A German carpenter has invented a valve which he claims will revolutionise contraception, by allowing a man to turn the flow of sperm from his testicles on and off at the flick of a switch. Clemens Bimek said the idea first came to him 20 years ago, when he was watching a documentary about contraception. When he discovered no one had ever filed a patent for such a device, he developed his own."Many of the doctors I consulted didn't take me seriously. But there were some who encouraged me to go on tinkering," Bimek said.Now his valve is to be implanted in 25 men in trials starting this year.It is about 2cm long and weighs less than 3g. It is surgically implanted on the vas deferens, the tube that carries sperm from the testicles, in a half-hour operation, and controlled by a switch beneath the skin of the scrotum.So far Bimek is the only man who has had the switch implanted.Hartwig Bauer, the urologist who carried out the surgery, told Spiegel magazine the valve was preferable to a vasectomy. "A third of patients want to have the [vasectomy] operation reversed later, but it doesn't always work," he said. ..