Siloxane-based thin films for biomimetic low-voltage dielectric actuators

Molecular beam deposition of siloxane-based polymer thin films was employed to realize single-layer dielectric elastomer actuators. With molecular weights of 6000 and 28,000 g/mol, vinyl-terminated polydimethylsiloxane (PDMS) was evaporated under high-vacuum conditions at crucible temperatures between 100 and 180 °C. Both deposition rate and realizable film thickness showed linear dependency with respect to the crucible temperature and were significantly higher for PDMS with lower molecular weight. Optimized growth conditions for 6000 g/mol were obtained at 180 °C with a deposition rate of (130 ± 5) nm per hour and a maximal film thickness of (530 ± 1) nm. Thermally induced polymerization was observed to limit the maximum accessible evaporation temperature for hydride-terminated PDMS above 180 °C and for vinyl-terminated PDMS above 230 °C. Ultraviolet (UV) light induced polymerization of vinyl-terminated PDMS was successfully established via radicalization at the functional vinyl end groups of the chains. Atomic force microscopy nanoindentation of the UV-polymerized network reveals that the oligomer chain length determines the elastic modulus of the polymer layer. Manufactured as asymmetric cantilever structures, the bending characteristic gave evidence that a 200 nm-thin film, activated in the voltage range between 1 and 12 V, maintains the actuation compared to a 4 μm-thick, spin-coated film, operated between 100 and 800 V. The force of the presented 200 nm-thin film cantilever actuator was about 10−4 N. This means that a multilayer actuator with more than 104 layers would reach forces comparable to natural muscles. Therefore, such nanostructures can qualify for medical applications for example to treat severe incontinence.