Shrinking materials hold big potential for smart devices, researchers say

UNIVERSITY PARK, Pa. — Wearable electronics could be more wearable, according to a research team at Penn State.

The researchers developed a scalable, versatile approach to designing and fabricating wireless, internet-enabled electronic systems that can better adapt to 3D surfaces, like the human body or common household items, paving the path for more precise health monitoring or household automation, such as a smart recliner that can monitor and correct poor sitting habits to improve circulation and prevent long-term problems.

The method, detailed in Science Advances, involves printing liquid metal patterns onto heat-shrinkable polymer substrates — otherwise known as the common childhood craft “Shrinky Dinks.” According to team lead Huanyu “Larry” Cheng, James L. Henderson, Jr. Memorial Associate Professor of Engineering Science and Mechanics in the College of Engineering, the potentially low-cost way to create customizable, shape-conforming electronics that can connect to the internet could make the broad applications of such devices more accessible.

“We see significant potential for this approach in biomedical uses or wearable technologies,” Cheng said, noting that the field is projected to reach $186.14 billion by 2030. “However, one significant barrier for the sector is finding a way to manufacture an easy-to-customize device that can be applied to freestanding, freeform surfaces and communicate wirelessly. Our method solves that.”

Yangbo Yuan, a graduate student in engineering science and mechanics and co-author on the study, explained that current production methods for wearable, internet-connected electronics include directly fabricating circuits onto target surfaces via 3D printing, a complex process that limits scalability and cost-effectiveness. He said that more cost-effective methods that use liquid metal and manipulating softened thermoplastic materials offer limited reconfiguration or customization due to the need for pre-conforming molds. Overall, no current commercial method solves the need for applying a smart, Wi-Fi-enabled device onto complex 3D surfaces or structures at scale.

“We’ve been working on approaches to get this circuit onto the human body or different 3D geometries, but the dream is always to find a solution that is super easy to fabricate, not just in a specialized lab, but also at home,” Cheng said.

The researchers’ dream came true, Cheng said, when they discovered the polymer used in shrink plastic craft kits, a commonly used children’s craft material used to create custom-cut items like keychains or jewelry. Packages of sheets can be bought online for under $15, so the sheets met their low-cost, readily available criteria. 

“When we saw this shrinkable craft, it seemed like the perfect fit,” Yuan said, explaining that when heated, the sheets shrink uniformly in the horizontal and vertical directions, allowing for a controlled shrinking process. “Our purpose is really to create a framework that is DIY and widely available to as many people as possible.”

With a substrate material selected, the researchers needed to determine a way to apply a circuit to the polymer that would withstand the heat-shrinking process without losing conductivity or structural integrity. Conductivity is key to minimizing power consumption and increasing data transmission efficiency, which is integral for connecting the circuit to a Wi-Fi network, the researchers said.

According to the team, traditional metals used in circuits, like gold or silver, are not only expensive, but they are also too rigid to withstand the shrinking process and could lead to wrinkled structures that reduce performance. The team had experience applying liquid metal — an alloy of gallium and indium with a melting point of about 60 degrees Fahrenheit — to stretchable structures and said they saw potential in attempting the reverse.

To better understand the liquid metal, they worked with Feifei Shi, assistant professor of energy engineering in the John and Willie Leone Family Department of Energy and Mineral Engineering and corresponding author. Shi’s lab was designed to eliminate the same environmental factors that affect liquid metal, such as air or humidity, for her research in electrochemistry of lithium-ion batteries that requires tight environmental controls. The team conducted their experiments inside an argon-filled glass box, allowing them to work with the sensitive material while preventing uncontrolled oxidation or other changes.

“Liquid metal is a magic metal,” Shi said. “But since the material is so new, an understanding of its intrinsic properties, such as surface tension or chemistry, is lacking. Its behavior can also be difficult to predict. So, at each step, we need to review things, layer by layer, to see what is happening.”

Shi helped the researchers observe how the liquid metal flowed when heat was applied to the polymer. 

