Scientists have created a skin implant they say could one day be used to help detect some of the most common cancers.
It works by looking for elevated levels of calcium in the blood, which is linked to some cancers.
When these levels go above a threshold, a response is triggered by the implant, leading the skin to form a brown mole.
Scientists say the technology could eventually help to diagnose cancer earlier, but experts say the research is still in its "early days".
For the study, published in the Science Translational Medicine journal, researchers engineered human cells to detect hypercalcaemia – or high calcium levels – which is linked to four of the most common cancers – breast, prostate, lung and bowel – as well as kidney failure.
The cells were also designed to produce melanin – a dark pigment in the skin, hair and eye – when higher than average calcium levels were detected.
These cells were then implanted under the skin of two sets of mice – one with cancerous tumours that cause hypercalcaemia and the other with tumours that do not affect calcium blood levels.
The artificial moles or "tattoos" appeared only on the skin of the hypercalcaemic mice, the study found.
The implant was also successfully tested in pig skin.
Used in people, the tattoo appearing would be a signal to consult a doctor for further tests, researchers said.
This could lead to earlier diagnoses and a better chance of successful treatment, they added.
Martin Fussenegger, professor at the department of biosystems science and engineering at ETH Zurich in Basel, who led the study, said: "Early detection increases the chance of survival significantly.
"Nowadays, people generally go to the doctor only when the tumour begins to cause problems. Unfortunately, by that point it is often too late."
The implant could be modified so that the mole was visible only under a red light, researchers say.
Prof Fussenegger said the technology could also potentially be used for other health conditions – such as neurodegenerative diseases – by looking for markers in the body other than calcium.
But authors said it was still a long way off being tested in people.
'Early days'
Dr Catherine Pickworth, from Cancer Research UK, said the type of "wearable technology" tested in the study was "exciting", but the research was still in its "early days".
She said: "This study in mice shows that a biomedical tattoo could detect changes in the amount of calcium in the blood, but we need to see if this holds true in people.
"High levels of calcium can be an indicator of cancer but also many other conditions, so this approach may one day help doctors identify when patients could benefit from further tests.
"Spotting cancer early is one of the most powerful ways of improving survival, so finding the best way to monitor people at high risk, or those in remission, is an important challenge."
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