Matrix-Type Membranes

Transport signals can be of the import or the export type. Import signals are contained in proteins that are transported into the individual compartments of mitochondria (matrix, inner membrane, intramembrane compartment, outer membrane), peroxisomes (lumen, boundary membrane) and into the interior of... 

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 2. Experimental system for the measurement of membrane potential by a PVC matrix liquid-membrane-type electrode (reprinted with permission from Yakugaku Zasshi 1995, 115, 432. Copyright 1995 The Pharmaceutical Society of Japan).

Filters can be divided into two types membrane (screen) filters and depth filters. Membrane filters, such as silver membrane filters, physically screen and retain particles on their surfaces. These filters have uniform pore sizes and are rated for absolute retention all particles larger than the pore size are retained. Depth filters, such as glass-fiber filters, consist of a matrix of fibers that form a tortuous maze of flow channels. The particulate fraction becomes entrapped by this matrix. These filters do not have a uniform pore size, and it is not possible to rate them for absolute retention. They are rated according to nominal pore size, which is determined by the particle size that is retained by the filter to a predetermined percentage. This percentage is usually given as 98 retention however, it can be as low as 90. ... 

The mechanical stability and ion exchange capacity of these condensation resins were modest. A better approach is to prepare a suitable crosslinked base membrane, which can then be converted to a charged form in a subsequent reaction. Ionics is believed to use this type of membrane in many of their systems. In a typical preparation procedure, a 60 40 mixture of styrene and divinyl benzene is cast onto a fabric web, sandwiched between two plates and heated in an oven to form the membrane matrix. The membrane is then sulfonated with 98 % sulfuric acid or a concentrated sulfur trioxide solution. The degree of swelling in the final membrane is controlled by varying the divinyl benzene concentration in the initial mix to control crosslinking density. The degree of sulfonation can also be varied. The chemistry of the process is ... 

To obtain a high selectivity, i.e., discrimination between the analytes and various unwanted matrix compounds, membrane extraction has a clear advantage over other sample preparation techniques, as all compounds that reach the analytical instalment must travel through the membrane. There is no direct connection and possibility for transferring compounds into the analytical instmment in other ways. This is not the case with other extraction techniques. With SPE, SPME, etc., there is a definite possibUity that matrix components are absorbed on the sorbing phase and subsequently being eluted into the extract. With LEE, such a transfer is less probable and it is generally considered that extracts after LLE are cleaner. The possible and common problem of the formation of emulsions at the phase interface with LLE, which is avoided with aU types of membrane extraction, is a source of contamination across the phase border. 

Fig. 1 Schematic of the three types of zeolite membranes (A) a polycrystalline zeolite membrane (B) a zeolite matrix composite membrane and, (C) a zeolite crystal layer.

The microsealed delivery device is a variation of the matrix-type transdermal system in which the drug is dispersed in a reservoir phase which is then immobilized as discrete droplets in a cross-linked polymeric matrix. Release can be further controlled by inclusion of a polymeric microporous membrane. This system therefore combines the principles of both the liquid reservoir and matrix-type devices. Rate of release of a drug from a microsealed delivery system is dependent on the partition coefficient between the reservoir droplets and the polymeric matrix the diffusivity of the drug in the reservoir, the matrix and the controlling membrane and on the solubility of the drug in the various phases. There are, obviously, many ways to achieve the desired zero-order release rate, but only nitroglycerin has been commercially formulated into this type of delivery device (Karim 1983). 

Matrix-type delivery systems are simple to make release is usually controlled by diffusion of drug through the polymer matrix. Mathematical descriptions of release are more complicated than are obtained for membrane-type devices, and it is difficult to produce devices that provide a constant rate of release. However, these materials are versatile and almost any compound can be formulated into a controlled-release matrix. 

Separation of different organic components from each other is still a matter of laboratory investigation. In the past 15 years considerable efforts have been devoted to develop polymeric membranes to separate, for example, aromatic hydrocarbons from aliphatic ones which resulted in several patents [25, 26], or olefins from paraffins or to separate isomers, e.g. para- and ortho-xylenes, from each other. In the last years additional membranes [27] have become available and the first industrial applications have been reported, e.g. the separation of sulfur-containing aromatics from gasoline [28] and of benzene from a stream of saturated hydrocarbons [29], Further development of membranes, especially of the mixed-matrix type, may lead to improved selectivity and a broadening of these applications. 

Nanoparticles are classified into two groups according to preparation techniques, which are nanocapsules and nanospheres (see Figure 11.3). Nanocapsules are vesicular systems enveloped with a polymeric membrane film. The active substances are encapsulated in the inner core. Nanocapsules consist of oily core and unilayer polymeric membrane or aqueous core and double layer polymeric membrane, called nanocapsule and polymersome, respectively. Nanospheres are matrix-type colloidal particles and they do not have an oily core, in contrast to nanocapsules. Nanoparticles can be prepared directly from cationic polymers such as chitosan, PEI or PLL. These cationic nanoparticles have been studied extensively for nucleic acid delivery in particular. 

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