Microencapsulation, controlled release

Freitas, S., Merkle, H. P., and Gander, B. (2004), Ultrasonic atomisation into reduced pressure atmosphere—envisaging aseptic spray drying for microencapsulation, /. Controlled Release, 95,185-195. 

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. 

Muramyl dipeptide derivatives have also been microencapsulated in lactide/glycolide copolymers for use alone as an immuno potentiator. L-lactide/glycolide copolymers were used to deliver MDP-B30, a lipophilic compound, from very small microspheres (less than 5 pm in diameter). The amount of MDP-B30 required for tumor growth inhibitory activity of mouse peritoneal macrophages was 2000 times less for the controlled release MDP-B30 microspheres than for the unen-capsulated drug (134). 

Lin, S. Y., Ho, L. T., Chiou, H. L., Microencapsulation and controlled release of insulin from polylactic acid microcapsules. Biomater. Med. Devices Artif. Organs. 86, 187, 1985. 

The microencapsulation and controlled release of nucleic acids, e.g., poly(I C), for the stimulation of interferon production has been patented (87). 

Yuksel N, Turkoglu M, Baykara T. Modelling of the solvent evaporation method for the preparation of controlled release acrylic microspheres using neural networks. J Microencapsulation 2000 17 541-51. 

Microencapsulation and characterization of tramadol-resin complexes. Journal of Controlled Release, 66, 107-113. 

Sol-gel microencapsulation in silica particles shares the versatility of the sol-gel molecular encapsulation process, with further unique advantages. Sol-gel controlled release formulations are often more stable, potent and tolerable than currently available formulations. The benefits of microencapsulation can be customized to deliver the maximum set of benefits for each active ingredient. Overall, these new and stable combinations of active pharmaceutical ingredients (APIs) result in improved efficacy and usability. 

Microencapsulated phase-change-material (PCM) slurry, 13 276 Microencapsulation, 16 438-463. See also Encapsulation processes controlled release pesticide applications, 7 561-566... 

Tablets, controlled-release 750 mg microencapsulated potassium chloride equivalent to 10 mEq potassium (Rx) K-Dur 10 (Key), Ten-K (Summit)...

Capsules, controlled-release 600 mg potassium chloride equivalent to 8 mEq potassium. Microencapsulated particles (Rx) Micro-K Extencaps (Robins)... 

Uludag H, Hwang JR, Sefton MV. Microencapsulated human hepatoma (HEPG2) cells— capsule-to-capsule variations in protein secretion and permeability. J Controlled Release 1995 33 273-283. 

Sugar-coated products have been marketed that contain KCl in a wax matrix (Slow-K and Kaon-Ct) and are purportedly slow- and controlled-release preparations. Available evidence indicates that these slow-release forms of KCl are occasionally capable of causing local tissue damage and therefore prol5ably should be used with caution for K+ supplementation. Solutions of potassium gluconate, like the tablets, also have been associated with intestinal ulceration. Microencapsulated KCl preparations Micro-K, K-Dur) that are neither enteric coated nor contained within a wax matrix appear to be superior to the wax matrix formulation. 

Several other investigators have reported microencapsulation methods based upon polyelectrolyte complexes [289, 343]. For example, oppositely-charged polyelectrolytes (Amberlite IR120-P (cationic) and Amberlite IR-400 (anionic)) were recently used along with acacia and albumin to form complex coacervates for controlled release microcapsule formations [343]. Tsai and Levy [344,345] produced submicron microcapsules by interfacial crosslinking of aqueous polyethylene imine) and an organic solution of poly(2,6 dimethyl... 

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. 

One particular example of controlled release is sustained release. In this form the desired material is continuously released over a period of time at a constant rate. Two timely publications (3)(4) cover the general area of controlled release, which can also include the controlled release of agricultural materials and biological materials, far example, pheromones. In using the term microencapsulation in this article, the author intends to refer to capsules in the size range of 1 micron to 1000 microns. Capsules below 1 micron in size are frequently referred to as nanocapsules and they are made by one or more very specialized methods (5). The term capsule refers to macro objects in the order of 1 millimeter or larger. This term of capsule is frequently used in the delivery of pharmaceuticals. 

