Biomimetic Preparation and Biomineralization of Bioceramics - Advanced Bioactive Inorganic Materials for Bone Regeneration and Drug Delivery - page 126

beneficial in enhancing the polarizability of pure HAp by inhibiting high-
temperature HAp phase decomposition, and established the significance of
dopants on polarized HAp for bone graft applications. Sol-gel chemistry can
be used to prepare Sr-doped CaP ceramics exhibiting a porous structure.
Doping with strontium ions has a clear effect on the proportions of the dif-
ferent CaP phases, increasing the amount of β-tricalcium phosphate (β-TCP).
Strontium ions also substitute for calcium in both HAp and TCP in specific
sites (Renaudin et al. 2008). Meanwhile, latteric parameters (a, c) unit cell
volume and density in Sr-substituted HAp (Sr-HAp), Sr x Ca 1- x (PO 4 ) 3 OH are
shown to increase linearly with strontium addition and are consistent to a
maximum at x = 0.5, then decrease with the increase of Sr content (O'Donnell
et al. 2008). Therefore, the solubility of Sr-HAp increases with increasing Sr
content and this can be interpreted as a destabilization of the crystal struc-
ture by the larger strontium ion (Pan et al. 2009).
Zinc can also be doped into the CaP structure by a wet chemical method
in aqueous solutions. The apatite lattice parameters and phase changes with
the inclusion of zinc due to the partially substituted Ca ions in the apatite
structure (Ren et al. 2009). Variation in the amount of ZnO present in TCP
and HAp ceramics shows differences in sintering behavior. At a sintering
temperature of 1250°C both TCP and HAp experience an increase in densifi-
cation with increasing ZnO content. The ceramics sintered at 1250°C proved
to be harder than those sintered at 1300°C. Analysis of microstructure using
SEM can reveal that grain size changes due to ZnO doping in TCP but have
limited influence on HAp (Bandyopadhyay et al. 2007).
Silicon (Si) substitution in the crystal structures of calcium phosphate
(CaP) ceramics generates materials with superior biological performance
to stoichiometric counterparts (Vallet-Regi and Arcos 2005). Si, an essential
trace element required for healthy bone and connective tissues, influences
the biological activity of CaP materials by modifying material proper-
ties and by direct effects on the physiological processes in skeletal tissue
(Pietak et al. 2007). The synthesis of Si-HAp and Si-α-TCP has focused on
wet chemical methods where Si is introduced as a chemical carrier such
as tetrapropyl orthosilicate (TPOS) or tetraethyl orthosilicate (TEOS), Si
IV acetate (Si(COOCH3)4) (Gibson et al. 1999, 2002; Kim et al. 2003), or as
some form of nanoparticulate silica during the precipitation or firing of an
amorphous CaP or nanocrystalline HAp (Reid et al. 2005; Li et al. 2006).
Hydrothermal (Tang et al. 2005) and solid state (Boyer et al. 1997) methods
of preparation have also been investigated. The materials are typically sin-
tered at temperatures between 700°C and 1200°C. TCP, unlike HAp, do not
precipitate from solution but rather are created from decomposition reac-
tions at temperatures exceeding 700°C. A calcium-deficient HAp or amor-
phous CaP material with a Ca/P ratio between 1.5 and 1.67 will decompose
into β-TCP or a biphasic system of β-TCP and HAp with sintering between
700°C and 1125°C. Sintering above 1125°C, the α-polymorph of TCP, is the
stable phase. However, the presence of Si stabilizes the α-TCP polymorph,