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66 The PCB Magazine • March 2017 tion. Another approach uses in-plane pattern designs to create metal traces capable of elon- gation. One pattern variant (variously termed sinusoidal, serpentine or meandering) uses in- terconnected "S" shaped circuit structures to accommodate deformation forces. This pattern scheme essentially creates a two-dimensional spring structure that reduces localized strain concentration on specific points along the trac- es. Other types of bulk metal stretchable pat- terns include mesh, spiral and fractal designs. Looking Ahead Many companies and universities are con- ducting novel research on new materials that may be used to create better stretchable circuits in the future. Super atoms are clusters of atoms that behave like single elemental units. Meth- ods for linking super atoms so that they may be used to imitate the properties of naturally occurring molecules have recently been discov- ered. This breakthrough may provide scientists the ability to tune properties like elasticity in metals or conductivity in polymers. Conductive polymers have long been a subject of electronic materials research and they may provide another avenue of research for developing stretchable conductors. Poly (3,4-ethylenediox-ythiophene) or PEDOT based compounds have been used in flexible display applications and are showing promise in other areas of electronics. Along with continued investigation of estab- lished metallic conductor morphology for creat- ing stretchable conductors, there is research fo- cusing on conductive nanomaterials to address the challenge of creating elastic circuits from inflexible conductors. Silver, copper and gold nanowires have been used to create stretchable circuitry by forming a flexible network of con- ductive filaments. Researchers are also investigat- ing carbon-based materials like carbon nanotubes and graphene stretchable circuitry applications. Work is being conducted on liquid metals such as gallium indium alloy (75.5% Ga and 24.5% In) and ionic fluids for use as stretchable circuit conductors. It's easy to envision how liq- uid conductors would fill a channel formed in a stretchable substrate matrix and maintain con- ductivity after repeated deformations without risk of conductor cracking. However, stress in- duced fractures in the surrounding matrix may allow the conductive material to leak. Stretchable film substrates have historically consisted of homogenous polymeric composi- tions. Researchers are investigating alternative constructions such as nano-layered co-extru- sion and the inclusion of polymeric and non- polymeric particles to improve mechanical and electrical performance and provide other func- tional advantages. Conclusion In the late 19th century, the famous Ameri- can architect, Louis Sullivan, coined the oft- used and frequently misquoted phrase "form follows function." This sentiment seems partic- ularly apt for the future development of stretch- able circuit materials. The PCB industry has en- tered the next phase of circuit board technology development. Driven by novel designs and new end uses in many markets, the era of stretch- able electronics is fully upon us. Like flex circuit boards and the rigid boards before them, there isn't likely to be a single "silver bullet" univer- sal substrate/conductor combination that will meet the needs of every end user. There is still much development work to be done that will probably include a significant degree of techno- logical cross-breeding. Ultimately, however, the required combination of materials and process- es will be developed for each end use. PCB Andy Behr is technology manager with Panasonic Electronic Materials Business Division. STRETCHING BEYOND FLEX " This breakthrough may provide scientists the ability to tune properties like elasticity in metals or conductivity in polymers. "