One example for brushite (CaHPO₄∙2H₂O) formation is the reaction of monocalcium phosphate monohydrate (MCPM, Ca(H₂PO₄)₂∙H₂O) and β-tricalcium phosphate (β-TCP, β-Ca₃(PO₄)₂), which react with H₂O according to (1). As Cu²⁺ is known to increase antibacterial activity, hence preventing inflammation [1], and has positive effects on angiogenesis and wound healing [2] it is a suitable ion as dopant for tricalcium phosphate. The incorporation of Cu²⁺ in the crystal structure of β-TCP has been confirmed in previous studies [1,3].

Ca(H₂PO₄)₂∙H₂O + β-Ca_3-xCu_x(PO₄)₂ + 7 H₂O → 4 CaHPO₄∙2 H₂O (1)

The effect of Cu²⁺ doping in β-TCP on the hydration was investigated by isothermal heat flow calorimetry, powder XRD of storage samples, in-situ XRD, ICP-MS measurements of the pore solution as well as ¹H-NMR for determination of different proton mobility in free H₂O and formed phases. β-TCP was mixed with MCPM with a molar ratio of β-TCP/MCPM = 1.63 and a water to solid ratio of 0.3 ml/g. Phytic acid (IP6, C₆H₆(OPO₃H₂)₆) was used as a setting retarder. The measurements were conducted at 23 °C and selected mixtures were measured at 37 °C. Additionally measurements without setting retarder were conducted at 8 °C. β-TCP with Cu²⁺ contents between 0 and 5 mole-% were used.

Heat flow calorimetry (Figure 1) and in-situ XRD measurements showed an increasing retardation of the reaction and a decreasing heat release with increasing amount of Cu²⁺ in β-TCP. This effect was detected in samples with and without setting retarder and can therefore be associated with the Cu²⁺ concentration in β-TCP. Brushite and small amounts of monetite were detected as reaction products in the storage samples. The samples with Cu²⁺ also show a secondary phase with a strong preferred orientation, which could not be identified, but is assumed to be a Cu²⁺ containing hydrate phase.

References

[1] N. Matsumoto, K. Sato, K. Yoshida, K. Hashimoto, Y. Toda. Acta Biomaterialia, 2009, 5(8): 3157–64.

[2] J. Barralet, U. Gbureck, P. Habibovic, E. Vorndran, C. Gerard, Ch. J. Doillon. Tissue Engineering Part A, 2009, 15(7): 1601–9

[3] K. Spaeth, F. Goetz-Neunhoeffer, K. Hurle.  Journal of Solid State Chemistry, 2020, 285, 121225.