American scientists are to widen trials of a male contraceptive, after having been approached by men eager to try it out as an alternative to a vasectomy.

The Intra Vas Device, or IVD, is a small plug of silicone gel, a pair of which blocks sperm travelling down each of the twin tubes connecting the testicles and penis.

If successful, it could offer an alternative to surgical vasectomy, where the tubes are tied and cut.

However, there is concern that inserting IVDs might lead to a build-up of pressure behind the plug, and damage the sperm production glands in the testes.

Initially the IVD manufacturer, Shepherd Medical Company, planned to test the plug in a single city, but after an impressive response have gone further.

Janelle Antil, from Shepherd, said: "We haven't even opened our doors yet, and men are already contacting us. We figured it was a pretty good sign, so we decided to take a chance on expanding."

Elaine Lissner, director of the Male Contraception Information project in San Francisco, said that the IVD might appeal to men who were worried that something was being taken away if they had a vasectomy.

"Somehow psychologically, it's a little easier to think about something being added than something being cut," she said. "They put in two tiny, soft silicone plugs per side, and any sperm that make it past the first plug are stopped by the second. It's not rocket science, but it seems to do the job.

"It's a lot easier to pull the plugs out than to find the best, most expensive micro-surgeon to sew a vas deferens back together. So that's a plus for the IVD.

"But we know that in a vasectomy, even if you can get sperm flowing again, the chances of a pregnancy go down about 10% for each year the man had the vasectomy. Only time will tell if it's the same for the IVD."

The IVD was effective in all cases in an earlier pilot study of 30 men, who with the plugs had either no sperm in their semen or too few to be capable of conceiving.