Spectroscopic ellipsometry on tailored siloxane-based nanostructures for low-voltage dielectric elastomer actuators

To convert electric energy into mechanical motion dielectric elastomer actuators (DEAs) are known for their high strains combined with low power consumption. Based on micrometer-thick elastomer films DEAs are an established technology to serve as artificial muscles for soft robotics. The ‘Smartsphincter‘ project aims to develop artificial muscles to treat fecal incontinence. However, to use DEAs as medical implants, operation voltages have to be reduced from the kV-range well below the medically approved limit of 42 V. Hence, this work pursues the fabrication of nanometer-thin elastomer layers to enables the low-voltage operation of DEAs. To guarantee the acceptance as medical implant DEAs are composed of biocompatible materials. The excellent elasticity of cross-linked polydimethylsiloxane (PDMS) in combination with its biocompatibility renders PDMS the polymer of choice for a large range of medical applications, including DEAs. Due to its inherent chemical inertness combined with outstanding conductance, Au is considered as biocompatible electrode. Organic molecular beam deposition (OMBD) is a versatile technique employed to produce nanometer-thin films with unique homogeneity, however, for polymer material OMBD faces the challenge of evaporating oligomers at a reasonable rate at temperatures well below thermal degradation. Herein, we present thermally evaporated linear PDMS serving as thin polymer films with a tailored molecular weight distribution (cp. Section 2.1). Limited by intermolecular interactions between the methyl side groups, molecular weights of up (6.000±100) g/mol corresponding to up to 80 repeating units of dimethylsiloxane are evaporated before thermal degradation. The functional group density per repeating
unit dimethylsiloxane is adjusted between (2.8±0.2) g/mol and (11±0.1) g/mol. These thermally evaporated PDMS fractions exhibits a narrowed MWD with polydispersity index (PDI) of 1.06±0.02 compared to the highly dispersive supplied polymer. The low PDI enables precise definition of functional group density anticipated to qualify for functionalized surfaces of biomedical devices e.g. microfluidic applications or tailoring cell-polymer interactions. Furthermore, we highlight the evaporation of high molecular weight PDMS chains to be of great importance to realize low elastic modulus cross-linked thin elastomer films. Cross-linking of these PDMS macromolecules is activated through ultraviolet (UV) radiation, which induces a photo-initiated reaction of radicalized functional vinyl terminations. Nano-indentation and infrared spectroscopic studies clarify the understanding of the cross-linking mechanism in section 2.2 and section 2.4. Due to the strong absorption coefficient of PDMS in the UV-range, the cross-linking density is increased within the first nanometers of the film compared to the bulk indicating a skin-like SiO2 surface. This stiffened silica-like surface combined with heat input through UV irradiation results in isotropic distributed wrinkled surface microstructures. Magnetron-sputtered gold coatings can release these wrinkled microstructures, which is accounted for a pre-stretching of the Au/SiO2/PDMS heterostructure. This suggests these heterostructures to qualify for stretchable electronics. One straightforward approach for increased electrode flexibility is a reduced metalfilm thickness, which is applicable for gold films above 10nm revealing the drawback
of reduced conductivity (cp. section 2.2). Regarding DEA-based artificial muscles, strains higher than 10% with maintained conductivity are needed to provide millisecond response time. As an important milestone within the ‘SmartSphincter‘-project an OMBD system was setup, combing the deposition of metal and polymer as well as the UV-polymerization. Simultaneous in situ spectroscopic ellipsometry (SE) monitoring paves the way to tailor soft multilayer metal/elastomer heterostructures on the nanometer scale (cp. section 2.5). Synthesized bi-functional thiol-terminated PDMS was specifically tailored concerning the MWD for thermal evaporation to serve as self-assembled adhesion monolayer (SAM) for gold. It is hypothesized that the SH-PDMS chains and the subsequently evaporated Au form a viscous composite. Cross-linked to the underlying PDMS membrane, the SH-PDMS induces stability, which permits the generation of confluent Au films with a thickness as low as (12±1) nm. We demonstrated a remarkably reduced percolation threshold at a film thickness of (4.4±0.3)nm if a nanometer-thin wrinkled Cr-interlayer is applied to the PDMS membrane. Our results suggest a dramatic improvement towards homogeneous Au growth on soft PDMS membranes. We claim the specifically tailored hetero nanostructures to retrieve some flexibility under strain due to either localized covalent gold-thiol bonds or pre-stretched nanometer-thin Cr-wrinkles, especially beneficial for soft and strechable nanoelectrodes. To proove the envisioned concept a single layer DEA is presented (cp. section 2.4). Manufactured on a 25 μm-thick polyetheretherketone (PEEK) cantilever the bending characteristic gave evidence of a maintained actuation efficiency for a 200 nm-thin film, activated with voltages from 1 to 12 V, compared to a 4 μm-thick, spin-coated film, operated between 100 and 800 V. The force of this 200 nm-thin film cantilever actuator was about 10−4N. Thus, a multilayer DEA with more than 104 layers would reach forces comparable to natural muscles. To tackle the major drawback of OMBD, exhibiting evaporation rates limited below 0.1 nm/s, a cooperative study with the EMAP in Dübendorf aimed to validate alternate current electro-spray (ACES) as a cost-effective growth technique for nanometer-thin PDMS films. As presented for OMBD, in situ SE real-time analysis approved ACES to qualify for DEA manufacturing (Florian Weiss et al., see publication list) permitting deposition rates above 10 μm/h. However, at these growth rates we obtained rather free-standing islands than a confluent film formation resulting in micrometer-rough surface morphologies. Thus, the outstanding homogeneity with sub-nanometer surface roughness accentuates OMBD better for soft nanotechnology.
Combining OMBD self-assembly with SE real-time tailoring of the electronic and optical properties allows the manufacturing of the nanometer-thin heterostructures with a unique precision. Soft, transparent, and biocompatible hetero-nanostructures based on electrically activated polymers are applicable to bio-MEMs, in nanophotonics as soft tuneable gratings or plasmonic absorbers, and flexible electronics. We propose a high impact for these nanostructures developing DEAs towards low-voltage operation. As unique property, sensing and actuating can be achieved concurrently within the same DEA hetero nanostructure. The fabrication of biocompatible actuator/sensor structures with compliance similar to that of human tissue is key to mimic natural muscles for biocompatible implants or soft robotics.