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28 The PCB Magazine • June 2015 sign requirements must be understood. Also, for any wearable device, the materials' exposure compatibility with the human body surface and fluids must be comprehended, especially for on- the-body and in-the-body applications. Referring earlier to the proximity categories (Figure 2), the choice of interconnect technol- ogy, design and material is defined by these categories. FPCs, RFPCs and printed electronics technologies are often used to take advantage of the flexibility requirements in wearable elec- tronic devices. There are two ways to incorpo- rate these technologies. 1. Flex to fit: The circuit is flexed once only to fit into the assembly. 2. Dynamic flex: This circuit will not only flex to fit into the assembly, but will be dynamic during operation [8] . Multiple applications in each proximity category have high mechanical demands on the interconnects. For example, interconnects in a wristband application must account for constant flexing and twisting during frequent application and removal from the wrist. The electronic patch (electronic tattoo), adhered to the skin, must move with the body, withstand human sweat, moisture and temperature during bathing and daily use. In each example, dynam- ic stresses are evident. Stretchable Electronics Advances in design and materials allow for stretching of the interconnects to mitigate the higher stresses and strains experienced by the circuit in certain wearable devices. In FPCs and RFPCs, higher ductility copper and optimizing the elongation of the dielectrics often addresses the interconnect reliability required. There are more harsh dynamic stress and strain condi- tions observed in a hinge area of smart eyewear, movement of smart apparel, or in soles of smart shoes. In these cases, the FPC and RFPC can be designed to stretch. The design employs mean- dering sections to allow a stretching and twist- ing movement. Figure 5 depicts a meandering design as one of many interconnect design op- tions. The current challenge of designing a me- andering structure is the limitation of commer- cial PCB/FPC design layout software to provide ease of use meandering solutions. The majority of the stretchable interconnect design software is proprietary and specific to a small application range. In printed electronics, several advances in pastes, inks and substrates are supporting the advancements in stretchable solutions. Polyes- ters and polyimides are common substrates for printed electronics. Elastomeric substrates such as polyure- thane and polydimethylsiloxanes are providing stretchable options. Silver inks form the major- ity of the conductive circuits in printed elec- tronics applied to stretchable substrates. The advanced nanoparticle technology of these inks supports a degree of movement of the intercon- nect. Figure 6 illustrates silver printed ink pat- terned on an elastomeric substrate. Stretchable electronics may combine tradi- tional printed circuits and printed electronics. For example, multiple PCBs can be connected ENABLING SMART WEARABLE TECHNOLOGY continues Figure 5: Flexible printed circuit designed with meander for stretch and twist. (courtesy Multek Inter- connect Technology center) FeAtuRe