Dissolution Characteristics of Bioactive Glasses

The formation of hydroxycarbonate apatite (HCA) layer on the surface of bioactive glasses and glass ceramics involves an ion exchange reaction, which increases the pH and causes cytotoxicity in the surrounding environment. Investigations into the in-vitro cell behaviour of bioactive glasses use a preconditioning step to reduce this pH excursion. This technique involves the immersion of the glass in either simulated body fluid (SBF) or culture medium for a pre-determined time period. Preconditioning of two types of bioactive glass particulates (melt-derived 45S5 Bioglass®, and 58S gel-glass) were examined to evaluate the effect on glass dissolution and subsequent HCA formation. This was achieved by immersing the glass powders in SBF for periods ranging from 30 minutes to 48 hours. The samples were evaluated using inductive coupled plasma atomic emission spectroscopy (ICP) and infrared spectroscopy (FTIR) to analyse the dissolution rates and HCA layer formation on the surface of the particles. Preconditioning was found to reduce the rate of dissolution for both glasses for calcium (by 20ppm for 45S5 and 75ppm for 58S). Sodium release from 45S5 was also diminished by 100ppm, whereas silica release was not affected for either glass. Results obtained from FTIR indicate that the rate of HCA layer formation was not significantly changed for either of the glass powders, implying that bioactivity has not been adversely influenced by the preconditioning step. Introduction One specific range of compositions known as the bioactive silicate glasses show a common behaviour of forming a mechanically strong bond to bone by means of a series of chemical reactions at the bone-implant interface. This results in the formation of a biologically active hydroxycarbonate apatite (HCA) layer on the surface of the glass that is responsible for the development of the strong interfacial bond [1]. One of the mechanisms responsible for HCA formation is the cation exchange between the bioactive glass surface and the aqueous environment (i.e. Ca, Na with H/H3O), which involves a pH excursion from de-alkalisation. Loss of soluble silica in the form of silicic acid will then occur due to the breaking up of the silicate network (Si–O–Si) that also contributes to a change in pH. It has been hypothesised that the ionic dissolution rate from the glass (in particular silicon) is critical for the gene expression of osteoblasts given that the change in ionic concentration along with the resultant alkalisation of the surrounding medium influences cell metabolism and function [2]. However, the concentration of ions released from a bioactive glass can reach levels at which cytotoxic effects may occur if the dissolution rate is too rapid [3]. One technique to avoid attaining a cytotoxic concentration is to immerse the glass in an aqueous environment such as culture medium or simulated body fluid (SBF). Removal of the glass after a defined time, prior to further studies can eliminate the initial rapid ion release and reduce the subsequent rate of dissolution. Such a technique has not been studied extensively although its usage is common. Key Engineering Materials Online: 2003-05-15 ISSN: 1662-9795, Vols. 240-242, pp 201-204 doi:10.4028/www.scientific. net/KEM.240-242.201 © 2003 Trans Tech Publications Ltd, Switzerland All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications Ltd, www.scientific. net. (Semanticscholar.org-13/03/20,21:20:34) In this work, the in-vitro preconditioning of two types of bioactive glass is studied to establish the effect on the dissolution rate and the HCA formation critical for bone growth. Methods Melt-derived 45S5 (46.1% SiO2, 24.4% Na2O, 26.9% CaO and 2.6% P2O5, in mol) (<5μm particle size) and sol-gel derived 58S (60% SiO2, 36% CaO and 4% P2O5, in mol) (<20μm particle size) bioactive glass powders were provided by US Biomaterials. 0.5g aliquots of each powder were immersed in 50ml of simulated body fluid under static conditions at 37C, for 0.5, 1, 2, 4, 8, 15, 24 and 48 hours. The powder samples were filtered, dried at 60C and analysed using FTIR whilst the recovered extract was analysed using ICP (Inductive Coupled Plasma Spectroscopy). For convenience, preconditioned powders will be referred to as Pre(X) where X is the length of time the powder was immersed in SBF for under static conditions at 37C. Pre(0) refers to unconditioned powder. 0.075g aliquots of each preconditioned powder (Pre(0) – Pre(48)) were then immersed in 50 ml of simulated body fluid at 37C at an agitation rate of 175 Hz. The samples were recovered at various time intervals and again analysed using FTIR and ICP. Results Static soak (Preconditioning) The static immersion of both powders in SBF resulted in similar dissolution profiles (see Fig. 1). The small particle size of the 45S5 powder results in a large surface area comparable to that of the mesoporous 58S powder. The pH profile for both powders is similar to that of [Ca] in Fig. 1, remaining steady at 8.5 (45S5) and 8.8 (58S). 0 100 200 300 400 500 600 700 800 90