Wearable devices like smart watches and fitness trackers help us measure and learn from physical functions such as heart rates and sleep stages. Now MIT researchers have developed a tiny equivalent for individual brain cells. These soft, battery-free wireless devices, actuated with light, are designed to wrap around different parts of neurons, such as axons and dendrites, without damaging them. They could be used to measure or modulate a neurons electrical and metabolic activity. They could also serve as synthetic myelin for axons that have lost this insulation, helping to address neuronal degradation in diseases like multiple sclerosis. The devices are made from thin sheets of a soft polymer called azobenzene, which roll when exposed to light. Researchers can precisely control the direction of the rolling and the size and shape of the tubes by varying the intensity and polarization of the light. This enables the devices to snugly, but gently, wrap around curved axons and dendrites. To have intimate interfaces with these cells, the devices must be soft and able to conform to these complex structures. That is the challenge we solved in this work, says Deblina Sarkar, an assistant professor in the Media Lab and the senior author of a paper on the research. We were the first to show that azobenzene could even wrap around living cells. The researchers, who developed a scalable fabrication technique that doesnt require the use of a cleanroom, have demonstrated that the devices can be combined with optoelectrical materials that can stimulate cells. Moreover, atomically thin materials can be patterned on top of the tubes, offering opportunities to integrate sensors and circuits. In addition, because they make such a tight connection with cells, they could make it possible to stimulate subcellular regions with very little energy. This could enable a researcher or clinician to treat brain diseases by modulating neurons electrical activity.