Controlled-release materials

Sulfur-coated urea (SCU) is a controlled-release material developed by TVA during the 1960s and 1970s.53 Sulfur was selected as the coating material on the basis of economy and efficiency after many coating materials were tried. Urea was selected as the material to be coated because of its high nitrogen content, low cost, and commercial availability. 

All the observed morphologies (including the phase separated parachutes and necklaces) are potentially useful in a variety of applications. German et al. have proposed that necklaces composed of a variety of different polymeric beads could be useful as controlled release materials [20]. Most importantly, the implications of polymerization in vesicles can reveal more about the fundamental properties of vesicles as well as provide information about polymerization reactions in confined media. 

C. Carraher, H. Stewart, S. Carraher, D. Chamely, W. Learned, J. Helmy, K. Abey, A. Salamone, Condensation polymers as controlled release materials for enhanced plant and food production influence of gibbeieUic acid and gibbereUic acid-containmg polymers on food crop seed in Functional Condensation Polymers , CE. Carraher, GG. Swift, eds. Springer, Chap 16, 223-234, 2002... 

Huang, L. Tonelli, A. E. Inclusion Compounds as a means to Fabricate Controlled Release Materials, Intelligent Materials for Controlled Release, Dinh, S. M., DeNuzzio, J. D.,Eds., ACS Symposium series 728, 1999, Chap. 10, p. 131. 

Mt and M q are the absorbed mass at time t and after equilibrium has been reached, respectively, k is a constant and n is the exponent, which indicates the type of diffusion transport in the hydration process. The kinetics was also dependent on the compositions of the prepared formulations as n was close to 0.5 (indicating Fickian type behaviour) for hydrogels containing MMA. When analysing formulations without MMA it was foimd that n was close to unity indicating non-Fickian behaviour and case II water transport mechanism, which is the most desirable kinetic behaviour for a swelling-controlled release material. ... 

These transport properties, being unique for polymeric materials, are associated with several practical advantages. In fact, the polymeric host framework can be obtained by a cheap commercial polymer and, due to its macromolecu-lar nature, it is robust (chemical and thermal resistant), easy to process (can be easily shaped as him, hber, membrane, foam, aerogel, etc.), and presents excellent mechanical properties. Hence, as discussed in detail in the section 10.3.4, SPS-based polymeric frameworks are suitable candidates as repeated use gas storage and controlled release materials. 

Cosmetics and Pharmaceuticals. The main use of hexadecanol (cetyl alcohol) is in cosmetics (qv) and pharmaceuticals (qv), where it and octadecanol (stearyl alcohol) are used extensively as emoUient additives and as bases for creams, Hpsticks, ointments, and suppositories. Octadecenol (oleyl alcohol) is also widely used (47), as are the nonlinear alcohols. The compatibiHty of heavy cut alcohols and other cosmetic materials or active dmg agents, their mildness, skin feel, and low toxicity have made them the preferred materials for these appHcations. Higher alcohols and their derivatives are used in conditioning shampoos, in other personal care products, and in ingested materials such as vitamins (qv) and sustained release tablets (see Controlled RELEASE technology). 

Phase Separation. Microporous polymer systems consisting of essentially spherical, intercoimected voids, with a narrow range of pore and ceU-size distribution have been produced from a variety of thermoplastic resins by the phase-separation technique (127). If a polyolefin or polystyrene is insoluble in a solvent at low temperature but soluble at high temperatures, the solvent can be used to prepare a microporous polymer. When the solutions, containing 10—70% polymer, are cooled to ambient temperatures, the polymer separates as a second phase. The remaining nonsolvent can then be extracted from the solid material with common organic solvents. These microporous polymers may be useful in microfiltrations or as controlled-release carriers for a variety of chemicals. 

