Scientists at Virginia Tech, Blacksburg, Va., synthesized a composite material whose properties make it an ideal candidate for the burgeoning field of flexible electronics.
“We discovered a liquid metal-based composite that is highly electrically conductive and strongly adhesive,” said Michael Bartlett, associate professor of mechanical engineering at Virginia Tech. “This enables robust mechanical and electrical connections between soft circuitry and rigid electrical components.”
This includes computer and mobile phone displays, wearable health-monitoring tools, various car, airplane, and environmental sensors, strain gauges on roads and bridges, and many others.
“[Our liquid metal composite] overcomes several challenges in next generation flexible and soft electronics and allows us to create flexible hybrid electronics, which combine the desirable properties of soft electronics and state-of-the-art rigid electronics into a single system,” added Bartlett.
To join various electronic components, scientists and engineers have commonly used hot solders — fusible alloys of metals such as tin, lead, bismuth and indium — but they pose challenges in flexible electronics.
Traditional solders are rigid and brittle, causes them to crack and delaminate when used with flexible and soft substrates as they are bent or stretched, which results in failure. Solder also requires high processing temperatures.
In addition to solders, polymer-based, electrically conductive adhesives can be used as interconnects in flexible devices, but they also have disadvantages. For example, their conductivity is relatively low and adhesive strength is generally poor.
To overcome these problems, the team developed a material of a different type, a composite of liquid metal microdroplets of eutectic gallium-indium (EGaIn), silver microflakes, and a flexible epoxy matrix. They call it E-CASE (electrically conductive adhesive with silver and EGaIn). It can be processed at low-temperatures through methods such as stencil printing and 3D printing and can be used with flexible circuit materials that traditional high-temperature solder cannot.
The team’s experimental study showcased not only E-CASE’s impressive adhesive strength and conductivity, but its remarkable mechanical properties. A strip of this material adorned with multiple LEDs could withstand a tight bending radius of about 1 mm without LED failure, and even supported the weight of a car driving over it.
“We hope that this material can play a role in several areas in electronics, robotics, and sensors,” stated Barlett. “As the use of flexible and hybrid circuits grows in fields such as wearable electronics, biomonitoring, soft robots and more, creating robust electrical and adhesive connections at soft-rigid interfaces will be key.”
Despite its success in laboratory experiments, the material’s journey to widespread industrial use could still face challenges. The material has been demonstrated to work in hybrid electronic systems, but processing parameters and an understanding of how the materials perform over time are needed for scalability in manufacturing.
This work was reported in the journal Advanced Functional Materials.
Image – E-CASE liquid metal adhesive enables flexible connections to make better, tougher flexible electronics for wearables and robotics. Courtesy of: Michael Bartlett et al.
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