An E-tattoo on the hand is worth two on the back

When interacting with another person, it is perfectly natural to change the way you communicate with them depending on their emotional state. You might change your tone of voice or decide that certain things should or shouldn’t be said based on their current mood. There are many clues, from facial expressions to abnormal behaviors, that tell us how another person is feeling. But computers, which are becoming an increasingly important part of our lives and with which we interact frequently, cannot understand these normal emotional signals. This is a factor that prevents many applications from reaching their full potential, in areas such as virtual reality, mental health devices and even games.

Since the usual signals that humans interpret so easily are difficult for computers to understand, proxy measurements are commonly used. Electrodermal activity, in particular, has been shown to be useful in measuring mental stress levels. However, collecting this data is not entirely straightforward. Palm trees are by far the best and most accurate place to collect this data, due to the high density of eccrine sweat glands, which fill up under conditions of mental stress. Easy, right? Nope! Solutions for capturing palm electrodermal activity are either large and invasive or small and fragile. The first group of devices interferes with normal daily activities and leads to social stigma, while the second is unreliable outside of carefully controlled conditions in a laboratory.

Given how much palm-based sensors disrupt a wearer’s normal activities, efforts have been made to pick up signals from other parts of the body, such as the wrists, shoulders, or back. Unfortunately, this turned out to be incorrect – the signals are frequently contaminated by secretions from the apocrine sweat glands that regulate our body temperature. A better way forward seems to have just arrived, thanks to work by researchers at the University of Texas at Austin and Texas A&M University. They developed a process to produce ultra thin graphene e-tattoos which can be stuck to the palm and are practically imperceptible. Due to their unique design, they are also strong enough to withstand the frequent bending and twisting that comes with territory.

The team had previously developed graphene electronic tattoos that were incredibly thin and very well suited to measuring the body’s electrical potential, but they were also too fragile to withstand the stress they would experience on the palm. The main innovation in this current work was to remove the wires in a straight line and instead construct a serpentine ribbon composed of two overlapping layers of graphene and gold. This arrangement provides a high degree of strain relief, allowing the conductor tracks to withstand the harsh environment of the human palm.

Using the researchers’ methods, the electronic tattoo can be transformed into sensors and wires that transfer their signals to proximal hardware components for processing and analysis. In this case, an e-tattoo on the palm was connected to a commercial smartwatch worn on the wrist. From there, the smartwatch itself could run apps that use that data, or it could be wirelessly transmitted to a virtual reality headset, smartphone, or just about any other device that needs it. Considering how discreet, flexible and durable these e-tattoos are, it’s easy to imagine a future where this technology is used in all sorts of portable electronic devices.