Written by Dr. Carson Bruns
Until recently, humans have used tattoos just to color the skin. By re-thinking the composition of tattoo pigments, bioengineers are beginning to develop a new wave of high-tech, smart tattoos that enhance the function of our skin. A tattoo technologist explains how science is bringing smart tattoos out of science fiction and into reality.
A Brief History of Tattoos
What is a Tattoo?
Tattooing is an ancient method of inserting pigment granules in the skin, historically achieved either by puncturing the skin with pigment-coated needles or by rubbing pigment into skin that has been opened by scratching or cutting. Polynesians have practiced tattooing for thousands of years, its history is much longer and more widespread. Indeed, in The Descent of Man, Charles Darwin noted: “not one great country can be named, from the polar regions in the north to New Zealand in the south, in which the aborigines don’t tattoo themselves.”
How Long Have Humans Practiced Tattooing?
The oldest direct evidence of tattooing comes from the remains of Ötzi, the Tyrolean Iceman – a 5,300-year-old European natural mummy bearing 61 black tattoos. Archaeologists discovered several tattooed mummies of similar age from Egypt and South America. Archaeological discoveries dating as far back as 38,000 BC – including those of apparently tattooed figurines and probable tattooing tools such as sharpened bones and cactus spines – indicate that tattooing may have been practiced for tens of thousands of years.
Tattoos in the West
In contrast with the east, where tattooing maintained a vibrant culture especially in the Pacific islands, societal acceptance of tattooing in the western world was low throughout the Middle Ages, mainly due to religious establishments. Pope Hadrian I imposed a ban on tattooing in 787 BC, although many Christians continued to tattoo themselves with religious symbols as professions of faith. A resurgence in tattoo popularity during the industrial revolution is attributed to the South Pacific voyages of Captain James Cook in the 1770s. In 1774, Cook brought a Tahitian man named Omai with him to London, where the aristocracy looked upon his tattoos with delight. It was at this time that “tattoo” (recorded by Cook as “tattow”) was adopted into the English language in favor of the terms “pricking” or “poking” that were in prior use.
Changing Views of Tattoos
Tattooing grew in popularity throughout the 19th century in Europe, reaching over 20% of the population, yet mostly lower classes and the aristocracy practiced tattooing (including monarchs King Edward VII, King George V, Kaiser Wilhelm II, Czar Nicholas II, King Constantine of Greece, King Oscar of Sweden). Ordinary people regarded tattoos as symbols of quirky aristocrats, sailors, or “degenerates” such as criminals, prostitutes, and the insane. This culture ultimately led to the demise of tattooing in the 20th century, with legal and social condemnations of tattooing emerging all over the world.
Tattoos: A Counterculture Resurgence
Beginning with the counterculture movements in the late 1960s, tattooing has gradually risen, once again, to prominence in popular western culture.
A European Commission Joint Research Centre Science for Policy Report estimates that more than 12% of Europeans and more than 24% of Americans are tattooed.
Tattooing is no longer relegated to specific socioeconomic or ethnoracial groups; it is widely practiced regardless of age, gender, background, or status. The reasons for getting a tattoo have also diversified. Historically, tattoos have either served a spiritual or therapeutic purpose, or marked an achievement, rite of passage, or affiliation with a group. Today, individuals also choose their own tattoos for very personal reasons, often involving sentimental, artistic, and/or aesthetic values.
How Tattoos Work
Tattooing methods have evolved very little over the past several thousand years; the primary technique still involves puncturing the skin repeatedly with needles dipped in the tattoo ink. Probably the biggest technological advance in the industry was the introduction of the tattoo machine, which was first patented by Samuel O’Reilly in 1891 based on an idea proposed by Thomas Edison. Improvements to the original design have been made over the past century, but the function remains essentially the same: a tattoo machine automatically oscillates the needles that deliver pigment into the skin, allowing the artist to tattoo much faster than by hand, and with more versatility.
A modern tattoo machine can puncture the skin approximately 100 times per second to a depth of up to 4 mm.
Therefore, tattooing remains a painful process, but there is room for improvement. For example, researchers are working on needle-free injection devices that fire microscopic tattoo ink droplets into the skin like a gun. Because the droplets are so small, they can feel almost imperceptible.
