Silicone elastomer matrices

The release of lipophilic steroids from silicone elastomer matrices is dependent on the cross linking density of the polymer and the content of filler, but also on the lipophilicity of the dmg. A relationship between the solubility parameters of a number of dmgs and their release rate is shown in Fig. 8.27. 

Nonoxynol-9 is an approved spermicide with strong antiviral activity. A vaginal device which facilitates the controlled release of nonoxynol-9 has been developed for contraceptive and anti-STD purposes. The device, available as a diaphragm or a disk pessary, is fabricated from silicone elastomer matrix system. The drag release profile demonstrates square root time kinetics (M co tV2) (see Section 4.4.2). 

Snorradottir, B.S., Gudnason, P.J., Scheving, R. et al. 2009. Release of anti-inflammatory dmgs from a silicone elastomer matrix system. Pharmazie. 64 19-25. 

Although a majority of these composite thermistors are based upon carbon black as the conductive filler, it is difficult to control in terms of particle size, distribution, and morphology. One alternative is to use transition metal oxides such as TiO, VO2, and V2O3 as the filler. An advantage of using a ceramic material is that it is possible to easily control critical parameters such as particle size and shape. Typical polymer matrix materials include poly(methyl methacrylate) PMMA, epoxy, silicone elastomer, polyurethane, polycarbonate, and polystyrene. 

Orug/Silicone Fluid Suspension e Drug/Sllicone Elastomer Matrix... 

Stress-strain properties for unfilled and filled silicon rubbers are studied in the temperature range 150-473 K. In this range, the increase of the modulus with temperature is significantly lower than predicted by the simple statistical theory of rubber elasticity. A moderate increase of the modulus with increasing temperature can be explained by the decrease of the number of adsorption junctions in the elastomer matrix as well as by the decrease of the ability of filler particles to share deformation caused by a weakening of PDMS-Aerosil interactions at higher temperatures. 

Considerable effort has been spent to explain the effect of reinforcement of elastomers by active fillers. Apparently, several factors contribute to the property improvements for filled elastomers such as, e.g., elastomer-filler and filler-filler interactions, aggregation of filler particles, network structure composed of different types of junctions, an increase of the intrinsic chain deformation in the elastomer matrix compared with that of macroscopic strain and some others factors [39-44]. The author does not pretend to provide a comprehensive explanation of the effect of reinforcement. One way of looking at the reinforcement phenomenon is given below. An attempt is made to find qualitative relations between some mechanical properties of filled PDMS on the one hand and properties of the host matrix, i.e., chain dynamics in the adsorption layer and network structure in the elastomer phase outside the adsorption layer, on the other hand. The influence of filler-filler interactions is also of importance for the improvement of mechanical properties of silicon rubbers (especially at low deformation), but is not included in the present paper. 

In this paper, the thermal stability of the silicone elastomers, base silicone resins, fillers, and their interactions with each other within the silicone matrix are described. Thermal decomposition volatiles, obtained indirectly through solvent extractables, reaction kinetics of the materials as integrated circuit (IC) devices encapsulants will be discussed. 

Finally, the iodine salt escapes from the matrix through the water-filled interconnecting network of cracks and cavities which is generated by this mechanism. The release rate of Nal is influenced by the mechanical properties of the silicone elastomer, the size and the shape of the matrix, the initial loading of iodine salt and the size distribution of the salt particles. 

A new approach to combating iodine deficiency through controlled release of iodide by a silicone elastomer has been reported (Fisch et ai, 1993). Silicone elastomers or silicone rubbers made from cross linked polymers are reinforced with silica. Silicones, being chemically inert, are safe and stable over a wide range of temperatures and have a high permeability to small molecules. Sodium iodide, being a small molecule, can be intercalated into the silicone matrix from where iodine is released in a controlled manner when water is passed through it. 

The formulations described in this paper were prepared using a dispersion disk and degassed In vacuum. The basic formulation of the resin matrix consisted of 33 parts of cycloaliphatic diepoxid, 33 parts of cresolnovolac epoxide, 33 parts of cycloaliphatic epoxy diluent, 0.5 parts of silane and 0.5 parts of photolnltlator (resin A). In addition fused silica 150 parts (resin B), 186 parts (resin C), 233 parts (resin D) was added. In the toughened mixture (resin E) the cycloaliphatic epoxide of resin D was substituted by 41.7 parts of a dispersion of 20Z silicone elastomer In epoxy resin. In the flexlbilized modification (resin F) 33 parts of epoxldlsed soya bean oil were added to resin D. 

They have been successfully utilized in a variety of forms including low and intermediate molecular weight fluid polymers and matrix polymers for silicone elastomers. This paper focuses particularly on a few interesting high performance and processing aspects of... 

Thermal analyses have been used for over 20 years to characterise heart-valve poppets (silicone elastomer) implanted into humans [29] and to evaluate the in vivo absorption of lipids and related compounds fix)m the body. Important deleterious effects on the implants, observed under in vivo conditions include changes in the hardness and mechanical properties, which appear to be associated with colour changes from white to yellow then brown. Because the thermal curves of the samples extracted from new and unused implants were almost identical, the differences were attributed to materials absorbed into the polymer matrix. Thermal analyses may therefore be useful in understanding the behaviour of these poppets in vivo. Table 1 shows the thermoanalytical data for new and used poppets, with those used poppets differing in terms of the extent extracted materials in vivo. 

Insoluble inserts are polymeric systems into which the drug is incorporated as a solution or dispersion. Ophthalmic inserts (ocuserts) have been reported using alginate salts, poly(A-vinyl pyrrol-idone), modified collagen, and hydroxyl propyl methyl cellulose. Ocufit is a silicone elastomer-based matrix that allows for the controlled release of an active ingredient over a period of at least 2 weeks. Osmotically controlled inserts have also been described, where release is by diffusion and is osmotically controlled. 

One particular hydrophobic polymer, EVAc, has been investigated extensively as a matrix system for protein delivery. This polymer is biocompatible, a major consideration because of the interest in developing systems for human health. Other classes of hydrophobic polymers, like silicone elastomers and polyurethanes, may also be useful for controlled protein delivery, although there are fewer examples available in the literature. Nondegradable, hydrophilic polymers, such as poly(2-hydroxyethyl methacrylate) [p(HEMA)], are also biocompatible but usually release proteins over a relatively short period. However, a few examples oflong-term release of peptides and proteins from hydrophilic polymers are available. Longterm release of peptides from devices that employ cross-linked p(HEMA) as rate-limiting barriers has been reported (Davidson et al, 1988). The use of hydrophilic polymers for protein release is discussed in more detail elsewhere in this volume. 

One of the simplest approaches relies on the realisation of composite materials by filling an ordinary elastomer with a highly dielectric component (e. g. ceramics), it is possible to obtain a resulting material showing the combination of the advantageous matrix elasticity and filler permittivity. As an example, promising results have been obtained with a silicone elastomer mixed with a titanium dioxide powder [284]. 

A system of continuous diffusion of iodine into the drinking water supply from boreholes (RHODIFUSE system) was tested in a zone of severe endemic goiter in Mali. The controlled release system consists of a matrix formed of a silicone elastomer, containing a dispersion of Nal in powder form, with a duration of action of one year. 

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