Silicon-based life forms

Science fiction books and television shows have fueled the life-form debate by creating a number of imaginary, silicon-based creatures. The Horta, perhaps one of the most well-known creatures, first appeared in 1967 on an episode of the original Star Trek series. These intelligent, silicon-based life forms looked a little like burnt marshmallows, yet intelligently defended their store of eggs against the unaware human invaders. 

The bottom line is that speculation about silicon-based life forms may make good television entertainment, but not good science. Experts at NASA s Astrobiology Institute say silicon-based life is not likely for two reasons. First, silicon chemistry Just does not work like carbon chemistry and, second, there is not much silicon in the universe. 

All life on Earth is based on the element carbon. This means that in living organisms on Earth, body structures and the chemicals that take part in life processes are almost all carbon compounds. But scientists and science fiction enthusiasts alike have long speculated on the possibility that elsewhere in the universe there could be life forms based on other elements, particularly silicon. 

Today all available evidence suggests that dimethylsiloxane is one of the most acceptable polymeric materials known from the standpoint of health and environmental impact. But not all silicon based compounds are inert. For instance, families of physiologically active silicon-based materials are well known today and several very active research programs focus on the discovery of effective silicon based pharmaceuticals [16, 17]. All evidence today suggests that there is a natural silicon cycle which incorporates varying amounts of silicon into all life forms, plant or animal [18]. So a major challenge is to educate the public that all silicon-based materials are not the same and certainly all of them are not inert ... 

The silicones (mainly polydimethyl siloxanes) are considerably more expensive than the polysulfides therefore, silicone use is restricted to more specialized applications where specific attributes of the materials are required. For example, virtually every U.S. missile or satellite contains some form of silicone sealant because of its long use life, low volatile content, and heat resistance. They are also used around aircraft windows and doors. High-temperature fuel tank sealants based on fluorocarbon-containing polysiloxanes have also been the subject of considerable government-sponsored research (21). 

From time to time, writers of science fiction have speculated about the possibility of life on other planets being based on the compounds of another element—for example, silicon, the element most like carbon. However, the bonds that silicon atoms form to each other are not nearly as strong as those formed by carbon, and therefore it is very unlikely that silicon could be the basis for anything equivalent to life as we know it. 

Group IV in the periodic table contains some of the elements that are most important for our daily life and, simultaneously, are capable of participating in very different bonding situations. Thus, carbon s ability to form sp, sp, and sp hybrids makes it possible for carbon to form bonds with very diverse elements and to have different coordinations. Moreover, whereas sp hybrids tend to lead to local, linear structures, sp hybrids are most often found in structures that are truly three-dimensional. In between, sp hybrids are found in structures that are locally planar. In the context of the present report, carbon-based nanostructures can be either quasi-O-dimensional or quasi-1-dimensional. Silicon and germanium, on the other hand, form mainly bonds based on sp hybrids. Here, the importance of, in particular, silicon for electronic devices has led to much interest in the properties of nanostructures based on this element. 

Factors other than pot life can affect the working time of an adhesive or sealant. Some silicones and isocyanates are cured by moisture which causes a skin to form on the surface, the thickness of which increases in proportion to the square root of time. Solvent-based adhesives can suffer from rapid evaporation, which limits their ability to wet the substrates. Hot-melt adhesives are prone to oxidative degradation at temperatures of application. 

The quality of the experimental evidence for nutritional essentiality varies widely for the ultratrace elements. The evidence for the essentiality of three elements, iodine, molybdenum and selenium, is substantial and noncontroversial specific biochemical functions have been defined for these elements. The nutritional importance of iodine and selenium are such that they have separate entries in this encyclopedia. Molybdenum, however, is given very little nutritional attention, apparently because a deficiency of this element has not been unequivocally identified in humans other than individuals nourished by total parenteral nutrition or with genetic defects causing disturbances in metabolic pathways involving this element. Specific biochemical functions have not been defined for the other 15 ultratrace elements listed above. Thus, their essentiality is based on circumstantial evidence, which most often is that a dietary deprivation in an animal model results in a suboptimal biological function that is preventable or reversible by an intake of physiological amounts of the element in question. Often the circumstantial evidence includes an identified essential function in a lower form of life, and biochemical actions consistent with a biological role or beneficial action in humans. The circumstantial evidence for essentiality is substantial for arsenic, boron, chromium, nickel, silicon, and vanadium. The evidence for essentiality for the... 

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