An E-Tattoo on the Hand Is Worth Two on the Back

When interacting with another person, it is perfectly natural to alter the way you communicate with them based on their emotional state. You might change your tone of voice, or decide that certain things should or should not be said when considering their present mood. There are many cues, from facial expressions to abnormal behavior that tip us off as to how another person is feeling. But computers, which are an ever larger part of our lives, and which we frequently interact with, cannot understand these normal emotional cues. This is a factor that is preventing many applications from reaching their full potential, in areas like virtual reality, mental health devices, and even gaming.

Since the usual signals that humans so effortlessly interpret are a challenge for computers to understand, proxy measurements are commonly used. Electrodermal activity, in particular, has proven to be useful in measuring levels of mental stress. Collecting this data is not entirely straightforward, however. The palms are by far the best, most accurate location from which to collect this data, owing to the high density of eccrine sweat glands, which fill up under conditions of mental stress. Easy, right? Nope! Solutions to capture the electrodermal activity of the palms are either large and obtrusive, or small and fragile. The former group of devices interferes with normal, daily activities and leads to social stigma, while the latter is unreliable outside of carefully controlled conditions in a laboratory.

Given how disruptive palm-based sensors are to a wearer’s normal activities, efforts have been made to capture the signals from other parts of the body, like the wrists, shoulders, or back. Unfortunately, this has proven to be inaccurate — signals are frequently contaminated by secretions from the apocrine sweat glands that regulate our body temperature. A better path forward appears to have just arrived, thanks to work done by researchers at The University of Texas at Austin and Texas A&M University. They have developed a process to produce ultrathin graphene e-tattoos that can be adhered to the palm and are virtually imperceptible. Due to their unique design, they are also strong enough to handle the frequent bending and twisting that comes with the territory.

The team had previously developed graphene e-tattoos that are incredibly thin and very well suited to measuring electrical potential from the body, but they were also too fragile to stand up to the strain they would experience on the palm. The primary innovation in this present work was to do away with straight-line wires, and instead build a serpentine ribbon consisting of two layers of graphene and gold that overlap one another. This layout offers a high degree of strain relief, which allows the conductive traces to stand up to the challenging environment of the human palm.

Using the researchers’ methods, the e-tattoo can be formed 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 applications that make use of this data, or it could be wirelessly forwarded along to a virtual reality headset, smartphone, or just about any other device that needs it. Considering how unobtrusive, flexible, and durable these e-tattoos are, it is easy to imagine a future where this technology is used in all manner of wearable electronic devices.