In every corner of the world for thousands of years, people have decorated their skin with colorful, permanent pigments. Despite how common tattoos are, scientists still know very little about how they interact with the body.

“Most of the research on the biology of tattooing is still sort of the old fashioned, ‘Oh my God, is it cancerous? Oh my God, are you gonna get an infection?’” said Christopher Lynn, a medical anthropologist at the University of Alabama. “The idea that tattooing is this dangerous thing is not really attested to by modern hygiene and sanitation.”

Tattoos are already used in the clinic to mark where doctors will apply radiation treatments, to correct skin discoloration, to mark potential tumors or other pathologies in endoscopic surgery, and for applying permanent makeup (1).

Now, researchers in fields spanning from chemical engineering and dermatology to medical anthropology are investigating tattoos not only as therapeutic in nature, but also as a means for better drug delivery and as “smart tattoos” to monitor and diagnose diseases. Tattoos are more than skin deep.

From the Iceman to today

When two hikers stumbled upon a body frozen in the Italian Alps in 1991 in a rocky gully about 3,200 meters above sea level, they had no idea that they had just discovered the oldest example of a tattooed human to date (2). Affectionately known as Ötzi, or the Iceman, the 5,300-year-old mummy has several tattoos decorating his body. Unlike the pictorial tattoos popular today, Ötzi’s tattoos consist of lines and crosses, and most are in locations that would have been covered by his clothes such as at the base of his spine, ankles, and around his knees. Because of this, researchers hypothesize that his tattoos were likely therapeutic in nature, and the fact that almost all of his tattoos line up with traditional acupuncture points to relieve pain supports this theory (3). Two other prehistoric mummies from Siberia and Peru also show evidence of tattoos on common acupuncture points.

Acupuncture is thought to have originated in China in 100 BCE (4). Through the process of inserting thin needles into the skin, an acupuncturist induces a microtrauma, which elicits a localized inflammatory response to promote healing.

Multiple indigenous groups around the world have traditions of tattoos on acupuncture points. Lars Krutak, a tattoo anthropologist at the Museum of International Folk Art in New Mexico, has documented medicinal tattoo practices in more than 30 cultures, from the Ainu in Japan, the Berber in Morocco, to the Chippewa in the United States and Canada (2). When he began working with the St. Lawrence Island Yupiget people in Alaska, he noticed that after people made significant hunting kills or when serving as pallbearers, they would get tattoos on their joints to prevent the spirits of the recently deceased from entering their bodies.

“If you were to be possessed by one of these powerful spirits, I was told that you would suffer extreme arthritis and severe pain in those joints,” said Krutak. “When I started lining up these primary tattoo locations, it was surprising to me that they lined up with classical acupuncture points to relieve rheumatism and arthritis in the primary joints.”

The practice of tattooing at acupuncture points continues around the world today, and it has ventured into the professional acupuncture space. Douglas Wingate, an acupuncturist at Oregon Health and Science University and a licensed tattoo artist, found that “tattooing essentially stimulates the acupuncture point to a greater degree than standard acupuncture does.” He has found that placing a tattoo at an acupuncture point is about equivalent to ten acupuncture treatments at that site, which is often enough to resolve someone’s pain.

Wingate has found that acupuncture with tattooing seems to work best in people with chronic pain conditions such as tattooing the ear to treat people with decades-long shoulder or back pain. Patients can pick whatever kind of tattoo they would like as long as it fits over the specific acupuncture point to be treated. In one memorable instance, a patient came to Wingate looking for relief from chronic headaches. As was his usual process, Wingate located the appropriate acupuncture points and administered the tattoo.

“[She] had the first period of time where she didn't have headaches at all in years,” said Wingate. But what the patient didn’t tell Wingate was that her headaches were due to a Chiari Malformation, which is a painful, congenital condition that causes part of the brain to poke out of the bottom of the skull. It can be treated with surgery in severe cases.

“If she would have come to me and told me that, usually my talk that I have to have with somebody is… acupuncture and tattooing is not going to change that,” said Wingate. “I was very surprised when she did have such a positive response and was able to get through without pain until she was able to get in for that surgery.”

