Biomedical Engineering Reference
In-Depth Information
EXAMPLE PROBLEM 5.1
A stent is a device that is implanted into an occluded artery to permit increased blood flow.
A permanent, yet flexible, device is needed for use as a vascular stent. What material meets that
need? In addition to information contained in this chapter, search the web for information on
current materials selections using keywords such as “stents” and “metals.” The National Institutes
of Health PUBMED website catalogs scientific publications within the biological, biomedical,
and medical sciences (http://www.ncbi.nlm.nih.gov/entrez). Corporate web pages can provide
additional information. Guidant and Boston Scientific are two companies that currently produce
coronary stents. The United States Patent and Trademark Office provides another very useful
web page for researching uses of materials in surgical and medical devices (www.uspto.gov).
Solution
The preferred materials for stents are metals such as platinum and titanium because they are
relatively inert and can be shaped into the helical woven device that can be collapsed and fed
through a small tube until the insertion point in the blood vessel. Shape memory alloys such as
nitinol also have been used. Stents made of nitinol are self-expanding and “remember” their
manufactured shape when they are deployed in the body. They are particularly good for curved
or tapered vessels. Metal materials have the strength required for this application.
5.2.2 Ceramics and Glasses
The advantages of the class of materials known as ceramics, which includes glasses, are
that they are very biocompatible (particularly with bone), are inert, have low wear rates,
are resistant to microbial attack, and are strong in compression. Some disadvantages
include brittleness, the potential to fail catastrophically, and being difficult to machine.
These properties arise from the atomic structure of ceramics. Unlike metal, in which atoms
are loosely bound and able to move, ceramics are composed of typically two different types
of atoms that are ionically and/or covalently bound into compound forms. This atomic
immobility means that most ceramics do not conduct heat or electricity. A ceramic that does
not have a crystalline structure and is amorphous is referred to as a glass. Glasses are often
silica-based. Silica is a network-forming oxide that can be heated to its melting point and,
unlike most ceramics, is more easily manufactured. Two very obvious properties of ceramics
that are different from metals are melting point and brittleness. Ceramics have very high
melting points, generally above 1,000
C, and are brittle. Examples of ceramics used in medical
devices are shown in Table 5.1. A photograph of a ceramic femoral head of a hip implant is
shown in Figure 5.6, and an example of a granular calcium phosphate bone graft substitute
is shown in Figure 5.7.
Certain compositions of ceramics, glasses, glass-ceramics, and composites have been
shown to stimulate direct bone bonding, which is important in securing orthopedic medical
devices such as replacement hips and knees and spinal fusion devices. These types of
materials are known as bioactive ceramics. Studies on retrieved implants have shown that
a biologically active calcium phosphate forms on the biomaterial surface upon implantation
in the body. Since the calcium phosphate that forms is much like that found in our bones,
