Could the Next Step in Wearables be E-Tattoos?

July 27, 2022 by Jake Hertz

Where other wearables falter, e-tattoos are showing major promise, especially in the medical field regarding blood pressure monitoring.

Wearable electronics is a subsect of consumer electronics that has grown by leaps and bounds in the past decade. One of the driving forces behind this growth has been the ability of devices like smartwatches to provide health monitoring functionality to the wearer.

Despite the impressive health monitoring abilities of current wearables, one place where they still fall short is measuring blood pressure. 


Overview of some applications, types, and purposes for wearable devices.

Overview of some applications, types, and purposes for wearable devices. Image used courtesy of Sam Solutions


To address this issue, the University of Texas at Austin researchers are looking at a new kind of wearable technology altogether. Recently, UT Austin researchers published a paper describing e-tattoos and their ability to monitor blood pressure, which traditional wearables typically cannot do.

This article will look at traditional wearable health monitoring, where they fall short, and how e-tattoos may be the answer. 


Wearables for Health Monitoring

Today, most smartwatches come equipped with health-monitoring capability and largely rely on the same optical approach.

That conventional method for health monitoring in smartwatches is known as photoplethysmography, which measures heart rate using light. The concept behind photoplethysmography is that our blood absorbs green light, so when the heart beats and more blood flows through our veins, more light is absorbed. In the period of time between beats, there is less absorption.


Smartwatches monitor our health using photoplethysmography.

Smartwatches monitor our health using photoplethysmography. Image used courtesy of the Conversation and Tim Collins


Smartwatches accomplish this by shining hundreds of tiny green LEDs onto our skin which are then coupled with sensitive photodiodes which detect the reflection of the green light. These measurements allow smartwatches to determine our heart rates and use advanced algorithms to infer other health information such as calories burned and even detect heart issues such as irregular beating. 

As powerful as this technique has proven to be, one place it is still lacking is in measuring blood pressure, which is among the most standard and important blood measurements in medicine. Smartwatches often fail to accurately measure it due to factors such as the movement of watches on the wrist and the far distance of a watch from major arteries. 


What is a Digital Tattoo or Electronic Tattoo (E-tattoo)?

To create a wearable capable of measuring blood pressure, many are looking to e-tattoos as the answer.


An example of an e-tattoo from TAU. Image used courtesy of TAU


Digital tattoos are the application of printed and flexible electronics applied in such a way as to exist and function on human skin. To do this, the circuitry and its underlying substrate must be flexible. Often, e-tattoos will leverage materials such as gold nanorods, graphene, or other polymers with rubber backing to apply the tattoo and then use thin-film transistors for the physical circuitry.

While the technology is still in development, it is already showing a lot of promise in medical applications where the direct application of circuitry to the skin can help passively monitor several health metrics in ways that are not currently possible.


A Step in E-tattoos and Monitoring Blood Pressure

Recently, researchers from UT Austin made headlines when they announced the successful application of e-tattoos for blood pressure monitoring.

In the paper, the researchers describe that the key to their work was a graphene tattoo applied to the subject's wrist that was fabricated on a 200 nm layer of polymethyl methacrylate using CVD graphene. The electronic tattoo worked to send AC currents into the subject's skin and analyzes the body's bioimpedance. This metric has a correlation with changes in blood pressure based on blood volume. 


The e-tattoo was developed by UT Austin researchers.

The e-tattoo was developed by UT Austin researchers. Image used courtesy of the University of Texas at Austin


To interact with the tattoo and control the device, the team developed a custom PCB to achieve low-noise bioimpedance sensing. They built the PCB around an ARM Cortex M4 MCU which was used to sense the bioimpedance signal via an onboard ADC and control the frequency and amplitude of the delivered electric current. The system then measures the voltage modulation detected by the injected current modulation, and bioimpedance is extracted using a specially trained machine learning model.

The results were impressive, as the researchers showed that the tattoo worked for over 300 minutes while continuously and non-invasively measuring blood pressure with an accuracy of 0.2 ± 4.5 mm Hg for diastolic pressures and 0.2 ± 5.8 mm Hg for systolic pressures.

All in all, these results show that e-tattoos can have promise. It will be interesting to see how this technology continues to grow and develop.