Biomedical Engineering Reference
In-Depth Information
Removing surface flaws by dissolving a small amount of surface can
increase strength greatly until new flaws are introduced. Mechanical
properties are also dependent on morphology; for example, the bending
strength (and flexibility) of a glass increases as fiber diameter decreases.
This is why endoscopes can be used in medicine. Fiber optics rely on
the optical properties of glasses and the fact that thin fibers can bend
without breaking as long as they do not get surface flaws. Surface flaws
are prevented by coating the glass fiber with a polymer sheath as soon
as the fiber is pulled from the melt. The optical fibers can then be used
for keyhole surgery.
1.3 HOMOGENEITY AND PHASE SEPARATION
We tend to think of glass as a very homogeneous material, but it can be
inhomogeneous to a greater or lesser extent. In some cases, for commer-
cial glasses widely used in the biotechnology industry, the inhomogeneity
is on quite an appreciable scale and does affect the behavior of biolog-
ical moieties quite significantly. Even silica, the most homogeneous of
glasses, is inhomogeneous on a scale smaller than the wavelength of
light. The fundamental random nature of glass means that there are
small fluctuations in density. However, there is evidence to suggest
that, even in commercial soda-lime-silica glass, the basis for most of
our commercial compositions, there is some regionality of the structure
consisting of compositional variations. We know for certain that most
borosilicate glasses (such as those used to hold high-temperature or
corrosive liquids, e.g. Pyrex
®
) exhibit compositional fluctuations on a
large scale that can ultimately grow, when heat-treated, manifesting as a
fogginess in the glass. This is liquid-liquid phase separation. The foggi-
ness results from the scattering of light from the interfaces between the
compositional fluctuations, which occurs because the refractive index
undergoes a sudden change. These types of microstructures, if nurtured
correctly, can ultimately help in making porous glasses. Phase separa-
tion is more the rule than the exception in glasses, but controlled phase
separation usually has to be metastable, that is, developed in the solid
glass. There are two types of immiscibility that occur: the formation of
a droplet phase in a matrix; and spinodal decomposition, which results
in an intertwined microstructure of two compositions. It is the latter
that can result in very well-defined porous glasses (see Chapter 6). In
addition to microstructural engineering by phase separation, where the
phases are both glass, one can also develop a phase within the glass that
is crystalline. These are called glass-ceramics and are used very widely
