From a distance, it looks like regular grey ink, but it is made of carbon nanotubes and liquid metal and is essentially a bioelectrode. There’s more information here, but it’s in Korean.
With it, it is possible to monitor heart rate, biomarkers in sweat, and perhaps more. Since there’s metal inside could even be heated up to have a physiotherapy effect (dilates blood vessels, bring more blood to promote healing).
The ink’s strong point is the sensor’s proximity to the skin, compared to a patch or distant sensor. As such, it is not only the most comfortable to “wear” but could be potentially more attractive from an accuracy perspective. However, no claims are made on that front.
The scientists say the ink is made of non-toxic materials and even comes off with soap in case you want to switch to a new design or remove it.
To be fair, this is more akin to paint applied on the skin rather than a “tatoo” if you look closely. Scientists paint an electronic circuit onto the skin rather than tattoo it. But it’s an excellent way to make the pitch more attractive.
The user’s comfort is the primary advantage here, and clinical applications might benefit from it. At least, it’s our hope. Several teams worldwide are working on similar concepts, including in Israel and in the United States.