When it appears, the hi-tech tattoo, which is still under development, looks like a large mole. It is placed on the bottom of the forearm, where the skin is usually paler, to make it easier to see. The tattoo will only become visible if it detects a rise in calcium levels in the blood and skin - which can be an indicator of cancer.
Scientists make the tattoo by removing patient’s skin cells using a local anaesthetic.
They then modify the cells in the laboratory to release large amounts of melanin, the pigment that causes skin to darken when we sunbathe, if there is an increase in calcium.
These altered cells are injected back into the skin where they remain the same colour as surrounding tissue unless calcium levels spike. Calcium is the most abundant mineral in the body and is needed by every cell to function effectively. The tattoo could be tested in humans within the next two to three years.