Bioactive Glass-Based Coatings and Modified Surfaces: Strategies for the Manufacture, Testing, and Clinical Applications for Regenerative Medicine - Biological and Biomedical Coatings: Processing and Characterization - page 386

into a high surface area layer on the glass. Between 10 and 20 h, macrophages attach to the
surface and then depart. By 100 h, stem cells have attached and are differentiating. After
100 h, bone matrix is generated and crystallized.
Each composition of bioactive glass exhibits specific interfacial reaction kinetics. For
materials in the apex of region B (Figure 9.4), surface hydration layer is formed between
the implant and the host. Materials in the lower area of region B (i.e., with sufficient SiO 2
in the glass network) form a dense SiO 2 rich film as a result of alkali-proton exchange and
repolymerization. This film protects the implant surface from further attack, resulting in
an inert tissue response. Region C glasses experience rapid and selective ion exchange of
alkaline ions with protons or hydronium ions forming a thick, porous, and subsequently
nonprotective SiO 2 -rich film. If the local pH rises above 9, the SiO 2 -rich film dissolves
rapidly allowing bulk network or stoichiometric dissolution to occur. Bioactive glasses
in region A form a dual protective film that is rich in Ca and P formed on an alkaline-
depleted SiO 2 -rich layer.
The interfacial reaction kinetics for 45S5 is well documented. They are summarized
below (Table 9.3). There are five reaction stages that include ion exchange (stage 1), silica
network dissolution (stage 2), silica repolymerization (stage 3), formation of an amorphous
CaP layer (stage 4), followed by crystallization of HA (stage 5).
The index of bioactivity I b is the inverse of the time required for more than 50% ( t 0.5bb )
of the materials' interface to be bonded where the I b value for 45S5 (Table 9.8) matches
bone. In addition, its I q (qualitative performance ratio; see Section 9.4.1) makes it suitable
for cancellous bone replacement. 45S5 can be combined with a polymeric binder (e.g., 0.4%
volume fraction of 45S5 plus polysulfone composite) to create a mechanically enhanced
composite material with an I q matching that of cortical bone. In addition, its bioactivity is
not compromised.

Bioactive Glass-Ceramics

Referring to Table 9.2, it is clear that the mechanical strength of bioactive glasses does
not match human bone. Seminal work by Brömer, Höland, Burger and Miller have all
addressed the improvement of mechanical strength by preparing bioactive glass composi-
tions that precipitate different crystalline phases [12, 13]. These compositions are known
as glass-ceramics.

TABLE 9.3
The Surface Reaction Stages for 45S5 Composition Bioactive Glass When Exposed/Immersed in an
Aqueous-Based Solution

Stage Reaction

1 Diffusion controlled rapid exchange of Na + or K + with H + or H 3 O + from solution

2 Interfacial breaking of Si-O-Si bonds, releasing soluble silica in the form of Si(OH) 4

resulting in the formation of silanols (Si-OH) at the implant surface

3 Condensation and reploymerization of a SiO 2 -rich layer depleted in alkalis and

alkaline-earth cations

4 Migration of Ca and P through the SiO 2 - rich layer forming a Ca-P rich film, followed

by growth of the amorphous Ca-P-rich film by incorporation of soluble Ca and P
from solution

5 Crystallization of the amorphous Ca-P film (HA)