Synthesis and characterization of zinc-containing whitlockite Ca 10 Zn 10 H 2 (PO 4 ) 14 for orthopedic applications Petr Melnikov, Diogo Muniz de Albuquerque ⇑ , Tayna Araujo Naves, Lincoln C.S. de Oliveira Federal University of Mato Grosso do Sul, 79080-680 Campo Grande, MS, Brazil article info Article history: Received 19 June 2018 Received in revised form 8 August 2018 Accepted 9 August 2018 Available online 10 August 2018 Keywords: Biomaterials Whitlockite Structural Functional abstract The present research is intended to provide a base for further investigation of the chemistry of whitlock- ites, and Zn-whitlockite, in particular. Zinc-containing whitlockite Ca 10 Zn 10 H 2 (PO 4 ) 14 was successfully prepared at room temperature by means of a chemical reaction between Ca(NO 3 ) 2 , Zn(NO 3 ) 2 and (NH 4 ) 2 HPO 4 at a pH around 11.0. The X-ray pattern of the compound is that of magnesium-iron whitlock- ite Ca 18 (Mg,Fe) 2 H 2 (PO 4 ) 14 or, in a broader aspect, solid solutions of the composition Ca 20Àx Me 2+ x H 2 (PO 4 ) 14 , structurally related to tricalcium phosphate. It is suggested that the compound, alone or as an additive to hydroxyapatite, might be suitable for the preparation of biodegradable orthopedic implants containing zinc. Ó 2018 Elsevier B.V. All rights reserved. 1. Introduction The primary requirement of any implantable device is that it is biocompatible, since the implant must be able to restore func- tion without adverse tissue response or chronic inflammatory reaction. The best biomaterials are those that are similar or have sufficient affinity with the compounds that normally exist in humans. The classic examples are hydroxyapatite (HA) and b-tricalciumphosphate (TCP) that take part in the bone mineraliza- tion and demineralization processes and, consequently, are widely used for biomedical applications [1]. Bone graft substitutes are fre- quently used as void fillers. Ideally, the material will dissolve and participate in bone remodeling, gradually replacing the bone graft substitute over time. In this way, it is gradually replaced by normal bone, and this restoration is not hampered by the presence of the starting material [2]. Strictly speaking, whitlockite is a mineral of the composition Ca 18 (Mg,Fe) 2 H 2 (PO 4 ) 14 , structurally related to b-Ca 3 (PO 4 ) 2 [3], while the synthetic compound known as Mg-whitlockite contains exclusively magnesium ions. It has rhombohedric lattice, space group R3c. This structure, although similar to b-Ca 3 (PO 4 ) 2 , differs significantly from the latter. It contains a chain of MgO 6 octahedra and PO 4 H groups which lie on three-fold axis [4]. An interest in whitlockite is due to its special features. On the one hand, magne- sium is easily replaced by divalent cations with close ionic radii. Thereby, in a broader sense, there is a series of solid solutions of the composition Ca 20Àx Me 2+ x H 2 (PO 4 ) 14 where X are divalent metals such as Mg, Fe, Cd, Mn, etc. On the other hand, easy substitution makes it possible to consider this matrix as a promising vehicle for introducing bioactive cations, particularly zinc, directly into the bone tissue in the form of inorganic grafting [5]. In this regard, it should be borne in mind that whitlockite fam- ily can also be considered as products of substitution of bivalent metals for calcium in b-Ca 3 (PO 4 ) 2 . Zinc can replace calcium in this structure up to about 20 at.% inducing a non-linear variation of the lattice constants [6]. In principle, at higher content of this element Zn-whitlockite formation could have been expected, but the authors do not seem to have sufficiently explored the borderline concentration range. On the other hand, b-Ca 3 (PO 4 ) 2 does not form in aqueous systems under normal laboratory conditions (i.e. up to 100 °C and at atmospheric pressures). Whitlockites can be obtained in such systems with the following molar percent substi- tution of Ca 2+ ions in solution: Mg 2+ 0.1–1%; Mn 2+ about 1%; and Fe 2+ about 10%. However, this methodology definitely failed in all attempts to prepare zinc-containing whitlockite [5]. In a recent patent the technique of preparation of whitlockites substituted with divalent cations is also proposed for large numbers of Me(II) and even of Me(III), but not for zinc [7]. By the way, this last infor- mation does not appear to be reliable since the authors do not make distinction between divalent and trivalent ions as candidates for replacements. Therefore, although it seems paradoxical, there does not seem to be any available technique so far for the prepara- tion of zinc-containing bioceramics. The aim of the present work is to propose a simple method for the synthesis of whitlockite solid solution containing calcium and zinc. https://doi.org/10.1016/j.matlet.2018.08.051 0167-577X/Ó 2018 Elsevier B.V. All rights reserved. ⇑ Corresponding author. E-mail address: diogomed1@yahoo.com.br (D.M. de Albuquerque). Materials Letters 231 (2018) 198–200 Contents lists available at ScienceDirect Materials Letters journal homepage: www.elsevier.com/locate/mlblue