Sol Gel coatings for protective and bioactive functionalisation of metals used in orthopaedic devices

A. DURÁN; A. CONDE; A. GOMEZ COEDO; T. DORADO; C. GARCÍA,; S. CERÉ

JOURNAL OF MATERIALS CHEMISTRY

Royal Society of Chemistry (RSC)

Año: 2004 vol. 14 p. 2282 - 2290

0959-9428

The aim of this work is the production and characterisation of sol–gel coatings for protection and bioactivationof metals used as standard surgical implant materials, such as stainless steel 316 L (ASTM F138), Co basedalloys (ASTM F75) and titanium alloy Ti-6Al-4V (ASTM F67). These films should both prevent degradationof the substrates by wear or corrosion, and bioactivate the material for inducing the formation of ahydroxyapatite (HA) rich layer onto the material surface, thereby permitting a natural bonding to living tissues.Formation of HA layers can be observed on performing in vitro tests by soaking the material in simulated bodysolutions. The work describes the development of coatings containing bioactive glass and glass-ceramic particlesin hybrid methyl-triethoxysilane (MTES) and tetraethylorthosilicate (TEOS) acidic sol, applied by dip-coatingto surgical alloys, AISI 316 L, ASTM F75 and ASTM 67, with the aim of accomplishing both high corrosionresistance of the metal in the body environment and adhesion of the implant to the surrounding tissue.The performance of the coated metal was evaluated in vitro by electrochemical techniques includingpotentiodynamic polarisation curves and electrochemical impedance spectroscopy, to follow the formation ofhydroxyapatite on the surface, as well as the in vitro release of ions by plasma atomic emission spectroscopy(ICP-MS) after up to one year of immersion. In vivo behaviour was evaluated by subcutaneous tests andendomedullar implantation in Hokaido rats to study possible rejection reactions and natural bonding toliving tissue.1. IntroductionBiomedical prosthetic devices are used in the human body tocarry out the functions no longer performed by the originalparts. Metals are used in the human body mainly fororthopaedic purposes and their degradation by wear and/orcorrosion must be negligible. Standard surgical implantmaterials include stainless steel 316 L (ASTM F138), Cobased alloys (mainly ASTM F75, and F799) and titaniumalloys; where Ti-6Al-4V (ASTM F67 and F136) are the mostemployed. The use of these alloys for orthopaedic appliancesrequires the formation of a passive film to prevent oxidation.These films consist of metal oxides which form spontaneouslyon the metallic surface to limit the transport of metal ionsacross the metal oxide–solution interface. These films shouldnot only be non-porous and fully cover the metal surface butmust also remain on the surface under mechanical stresses. Inaddition the surface should allow repassivation in case of filmremoval or damage.1Cobalt based alloys are widely used for fabrication of diversesurgical devices due to their good corrosion and wear resistanceas well as their good biocompatibility. The main corrosionproducts, CoO and Cr2O3, may react with water to formCo(OH) and Cr(OH)3.2 Cell culture has demonstrated thatchromium species can stimulate osseous resorption andnecrosis of periprosthetic tissue.3Titanium alloys are employed for orthopaedic implants dueto their excellent combination of biocompatibility, corrosionresistance and mechanical properties. However, toxicity ofalloying elements and a high elastic modulus compared to bonehave been reported as possible disadvantages.4 Althoughtitanium