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* '''Electronic Fillers''': This is the oldest approach to making an [[elastomeric]] material elastically stretchable. In principle, rigid and [[electrically conductive]] materials and mixed with an elastomeric [[polymer]] before curing to create stretchable composites. If the concentration of the electrically conductive filler is high enough they form a mesh-like [[percolation]] network that facilitates the free movement of charge carriers (ions, electrons) through contact junctions. The minimum concentration of the electronic filler material that is required to create conductive pathways for [[charge carrier]] transport through the elastomer <ref>Kyrylyuk, A. V., and P. van der Schoot. "Proc. Natl. Acad. Sci. USA." Proceedings of the National Academy of Sciences of the United States of America, vol. 105, 2008, p. 8221.</ref> is called the percolation threshold.<ref>"Percolation Threshold." ScienceDirect, Elsevier, www.sciencedirect.com/topics/engineering/percolation-threshold. Accessed 10 Nov. 2024.</ref> The [[percolation threshold]] is usually indicated as weight percentage (wt%) or volume percentage (vol%) of the filler material, and ranges from less than 1wt% for high aspect ration carbon [[nanotubes]] to over 15wt%. The type of filler materials ranges from metals in powder or [[nanowire]] form, [[carbon]] as [[graphite]] or [[nanotubes]], to electrically conducting polymers.
* '''‘Wavy’ [[Nanowires]] and Nanoribbons''': The spontaneous formation of wavy patterns of aligned [[buckles]] that is caused by the deposition of a thin gold film on the surface of the [[elastomer]] [[PDMS]] was first described by the group of George Whitesides at Harvard University in 2000.<ref>Huck, Wilhelm T. S., et al. "Ordering of Spontaneously Formed Buckles on Planar Surfaces." Langmuir, vol. 16, no. 7, 2000, pp. 3497-3501.</ref> The gold was deposited on warmed PDMS (100 °C), and, upon cooling and the associated thermal [[shrinkage]] of the elastomer, the gold film comes under compressive stress which is relieved by creating [[buckles]]. In subsequent years, the group of John Rogers at the University of Urbana Champaign (now at Northwestern University) has developed the technology to bond very thin silicon ribbons to a pre-stretched [[PDMS]] membrane. Upon relaxation of the per-stretch, the compressive [[mechanical stress]] in the [[silicon]] ribbons is relieved by creating wavy buckles in the PDMS. As silicon is a brittle material, the ribbons need to very thin (about 100nm) to stay intact during buckling.<ref>Kim, Dae-Hyeong, and John A. Rogers. "Stretchable Electronics: Materials Strategies and Devices." Advanced Materials, vol. 20, no. 24, 2008, pp. 4887-4892.</ref>
* '''Liquid Metals''': A [[metal]] or [[alloy]] that is liquid at room temperature can be enclosed in [[PDMS]] and used as a stretchable [[conductor]]. [[Mercury (element)|Mercury]] is the only pure metal that is liquid at room temperature but has limited application due to its [[neurotoxicity]]. Cesium melts at 28.5°C, but reacts violently when exposed to air and is therefore not suitable for this application. Most researchers therefore use an [[eutectic]] mixture of Indium and Gallium, so called EGaIn, which has a melting point is 15.7°C and consists of 75.5% Gallium and 24.5% Indium. A eutectic mixture of Ga (68.5%), In (21.5%) and Sn (10.0%), also known as [[Galinstan]], is another popular choice and has a melting point of 10.5°C.
* '''Microcracked gold thin film''': When a thin gold film is deposited on [[PDMS]] under certain conditions<ref>Stretchable and Foldable Silicon Integrated Circuits." ACS Publications, American Chemical Society, pubs.acs.org/doi/abs/10.1021/am1002537. Accessed 10 Nov. 2024.</ref>, the gold film adopts a microcracked morphology<ref>Lacour, Stéphanie P., et al. "Mechanisms of Reversible Stretchability of Thin Metal Films on Elastomeric Substrates." Applied Physics Letters, vol. 88, no. 20, 2006, p. 204103. </ref> which makes the gold stretchable. The maximum [[strain]] of the film decreases with the length and increases with the width of the conductor.<ref>Graudejus, O., et al. "Size-dependent rupture strain of elastically stretchable metal conductors." Scripta Materialia, vol. 66, no. 11, 2012, pp. 919-922. ScienceDirect, www.sciencedirect.com/science/article/abs/pii/S1359646212001364. Accessed 10 Nov. 2024.</ref>
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