Liquid metal microdroplets enable smooth, flexible electrical connections across and across circuit layers

If a phone or other electronic device were made from soft materials, how would that change its use? Would it be more durable? If hospital health monitoring equipment were made of less rigid components, would they be easier for patients to wear?

Although this type of electronics is still far in the distant future, Virginia Tech researchers have developed an innovative method for building the flexible electronic components that make them up. The project from the team of Michael Bartlett, principal investigator and associate professor in the Department of Mechanical Engineering, focuses on the circuits that manage all the electronic connections inside.

Published inNatural electronicsThis new technique uses microdroplets of liquid metal to create a staircase-like structure that forms small conductive passages called vias. These vias create electrical connections across circuit layers without the need to drill holes in the hardware, as previous techniques did.

“This brings us closer to exciting possibilities such as advanced soft robotics, wearable devices and electronics capable of stretching, bending and twisting while maintaining high functionality,” Bartlett said.

The publication’s first author was Dong Hae Ho, a postdoctoral researcher working with Bartlett. The Virginia Tech team was joined in this study by colleagues Ling Li, an associate professor at the University of Pennsylvania, and Chenhao Hu, a Ph.D. student in Li’s team.

Soft electronics, vias and interconnections

Previous soft circuit research developed by Bartlett’s team replaces rigid materials with soft electronic composites and tiny electrically conductive liquid metal droplets. These soft electronics are part of an emerging area of ​​technology that gives gadgets a new level of durability.

In this project, researchers tackled the problem of flexible printed circuits, in particular the passage of electric currents between layers stacked on top of each other. It is important to make good use of electrical current in the limited space occupied by printed circuit boards.

While conventional rigid electronics uses well-established techniques to create vias, essential to manufacturing the multilayer electronics common today, it often requires drilling holes through a circuit board, which works when materials Rigid wires are used to connect these layers. In a flexible material where a punched hole can open, controlling this current requires a different approach.

The team’s new technique makes no holes and uses microdroplets of liquid metal to form flexible vias and planar interconnects, creating electrical connections across circuit layers, overcoming these challenges. The process involves the directed layering of liquid metal droplets into a photoresist. By exploiting the irregularities that arise during ultraviolet exposure, the researchers create a staircase-like structure that allows droplets to assemble in a controllable 3D manner.

This approach is very versatile and these liquid metal vias and interconnects can be implemented in several types of materials. They can go further and perform the manufacturing approach multiple times and create more and more layers.

Use a bug as a feature

In known methods of creating electronic components and other micro- and nanotechnologies, upon exposure to ultraviolet, imperfections known as mask edge anomalies, or undercutting, typically present challenges in the standard manufacturing. However, the researchers turned this bug into a feature: the edges of the ultraviolet-exposed regions cause liquid metal droplets to settle and stratify in a vertical, step-like pattern.

This directed assembly allows the droplets to form a continuous path through the photoresist, connecting the top and bottom layers, which is then fully cured to lock the configuration in place. This process occurs simultaneously and droplet settling is rapid, so the process of creating multiple vias takes less than a minute.

“By exploiting these otherwise undesirable edge effects, we can create flexible, conductive vias that connect different circuit layers in a fast, parallel manner,” Ho said. “We can do all this while maintaining flexibility and integrity mechanics of the software device.”

“By integrating layers of circuits in-plane and across-plane, it is possible to create soft, flexible circuits with complex multilayer architectures,” Bartlett said. “This enables new forms of soft electronics, in which multiple vias and soft interconnections are created in a parallel and spatially controlled manner. This is crucial for advancing the field.”