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Flexible Thinking: Stretching Conductors—and Design Possibilities

Stretchable circuits, also referred to as elastic circuits or even “elastronics,” are a subset of the venerable flexible circuits that have enabled countless numbers of today’s electronics products, from toys to smartphones to the International Space Station. This most recent “member” of the interconnection family—stretchable circuits—has been designed and engineered to be resiliently bent, twisted, and/or stretched in support of the end product’s need without negatively affecting its electrical function.

The idea of creating an interconnecting circuit that can be stretched and resiliently (or forcibly) returned to its original position has been around for many years. Ones that caught my attention when the industry first began to “pile on” the concept by creating circuits on elastomeric base films were those where the circuit was formed in an accordion-like fashion. This was used to allow access to electronics housed in file cabinet-like drawers since at least the 1970s and, on a much larger scale, the jetways used to get passengers on and off passenger planes. You can see the undulating cables hanging beneath the jetways.

However, the pursuit of using elastomeric substrates for creating stretchable circuits arguably began in earnest in Europe, funded by the European Union’s shared scientific funding called Project STELLA, which ran from February 2006 to January 2010. I was fortunate enough to give a keynote address at the first conference on flexible and stretchable circuits in Gent, Belgium in 2008.

There were many excellent papers given at the inaugural conference, which described research on how to design and fabricate the circuits, typically using copper, and mitigating the mechanical damage inevitably caused by repeated stretching. Solutions were both in-plane (using undulating traces) and out-of-plane (using humped circuits that flattened when stretched). There were also examinations of stretchable conductors comprised of elastomers loaded with conductive metals.

Most interesting at the time were the applications that were shared, especially wearable electronics products. At the time, fashion apparel was most alluring. Affixing rows or arrays of LEDs to clothing was eye catching and became somewhat of a poster child for the nascent technology. However, it was the fact that stretchable circuits could conform to irregular contours of the human body, thus allowing for comfortable and unobtrusive integration into clothing, accessories, or directly on the skin; this opened the doors to the new interconnection option. They soon found their way into many new products, including a variety of health monitoring, fitness tracking features, and smart textiles which integrated electronic functionality into clothing.

Stretchable circuits were also deemed to have significant potential in healthcare. They are currently used in novel medical devices, including electronic skin patches, which can monitor vital signs, like blood pressure; measure blood chemistry, such as sugar levels; and deliver drugs such as insulin to correct the levels. They can also provide real time feedback during physical rehabilitation sessions. Some researchers have been looking into using stretchable circuit technology for implantable applications that would enable seamless integration with soft tissues, providing the potential to restore use of paralyzed limbs and mobility to accident victims. In this regard, stretchable circuits show promise in the manufacture of prosthetics to restore limbs of amputees, many the victims of wars. Stretchable circuits integrated into such products allow for more natural movement and allow wearers to move back into society with the anonymity they enjoyed before they lost their limbs. All appendages are subject to development, and it can be expected they will have sensors integrated into them that allow the wearer to sense touch, pressure, and temperature; perform delicate tasks; and experience again the joy of holding their loved ones or a newborn child.

Not surprisingly, stretchable circuits are being employed to create smart and flexible sensor systems for all manner of products from automobiles and appliances to household furniture, pet monitoring, and security systems—all connected to the internet and monitored by computers. Getting sleepy at the wheel? Your car will direct you safely off the road.

Of course, stretchable circuits will, without question, be employed in the design of increasing numbers of consumer electronics, from video gaming appliances and virtual reality headsets to exercise and relaxation devices as well as an unimaginable range of devices locked—only for this moment in time—inside the minds of the countless present and future inventors, who will bring them to the public to improve everyday lives.

In summary, while stretchable circuits are still a relatively new kid on the block, we can expect to see much more use of the technology in the future. I am doing my part. In my most recent Occam/SAFE technology patent, I described a way to make such products without dealing with the inherent instability that stretchable materials present to manufacturing. I hope to soon get the opportunity to build a demonstration circuit as envisioned and open doors to even more applications for this wonderful new member of the flexible circuit family.

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.”

This column originally appeared in the July 2023 issue of Design007 Magazine.