The touch screens used in today's smartphones and tablets give users feedback through diffuse vibrations. One of the challenges researchers are trying to overcome is to provide more accurate feedback by generating a sensory signal only at a precise location on the screen's surface. A team of researchers at List successfully used time-reversal wave focusing—well-known and widely used in medicine for two decades—to improve haptic feedback.

The researchers placed piezoelectric actuators around the edges of a planar surface to generate waves that come together at a specific point on the surface. They also developed and patented a method for the real-time interpolation of the command signals that can focus the waves on any point on the surface based on a reduced set of pre-calibrated points. The result is a very precise localized impulse that can move quickly and continuously.

The rapid movement of the focus point under the user's fingertip and the modulation of the point's amplitude over time vary the touch sensations produced, making it possible to create touch icons with the "feel" of virtual buttons, for instance. List is now working with Leti on integrating these advances into electronic devices using thin-layer actuator deposition technologies.