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30 The PCB Magazine • June 2014 disk. A gradual increase in the size of the cracks with increasing strain is observed. After about a thousand stretch cycles, these cracks may merge to a length that may be as long as the width of the gold stripe leading to electrical failure. The nature of the cracks' progression at the disk edge is different than that of the cracks on the region above the bigger disk, where they originate at the pillars but terminate after a short distance between the pillars, as shown in row (d). Poisson compression, normal to the stretch direction, can also be very clearly seen at 10%, 15%, and 20% stretch in row (d), and results into a small buckling the gold layer orig - inating from the microposts. The wrinkles run from one post to the immediately adjacent one. Investigation of series of ESEM images shows however that the main mechanical failure still occurs at the edge of the smaller disk but its occurrence is delayed thanks to the presence of the larger PI disk. IV. Stretchable Hybrid Circuits A multivibrator circuit with a flashing LED was then implemented combining the stretch- able circuit board and discrete electronic com- ponents (Figure 4). The circuit is operating so that the rate of a flashing LED decreases when the circuit is stretched. The oscillating frequen- cy is defined as where R2, R3, R4, and C1 are discrete resis- tors and capacitance while R is a variable ser- pentine resistance patterned with stretchable thin gold film on the PDMS substrate. HyBRID STRETCHABLE CIRCUITS oN SILICoNE SUBSTRATE continues Figure 4: Hybrid stretchable circuit. (a) Pictures of the hybrid multivibrator prepared on the stretchable circuit board. the larger polyimide disks are 8 mm in diameter. close up of some components (top); circuit mounted in the uniaxial stretcher (bottom). (b) electronic diagram of the multivibrator circuit. (c) Flashing rate of the led as a function of applied uniaxial strain.