Mike Lambourn, an industrial designer, has developed a bike theft deterrent — or at least a startling telltale that brands a thief temporarily.

From Lambourn’s site:

SmartLock is a cable lock that has cores of compressed air and liquid running through its body. If cut, the liquids spray out over the perpetrator, his tools, the bike and the scene of the crime. A bike that has been stolen will be covered in coloured dye (the dye renders the bike undesirable and therefore unsellable) as well as transluscent Smartwater - an invisible forensic property marking liquid.

Source: mikelambourn.com and treehugger.com