmitoylphosphatidylcholine (R = R = palmitic R" = choline), form bilayer stmetures known as vesicles or Hposomes. The bdayer stmeture can microencapsulate solutes and transport them through systems where they would normally be degraded. This property allows their use in dmg deHvery systems (qv) (8). 

Coa.cerva.tlon, A phenomenon associated with coUoids wherein dispersed particles separate from solution to form a second Hquid phase is termed coacervation. Gelatin solutions form coacervates with the addition of salt such as sodium sulfate [7757-82-6] especially at pH below the isoionic point. In addition, gelatin solutions coacervate with solutions of oppositely charged polymers or macromolecules such as acacia. This property is useful for microencapsulation and photographic apphcations (56—61). 

Slow release formulations incorporate nonpersistent compounds, eg, methyl parathion, insect growth regulators, and sex pheromones, in a variety of granular, laminated, microencapsulated, and hoUow-ftber preparations. 

Microencapsulation is the coating of small solid particles, liquid droplets, or gas bubbles with a thin film of coating or shell material. In this article, the term microcapsule is used to describe particles with diameters between 1 and 1000 p.m. Particles smaller than 1 p.m are called nanoparticles particles greater than 1000 p.m can be called microgranules or macrocapsules. 

Fig. 6. Flow diagram of microencapsulation process that utilises acid-cataly2ed in situ polymerisation of melamine or urea with formaldehyde to form a...

Microcapsules are used in a number of pharmaceutical, graphic arts, food, agrochemical, cosmetic, and adhesive products. Other specialty products also exist, thus the concept of microencapsulation has been accepted by a wide range of industries. In order to illustrate how microcapsules are used commercially, it is appropriate to describe a number of commercial microcapsule-based products and the role that microcapsules play in these products. 

The number of microencapsulated commercial oral formulations available and the volume of these formulations sold annuaUy is comparatively smaU. This may reflect the difficulty of developing new dmg formulations and bringing them successfully to market or the fact that existing microencapsulation techniques have had difficulty economically producing mictocapsules that meet the strict performance requirements of the pharmaceutical industry. One appHcation that is a particularly active area of development is mictocapsules or microspheres for oral deUvery of vaccines (45,46). 

Several parenteral microencapsulated products have been commercialized the cote materials ate polypeptides with hormonal activity. Poly(lactide— glycohde) copolymers ate the sheU materials used. The capsules ate produced by solvent evaporation, polymer-polymer phase separation, or spray-dry encapsulation processes. They release their cote material over a 30 day period in vivo, although not at a constant rate. 

Another biomedical appHcation of mictocapsules is the encapsulation of Hve mammalian ceUs for transplantation into humans. The purpose of encapsulation is to protect the transplanted ceUs or organisms from rejection by the host. The capsule sheU must prevent entrance of harmful agents into the capsule, aUow free transport of nutrients necessary for ceU functioning into the capsule, and aUow desirable ceUular products to freely escape from the capsule. This type of encapsulation has been carried out with a number of different types of Hve ceUs, but studies with encapsulated pancreatic islets or islets of Langerhans ate most common. The alginate—poly(L-lysine) encapsulation process originally developed in 1981 (54) catalyzed much of the ceU encapsulation work carried out since. A discussion of the obstacles to the appHcation of microencapsulation in islet transplantation reviewed much of the mote recent work done in this area (55). Animal ceU encapsulation has also been researched (56). 

Microencapsulation has much hidden potential for the food industry which promises to be tapped in the future (62). An interesting discussion of the problems that have been encountered while attempting to develop microcapsule formulations for commercial use in food products has been presented (65) and a review provides a number of references to food encapsulation studies (66). 

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