Silicon

Silicon is an essential element in humans, found in significant concentrations
in hair, bone, epidermis and dental enamel. It is also the second most abundant
element on the earthís crust, constituting about 28% by weight.(cite) Many
foods and beverages, including certain vegetables, grains, rice, and beer have
been shown to contain significant amounts of silicon. Silicone is a synthetic
form of silicon and includes 40% silicon by weight. The silicones are synthetic
polymers and are not therefore found naturally. They have a linear, repeating
silicon-oxygen backbone akin to silica. However, organic groups attached
directly to the silicon atoms by carbon-silicon bonds prevent formation of the
three-dimensional network found in silica. These types of compound are also
known as polyorganosiloxanes. Certain organic groups can be used to link two or
more of these silicon-oxygen backbones and the nature and extent of this cross
linking enables a wide variety of products to be manufactured.(cite) The most
important materials used in medical implants are fluids, gels and rubbers (elastomers)
whose physical and chemical properties include, amongst others, a high degree of
chemical inertness, thermal stability and resistance to oxidation. Silicone is
used by many prosthesis, medical devices, and pharmaceutical products. The many
silicone containing medical devices include artificial heart valves, artificial
joints, Norplant contraceptive implants, pacemaker wires, and dialysis tubing.

Of course silicone is probably best known for its use in breast implants. In

1992 the FDA pulled silicone-gel filled breast implants off the market as they
were alleged to cause "connective-tissue disorders such as systemic lupus
erthematosus, rheumatoid arthritis and scleroderma, a hardening of the
skin."(cite) Recent studies have disproven this, showing that
connective-tissue diseases were no more common in women with implants than those
without. Also a study by the U.S. National Cancer Institute showed a lower
cnacer risk amoung women However, tests looking with "reliable, validated
analytical techniques for the dissemination of silicones from implants in the
body, including breakdown products of the polymers, have shown either no
dissemination, or the presence of only very small amounts at distant sites
following rupture of gel-filled implants, or after deliberate injection of the
gel."(cite) The risks of these implants, as shown in laboratory studies as
well as in real life, are local inflammatory and scarring reactions, and local
infection, as around any foreign body in the tissues. If a silicone fluid is
released from a ruptured gel-containing implant, the inflammatory and fibrotic
reaction will affect a wider area. There is no evidence of any type of"systemic reaction, or of abnormalities of the immune system in subjects who
have received implants."(cite) Perhaps one of the best known biomaterials
today is titanium and its alloys. Commercially pure titanium, also known as F67,
is non-magnetic, and there is no harmful additives or alloying. The most common
alloy used is called F136, or Ti-6Al-4V. This alloy is an alpha-beta alloy,
meaning the properties will vary depending on treatments. However usually this
alloy is corrosion resistant but not ware-resistant and has a higher strength
than when in its pure form. The major drawback of this alloy is in its long-term
usage. The vanadium is biocompatible only in the short term.(3,pg. 2) There are
four grades of titanium, 1-4 with four being the strongest but least ductile.

The amount of oxygen in the CP titanium is a major force on how strong the yield
and fatigue strengths will be, and also determines the grade of the alloy.

Titanium demonstrates exceptional resistance to a broad range of acids, alkalis,
natural waters and industrial chemicals. It also offers superior resistance to
erosion, cavitation or impingement attack. Titanium is at least 20 times more
erosion resistant than the coppernickel alloys.(cite) The low density of
titanium makes it significantly lighter when compared to the stainless steels
and cobalt-alloys. The densities of titanium-based alloys range between .160
lb/in3 and .175 lb/in3. Titanium also has a higher fatigue strength than many
other metals. Yield strengths range from 25,000 psi commercially pure(CP) Grade

1 to above 200,000 psi for heat treated beta alloys. (cite) The combination of
high strength and low density results in exceptionally favorable
strength-to-weight ratios for titanium-based alloys. These ratios are superior
to almost all other metals and become important in such applications as the
surgical implants in the plastic and reconstructive surgery fields of medicine.

Titaniumís higher strength permits the use of thinner walled equipment. Due to
the difficulty in electropolishing titanium, it is anodized, this is an
electrochemical process which increases the thickness of the oxide film that
lies on titanium. Here is where the colors that are associated with titanium,
most often gold, is produced. In addition, the unique qualities of titanium
prove to be MRI and