Thermal crystallization of amorphous calcium phosphate combined with citrate and fluoride doping: a novel route to produce hydroxyapatite bioceramics.

Lorenzo Degli Esposti,Julietta V Rau,Alessio Adamiano,Nenad Ignjatovic,Vuk Uskoković,Silvia Panseri,Smilja Markovic,Marco Fosca,Monica Montesi,Michele Iafisco

doi:10.1039/d1tb00601k

Abstract

Amorphous calcium phosphate (ACP) is a material of high interest for dentistry, orthopedics, and other biomedical sectors. Being intrinsically metastable, the process of transformation of ACP into a crystalline phase upon heating is of high relevance for the development of innovative bioceramics. Here we have first studied the thermal behavior of a citrate-stabilized ACP (Cit-ACP) also doped with fluoride ions (Cit-FACP) prepared at three different nominal Cit/Ca ratios (i.e. 4, 2, 1) by differential thermal analysis. Next, the physico-chemical features of the crystalline products as well as the in vitro cell response to the materials were investigated. A citrate and fluoride free ACP sample was also tested as the blank. We have found that the activation energy of crystallization of Cit-(F)ACP samples is lower in comparison to the blank ACP and this is influenced by the nominal Cit/Ca molar ratio. Interestingly, we have discovered that the thermal treatment of Cit-(F)ACP at 800 °C yields hydroxyapatite (HA) or fluorapatite (FHA) as the main products differently from blank ACP that, like most of the ACPs reported in the literature, yields β-tricalcium phosphate. This was attributed to the Ca/P ratio of Cit-(F)ACP, which is similar to HA. A study of the crystalline products has revealed that all the (F)HA samples were non-cytotoxic, and retained carbonate ions in the crystal structure despite the heat treatment that should have induced decarbonation. The morphology of the products is influenced by the nominal Cit/Ca ratio and the presence of fluoride, ranging from spherical nanoparticles to micrometric hexagonal rods. Overall, our results prove that the thermal crystallization of Cit-(F)ACP is markedly different from classic ACP based materials and the thermal treatment of Cit-(F)ACP represents an attractive route for producing pure bioactive HA ceramics.

Highlights

Amorphous calcium phosphate (ACP) into all these crystalline phases is thermodynamically favored.[10,11] ACP is intrinsically metastable and given sufficient time, temperature and/or humidity, it will crystallize into one of the aforementioned calcium phosphate phases
The long-term stability of dry Cit-(F)ACP samples as amorphous materials shown in our previous work[38] and ascribed to the presence of citrate ions was further confirmed since the Powder X-ray diffraction (PXRD) patterns of the samples collected after four years of storage at room temperature (Fig. 1B and C) are identical to those immediately after preparation
Cit-(F)ACP samples have SSABET values that range from 200 up to 330 m2 gÀ1, depending on the nominal Cit/Ca ratio, while the SSABET value of blank-ACP sample is 151 m2 gÀ1. This latter value is similar to the SSABET of other ACPs reported in the literature, which are typically between 80 and 160 m2 gÀ1.43,44 The nominal Cit/Ca molar ratio does not influence the stability of Cit-(F)ACP in the dry state nor their structural and compositional features, but the SSABET changes significantly when the nominal Cit/Ca ratio

Summary

Introduction

ACP into all these crystalline phases is thermodynamically favored.[10,11] ACP is intrinsically metastable and given sufficient time, temperature and/or humidity, it will crystallize into one of the aforementioned calcium phosphate phases. ACP is of high biological relevance because it is the precursor of biogenic HA of bones and teeth.[12,13,14,15] It was demonstrated by Robinson et al.[16] and Beniash et al.[17] that the mineralization of tooth enamel occurs through the deposition of spherical ACP nanoparticles into chains that crystallize to HA, and a similar mechanism was proposed for bone formation.[18] Apart from their biological relevance, ACP nanoparticles were extensively studied as remineralizing agents in dentistry and as biomaterials for bone repair.[1,2] They are used, for example, in remineralizing toothpastes, coatings for prostheses, self-setting injectable cements, and hybrid composites.[1,2,15] All these applications rely on the capacity of ACP to crystallize and form HA in the presence of water, to the biogenic formation of HA. Cit-ACP and Cit-FACP will be defined together as Cit-(F)ACP

Objectives

Results

Conclusion