Biomedical Engineering Reference
In-Depth Information
TABLE 9.19
Composition and Main Properties Used for Bioactive Particle Embedding Approach
ThermalExpansion
Coefficient
(200-400ºC)
Young's
Modulus
(GPa)
SiO
2
(wt.%)
Na
2
O
(wt.%)
K
2
O
(wt.%)
CaO
(wt.%)
MgO
(wt.%)
P
2
O
5
(wt.%)
Material
Bioglass
(45S5)
45.0
24.5
0.0
24.5
0.0
6.0
15.1
70
6P57
56.5
11.0
3.0
15.0
8.5
6.0
10.8
80-90
6P68
67.7
8.3
2.2
10.1
5.7
6.0
8.8
80-90
strengths could be doubled (Figure 9.11). In addition, interfacial strengths could be increased
with time of oxidation. However, they found that there was a critical oxidation tempera-
ture where higher than 650ºC, glass layers were created with decreasing shear strength
with increasing oxidation time.
Hench describes a two-layer glass coatings approach where the first glass layer provides
a direct interfacial bond with the metallic substrate (the glass has substrate-matching ther-
mal expansion coefficients coupled with a low
T
g
). The second glass layer is composed of a
bioactive glass fused to the first layer (again with a matching thermal coefficients of expan-
sion coupled with slightly lower
T
g
).
An alternative to making the second bioactive glass coating is the embedding approach
developed by Gomez-Vega and coworkers [30] and is interesting because:
1. Substrates were first coated* with specially formulated glass compositions (6P57,
6P58; see Table 9.19) with thermal expansion coefficients matched to the titanium
substrates.
2. Coating bioactivity was introduced via embedded HA or BG particles on the coat-
ing. They found that HA particles were immersed partially during heating and
remained firmly embedded on the coating after cooling, whereas the BG particles
softened and some infiltration into the glass coating took place during heat treat-
ment (800-840ºC). In addition, when the particle concentration exceeded 20% sur-
face, cracked coatings due to excessive induced stress were observed.
Due to limitations in the size of this chapter, I will not repeat the excellent review by
Hench and Andersson [27], who offer an excellent explanation of the need for composi-
tional optimization for bioactive glass coatings on metallic implants.
Ideal Thick Coating
There remains significant interest in developing thick bioactive glass coatings on metallic
implants.
*
Coating was via sedimentation of isopropyl/glass powder slurries on titanium alloy substrates, followed by
firing at 600ºC. Coatings were then thermally soaked at 1.3 × 10
4
Pa at 700-820ºC and then allowed to cool
to room temperature [45]. Since Bloyer and coworkers classify this coating method as enameling, it has been
included in this section.
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