Rendering Spatiotemporal Haptic Effects Via the Physics of Waves in the Skin

A major challenge in haptic engineering has been to design practical methods to efficiently stimulate distributed areas of skin. Here, we show how to use a single actuator to generate vibrotactile stimuli which cause sensations of temporally varying spatial extent. Through optical vibrometry methods, we show that vibrational stimuli applied at the fingertip elicit waves in the finger that propagate proximally toward the hand and show how the frequency-dependent damping behavior of skin causes propagation distances to decrease rapidly with increasing frequency of stimulation. Utilizing these results, we design haptic stimuli applied through a single actuator that produces wavefields that expand or contract in size. In a perception experiment, participants accurately (median $>$95%) identified these stimuli as expanding or contracting without prior exposure or training. As a potential application, we used these effects as haptic cues for interactions in virtual reality. We show through a second experiment that the spatiotemporal haptic stimuli were rated as significantly more engaging than conventional vibrotactile stimuli. These findings demonstrate how the physics of waves in skin can be utilized to excite spatiotemporal tactile effects over large surface areas with a single actuator, and inform methods to utilize the effects in practical applications.