(Nanowerk Spotlight) In many of our electronic devices, touchscreens have been replacing physical buttons and knobs as our primary means of interfacing with electronics. But touchscreens suffer a major downside: they rely entirely on vision and provide no tactile feedback. We lose the ability to find controls by feel, press buttons with a satisfying “click,” and receive textured cues that confirm our actions.
Researchers have pursued numerous approaches to restore tactile sensations to touch screens. But technical constraints and complex engineering have hindered progress. Solutions like electrodes that mimic textures and actuators that locally vibrate the screen can crudely approximate some tactile effects. Yet they remain far from replicating the intuitive feel and functionality of pressing real mechanical buttons.
Recent materials advances have brought key components needed for versatile and rapid touch feedback closer to reality. Novel thin-film polymers allow large deformation and high voltage operation unattainable with conventional elastic materials. Advanced manufacturing methods enable the assembly and precise structuring of films under a millimeter thick. And developments in flexible printed electronics facilitate seamless integration with touch surfaces.
Capitalizing on these breakthroughs, researchers at EPFL in Switzerland have created “PopTouch” – an ultra-thin haptic interface that makes touchscreens tactile by generating physical buttons on demand. The interface consists of an array of tiny “taxel” actuators under a half-millimeter elastic skin. Applying over a thousand volts zippers the taxels shut, displacing liquid into an expanding upper bubble that pops up as much as 1.5 millimeters to form a button. Users can explore these buttons by feel and press them with a solid mechanical click just like conventional controls.
a) A hydraulically amplified electrostatic zipping taxels (HAXEL) is a pouch, composed of layers of flexible dielectrics and electrodes, filled with a dielectric liquid. Anchored HAXELs can be either transparent or opaque, depending on the materials used. b) Actuation mechanism of a HAXEL: applying a high voltage between the electrodes generates an attractive electrostatic force between them, pulling the two zipping layers together, pushing the liquid into the central stretchable silicone cap, thus creating the raised button the user can feel and press. (Reprinted with permission from Wiley-VCH Verlag)
The key innovation enabling this breakthrough haptic interface is the addition of anchored tabs in the electrostatic zipping actuators. These anchors completely change the mechanics of how the zipping layers peel apart under load, increasing the button’s holding strength from under 300 millinewtons to over 4 newtons – allowing realistic click sensations. And unlike earlier zipping interfaces that required complex driving signals to dynamically maintain position, the anchored taxels operate at only 6 milliwatts per button using simple DC voltage.
The researchers implemented PopTouch two ways: as a transparent interface placed overtop displays to create reconfigurable tactile buttons matched to virtual GUI elements, and as a thin sensorized skin able to detect interactions and render buttons on any product surface. The transparent version uses novel transparent conducting and dielectric polymers that still permit 75% visible light transmission despite the high voltage operation. The sensorized variant integrates printed piezoelectric films configured as strain relieved floating elements that transduce applied force while moving freely with the button displacement.
PopTouch demonstrates a promising step toward mass deployment of versatile haptic feedback for touchscreens. Its low power budget, thin and flexible form factor, transparency, and sensor integration capabilities overcome multiple barriers facing previous approaches. The researchers aim to enhance robustness for real-world application through further material and reliability optimization. But already, PopTouch points toward a future where users need not sacrifice the satisfying feel of mechanical interfaces for the flexibility of virtual touchscreens.
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