The Higuchi model applies under which conditions?

• A. Spherical matrices with sink conditions
• B. Planar matrices with uniform dispersion and sink conditions
• C. Cylindrical matrices with non-sink conditions
• D. Planar matrices with non-uniform dispersion

Answer: (B)

Higuchi’s diffusion model is built on a planar (slab) geometry with uniform initial distribution of drug inside the matrix and with sink conditions in the surrounding medium. Under these conditions, the drug diffuses out of the flat slab into an infinite reservoir where the external concentration remains effectively zero. This keeps the driving force constant and leads to a release that follows the square root of time.

If the geometry were spherical or cylindrical, or if the drug were not uniformly dispersed, the diffusion path and concentration gradients change, so the same square-root-of-time relationship doesn’t apply in the same way. Likewise, if sink conditions aren’t maintained (the external medium becomes saturated), the driving force decreases over time and the simple Higuchi form no longer holds.

So the best description is planar matrices with uniform dispersion and sink conditions, because those specific assumptions produce the classic Higuchi, diffusion-controlled release behavior.