Page 43 - Academic Press Encyclopedia of Physical Science and Technology 3rd BioTechnology

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Encyclopedia of Physical Science and Technology EN002G-62 May 19, 2001 19:27

Biomaterials, Synthetic Synthesis, Fabrication, and Applications 183

(wires and sheets formed from metallic ingots), forg- strengths but they are difficult to machine and much more
ing (metallic forms obtained from dies), machining (for expensive to produce. Titanium-base alloys are found in
complex geometries), and welding (local heating and fu- many commercial medical devices and they are also used
sion to produce complex parts) may additionally modify as coatings. For dental implants bone is found to grow best
the physical properties of the metal or alloy being used. in the presence of titanium or materials coated with tita-
The materials currently used in the production of med- niumwheresurfacerougheningduringmanufactureisalso
ical devices include stainless steels, cobalt-base alloys, found to improve the performance. Platinum-base alloys
titanium-base alloys, platinum-base alloys, and nickel– are used primarily in electrodes for electrical stimulation
titanium alloys. Steels were the first modern metallic al- for although they show excellent strength and corrosion
loys to be used in orthopedics and initial problems with resistance they are very expensive materials to produce
corrosion were overcome by modifying the composition and machine.
of the steel with the addition of carbon, chromium, and Figure 6 shows the clinical uses of metals in the hu-
molybdenum. Carbon was added at low concentrations man body. In many instances metallic implants have to
(ca. 0.03–0.08%) to initiate carbide formation, while the be fixed in to the body and the implants must be com-
addition of chromium (17–19%) facilitated the forma- patible with the fixative which may be metallic (screws),
tion of a stable surface oxide layer and the presence of ceramic (screws and other components) and/or polymer
molybdenum (2.0–3.0%) was found to control corrosion. phases (e.g., glue). In the design of replacement compo-
The compositions of stainless steels used can vary widely. nents with high strength it is important that the compati-
Table V shows the limits for the chemical compositions of bility of all of the biomedical components is required and
three different alloys containing eleven different elements investigated before novel implants are placed in the human
together with the mechanical properties for the samples body.
after annealing and cold working.
There are at least four compositions of cobalt-base al-
D. Ceramics
loys in use which are similarly designated by code num-
bers such as F75, F90, F562, and F563. Again, these During the last 40 years a revolution in the use of ceram-
differ in the relative composition of the following ele- ics has occurred. The revolution is the development of
ments: manganese, silicon, chromium, nickel, molybde- specially designed and fabricated ceramics, termed “bio-
num, carbon, iron, phosphorus, sulfur, tungsten, titanium, ceramics” when used in the body for the repair and re-
and cobalt. These alloys are used because of their superior construction of diseased, damaged, and “worn out” parts

TABLE V Chemical Composition and Tensile Strength of Standard
Stainless-Steel Alloys Used in Biomedical Applications
F55(%) F138(%)
Composition
Element Grade 1 Grade 2 Grade 1 Grade 2 F745(%)
Carbon 0.08< 0.03< 0.08< 0.03< 0.06<
Manganese 2.0< 2.0< 2.0 2.0 2.0<
Phosphorus 0.03< 0.03< 0.025< 0.025< 0.045<
Sulfur 0.03< 0.03< 0.01< 0.01< 0.03<
Silicon 0.75< 0.75< 0.75< 0.75< 1.0
Chromium 17–19 17–19 17–19 17–19 17–19
Nickel 12–14 12–14 13–15.5 13–15.5 11–14
Molybdenum 2.0–3.0 2.0–3.0 2.0–3.0 2.0–3.0 2.0–3.0
Nitrogen 0.1< 0.1< 0.1< 0.1<
Copper 0.5< 0.5< 0.5< 0.5<
Iron Balance Balance Balance Balance Balance

Ultimate tensile
strength MPa MPa MPa
Annealed 480–515 480–515 480>
Cold-worked 655–860 655–860