Stretchable gold conductors embedded in polydimethylsiloxane (PDMS) films were successfully prepared using standard photolithography. The minimum feature sizes patterned in the metal film and PDMS encapsulation are 10 μm and 20 μm, respectively. The micro-patterned conductors are robust to uni-axial (1D) and radial (2D) stretching with applied strains of tens of percent. The electrical response of the conductors follows a nonlinear increase with strain, and is reversible. The extensive stretchability of the conductors relies on a randomly and independently distributed network of micro-cracks (∼100 nm long) in the metal film on PDMS. The micro-cracks elongate to a few microns length both in the stretching and normal directions in 1D stretching but during 2D stretching, the micro-cracks grow and form 'dry mud' islands leaving the gold microstructure inside the islands intact. Patterning metallic thin films directly onto elastomeric substrates opens a promising route for microelectrodes and interconnects for soft and ultra-compliant MEMS and electronic devices.

The resonance of electrostatically actuated amorphous silicon microbridges immersed in de-ionized water is characterized. It is shown that under correct actuation conditions resonance can be measured in water without electrolysis or electrode screening. The resonance frequency of these nonpassivated structures is also studied in air and under vacuum for microbridges with different lengths. When the operating medium is changed from vacuum to air, the resonance frequency decreases by 5% and the quality factor decreases from approximately 1000 to 100. Operation in de-ionized water produces a 60% shift in resonance frequency to lower values and the quality factor decreases to 10.

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The resonance of electrostatically actuated thin-film hydrogenated amorphous silicon microbridges in air and immersed in aqueous solutions is measured and characterized. The detection of the resonance frequency of a microbridge operating in aqueous solutions with high electrical conductivities, up to 8 mS/cm, and high viscosities, up to 0.15 Pa s is demonstrated. The range of actuation voltages needed to excite resonance frequencies in the megahertz in these microstructures under aqueous solutions allows electrostatic actuation without electrode screening and electrolysis. Values of the quality factor between 1 and 10 in aqueous solutions will allow the detection of ∼1 pg of added mass to the resonators.

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