Biomedical Engineering Reference
In-Depth Information
Glass surface reactivity is the biggest problem. Bioactive glasses (less than 60 mol% SiO
2
)
have a random, sheetlike network that is open for ion transport, resulting in the formation
of CaP and HA reaction layers (see Table 9.3). In addition, this open network allows other
cations (e.g., Fe, Cr, Ni, Ti, Mo, and Ta) to pass though the glass, thus altering the glasses
ability to form either or both the CaP and HA reaction layers. Surface bioactivity is altered
and in some cases lost. Only a few percent of migrated cations are needed to make the
glass nonbioactive.
Glazing (Enameling)
Early attempts by Hench to directly glaze (see Table 9.18) stainless-steel substrates with
bioactive glasses were not successful due to:
1. Rapid diffusion of multications into the glass, resulting in a loss of bioactivity [27]
2. Extensive crystallization in the glass layer, resulting in lack of surface adhesion [28]
Dual layering technique of glass coatings is one strategy that has been investigated to
address this problem (see Figure 9.11). Dual glass layers have been applied by either:
1. Creating a preoxidized (TiO
2
layer) first layer on a titanium substrate where the
second layer is formed by directly enameling it with liquid glass
2. Directly enameling a metallic substrate with liquid glass with matching thermal
expansion coefficients to create the first layer, followed by embedding either hydroxy-
apatite (HA) or bioglass (BG) particles onto TEC substrate-matching glass coatings
[27, 29]
Dual-layer approaches address the limited adhesion strength of coatings with titanium
substrates. For example, Hench and Wilson [9] demonstrated that when Co-Cr alloy sub-
strates oxidation temperatures were raised from 350°C to 500°C, glass-metal interfacial
500
º
C
80
650
º
C
40
800
º
C
350
º
C
0
0
5
10
15
20
Initial oxidation time (min)
FIGURE 9.11
Glass-metal bond strength as a function of time and temperature of oxidation [9].
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