Page 42 - 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

182 Biomaterials, Synthetic Synthesis, Fabrication, and Applications

TABLE IV Metals Used in Medical Devices
Metal Medical device applications
Cobalt–chromium alloys Dental applicances, fracture plates, heart
valves, joint components, nails, screws
Titanium alloys Conductive leads, joint components,
pacemaker cases, nails, screws
Stainless steel Fracture plates

and benzyl hyaluronic acid ester (HYAFF-11) are used to FIGURE 5 Schematic diagram of grain boundaries between crys-
replace short nerve segments and in wound healing. The tallites. The boundaries may occupy only one row of atoms or
more.
materials can be used as threads, films, fabrics and sponges
and additional aplications are expected in plastic surgery
and orthopedics.
incorporate a wide range of different atoms either within
the crystal lattice, at interstitial sites within the crystal
C. Metals
lattice (spaces/sites where atoms or ions are not normally
Metals have a large range of applications as devices for located), or at grain boundaries and thus a multitude of
fracture fixation, joint replacement, surgical instruments, metal-containing materials can be made. The deformation
external splints, braces and traction apparatus, as well as characteristics of a metal are determined by the grain size
dental amalgams (Table IV). The high modulus and yield of the crystals as imperfections are concentrated at the
point coupled with the ductility of metals makes them grain boundaries. Mixing of metal atoms of different sizes
suitable for bearing heavy loads without large deforma- as in the production of an alloy can serve to modify the
tions and permanent size changes. Metals are generally properties of the metallic phase. Metallic elements used in
inert and if the composition is chosen carefully do not the formation of implants include: aluminium (Al), cobalt
degrade in a saline environment. Metals are crystalline (Co), chromium (Cr), copper (Cu), gold (Au), iridium
materials with a specific arrangement of metal atoms (Ir), iron (Fe), manganese (Mn), molybdenum (Mo),
within a crystal lattice. Figure 4 shows the arrangements nickel (Ni), niobium (Nb), palladium (Pd), platinum
of atoms in the common crystal packing arrangements (Pt), silver (Ag), tantalum (Ta), tin (Sn), titanium (Ti),
adopted by metals. The limits of a perfect crystal vanadium (V), tungsten (W), zinc (Zn) and zirconium
lattice are defined by grain boundaries where individual (Zr). Nonmetallic elements that are used to modify the
perfect crystals come together (Fig. 5). It is possible to properties of the metallic phases include carbon (C),
nitrogen (N), phosphorous (P), sulfur (S), and silicon (Si).
The metals and alloys that were originally used in the
medical environment were selected on the basis of their
strength and ductility although their original genesis may
have been for a totally different purpose. To surmount
problems due to corrosion under saline conditions alloys
(homogeneous mixtures of the metallic elements at the
atomic level) have been developed to render materials
passive to corrosion. Another method is to promote the
formation of an adherent oxide layer, often by acid treat-
ment of the metallic phase. Both methods are currently
used.
Although the properties of metals can be modified by
the chemical addition of different atoms to the alloy mix-
ture, physical treatments such as annealing, precipitation
hardening, tempering, work hardening, and polishing can
also modify the modulus, toughness, and surface proper-
ties of the metallic phase. Processing of the metallic ma-
terial is necessary to produce functional components and
a combination of brazing (complex part formed by heat-
FIGURE 4 Common crystal packing arrangements for metals. ing in the presence of another metallic material), drawing