Effects of polymer architecture and charge density on the pH-responsive Ca(II) release from brushite

Calcium is one of the major components in the biomineralization of living species, where the transport and release of calcium as well as the consequent nucleation and crystallization are required to be controlled. In this report, the pH-responsive release of calcium from brushite, dicalcium phosphate dihydrate, was studied with addition of some structurally modulated poly(ethylene imine). Poly(ethylene imine) made the calcium delivery preferential at the physiological pH at the same time promoting the sustained behavior, while the release was intrinsically faster at acidic conditions without additive. The release at pH 7 was more than three times faster than at pH 3 for the best case among the systems studied in the present study. The observed phenomenon was attributed to the altered boundary layers of the calcium diffusion at the brushite surfaces when the pH changed, arising from the conformational change of poly(ethylene imine) combined with the charge reversal of brushite surfaces. The current results could be of critical implications in the fields of bioinspired and biomimetic mineralization as well as the controlled drug release from mineral carriers.