Biomedical Engineering Reference
In-Depth Information
SiO
2
B
C
A
D
CaO
Na
2
O
FIGURE 2.14
Approximate regions of the tissue−glass−ceramic bonding for the SiO
2
-CaO-Na
2
O system. A:
Bonding within 30 days. B: Nonbonding; reactivity is too low. D: Bonding does not form glass. (From Hench L.L.
and Ethridge E.C. 1982.
Biomaterials: An Interfacial Approach
, p. 147. Academic Press, New York. With permission.)
inducing direct bonding with bone (Table 2.14). The bonding to bone is related to the simultaneous
formation of a calcium phosphate and SiO
2
-rich film layer on the surface, as exhibited by the 46S5.2-
type Bioglass. If a SiO
2
-rich layer forms first and a calcium phosphate film develops later (46-55 mol%
SiO
2
samples) or no phosphate film is formed (60 mol% SiO
2
), then direct bonding with bone does not
occur (Park and Lakes, 1992). The approximate region of the SiO
2
-CaO-Na
2
O system for the tissue-
glass-ceramic reaction is shown in Figure 2.14. As can be seen, the best region (region A) for good tissue
bonding is the composition given for 46S5.2-type Bioglass (see Table 2.14) (Park and Lakes, 1992).
2.4.2 Ceravital
The composition of Ceravital is similar to that of Bioglass in SiO
2
content but differs somewhat in other
components (see Table 2.14). In order to control the dissolution rate, Al
2
O
3
, TiO
2
, and Ta
2
O
5
are added
in Ceravital glass ceramic. The mixtures, after melting in a platinum crucible at 1500°C for 3 h, are
annealed and cooled. The nucleation and crystallization temperatures are 680°C and 750°C, respec-
tively, each for 24 h. When the size of crystallites reaches approximately 4 Å and the characteristic nee-
dle structure is not formed, the process is stopped to obtain a fine-grain structured glass ceramic (Park
and Lakes, 1992).
Glass ceramics have several desirable properties compared to glasses and ceramics. The thermal coef-
ficient of expansion is very low, typically 10
−7
-10
−5
°C
−1
, and in some cases it can even be made negative.
Due to the controlled grain size and improved resistance to surface damage, the tensile strength of these
materials can be increased by at least a factor of 2, from about 100 to 200 MPa. The resistance to scratch-
ing and abrasion of glass ceramics is similar to that of sapphire (Park and Lakes, 1992).
A transmission electron micrograph of Bioglass glass ceramic implanted in the femur of rats for
6 weeks showed intimate contacts between the mineralized bone and the Bioglass (Figure 2.15). The
mechanical strength of the interfacial bond between bone and Bioglass ceramic is on the same order
of magnitude as the strength of the bulk glass ceramic (850 kg/cm
2
or 83.3 MPa), which is about three-
fourths that of the host bone strength (Park and Lakes, 1992).
A negative characteristic of the glass ceramic is its brittleness. In addition, limitations on the compo-
sitions used for producing a biocompatible (or osteoconductive) glass ceramic hinders the production
of a glass ceramic which has substantially higher mechanical strength. Thus, glass ceramics cannot be
