Male birth control might be closer than you think. A new type of reversible, condom-free male contraceptive just passed a major hurdle, after successfully blocking male reproduction in a group of rhesus monkeys for more than a year.

The contraceptive is known as Vasalgel, and it works by blocking the tube that sperm travels down - the vas deferens - with a flexible, spongy, hydrogel material. This allows all the important bodily fluids to get through while blocking sperm cells, which means men could theoretically still ejaculate but not get a woman pregnant.

Importantly, animal studies so far suggest that the treatment is fully reversible. And because it only requires men to get one long-lasting injection, it doesn't rely on them remembering to take a nightly pill.

In the latest trial, 16 adult, male rhesus monkeys received injections of Vasalgel, and were then housed with three to nine fertile females for at least one breeding season. Seven of the monkeys were housed with females for up to two years.

No pregnancies were reported after the Vasalgel injections in any of the animals, despite the fact they were housed outdoors in a free-living environment where they had unlimited access to the females.

Similar to earlier animal trials, there was no evidence of complications, or the monkeys' immune systems reacting badly to the Vasalgel.

To be clear, this is a very small trial, and the results weren't compared against a control group, so it's not enough to say for sure that the product works in monkeys just yet. And researchers regularly struggle to replicate animal results in human trials.

But based on these results - and coupled with earlier tests in rabbits - preparations are now being made for the first clinical trials to take place, according to the company behind the contraceptive, the not-for-profit Parsemus Foundation.

The Parsemus Foundation said last year that it hopes Vasalgel could be on the market as early as 2018, with an international pricing structure to ensure affordability for all men.

"Contraceptive development is a hugely expensive project," Parsemus executive director, Elaine Lissner, said last year.

"But this is not just another early-stage lead; we're so close on this one. It's time to finish the job we've started."

That 2018 goal is probably a little ambitious, but any progress at this stage is a big deal, seeing as male contraceptive options have hardly changed in more than a century. 

Right now, men who want to take contraception into their own hands either need to use condoms, which have a relatively high failure rate of around 15 percent, or get a vasectomy - a surgical procedure that's designed to be irreversible.

Instead, many couples choose to rely on female birth control options such as the pill or IUDs, which come with their own side effects and complications.

Several research groups are currently testing out alternative, reversible male birth control options, but most options involve hormonal or chemical regulation of sperm, which comes with unpredictable side effects.

One of the most promising trials was stopped early last year due to severe side effects, including depression and mood disorders. 

Vasalgel takes a different approach by physically blocking sperm, sort of like a condom or female diaphragm. But instead of having to use something every time you have sex, the Vasalgel polymer is injected into the vas deferens as a one-off.

It's not yet clear how long the Vasalgel will last - longer term trials are needed to figure that out. 

But rhesus monkeys are known for being similar to humans in the way their bodies respond to new medications and treatments, so this new study is a positive first step.

The researchers have also previously shown that the Vasalgel blocked 12 rabbits from getting females pregnant throughout a year-long trial, without any abnormal responses.

A follow-up study on seven of the rabbits showed that after Vasalgel was flushed from their vas deferens, sperm production quickly returned to normal - but those results are pending publication, so we have to take the researchers' word on that for now.

There's still a long way to go before we can get too excited about the potential of Vasalgel - at least three rounds of human clinical trials are planned before the drug is considered for regulatory approval.

But it's very rare for a male contraceptive to make it this far in the first place, so we'll be watching the results closely.

The next step is for the Parsemus Foundation team to attempt to reverse the Vasalgel in some of the monkeys used in this trial to see if there are any long-term effects of the treatment.

As an unexpected benefit, veterinarians working on the project think that, if nothing else, Vasalgel could offer an effective solution for birth control in zoo animals. 

"While vasectomy is a quick and relatively simple procedure in humans, in monkeys there can be additional complications, as it is inherently more complex," said Angela Colagross-Schouten, lead veterinarian on the project.

"We were impressed that this alternative worked in every single monkey, even though this was our first time trying it. … Hopefully, Vasalgel placement can be an option for other captive colonies, including zoos, that want to manage reproductive rates while allowing for social housing."

The research is due for publication in Basic and Clinical Andrology this week.