Wingate has seen much success with his tattoo and acupuncture treatment combinations but he is also interested in studying this phenomenon more formally. For now, he’s excited to provide people with a beautiful tattoo and some relief.

While many traditionally medicinal tattoos line up with acupuncture points, not all of them do.  Researchers have seen that tattoos can have medical benefits that have nothing to do with pain.

Exercise for the immune system

Before the beautiful final result, people must go through the undeniably painful process of getting a tattoo. In the face of a stressful situation like this, the body ramps up production of stress hormones and dampens the immune system, readying the fight or flight response. Despite the pain and the stress, once people get their first tattoo, many go back for more. How multiple tattoos influence the body’s stress and immune responses, however, remains an open question.

“Cultures around the world see tattooing as a way to toughen up the body or make it stronger, and I think of that very biologically,” said Lynn. “I want to understand how that health happens… How can your cultural practice and, ironically, this injury to your body make you healthier?”

To investigate the connection between tattooing and the immune system, Lynn and his team sampled saliva from people with different levels of tattoo experience (5). They measured cortisol, an immunosuppressant, and immunoglobulin A (IgA) antibody levels just before and immediately after people got a tattoo. Cortisol levels in the blood peak in response to stress, which triggers the dampening of the immune system, indicated by a drop in IgA levels. 

Lynn and his team reported that people with little tattoo experience had decreased IgA levels after their tattoos, indicating a stress-induced suppression of the immune system. For people with lots of tattoo experience, however, the researchers saw an increase in IgA levels immediately after the tattoo, indicating that there was no immune suppression in frequent tattoo-getters.

“Think about it in terms of exercise,” said Lynn. “We oftentimes will push ourselves and expect to feel pain or feel sore, and the idea is to push our bodies so that ultimately, we're not continually reinjuring it, but we're building up our muscles…and in the process, we're building up our immune systems.”

Lynn and his team have since replicated these results in a Samoan population (6), and they have additional data from an even larger population that await analysis. Lynn plans to assess additional immune biomarkers and common health biomarkers like cholesterol levels in the context of tattooing.

In particular, he and his team are studying how the levels of these biomarkers change in a person undergoing a very painful and long tattoo experience such as the traditional Samoan pe’a. This tattoo uses the hand tapping tattooing technique, which relies on handheld tools to pierce and transfer the ink into the skin. Hand tapping is more painful than a tattoo applied with a typical electric tattoo machine, and the pe'a  can take as many as 20 to 30 hours to complete. The final tattoo covers the entire lower torso and thighs.

“We're trying to understand not just the on-off switch, but the interaction of mechanisms within physiology,” said Lynn. These large, traditional tattoos also hold important cultural significance for the wearer. Lynn is interested in uncovering how the cultural meaning of a tattoo influences an individual’s immune response.

While it may be tempting, Lynn does not recommend that people go out and get a tattoo for the sole purpose of boosting their immune systems. Exercise and eating well accomplish the same thing just fine.

“It's one thing to go and get a tattoo because it's cool and you'll have an immune response. It's another thing altogether to get a tattoo that says, ‘I am of service to my community, and I am wearing something that my ancestors have worn for at least 1000 years. And it's extraordinarily visible and very painful,’” said Lynn. “There's an added psychological benefit to that, and that's what I'm trying to piece together now.”

Blue lymph nodes 

The dermal layer of the skin, the dermis, resides just underneath the outer epidermal layer and contains blood and lymph vessels along with hair follicles and sweat glands. When a tattoo artist pokes into the skin with ink-laden needles, the cells in the dermis spring into action. 

Once the tattoo ink enters the skin, macrophages gobble up the insoluble pigment particles as if they were invading microbes (7). When the pigment-containing macrophages eventually die, they release the pigment particles back into the dermis, and other macrophages engulf them. Scientists think that this process of “capture-release-recapture” is what leads to the permanence of tattoos. But sometimes pigment particles can evade capture and trickle into the lymphatic system (8).

“If you have a nice blue whale on your shoulder or on your arm, it's very likely that the lymph nodes in your armpit are also blue,” said Ines Schreiver, a tattoo toxicologist at the German Federal Institute for Risk Assessment (BfR).

