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Jan 19, 2024

What Is an Electronic Tattoo?

Imagine a world where clinical-grade medical equipment — brain activity

Imagine a world where clinical-grade medical equipment — brain activity scanners, heart rate monitors and blood pressure sensors — could be fit onto a tiny sticker that's imperceptible to the wearer. That's the promise of electronic tattoo technology.

An electronic tattoo is a soft wearable device with integrated sensors that attaches to the wearer's skin and transmits data wirelessly. Electronic tattoos can attach anywhere on the user's body, and can be used to track anything from the body's electric impulses to the chemical composition of the wearer's sweat.

Electronic tattoos are soft, sensor-equipped wearable devices that attach to a person's skin and are typically used to gather data. These devices are often made from conductive materials, like graphene, carbon or conductive polymers, which allow them to measure biopotentials, or electric signals emanating from the wearer's body. (For example: muscle impulses, heart rate and brain activity.) They can be equipped with other sensors as well, including accelerometers, which track motion, temperature sensors, or even sensors that measure the chemical composition of the wearer's sweat.

The key difference between electronic tattoos and traditional wearables — like smartwatches and chest-strap heart rate monitors worn during exercise — is the soft form factor, allowing them to attach to parts of the body where you couldn't easily attach a rigid device for extended periods (basically anywhere but the wrist). The use of flexible and thin materials also allows for much closer contact with the wearer's skin than a rigid sensor could accomplish, which is essential for reliably measuring the body's electrical impulses. The result is a device that is more comfortable to wear than a traditional wearable and that in many cases can capture data that could until recently only be gathered in a lab or hospital setting.

Because electronic tattoos are designed to be worn continuously, they allow researchers and medical professionals to collect data that would previously have been impossible, or prohibitively expensive, to track. Electronic tattoos that capture brain activity and eye movement, for example, could be applied to research subjects in a distracted driving simulation that closely resembles a real-world scenario. And in a healthcare setting, an electronic tattoo could be used for continued at-home monitoring of a patient who might otherwise have had to stay at the hospital for observation.

"There's a lot of momentum building in this space, and a successful translation of this technology for the broader population will really change the way we think about monitoring health status and delivering healthcare," said John Rogers, director of the Querrey Simpson Institute for Bioelectronics at Northwestern University and the co-author of a major research paper published in Science in 2011 outlining the emerging field of electronic tattoos. "It has the potential to reduce costs and improve outcomes for patients."

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Electronic tattoos attach sensors to the skin with an adhesive — essentially like an ultra-thin sticker. These sensors wirelessly transmit data to a device with a receiver, like the wearer's smartphone or a dedicated device. Some electronic tattoos also incorporate thin layers of conductive materials that pick up the subtle electrical impulses, or biopotentials, our bodies emit when we move, think and interact with the world through our senses.

"The body is really an electrical machine," said Deji Akinwande, a nanotechnology researcher and professor at the University of Texas at Austin. "Every part of the body has characteristic electrical signals that correlate with specific functions."

Because electricity seeks the path of least resistance and because the human body is not fully insulated for electricity, a portion of these signals jump from the body to the electronic tattoo's conductive layer. The signals can then be measured by a sensor that attaches to the conductive material.

"The body is really an electrical machine ... Every part of the body has characteristic electrical signals that correlate with specific functions."

For devices that measure biopotentials — and especially faint ones, like the signals emitting from the brain — it is important that the conductive material conforms as closely with the skin as possible, thus maximizing the contact surface. This poses a greater challenge than one might imagine because even in areas where the skin appears relatively flat, like your arm or forehead, there are a lot of tiny irregularities that can result in poor contact or noisy data caused by the materials rubbing against each other as the wearer moves.

Historically, this challenge has been overcome in clinical settings by using conductive gel that fills these gaps. But thanks to advances in nanotechnology, researchers are now able to create membranes that are thin enough — in some cases the thickness of only a single carbon atom — to eliminate the need for gels.

"Depending on the material and depending on the skin condition, we can achieve contact impedance that is as low, or sometimes even lower, than gel electrodes," said Nanshu Lu, principal investigator at UT Austin's Lu Research Group and a co-author of the 2011 Science paper.

Many electronic tattoos use cheaper and thicker conductive materials, however — especially if they’re capturing stronger signals, like those emitted from the heart. And some don't measure electric signals at all, relying instead on accelerometers or temperature sensors, or the chemical composition of the wearer's sweat.

The conductive layers of electronic tattoos can be made using pure conductive materials, like gold or carbon. To attain the flexibility required for these applications, gold is stretched into thin, s-shaped ribbons and carbon is arranged in a two-dimensional hexagonal lattice (carbon arranged in this way is commonly referred to as graphene).