Tattoos and Your Skin
Your skin is made up of three layers: epidermis, dermis, and subcutis. The epidermis is the outermost layer. You are constantly developing new cells called keratinocytes that migrate gradually to the skin’s surface where they ultimately die, slough off, and become dust. Underneath the epidermis lies the dermis, a layer composed mostly of connective tissue (fibers like collagen and elastin) and important structures like follicles, glands, nerve fibers, and blood vessels. Below the dermis, the subcutaneous tissue (subcutis, also known as the hypodermis) houses larger vessels and nerves and comprises mostly fat-storing cells called adipocytes.
What Makes a Tattoo Permanent?
The pigment granules contained in tattoo ink are deposited in both the epidermis and the dermis. While the skin cell regeneration cycle pushes pigments out of the epidermis within a few weeks, most pigments in the dermis stay put.
Immune cells called macrophages visit the tattoo site during wound healing, recognize the presence of foreign materials, and engulf the pigment particles to isolate them from the body.
Normally, macrophages will carry their cargo to the lymph nodes, the body’s filter for foreign substances. Only some macrophages actually reach the lymph nodes, reducing the pigment content in the tattoo by up to 30%. Many macrophages get stuck in the dermis until they die. Tattoo pigments can undergo multiple cycles of capture and release from these macrophages without ever leaving the dermis. However, this macrophage activity does cause the pigments to migrate slowly, leading to the gradual fading and blurring of the tattoo over the course of one’s life.To see an animation of this process, check out this cool TED video.
Pigments are solid particles that change the color of reflected light, and ideally, remain stable for a lifetime. In tattoo inks, pigment particle diameters are typically less than 1 micrometer (roughly 100x smaller than the width of a single human hair), yet larger than 20 nanometers (roughly the size of a single transistor in a modern computer processor). If they are too small, particles will be cleared easily by macrophages leading to impermanent tattoos, yet particles that are too large become increasingly difficult, painful, and damaging to tattoo. The reason tattoo removal works is that high-energy lasers can break down pigments into tiny pieces small enough to be eliminated by macrophages. A similar process can happen (much slower) in ordinary sunlight, which is one reason why it is advised to protect tattoos from sunshine.
Tattoo Ink Technology
The technology of tattoo ink has also evolved. Throughout history, most tattoo inks have been limited to black pigments. Indeed, modern black tattoo pigments are still made of the same material (carbon soot) as the 5,300-year-old tattoos of Ötzi the Iceman. With the advent of industrial pigment manufacturing, however, we can now access tattoo pigments of virtually any color.
At least 50 different chemical compounds are in regular use as pigments for tattoo inks.
Manufacturers have also done much to optimize tattoo ink formulations with respect to particle sizes, fluid viscosity, and ink stability, in order to maximize tattooing efficiency and tattoo lifetimes.
Until recently, we’ve used tattoos only to color the skin. Through advancements in tattoo technology, bioengineers are beginning to develop a new wave of smart tattoos that enhance the function of our skin.
The skin serves many important protective, regulatory, and sensing functions for the body. Wearable technologies can provide additional enhancements to our protection (such as helmets, sunscreen), regulation (such as clothing), and sensing (such as hearing aids, eyeglasses). Scientists have even developed temporary tattoos to help the wearer control computing devices, as well as monitor UV exposure, heart rate, or lactate and glucose levels in sweat.
The Limitations of Wearable Technology
Wearables are inherently limited, however, because being worn outside of the body creates points of physical contact that become uncomfortable, and are necessarily temporary. Subcutaneous implants do not share this limitation, yet they require invasive, costly surgical installation procedures and can also cause discomfort.
Tattoos combine the advantages of wearables and implants; they are comfortable and permanent, yet relatively non-invasive and inexpensive.
We, therefore, consider tattooing as a delivery method for functional, microscopic dermal implants – or “smart” tattoos.
What Is a Smart Tattoo?
Let us define a smart tattoo as a tattoo designed to serve a technological function beyond skin coloration. The idea of a smart tattoo is not new. It has circulated science fiction literature for some time; a noteworthy example is a mediatronic tattoo described in Neal Stephenson’s 1996 novel The Diamond Age. In his 1999 book Nanomedicine, Freitas proposed a dermal display made of tattooed nano-robots that could act like pixels and cooperatively project information through the skin.
