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Percolation theory has been applied in order to study the existence of critical points as well as the possibility to find a "combined percolation threshold" for ternary hydrophilic matrices prepared with HPMC, NaCMC, and theophylline. For this purpose, different batches of ternary as well as binary hydrophilic matrices have been prepared. Critical points have been found for binary hydrophilic matrices between 21.5 and 31.3% (v/v) of HPMC and between 39 and 54% (v/v) of NaCMC, respectively. In a previous work carried out with the same polymers but a much more soluble drug (KCl), it was demonstrated the existence of a partial collaboration between the polymers in order to establish the gel layer. In this work, it has been observed for the first time the need of a minimum concentration of one of the matrix-forming polymer (between 10 and 20% v/v, approximately) for establishing an effective collaboration.

The purpose of the present work was to study the existence of critical points on the drug release and water uptake behaviour of ternary hydrophilic matrix tablets and to study the possibility of simplifying a ternary to a binary system. The ternary hydrophilic matrix tablets were prepared between 40 and 100% (w/w) of KCl, HPMC and NaCMC. Dissolution studies were carried out using the paddle method and the water uptake studies were measured using the modified Enslin apparatus and the behaviour of the kinetic parameters was studied. According to the percolation theory, both studies confirm the existence of critical points; those were related to the excipients percolation tresholds. The percolation thresholds for the binary hydrophilic matrix tablets were found between 28.7-40.7% (v/v) of HPMC and 38.6-53.9% (v/v) of NaCMC. For the ternary hydrophilic matrix tablets, the existence of a critical barrier between 54 and 61% (v/v) KCl (60-70%, w/w of KCl) was found. In the studied ternary systems HPMC and NaCMC showed that is not possible to simplify the system but they present a partial collaboration in order to establish the gel layer. The knowledge of this critical barrier will be useful in order to optimize the design of the ternary hydrophilic matrix systems., (Copyright 2009 Elsevier B.V. All rights reserved.)