The surface morphologies of Er-HA series before and after immersion in SBF for various days are shown in (Fig. 8). After 3 days of immersion in SBF an apatite layer was observed on the surface of pellets (Fig. 8b–e). When the incubation time was increased to 7 and 14 days, the phase grain indicated a gradual growth that EPZ004777 further showed the formation of new apatite layer composed of irregular agglomerates (Fig. 8bi, bii, ci, cii, di, dii, ei and eii). These particles became more densely packed as the Er3+ concentration increased inside the HA structure, suggesting that the increased Er3+ concentration in HA promoted the formation of the apatite layer. Mechanism of apatite formation can be described through surface chemistry of Er-HA, negatively charged surface of Er-HA can attract positive Ca2+ ions present in the SBF, resulting in the formation of Ca-rich amorphous calcium phosphate (ACP). The newly formed positively charged ACP layer can attract negatively charged PO43− ions from SBF to form Ca-poor ACP. This type of Ca-deficient ACP has been observed as a precursor, which eventually crystallizes into bonelike apatite on various bioactive ceramics , , ,  and . In addition Er-HA series provides more nucleation sites for the deposition of apatite particles due to histone proteins has high surface area and mesoporous nature (Table 4).