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 told Spiegel magazine the idea first came to him some 20 years ago, when he was watching a television documentary about contraception, and wondered whether it would be possible to control the flow of sperm with a simple valve.

When he discovered that no one had ever filed a patent for such a device, he decided to develop his own.

Clemens Bimek's device is to be implanted in 25 men in trials starting this year Photo: Bimek SLV

“Many of the doctors I consulted didn’t take me seriously. But there were some who encouraged me to go on tinkering and helped me with their expertise,” Mr Bimek said.

Now the valve he has developed is to be implanted in 25 men in trials starting this year.

The tiny valves are less than a inch long and weigh less than a tenth of an ounce. They are surgically implanted on the vas deferens, the ducts which carry sperm from the testicles, in a simple half-hour operation.

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They are controlled with a simple on-off switch which the man can reach under the skin of his scrotum.

So far Mr Bimek himself is the only man who had them implanted. He underwent the operation under a local anaesthetic so he could help direct the surgeon.

Hartwig Bauer, the urologist who carried out the surgery, told Spiegel the valve was preferable to a vasectomy.

“A third of patients want to have the operation reversed later, but it doesn’t always work,” he said.

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But other doctors have expressed concerns over the new technology.

“My assessment is that implanting the valve could cause scarring where it meets the vas deferens,” Wolfgang Bühmann, spokesman for the Professional Association of German Urologists, said.

This scarring could prevent sperm from flowing even when the valve is open, he warned.

He also warned the valve could become clogged over time if left in the closes position for too long.

"Other implants made of this material have been well tolerated elsewhere in the body,” Anneke Loos, head of a testing centre for medical products in Hannover, said.

“The question is whether it will cause problems when it is implanted in this area.”