“Liquid metal flows very nicely as droplets,” Cheng said. “Our challenge was we needed to consider how to make sure the droplets maintain a connection with each other to provide a conductive pathway for an electric current while staying bonded to the substrate. Otherwise, the substrate shrinks and the circuit does not — and that’d be a big problem.”

The solution was to modify the pristine liquid metal.

The researchers addressed the adhesive issue by encapsulating and dispersing liquid metal droplets via ultrasonication — a process that uses high-frequency sound waves to generate ultrasonic vibrations that turn the liquid into small particles — and sodium dodecylbenzene sulfonate, a chemical similar to a standard detergent. The modification steps switched the liquid metal’s hydrophobic nature of repelling water to one that is hydrophilic and attracts water, creating an environment more conducive to bonding.

They also applied a plasma treatment to the plastic sheet to encourage the formation of hydrogen bonds. The researchers found that the combination of the modified liquid metal and the plasma coating enabled both the printed circuit pattern to survive the heat-shrinking process and produced performance advantages, Yuan said. The ultrasonic process produced a liquid metal containing both solid and liquid components — the solid component improved conductivity and mechanical robustness and the liquid component maintained the metal’s fluidity. Additionally, they found that the liquid metal filled microscopic holes in the plastic sheet before hardening, forming an interlocked structure. Overall, tests revealed that the modification steps increased adhesion by 20%.

“We are still investigating the substrate; we don’t know its entire morphology but seeing how the liquid metal went from planar to a dome-like structure, coupled with the improved performance, shows how 3D architecture and geometry can improve circuit designs,” Cheng said.

By printing black ink to make the circuit on the substrate and then using near-infrared light, the team could direct heat to shrink the material into a target geometric shape.

“Fabricating small circuits requires very expensive equipment, infrastructure, and delicate hands with a lot of experience and skill,” Cheng said. “The ability to start with a larger surface area and then reduce the size opens interesting possibilities.”

Next, the team used repeating geometric patterns to create a more compact antenna. Yuan explained the ability to control how the material folds and transforms allows them to design shapes that reduce interference and conform to various surfaces and shapes. Their process of utilizing thermally induced shrinking to conform antennas onto household objects, could significantly reduce manufacturing costs compared to designing customized antennas for each individual object, Yuan said.

“In this context, the shrinkable antenna, together with compatible sensor modules, can be deployed as a plug-in solution to update conventional daily objects with intelligent wireless capabilities, demonstrating high potential for retrofitting the current home stock into smart homes,” Yuan said.

As a proof-of-concept test device, the team developed a wearable ring with an embedded, miniaturized accelerometer that captured and successfully communicated gesture-based movements over a network. The initial results showed potential for wide-ranging applications, Yuan said, but noted that specific use cases, such as sign language recognition from finger movement, would require further testing.

“We provide an exciting framework with broad material compatibility, precision and low manufacturing costs that provide a promising route towards a more accessible, next generation of 3D electronic devices,” Yuan said.

Cheng said he is already thinking about next steps and future collaborations with researchers in the Penn State College of Engineering and the College of Medicine to improve antenna designs and biomedical applications.

“The ability to create a custom, low-cost device that monitors patient information is an exciting, potentially life-saving possibility — no matter the patient's shape or size,” Cheng said. 

Other collaborators include Dongliang Chen and Jianyu Li, both graduate students in the Department of Energy and Mineral Engineering; Xin Xin, graduate student in the Department of Engineering Science and Mechanics; Ankan Dutta, doctoral student in the Department of Mechanical Engineering; Mohammad Ali Amidia, Fatema Tuz Zohra and Wanqing Zhang, doctoral students in the Department of Engineering Science and Mechanic; Xianzhe Zhang and Abu Musa Abdullah, recent graduates from Department of Engineering Science and Mechanic; and Bowen Li, assistant teaching professor of mechanical engineering at Penn State. Cheng, Yuan and Xin are also affiliated with the Neuroscience Institute, Huck Institutes of the Life Sciences. Cheng is also affiliated with the Center for Chemical Ecology, Huck Institutes of the Life Sciences. 

This work was supported by funding from the National Institutes of Health and Penn State. 

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