Engineered substrates with embedded strain relief for stretchable thin-film electronics

Stretchable electronics are integrated circuitry that can reversibly expand and relax while retaining their functionality. This emerging technology has great potential in unconventional electronic application areas, especially in biomedical sector. To attain stretchability, new class of substrates, based on highly elastic polymers, have to be used. The resulting stretchable circuits are hybrid systems, combining hard functional materials on soft substrates. However, manufacturing of stretchable electronics sets new challenges due to significantly different material properties between the stretchable substrates and electronic devices, and,therefore, optimized mechanical architectures are required. In this thesis a simple and robust approach to design and manufacture stretchable substrate for stretchable electronic applications is reported. The proposed solution is based on engineered stretchable substrates with embedded strain relief. The substrate is engineered by embedding stiff platforms within a soft elastomer. Since the platforms are signicantly stiffer than the surrounding silicone matrix, the stiff regions distributed across the substrate allow for large global strains with local areas that remain un-strained. These local, un-strained regions act as non-stretchable areas on the plain top surface of the elastomer, which can host brittle devices and protect them from exceeding their fracture strain. Optimization of the platforms geometry and layout, as well as, grading of the mechanical compliance at the rigid-to-soft transition zones have been performed to adjust the strain distribution on the top surface. Associated design rules to produce stretchable circuits based on experimental as well as modeling data are presented. This innovative approach is compatible with conventional, additive thin-film processing. Direct integration of metal oxide thin film transistors onto the planar but mechanically engineered heterogeneous elastic substrate is demonstrated. IGZO TFTs interconnected with stretchable metallization spanning across the rigid platform were manufactured directly on the non deformable elastomer regions, using standard, dry and low temperature processing. IGZO TFT could sustained applied tensile strain up to 20% without electrical degradation and mechanical fracture. Elastomeric substrates with engineered strain relief are, thus, a promising solution to carry reliable and durable stretchable electronics. Additionally, concurrent mechanical and structural photopatterning in photosensitive elastomer was discovered. To date, photosensitive elastomers have mainly been implemented in soft lithography or to locally modulate the elasticity. This thesis demonstrates that shape and stiffness can be engineered in a single elastomeric membrane using simple UV exposure through standard photolithography mask. Modulating the UV dose defines mechanical stiffness gradient within the elastomer as well as topography across the elastomer surface. This process eliminates lithography wet development step therefore offers a new technique to silicone microstructuring, allowing for mold and development free 3D patterning of microfluidic channels. Therefore, single-step photopatterning of photosensitve elastomers enables for rapid prototyping approach for soft MEMS, micro uidics and stretchable electronic.