Trials of a new male contraceptive are being expanded in the US after overwhelming initial interest.
The minute device blocks the path of sperm
Men have been eager to test the device which is designed as an alternative to surgical vasectomy.
The Intra Vas Device or IVD, inserted via a small hole made in the scrotum, is a tiny silicone plug that blocks the tube sperm travel along in the body.
In a pilot study involving 30 men the IVD was effective. Studies in monkeys also showed it was reversible.
Extensive tests are now needed to check the same would be true in men using the device for years rather than months.
Traditional vasectomy - where the two vas deferens tubes connecting the testicles and the penis are cut - can be reversed in some men to restore fertility, but it is designed to be a permanent contraceptive.
Elaine Lissner, from the non-profit US organisation Male Contraception Information Project in San Francisco, said: "It is a lot easier to pull the plugs out than to find the best, most expensive micro-surgeon to sew a vas deferens back together.
"But we know that in vasectomy, even if you can get sperm flowing again, the chances of pregnancy go down by about 10% for each year the man had the vasectomy. Only time will tell if it's the same for IVD."
There are many contraceptive choices available to couples, but currently only really two that rely on the man - condoms and vasectomy.
Ms Lissner said men want to take responsibility and control of contraception.
She said: "At least in the US, the idea that men aren't willing to participate is clearly out of date.
"Men want to control their own destinies."
A recent study of over 9,000 men in nine countries on four continents showed more than 60% of men in Spain, Germany, Mexico and Brazil expressed willingness to use a new male contraceptive.
These men said they would like to relieve their partners of some of the contraceptive burden in their relationship or would simply like a reliable backup to condoms.
Investigators are looking at using hormonal methods, similar to the female pill or implants.
In the pilot, men had no sperm or too few in their semen to cause pregnancy
These have the advantage of being readily reversible, meaning a man could use it repeatedly at different times in his life, stopping to have children in between.
But these act on the whole body and can have unwanted side effects, like the female pill. Some men also say they do not find hormonal methods acceptable because they feel it somehow threatens their masculinity.
Scientists have been searching for less invasive, localised, non-hormonal and reversible male contraceptives.
Surgeons have tested a silicone gel that can be injected through the skin of the scrotum directly into the vas deferens where it will block sperm.
The main concern about the IVD and the gel is the build up of pressure behind the plug, which, theoretically, could damage the sperm production houses in the testes.
Researchers in China are working on a mesh-like vas deferens plug that intentionally allows small amounts of sperm through, which would not be plentiful enough to cause a pregnancy but may get round the problem of back pressure.
Shepherd Medical Company will begin testing its IVD on October 16. Originally the researchers had planned to include male volunteers from St Paul, Minnesota, but have expanded trials due to demand.
All of the men taking part have finished having children and would otherwise desire a vasectomy. They will test the IVD for two years.
UK regulators said they were not aware of any similar trials planned in the UK.
Dr Richard Anderson from the University of Edinburgh, who has been investigating hormonal male contraceptives in the UK, said: "This would be a very attractive alternative to vasectomy.
"But even if it is potentially reversible after a few years, it would still not be a contraceptive that could be put in and taken out repeatedly for spacing your family, for example."
Dr Allan Pacey, a fertility expert from the University of Sheffield, said: "The current options for men are somewhat limited or unacceptable so there is a big market for an effective, reversible male contraceptive."
However, he was concerned that a device that blocks the passage of sperm could potentially result in later fertility problems - even after it was removed from the body.