When interacting with another person, it is quite natural to change the way you communicate with them based on their emotional state. You can change the tone of your voice, or decide whether or not to say certain things based on their current mood. From facial expressions to unusual behavior, there are many cues that let us know how another person is feeling. But computers, which are an ever-larger part of our lives and with which we interact frequently, cannot understand these common emotional cues. This is a factor that prevents many applications from reaching their full potential in fields such as virtual reality, mental health devices, and gaming.
Because common signals that humans interpret so easily are challenging for computers to understand, proxy measurements are commonly used. Electrodermal activity, in particular, has proven useful for measuring stress levels. However, collecting this data is not entirely straightforward. Because of the high density of eccrine sweat glands that fill up under conditions of mental stress, the palms are the best and most accurate place to collect this data. Easy, right? No! Solutions for capturing the electrodermal activity of palms are either large and disruptive or small and weak. The former group of devices interferes with normal daily activities and leads to social stigma, while the latter is unreliable outside the carefully controlled conditions of a laboratory.
Serpentine ribbons that deform without breaking (📷: H. Jang et al.)
Given the extent to which palm-based sensors interfere with the wearer’s normal activities, attempts have been made to capture signals from other parts of the body, such as the wrists, shoulders, or back. Unfortunately, this has proven inaccurate—the signals are frequently contaminated by secretions from the apocrine sweat glands that regulate our body temperature. Thanks to work by researchers at the University of Texas at Austin and Texas A&M University, a better path forward appears to have been reached. They have developed a process to produce Ultrathin Graphene E-Tattoos Sticks to the palm and is virtually invisible. Because of their unique design, they are strong enough to handle the regular bending and twisting that comes with the territory.
The team had previously developed graphene e-tattoos that were incredibly thin and suitable for measuring electrical potentials from the body, but they were too weak to withstand the strain felt on the palm of the hand. The primary innovation in this work was to avoid straight wires and instead produce a serpentine ribbon consisting of two layers of graphene and gold that overlap each other. This layout provides a high degree of strain relief, allowing conductive traces to withstand the challenging environment of the human palm.
Using the researchers’ methods, e-tattoos can be formed into sensors and wires that transmit their signals to proximal hardware components for processing and analysis. In this case, an e-tattoo on the wrist is connected to a commercial smartwatch worn on the wrist. From there, the smartwatch itself can run applications that make use of this data, or forward it wirelessly to a virtual reality headset, smartphone, or any other device that needs it. Considering how seamless, flexible, and durable these e-tattoos are, it’s easy to imagine a future where this technology is used in all wearable electronic devices.