TEM analysis of apatite surface layers observed on zinc based glass polyalkenoate cements

Abstract

Glass polyalkenoate cements (GPCs) are acid base cements formed by the reaction of an aqueous solution of polyalkenoic acid, usually polyacrylic acid (PAA) with an acid degradable aluminosilicate glass. The result of the reaction is cement consisting of reacted and unreacted glass particles embedded in a polysalt matrix. In addition to these conventional GPCs, aluminium-free glass polyalkenoate cements based on zinc silicate glasses (Zn-GPCs) exhibit significant potential as bone cements for several reasons. Primarily, they are formulated without the inclusion of aluminium (Al) [1] in the glass phase and thus eliminate clinical complications arising from the release of the Al ion from the cement in vivo. Such complications have, in the past, included aluminium-induced encephalopathy [2– 5] and defective mineralization of cancellous bone [6]. Secondly, Zn-GPCs set without a significant evolution of heat, when compared with commercial bone cements such as Spineplex (Stryker, Limerick, Ireland). Finally, these materials can be tailored to release clinically beneficial ions into the surrounding tissues [7]. In addition to Zn, these cements have been synthesized to contain strontium (Sr) [8, 9]. Both Sr and Zn inhibit osteoclastic turnover and promote osteoblastic turnover, resulting in increased bone strength and decreased fracture risk [10–14]. Conventional GPCs bond directly to hydroxyapatite (HA), the mineral phase of tooth and bone, via the adsorption of carboxylate groups of the polyacid chains into the HA structure [15]. This indicates all conventional GPCs are capable of bonding to the living bone. However, it has been postulated that an essential requirement for a material to bond to living bone lies in its ability to form a bone-like crystalline apatite layer at its surface in vivo and that such a system can be replicated using simulated body fluid (SBF) [16]. Kamitakahara et al. recently used SBF to evaluate the bone bonding ability of conventional GPCs and found that PAA inhibited the formation of an apatite layer at the surface of GPCs and thus concluded that they were unlikely to bond to bone in vivo [17]. Given these conflicting viewpoints, the authors have previously evaluated the ability of Zn-GPCs to form a bone-like apatite layer in vitro using SBF [1, 9]. It was shown that calcium phosphate layers are produced on ZnGPCs immersed in SBF within 24 h. However, thin film Xray diffraction (TF-XRD) indicates no crystallinity within the surface layer, and chemical analysis using scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) of the surface layers is complicated by the collection of elemental signatures from the cement sample itself. In order to eliminate these issues, this letter seeks to validate the amorphous nature and composition of the surface layers observed on Zn-GPCs after immersion in SBF using transmission electron microscopy. One glass composition 0.08SrO/0.08CaO/0.36ZnO/ 0.48SiO2 (mol. fraction), was synthesized. Appropriate amounts of analytical grade strontium carbonate, calcium carbonate, zinc oxide and silicon dioxide (Sigma Aldrich, Dublin, Ireland), were weighed out in a plastic tub and mixed in a ball mill for 1 h, then dried (100 C, 1 h). The D. Boyd (&) M. R. Towler A. W. Wren O. M. Clarkin D. A. Tanner Materials and Surface Science Institute, University of Limerick, National Technological Park, Plassey Park, Castletroy, Limerick, Ireland e-mail: Daniel.Boyd@ul.ie