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 evolve, the touchscreens we will need in the future will need to be both flexible and biocompatible. In a recently published study in Scienceresearchers have designed an ionic touchscreen that features stretchability and biocompatibility, making it easy to integrate into the human body.
The team chose 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 are very transparent. In this case, the scientists chose an ionic hydrogel – a polyacrylamide base containing lithium chloride salts.
For a gel to function as a touchscreen panel, it must conduct electricity, which is why lithium chloride is present. To produce a uniform electrostatic field across the panel, voltage was applied to all panel corners. When a person touches the panel with their finger, the finger acts as a grounded conductor. This creates a potential difference between the electrode and the touch point, causing 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 changes and increases 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 with platinum electrodes. Each time a finger touched the strip, a closed circuit formed, allowing current to flow from both ends of the strip to the point of contact.
Next, the team developed a 2D hydrogel panel. A thin rectangular hydrogel film was bonded to platinum electrodes and fitted with flow 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 turned this into an ionic touchscreen panel by placing it over a computer screen. The monitor was then connected to a controller that sent the information from the touchscreen panel to a computer. The computer then indicated via a second monitor where the touches took place. This allowed people to write or draw on the panel.
Their first tests, using a stick-figure person, seemed to show slight distortions. But through experiments 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 to a biaxial stretcher and connected to the controller board with platinum electrodes. Using this system, the researchers increased the diameter of the circle to triple its original value, increasing the surface tension by 1,000 percent. Even in this highly stretched state, the touchpad was still operational.
The researchers also examined how the hydrogel panel would react if stretched in only one direction (anisotropic deformation) and found that it was operational in that configuration as well.
Demonstrate integration with the human body
Finally, the team developed a transparent touch panel that could be applied directly to the skin. To fix and insulate the panel to the skin, a 1 mm thick foil was used. The hydrogel panel was soft and stretchy, as expected, allowing for comfortable movement.
The team evaluated the panel’s electrical response before and after attachment and found that it operated normally. People could use it to write, play music, and play chess when connected to a monitor.
Science2016. DOI: 10.1126/science.aaf8810 (About DOIs).