Electrode layers can also be made using conductive polymers, like PEDOT, which are not as thin as graphene but have the benefit of being both ionically and electronically conductive. Ionic conductivity makes devices more sensitive because the body's own signals are ionic, making up for the added thickness and decreased contact surface, Lu said. But these materials are all expensive, causing some researchers to opt for composite materials instead.

"You can use an electrically non-conductive polymer matrix — like a rubber matrix or PDMS, or Ecoflex or polyurethane — and then you add conductive fillers, for example carbon nanotubes, silver nanowires, carbon black particles or graphene flakes," Lu said. "The benefit is that it's very cheap. You don't need a large-area sheet of flawless graphene or silver. You just need some nanoparticles and then you mix it."

Electronic tattoos in use today are temporary, and in many cases only designed to last for a few days. This is primarily because an electronic tattoo's electrode layer — the part containing the adhesive and conductive materials — degrades and detaches from the skin over time. For this reason, reducing the cost of materials will be important to widespread adoption.

Cheaper materials aren't the only way to bring costs down, though. Lu said researchers in her lab are also working hard to separate the electronics that read and transmit signals from the disposable layer, so that these components can be re-used or recycled, rather than thrown out with the rest of the tattoo. In the future, this might lead to devices with durable, rechargeable electronic components and disposable electrode layers that attach to the skin.

And because electronic tattoos are still a relatively new kind of technology, researchers are still more focused on functionality than durability and longevity. Akinwande expects a wave of innovation aimed at making electronic tattoos last longer once these devices reach wider adoption.

"That just requires a lot of partnerships between academics and end-use entities like hospitals and companies," he said.

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Electronic tattoos have a wide range of potential applications, including behavioral research and input controls for robots and video games. But like with other wearable technologies, electronic tattoo innovation today is primarily focused on health and wellness.

Electronic tattoos can be used to monitor vital signs for babies born prematurely. Due to their increased risk of illness, disability and death, babies born before 37 weeks of pregnancy typically have their vital signs continuously monitored in a neonatal intensive care unit, or NICU. This monitoring usually involves taping wired sensors to the patient's skin, which can lead to pain and injuries because the skin is underdeveloped and fragile.

To address this problem, Rogers and a team of researchers at Northwestern University have developed electronic tattoos that monitor all the signals captured by traditional NICU sensors — heart rate, respiration rate, body temperature and blood oxygen — plus additional data points like blood flow, blood pressure and separate measures for core body temperature and surface temperature. And because they are lightweight and wireless, these tattoos can use adhesives only one tenth as strong as the ones used for traditional sensors, making them milder on the patient's skin. Moreover, wireless monitoring devices can reduce stress for parents because they can allow for continued monitoring after leaving the hospital.

Once such devices reach wider adoption, adding new sensors and analyzing the results with machine learning could also allow doctors to catch signs of trouble earlier, according to Rogers.

"We have a high-bandwidth accelerometer that captures vibrations on the skin associated with gastrointestinal sounds, respiratory sounds and crying," he said. "I think that's a really powerful trajectory to think about — not stopping with just traditional vital signs but capturing a whole array of characteristics that can be churned through neural networks."

Right now, we don't know exactly how crying patterns, gastrointestinal activity and muscle twitches correlate with health issues that require intervention. But over time, algorithms can be trained to pick up on patterns that would be too subtle for a human observer to notice, which in turn could lead to earlier interventions and better health outcomes.

Electronic tattoos can be used to improve healthcare for patients affected by neurological disorders like Parkinson's disease, cerebral palsy and Alzheimer's. By measuring electrical impulses or physical movement through accelerometers, these devices can help care providers understand the frequency and severity of involuntary movements — data that can be used to adjust medications or change a patient's schedule to make daily tasks more manageable. Future devices could also be used to measure Parkinson's medication in the patient's sweat, which could add another data point that can help calibrate patient care, Rogers said.

"This is really uniquely enabled by the ability to place these devices in any location of the patient's anatomy ... It's hard to imagine strapping an Apple Watch to your neck."

Rogers’ team has also worked on devices that mount to the base of the wearer's neck to identify issues with swallowing — a common complication for patients with a range of neurological disorders including Alzheimer's — and provide haptic or visual feedback that reminds patients to swallow more frequently and helps them swallow at the right time in the breathing cycle.

"This is really uniquely enabled by the ability to place these devices in any location of the patient's anatomy," Rogers said. "It's hard to imagine strapping an Apple Watch to your neck."

