Narrow-dispersed calcium alginate microspheres were prepared by a membrane emulsification-gelation method.The microspheres were treated with NaCl solutions of different concentrations to control the amount of free carboxylate groups.The size of the microspheres increased simultaneously after salt treatment.The quantity of calcium and sodium ions in the microspheres was measured by atomic absorption spectrometer

and the change of functional groups was determined by Fourier transform infrared spectroscopy.During the treatment process calcium ions in the microspheres were replaced by sodium ions

and the carboxylate groups were converted from a chelate to a free state.As doxorubicin hydrochloride was positive in water while alginate was negative

doxorubicin was loaded into the alginate microspheres via electrostatic interaction.The loading capacity of microspheres was enhanced greatly after treatment with NaCl solution.When the microspheres were treated with 1.8% NaCl solution

the drug content increased remarkably to 1310 g/mg

which was nearly 10 times the amount of the untreated ones.In vitro release tests confirmed that relative smaller percentage of drug was released from the microspheres containing more drug.The release behavior of the drug-loaded microspheres was pH-sensitive

i.e.slower at pH 7.4 but faster at pH 5.5.After salt treatment

the drug release mechanism was transformed from diffusion alone to the combination of diffusion and matrix erosion.