Doctors have reported that minimally invasive surgery suffers from a lack of the sense of touch, causing frustration and even errors during these procedures. Incorporating a high-resolution sensor that provides an analogue to touch into an endoscope would, for example, enable surgeons to superimpose the representation of a tumor’s texture over its optical image. With such an instrument, surgeons could determine during a procedure whether all cancerous tissues had been removed, rather than waiting days for results of a pathological test. According to Ravi F. Saraf, a chemical engineer at the University of Nebraska-Lincoln, proposed touch sensors can be described by two main characteristics: sensitivity and resolution. Sensitivity is the threshold, or the lowest level, of stress that the sensor can measure, and resolution is how finely it can measure the stress distribution. Engineers have developed a large-area, high-resolution touch sensor based on a thin-film design. Pressure images of portions of coins obtained with the touch sensor show remarkable correspondence with their optical images (left pair). The device can capture images with such detail that the letters “TY” in “LIBERTY” and the outline of the embossed number 5 are clearly visible (right pair). Although touch sensors currently offer sensitivity comparable to that of the human finger during a caress (an applied load of ~10 kPa or higher), emulating the finger’s resolution of about 40 μm has been a challenge for researchers. For example, small-area devices, such as an 8 x 8 array of capacitance sensors, have demonstrated 100-μm spatial resolution. Large-area (>1 cm2) sensors, however, provide only 1- to 2-mm resolution. Saraf and his colleague, Vivek Maheshwari, have developed a 2.5 x 2.5-cm sensor that offers spatial resolution of ~40 μm. Their goal was to take a different approach by making a “continuous sensor” — similar to the human finger — as opposed to using an array of devices, to image stress distribution. The electro-optical device comprises stacks of alternating layers of 10-nm gold and 3-nm semiconducting cadmium-sulfide nanoparticles sequentially deposited by dipping in four solutions. The layers are separated by ~3-nm-thick polymer dielectric barriers and are flanked by two electrodes — one gold, coated with flexible plastic, and one transparent, on glass — resulting in a thin-film sensor about 100 nm thick. To test the device, the scientists pressed a penny against the flexible electrode. Current increases locally where pressure is applied, and the semiconducting particles respond to the increased flow. At biases exceeding 8 V, the applied force enhanced electron tunneling between the layers, inducing electroluminescence from the cadmium-sulfide nanoparticles that was detected through the transparent electrode with a CCD camera. The optical intensity and current were linearly proportional to the applied stress, resulting in a touch sensor that was not jumpy or hypersensitive. The sensor overcomes several of the challenges facing its precursors. Devices based on silicon technology, such as organic thin-film flex circuits, rely on expensive lithographic or chemical vapor deposition methods. Also, flex circuit devices are limited in the degree to which they can conform to a surface because bending produces internal stresses that cause a background signal. The team’s sensor can be fabricated in ambient conditions and is self-assembled, costing significantly less, and it could be deposited onto large-area, complex-shaped surfaces. The researchers’ next goal is to map temperature and ultrasound radiation using a similar device architecture. Science, June 9, 2006, pp. 1501-1504.