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102 DESIGN007 MAGAZINE I JUNE 2020 these products. Generating fine traces from the circuits' base films relied on dimensional sta- bility, but new applications are completely dif- ferent. Several circuit manufacturers and materi- al suppliers are collaborating to develop new elastic, flexible circuits. Urethane rubber could be an appropriate base material once its rela- tively low-heat resistance is improved. Silicone rubber is the perfect material to use as the base layer and cover coating, due to its heat resis- tance and transparency. The increase in cost is the only problem. Elastic conductor material has to be ad- dressed. One solution is to have laminate suppliers develop copper-clad laminates with elastic sheets. Circuit manufacturers could etch the copper foil to generate the circuits using a typical manufacturing process for flexible circuits. Copper foils do not have the equivalent elasticity as rubber materials, so conductors have to use a meander pattern (Figure 1). This pattern allows the circuits to stretch by 50%. Creating flexible conductors using a screen- printing process could be another choice for elastic conductors. Printed conductors can have a much higher elasticity compared with copper foils depending on the ink used for the binder material, but the circuit's conductivity can decrease by one hundredth. Another challenge with elastic conductors is they are not very stable. The resistance of the conductor is proportional to the elongation of the circuit. For example, when the conductor is stretched by 100%, the resistance doubles. This necessitates the use of meander patterns for printed conductors. Unfortunately, when using meander patterns instead of straight pat- terns, the amount of space needed for circuits increases. The circuits are larger, and a rise in costs is inevitable due to an increase in the us- age of materials. It may need supplemental ideas and experience to build high-density cir- cuits on the elastic substrates. Creating elastic circuits does not employ conventional thinking. Because the resistance from the elastic conductor is proportional to its elongation, we can estimate how much the conductor is stretched measuring its re- sistance. This is the basic mechanism of the strain gauge. The demands for elastic, flexible circuits will grow significantly in the next few decades as the market for wearable, and medical electron- ics will continue to grow. This segment forces us to think outside of the box for material us- age, design ideas, and manufacturing process- es. Anything is possible. FLEX007 Dominique K. Numakura is the man- aging director of DKN Research LLC. Contact for further information and news. Figure 1: Elastic, flexible circuits with meander pattern provided by Oki Cable. Several circuit manufacturers and material suppliers are collaborating to develop new elastic, flexible circuits.

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