Diffusion reservoir-type

Buccal dosage forms can be of the reservoir or the matrix type. Formulations of the reservoir type are surrounded by a polymeric membrane, which controls the release rate. Reservoir systems present a constant release profile provided (1) that the polymeric membrane is rate limiting, and (2) that an excess amoimt of drug is present in the reservoir. Condition (1) may be achieved with a thicker membrane (i.e., rate controlling) and lower diffusivity in which case the rate of drug release is directly proportional to the polymer solubility and membrane diffusivity, and inversely proportional to membrane thickness. Condition (2) may be achieved, if the intrinsic thermodynamic activity of the drug is very low and the device has a thick hydrodynamic diffusion layer. In this case the release rate of the drug is directly proportional to solution solubility and solution diffusivity, and inversely proportional to the thickness of the hydrodynamic diffusion layer. 

Figure 4.2 Released amount Qt versus time t plots. Illustration of time lag tL and burst effect tB in a reservoir-type diffusion-controlled drug delivery system.

As with oral diffusion-controlled systems, there are two basic designs for transdermal diffusion-controlled systems matrix-type and reservoir-type systems. The matrix-type systems can be further classified as... 

Diffusion controlled drug elution. Left panel shows a matrix diffusion type, the right panel shows a reservoir type diffusion control. Source From Ref. 34. 

In reservoir-type TDS, heat scalability is an additional necessary condition. The laminated film, Al/EVA and PET/PE, is the typical backing material, where Al and PET are in charge of drug storing, whereas EVA and PE are responsible for heat sealability. With these materials, there is still a possibility that drugs and nonactive ingredients dissolve in EVA and PE from the reservoir, and diffuse to the peripheral part of the system. 

Most research on controlled release polymeric systems has, however, centered on compositions in which a drug is either encapsulated in the center of a polymeric membrane (reservoir type) or dispersed throughout the polymer (monolithic type). The drug diffuses through the polymeric material to the surface where it is released to the body fluids. Such systems have been used to give... 

For the reservoir-type di sion device the rate of diffusion dm/di) can be calculated using Eqn (6.1),... 

With the above assumptions, the cumulative amount Q of drug released from a diffusion-controlled reservoir-type drug delivery device with a unit surface area can be described as follows ... 

The design of biodegradable implant devices based on PLA and PGA, noted earlier, have been mainly limited to reservoir types where the drug, in suspension or crystalline state, is sealed within a polymeric capsule so that the rate of release is regulated by diffusion characteristics. Another approach involves the use of poly(a-amino acids) in which the drug is encapsulated in polymeric shells, intermixed with the polymer, or covalently bound to the polymer backbone. i- ... 

Diffusion systems are characterized by the release rate of a drug being dependent on its diffusion through an inert membrane barrier. Usually this barrier is an insoluble polymer. In general, two types or subclasses of diffusional systems are recognized reservoir devices and matrix devices. These will be considered separately. 

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