**4.2 Titanium dioxide (TiO2) as a sol-gel coating**

TiO2 is a very interesting material due to its technological applications in biomaterials. An example of this is sol-gel-derived titania and its coatings, which are bioactive materials, and are used in applications as diverse as biomaterials [79, 80]. Advantages of sol-gel processes over other methods are their controlled transformation of the microstructure of the deposited film. Today microstructure control is needed for many applications and the use of sol-gel routes opens up new possibilities.

In common sol-gel methods for preparing TiO2 materials, highly reactive alkoxide titanium precursors are violently hydrolyzed and further condense to form a Ti▬O▬Ti network. Unfortunately, this route can lead to precipitation of amorphous particles with uncontrolled structures. To overcome many of the specific problems of sol-gel methods employing water as the hydrolysis agent and to control hydrolysis and polycondensation reactions, the application of nonhydrolytic methods, ionic liquids, organic additives, and coordination chemistry have been attempted [81–83].

On the other hand, it has been reported that TiO2 films can effectively improve corrosion resistance and biocompatibility [84–86]. An example of this is given in titania films on biomedical steel exhibit anticorrosion properties in physiological solutions [87, 88]. At present, resistant sol-gel TiO2 coatings synthesized by heat treatment under low temperature have received good results for the preparation of anticorrosive and bioactive coatings to protect metal implants.

The desired properties can be obtained by careful control of reaction conditions or by the use of suitable additives [89]. Moreover, sol-gel processing offers a unique opportunity to prepare layers at low temperatures so that an essential part of the sol-gel preparation process is the thermal treatment that is necessary to form pure TiO2 films.

Conventional metal implants can be surface modified with sol-gel coatings, but organic synthetic polymers such as polyether-ether-ketone (PEEK) can also be surface modified with TiO2 sol-gel-derived coatings to overcome low bioactivity due to their chemical inertness, which is characteristic of most synthetic organic polymers. These sol-gel-derived TiO2 coatings showed greater bone bonding ability than PEEK [90].

However, to avoid corrosion, a TiO2 dip-coating method and its variants are commonly used for the deposition of TiO2 sol-gel-derived coatings onto metal implants [91]. For example, an NiTi surgical alloy was surface modified with thin films of TiO2 to improve corrosion resistance, but these films also showed blood compatibility in vitro [92]. Avoiding corrosion and improving biocompatibility with surrounding tissues are not the only ultimate goals of sol-gel-coating metals of biomedical interest, avoiding the release of toxic ions, possibly through degradation, that some alloys may contain is another such goal [92].

With respect to annealing time and temperature in sol-gel coatings, both parameters exhibit a dependence with the degradation rate, as studied in the TiO2 sol-gel deposited onto a magnesium alloy (AZ31), where the treatment with low annealing temperatures decreased the corrosion rate. Long-time treatment of annealing helped to enhance corrosion resistance [93].

**57**

applications [104].

and industrial feasibility [110, 111].

tissue while stimulating bone regeneration [97].

*Surface Science Engineering through Sol-Gel Process DOI: http://dx.doi.org/10.5772/intechopen.83676*

**4.3 Sol-gel-derived bioactive glass coatings**

TiO2 sol-gel coatings doped with a high calcium ion concentration showed better corrosion resistance for M30NW biomedical alloy substrates in a simulated body fluid (SBF) test than similar coatings with low calcium ions concentrations [94].

Bioglasses are a family of materials that have shown bioactivity for bone repair and can bond with living bone [95]. In 1971, the first bioglass named 45S5 was discovered by Hench, and since then, many other glass compositions have been developed. Bioglass 45S5 is composed of 45 wt% SiO2, 24.5 wt% CaO, 24.5 wt% Na2O, and 6.0 wt% P2O5, but other similar composition has been used and in some cases enhancing components can be added [96]. Bioglass 45S5 compositions have been shown to be optimal for biomedical applications because it is similar to that of HAp, the mineral component of bone. Ca/P ratios in SiO2·CaO·P2O5 glasses coatings can be controlled with stoichiometric control of TEOS, calcium nitrate tetrahydrate (Ca(NO3)2·4H2O), and triethyl phosphate (TEP) as sol-gel precursors [97]. Coatings with higher Ca/P ratios showed that greater cell proliferation, however, growth inhibition was observed in response to a low Ca/P-ratio in coating compositions [98]. The surface of a bioglass implant, when subjected to an aqueous solution or body fluids, converts to a silica-CaO/P2O5-rich gel layer that subsequently mineralizes into hydroxycarbonate in hours [99–101]. Bone tissue growth improved with increasing dissolution [102]. This gel layer resembles the HAp matrix so much that

osteoblasts were differentiated and new bone was deposited [103].