Earlier this year, Akinwande and a team of researchers at UT Austin and Texas A&M University developed an electronic tattoo that can continuously measure the wearer's blood pressure with a high degree of accuracy. Akinwande said his team decided to focus on blood pressure because it is the only one of the four commonly recognized vital signs — body temperature, pulse rate, respiration rate and blood pressure — that cannot be easily monitored throughout the day in a real-life setting.

The electronic tattoo designed by Akinwande's team stimulates the skin with a mild electric current and measures the electrical impedance, or resistance, as the current travels through the body. These measurements are fed into a machine learning algorithm that uses this data to infer the wearer's blood pressure.

Blood pressure is traditionally measured using an inflatable cuff, which requires the patient to sit still and is too bulky to carry around for repeat measurements throughout the day. A device designed for continuous measurement, by contrast, would allow blood pressure measurements to be cross-checked against other data to provide insights about the impact of diet, exercise, sleep and stress levels on blood pressure in a real-life setting. This could help doctors recommend specific lifestyle changes and track their impact, or even measure the effectiveness of medications the patient is using to treat high blood pressure.

Signals gathered from the wearer's muscles by electronic tattoos can be used to control robots with gestures. That could look like a robotic arm that mirrors the movements of the wearer's real arm, or a drone that can be controlled by subtle hand movements. The technology could also be used to control prosthetic limbs.

Although electronic tattoo technology could theoretically replace smartwatches and other wearables, they’re not likely to do so anytime soon. Heart rate and respiration, the vital signs most relevant for fitness tracking, are strong enough to pick up with a wristwatch or a chest strap, and a larger form factor makes it easier to add features like accelerometers and location tracking, not to mention the batteries required to power them all.

But electronic tattoos can be used to measure things traditional wearables can't, like brain activity during meditation, for example. And Epicore Biosystems, a tech startup of which Rogers is a co-founder, has partnered with the sports nutrition brand Gatorade to make a smart bottle and companion skin patch that measure water intake and electrolyte loss through the wearer's skin.

Because they allow for continued measurements as the wearer moves around, electronic tattoos can be used to research human behavior and attention in ways that were not previously possible. Electronic tattoos could be used to measure brain activity and eye movement for truck drivers, airline pilots or heavy machinery operators to determine how long they can safely go between breaks before their attention starts to drift.

By applying an array of electronic tattoos and other sensors to a person's body, Akinwande said researchers can also come closer to realizing the vision of a digital twin: a virtual representation of a person that can be used to identify personal lifestyle changes that can improve longevity, or even choose between treatment options for an illness based on the results of patients with similar characteristics.

Lu, for her part, said the technology could be used to develop better interfaces between humans and robotic assistants. By monitoring humans as they interact with robots at home and in workplaces, engineers can more easily identify moments of stress and communication breakdowns and adapt the robot's behavior and design to avoid them.

Electronic tattoos could be used to steer an avatar in a virtual reality or metaverse setting with physical gestures, or even deliver targeted haptic feedback, allowing for more immersive VR experiences. But the technology could also be used to improve telemedicine through providing doctors with real-time vital sign data throughout a virtual exam.

"You could take telemedicine to the next level," said Akinwande. "You could almost replicate an in-person visit. This is the idea behind meta-health."

Because electronic tattoos are so versatile, it is likely that their use will become more widespread in coming years — especially in healthcare settings where effective patient monitoring can be a matter of life and death. Wide-scale adoption of electronic tattoos at the consumer level, however, will likely require devices to become longer-lasting or so cheap and easily recyclable that consumers won't think twice about using and disposing of them. (According to a Northwestern University press release, Rogers’ NICU sensor costs about $10 — cheap for a medical device but expensive for a single-use exercise tracker or VR accessory.)

Beyond monitoring, electronic tattoos could be used to deliver pharmaceuticals through the skin or stimulate parts of the body to support rehabilitation. As electronic tattoo technology advances, one could imagine a self-contained device that measures vital signs related to a chronic health condition and automatically releases medication as needed.

"You’re riding the wave of development activity in the consumer electronics and gadgetry industry. All the trend lines are in the right direction: smaller batteries, more power-efficient electronics, smaller footprints. That will work itself out pretty naturally."

The way Rogers sees it, the challenge of making electronic tattoos more widespread largely comes down to logistics and operations, as many hospitals lack the wireless equipment and data storage capabilities required to make effective use of such devices. To overcome this hurdle, his team has focused on building relationships with existing hospital monitoring equipment vendors who can partner with them in creating the required infrastructure.

And on the engineering side, electronic tattoos will only become easier to make over time.

"We don't have to break any laws of physics," Rogers said. "You’re riding the wave of development activity in the consumer electronics and gadgetry industry. All the trend lines are in the right direction: smaller batteries, more power-efficient electronics, smaller footprints. That will work itself out pretty naturally."