But macrophages are not the only immune cells in the skin (9). Keratinocytes, often called skin sentinels, induce an inflammatory response when skin is cut or injured. Specialized macrophages called Langerhans cells interface with the lymphatic system and promote an immune response in the skin. The dermal layer also contains many antigen-presenting cells like dendritic cells and T cells that are ready to respond to an infectious threat.

The interaction of tattoo pigments and these immune cells is normally benign, but sometimes the immune cells in the skin overreact and produce an allergic reaction. Schreiver and her team investigate how different pigment particles in tattoo inks interact with the immune system to determine what it is  about certain pigments that causes these reactions.

Using skin biopsies of people who had allergic reactions to tattoo ink, Schreiver and her colleagues noticed that bright red and pink pigments made of organic azo compounds, which are pigments often used in the textile and printing industry, seem to cause the most allergic reactions (10).

Prior research using patch tests noted that the red pigments might not cause these allergic reactions themselves, but in the skin, they may breakdown into smaller metabolites or degradation products that could trigger an immune response. UV light, for example, can easily cleave azo pigments into smaller molecules.

“There are by now a few hints, but not really proof yet, that maybe tattoos that are more exposed to sun are the ones that are reacting when they're red,” said Schreiver. To better understand the interaction between immune cells and tattoo pigments, Schreiver and her team are developing a 3D-skin model for tattooing (11). 

Schreiver and her team recently used their model to understand how tattoos interact with UV light. A prior study reported that mice tattooed with black pigment and then exposed to UV light developed skin cancer more slowly than mice that were exposed to UV light but not tattooed (12).

To determine the mechanism underlying this UV-protective effect, Schreiver and her colleagues tested how UV light affected human skin that had been tattooed with black, white, or orange pigments (13). To their surprise, the white and black pigments protected the dermal cells in the model from UV-irradiation.

“If you think about it, it actually sounds logical afterwards because everything that's beneath this black layer and also with other pigments that are very good in absorbing or scattering light, they protected the underlying cells,” said Schreiver.

She and her team are now adding additional components to their 3D skin models such as macrophages to get as close to human skin as possible and better understand how tattoo pigments interact with human skin.

Tattooing vaccines

While some tattoo pigments elicit an allergic reaction, researchers wondered whether they could tip the immune scales in the opposite direction and use tattooing as a mode of vaccination. With the skin’s unique composition of immune cells, it is an excellent place to mount a multi-faceted immune response of antibodies and immune cells (14). Capitalizing on this, researchers have developed multiple methods to target immune cells in the skin, including a gene gun (which shoots DNA-coated gold particles and was originally developed to genetically modify plants), microneedles, a jet injector (a high-pressure stream of liquid containing vaccine components), and a tattoo machine (15).

Researchers have mostly investigated the potential of tattoo-based vaccination in the context of DNA vaccines because DNA vaccines delivered via the skin generally induce a stronger immune response than vaccines injected into the muscle. While there are multiple DNA vaccines in clinical trials for HIV, human papilloma virus (HPV), Zika fever, and cancer, the only DNA vaccine approved for human use so far is India’s ZyCoV-D DNA vaccine against SARS-CoV-2.

In a study comparing intramuscular vaccination to tattoo-based vaccination, researchers found that the tattoo delivery strategy led to a faster T cell response to HPV and protection against an influenza virus challenge (16). More T cells encountered the vaccine antigen via tattooing than when the same vaccine was given via a standard injection into the muscle.

“People study tattoo delivery as a way to deliver more DNA over larger areas,” said David Weiner, an expert in DNA vaccines at the Wistar Institute. Tattoo delivery “can cover larger areas of the skin, and so you can deliver a lot more product.”

Samir Arbache, a dermatologist at São Paulo Federal University, uses tattoo machines to treat a variety of dermatological conditions including alopecia and idiopathic guttate hypomelanosis, a disease that causes the loss of pigmentation in spots on the skin (17). Arbache started the company Traderm, which commercializes tattoo medical supplies, and so far, he has trained more than 2,000 doctors on how to use tattoo machines for drug delivery and vaccination.

