Experimental study on the interaction of insulin with apatitic calcium phosphates analogous to bone mineral: adsorption and release

The present work investigated the interaction of human insulin with synthesized poorly crystalline apatitic calcium phosphates containing simultaneously HPO42- and CO32- ions in various amount. The adsorption kinetics is very fast, while the release kinetics is generally slow. The chemical composition of apatite has an influence on both the adsorption and release processes. The experimental results show that the percentage of insulin adsorption and release decreased with the increase of the content of carbonate. The equilibrium adsorption data are fitted into Langmuir, Freundlich, Elovich, Temkin, and Dubinin–Radushkevich isotherms. The Langmuir model is best suited with a maximum monolayer adsorption capacity of 33.20 and 25.08 mg/g at 310 K corresponding to the carbonated and octocalcium phosphate apatite respectively. Isotherms parameters have revealed that the adsorption of insulin on these apatites is a feasible, spontaneous, and exothermic process. Fourier-transforms infrared confirm the fixation of insulin on non-carbonated and carbonated apatite. The adsorption and release of insulin molecules can be well described as an ions exchange-reaction between species in the hydrated layer of apatite and other species in solution. All of these results suggested that apatitic calcium phosphates can be used as systems for insulin delivery.