Which polymer provides a hydrophobic diffusion barrier in oral controlled-release matrices?

• A. HPMC
• B. Ethyl cellulose
• C. Alginate
• D. Eudragit

Answer: (B)

The key idea is using a hydrophobic, water-insoluble polymer to slow drug release by diffusion rather than by swelling or erosion. Ethyl cellulose fits this role because it is hydrophobic and does not readily take up water. When it’s part of a matrix, it forms a relatively non-wetting, diffusion-friendly barrier that slows both water ingress and drug diffusion, leading to a diffusion-controlled release profile with minimal swelling.

In contrast, the other polymers are hydrophilic or gel-forming: HPMC and alginate swell and create gel layers that govern release through gel diffusion and erosion rather than a true hydrophobic barrier. Eudragit polymers can be used for coatings or certain matrix systems with different release mechanisms, but the classic hydrophobic diffusion barrier in oral controlled-release matrices is provided by ethyl cellulose.