Bioglasses are the most promising materials for bone grafting in several clinical applications such as orthopedic, dental, maxillofacial, and otolaryngological

From a synthetic point of view, bioglasses can be prepared either by melting or sol-gel methods, which affect their physical and biological properties [105], but it is also important to define the methods of coating preparation or deposition affecting the ultimate performance of the coatings. Bioglass coatings are usually deposited onto metals or alloys using sol-gel [106], electrophoretic deposition [107], laser cladding [108], and thermal spraying (plasma spraying and high-velocity oxy-fuel) techniques [109]. The technique most employed to spray bioglass since 1980 is thermal spraying and specifically atmospheric plasma spraying, due to its low cost

Sol-gel-derived bioglasses are excellent materials for use in tissue engineering applications, such as covering prosthetic metallic implants. Recently, porous bioactive glasses have been derived through sol-gel processing in an attempt to increase the specific surface area and thus the surface reactivity and degradability of the material. This approach allows the material to be replaced ultimately by natural

The sol-gel technique can be used to coat 316 L SS [112], titanium [4], and magnesium biomedical alloys [106] with bioactive glass or derived glass-ceramic. For example, the formation of an apatite layer assures the bioactivity of the bioglass coating, which also improves the corrosion resistance of 316 L SS substrates. Bioactive glass-coated 316 L SS showed greater pitting corrosion resistance than pristine samples. It was concluded that by using the bioactive glass-coated 316 L SS as a human body implant, improvement of corrosion resistance, as an indication of biocompatibility, and bone bonding could be obtained simultaneously [112]. Uncoated 316 L SS possesses high corrosion current density (Icorr = 265 nA/cm2

Relatively dense sol-gel coatings can be obtained with postheat treatment, causing a substantial volume contraction. In parallel, residual stress gradually

and thus low corrosion resistance in normal saline solution [112].

)

*Applied Surface Science*

implants [76–78].

possibilities.

attempted [81–83].

implants.

TiO2 films.

than PEEK [90].

**4.2 Titanium dioxide (TiO2) as a sol-gel coating**

Out of these, the sol-gel route for surface modification of Ti implants is of particular interest because of simple and inexpensive methodology, low temperature processing, and suitability for coating substrates of irregular shapes, such as

TiO2 is a very interesting material due to its technological applications in biomaterials. An example of this is sol-gel-derived titania and its coatings, which are bioactive materials, and are used in applications as diverse as biomaterials [79, 80]. Advantages of sol-gel processes over other methods are their controlled transformation of the microstructure of the deposited film. Today microstructure control is needed for many applications and the use of sol-gel routes opens up new

In common sol-gel methods for preparing TiO2 materials, highly reactive alkoxide titanium precursors are violently hydrolyzed and further condense to form a Ti▬O▬Ti network. Unfortunately, this route can lead to precipitation of amorphous particles with uncontrolled structures. To overcome many of the specific problems of sol-gel methods employing water as the hydrolysis agent and to control hydrolysis and polycondensation reactions, the application of nonhydrolytic methods, ionic liquids, organic additives, and coordination chemistry have been

On the other hand, it has been reported that TiO2 films can effectively improve corrosion resistance and biocompatibility [84–86]. An example of this is given in titania films on biomedical steel exhibit anticorrosion properties in physiological solutions [87, 88]. At present, resistant sol-gel TiO2 coatings synthesized by heat treatment under low temperature have received good results for the preparation of anticorrosive and bioactive coatings to protect metal

The desired properties can be obtained by careful control of reaction conditions or by the use of suitable additives [89]. Moreover, sol-gel processing offers a unique opportunity to prepare layers at low temperatures so that an essential part of the sol-gel preparation process is the thermal treatment that is necessary to form pure

Conventional metal implants can be surface modified with sol-gel coatings, but organic synthetic polymers such as polyether-ether-ketone (PEEK) can also be surface modified with TiO2 sol-gel-derived coatings to overcome low bioactivity due to their chemical inertness, which is characteristic of most synthetic organic polymers. These sol-gel-derived TiO2 coatings showed greater bone bonding ability

However, to avoid corrosion, a TiO2 dip-coating method and its variants are commonly used for the deposition of TiO2 sol-gel-derived coatings onto metal implants [91]. For example, an NiTi surgical alloy was surface modified with thin films of TiO2 to improve corrosion resistance, but these films also showed blood compatibility in vitro [92]. Avoiding corrosion and improving biocompatibility with surrounding tissues are not the only ultimate goals of sol-gel-coating metals of biomedical interest, avoiding the release of toxic ions, possibly through degrada-

With respect to annealing time and temperature in sol-gel coatings, both parameters exhibit a dependence with the degradation rate, as studied in the TiO2 sol-gel deposited onto a magnesium alloy (AZ31), where the treatment with low annealing temperatures decreased the corrosion rate. Long-time treatment of annealing

tion, that some alloys may contain is another such goal [92].

helped to enhance corrosion resistance [93].

**56**

TiO2 sol-gel coatings doped with a high calcium ion concentration showed better corrosion resistance for M30NW biomedical alloy substrates in a simulated body fluid (SBF) test than similar coatings with low calcium ions concentrations [94].