“The future is vaccination,” said Arbache. Tattoo vaccine delivery is “becoming very popular [in Brazil]. Several doctors began clinical trials, and we need to spend some time [with] regulatory” groups, he added.

Smart sensors

After Carson Bruns, a chemical engineer at the University of Colorado Boulder, got his first tattoo at age 19, he quickly became a fan of body art. But he didn’t marry engineering and tattoos until he started research as a chemist building color-changing molecular machines.

“I just had this idea that maybe some of the color changing compounds I was used to working with could be exchanged for the colorants that are used in normal tattoo pigments,” said Bruns. “As I started working on it and started my lab, we quickly realized that there were probably a lot of biomedical applications of this idea too.”

Bruns and his team started out by designing what they called “solar freckles,” which are small, tattooed dots containing a pigment that changes color in response to UV light (18). These dots could tell the wearer when it’s time to apply more sunscreen. The solar freckle tattoos worked well when the researchers tested them on dead pig skin, but they wanted to see if it would work in humans. Bruns volunteered.

“It was really exciting. I was very thrilled when the first time I tested it on my skin and I saw the tattoo change color,” said Bruns.

Bruns has now expanded on the idea of solar freckles to design another sensor, in this case for measuring exposure to high energy radiation (19). These sensors would be useful to people likely to be exposed to radiation, like astronauts in outer space, sailors on nuclear submarines, and people undergoing radiation treatment for cancer.

“The thing we've been working on the hardest is a UV protective tattoo,” said Bruns. “What we're doing now is developing a completely invisible tattoo ink. So, it won’t change the color of your skin, but you would get this tattoo wherever your skin is exposed frequently to sunshine.”

Because most skin cancers occur on the face and the hands, Bruns suggests that people could get tattooed in those areas, and the tattoo would act as a permanent sunscreen. But people are likely going to be reluctant to tattoo their entire faces.

“We consider that the main barrier to people actually adopting this technology if it becomes available. It's not very fun or easy to get your whole face tattooed, so we're also really working on new methods of tattooing that are much faster and less painful. We want to make tattooing more efficient so that treatments like that don't seem so scary,” Bruns said.

While Bruns uses tattoos as interfaces between the body and the environment, Ali Yetisen, a chemical engineer at Imperial College London designs tattoos that sense the inner workings of the human body. Yetisen and his team used existing color-changing reactions and modified them to produce tattoo inks that change color in response to changes in interstitial fluid, the fluid that transfers nutrients and waste products between capillaries and cells.

“We developed electrolyte-based sensors. So, these are based on sodium, potassium, calcium, magnesium, and zinc,” said Yetisen. By monitoring these biomarkers, the tattoo sensors could tell people about their liver and kidney function or if they have a hydration imbalance (20). “For example, if you're exercising, your electrolyte levels may change in the blood,” he added.

There are still significant hurdles. One is figuring out how to reverse the sensor so that once it reacts to a stimulus in the body, it can reset and be ready to react again. A second is how to implant the sensors to make them as biocompatible as possible. While all of their tattoo sensor research has taken place in animals, Yetisen and his team plan to start tests in humans later this year.

Like Bruns, Yetisen knows that user acceptance will be a large impediment to the feasibility of his tattoo sensors.

“Maybe [for] chronic conditions such as diabetes, it can be useful, but if someone were to use it for a couple of weeks, maybe it may not be the best approach,” he said. Since these tattoos would serve a medical function more than an artistic one, Yetisen hopes that people in cultures that may have a stigma against tattooing will have a different perspective on these sensor-based tattoos.

Both Bruns and Yetisen see tattoo-based sensors as an important middle ground between wearable sensors and surgery. While surgery often permanently fixes a problem, it is invasive. Wearables are convenient and able to be taken on and off, but they can cause discomfort such as skin conditions like atopic dermatitis. And devices that sit in the skin but are open to the outside environment have a risk for infection. 

“What excites me is thinking about what types of technologies we can implant in the body by tattooing so as to avoid a more invasive intervention like surgery,” said Bruns.

Yetisen echoed this same sentiment. He sees tattooed sensors extensions of human senses.

