Biomedical Engineering Reference
In-Depth Information
5.2 Biomimetic Preparation of Bioceramics
In nature, there are numerous complex inorganic and inorganic-organic
hybrid forms by natural evolution over millions of years. Calcium phos-
phates (CaPs) are the principal inorganic constituents of normal (bones,
teeth, deer antlers, and some species of shells) and pathological (dental
and urinary calculus and stones) calcifications. Nearly all hard tissues of
the human body are made of CaPs. Structurally, they occur mainly in the
form of poorly crystallized nonstoichiometric fluoride, sodium, magne-
sium, and carbonate-substituted hydroxyapatite (HAp) (Pasteris et al. 2008).
Biomimetism of synthetic CaPs can be carried out at different levels, such
as composition, structure, morphology, bulk, and surface chemical-physical
properties. Biomaterials can copy these characteristics to not only optimize
their interaction with biological tissues but also mimic biogenic materi-
als in their functionalities. Therefore, mimicking “nature” and designing
biomimetic bioceramics represent a promising way to reach technological
innovations in biomedical fields, since biological materials exhibit a high
degree of hierarchical architecture, hybridization, bioactivity, adaptability,
and porosity. Detailed information on CaPs, their structure, chemistry, other
properties, and biomedical application have been comprehensively reviewed
(Dorozhkin and Epple 2002; Dorozhkin 2009). In this section some methods
of synthesizing biomimetic CaP bioceramics will be described.
5.2.1 Transformation and Replication of Biological
Origin into Calcium Phosphate Ceramics
Porous solids have the advantage of allowing circulation of body fluids and
of increasing the potential for firm attachment of body tissue. Porous biomi-
metic CaP ceramics in simulating spongy bone morphology (porosity vary-
ing from a microporosity >1 µm to a macroporosity ranging from 100 to 1000
µm) has been prepared using various technologies to control pore dimen-
sion, shape, distribution, and interconnections. The earliest replamineform
process (meaning replicated life forms) for porous metal and ceramic mate-
rials using natural marine invertebrates as templates was reported in the
early 1970s (White et al. 1972). The porous calcium carbonate (CC) skeletal
structure of some marine invertebrates has been achieved through specific
building principles selected by evolution, and can be partially templated
in man-made materials for the fabrication of porous bioceramic scaffolds
for bone repair. The first reported coral-to-CaP conversion by employing
exchange reactions at elevated temperatures and pressures was published
by Roy et al. in the 1970s (Roy and Linnehan 1974). The concern of the work,
of course, is the exchange process by which the porous coral structure
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