Researchers at the Technische Universität Berlin propose a means to add temporary tactile buttons to a touchscreens-- the GelTouch, a display with a layer of gel that hardens with the application of heat.
GelTouch uses a heat-responsive hydrogel. The gel is fluid and transparent at room temperature, but turns stuff and opaque when heated above 32°C. A layer of conductive indium tin oxide (ITO) heats discrete areas of the display via electric current, turning the gel solid to create any number of button layouts.
A 7-inch prototype shows a grid of buttons, a slider and a 1-finger joystick buttons, all usable with applications such as a phone dialler and simple games. Buttons disappear once heat is removed.