Biomedical Engineering Reference
In-Depth Information
kinetics that can be modeled by the contracting volume model (surface
dissolution). Each of the three glasses containing silica initially starts
off with similar kinetics (solid line), but, owing to the formation of a
silica-rich layer, the reaction of each glass slows and the data are fit by
the slower diffusion model (dashed line). This means that the silica-rich
layer is acting as the conduit for the dissolved ions from the glass to
diffuse and either be released to the body fluids or form HCA. The slower
diffusion kinetics for silicate glasses can be a distinct disadvantage in
achieving full conversion of the bioactive glass, especially in larger glass
particles (
m) or monolithic implants. It has been documented
that silicate-based bioactive glass implanted in human tooth sockets still
had unreacted glass particles present four years after implantation [6].
Of course, the choice of the best glass will depend on the intended
application. The rate of degradation or conversion to apatite that is
required may differ depending on the clinical indication.
300
μ
>
6.3 EVALUATING REACTIVE MATERIALS (
IN VITRO
VERSUS
IN VIVO
TESTING)
It is extremely challenging to create an
in vitro
test for potential medical
devices that mimics the
in vivo
environment. One of the main difficulties
is that body fluid flows
in vivo
and most
in vitro
systems are static.
However, there are a host of other factors that are difficult to emulate,
such as the mechanical forces that implants will experience, and the
number of different cell types, proteins, and growth factors that are
present in the blood.
Reactive materials such as bioactive glasses are especially difficult to
evaluate
in vitro
, because the environment, such as ion concentration of
the media and pH, can change drastically from the initial environment as
the materials degrade, which can lead to false interpretation [7]. Much
work has been published on the bioactive silicate glasses
in vitro
[8], but,
as shown in Figure 6.2, these silicate glasses react one to two orders of
magnitude slower than some of the borate glasses that will be described
later in the chapter. This high rate of ion release makes cell culture
studies with borate glasses difficult to evaluate. Traditional methods of
cellular compatibility testing in static cell cultures will show in general
that borate glasses are toxic, but this turns out to be a false negative [7].
In vivo
evaluation of borate glasses shows the exact opposite of the
in
vitro
tests, in that bone and soft tissue not only can survive in the presence
of borate glass, but also can regenerate tissue just as effectively as silicate-
based bioactive glasses [9]. The major difference between the
in vivo
