Transformation kinetics of potassium and magnesium modified high-performance brushite cement to carbonated apatite in blood-mimicked condition

Abstract

As a starting cement material, brushite is beneficial due to its short setting time and fast resorbability, and is metastable, allowing in vivo and in vitro transformation into a more stable calcium phosphate (CaP) phase. The challenges for a successful application of brushite include tissue inflammation from its acidic nature, and poor mechanical strength from weakly compacted crystals. Thus, in this study, it is aimed to avoid these drawbacks through a chemical modification of brushite cement. To circumvent tissue inflammation, brushite was modified with 5% of K+ or Mg2+ to induce the transformation into a more stable phase like hydroxyapatite (HAp). Surface transformation in a Tas-stimulated body fluid (Tas-SBF) solution occurred faster for K+-modified cements (1 week, to HAp and β-TCP) than for unmodified brushite (2 weeks, to HAp), and Mg2+-modified brushite (2 weeks, to HAp and β-TCP). Bulk transformation depended on the penetration of the Tas-SBF solution. Thus, Mg2+-substituted cement exhibited the most apatitic structure (26 wt%) due to the highest porosity. To improve the mechanical properties of brushite, setting retardants (acid pyrophosphate, citric acid) and an excess amount of β-TCP as a filler material were included in the cement’s composition. Diametral tensile strength (DTS) was the highest for unsubstituted cements (4.96 ± 1.04 MPa), slightly lower for K+-enriched cement (4.01 ± 0.71 MPa), and lowest for Mg2+-doped cements (1.34 ± 0.23 MPa). This study shows that it is possible to induce the transformation of brushite cement into a more biocompatible bone-like cement and simultaneously achieve enhanced mechanical properties through compositional modification of the cement.