Biomedical Engineering Reference
In-Depth Information
Figure 8.2
A backscatter electron micrograph of a zirconia cylinder coated with
bioactive glass 30 days after surgery. Scale bar is 500
μ
m. (Reprinted with permission
from [8]. Copyright (2002) Elsevier Ltd.)
Figure 8.2 shows a backscatter electron micrograph of a zirconia cylinder
coated with bioactive glass 30 days after surgery. Four regions can be
evidenced at the interface between the implant and the bone: 1, zirconia
cylinder; 2, region of glass diffusion through the substrate; 3, bioactive
glass; and 4, bone.
Although alumina substrates have been successfully coated by bioac-
tive glasses [9], there are several issues that remain to be solved. As
already discussed for enameling, most of the bioactive glasses for biomed-
ical applications have a high thermal expansion coefficient, much higher
than that of alumina. Alumina could be coated by glasses having a low
expansion coefficient, which could be obtained by using a higher content
of silica. Increased silica contents would require a higher processing tem-
perature and, therefore, an extensive reaction between the substrate and
the glass would occur. This, in turn, would lead to undesirable changes
in the glass structure (crystallization), which would adversely affect its
bioactivity, owing to stabilization of the glass network. Alumina may
also slow bone mineralization owing to the precipitation of multi-valent
ions such as hydroxides or carbonates, which are not compatible with
the bone growth process.
One approach to solve this problem is based on multi-layer coatings
to accommodate the challenges described above [10]. Bioactive coatings
on alumina can be produced using graded structures by means of
different techniques. One approach is based onmulti-layer glass coatings,
