Thin, Stretchable Circuits for Transparent Display and Soft Robotics

An international research team has developed conductive thin films that are essentially invisible, stretchable circuits, according to a new study in the journal Advanced Materials.

These cutting-edge transparent electronics could be used in countless electronics applications, from touchscreens to solar power.

Many electrically conductive films have already been developed. These materials can function as circuit wiring or electrical charge collection. Most transparent conductors are quite rigid: stretching them causes damage and a loss of electrical functionality. This lack of ability significantly restricts potential uses for these existing materials. For up and coming applications in wearable computing, bioelectronics, and biologically-inspired robotics, engineers need computing technology that is supple, elastic, and very stretchable.

In the new study, researchers described how they developed thin films with this very distinct combination of qualities. The study team said their materials have a high degree of electrical conductivity, flexibility, and elasticity. They are also nearly invisible to the naked eye.

The researchers said they were able to develop these materials using a laser-based production technique to lay down a very fine metal grid on a thin layer of rubber. An alloy of gallium and indium, the metal grid is in a liquid state at room temperature.

“We have developed a laser ablation processing technique to direct-write visually imperceptible liquid metal circuits on soft elastomers to create transparent, stretchable conductors,” Carmel Majidi, an Associate Professor of Mechanical Engineering and director of the Soft Materials Laboratory at Carnegie Mellon University, told AZoNetwork. “A laser is an unlikely tool for this because lasers are typically known to thermally damage the optical, mechanical or properties of soft materials."

“The short laser pulses allowed us to selectively ablate liquid metal from the transparent elastomeric substrate while preserving the mechanical stretchability and optical transparency. Further, with short laser pulses, we were able to narrow down the liquid metal wire width to a few microns - below the acuity threshold of human eyes.”

To test their novel material, the research team created a system that could track air quality and visually indicate pollutant levels on a contact lens display.

This represents an emerging new field: laser processing of soft materials. We have integrated the fundamentals in laser-material interaction, materials science and soft matter to create something that was considered impossible before - visually imperceptible transparent conductors that stretch like rubber.

We plan to extend this emerging engineering framework to develop new classes of circuits, displays, and optical materials that are highly soft and stretchable and extend it to build three-dimensional transparent conductors.

Carmel Majidi, Associate Professor of Mechanical Engineering

Two more recently-published studies described the development of ultrathin films of crystalline materials that could also be used in next-generation electronics. The studies described how scientists created a multistep sequence to fabricate single-crystal, atomically-thin layers of tungsten diselenide over large sapphire substrates.

Past efforts to make similar materials were implemented using small flakes scraped off of larger crystals. The new system uses sapphire as the substrate due to its crystalline composition. The sapphire substrate positions the film, so that it grows in a crystal pattern in a method known as epitaxy.

As seeds of the crystalline material appear on the substrate and the substrate is warmed, the initial seeds stretch over the substrate in a consistent pattern, creating a film without breaks and hardly any defects. The study team said their crucial advancement was the utilization of gas-source chemical vapor deposition to exactly control the seed density and pace of spreading to attain a single two-dimensional layer.

The study team also found a powerful interaction involving the sapphire substrate and the crystalline film, with the substrate directing the qualities of the film. To address these difficulties, the scientists grew two or three layers, which enhanced the performance by desired factors like electron mobility by 20 to 100 times.

Thumbnail Image Credit: inroad/shutterstock

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