Silicon Essays - Prosthetics, Biomaterials, Medical Technology

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 earth's 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. Titanium's 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