Jose M. Gomez-Vega

Bioactive glass coatings with hydroxyapatite and Bioglass particles on Ti-based implants. 1. Processing.

Silicate-based glasses with thermal expansion coefficients that match those of Ti6A14V were prepared and used to coat Ti6A14V by a simple enameling technique. Bioglass (BG) or hydroxyapatite (HA) particles were embedded on the coatings in order to enhance their bioactivity. HA particles were immersed partially during heating and remained firmly embedded on the coating after cooling. There was no apparent reaction at the glass/HA interface at the temperatures used in this work (800-840 degrees C). In contrast, BG particles softened and some infiltration into the glass coating took place during heat treatment. In this case, particles with sizes over 45 microm were required, otherwise the particles became hollow due to the infiltration and crystallization of the glass surface. The concentration of the particles on the coating was limited to 20% of surface coverage. Concentrations above this value resulted in cracked coatings due to excessive induced stress. Cracks did not propagate along the interfaces when coatings were subjected to Vickers indentation tests, indicating that the particle/glass and glass/metal interfaces exhibited strong bonds. Enameling, producing excellent glass/metal adhesion with well-attached bioactive particles on the surface, is a promising method of forming reliable and lasting implants which can endure substantial chemical and mechanical stresses.

Glass-based coatings for titanium implant alloys

Glass coatings on Ti6Al4V were prepared using a simple enameling technique. The composition of the glasses was tailored to match the thermal expansion of Ti6Al4V. By controlling the firing atmosphere, time, and temperature, it was possible to control the reactivity between the glass and the alloy and to fabricate coatings (25-150 micron thick) with excellent adhesion to the substrate. The optimum firing temperatures ranged between 800 and 840 degrees C at times up to 1 min in air or 15 min in N2. The same basic technique was used to prepare multilayered coatings with graded composition or with hydroxyapatite (HA) particles embedded in the outer layer. Some of these coatings presented excellent adhesion to the substrate and were able to form HA during in vitro tests in simulated body fluid.

In vitro behavior of silicate glass coatings on Ti6Al4V

The in vitro response in simulated body fluid (SBF) of silicate glass coatings on Ti6A14V was evaluated. Glasses belonging to the SiO2-CaO-MgO-Na2O-K2O-P2O5 system were used to prepare 50-70 m thick coatings on Ti6Al4V, employing a simple enameling technique. Glasses with silica content higher than 55 wt% can be used to prepare coatings that do not crack or delaminate and exhibit good adhesion to the alloy. It has been found that coatings with silica content lower than 60 wt% are more susceptible to corrosion and precipitate carbonated hydroxyapatite on their surface during in vitro tests. However, these coatings have a higher thermal expansion than the metal and are under tension. After 2 months in SBF cracks grow in the coating that reach the glass/metal interface and initiate delamination. Glasses with silica content higher than 60 wt% are more resistant to corrosion and have lower thermal expansion. These coatings do not crack but they do not precipitate apatite even after 2 months in SBF.

Novel Bioactive Functionally Graded Coatings on Ti6Al4V

Bioactive glass coatings are very promising for implant materials due to their good adhesion, mechanical stability, and bioactivity. A new family of silicate-based glasses has been prepared and applied to metallic implants using a simple enameling technique. The graded approach used here reduces stress between layers, and preliminary indentation tests indicate a strong glass/metal adhesion.

Bioactive glass-mesoporous silica coatings on Ti6Al4V through enameling and triblock-copolymer-templated sol-gel processing

The combination of thick glass coatings that can protect Ti6Al4V from corrosion in the body fluids, and mesoporous silica films able to readily induce the formation of apatite when immersed in a simulated body fluid (SBF), has been investigated in this work as a possible route towards more resistant and long-lasting implants. Glasses in the system Si-Ca-Mg-Na-K-P-O with thermal expansion coefficients close to that of Ti6Al4V were prepared and used to coat this alloy by an enameling technique. However, the glasses apt to coat Ti6Al4V exhibited a very limited capacity to induce apatite formation in SBF. In order to enhance their bioactivity, a thin film of mesoporous silica was applied on the exterior of the specimens by spin coating a sol-gel solution. When tested in SBF, these coatings induced apatite formation after 7 days. The mesoporosity of the silica film was created through a triblock-copolymer-templating process. The diameters of the mesochannels could be adjusted by changing the size of the directing agent. A preferred alignment of the mesostructure was observed. The removal of the organic templates could be achieved through a photocalcination treatment, which, compared to conventional thermocalcination, offered several advantages.

