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Flexible Thinking: The Rapidly Expanding Realm of Stretchable Circuits

Flexible circuit technology has been rising ever higher on the radar of those charged with designing next-generation electronic products for every imaginable application, from the mundane to the highly exotic. The technology is being embraced by a growing fan base as they become increasingly aware of flex circuit technology’s numerous benefits. They are being driven to new heights by industry and government collaborations such as NextFlex and FlexTech, and their laser-like focus on what was formerly called printed polymer thick film circuits—now rebranded as flex hybrid electronics (FHE) and printed electronics.

More than a simple technology rebranding and marketing effort, NextFlex has created an impressive operation in San Jose, California, where material equipment and process solution providers can showcase their offerings to members and provide space where engineers and local students can familiarize themselves with them. Given the looming talent gap in the field of interconnection fabrication technologies within the United States, this development is most welcome.

The general focus on printing technologies for their flexible circuit products has boosted enthusiasm among designers looking for new ways to use the technology to realize their visions—from the practical to the fanciful. According to a recent study and report by market research group Lucintel, the stretchable and conformal electronics market is expected to reach approximately $479 million by 2025 through a compound annual growth rate (CAGR) of 89%¹. If it holds true, that is an impressive number.

One area receiving ever greater attention in recent times has been wearable electronics, especially regarding physical wellness monitoring for sports and medicine. While not an imperative (wireless solutions are an option), a key feature increasingly sought in such products has been stretchability and conformability to the contours of the subject being monitored—the human body. We take for granted the various movements of our physical bodies, especially our appendages. However, we less often think about the rising and falling of our chests as we breathe, or movements in our throats as we swallow, but these motions should be accounted for if we need to reliably keep contact with the skin during use to monitor internal functions such as pulse and/or heart rhythm. A circuit that can stretch, and repeatedly and reliably recover during operation, can be a very important feature.

In my book Flexible Circuit Technology 4th Edition, I co-authored a chapter on stretchable circuits with Professor Jan Vanfleteren. At the time, the topic was new, but nevertheless, there was a great deal of interest in stretchable circuit technology in the European Union, with research funded by the same. That interest has remained over the past decade and a range of new materials, both substrates and conductors, have been developed and entered the fray.

Developing stretchy materials has been less of a challenge than making printed conductors reliably adhere to them over repeated stretching events. The continuing effort to create better and more stretchable conductors has yielded some interesting results, including some liquid metal solutions.

The stretchable circuit branch of flexible circuits has resulted in the need to create dynamic testing tools to account for the different stresses that might occur with these newest members of the flexible circuit family. During my visit to FlexCon, co-located with SEMICON® West in San Francisco last month, I visited the Bayflex Solutions booth. The company is the exclusive distributor of mechanical endurance testing equipment for North America and Europe for Yuasa System Co., Ltd. Yuasa describes itself as a global leader in accurate, repeatable, and reliable mechanical test equipment capable of flexing, bending, folding, stretching, rolling, and twist testing flexible circuits, including data collection and analysis. In the real world, however, stretching can be more complex than simple linear stretching. The company displayed a device where a dome-shaped tool was pressed down into a stretch circuit and then lifted upward in a repeating motion. This machine was not found on the company website but appeared to be a new and legitimate type of test, capable of approximating the motion of a circuit placed, for example, over a knee or elbow during flexing.

Finally, they displayed a twisting test machine, which I found interesting because testing was not a normal part of the traditional mechanical testing regimen for flexible circuits, but makes sense when you consider the wider range of motions sought from flexible/stretchable circuits both for monitoring motion in humans and enabling motions in robots.

In summary, stretchable circuit technology is rapidly expanding its position and role in the realm of flexible circuitry, enabling a host of new applications, including consumer electronics, healthcare monitoring, artificial skins for robots, decorative textiles, automotive systems, and aerospace and defense. It is a welcome and enabling addition to the family of flexible circuit technologies.

References

1. “Stretchable and Conformal Electronics Market, Global Forecase to 2022, Technology & Telecommunications, Lucintel.

Joe Fjelstad is founder and CEO of Verdant Electronics and an international authority and innovator in the field of electronic interconnection and packaging technologies with more than 185 patents issued or pending. Download your copy of Fjelstad’s book Flexible Circuit Technology, 4th Edition, and watch his in-depth workshop series, “Flexible Circuit Technology.”