2, Stanford University, Stanford, California, United States
This work demonstrates fully stretchable (>140%) organic electrochemical transistors (OECTs) by the development of highly stretchable Au conductors. OECTs are the ideal platform to realize high-sensitivity wearable/implantable sensors for electrophysiology and chemicals because of the high transconductance. However, stretchable OECTs have not been demonstrated due to the limited stretchability of Au films which are utilized for source/drain contacts and wirings in OECTs. To solve this issue, we first developed highly stretchable Au which has sheet resistance of 11.8 Ohm/sq. at 0% strain and 28.5 Ohm/sq. at 100% strain. Remarkably, the high performance was maintained even after 10,000 cycles of 50% strain. This highly stretchable Au conductor was realized by controlling the Au thin film growth on elastomer substrates during the vacuum deposition. In contrast to conventional Au having limited numbers of short (< 1 μm) microcracks, newly developed stretchable Au film possesses many initial microcracks longer than 1 μm. Larger numbers of long initial microcracks resulted in reduced crack propagation by strain, leading to the small resistance change by strain. Furthermore, stretchable OECTs was fabricated using this highly stretchable Au conductor. Poly(3,4-ethyle nedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) was used as an active layer and showed high stretchability in electrolyte solution. The transconductance is as high as 0.54 mS at 0% strain and 0.22 mS at 100% strain. The stretchability of OECTs would improve the conformability to human bodies, leading to reduction of the discomfort of wearing and improved signal integrity.
 D. Khodagholy, et al. Nature Communications 4, 1575 (2013).
 S. Lacour, et al. Proceedings of the IEEE 93, 1459-1467 (2005).