Which polymer forms swelling gels that slow diffusion in oral controlled-release matrices?

• A. Ethyl cellulose
• B. HPMC
• C. Karaya gum
• D. Eudragit

Answer: (B)

When a polymer swells in the presence of water, it can form a gel layer at the tablet surface that acts as a barrier to drug diffusion. The swollen gel becomes viscous and forms a tortuous path, so drug molecules must diffuse through a thicker, more resistant medium, slowing the release and helping achieve a controlled, extended release in the GI tract. Hydroxypropyl methylcellulose is a classic example of this behavior: it hydrates and swell rapidly to create a robust gel barrier, providing a reliable, diffusion-controlled release mechanism in oral matrices.

Ethyl cellulose is largely hydrophobic and does not form a swelling gel barrier; instead, it creates a non-swelling, diffusion-controlled matrix where release is governed mainly by diffusion through the solid polymer or by erosion, not by a gel layer. Karaya gum does swell and form gels, but its gel strength and release characteristics are more variable and sensitive to environmental factors, making it less predictable for sustained-release applications. Eudragit polymers can be tailored for pH-dependent solubility or for insoluble but permeable matrices, releasing drugs by mechanisms other than a swelling gel barrier, so they don’t primarily rely on swelling to slow diffusion.