Fabrication of Electrode Structures on Polyimide. A layer of gold (thickness: 50 nm) was deposited onto polyimide (PI) via thermal evaporation. The substrates were then coated with the negative-tone photoresist AZ nLOF 2020 via spin-coating (3000 rpm, 60 s, Suss MicroTec LabSpin 6 TT), and soft-baked for 60 s at 110 • C. Next, the resist-coated PI substrates were exposed through a photomask for 12 s using a mask aligner (Karl Suss MJB-3). A post-exposure bake was conducted for 60 s at 110 • C. The photoresist was developed for 60 s by immersion in the AZ 726 MIF developer, and rinsed with DI water and dried in a stream of nitrogen. Then, the exposed gold areas were removed by immersion in a gold etchant solution (KI/I 2 /H 2 O, 4/1/400, m/m/m) for approximately 2 min. The substrates were rinsed with DI water. The photoresist was removed by immersion in TechniStrip NI155 for 30 min at 65 • C, yielding the gold interdigitated electrode structures (IDEs) on PI. The substrates were cleaned in a final step by subsequent immersion in acetone, 2-propanol, water, and then dried in a gentle stream of nitrogen.