Bioactive glass coatings for orthopedic metallic implants

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Abstract

The objective of this work is to develop bioactive glass coatings for metallic orthopedic implants. A new family of glasses in the SiO2–Na2O–K2O–CaO–MgO–P2O5 system has been synthesized and characterized. The glass properties (thermal expansion, softening and transformation temperatures, density and hardness) are in line with the predictions of established empirical models. The optimized firing conditions to fabricate coatings on Ti-based and Co–Cr alloys have been determined and related to the glass properties and the interfacial reactions. Excellent adhesion to alloys has been achieved through the formation of 100–200 nm thick interfacial layers (Ti5Si3 on Ti-based alloys and CrOx on Co–Cr). Finally, glass coatings, approximately 100 μm thick, have been fabricated onto commercial Ti alloy-based dental implants.

Introduction

Since the discovery of Bioglass® by Hench, bioactive glasses have been used in many medical applications, such as drug delivery systems, non-load-bearing implants, and bone cements.1, 2, 3 However, due to their poor mechanical properties, these glasses cannot be used in load-bearing applications, where metallic alloys are still the materials of choice. It was recognized early on that one of the main applications of bioactive glasses could be coatings for prosthetic metallic implants.4 These coatings would serve two purposes: improving the osseointegration of the implants, and protecting the metal against corrosion from the body fluids and the tissue from the corrosion products of the alloys. Unfortunately, most of the attempts to coat metallic implants with bioactive glasses have had limited success. This is due to poor adhesion of the coating and/or degradation of the glass properties during the coating procedure (typically enameling, or flame or plasma spray coating).4

Several key design criteria are necessary for the successful development of new bioactive coatings for implants: (1) the firing cycle should not degrade the properties of the metal or the glass; (2) the thermal expansion of the glass and the metal should be similar in order to avoid the generation of large thermal stresses that can result in coating cracking or delamination during fabrication (typically it is preferred that the coating has a slightly lower thermal expansion than the metal, resulting in small compressive stresses); (3) the firing cycles should result in interfaces with optimum adhesion; and (4) the glass coatings should form hydroxyapatite when in contact with body fluids. The Ti alloys used in the fabrication of prosthetic implants are very reactive, and the glass/metal reactions that occur during firing are detrimental to adhesion and bioactivity. Thus, coating titanium with bioactive glasses is challenging. Additionally, firing temperatures below the α → β transformation of Ti (between 885 and 950 °C for unalloyed Ti, depending on the impurity content,5, 6 and between 955 and 1010 °C for Ti6A14V6) are required to avoid a degradation of the mechanical properties of the implant. Bioactive glasses are typically silicate glasses with silica content below 60 wt.%. Most of these glasses are hygroscopic and crystallize readily at the firing temperatures required for enameling. The thermal expansion coefficients of bioactive glasses are typically much larger than those of Ti alloys. The simplest way to reduce thermal expansion is to increase the SiO2 content of the glass; unfortunately, this is at the expense of bioactivity, which is significantly reduced.1, 2, 3

This work examines the properties of a new family of bioactive glasses designed to coat Ti-based alloys. The results are compared with existing models for calculating properties of silicate glasses and with the optimum enameling conditions needed for the fabrication of bioactive coatings on Ti-based and Co–Cr alloys with good adhesion to the metal.

Section snippets

Experimental

Glasses in the system SiO2–Na2O–K2O–CaO–MgO–P2O5 were prepared by mixing SiO2 (99.5%) (Cerac, USA), CaCO3 (99.9%) (JT Baker, USA), MgO (98.6%) (JT Baker, USA), K2CO3 (99%) (Allied Chemical, USA), NaHCO3 (99.5%) (JT Baker, USA) and NaPO3 (99.7%) (Allied Chemical, USA) in ethanol using a high-speed stirrer. The compositions of the glasses are summarized in Table 1. The mixtures were dried at 80 °C for 12 h and then fired in air at 1400–1500 °C for 4 h in a Pt crucible (glasses with larger silica

Glass properties

XRD analysis of the synthesized glasses did not show any crystalline phase, with the exception of 6PM and 6P68, where small amounts of sodium calcium phosphate crystals (2.4 CaO·0.6Na2O·P2O5) were found.

Table 2 summarizes the thermal expansion (α), softening (Ts), and transformation (Tg) temperatures of the glasses. The thermal expansions of the glasses cover a wide range, which includes the thermal expansion of Ti and Ti6A14V (9.5–10.5×10−6 °C−1 at 400 °C)6 and Vitallium© (∼14 ×10−6 °C−1),7 a

Conclusions

A new family of glasses in the SiO2–Na2O–K2O–CaO–MgO–P2O5 system has been formulated so that bioactive glass coatings can be prepared on orthopedic metallic implants by enameling. The partial substitutions of CaO by MgO and Na2O by K2O are required to match the thermal expansion of the coatings to that of Ti-based alloys. In that way, coatings with silica contents below 60 wt.% that do not crack or delaminate can be prepared. This is an important requirement since glasses with silica contents

Acknowledgements

The authors would like to acknowledge Professor K. Hiraga and Mr. B. Aoyagi for allowing us to use the ARM-1250 kV microscope and Drs. S Hata and K. Kaneko for TEM assistance. S. Fujino wishes to thank the Japanese Ministry of Education, Culture and Sciences for a fellowship given to him by the National Program of Fellowships for young researchers in foreign countries, 2000. S. Lopez-Esteban was partly supported by a Fulbright Grant and wishes to thank the Spanish Ministry of Education, Culture

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