Biomedical Engineering Reference
In-Depth Information
polymers for bone-healing applications. The discontinuous phase is typically harder and
stronger than the continuous phase and is called the reinforcement.
Composites are made by mixing two components and molding, compacting, or chemi-
cally reacting them together. If the reinforcing primary phase is made of fibers, they are
typically coated or impregnated with the second phase so the composite can be heated
and pressurized to densify the assembly. A chemical reaction may be utilized to form
composites in which a second phase precipitates or forms in place upon reaction. There
may be a filament winding process of the first phase around a core if high-strength hollow
cylinders are being formed, followed by densification of the construct.
Composites are well suited for devices that require a combination of properties not pos-
sible from one single material such as total joint replacements, dental fillings, and bone
plates. The advantages of composites are that the properties can be tailored to fit nearly
any application; however, it is difficult to make a composite with an ideal structure. There
are typically problems with dispersion of the second phase or weak interfacial bonds
between the two phases, which leads to less than ideal mechanical properties and thus poor
product performance. However, in many cases the actual performance is still much better
than any single component biomaterial, so composites are becoming more widely used in
biomedical applications.
EXAMPLE PROBLEM 5.4
What materials are preferred for reconstructive dental applications? What are some advantages
of the composite structure over a monolithic structure?
Solution
Mercury amalgams made of mercury, silver, and tin are the most commonly used filling mate-
rials. Aesthetically pleasing tooth-colored filling materials also can be used and are made of filled
resins (e.g., large molecule bifunctional methyacrylates (BisGMA) filled with micro and nano-
particulate silica). PMMA is the predominant material used for complete and partial dentures.
Chrome-based alloys are used for the framework of removable dentures. Crowns and bridges
are made of a cast metal frame veneered with tooth-colored porcelain. All-ceramic systems are
available as well. Recently, composites made of light curing resins reinforced with glass fibers
have been developed for dental bridges. The metallic post typically used to provide structural
support for crowns is now being replaced with this type of glass fiber reinforced composite,
primarily for better aesthetic results; the polymer post does not show through the porcelain crown
like the metallic post does.
5.2.5 Natural Materials
Natural materials are synthesized by an organism or plant and are typically more
chemically and structurally complicated than synthetic materials. Examples of natural
biomaterials currently used in medical devices are listed in Table 5.1. Proteins and poly-
saccharides are nature's form of polymers and thus can actually be classified into the
polymer category. Proteins, the primary group of molecules responsible for making life
