Successful experiments on mice bode well for a future human contraceptive - if men can stomach the injections

Researchers in China have discovered a new method of male contraception: a quick injection of gold nanorods into the testes, followed by a 10 minute dose of infrared light. The procedure has only been demonstrated in mice, but the researchers believe it could be used for dogs and cats – and even humans.

Pet contraception is considered an important topic, given the four million unwanted dogs and cats that are thought to be put down every year in the US alone. Many vets routinely sterilise pets, but since surgery requires time and expertise scientists have been looking for cheaper, simpler alternatives.


Functionalising the nanorods with methoxy poly(ethylene glycol) enables them to be used for contraception or even sterilisation

One promising route has been in heating a male animal’s testicles using, for example, hot water baths or microwaves. The heat suppresses the function of the testicles and can even destroy the so-called seminiferous tubules, where sperm is created, leading to infertility. Now, Fei Sun at the University of Science and Technology of China and colleagues have developed the heating idea into what they claim is a convenient, efficient and cheap sterilisation procedure.

The process involves a local anaesthetic for the testes before injecting them with gold nanorods. The nanorods are functionalised with methoxy poly(ethylene glycol), which makes the nanorods resonate in infrared light. When the testes are exposed to an infrared laser, the nanorods gradually heat up.

The researchers tested the procedure on six groups of mice, which received varying concentrations of gold nanorods. The lower concentrations led the mice’s testes to heat up to a maximum of 40°C, so that the mice’s fertility shrank to almost zero for a week before climbing back up to 50% after 60 days. Higher concentrations led to temperatures of 45°C, however, which rendered the mice infertile. None of the mice exhibited any change in behaviour.

Sun believes that the apparently temporary contraception offered by lower doses of gold nanorods would not be of interest to pet owners – but it might be to humans. ‘Such an approach can be used for human contraception,’ he says. ‘Currently, there are no efficient and suitable contraception methods for men, except for condoms … We intend to extend the mouse contraceptive strategies to non-human primates, and finally on humans to check their safety and effectiveness.’

William Holt at the Zoological Society of London points out that the need for an injection means a vet would still be needed, which would make it unsuitable for large-scale wildlife control. But he says that the lower doses show promise for human contraception – provided the procedure passes safety tests. ‘The method might be applicable in western cultures … as an alternative to vasectomy,’ he says. ‘It would be too invasive for use in third world countries, where women have to hide the fact that they are [benefitting from] contraception.’