“Our bodies have been evolving over centuries now, and to [get to] the next level, it is going to be the integration of new types of materials, including electronic technologies inside the skin to create new senses that can transcend the ability of a conventional human,” he said.

From Ötzi’s delicate line tattoos to next-generation tattooed sensors, tattoos have been a part of human existence from the beginning and will likely be with us long into the future, enhancing human health and telling a story in vibrant color.

References

1. Vassileva, S. & Hristakieva, E. Medical applications of tattooing. Clinics in Dermatology  25, 367-374 (2007).
2. Piombino-Mascali, D. & Krutak L. Therapeutic Tattoos and Ancient Mummies: The Case of the Iceman. In: Sheridan, S., Gregoricka, L. (eds) Purposeful Pain. Bioarchaeology and Social Theory. Springer, Cham. (2020).
3. Dorfer, L. et al. A medical report from the stone age? The Lancet  354, 1023-1025 (1999).
4. White, A. & Ernst, E. A brief history of acupuncture. Rheumatology  43, 662-663 (2004).
5. Lynn, C.D., Dominguez, J.T., & DeCaro, J.A. Tattooing to “Toughen up”: Tattoo experience and secretory immunoglobulin A. Am J Hum Biol  28, 603-609 (2016).
6. Lynn, C.D. et al. The evolutionary adaptation of body art: Tattooing as costly honest signaling of enhanced immune response in American Samoa. Am J Hum Biol  32, e23347 (2020). 
7. Baranska, A. et al. Unveiling skin macrophage dynamics explains both tattoo persistence and strenuous removal. J Exp Med  215, 1115-1133 (2018).
8. Schreiver, I. et al. Synchrotron-based ν-XRF mapping and μ-FTIR microscopy enable to look into the fate and effects of tattoo pigments in human skin. Sci Rep  7, 11395 (2017).
9. Nestle, F. et al. Skin immune sentinels in health and disease. Nat Rev Immunol  9, 679-691 (2009). 
10. Serup, J. et al. Identification of pigments related to allergic tattoo reactions in 104 human skin biopsies. Contact Dermatitis  82, 73- 82 (2020). 
11. Hering, H. et al. TatS: a novel in vitro tattooed human skin model for improved pigment toxicology research. Arch Toxicol  94, 2423-2434 (2020).
12. Lerche, C.M. et al. Black tattoos protect against UVR-induced skin cancer in mice. Photodermatol Photoimmunol Photomed  31, 261-268 (2015).
13. Hering, H. et al. Phototoxic versus photoprotective effects of tattoo pigments in reconstructed human skin models: In vitro phototoxicity testing of tattoo pigments: 3D versus 2D. Toxicology  460, 152872 (2021). 
14. Hettinga, J. & Carlisle, R. Vaccination into the Dermal Compartment: Techniques, Challenges, and Prospects. Vaccines  8, 534 (2020).
15. Kim, YC. Skin Vaccination Methods: Gene Gun, Jet Injector, Tattoo Vaccine, and Microneedle. In: Dragicevic, N., I. Maibach, H. (eds) Percutaneous Penetration Enhancers Physical Methods in Penetration Enhancement. Springer, Berlin, Heidelberg (2017).
16. Bins, A. et al. A rapid and potent DNA vaccination strategy defined by in vivo monitoring of antigen expression. Nat Med  11, 899-904 (2005).
17. Arbache, S. et al. Treatment of idiopathic guttate hypomelanosis with a tattoo device versus a handheld needle. JAAD Int  3, 14-16 (2021). 
18. Butterfield, J.L. et al. Solar Freckles: Long-Term Photochromic Tattoos for Intradermal Ultraviolet Radiometry. ACS Nano  14, 13619-13628 (2020).
19. Butterfield, J.L. et al. Photochromic Intradermal Smart Tattoo Based on Diarylethene-Doped Polystyrene Nanoparticles for Personal γ-Ray Dosimetry. ACS Appl Nano Mater (2022). 
20. Jiang, N. et al. Fluorescent dermal tattoo biosensors for electrolyte analysis. Sensors and Actuators B: Chemical  320, 128378 (2020).