In vitro and mechanism study of poly(ethylene-co-vinyl acetate)-based implant for sustained release of vitamin B12

A polymer-vitamin B12 implant system has been developed to overcome the disadvantages of traditional vitamin B12 administration to sheep. The ethylene/vinyl acetate (EVAc) copolymer was pre-blended with crystalline vitamin B12. The blends were then extruded at 100 °C at a constant pressure and rate using a piston extruder to form a polymer-vitamin B12 composite cord. The final implant was a cylinder 2 mm in diameter and 4 mm in length. Determination of the rate of vitamin B12 release from the implant into a pH7.4 phosphate buffer at 37 °C showed that the release rate was strongly dependent on the feed concentration, size and shape of vitamin B12 crystals. The well-known Korsmeyer-Peppas exponential equation was applied to and the release mechanism was found to be a typical anomalous transport mechanism with a strong diffusion-controlled feature between pure diffusion-controlled and Case-II transport mechanisms because the n values obtained were between 0.45 and 0.51 for most polymer-vitamin B12 cylindrical implant systems.