Touchscreens have transformed the way we interact with electronics, enabling the development of elegant handheld devices. But currently, their screens are limited to a fixed size. As flexible and wearable electronics are in development, the touchscreens we'll need in the future will have to be both flexible and biocompatible. In an investigation recently published in Science, researchers have designed an ionic touchscreen that boasts stretchability and biocompatibility, allowing easy integration with the human body.

The team selected a hydrogel-based material for their work. Hydrogels are soft, water-filled polymer networks; their mechanical properties are similar to those of certain tissues, and they can be made of biocompatible materials. As an added bonus, they’re highly transparent. In this case, the scientists selected an ionic hydrogel—a polyacrylamide base containing lithium chloride salts.

For a gel to function as a touchscreen panel, it has to conduct electricity, which is why there's lithium chloride present. To produce a uniform electrostatic field across the panel, voltage was applied at all the panel corners. When a person touches the panel with their finger, the finger acts as a conductor that is grounded. As a result, a potential difference is generated between the electrode and the touch point causing the current to flow from the electrode through the finger.

Depending on the distance between the touch point and the electrode, the magnitude of the current is altered, increasing with decreasing distance.

An incremental approach

To test the concept, the team first developed a 1D touchpad by connecting both ends of a conductive hydrogel strip to platinum electrodes. Every time a finger touched the strip, a closed circuit formed, allowing current to flow from both ends of the strip to the touch point.

Next, the team developed a 2D hydrogel panel. A thin rectangular hydrogel film was connected to platinum electrodes and equipped with current meters at each corner. Through testing, the researchers found that a touch caused the current recorded by each of these meters to be proportional to the distance between the electrode and the touch. As a result, these current values could be used to determine the position of the touch.

As a next step, the team made this into an ionic touchscreen panel by placing it over a computer monitor. The monitor was then attached to a controller that sent the information from the touchscreen panel to a computer. The computer then indicated where touches took place using a second monitor. This allowed people to write or draw on the panel.

Their first tests, using a stick-figure person, appeared to have minor distortions. But through experimentation and simulations, the team was able to reduce the distortion.

Hydrogels are flexible, so the scientists looked at how the touchpad performed in a stretched configuration. A small, circular touchpad was glued onto a biaxial stretcher and connected to the controller board using platinum electrodes. Using this system, the researchers increased the diameter of the circle to triple its native value, causing a 1,000 percent increase in areal strain. Even in this highly stretched state, the touchpad was still operational.

The researchers also investigated how the hydrogel panel would respond to being stretched in only a single direction (anisotropic deformation) and found that it was operational in that configuration, as well.

Demonstrating integration with the human body

Finally, the team developed a transparent touch panel that could be applied directly to the skin. A 1-mm thick film was used to attach the panel to the skin and to provide insulation. The hydrogel panel was soft and stretchable, as expected, enabling comfortable movement.

The team evaluated the electrical response of the panel before and after attachment and found that it operated normally. People were able to use it to write, play music, and play chess when it was hooked up to a monitor.

Science, 2016. DOI: 10.1126/science.aaf8810 (About DOIs).