What if phones and electronic devices were crafted from soft materials? How would this impact their functionality? Would they be more resilient? If health monitoring gadgets in hospitals were designed with softer components, would that make them more comfortable for patients to wear?
Although we may not be near this reality yet, researchers at Virginia Tech have made significant strides in developing soft electronic components. Led by Michael Bartlett, a principal investigator and associate professor in the Department of Mechanical Engineering, the project centers on the circuits that facilitate all electronic connections.
The innovative technique, recently published in Nature Electronics, employs liquid metal microdroplets to create a stepped formation that establishes small conductive pathways known as vias. Unlike traditional methods that involve drilling holes in hardware, these vias enable electrical connections to be formed across circuit layers without the need for drills.
“This advancement brings us closer to promising innovations such as advanced soft robotics, wearable technologies, and electronics that can flex, twist, and stretch while retaining high performance,” Bartlett stated.
Dong Hae Ho, a postdoctoral researcher collaborating with Bartlett, served as the lead author on the publication. The Virginia Tech team collaborated with Ling Li, an associate professor at the University of Pennsylvania, and Chenhao Hu, a Ph.D. candidate on Li’s team.
The research received support from the Office of Naval Research Young Investigator Program award and the National Science Foundation’s Early Faculty Career Development (CAREER) award, along with assistance from Virginia Tech.
Soft electronics, vias, and interconnects
Past research by Bartlett’s team focused on swapping rigid components for soft electronic composites and minuscule liquid metal droplets that conduct electricity. This area of soft electronics is rapidly evolving, providing gadgets with enhanced durability.
The project tackled challenges surrounding soft circuit boards, particularly the transmission of electrical currents between layered components. Efficiently managing electrical flow in the compact space of circuit boards is vital.
While traditional rigid electronics utilize well-established methods for creating vias—essential for constructing multilayer electronics—these methods typically necessitate drilling holes through circuit boards, effective when using hard materials. In flexible materials, holes may stretch, requiring a novel approach to control electrical current.
The team’s innovative method eliminates the need for drilled holes, instead using liquid metal microdroplets to create flexible vias and planar interconnects that facilitate electrical connections between circuit layers. The process involves a directed arrangement of liquid metal droplets within a photoresin. By taking advantage of irregularities that occur during ultraviolet exposure, researchers form a stepped structure, enabling droplets to be strategically assembled in three dimensions.
This approach is impressively adaptable, allowing for the use of liquid metal vias and interconnects in various materials. Moreover, the fabrication technique can be repeated multiple times to produce several additional layers.
Turning a flaw into a feature
During conventional electronic manufacturing, inconsistencies known as mask-edge abnormalities or undercutting frequently pose challenges. However, these researchers flipped the issue into an advantage: the edges of the ultraviolet-treated areas cause the liquid metal droplets to position and form a step-like pattern. This directed assembly enables the droplets to connect the upper and lower layers, which are then fully cured to secure the design. This entire process occurs concurrently, with the droplet arrangement completed in under a minute.
“By exploiting these unintended edge effects, we can quickly produce soft, conductive vias that link different circuit layers while preserving flexibility and mechanical integrity,” Ho explained. “This allows for the parallel creation of these connections effectively.”
“Integrating both in-plane and through-plane circuit layers enables the production of soft, flexible circuits with intricate, multilayer designs,” Bartlett noted. “This capability paves the way for new categories of soft electronics, as it allows for the spatially controlled parallel creation of multiple soft vias and interconnects, which is vital for progress in this field.”
