Biomimetic Preparation and Biomineralization of Bioceramics - Advanced Bioactive Inorganic Materials for Bone Regeneration and Drug Delivery - page 124

5.2.3 Nanocrystalline Calcium Phosphate Bioceramics
The mineral phase of human bone and teeth consists primarily of substi-
tuted apatite crystals with size <100 nm. Synthetic HAp ceramic is chemi-
cally equivalent to these apatite crystals with subtle differences. Thus, HAp
is a preferred material for bone repair because of its stability under in vivo
conditions, compositional similarity, excellent biocompatibility, osteoconduc-
tivity, and ability to promote osteoblast functions. Nanocrystalline HAp is
expected to have homogeneous resorption and better bioactivity than coarser
crystals (Stupp and Braun 1997; Webster et al. 2001). Nanostructured biomate-
rials promote osteoblast adhesion and proliferation, osteointegration, and the
deposition of calcium-containing minerals on the surface of these materials
(Xu et al. 2005). Also, nanostructured bioceramics can be sintered at lower
temperatures; thereby, problems associated with high temperature sintering,
such as abnormal grain growth and formation of microcracks can be avoided.
Properties and performance of HAp bioceramic can be improved by chang-
ing powder particle size and shape, their distribution, and agglomeration.
The biomimetism of bioceramics based on HAp could be carried out at
different levels: composition, particle size and morphology, microstructure,
and surface reactivity. The aim of a researcher is to realize a preparation of
bioceramics that is biomimetic in all these characteristics in order not only
to optimize the microstructure but also, and more ambitiously, to mimic the
biogenic apatitic composition and particle dimensions in its functionality.
This concept should be utilized in designing and preparing nanocrystalline
biomimetic bioceramics in replacing hard tissues. In fact, the nanosize of bio-
logical tissue building blocks is one of the bases of their self-assemble ability
and one that needs to be replaced by synthetic bioceramics in the synthesis
of structured architectures on a multiple-length scale.
Many different methodologies have been proposed to prepare nanosized
HAp (Schmidt 2000). They include wet chemical precipitation (Buehler et al.
2001), sol-gel synthesis (Kuriakose et al. 2004), solid state synthesis (Rao et al.
1997), coprecipitation (Kong et al. 2002), hydrothermal synthesis (Bonnelye
et al. 2008), microwave processing (Siddharthan et al. 2006), high tempera-
ture flame spray pyrolysis, and several other methods by which nanocrys-
tals of various shapes and sizes can be obtained. In general, the shape, size,
and specific surface area of the HAp appear to be very sensitive to both the
reaction temperature and the reactant addition rate. Depending upon the
technique, materials with various morphology, stoichiometry, and level of
crystallinity have been obtained. In order to optimize its specific biomedical
applications, especially bioceramics function, the physical-chemical features
that should be tailored in synthetic biomimetic HAp are dimensions, poros-
ity, morphology, and surface properties.
Sintering of HAp is a complex process since many parameters influence
the sintering process. Given its poor sinterability, HAp ceramics show low
strength, especially in wet environments as would be found in physiological