The medical sensors in these temporary tattoos can detect changes in the skin's pH levels caused by metabolic stress.
Researchers at University of Toronto Scarborough invented the sensor, which comes in a thin flexible transfer adorned with a cheerful smiley-face design, the 'Daily Mail' reported.
"We wanted a design that could conceal the electrodes," said Vinci Hung, from the university's Department of Physical & Environmental Sciences, who was part of the team.
"We also wanted to showcase the variety of designs that can be accomplished with this fabrication technique," Hung said.
The new tattoo-based solid-contact ion-selective electrode (ISE) is made using standard screen printing techniques and commercially available transfer tattoo paper.
That's the same kind of paper that usually bears pictures of Spiderman or Disney princesses for youngsters to stick on to their arms.
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In the case of the smiley face sensor, the 'eyes' function as the working and reference electrodes, and the 'ears' are contacts to which a measurement device can connect.
These sensors can detect changes in the skin's pH levels in response to metabolic stress from exertion.
Similar devices, called ion-selective electrodes (ISEs), are already used by medical researchers and athletic trainers.
They can give clues to underlying metabolic diseases such as Addison's disease, or simply signal whether an athlete Is tired or dehydrated during training.
The devices are also useful in the cosmetics industry for monitoring skin secretions. However existing devices can be bulky, or hard to keep stuck to sweaty skin.
The new tattoo-based sensor stayed in place during tests, and continued to work even when the people wearing them were exercising and dripping with sweat.
They were applied in a similar way to regular transfer tattoos, right down to using a paper towel soaked in warm water to remove the base paper.
To make the sensors, Hung and her colleagues used a screen printer to lay down layers of silver, carbon fibre-modified carbon and insulator inks, followed by electropolymerisation of aniline to complete the sensing surface.
By using different sensing materials, the tattoos can also be modified to detect other components of sweat, such as sodium, potassium or magnesium, all of which are of potential interest to researchers in medicine and cosmetology.
The details will be published by the Royal Society of Chemistry's journal, Analyst.