A <perfect sink condition in drug release> is an idealized receiving boundary that removes released drug fast enough for its concentration there to be treated as zero. In <diffusion-controlled drug release>, this preserves the largest concentration drop available at the exposed boundary. It is a condition on transport at the release surface, rather than a statement that the entire apparatus contains no drug.

For a planar matrix occupying $x>0$, a common <diffusion equation> model uses
$$
c(0,t)=0,\qquad J=-D\frac{\partial c}{\partial x},
$$
where $c$ is dissolved-drug concentration and $D$ is its <diffusion coefficient>; the outward flux magnitude is used when reporting a positive <drug release rate>. A <finite mass-transfer boundary in drug release> instead allows a nonzero surface concentration. If the same concentration scale applies on both sides of the interface, an external transport model is
$$
J_{\mathrm{out}}=h(c_s-c_b),
$$
with transfer coefficient $h$, surface concentration $c_s$ and well-mixed bulk concentration $c_b$. A small $c_b$ does not force $c_s$ to be small when a stagnant layer or membrane imposes substantial resistance.

An original finite-reservoir example separates capacity from transport. Suppose a formulation can release $10\,\mathrm{mg}$ into $100\,\mathrm{mL}$, and measured <solubility> in the chosen medium is $1\,\mathrm{mg/mL}$. Even after complete release the well-mixed bulk concentration is only $0.10\,\mathrm{mg/mL}$. In a simple saturated-interface dissolution model $J=h(c_{\mathrm{sat}}-c_b)$, that concentration still reduces flux to $90\%$ of its zero-bulk-concentration value. Large capacity makes a zero-concentration approximation plausible to a chosen tolerance; it does not make the approximation exact.

A useful experimental test compares <in vitro drug release test> curves while increasing receiving volume or flow, with temperature and medium chemistry controlled. Sampling and medium replacement require <sampling correction in drug release testing>. If increasing external transport changes the curve, apparatus resistance or accumulation may still matter. The FDA’s primary account of adaptive perfusion describes pressure-driven filtration designed to avoid rate limitation by a passive dialysis membrane.<Adaptive Perfusion: A Novel In Vitro Drug Release Testing Method for Complex Drug Products by the FDA>[Technology Summary, membrane transport and tangential-flow filtration]{ref}

The ideal boundary should be distinguished from a method’s practical sink criterion. The FDA’s 1997 dissolution guidance discusses medium, apparatus and agitation together and explicitly allows justified non-sink tests; a zero boundary is a modelling choice to be checked against the actual experiment.<Dissolution Testing of Immediate Release Solid Oral Dosage Forms by the FDA>[Appendix A, pp. A-1–A-2, Dissolution Medium and Agitation]{ref}


 Perfect sink condition in drug release (source code)