Scientists have developed tattoos that change colour depending on the interstitial fluid concentration of key biomarkers. The development is a step towards developing quicker diagnostics.

The biomarkers studied were pH, glucose and albumin. Variation of blood pH can be indictive of a range of health conditions and glucose sensing is extremely useful for monitoring diabetes. Albumin is a blood protein and high concentrations can indicate heart problems, whilst low concentrations can indicate kidney or liver problems.

The sensor tattoos were tattooed onto pig skin and exposed to different concentrations of the relevant biomarker. The tattoo ink infiltrates the interstitial fluid, which bathes the skin cells and has a composition extremely similar to the blood.

The pH measuring tattoo contained three indicator dyes; methyl red, bromophenol blue and phenolphthalein. The sensor is yellow at pH 5, blue at pH 9 and different shades of green at pHs in between. As the normal pH of the blood is 7.4, the tattoo sensor can measure if the blood pH deviates from the normal range.

The glucose sensor tattoo consists of a colour changing pigment and the enzymes glucose oxidase and peroxidase. The glucose oxidase metabolises the glucose to hydrogen peroxide, which is oxidised to water. The amount of water present changes the structure of the pigment, which causes a colour change from yellow to green as the glucose concentration increases up to 50g/L-1.

The albumin sensor tattoo changed colour from white to blue depending on the albumin concentration, due to structural change in the sensor pigment.

However, only the pH sensor was reversible. Once the other two sensors had been exposed to a set biomarker concentration the colour would not change even if it was exposed to a different concentration. If a sensor is not reversible then the biomarker cannot be continuously monitored. No sensor could provide a quantitative measurement of the pH, which limits the accuracy.

Although sensor tattoos are an intriguing concept, the reversibility and accuracy must be improved to ensure widespread use.