The rationale for the development of such fibers is demonstrated by their appHcation in the medical field, notably hemoperfusion, where cartridges loaded with activated charcoal-filled hoUow fiber contact blood. Low molecular weight body wastes diffuse through the fiber walls and are absorbed in the fiber core. In such processes, the blood does not contact the active sorbent direcdy, but faces the nontoxic, blood compatible membrane (see Controlled RELEASE TECHNOLOGY, pharmaceutical). Other uses include waste industrial appHcations as general as chromates and phosphates and as specific as radioactive/nuclear materials. 

Release coatings are important components of pressure sensitive adhesive (PSA) products such as tapes and labels [1]. Release materials are coated onto the backside of PSA tape backings (often called low adhesion backsizes or LABs in this form) to provide the desired tape roll unwind force. They are also coated onto various substrates to form release liners for PSA products such as labels and transfer tapes. Typically the thickness of the release coating is less than 1 p,m, and often times less than 0.1 jLm. Release coatings can be thought of as the PSA delivery system, providing a controlled unwind or release force and protecting the adhesive from contamination and unintentional contact until it is applied. 

The second line is the controlled release of materials through venting to containment/ confinement or through filters, scrubbers, flares, or overflow tanks. 

Microcapsules can be used for mammalian cell culture and the controlled release of drugs, vaccines, antibiotics and hormones. To prevent the loss of encapsulated materials, the microcapsules should be coated with another polymer that forms a membrane at the bead surface. The most well-known system is the encapsulation of the alginate beads with poly-L-lysine. 

Occasionally in the synthesis of the copolymers, insoluble material is produced. This results from polymer containing blocks of polyglycolide rather than the desired random structure. Obviously, such compositions would have considerable effect on the performance of controlled release formulations utilizing those polymers. This problem is particularly evident when one is seeking to utilize the 50 50 glycolide/lactide copolymer as a biodegradable excipient. However, with carefully controlled polymerization conditions, useful 50 50 polymer is readily produced. 

Developers of controlled release formulations have employed polymers produced from both L-lactide and Dl -lactide. In terms of clinical studies, however, it appears that perhaps the DL-lactide formulations have been somewhat more successful. It is unclear if this is due to the DL-lactide materials being less crystalline and more permeable to most drugs or perhaps more sophisticated techniques and... 

Baker, R., Controlled Release of Biologically Active Materials, John Wiley and Sons, New York, 1987. 

B. M., Price, M. W., and Stoner, W. C., Jr., Polymeric-pellet delivery systems for controlled release of aquatic herbicides, Toronto Controlled Release Society, Proc. 14th Int. Symp. Control. Rel. Bioact. Materials, 291-292, 1987. 

Pitt, C. G., Marks, T. A., and Schindler, A., Biodegradable drug delivery systems based on aliphatic polyesters application to contraceptives and narcotic antagonists, in Controlled Release of Bioactive Materials (R. Baker, ed.). Academic Press, New York, 1980, pp. 19-43. 

Jaffe, H., Hayes, D. K., Luthra, R. P., Shukla, P. G., Amar-nath, N., and Chaney, N. A., Controlled-release microcapsules of insect hormone analogues, Geneva, Switzerland Controlled Release Society, Proc. 12th Int. Symp. Control. Rei. Bioact. Materials. 282-283, 1985. 

Polymers used in medicine fall into two main categories those that are sufficiently inert to fulfill a long-term structural function as biomaterials or membranes, and those that are sufficiently hydrolytically unstable to function as bioeradible materials, either in the form of sutures or as absorbable matrices for the controlled release of drugs. For the synthetic organic polymers widely used in biomedicine this often translates to a distinction between polymers that have a completely hydrocarbon backbone and those that have sites in the backbone that are hydrolytically sensitive. Ester, anhydride, amide, or urethane linkages in the backbone usually serve this function. 

Kohn, J., and Langer, R., A new approach to the development of bioerodible polymers for controlled release applications employing naturally occurring amino acids, in Proceeding of the ACS Division of Polymeric Materials. Science and Engineering. American Chemical Society, 1984, Vol. 51, pp. 119-121. 

---