Nanostructured dielectric elastomer transducers for smart implants

Nanotechnology based dielectric elastomer transducers (DETs) are used for a grow- ing number of applications, including tunable optics, microfluidics, soft robotics, and haptic devices. DETs consist of non-conductive soft membranes sandwiched between compliant electrodes. They show unique performance characteristics, i.e. actuation strains similar to human skeletal muscles and millisecond response times which make them promising for applications as artificial muscles. Their main drawback, how- ever, are the high driving voltages of several kilovolts. For medical applications their operation voltage is limited to a few tens of volts. Within the SmartSphinc- ter project of the nano-tera.ch initiative, we have developed techniques to fabricate and characterize DET structures to be used as implants to treat fecal incontinence. Polydimethylsiloxane (PDMS) was selected as the material of choice for the mem- branes due to its biocompatibility and excellent elastic properties. The low adhesion of Au to PDMS has been reported in the literature and leads to the formation of Au-nanoclusters. The diameter of these nanoclusters was found to be (25 ± 10) nm and can be explained by the low surface energy of PDMS resulting in a Volmer- Weber growth mode. Therefore, we show that (3-mercaptopropyl)trimethoxysilane (MPTMS) can be used as a molecular glue to promote the adhesion between the plasma treated PDMS and Au electrode.
In order to reach actuation forces of several newtons as required for artificial fecal sphincters, multi-layer DETs have to be built. As a first step, a compact apparatus was designed and brought into operation to measure the exerted forces of planar DETs. DETs with an active area of 4 mm × 12 mm were fabricated on polyethy- lene naphthalate (PEN) cantilevers. The bending of the asymmetric cantilever was detected using a optical beam-deflection readout. The actuation force of such a single-layer DET was found to be below 1 mN. Therefore, thousands of DET layers have to be stacked to generate the required forces. Real-time measurements of the PEN cantilever curvature allow to extract the expected millisecond response time as well as to detect self-clearing effects of the electrode.
The electrostatic pressure in DETs drives the incompressible PDMS elastomer to expand laterally by tens of percent. A major goal is the development of nanometer- thin flexible and stretchable electrodes. The two main paths employed to increase an electrode’s compliance involve either the manipulation of its intrinsic material properties or its structural features, such as the introduction of wrinkles, which arise above the critical stress of metal films on elastomer substrates. We have demon- strated that DETs with wrinkled Cr/Au electrodes show a drift of only 2 % compared to 15 % for the ones with planar electrodes. The effective elastic modulus of DETs was measured using state of the art nanoindenting techniques and atomic force mi- croscopy (AFM) based nanoindentations. The elastic modulus of the PDMS film for single-layer DETs was found to increase by a factor of 2.3 after depositing a 10 nm-thin Au electrode.
We demonstrate that the interplay between functionalized, oxygen plasma-treated PDMS films and sputter-deposited Au electrodes allows to conserve the compressive stress within the electrode. Insulator-metal transition occurs at only 10 nm-thin Au electrodes and below this electrode thickness, AFM nanoindentations reveal no stiffening of the Au/PDMS heterostructure. Probing a DC-powered thin-film DET with an AFM spherical tip leads to increased indentation depths by several ten percent, as verified by dynamic FE models. Charge accumulation is found to be responsible for a softening effect of the DET structure. In conclusion, electrodes with a controlled topology can be prepared to trigger a preferred direction of the actuation. We show that wrinkle patterns formed on the elastomeric membrane depend on the plasma treatment parameters as well as on the elastic modulus of the bulk PDMS film. Nanomechanical mapping with sub-micrometer resolution reveals topology dependent elastic properties of plasma treated PDMS membranes. The material on the nano-hills is significantly stiffer with respect to the one located in the nano-valleys.
Operation voltages of DETs at a few volts require PDMS membranes with some hundred nanometers in thickness. Thin PDMS films were fabricated using in-house built electro-spray (ESD) or organic molecular beam deposition (OMBD) techniques. Although the homogeneity of thermally evaporated PDMS films using OMBD is su- perior with respect to spin-coating or electro-spraying, its applicability for DETs is restricted. The evaporation of PDMS prepolymers is limited to oligomers at low deposition rates, due to the thermal degradation of the used polymer. We show that by using SH-functionalized PDMS with a low molecular weight of 3’600 g/mol, one can electro-spray smooth PDMS membranes at growth rates 10-times higher compared to OMBD. An adhesive layer between the electrode and the elastomer is not required, as the Au clusters form strong covalent bonds to the SH-groups within the elastomer network. We also show that PDMS films can be cured in air using a high-power Xe-Hg ultraviolet light (UV) lamp, and tune the elastic modulus between 165 and 1’300 kPa in less than a minute. This production speed makes ESD and in-situ UV curing promising for fabricating of multi-layer DETs that can be operated at low voltages.