Table 1.
| Ca/P | Compound | Formula | Stability (solubility/g l−1 at 25 °C) | Characteristics |
|---|---|---|---|---|
| 0.5 | Monocalcium phosphate monohydrate (MCPM) | Ca(H2PO4)2·H2O | pH 0–2; (∼18) | The most acidic and water-soluble CaP phase; sealer in dentistry; bone cement with β-TCP; |
| 0.5 | Monocalcium phosphate anhydrous (MCPA) | Ca(H2PO4)2 | >100 °C; (∼17) | Slightly inferior solubility and similar properties to MCPM; |
| 1.0 | Dicalcium phosphate dihydrate (DCPD), mineral brushite, | CaHPO4·2H2O | pH 2–6; (∼0.088) | Greater solubility; Higher supplement for Ca2+ and PO43− ions; precursor to DCPA (pH < 6), OCP (pH ≈ 6–7), or HA (pH > 7); |
| 1.0 | Dicalcium phosphate anhydrous (DCPA), mineral monetite, | CaHPO4 | >100 °C and pH 4–5; (∼0.048) | Slightly inferior solubility to DCPD; higher release of Ca2+ and PO43− ions; Precursor to HA; |
| 1.33 | Octacalcium phosphate (OCP) | Ca8(HPO4)2(PO4)4·5H2O | pH 5.5–7.0; (∼0.0081) | Most stable at a physiological pH and temperature; the initial crystalline phase in the in vivo formation of HA; transform to HA at alkali conditions; |
| 1.5 | α-Tricalcium phosphate (α-TCP) | α-Ca3(PO4)2 | Only obtained when sintered at above 1250 °C; (∼0.0025) | Greater solubility than HA; a precursor of OCP or CDHA via hydrolysis in phosphoric acid; quick resorption rate—faster than the formation rate of new bone; common component of CaP cement; |
| 1.5 | β-Tricalcium phosphate (β-TCP) | β-Ca3(PO4)2 | Only obtained when sintered at 900–1100 °C; (∼0.0005) | Greater solubility than HA; superior stability to α-TCP; CaP bone cement; dietary food supplement; biphasic bioceramic or coating in combination with HA; |
| 1.2–2.2 | Amorphous calcium phosphates (ACP) | CaxHy(PO4)z·nH2O n = 3–4.5; 15–20% H2O | pH ∼5–12; pH-depending solubility: 25.7 ± 0.1 (pH 7.40) | Glass-like physical properties; a transient precursor phase of other CaPs in aqueous systems; release calcium and phosphate ions in the acidic environment; |
| 1.5–1.67 | Calcium-deficient hydroxyapatite (CDHA or Ca-def HA) | Ca10−x(HPO4)x(PO4)6−x(OH)2−x (0 < x < 1) |
pH 6.5–9.5; (∼0.0094) | Poorly crystalline and of submicron dimensions; convert to β-TCP or HA+β-TCP when heating above 700 °C; a compound of all commercially available CaP cement; |
| 1.67 | Hydroxyapatite (HA, or HAp) | Ca10(PO4)6(OH)2 | pH 9.5–12; thermally stable; (∼0.0003) | Naturally occurring mineral form of calcium apatites; major mineral component of bones and teeth; bioactive and osteoconductive; coating on orthopedic and dental implants; slower resorption rates in vivo; |
| 2 | Tetracalcium phosphate (TTCP, or TetCP) mineral hilgenstockite, | Ca4(PO4)2O | Only obtained when sintered at above 1300 °C without water vapor; (∼0.0007) | Metastable in wet environments and slowly hydrolyzes to HA and calcium hydroxide; combine with other CaPs or polymers to form various self-setting cement and biocomposites. |