How Do We Bring Smart Tattoos Out of Science Fiction and Into Reality?
There are at least two ways: synthetic biology and nanotechnology. The synthetic biology approach involves engineering cells, through genetic manipulation, to express new functions in the skin. In a pioneering study in this area, published in 2018, researchers engineered a new type of skin cell that accumulates melanin (the natural pigment responsible for skin darkening) only when it detects abnormal blood calcium levels. The mice used to test these synthetic biology tattoos developed freckle-like markings upon developing a certain type of cancer.
The nanotechnology approach to smart tattoos involves engineering tiny synthetic particles to implant in the skin.
Nanotechnology is the practice of manipulating matter on the nanoscale (1–100 nanometers), which overlaps with the length scale (20-1000 nanometers) of tattoo particles. If we can design particles to visibly interact with something in the environment or the biological soup (known as the interstitial fluid) in our skin, then perhaps we can implant those particles in the dermis to make a smart tattoo.
Tattoos and Nanotechnology
In my laboratory, we have been using nanotechnology to develop tattoo inks that change color in response to external cues in the environment. In my TEDxMileHigh talk, I describe tattooable UV sensors and tattooable thermometers that could help you detect when you are in danger of sunburn/skin cancer or when you have a fever. I tested the UV-sensing tattoos on myself and they really work!
My lab is also working on a tattooable wire that conducts electricity, looking toward a future of tattooable electronics. Prepare for the cyborg revolution!
Tattoo Technologists Worldwide
While tattoo nanotechnologists like me are a rare breed for the time being, I am not the only player in the game. A European group of researchers led by Ali Yetison recently reported nano-sensors that undergo color changes in response to pH, glucose, and albumin when tattooed in butchered pigskin. The conceptual predecessor of this work was an MIT Media Lab project called Dermal Abyss. In principle, these molecular sensors could help detect problems like acidosis/alkalosis, or high/low blood sugar. Unfortunately, these particular sensors are probably too small (~1 nanometer) to remain in the dermis as permanent tattoos, but they do represent an important first step toward the vision of tattooable biosensors.
The Future of Smart Tattoos
Looking further into the future, we can do a lot more with biosensing tattoos.
It might soon be possible to monitor our blood alcohol or our hydration levels with a tattoo or even detect early signs of cancer.
I also believe that we can design tattoos to enhance skin function beyond the domain of sensing. Perhaps we can make tattoos that help our skin’s protective function, guarding us against damage from different types of radiation, or even forces that would normally wound us. Perhaps we can also engineer tattoos that enhance our skin’s regulatory function, helping us maintain healthy body temperature, for example, or releasing microscopic doses of hormones and drugs to help us maintain balance in our body chemistry. In the future, tattoos will turn our skin into a seamless interface for maintaining good health, as much as for expressing our creativity.
A Note on Tattoo Safety
The U.S. Food and Drug Administration (FDA) considers tattoo inks to be cosmetic products containing color additives. While the FDA has the authority to monitor and regulate color additives, traditionally this authority has not been exercised.
The Need for Safety Regulations
Ink manufacturers are not required to have their ingredients approved, nor to disclose them publicly. They are not even required to sterilize the inks, leading to cases of ink recalls due to bacterial contamination. Some inks contain pigments used in printer toners and car paints. With no federal laws regarding tattooing, tattoo safety laws vary from state to state. The lack of oversight is shocking, considering that tattoo inks become implanted permanently in the body. According to the Coalition for Tattoo Safety, industry leaders have been discussing pending congress bills to revise the FDA Cosmetic Act of 1938 in order to raise safety standards.
The Need for Scientific Studies
While more regulation of the tattoo industry may be forthcoming, there remains a deficit of fundamental knowledge needed to inform those decisions. As few studies have been done on tattoo safety, we know surprisingly little about tattoo biocompatibility. A variety of complications arise in approximately 2% of tattoos – most commonly, infections, granulomas, allergic reactions, cutaneous lymphoma, and UV-induced phototoxicity. However, researchers’ attempts to find correlations between tattoos and cancer have been inconclusive. For regular tattoos and smart tattoos alike, it is imperative that we invest resources into understanding their short- and long-term effects on the body.