The implant, or "biomedical tattoo," as researchers call it, has been tested in lab animals, lasts about a year and recognises the four most common types of cancer: prostate, lung, colon and breast cancer.
It works by reacting to the level of calcium in the blood, which rises when a tumuor is developing. About 40 percent of cancers could theoretically be detected this way, researchers said.
"The biomedical tattoo detects all hypercalcemic cancers at a very early, asymptomatic stage," lead author Martin Fussenegger, Professor at the Department of Biosystems Science and Engineering at ETH Zurich, told AFP by email.
"If blood calcium levels remain high over longer periods of time, the calcium sensor in the biomedical tattoo cells produces an enzyme, tyrosinase, which converts the amino acid into the black skin pigment, melanin."
If the wearer notices the spot darken, they should see a doctor to clarify the reason for the change and determine if or what treatment is warranted, he said.
"Early detection increases the chance of survival significantly," he said.
"Nowadays, people generally go to the doctor only when the tumor begins to cause problems. Unfortunately, by that point it is often too late."
The implant was tested in mice with either cancerous tumors that cause hypercalcemia or tumors that do not affect calcium blood levels.
During a 38-day experiment, the tattoos appeared only on the skin of the hypercalcemic mice, which showed no symptoms of illness.
More research and funding is needed to advance the tattoo to clinical trials in people, and the process could take a decade, Fussenegger said.
A paper describing the prototype was published in the journal Science Translational Medicine.