Journal of Controlled Release 61 (1999) 83–91 Observation of swelling process and diffusion front position during swelling in hydroxypropyl methyl cellulose (HPMC) matrices containing a soluble drug a, a b * Paolo Colombo , Ruggero Bettini , Nikolaos A. Peppas a Department of Pharmacy, University of Parma, 43100 Parma, Italy b Biomaterials and Drug Delivery Laboratories, School of Chemical Engineering, Purdue University, West Lafayette, IN 47907-1283, USA Received 18 September 1998; accepted 14 April 1999 Abstract The behavior of gel layer thickness in swellable hydroxypropyl methyl cellulose matrices loaded with increasing amounts of soluble and colored drug and exhibiting swelling, diffusion and erosion fronts, was studied using a colorimetric technique. The effect of the drug loading on the front position in the gel layer, in particular, on the presence of a diffusion front and its movement, was investigated. In addition, the swelling, diffusion and erosion front positions at different releasing times were measured and a theoretical analysis of the overall process was provided. It was found that the diffusion front was visible in systems with more than 30% drug, due to the presence of an undissolved drug layer. The physical analysis of such systems clearly showed the importance of drug solubility and loading in the observation of the diffusion front.  1999 Elsevier Science B.V. All rights reserved. Keywords: Swellable matrices; Drug loading; Diffusion front 1. Introduction studies on the swelling and subsequent dissolution of hydroxypropyl methyl cellulose (HPMC) tablets or Swelling of hydrophilic polymeric matrices has matrices. For example, Melia et al. [7,8] and Rajabi- been the subject of significant research in the last Siahboomi et al. [9] have investigated the swelling few years. In particular, there have been several process using scanning electron microscopy and studies trying to relate molecular characteristics of NMR spectroscopy. Gao and associates [5,10] pre- the swelling process (macromolecular chain exten- sented studies on HPMC swelling using an optical sion, solvent accommodation) to macroscopic imaging technique. Konrad et al. [11] measured the characteristics [1–6]. eroding and swelling front advancement during drug Of particular interest are recent experimental release using ultrasound. In recent years, our group has also examined various aspects of the swelling behavior of such *Corresponding author. Tel.: 139-0521-905086; fax: 139- tablets in terms of front positions [3,12–14]. Indeed, 0521-905085. E-mail address: farmac2@unipr.it (P. Colombo) during contact between a dry HPMC matrix and a 0168-3659 / 99 / $ – see front matter  1999 Elsevier Science B.V. All rights reserved. PII: S0168-3659(99)00104-2