Engineers at the University of Nebraska in Lincoln have developed a large-area, thin-film-based touch sensor with spatial resolution of about 40 μm — comparable to that of a human fingertip. As reported in the June 9 issue of Science, the electro-optical device comprises alternating self-assembled layers of gold and CdS nanoparticles separated by approximately 3-nm-thick polymer dielectric barriers. The layers are flanked by a flexible electrode constructed of gold on plastic and a transparent electrode of ITO on glass. To test the device, investigators Vivek Maheshwari and Ravi F. Saraf pressed a coin — similar to the Indian 5 rupee piece shown here — against the flexible electrode. At biases exceeding 8 V, the applied force enhanced electron tunneling between the layers, inducing electroluminescence from the CdS nanoparticles, detected through the transparent electrode with a CCD camera. The optical intensity and current were linearly proportional to the applied stress.Measuring 2.5 × 2.5 cm in area and 100 nm in thickness, the sensor is large and thin enough to coat the fingertip of a robotic system. The advantage of the device is that it can measure force as well as spatial information, such as texture. With such a tactile sensor, a robot could quickly determine whether an object has smooth or rough edges, a property that optical systems would require more time and more computing power to detect. Touch sensors also would enable robots to function in the dark. Other potential applications include minimally invasive surgery.