Atsushi Hozumi

ζ-Potentials of planar silicon plates covered with alkyl- and fluoroalkylsilane self-assembled monolayers

Abstract ζ-potentials of planar Si plates covered with an alkyl- or a fluoroalkyl organosilane self-assembled monolayer (SAM) were measured over a pH range of 3–11 by means of an electrophoretic light scattering spectrophotometer, and were compared with those of Si plates covered with native oxide (SiO 2 ), polyethylene (PE) and polytetrafluoroethylene (PTFE). The SAMs were prepared on SiO 2 /Si substrates by a chemical vapor deposition (CVD) method in which an alkylsilane, that is, octadecyltrimethoxysilane [CH 3 (CH 2 ) 17 Si(OCH 3 ) 3 , i.e. ODS] or a fluoroalkylsilane, that is, (heptadecafluoro-1,1,2,2-tetrahydrodecyl) trimethoxysilane [CF 3 (CF 2 ) 7 CH 2 CH 2 Si(OCH 3 ) 3 , i.e. FAS] was used as a precursor. The SAM-covered Si plates were found to be 35–65% less charged than the SiO 2 /Si plate. Furthermore, SAM formation reduced surface acidity. The isoelectric points (IEPs) of the ODS- and FAS-SAM surfaces observed at pH 3.5–4 were higher than that of SiO 2 /Si (∼pH 2.0). These results are attributable to a reduction in the density of surface silanol (Si–OH) groups on the SiO 2 /Si substrate. Si–OH groups were consumed due to the formation of siloxane bondings with the organosilane molecules which compose the SAMs. Finally, the IEPs of the ODS- and FAS-SAM covered surfaces were found to be almost identical to those of the PE and PTFE bulk surfaces.

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.

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

Improvement of chemical resistance of apatite/titanium composite coatings deposited by RF plasma-spraying: surface modification by chemical vapor deposition

Abstract The chemical resistance of plasma-sprayed hydroxyapatite (HA) coatings on titanium substrates was successfully improved by fluoroalkylsilane (FAS), that is, 1H,1H,2H,2H-perfluorodecyltrimethoxysilane (FAS17) self-assembled monolayers (SAMs) formed through chemical vapor deposition. FAS17-SAMs were formed on the topmost HA layer of HA/Ti composite coatings prepared with rf input powers of 12, 17 or 27 kW. Each sample was immersed in a pH 6 buffer solution and its solubility investigated by an ion chromatography. All of the FAS17-coated samples became very hydrophobic with contact angles of more than 130°. The total amount of Ca2+ ions released from the FAS17-coated HA surface decreased in comparison with the samples not coated with FAS17. In addition, the amount of Ca2+ ions released was significantly influenced by the base HA coatings. Scanning electron microscopic observation showed less damage on the FAS17-coated surfaces even after 12 weeks’ immersion in the acidic solution, demonstrating that the effectively protected the underlying HA layer.