Classes of materials used in medicine - Biomedical Engineering Desk Reference - page 255

Ti 4 þ are present in the glass or solution, multiple layers
form on the glass as the saturation of each cationic
complex is exceeded, resulting in a type IIIB surface
( Fig. 3.2.10-6 ), which does not bond to tissue.
A general equation describes the overall rate of change
of glass surfaces and gives rise to the interfacial reaction
profiles shown in Fig. 3.2.10-6 . The reaction rate ( R )
depends on at least four terms (for a single-phase glass).
For glass-ceramics, which have several phases in their
microstructures, each phase will have a characteristic
reaction rate, R i .

The surface chemistry of bioactive glass and glass-ce-
ramic implants is best understood in terms of six possible
types of surface reactions (Hench and Clark, 1978). A
high-silica glass may react with its environment by de-
veloping only a surface hydration layer. This is called
a type I response ( Fig. 3.2.10-6 ). Vitreous silica (SiO 2 )
and some inert glasses at the apex of region B
( Fig. 3.2.10-5 ) behave in this manner when exposed to
a physiological environment.
When sufficient SiO 2 is present in the glass network,
the surface layer that forms from alkali-proton exchange
can repolymerize into a dense SiO 2 -rich film that pro-
tects the glass from further attack. This type II surface
( Fig. 3.2.10-6 ) is characteristic of most commercial sili-
cate glasses, and their biological response of fibrous
capsule formation is typical of many within region B in
Fig. 3.2.10-5 .
At the other extreme of the reactivity range, a silicate
glass or crystal may undergo rapid, selective ion exchange
of alkali ions, with protons or hydronium ions leaving
a thick but highly porous and nonprotective SiO 2 -rich film
on the surface (a type IV surface) ( Fig. 3.2.10-6 ). Under
static or slow flow conditions, the local pH becomes suf-
ficiently alkaline (pH > 19) that the surface silica layer is
dissolved at a high rate, leading to uniform bulk network or
stoichiometric dissolution (a type V surface). Both type
IV and V surfaces fall into region C of Fig. 3.2.10-5 .
Type IIIA surfaces are characteristic of bioactive sili-
cates ( Fig. 3.2.10-6 ). A dual protective film rich in CaO
and P 2 O 5 forms on top of the alkali-depleted SiO 2 -rich
film. When multivalent cations such as Al 3 þ ,Fe 3 þ , and

R ¼k 1 t 0 : 5 k 2 t 1 : 0 k 3 t 1 : 0 þ k 4 t y þ k n t z

(3.2.10-1)

The first term describes the rate of alkali extraction from
the glass and is called a stage 1 reaction. A type II non-
bonding glass surface (region B in Fig. 3.2.10-6 ) is pri-
marily undergoing stage 1 attack. Stage 1, the initial or
primary stage of attack, is a process that involves an ex-
change between alkali ions from the glass and hydrogen
ions from the solution, during which the remaining con-
stituents of the glass are not altered. During stage 1, the
rate of alkali extraction from the glass is parabolic ( t 1/2 )
character.
The second term describes the rate of interfacial
network dissolution that is associated with a stage 2 re-
action. A type IV surface is a resorbable glass (region C in
Fig. 3.2.10-5 ) and is experiencing a combination of stage
1 and stage 2 reactions. A type V surface is dominated by
a stage 2 reaction. Stage 2, the second stage of attack, is
a process by which the silica structure breaks down and
the glass totally dissolves at the interface. Stage 2 kinetics
are linear ( t 1.0 ).
A glass surface with a dual protective film is desig-
nated type IIIA ( Fig. 3.2.10-6 ). The thickness of the
secondary films can vary considerably

from as little as
0.01 m m for Al 2 O 3 -SiO 2 -rich layers on inactive glasses,
to as much as 30 m m for CaO-P 2 O 5 -rich layers on bio-
active glasses.
A type III surface forms as a result of the repolyme-
rization of SiO 2 on the glass surface by the condensation
of the silanols (Si < pisbOH) formed from the stage 1
reaction. For example:

d

Si OH þ OH Si/Si O Si þ H 2 O

(3.2.10-2)

Stage 3 protects the glass surface. The SiO 2 poly-
merization reaction contributes to the enrichment of
surface SiO 2 that is characteristic of type II, III, and IV
surface profiles ( Fig. 3.2.10-6 ). It is described by the
third term in Eq. 3.2.10-1 . This reaction is interface
controlled with a time dependence of þk 3 t 1.0 . The

Fig. 3.2.10-6 Types of silicate glass interfaces with aqueous or
physiological solutions.