A little gold in your family jewels: nanotechnology at the service of male contraception

Researchers are currently developing a nanotechnology that uses gold particles to enable male contraception.

By Jeffrey Mo, University of Toronto

ANALYSIS – While women have access to several methods of contraception, men have only two means at their disposal, the condom and the vasectomy. Like abstinence, they come with their own set of drawbacks.

Condoms can tear, and some men are allergic to commonly used latex. As for vasectomy, this sometimes painful surgery is usually irreversible.

So the research continues, and one of the methods being investigated is “nanocontraception.”

Like a switch

The process by which the testicle produces sperm is very sensitive to heat. This is why, in mammals, the genital glands are usually located in a sac outside the body, the scrotum, which ensures that they are kept at an optimal temperature, slightly lower than that of the rest of the body.

Nanocontraception consists of turning the genital gland on or off at will, much like a switch. This is achieved by implanting nanoparticles in the testicle with a diameter of about 100 nanometers – one thousandth the thickness of a human hair – which are then heated by magnetic or photothermal effect.

In practice, however, the exercise is rather tricky: too much heat can irreversibly destroy sperm-producing tissue.

Gold and iron

In 2013, biologist Fei Sun and his multidisciplinary research team carried out the first nanocontraception experiments on mice. They first injected nanoparticles directly into the testicles. They were gold nanotubes — 120 atoms long and 30 atoms wide — covered with a few chains of polymer that gave them the appearance of oblong bacteria bristling with hair.

Secondly, they subjected the testicles to infrared, which made it possible to raise the temperature of the nanotubes by 30°C to a level between 37 and 45°C — a variation which depended on the concentration of the nanoparticles and on the intensity of the radiation.

Unfortunately, the radiation caused burns. Although there was no reliable way to measure the level of pain, the team preferred to start from scratch.

In July 2021, the Fei Sun team published an article about their latest progress. The new nanotubes are similar to previous ones, except they're instead made of iron oxide, which has magnetic properties, and they're coated with citric acid rather than ethylene glycol.

These magnetic nanoparticles are simply injected into the bloodstream and then the animal is anesthetized. Every four hours for one to four days, the nanotubes are attracted to the testicles using a magnet placed near the scrotum.

Then the scrotum is wrapped in electric wire. When energized, the wire then induces a magnetic field which heats the nanotubes.

This method produced temperature differences similar to the previous process, between 37 and 42°C – the variation depends on the quantity injected.

The testicles, thus warmed up, atrophied and their fertility decreased seven days after the treatment – ​​to the point of making them completely infertile for some. After discontinuation of treatment, the testicles returned to normal shape and function within 30 to 60 days.

Although fertility has not returned to previous levels, sperm quality does not appear to have been affected. In mated females, the researchers found no difference in litter size or infirmities in their offspring.

Finally, Fei Sun and his colleagues have found that this process reduces the risk of long-term toxicity. While the gold nanotubes were housed indefinitely in the testicles, the iron-based ones are gradually broken down by the liver and spleen before being eliminated by the body.

Controlled reproduction

The first applications of nanocontraception concerned pets, as an alternative to surgical castration, which is costly and irreversible. Nanocontraception would already be used on cats in China, according to Fei Sun.

This method is more likely to find takers in Europe, where surgical castration is less common, than in North America, predicts David Powell, director of the Center for Reproductive Management at the Association of Zoos and Aquariums in St. Louis, Missouri.

The animal market is all in all limited, he explains, in the absence of an outlet on the livestock side. “The agriculture industry does very little research on animal contraception because cattle and sheep are raised for food and slaughter.”

“And zoos are a very small market on top of that, so pharmaceutical companies aren’t very motivated to make animal contraceptives,” says David Powell. Still, some do, and the Center for Reproductive Management collects data to assess the effectiveness of contraceptives on different species.

Nanocontraception by ferrous nanotubes could one day be part of the panoply of zoo reproduction tools. But, warns David Powell, it will first be necessary to establish whether it is painful, and in which species there would be a contraindication. Iron can be toxic in large amounts. However, according to studies, certain mammals – rhinos, lemurs and dolphins – do not eliminate this mineral in a normal way in captivity.

Reversible options

Reversibility would be the main potential advantage of nanocontraception: it would allow zoos, in particular, to precisely time reproductive events during the life cycle of animals.

But the thing remains to be studied. The mice in Fei Sun's experiments only underwent the treatment once; they have never been subjected to a second injection of nanoparticles after recovery of the testicles.

The researcher's ultimate goal is human nanocontraception, but he admits that he's still a long way off. But besides the fact that its level of toxicity in humans has not been established, it is not certain that gentlemen would agree to undergo four hours of anesthesia before having their scrotum wrapped in electric wire.

Fei Sun, who plans to administer the magnetic nanotubes orally, will also have to find something other than a magnet to make them migrate to the genital glands.

And even then, it remains to be seen how many males will be comfortable with the idea of ​​shrunken testicles, even if these return to their original size over time.

We will therefore have to stick to condoms for a while.

Jeffrey Mo, Global Journalism Fellow, Dalla Lana School of Public Health, University of Toronto

This article is republished from The Conversation under a Creative Commons license. Read the original article.