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86 DESIGN007 MAGAZINE I JULY 2019 of their construction. The European Union took point in helping to grow the early market by making a substantial investment of euros to fund research and jump-start the technol- ogy. They also sponsored a series of bi-annu- al conferences over six years (I had the honor and privilege of delivering one of the keynotes at the inaugural 2007 conference held in Leu- ven, Belgium, at the IMEC Research Center). The key challenge was and has always been not so much one of metallizing an elastic sub- strate but in making metal circuits that could be stretched. Flexible circuits have arguably been stretched for a few decades when they have been fold- ed in an accordion fashion to allow for servic- ing electronic systems contained in drawers as they are opened and closed while maintaining a connection. This method was one of the ap- proaches used for the metal circuits but where the circuit elements are created with "humps" on stretchable materials. Thus, when the ma- terial is stretched, the circuit humps are flat- tened. Fundamentally the same principle as used in the original but on a much smaller scale. However, many applications were think- ing more along the lines of a traditional planar construction. Designers figured out that stretching of cir- cuits could be accomplished by creating ser- pentine circuit patterns. Other researchers sought ways to make circuits that did stretch without relying on in- or out-of-plane reshap- ing of the circuits, focusing on how to make cir- cuits stretch reliably without breaking. What they came up with were polymer conductors that were resilient. Mixtures of elastic mate- rials infused or mixed with conductive pow- ders comprised of microscopic metals such as silver, copper, and gold. Others demonstrated the ability to contain liquid metals in channels formed in an elastic material. There has clear- ly been a great deal of cerate thinking applied to serve the growing interest and need. Since the organized roll-out of stretchable circuits in the EU a little over a decade ago, the areas of actual or prospective applications have grown. Stretchable circuits are still be- ing used in fashion, but the applications have been (if you can forgive the pun) stretched considerably as well. Many of the applications still center on interfaces with humans in the form of electronic diagnostic and monitoring devices with integral sensors (think mobile heart and breathing monitors and the like). While stretchable circuits clearly have a cer- tain appeal, they usually are not as cheap as traditional flexible circuits owing to the more limited manufacturing infrastructure. There are other ways to achieve the objectives if one thinks the problem through. As I have sug- gested in my flexible circuit seminars over the years, the most "flexible" interconnection "medium" is the air through which virtually all of the information in the world can now be accessed on a smart device. With the continu- ing growth and advancement of wireless tech- nologies and near-field communication devic- es, such as IoT tags and smart devices, wire- less solutions may well provide a simple an- swer to an otherwise complex interconnection construction. It is up to you to make the de- cisions based on the needs of the product in consideration. One must be flexible in one's thinking first before setting out to making a new design, either flexible or stretchable. Enjoy your journey. FLEX007 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 150 patents issued or pending. To read past columns or contact Fjelstad, click here. Designers figured out that stretching of circuits could be accomplished by creating serpentine circuit patterns.

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