Chemistry engraved on the skin

There is nothing deeper in Mankind than their own skin, the French philosopher and poet Paul Valéry once wrote. It is on the skin - through tattoos - that people preserve the marks of their journey, their identity, their emotions.

Tattoos tell ancient stories. The geometric patterns of Polynesia symbolise the tribal and social identity of its inhabitants; the crude drawings of ancient Greece (1100 BC to 146 BC) identify prisoners, slaves or criminals; dragons, carp and coloured tigers perpetuate the crimes of members of the Japanese Yakusa mafia; and in Portugal, tattoos are associated with marginalisation, prostitution, fado and the sailors of bohemian Lisbon during the early 20th century.

But the oldest recorded tattoos are approximately 5300 years old and are speculated to have had therapeutic purposes. They belong to Otzi, the famous "Ice Man", found in the Italian Alps in 1991. The mummy preserves 61 perfectly visible tattoos located near the joints.

Throughout history, the tattoo tradition has alternated between being socially valued or repressed. Today the art of tattooing the body has been democratised, establishing itself as a common form of expression and artistic practice.

According to the European Chemicals Agency (ECHA), about 12% of European citizens have tattoos, but the percentage in the 18-35 age group is double that. In the US, an estimated 24% of Americans are tattooed.

Tattoo inks are usually obtained by creating a solution with a dye and an appropriate liquid: water (H2O), alcohol(CH3CH2OH or C2H6O), glycerine(C₃H₈O₃), or a mixture of these.

Pigments and dyes are produced to give colour to materials and vary greatly in their composition: black inks are usually carbon oxides, while blue inks are obtained using copper salts or cobalt oxides. White can be titanium dioxide, zinc oxide or lead carbonate.

So when tattoo needles pierce the skin at a rate of between 50 and three thousand pricks per minute, they inject a combination of insoluble chemical compounds (the body does not absorb them) that permanently penetrate the dermis.

In the past, most colour pigments contained heavy metals such as lead(Pb, 82), chromium(Cr, 24), titanium(Ti, 22) and others. The discovery of the toxicity of these compounds in the human body pushed the chemical industry to look for organic pigments that could be used on the body while achieving the same result: permanence, brightness and well-defined designs. 

It is also thanks to the efforts of the scientific community and the development of chemistry that tattoos are no longer a lifelong commitment: in the first decade of the 2000s, inks were discovered that can be removed using lasers and, more recently, in 2017, ephemeral inks that disappear after a year.

The invention came from a group of chemistry and molecular bioengineering students from New York University, in the USA, who wanted to respond to all those people who want to get a tattoo, but who are discouraged, either by its permanent nature, or for cultural or religious reasons, or simply because they are undecided.

The group - which created the startup "Ephemeral Tattoos" - needed six years, 50 formulas and hundreds of tattoo tests to perfect the ink, which is composed of bioabsorbable polymers that are slowly dissolved and completely removed by the body's immune system over a period of 15 months.

The tattoo is made with conventional needles, but the ink is special and can be removed from the skin easily, without the need to go through the painful process of laser removal. It is possible to remove the tattoo at any time using a chemical solution, created by the same group of scientists. This solution is tattooed onto the area of the tattoo and accelerates the pigment expulsion process.

Currently, it is only possible to tattoo with black removable ink, but the development of coloured inks with these characteristics is already being studied.

Permanent tattoos have become safer, but still the scientific community has been arguing that there needs to be a single standard in Europe, such as a list of ingredients and pigments that can be used, as well as long-term toxicity testing of inks, in order to avoid allergic reactions and other serious effects on the skin.

This single list has not yet been achieved, but inks will be subject to tighter restrictions as of 4 January 2022 in the European Union with the REACH regulation.

The restriction covers, for example, chemicals that cause cancer or genetic mutations, and toxic chemicals, as well as skin sensitisers and irritants.  The European regulation also requires that mixtures intended for tattooing and permanent make-up are specified on the labels of the bottles, as well as the list of ingredients used and relevant safety indications.

However, safer and technically suitable alternatives do not yet exist for two pigments: blue 15 and green 7. The European Commission and Member States have agreed on a transition period of 24 months beginning on 4 January 2023 to replace them with safer substances.

Although tattooing dates back thousands of years, safety issues show that there are many points that are still unclear and need to be clarified. According to chemical scientists, the raw materials purchased to make the ink may contain impurities and the actual labels are not always correct. Therefore, this new restriction aims to protect human health first and is a significant step towards making the industry safer.