Page 391 - Biomedical Engineering and Design Handbook Volume 1, Fundamentals

368 BIOMATERIALS

Synthetic apatites are processed via hydrolysis, hydrothermal synthesis and exchange, sol-gel
techniques, wet chemistry, and conversion of natural bone and coral (Koeneman et al., 1990).
Differences in the structure, chemistry, and composition of apatites arise from differences in pro-
cessing techniques, time, temperature, and atmosphere. Understanding the processing-composition-
structure-processing synergy for calcium phosphates is therefore critical to understanding the in vivo
function of these materials. For example, as stoichiometric HA is heated from room temperature, it
becomes dehydrated. Between 25 and 200°C, adsorbed water is reversibly lost. Between 200 and
400°C, lattice-bound water is irreversibly lost, causing a contraction of the crystal lattice. At tem-
peratures above 850°C, reversible weight loss occurs, indicating another reversible dehydration
reaction. Above 1050°C, HA may decompose into β-TCP and tetracalcium phosphate (Van
Raemdonck et al., 1984), and at temperatures above 1350°C, β-TCP transforms into α-TCP.
Analogous reactions occur with nonstoichiometric HA, but the reaction products differ, as a function
of the Ca/P ratio (Van Raemdonck et al., 1984).
The mechanism of biological bonding to calcium phosphates is as follows (de Bruijn et al., 1995).
Differentiated osteoblasts secrete a mineralized matrix at the ceramic surface, resulting in a narrow,
amorphous electron-dense band approximately 3 to 5 μm thick. Collagen bundles form between this
zone and cells. Bone mineral crystals nucleate within this amorphous zone in the form of an octa-
calcium phosphate precursor phase and, ultimately, undergo a conversion to HA. As the healing site
matures, the bonding zone shrinks to about 0.05 to 0.2 μm, and bone attaches through a thin
epitaxial layer as the growing bone crystals align with apatite crystals of the material.
Calcium-phosphate-based bioceramics have also been used as coatings on dense implants
and porous surface layers to accelerate fixation to tissue (Kohn and Ducheyne, 1992; Cook
et al., 1992; Ducheyne et al., 1980; Oonishi et al., 1994). Bond strength to bone, solubility, and
in vivo function vary, suggesting a window of material variability in parallel with a window of
biological variability.
Processing techniques used to bond Ca-P powders to substrates include plasma and thermal-
spraying (de Groot et al., 1987; Koch et al., 1990), sintering (de Groot, 1983; Ducheyne, et al., 1986,
1990), ion-beam, and other sputter techniques (Ong et al., 1991; Wolke et al., 1994), electrophoretic
deposition (Ducheyne et al., 1986,1990), sol-gel techniques (Chai et al., 1998), pulsed laser deposi-
tion (Garcia et al., 1998), and chemical vapor deposition (Gao et al., 1999).
Different structures and compositions of Ca-P coatings result from different processing approaches,
and modulate biological reactions. For example, increased Ca/P ratios, fluorine and carbonate con-
tents, and degree of crystallinity lead to greater stability of the Ca-P (Posner, 1985b; Van Raemdonck
et al., 1984). Calcium phosphates with Ca/P ratios in the range 1.5 to 1.67 yield the most beneficial
tissue response.

15.3.3 Bioactive Ceramic Composites
Bioactive ceramics typically exhibit low strength and toughness. The design requirement of bioac-
tivity supercedes any mechanical property requirement and, as a result, mechanical properties are
restricted. Bioceramic composites have therefore been synthesized as a means of increasing the
mechanical properties of bioactive materials. Three approaches are used in developing bioceramic
composites: (1) utilize the beneficial biological response to bioceramics, but reinforce the ceramic
with a second phase as a strengthening mechanism; (2) utilize bioceramic materials as the second
phase to achieve desirable strength and stiffness; and (3) synthesize transient scaffold materials for
tissue (re)generation (Ducheyne, 1987).
Bioactive glass composites have been synthesized via thermal treatments that create a second
phase (Gross and Strunz, 1980, 1985; Kitsugi et al., 1986). By altering the firing temperature and
composition of the bioactive glass, stable multiphase bioactive glass composites have been produced.
Adding oxyapatite, fluorapatite, β-Wollastonite, and/or β-Whitlockite results in bending strengths
2 to 5 times greater than that of unreinforced bioactive glasses (Kitsugi et al., 1986). Calcium phos-
phates have been strengthened via incorporation of glasses, alumina, and zirconia (Ioku et al., 1990;
Knowles and Bonfield, 1993; Li et al., 1995).