to increase shear strength, creating a more impact resistant bond. Such additives may include rubber, as in Loctite's "Ultra Gel", or others which are not specified. In general, the C=C groups rapidly undergo chain-growth polymerisation in the presence of water (specifically hydroxide ions), forming long, strong chains, joining the bonded surfaces together. Because the presence of moisture causes the glue to set, exposure to normal levels of humidity in the air causes a thin skin to start to form within seconds, which very greatly slows the reaction; hence, cyanoacrylates are applied as thin coats to ensure that the reaction proceeds rapidly for bonding. Cyanoacrylate adhesives have a short shelf life—about one year from manufacture if unopened, and one month once opened. General properties Cyanoacrylates are mainly used as adhesives. Thin layers bond effectively, thick layers much less so. They bond many substances, including human skin and tissues, natural fibres, cotton, wool, and leather. Cyanoacrylate glue has a low shearing strength, which has led to its use as a temporary adhesive in cases where the piece needs to be sheared off later. Common examples include mounting a workpiece to a sacrificial glue block on a lathe, and tightening pins and bolts. It is also used in conjunction with another slower, but more resilient, adhesive as a way of rapidly forming a joint, which then holds the pieces in the appropriate configuration until the second adhesive has set. Cyanoacrylate-based glue has a weak bond with smooth surfaces and as such easily gives to friction; a good example of this is the fact that cyanoacrylates may be removed from human skin by means of abrasives (e.g. sugar or sandpaper). Uses Medical and veterinary Cyanoacrylate glue is widely used in human and veterinary medicine. It was in veterinary use for mending bone, hide, and tortoise shell by the early 1970s or before. A cyanoacrylate spray was used in the Vietnam War to reduce bleeding in wounded soldiers until they could be taken to a hospital. n-Butyl cyanoacrylate has been used medically since the 1970s. In the US, due to its potential to irritate the skin, the U.S. Food and Drug Administration (FDA) did not approve its use as a medical adhesive until 1998 with Dermabond (2-octyl cyanoacrylate). A 1986 independent study suggests that cyanoacrylate can be safer and more functional for wound closure than traditional suturing (stitches). Although cyanoacrylates are widely used for sutureless