Touchscreens are integral to modern life. The infinitely reconfigurable displays mean custom interfaces for every conceivable purpose appear in a fraction of a second in beautiful, vibrant color.

But while screens may catch the eye, they offer little to our sense of touch — tapping a button on a flat screen can never match the tactile satisfaction and intuitive feedback that a traditional mechanical button provides.

Researchers aiming to imbue flat surfaces with a more physical interface have developed “PopTouch”: a millimeter-thin transparent layer that can raise clickable buttons on demand, anywhere on a screen.

The technology, outlined in the journal Advanced Materials, marries the infinite reconfigurability of flat screens with the tactile interaction of a classic technology in a responsive surface that could be applied to everything from phones to car steering wheels.

Getting in touch

“We all love the mechanical feel and click of a button or a switch, because it allows us to unambiguously feel that our input has been registered,” said study author Herbert Shea, a professor at the École Polytechnique Fédérale de Lausanne. “This is lacking on a flat screen, forcing us to look to see if a virtual button push has been correctly accepted.”

Although the team has long worked on using haptics as a means to improve accessibility for the visually impaired, PopTouch was developed with a more general user of “smart objects” in mind, enhancing the user experience of the devices many of us interact with hundreds of times a day, making interactions more intuitive and accessible.

“We developed PopTouch as a haptic interface that can be integrated on any smooth object with minimal influence on its thickness and appearance,” said Shea. “PopTouch adds haptics to existing objects by simply sticking it to the surface.

Developing a thin, transparent device that can create raised, dynamic buttons with tangible resistance to a “press” was not completely straightforward. For one, how could the team make a flexible material that also generates the high forces needed to produce a button that offers resistance to a user’s press?

Poptouch is a development of technology previously developed in Shea’s lab driven by electrostatic zipping – a way of converting electrical energy into mechanical movement by harnessing attractive forces between charged components, resulting in a zipping motion that displaces the liquid into the button, causing it to rise. The buttons must pop up and stay sufficiently stable that they provide satisfying resistance to a press.

Generating and maintaining that strength — the team are aiming for resistance to at least one Newton of pressing force, which is roughly the amount of force needed to lift an apple just off a table — on a small, dynamic button was a particular challenge.

“To reach the several Newton holding force, needed for the PopTouch button to feel like a real spring-loaded physical button, we made important changes to the peeling mechanics of the existing hydraulically amplified electrostatic zipping actuators,” said Shea.

“This new geometry resulted in a tenfold increase in the holding force of the actuator. This serves as the cornerstone of our research and holds the potential for application to various electrostatic zipping actuators, enhancing their performance in terms of holding force.”

To pair this strength with the transparency needed if the buttons are to appear over a touchscreen, the electrodes in the Poptouch buttons are made from thin indium tin oxide layers – a material often used in transparent displays themselves. The result is a clear layer from which clickable buttons can arise with different configurations.

Traditional sensation in modern devices

“PopTouch combines the advantages of touchscreens and mechanical buttons, offering dynamically reconfigured, pressable buttons,” explained Shea. “It delivers the tactile richness associated with traditional buttons while retaining the key attributes that have propelled the popularity of touchscreens—namely, a thin form factor and reconfigurability.”

The team outline two versions of PopTouch. The first is a transparent overlay for touchscreens. The alternative, a set of buttons with additional embedded sensors that measure the proximity of the user’s fingers and the force of click, provide additional use possibilities such as buttons that appear wherever a user hovers their hand on a steering wheel, and that detect subtleties of a button press from a light tap to harder clicks.

“The main integration challenge is further miniaturizing the high voltage power supply to enable embedding it inside the device. This is easy for instance in a car scenario, as there is ample room for the power supply. It is more challenging for a mobile phone.”

What’s next…

Whether the technology is integrated into devices or applied as an additional layer, the next step is to refine the approach to allow smaller, closer-together buttons, which would increase the resolution of button shapes that could materialize.

Other areas for development, Shea says, include improving the transparency of the interface across the layers of the device to minimize any distortion of the screen below, and boosting the resilience of the interface to withstand the rough treatment of constant use.

If this approach can enable seamless integration of clickable buttons onto the flat surfaces we are forever tapping away at, it could bring a more traditional feel to technology of the future.

Reference: Herbert Shea, et al., PopTouch: a submillimeter thick dynamically reconfigured haptic interface with pressable buttons, Advanced Materials (2023). DOI: 10.1002/adma.202307636