"This is a breakthrough that opens a door to infinite possibilities," said Liu Jing , a biomedical engineering professor and the lead scientist on the project.

"With this technology we will not only revolutionise the treatment of bone injuries or diseases, but create super-fighters, as in science fiction, such as Wolverine," Liu said.

Wolverine is a comic-book hero whose skeleton is laced with a fictional metal that makes him indestructible. The character has featured in several Hollywood films.

Liu's team published their research on the alloy in the international academic journal last month. They have tested the technology on pigs and mice, but have yet to conduct clinical trials on people.

"The results are encouraging. The alloy should be safe, with low toxicity," Liu said, adding that he was not aware of similar technology in other countries. "We are making the first step," he said.

Solid metal bone implants have been in use for years, but require surgery.

The new alloy can fill small cracks by injection, according to the researchers.

The alloy consists of bismuth, indium, tin and zinc.

"We can increase the alloy's strength by adjusting the metallic components. We can also control its melting point," Liu said.

The technology could have several military applications, according to the researchers.

Soldiers could be injected with the metal instead of undergoing surgery to repair wounded bones, making them fit for service much more quickly.

They could also endure stronger blows with metal strengthening parts of the skeleton, the researchers said.

Mao Zhengwei , an associate professor of materials science at Zhejiang University, said the new technology showed promise, as it could be an improvement on existing surgery to heal bones.

But he said the technique needed several more years' development to prove it was safe and practical.

"What they have conducted in the laboratory so far does not count in the long run," Mao said.

"They must obtain government authorisation for long-term animal tests before moving on to many rounds of human trials."

The Tsinghua University team is also working on another alloy to create a human exoskeleton. The aim is to increase people's power and strength through mechanical parts joined to the body and limbs. Details of the research were published last month in the .

Liu's team created headlines around the world last year after they developed a metallic ink to paint electrodes onto the human body. The technology was aimed at making it easier for doctors to hook up machines to the body to monitor muscle, brain or heart activity.

Liu said another use of the technology was putting printed circuits on the body that could be used to energise the heart through a small external power source. "If we combine all these technologies together, we can create a superhuman," he said.