Spin casted mesoporous silica coatings for medical applications

Self-assembling organic/inorganic sol–gel systems were used to prepare mesoporous silica coatings that can qualify for medical applications. The cationic surfactant cetyltrimethyl ammonium chloride (CTAC) or an amphiphilic triblock copolymer were utilized as templates or structure-directing agents, and tetraethylorthosilicate (TEOS) as the silica precursor. Thin films could be applied on different substrates (glass, silicon, and titanium) by spin casting of the sol–gels. X-Ray diffraction analyses indicated that well ordered hexagonally packed mesostructures with unit cells of 3 and 13 nm, when CTAC and triblock copolymer were used, respectively, could be fabricated. Parameters such as CTAC/TEOS molar ratio and gel formation time highly affected the resulting structure, so the optimum values were established. A competition occurs between the formation of cylindrical mesochannels on the substrate and homogeneous nucleation of the silica in the sol–gel solution to form spherical particles. Therefore, a growing presence of silica particles on the spin casted coatings happens as the gel formation time is increased above ∼60 s, which results in poorer mesoporous films. When a triblock copolymer was used as template, a preferred alignment of the mesostructure was observed independently of the substrate. The removal of the organic template to hollow the pores was accomplished by photocalcination (selective ultraviolet irradiation). The resulting mesoporous silica coatings were able to induce apatite formation after 1 week of immersion in a simulated body fluid in physiological conditions, which is a sound indication of a bioactive behavior when tested in vivo. These results indicate that the coatings prepared by the methodology described in this work may be valid candidates to be used on implants.

Mesoporous silica thin films produced by calcination in oxygen plasma

Abstract A self-assembling amphiphilic triblock copolymer was utilized as template or structure-directing agent to apply mesoporous silica thin films on silicon by spin casting of an aqueous acidic sol–gel solution. Hexagonally packed mesoporous structures could be prepared, as indicated by X-ray diffraction analysis. Calcination in oxygen plasma was investigated as a possible method to hollow the pores. The resulting coatings were compared to those obtained through thermo- and photocalcination (selective ultraviolet irradiation), which is also a method recently developed in our group. Once the conditions were optimized, the oxygen plasma treatment proved to be a valid method to remove the organic template. Nevertheless, this methodology is comparatively more aggressive than photocalcination, resulting in flawed coatings.

Aligned bioactive mesoporous silica coatings for implants

Ongoing research is reported aimed at preparing mesoporous silica coatings on various substrates for medical applications by a biomimetic approach (self-assembling of organic/inorganic sol-gel systems into ordered structures). Tetraethylorthosilicate (TEOS) was selected as the silica precursor, and amphiphilic triblock copolymers poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide), and the cationic surfactant cetyltrimethyl ammonium chloride (CTAC), as structure-directing agents. The mesochannels diameter could be adjusted by changing the directing agent, and a preferred alignment of the mesostructure was observed independently of the used substrate (glass, silicon, Ti or Ti6Al4V). Three different treatments (thermocalcination, photocalcination, and solvent extraction) have been also studied to remove the organic templates, of which photocalcination showed to be the most versatile. When soaked in a simulated body fluid, mesoporous silica coatings induced apatite formation after seven days.© 2001 Kluwer Academic Publishers

Structural characterization of the metal/glass interface in bioactive glass coatings on Ti-6Al-4V

Coating Ti-based implants with bioactive materials promotes joining between the prostheses and the bone as well as increasing long-term implant stability. In the present work, the interface between Ti-6Al-4V and bioactive silicate glass coatings, prepared using a simple enameling technique, is analyzed. High-resolution transmission electron microscopy of the glass/alloy interface shows the formation of a reaction layer (∼150 nm thick) composed of Ti5Si3 nanoparticles with a size of ∼20 nm. This nanostructured interface facilitates the formation of a stable joint between the glass coating and the alloy.