Microcapsules, controlled release formulations

Microspheres and microcapsules of lactide/glycolide polymers have received the most attention in recent years. Generally, three microencapsulation methods have been employed to afford controlled release formulations suitable for parenteral injection (1) solvent evaporation, (2) phase separation, and (3) fluidized bed coating. Each of these processes requires lactide/glycolide polymer soluble in an organic solvent. 

The racemic form is as effective as the (+) enantiomer for disrupting mating. This technique for population suppression has been evaluated in experiments to compare the effects of formulation, dose rate and population density on its efficacy. In light infestations, gypsy moth mating is effectively suppressed. Microcapsules, laminated polymeric "flakes" and hollow fibers were compared as controlled-release formulations. 

Controlled release formulations are a recent innovation in which the pesticide is incorporated into a carrier, generally a polymeric material (Scher, 1999). The rate of release of the pesticide is determined by the properties of the polymer itself as well as environmental factors. There are mainly two types of CR formulations reservoir devices and monolithic devices. As shown in Figure 2.1, in the reservoir device, the toxicant is enclosed in capsules of thin polymeric material to become microcapsules (1-100 pm in diameter), e.g., Penncap-M microcapsules (methyl parathion). In the monolithic device, the toxicant is uniformly... 

Because of its inherent costs, spray drying is not always considered as a processing option for many conventional formulations. However, when a specialized particle type is required by the active ingredient or dosage form, spray drying can become a feasible alternative to more conventional manufacturing processes. Such particle types include microcapsules, controlled release particles, nanoparticles, and liposomes. The application of spray drying to pharmaceuticals has been extensively discussed in review articles (21,22). 

An important application in agrochemicals is that of controlled-release formulations. Several methods are used for controlled release, of which microcapsules (CS) are probably the most widely used. These are small particles with size range 1-1000 pm consisting of a core material and an outer wall. The latter isolates the core material from the environment and protects it from degradation and interaction with other materials. The core active ingredient is designed to be released in a controlled manner as required. Microencapsulation of agrochemicals is usually carried out by interfacial condensation, in situ polymerization or coacervation, all of which are determined by the interfacial properties. 

Lidocaine (112), xyloceiine, and dibucaine (113) have been formulated in homo- and copolymers of lactide and glycolide. The goal of these studies has been relatively short-term (24-hr) controlled release of the anesthetic. Injectable microcapsules of lidocaine hydrochloride were produced by an air suspension coating technique and administered i.m. to rabbits (112). Serum levels of Udocaine indicated an initial rise over the first 2 hr and then a gradual decline with clearance after about 8-10 hr. 

Microcapsules represent an extra degree of freedom in the formulation or development of these food products. Many of the reasons or causes for the use of microcapsules are covered in a previous symposium (1) and a continued updated review on this subject (2). The use of microcapsules is one means of achieving controlled release of the core or inner material. The term controlled release actually covers a wide range of technologies and microencapsulation is one way of achieving controlled release. In fact, microencapsulation is the dominant means for achieving controlled release both in product volume and dollar value. 

The first plot received 500 g.a.i./h of displarlure as NCR gelatin-walled microcapsules containing 2% ai. The formulation, applied as an aqueous suspension, also contained 1 of sticker to aid adhesion of the formulation to foliage. The second plot received 500 g./h. as Herculite Corporation sprayable laminate flakes containing 9.1 ai. The flakes consisted of two layers of vinyl, each 0.08 mm thick on both sides of a central porous layer containing the disparlure the surface area of the flakes was between 7 and 35 mm2 per side. The same sticker as that in the microcapsules was used. The third plot received 330 g.a.i./h as "Conrel" controlled release hollow fibers containing nominally 11.5% ai. a suitable sticker was also incorporated in the formulation. (Note that the use of trade or proprietary names here or elsewhere does not constitute an endorsement by the USDA). 

The term microcapsule is defined, as a spherical particle with the size varying between 50 nm and 2 mm containing a core substance. Microspheres are, in a strict sense, spherically empty particles. However, the terms microcapsules and microspheres are often used synonymously. In addition, some related terms are used as well. For example, microbeads and beads are used alternatively. Spheres and spherical particles are also employed for a large size and rigid morphology. Due to attractive properties and wider applications of microcapsules and microspheres, a survey of their applications in controlled drug release formulations is appropriate. 

Sah, H. Toddywala, R. Chien, Y.W. TTie influence of biodegradable microcapsule formulations on the controlled release of a protein. J. Controlled Release 1994, 30, 201-211. 

Dibutyl sebacate is used in oral pharmaceutical formulations as a plasticizer for film coatings on tablets, beads, and granules, at concentrations of 10-30% by weight of polymer. It is also used as a plasticizer in controlled-release tablets and microcapsule preparations. 

Sodium chloride has been used as a lubricant and diluent in capsules and direct-compression tablet formulations in the past, although this practice is no longer common. Sodium chloride has also been used as a channeling agent and as an osmotic agent in the cores of controlled-release tablets. It has been used as a porosity modifier in tablet coatings,and to control drug release from microcapsules. 

Scher, H.B. Development of herbicide and insecticide microcapsule formulations. In Proceedings of International Symposium on Controlled Release of Bioactive Materials, 1985, pp. 110-111. 

However, functions of these smart textile stmctures could still be improved and their role optimized. Bringing the textile/microcapsule systems for controlled release of active agents to a higher level demands a multidisciplinary approach. Considering all aspects of the controlled release systems such as active agent formulations, microcapsule polymer wall composition, encapsulation approach, and embedding loaded microcapsules into textile structures could push the limits of smart textiles even further. 

However, these advantages have been known for some decades (in fact an early publication on a site-specific release formulation of an insecticide dates back to 1948 (3)) but their exploitation has been slow to develop in commercial practice. The first microcapsule formulation came on the market in 1974 (4) since then, uptake represents only a small portion of total pesticide formulations. This is in contrast to the drug sector where controlled release and delivery has been rapidly expanding. 

A variety of different formulations for controlled release polymers exist including microparticles, microcapsules and microspheres. Microparticles range in size from 1-200 pm, while particles with a diameter smaller than 1 pm are called nanoparticles. Microcapsules are microparticles which have the substance of interest enclosed in a shell of degradable polymer. Microcapsules however are characterized by a relatively fast release of large amounts of the enclosed substance. Microspheres (Figi 1), on the other hand, are monolithic in structure, Le. have the substance unifomdy distributed within the polymer layer. This distribution results in a more uniform release over longer periods of time. We selected such microspheies prepared from poly(lactide-glycolide) copolymers to develop our sensors. 

Also known as bhara gum, the exudate from the incised trunk of some tropical trees of Terminalia genus (T. bellerica Roxb., T. catappa L., T. randii Baker f.), Combretaceae family, may be used in the formulation of some sustained release pharmaceutical forms famotidine microcapsules (ionic gelation technique) [296] dextromethorphan hydrobromide tablets obtained by direct compression, wet granulation and solid dispersion techniques [297] carvedilol (water insoluble) and theophylline (water soluble) tablets prepared by direct compression - Terminalia gum was assessed as controlled release matrix against xanthan gum and hydroxypropyl methylcellulose [298]. 

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