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24 The PCB Magazine • June 2014 HyBRID STRETCHABLE CIRCUITS oN SILICoNE SUBSTRATE continues on top of the substrate), and the strain within the elastic wiring, (i.e., the top surface strain) smoothly increases across the rigid-to- elastic interface with the applied strain and does not exceed the applied strain. Non-deformable re- gions are prepared by embedding two concen- tric disks of stiff plastic foil within an elastomer matrix. We demonstrate the dual disks design is a straightforward way to keep the strain above the smaller disk to 0% and suppress the strain peak across the hard-soft transitions. We moni- tor the surface strain within elastic wiring pre- pared with stretchable thin gold film intercon- nects and highlight minimal mechanical fatigue in the metallic conductors upon cyclic stretch- ing to 20% uni-axial strain. Packaged compo- nents (i.e., an operational amplifier, or op-amp) LED, and resistors, are subsequently mounted and interconnected on the stretchable circuit board to form an oscillating circuit, which fre- quency is designed to linearly decrease with the applied strain. The electromechanical integ- rity of the circuit is maintained over repeated stretch cycling demonstrating the potential of our stretchable circuit boards. II. overview of the Hybrid Stretchable Circuit Board Stretchable substrates may be formed as (i) a homogeneous elastomer membrane, (ii) a mechanically graded elastomer, or (iii) an elas- tomeric membrane carrying embedded rigid platforms. A bulk elastomer membrane stretch- es uniformly so large strains appear across its surface. Macroscopic strain applied across a me- chanically graded elastomer (prepared, for ex- ample, with photo-patternable PDMS14) is dis- tributed through the polymer so that the stiffest, patterned regions stretch less than the softest ones. Yet, truly rigid regions (i.e., non-stretch- able) cannot be prepared with this approach. Embedding stiff platforms within the thickness of the stretchable substrate can provide "zero- strain" zones on the surface of the substrate [12,15] . The strain profile on the surface of such engineered substrates may present a sharp peak close to the edge of the platform and the effec- tive "zero-strain" surface area depends strongly on the geometry and density of the embedded rigid platforms material. Here, we propose the use of concentric rigid disks embedded in the elastomeric substrate to maximize the no-strain surface area and modulate further the surface strain across the soft-hard boundary. Figure 1 presents a schematic cross-section of the hybrid stretchable circuit board. It consists of a millimetre thick PDMS substrate within which two concentric discs of polyi- mide (PI) foil (50 lm thick) are embedded. The upper disk is the smallest and is positioned towards the PDMS sur- face. The second disk has a bigger diameter than that of the first one and is embedded deeper in the PDMS membrane. The top disk ensures the strain at the PDMS surface immediately above Figure 1: schematic cross-section of the double-disk stretchable substrate.