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76 DESIGN007 MAGAZINE I SEPTEMBER 2022 able 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 moni- toring for sports and medicine. While not an imperative (wireless solutions are an option), a key feature increasingly sought in such prod- ucts has been stretchability and conformabil- ity to the contours of the subject being moni- tored—the human body. We take for granted the various movements of our physical bod- ies, especially our appendages. However, we less oen 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 mon- itor internal functions such as pulse and/or heart rhythm. A circuit that can stretch, and repeatedly and reliably recover during opera- tion, 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 cir- cuit technology in the European Union, with research funded by the same. at interest has remained over the past decade and a range of new materials, both substrates and conduc- tors, have been developed and entered the fray. Developing stretchy materials has been less of a challenge than making printed conduc- tors reliably adhere to them over repeated stretching events. e continuing effort to cre- ate better and more stretchable conductors has yielded some interesting results, including some liquid metal solutions. e stretchable circuit branch of flexible circuits has resulted in the need to create dynamic testing tools to account for the dif- ferent stresses that might occur with these newest members of the flexible circuit fam- ily. During my visit to FlexCon, co-located with SEMICON® West in San Francisco last month, I visited the Bayflex Solutions booth. e 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 collec- tion and analysis. In the real world, however, stretching can be more complex than simple linear stretching. e company displayed a device where a dome-shaped tool was pressed down into a stretch circuit and then lied upward in a repeating motion. is 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 tradi- tional mechanical testing regimen for flexible circuits, but makes sense when you consider the wider range of motions sought from flex- ible/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 flexile circuitry, enabling a host A circuit that can stretch, and repeatedly and reliably recover during operation, can be a very important feature.