Biomedical Engineering Reference
In-Depth Information
in medicine and biotechnology (see Chapter 7). The chief advantages
of devitrifying glasses (devitrifying means nucleating and growing a
crystalline phase in an amorphous glass) are usually manifest in their
mechanical properties, where crack propagation can be inhibited by
the presence of the glass-crystal interface, and also in thermal expan-
sion, where growth of a crystal phase can help control the net thermal
expansion of a glass.
1.4 FORMING
Glass affords enormous flexibility in terms of forming, that is, making
different shapes. This is in part due to the ability to cast glass in near
net shapes. Commercial glasses tend to be categorized as flat, container,
fiberglass, and specialty glass, where ''specialty glass'' is more or less
everything else. Flat glass is generally made three ways: floating on a
bath of molten tin (possible for a limited range of compositions, but very
efficient and perfectly flat, and usually used for windows, etc.); pulling up
from a melt through a rectangular hole; and allowing glass to overflow
from a trough (e.g. used for liquid-crystal displays). Automotive glass,
even though rarely ''flat,'' comes under this category - it starts out as
flat, but is slumped (heated slowly over a shaped mold) to get the
aerodynamic shapes required for modern vehicles.
Container glasses are usually soda-lime-silica glasses, except for
pharmaceutical glasses, which are type I (borosilicate, chemically resis-
tant), type II (dealkalized soda-lime-silicate glass), or type III (low-alkali
soda-lime-silicate glass). Most soda-lime-silicate containers are very
durable in water and slightly acidic solutions but are attacked by basic
solutions. Dependent upon the contents, the glass may be doped to be a
certain color if the products within are sensitive to light. Containers are
made in an automated process that blows them into a mold. Another
forming method that has recently been finding new application in the
field of biotechnology and medicine is the processing of glass micro-
spheres. These microspheres are a fraction of the thickness of a human
hair and can be injected into the bloodstream or their large controlled
surface area can be used to stabilize biological molecules. Glass can
also be made into fibers, both as a structural material and as an optical
material. We are used to the former as being a component that keeps
our homes well-insulated from heat and cold, and also as ''fiberglass,''
which is a composite of a polymer matrix reinforced with glass fibers for
a lightweight but strong material. The glass increases the stiffness of the
polymer and the polymer matrix helps prevent the fibers from cracking.
