Controlled Release and Related Phenomena

The actual release tends to occur from two types of systems. The first is a microcapsule. In its simplest form, this is just a small bubble with a solid, usually glassy, shell and a fluid or gel core. Because diffusion of the active species is much slower in the shell than in the core, release is often controlled by diffusion of the active species through the shell. In the second type of system, the active species is distributed throughout sohd particles. Release is controlled either by diffusion out of the active species or because of diffusion in a solvent, commonly water, which releases the active species. 

In this chapter, we have divided our discussion as cases of diffusion out of the active species, and of diffusion in solvent. More specifically, in Section 19.1, we describe cases 

We should stress that some cases of controlled release are not regulated by diffusion and are not detailed in this chapter. Some involve release controlled by chemical kinetics. For example, imagine a drug suspended in a water-insoluble polymer. The polymer slowly reacts with water and then dissolves. As the polymer dissolves, the drug will be released at a rate controlled by the polymer s hydrolysis. Alternatively, imagine an implantable pump that releases a drug in response to the patient s demand. While these cases involve controlled release, neither is controlled by diffusion, so neither is discussed here. 

Other applications of diffusion are also discussed in this chapter. In Section 19.3, we discuss diffusion barriers. These are the antithesis of controlled release because our objective is to retard diffusion, rather than regulate it. The most obvious method of retardation is to choose the barrier so that it is impermeable to any solute. For example, 

Single compound Organic and inorgani solids and liquids M 200 Zero order or burst 

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Diffusion is the main regulation phenomena for controlled release in stable matrix. The diffusion process is well known, thanks to thousands of experimental studies related to mass transfer through films, coatings, and capsules. The macromolecular network density of the matrix and the molecular weight of the encapsulated compounds are the key factors for release controlled by diffusion. The chemical potential difference at each side of the matrix is the major driving force influencing diffusion. The principal steps in the release of a compound from a matrix system are diffusion of the active agent to matrix surface component partition between matrix and food and finally, transport away from the matrix surface. 

If LRF and FRF acts only on the release of LH and FSH, how is the fact explained that synthesis of these hormones is enhanced in the pituitary incubated with RF s Several authors reported recently (20, 21, 22, 23) that intracarotid or intrajugular injection of a hypothalamic extract or a purified FRF preparation in the rat induced, 15 to 45 min after treatment, a depletion of pituitary FSH, This initial effect was followed by the resynthesis of this hormone in the pituitary (22), It is possible consequently that the phenomena observed in vitro in our work are composite results RF s inducing the release of gonadotropins, which in the second stage, are resynthesized in the tissue. Both phenomena of synthesis and release of gonadotropins are probably closely related the resynthesis being consequential of the release. If so, the precise mechanism that controls the synthesis of gonadotropins still remains to be discovered,... 

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