Strontium-loaded magnesium phosphate bone cements and effect of polymeric additives

Magnesium phosphate-based cements (MPCs) are nowadays regarded as promising materials in the field of bone repair. The inclusion of Sr ions in the formulations may represent a valuable strategy to improve their bone regeneration performances, but the effect that such ion exerts on the physico-chemical properties of the material have not been investigated so far. In this work we describe the development of Sr-MPCs obtained including Sr ions in different forms, i.e., using Sr-substituted tri-magnesium phosphate precursor powder or including in the formulation Sr-based salts (SrCl2 and SrHPO4). The materials were characterized both in the form of pastes and hardened cements, finding that according to the type of Sr precursor used we can tune the setting time, the amount of binding phases in the cements, their morphology and thermal behavior. The dissolution behavior and the release kinetics of Mg2+ and Sr2+ can as well be modulated, and in particular the use of SrCl2 in the formulation leads to a higher dissolution and a faster release of a significant amount of both Mg2+ and Sr2+, compared to the other samples. Given the unsatisfying performances obtained during the injectability and anti-washout tests, we also included two polymeric additives, namely poly(N-isopropylacrylamide) and mucin, in the Sr-MPCs formulations. The results demonstrate that it is possible to obtain Sr-MPCs with promising properties for applications as bone cements, that can be tuned according to the form under which Sr is included in the formulation. In addition, mucin markedly improves the cohesion and injectability of the Sr-MPC pastes, providing a simple but effective strategy to develop materials of interest in the orthopedic field.