Retarded release

Methods were described for the incorporation of proteins in the form of noncovalent complexes with polycationic reagents, into sustained release systems where the polycation stabilizes the protein against inactivation while it resides in the delivery device, and retards release of the protein from the delivery device [469,470]. A variety of polycations have been used, including simple polyamino acids such as polylysine or polyarginine, protamine and chitosan. The end result was the release of the active agent with retention of biological activity, with a high cumulative field and over a sustained period of time. 

Drug release profiles from the tablets in various dissolution media are shown in Fig. 2. In all cases the release rates decreased initially from the control (distilled water) as electrolyte concentration increased, until a minimum release rate was obtained. As the electrolyte concentration further increased the release rates similarly increased until a burst release occurred. These initial decreases in release rates were probably coincident with a decrease in polymer solubility, in that as the ionic strength of the dissolution medium is increased the cloud point is lowered towards 37°C. It may be seen from Table 5 that minimum release rates occurred when the cloud point was 37°C. At this point the pore tortuosity within the matrix structure should also be at a maximum. It is unlikely to be an increase in viscosity that retards release rates since Ford et al. [1] showed that viscosity has little effect on release rates. Any reduction in hydration, such as that by increasing the concentration of solute in the dissolution media or increasing the temperature of the dissolution media, will start to prevent gelation and therefore the tablet will cease to act as a sustained release matrix. 

Total S content cannot indicate whether increased carbon inputs to sediments cause increased diffusion of sulfate into sediments or restrict reoxidation and release of S from sediments, because the net effect is the same. In a survey of 14 lakes, Rudd et al. (80) did not observe a strong correlation between organic matter content per volume and net diffusive flux of sulfate. However, in English lakes the lowest C S ratios occur in the most productive lakes (24) whether this represents enhanced influx or retarded release is not clear. Among 11 Swiss lakes, ratios of C to S sedimentation rates are relatively constant and substantially below C S ratios in seston net S fluxes... 

It was shown that liposomes, due to their structure, have a retarding effect on the incorporated drug release. In early studies, Knepp et al. reported that progesterone release from agarose gel was faster than from liposomes embedded in the gel [29]. This retarding release behavior from liposomes was further confirmed by a lower drug transport rate as compared to the gel measured across hairless mouse skin [30], Another study by Foldvari et al. [8] examined the... 

The diffusion equations just used are simplifications of more complex processes. The F factor was empirically derived and must take into account those matrix pore geometric factors contributing to decreases in diffusion rates. Such factors may include pore tortuosity, dead-end pores, and pore constrictions. Initial modeling studies suggest that constrictions, in particular, have large effects in retarding release (8,9). 

Consequences of change 1 Increases bioavailability 2 Enables possibly liquid formulation 1 Suspensions physically more stable (reduced rate of particle growth) 2 Solid dosage forms retarded release... 

Impregnated Activated Carbons (lAC) Activated carbons impregnated with organic molecules (often from supercritical C02 atmosphere) to provide anchor sites for (big) biochemical molecules Substrates for retarded release of organic or pharmaceutical substances Removal of heavy metal ions from solutions [1.36]... 

Collagen-based hydrogels as drug delivery systems exhibit an ability to entrap different size molecules and also show advantages such as ease of application, high bioadhesion, acceptable viscosity, compatibility with numerous drugs contributing to patients compliance and product efficiency, controllable retarded release, etc. [61,93],... 

Blends of pectin with chitosan have been studied for use in nasal [39] and oral [10] drug delivery systems. The crosslinked mixtures of pectin with high amylose starch exhibited suitable performance in retarding release rates [43]. 

Recently, Chen and Du designed a novel polymer vesicle that respond to both physical [ultrasound] and chemical [pH] stimuli based on a PEO-b-P[DEA-stat-TMA] block copolymer, [PEO— poly[ethylene oxide] DEA—2-[diethylamino]ethyl methacrylate TMA—[2-tetrahydrofuranyloxy]ethyl methacrylate] [180], These dually responsive vesicles exhibiting retarded release profile and controllable release rate when subjected to ultrasound radiation or varying pH. 

Thus far in this chapter, we have described diffusion as a means of controlling or retarding release of solutes like drugs or flavors. These solutes may initially be present as a solution or a suspension. If the suspension dissolves quickly, diffusion is key. We now turn to other cases of diffusion in two-phase systems. Like controlled release, these cases can show dramatic and unexpected behavior. Like controlled release, this behavior is the consequence of rapid chemical reactions. 

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