Dissolution-controlled

Peppas, N. A., A model of dissolution-controlled solute release from porous drug delivery polymeric system, J. Biomed. Mater. Res., 17. 1079, 1983. 

Fig. 2 Two types of dissolution-controlled, pulsed delivery systems (A) single bead-type device with alternating drug and rate-controlling layers (B) beads containing drug with differing thickness of dissolving coats.

Microreservoir dissolution-controlled system in which microscopic spheres of drug reservoir are dispersed in a polymer matrix Nitrodisc (nitroglycerin Searle). 

Dissolution, of viscose, 11 254-255 Dissolution-controlled drug delivery, degradation/ erosion-based drug delivery systems, 9 11-19 Dissolution inhibition, by... 

The weathering of silicates has been investigated extensively in recent decades. It is more difficult to characterize the surface chemistry of crystalline mixed oxides. Furthermore, in many instances the dissolution of a silicate mineral is incipiently incongruent. This initial incongruent dissolution step is often followed by a congruent dissolution controlled surface reaction. The rate dependence of albite and olivine illustrates the typical enhancement of the dissolution rate by surface protonation and surface deprotonation. A zero order dependence on [H+] has often been reported near the pHpzc this is generally interpreted in terms of a hydration reaction of the surface (last term in Eq. 5.16). 

Keir, R.S., and R.L. Michel. 1993. Interface dissolution control of the C profile in marine sediment. Geochimica et Cosmochimica Acta 57 3563-3573. 

Growth or dissolution controlled by diffusion or heat conduction... 

Later, there is a period of sustained release, typically showing release proportional to indicating that the process of fluoride release is diffusion controlled [228]. Alternatively, in some studies, release has been shown to be proportional to t, which means that for those cements, release is dissolution controlled [232]. This sustained release phase is the one that prevails over long periods of time. For example, Forsten [225] carried out a study in which cement specimens were exposed to running tap water for 5 years, with short periods in static water to allow samples to be collected for analysis, after which they were returned to the running tap water. The data he obtained have been analysed by Billington et al. [233], who showed that the pattern of fluoride release right up to the end of the experiment (i.e., 5 years) continued to show a linear relationship with Fluoride release is thus diffusion controlled for a considerable period of time. 

Another important consequence of the constant rate of release diffusion model is that it mimics many of the features that have commonly been attributed to surface reaction (matrix dissolution) control. If one were to account for changes in surface area over time, the predicted long-term dissolution rate due to surface reaction control would also yield constant element release. In surface reaction controlled models, the invariant release rate with respect to time is considered to be the natural consequence of the system achieving steady-state conditions. Other features of experiments commonly cited as evidence for surface reaction control, such as relatively high experimental activation energies (60-70 kJ/ mol), could be explained as easily by the diffusion-control model. These findings show how similar the observations are between proponents of the two models it is only the interpretation of the mechanism that differs. 

As we have demonstrated, IEX reactions occur independent of the saturation state of the aqueous solution, raise the local solution pH, and so cause additional glass dissolution via reaction (6). The residual rate for alkali-rich glass compositions, therefore, is simply the net rate of glass dissolution controlled by the rate at which the ion-exchange reaction proceeds. 

If the growth rate of particles B is governed by the dissolution rate of particles A (dissolution-controlled growth), it must holds that KASA KBSB. In this case, Eq. (4) reduces to... 

Consequently, drA/dt is proportional to SA/SK for the dissolution-controlled growth, whereas it is independent of SA/SB for the deposition-controlled growth. Obviously, information on the growth mechanism is obtained only in the case of deposition-controlled growth. 

Differences in solubility between different crystal forms alter the driving force for dissolution, controlled by the difference between the solution concentration and the saturation concentration (Cs - C). Hamlin et al. (1965) have shown that dissolution rate correlates well with solubility for a large number of pharmaceutical compounds varying in solubility from 0.01 to 10 mg/mD t 37 Nicklasson and Brodin (1984) have shown that using cosolvent mixtures fordrugs with poor aqueous solubility produces a good correlation between dissolution rate and solubility. 

Several known mechanism-based approaches, such as dissolution-controlled, diffusion and/or erosion-controlled, combination of dissolution and diffusion-controlled, and osmotically-controlled... 

For water-insoluble drugs, dissolution-controlled systems are an obvious choice for achieving sustained-release because of theirslow dissolution rate characteristics. Theoretically, the dissolution process at steady state can be described by the Noyes-Whitney equation as shown in Equation 22.7. The rate of dissolution of a compound is a function of surface area, saturation solubility, and diffusion layer thickness. Therefore, the rate of drug release can be manipulated by changing these parameter. 

Delivery systems based on dissolution controlled 156 release coated technologies... 

The dissolution controlled release matrix systems provide sustained release profiles i.e., the active drugs in these systems are released continuously at a slow rate to provide a long-term therapeutic effect. Unlike diffusion controlled release coated systems, release profiles from dissolution controlled release coated systems do not follow zero-order kinetics but fall within the classification of delayed release systems,4 pulsatile or repeat-action systems,5 and sustained release systems.3... 

Although examples of delivery systems using the parenteral and oral (solid) routes are presented in this chapter, application of dissolution controlled release matrix and coated systems concepts can extended easily (and has been) used for many other delivery routes. 

Theoretical Considerations for Dissolution Controlled Release Matrix and Coated Systems... 

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