A stretchable light-emitting device that operates at low voltages and is safe for human skin, developed by researchers at Nanjing University, could one day provide runners and others with a convenient alternative to a stopwatch or cellphone, allowing them to check their running times with a flick of the wrist. Alternating-current electroluminescent (ACEL) displays currently exist that can be stuck like a temporary tattoo onto skin and other surfaces. However, these displays require relatively high voltages to achieve sufficient brightness, which can create safety concerns. The Nanjing team wanted to develop an ACEL display that could operate at lower voltages and thus be safer for humans to wear on their skin. A stretchable light-emitting device becomes an epidermal stopwatch. Courtesy of ACS Materials Letters, 2019, DOI: 10.1021/acsmaterialslett.9b00376. To make their device, the researchers sandwiched an electroluminescent layer, made of light-emitting microparticles dispersed in a stretchable dielectric material, between two flexible silver nanowire electrodes. To make their display brighter than existing ACEL displays, they used a new type of dielectric material formed from ceramic nanoparticles embedded in a rubbery polymer, which exhibited high permittivity, mechanical deformability, and solution processability. The dielectric nanocomposite effectively concentrated electric fields onto phosphor to enable low-voltage operation of a stretchable electroluminescent display, thereby alleviating safety concerns toward its use for wearable applications. The researchers created a four-digit stopwatch display, which they mounted onto a volunteer’s hand. At low voltages, the stretchable display was sufficiently bright to be seen under indoor lighting. The bright stretchable display could find a broad range of applications in smart wearables, soft robotics, and human-machine interfaces, the researchers said. The research was published in ACS Materials Letters (https://doi.org/10.1021/acsmaterialslett.9b00376).