What is Silicone? The difference between silicon, silica and silicone.
Silicon is the second most abundant element in the earth's crust, comprising around 28% of it. It is not found in its elemental form but occurs mainly as oxides and silicates. In contrast to carbon, silicon-silicon bonds are uncommon. Natural silicon-carbon bonds are extremely rare but they can be created synthetically.
Silica is a three dimensional network of silicon dioxide, most commonly encountered as sand. Silica exists in crystalline and amorphous forms. Silica is chemically resistant at ordinary temperatures but can undergo a variety of transformations at high temperatures (greater than 500ºC) and pressures. The industrial production of amorphous silica requires temperatures of 500ºC and much higher temperatures are required to produce crystalline silica.
The prolonged inhalation of crystalline silica dust is associated with silicosis. Amorphous silica is much less pathogenic than crystalline forms. Conversion of amorphous to crystalline silica cannot occur at body temperature. High purity amorphous silica is used as a reinforcing agent to increase the tear resistance of silicone rubbers used in medical devices and implants.
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 crosslinking enables a wide variety of products to be manufactured. 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 fluids (oils) are usually linear chains of polydimethylsiloxane (PDMS) which have a wide range of chain lengths and molecular masses. Cyclic polydimethylsiloxanes also occur and are important intermediates in the manufacture of the linear chain fluids. They are virtually insoluble in water.
Silicone gels have lightly cross-linked polysiloxane networks, swollen with PDMS fluid to produce a cohesive mass. The PDMS fluid is not chemically bound to the crosslinked network but is retained only by physical means, as water is in a sponge, and there is a tendency for the fluid to "bleed". The degree of cross-linking and amount of fluid affects the physical properties of the gel and the rate at which fluid "bleeds" from it. Once suitably cross-linked, silicone gels retain their form without external containment.
Silicone elastomers are extensively cross-linked and contain little free PDMS fluid. The barrier coating of breast implant shells is a special silicone elastomer which is selected specifically to minimise migration of PDMS from the implants. The tensile strength and tear resistance of silicone elastomers may be increased by addition of amorphous silica which is usually pre-treated with organosilicon compounds to enable it to be tightly incorporated into the polymer network.
