Synthesis and Characterization of Hydroxyapatite Nanocoatings by Sol–Gel Method for Clinical Applications - Biological and Biomedical Coatings: Processing and Characterization - page 44

and a Ca(OH) 2 saturated solution. The pretreated samples were then soaked in SBF of two
different concentrations, 1.5 times and 5.0 times the ion concentration of blood plasma. The
cotton was then burnt out via sintering of the ceramic coating at 950°C, 1050°C, 1150°C,
and 1250°C. Hollow calcium phosphate fibers approximately 25 μm in diameter and with
a 1-μm wall thickness were successfully manufactured. 5.0 × SBF produced a thicker and
more crystalline coating of greater uniformity.
Although in micron size rather than nanosize, a bioactive CHA layer on cellulose fabrics
was developed by Hofmann et al. (2006). Nonwoven cellulose (regenerated, oxidized) fab-
rics were coated with CHA using a procedure based on the SBF method. SBF with a high
degree of supersaturation (5 × SBF) was applied to accelerate the biomimetic formation of
bonelike apatite on the cellulose fabrics. After creating calcium phosphate nuclei on the
cellulose fibers in an initial 5 × SBF with high Mg 2+ and HCO 3 - concentrations, the cellu-
lose fabrics were additionally soaked in a second 5 × SBF that was optimized with regard
to accelerated crystal growth by reduced Mg 2+ and HCO 3 − concentrations. The carbonated
apatite layer thickness increased from 6 μm after 4 h of soaking in the latter solution, to 20
μm after 48 h. The amount of CO 3 2− substituting PO 4 3− in the hydroxyapatite (HAp) lattice
of the precipitates can be varied by changing the soaking time.
TheSol-GelProcess
Sol-gel processing is a versatile and attractive technique since it can be used to fabricate
ceramic coatings from solutions by chemical means. The sol-gel process is relatively easy
to perform and complex shapes can be coated, and it has also been demonstrated that the
nanocrystalline grain structure of sol-gel coatings produced results in improved mechan-
ical properties (Kirk and Pilliar 1999; Chen and Lacefield 1994; Anast et al. 1992; Roest et
al. 2004; Roest 2010).
The sol-gel process goes back to the genesis of chemistry. It was first identified in 1846 as
an application technology, when Ebelmen (1846) observed the hydrolysis and polyconden-
sation of tetraethylorthosilicate (TEOS). In 1939, the first sol-gel patent was published cover-
ing the preparation of SiO 2 and TiO 2 coatings (Geffcken and Berger 1939). Roy and Roy (1954)
recognized the potential for producing high-purity glasses using methods not possible with
traditional ceramic-processing techniques. In doing so, they generated the first report on the
use of sol-gel technology to produce homogeneous multicomponent glasses.
Schroeder reported the first investigation conducted by Schott Glass involving sol-gel
synthesized coatings (Schroeder 1965). Mixed-oxides coatings were developed although
they were mainly interested in single-oxide optical coatings of SiO 2 and TiO 2 .
During the late 1980s and 1990s, sol-gel technology also found applications in a number
of technology fields, such as biomedical applications (Ben-Nissan and Chai 1995; Chai et
al. 1998), in optoelectronics, smart windows, electronic materials, for stabilization of radio-
active isotopes (Bartlett and Woolfrey 1990), the laser industry (Yoldas 1984; Floch et al.
1995), and a range of electronic and optoelectronic applications (Anast 1996).
A number of excellent review articles, topic chapters, and topics cover the science
and technology of the basics of sol-gel technology for various ceramic oxide systems
(Mazdiyasni 1982; Sakka et al. 1984; Yoldas 1984; Roy 1987; Klein 1988; Scriven 1988; Brinker
et al. 1988; Brinker and Scherer 1990; Hench and West 1990).