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Among other controlled drug delivery systems, hydrophilic matrix systems are the most popular ones due to their simplicity and ease in formulation, low cost and acceptability with wide range of drugs (Wen & Park, 2010). Polyethylene oxide (PEO) WSR and WSR-N are water soluble resins, used to produce [url=http://www.gotprintsigns.com/abercrombiepascher/‎]abercrombie soldes[/url] swellable-soluble hydrophilic colloidal matrix for the purpose of extended release tablets (Howard, 2004). The matrix formed from such polymer is capable of forming a swelled gel that is produced when the polymer comes in contact with the water (Parajapati et al., 2011). This [url=http://www.1855sacramento.com/moncler.php]moncler outlet[/url] is formed due to the cross [url=http://www.mquin.com/saclancel.php]lancel pas cher[/url] linking between the polymeric structure. This cross linking is due to hydrogen bonding between adjacent polymer chains, which limits the mobility of the polymer and give it a gel like structure. Portions of polymer can move, but overall, limit its degradation (Wilson & Crowly, 2011; Nie et al., 2005; Parajapati et al., 2011). This hydrated gel is then subjected to erosion and rate of release of drug from the tablet is dependent on the erosion of the tablet and diffusion of the tablet from the eroding mass (Anna et al., 2009). Various release patterns are obtained depending upon the drug's physicochemical properties [url=http://www.rtnagel.com/louboutin.php]louboutin[/url] and molecular mass of the PEO, making it a good candidate for control release tablets (Nie et al., 2005). Release retarding capability of a polymer also depends on the physicochemical property, particularly size, molecular weight, and solubility profile of the drug (Kiss et al., 2008). The type of polymer and drug used imparts [url=http://www.moncleroutletosterblade.com]moncler outlet[/url] a conspicuous effect on the mechanism of drug released from the polymer (Reza, Qadir & Haider, 2003).
In case a hydrophilic drug is being used in a hydrophilic polymer, its release rate will increase further as soon as the polymer is hydrated, especially when there is a high dose of drug involved. Polymer parameters known to influence the release rate of a drug are chemical properties of the polymer, hydrophilicity, its molecular weight, degree of cross linking, and degree of substitution (Anna et al., 2009). PEO of higher molecular weight (like WSR-303) are largely dependent on swelling for the controlled release of active moiety. This principle leads to decrease in diffusion of the drug in the swollen layer, and hence, a non-uniform release rate is imparted by the diffusion control (Kiss et al., 2008). In lower molecular weight PEOs, both swelling and erosion takes place to control the release of active moiety, and active's release rate is in strict control of polymer dissolution mechanism. (Pinto et al., 2004). The combination of two blends of the polymer can give intermediate results (Kojima et al., 2008). The stationary condition, where a swelling rate equals the rate of dissolution is the target of the sustained release drug. This ensures uniform release rate of the drug (Apiecella et al., 1993).
Liquid penetration and interaction between the polymer [url=http://www.jordanpascherofficiele.com]air jordan pas cher[/url] molecules are the basic rate limiting step (Kojima et al., 2008). PEO requires excessive amount of water to hydrate and form gel. Water content in GIT is different at different sites. [url=http://www.mxitcms.com/abercrombie/]abercrombie[/url] Consequently, PEO sometimes give erratic [url=http://www.jordanpascherofficiele.com]air jordan[/url] absorption invivo, even though it gives consistent results during invitro experiments (Sako et al., 1996). In some formulations, PEO is accompanied with hydration promoters to increase its hydration tendencies. In a study, PEO/PEG matrixes have shown rapid water uptake by PEG and resulting in complete gelation of PEO within few hours. Various other studies have shown that it is possible to obtain different release rates of drugs by altering the molecular weight of the polymer matrix (Apeicella et [url=http://www.sandvikfw.net/shopuk.php]hollister outlet sale[/url] al., 1993). Combination of hydrophobic carrier with hydrophilic polymer in a tablet formulation, also effectively control the initial rapid release of a highly water soluble drug (Nanjawade et al., 2011).
A study has shown increased crystallinity and volume relaxation in the two types of the PEO, even after a short storage interval of four weeks. This factor highlights the importance of storage condition in the PEO containing dosage forms (Kiss et al., 2008). The effect of this physical ageing can be observed in other polymers such as lipids etc., but not in PEO. Moreover, thermal stability studies of the PEO have shown that the chemical stability of a polymer is dependent on its processing and storage temperature, chemical properties of the polymer and also on its molecular weight (Maggie et al., 2002). WSR-PEO is more thermo-stable than WSR-N (Affify & Hay, 1972).Therefore, PEO grade selected should present a desirable rate of release of drug, with inherent physicochemical properties that help in attaining and maintaining a high gel state over a specified period of time. Since PEO is non-ionic, [url=http://www.mnfruit.com/louboutinpascher.php]louboutin[/url] it is non-irritant, highly stable and highly soluble polymer, which makes it ideal for the use in tablet preparation (Kiss et al., 2008).
The gel that is formed during the hydration of the hydrophilic matrix is a viscous structure, the viscosity of which depends on parameters like molecular weight, hydrophilicity, etc. Concentration of used matrix also [url=http://www.mxitcms.com/tiffany/]tiffany[/url] produces effect on the threshold viscosity on the surface of the gel. (Anna et al., 2009) A study reveals that the polymer release rate will alter with altering molecular weight of the polymer and agitation of [url=http://www.louboumaterialistanyc.com]louboutin pas cher[/url] the mixture, whereas, molecular weight will not produce much effect on dissolution of the solid core (Korner et al., 2010). This study also supports that polymers of higher molecular weight (like PEO WSR-303) will produce more viscous gel structure and will limit the release of the drug more effectively than lower molecular weight polymers. Thus, the average intrinsic viscosity of a polymer can easily predict the release rate from a tablet of given polymer (Anna et al., 2009). The result of another study show that the gel formed with low molecular weight PEO as WSR-N 750, 10, and 80, is usually very thin and the release rate is between 2-3hrs. The release rate of WSR category ranged from 9-14 hrs (Anna et al., 2009). The release rate of long and short chain polymers is found to be equal in the same study, as there was not much difference in their viscosities due to negligible polymer-polymer movement (Korner et al., 2005).
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