K. Aiedeh, E. Gianas, I. Orienti, V. Zecchi, Chitosan microcapsules as controlled release systems for insulin, J. Microencapsulation 14 567-576 (1997). 

Microencapsulation can be used to provide a temporary barrier between a chemical species and its surrounding environment see also Section 14.3). This permits controlled (slow) release of the active agents following application. Depending on the product and the situation, an active ingredient such as a pesticide may need to be released slowly at low concentration, or slowly at high concentrations. Such controlled release can both reduce the number of crop applications that are required and also help prevent over use and subsequent run-off. The barrier can be provided by a polymer film, in the case of suspensions [867], or a liquid membrane, in the case of single or multiple emulsions [865], Microemulsions have also been used [234,865],... 

Price, R.R., Patchen, M., Rittschof, D., Clare, A.S., and Bonaventura, J., Performance enhancement of natural antifouling compounds and their analogs through microencapsulation and controlled release, Biofouling, 6, 207, 1992. 

Microencapsulation has been the subject of massive research efforts since its inception around 1950. Today, it is the mechanism utilized by approximately 65% of all sustained-release systems [35], The technique s popularity can be attributed mainly to its wide variety of applications. Hundreds of drugs have been microencapsulated and used as controlled-release systems. Some examples are Arthritis Bayer, Dexatrim Capsules, and Dimetapp Elixir. 

Mathir, Z. M., Dangor, C. M., Govender,T., and Chetty, D. J. (1997), In vitro characterization of a controlled-release chlorpheniramine maleate delivery system prepared by the air-suspension technique, / Microencapsul., 14,743-751. 

Lam, X. M., Duenas, E.T., and Cleland, J. L. (1998), Stabilization of nerve growth factor during microencapsulation and release from microspheres [abstract], Proc. Int. Symp. Controlled Release Bioactive Mater., 25,491. 

Bakan JA (1980) Microencapsulation Using Coacervation/phase Separation Techniques in Controlled Release Technologies Method, Theory, and Applications (Ed Kydoneius AF), CRC Press, Florida... 

Another area of concern is the residual life of a pesticide once it is brought back to the hive. Microencapsulated methyl parathion was at one time believed to represent a special hazard because of its controlled release feature. Thus methyl parathion from MMP was reported to persist in stored pollen for up to 17 months.( ) Unfortunately, little is known about the persistence of insecticides in honey bee combs etnd the subsequent effects of their residues on the honey bees. Carbaryl has been shown to persist for at least eight months in colonies ( ) euid permethrln for at least seven months.O) Recently, USDA researchers at the University of Wisconsin studied samples from two bee kills that apparently Involved methomyl and MMP applied to sweet com. Samples were collected to determine, among others, whether methomyl persisted in combs. Analysis demonstrated that eight months after the insecticide application, residues of 0.03 ppm of methyl parathion and 0.03 ppm of methomyl ( 5) remained, even though the latter is considered to be a short-residual pesticide. 

Two distinct controlled release technologies are encapsulation of liquid pesticides and the coating of individual pesticide crystals. Encapsulation of liquid pesticides is an established tool for modem formulators. Commercial microencapsulated pesticide products exist and new developments continue to be made. Coating of individual pesticide crystals without their aggregation is more difficult. While new processes do exist to coat pesticide crystals without aggregation these processes have not yet been utilized to create commercial pesticide products. 

R. E. Sparks, I. C. Jacobs, Selection of Coating And Microencapsulation Processes, Controlled Release Delivery Systems, Marcel Dekker, Inc., USA, 1999, pp. 3-29. 

In addition to microelectronic and optical applications, polymers deposited using thermal and plasma assisted CVD are increasingly being used in several biomedical applications as well. For instance, drug particles microencapsulated with parylenes provide effective control release activity. Plasma polymerized tetrafiuoroethylene, parylenes and ethylene/nitrogen mixtures can be used as blood compatible materials. An excellent review of plasma polymers used in biomedical applications can be found in reference 131. 

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