Reversible immobilization of drugs on a hydrogel matrix, 2. Diffusion of free chloramphenicol from poly(2‐hydroxyethyl methacrylate) hydrogels

The diffusional behaviour of chloramphenicol D ‐(–)‐threo‐ N ‐[2‐hydroxyl‐1‐hydroxymethyl‐2‐(4‐nitrophenyl)ethyl]dichloroacetamide. ( 1 ) from poly( 2 ‐hydroxyethyl methacrylate) (pHEMA) hydrogels is reported. The drug was trapped in the matrix cross‐linked with various amounts of ethylene dimethacrylate (0,5–18 mol‐%), and the diffusion was followed from disc‐shaped slabs at swelling equilibrium in water (phosphate buffer, pH 7) at 37°C. The diffusion obeys the second Fick law and is rapid ( D ≈ 10 −8 cm 2 s −1 ) in the case of the slightly crosslinked hydrogels used for corneal lenses. The diffusion coefficients decrease almost exponentially with increasing cross‐linking ratio, and the process appears predominantly controlled by the free volume contribution of the diluent rather than by the sieving of the network.