title: Polymer Percolation Threshold in Multi-Component HPMC Matrices Tablets.
abstract: BACKGROUND
  The percolation theory studies the critical points or percolation thresholds of the system, where one component of the system undergoes a geometrical phase transition, starting to connect the whole system.The application of this theory to study the release rate of hydrophilic matrices allows to explain the changes in release kinetics of swellable matrix type system and results in a clear improvement of the design of controlled release dosage forms.
  
  
  METHODS
  In this study, the percolation theory has been applied to multi-component hydroxypropylmethylcellulose (HPMC) hydrophilic matrices. Matrix tablets have been prepared using phenobarbital as drug, magnesium stearate as a lubricant employing different amount of lactose and HPMC K4M as a filler and matrix forming material, respectively. Ethylcelullose (EC) as a polymeric excipient was also examined. Dissolution studies were carried out using the paddle method.In order to estimate the percolation threshold, the behaviour of the kinetic parameters with respect to thevolumetric fraction of HPMC at time zero, was studied.
  
  
  RESULTS
  In both HPMC/lactose and HPMC/EC/lactose matrices, from the point of view of the percolation theory, the optimum concentration for HPMC, to obtain a hydrophilic matrix system for the controlled release of phenobarbital is higher than 18.1% (v/v) HPMC. Above 18.1% (v/v) HPMC, an infinite cluster of HPMC would be formed maintaining integrity of the system and controlling the drug release from the matrices. According to results, EC had no significant influence on the HPMC percolation threshold.
  
  
  CONCLUSIONS
  This may be related to broad functionality of the swelling hydrophilic matrices.
authors: School of Pharmacy and Drug applied Research Center, Tabriz University of Medical Sciences, Tabriz, Iran.
authors: Maryam
authors: M
authors: Maghsoodi
authors: Leila
authors: L
authors: Barghi
keywords: HPMC; Percolation theory; Percolation threshold; Phenobarbital