Synthesis and Characterization of Hydroxyapatite Nanocoatings by Sol–Gel Method for Clinical Applications - Biological and Biomedical Coatings: Processing and Characterization - page 41

Thick-andThin-FilmCoatingMethods
Although other oxides and mixed oxides were trialed, hydroxyapatite nanocoatings
research and development did not initiate until the early 1990s (Ben-Nissan and Chai
1995).
Coatings offer the possibility of modifying the surface properties of surgical-grade mate-
rials to achieve improvements in performance, reliability, and biocompatibility. Techniques
such as physical vapor deposition (PVD), chemical vapor deposition (CVD), thermal and
electron beam evaporation, plasma metalorganic chemical vapor deposition (MOCVD),
electrochemical vapor deposition, thermal or diffusion conversion electrophoretic coat-
ing, simulated body fluid (SBF), and sol-gel processing, have been used to produce both
macro- and nanocoatings. Except for a few specific sol-gel compositions, all these meth-
ods have problems regarding their stoichiometry and chemical composition control and
their dissolution within the physiological environment due to the production methods
and high-temperature heat treatments used.
In the context of biomedical applications, the definitions of macro-, micro-, thin film,
and nanothickness, or more generally, thin films, have been used interchangeably and/or
wrongly, the authors of this chapter believe that coatings greater than 1000 μm should be
considered thick or macrocoatings, 1 to 1000 μm should be considered thin-film coatings
or microcoatings, and below 1 μm should be considered nanocoatings.
Taking into account that most of these coating methods are well covered within this
topic and a large number of excellent review papers have appeared during the past two
decades, only a brief summary of the SBF and biomimetic approach is covered in the next
section.
Biomimetics and SBF Hydroxyapatite
Biomimetics Approach
The technology of coating can benefit from the study of the natural world, as a model for
biomimicry or as a source of inspiration for new designs. One of the ways the study of
nature could augment strategies in thin film coating is by mimicking the growth of biocer-
amics in ambient conditions in water.
Nature is a consummate problem solver. We see exquisite, almost perfect designs all
around us. There are now systematic ways of harnessing these designs for all sorts of
challenges in regenerative medicine. A vital part of making regenerative medicine more
of a future clinical success is the production of highly proficient scaffolds that function
at many different levels: the nanoscale, microscopic, and macroscopic. It can be envis-
aged that these proficient scaffolds incorporated with biogenic additives such as BMP and
MSC will generate macro structures that can pull cells into the hard tissue and in nano-
scale release encapsulated chemical signals in a targeted way, and convey them into the
body. Why not reverse engineer the structural fabric of human tissues and faithfully copy
them artificially? This is not only improbably difficult but is also too time-consuming a
task. Instead we extract the key essence of what we are trying to reassemble. There are
abundant sources of structures and materials that can be used for a different function to
their evolved intended one. The simplest strategy is to select a predesigned, preformed
structure but modify it in a directed way specifically for its new intended function. In
other strategies we pull out inventive principles to solve a similar problem we are faced