Multilayer soft dielectric elastomer actuators (DEAs) consist of stacked elastic capacitors that convert electrical energy into mechanical work. The generated mechanical work and force can be increased by reducing the dielectric layer thickness, increasing the material's dielectric permittivity, or increasing the number of layers. Despite major progress in developing high-permittivity elastomers, integrating these materials into multilayer devices remains challenging. To date, large-scale production of stack DEAs has been achieved only with commercial polydimethylsiloxane (PDMS), which has a low dielectric permittivity of about 3. Here, we report a solvent-free, high-dielectric-permittivity capillary ink that combines a pot life of more than 300 days at -30 °C, good processability comparable to that of commercial PDMS, and rapid thermal crosslinking, fulfilling essential criteria for the industrial-scale fabrication of stack actuators. The ink wets a preheated metal substrate at 100 °C to form highly uniform ultrathin films that cross-link in just 2 minutes. The resulting elastomer exhibits a dielectric permittivity of 11 at 10 Hz, a storage modulus of 350 kPa, and negligible mechanical losses. Single-layer circular DEAs exhibit a 5.7% lateral strain at 26.2 V µm-1 and a stable actuation over 5000 cycles at 24.7 V µm-1. At an electric field of 19.0 V µm-1, a stripe actuator exhibits 5.5% lateral strain and 1 Hz, which increased to 9% at 5 Hz. The actuators respond to a low voltage of 500 V, corresponding to 25 V µm-1, and provide fast, reversible actuation with a displacement of 25 µm. These results demonstrate a scalable route to high-performance DEAs, marking a significant step towards the industrial application of high-dielectric permittivity polysiloxanes.

Addressing global challenges such as energy production and storage, climate change, and the aging population requires innovative solutions. Dielectric elastomer transducers (DETs) have the potential to make significant contributions to tackle these issues. They are electrically driven elastic capacitors capable of converting one form of energy into another and can be used as actuators, generators, sensors, and capacitive light-emitting devices. Actuators convert electrical into mechanical energy and have various applications, including valves, pumps, optical lenses, engines, and artificial muscles. Generators convert mechanical or thermal energy into electricity. Sensors convert mechanical deformation into an electric signal. Finally, capacitive light-emitting devices convert electrical energy into light. Although any elastomer can function as a dielectric, enhancing the performance of DETs to be used in various applications requires dielectric elastomers with dielectric permittivity that is as high as possible and carefully tailored mechanical properties. This chapter emphasizes the molecular structure of elastomers and discusses design parameters leading to new materials for DETs with improved performance.

The main drawback of polymers in a wide range of soft electrical applications is their low dielectric permittivity. The chemical modification of polymers with organic dipoles has been a successful strategy to increase their dielectric permittivity. However, what is the maximum achievable dielectric permittivity by this method? We present four novel polysiloxanes with relative permittivities ranging from 23 to 31 at room temperature (RT), reaching 34 at 40 °C. These are the highest dielectric permittivity values reported for any amorphous filler-free elastomer. Additionally, we derive a universal guiding principle in designing future elastomers, with an ideal trade-off between elasticity and permittivity at an operating temperature of Tg + 60 °C. We further explore the resulting composites with SiO2 and TiO2 and show that two glass transitions (Tgs) occur due to the interfacial layer and the bulk phase. The two phases show distinct dielectric behavior, which we demonstrate as useful in achieving pyroelectric materials. The materials exhibit the highest reported pyroelectricity of any crystal-free, fully amorphous polymer with a stable quasi-static pyroelectric coefficient of 3.4 μC m-2 K-1 at 30 ± 0.5 °C.

The fast and scalable production of dielectric elastomer actuators (DEAs) remains the major bottleneck preventing the widespread use of DEAs. In this work, an ultra-fast production method is presented for dielectric elastomer fibers which can reach industrial-like extrusion speeds of up to 60 mm s-1 of fiber, leading to a production speed of up to 16.7 m of fiber per second or 216 m per hour. Electrode and high permittivity (epsilon${\varepsilon}$' = 11.2) dielectric inks are used with a long pot life, but ultra-fast cross-linking at elevated temperatures. The process eliminates the need for tedious and time-consuming post-processing, as simple co-extrusion and laser ablation enable the production of fully functional, high-permittivity DEA fibers within seconds. This work represents a significant advancement in DEA manufacturing, transitioning from conventional layer-by-layer batch production to continuous co-extrusion-based manufacturing. To the best of the knowledge, this is currently the fastest method for fabricating fully functional DEAs in a single processing step.

High-permittivity polysiloxanes are attractive for applications in dielectric actuators, sensors, energy devices, and electrolytes. A major challenge is the synthesis of polar polysiloxanes with well-defined end groups suitable for controlled cross-linking while minimizing cycle content that can compromise performance. Here, we report the synthesis of polysiloxanes bearing 3-cyanopropyl side groups and aminopropyl or vinyl end groups, with controlled molecular weights and reduced cyclic byproducts. While the reactions in chlorinated solvents predominantly give short chains, the nonchlorinated solvents favor cycle formation. In contrast, hydrolysis-condensation of (3-cyanopropyl)-methyldichlorosilane under solvent-free conditions yields high-molecular-weight polymers (≈14 kg mol-1) with only 11% cycles, which are readily removed by toluene extraction. Additional polymer growth is achieved via anionic ring-opening polymerization of isolated cycles, yielding polymers up to 25 kg mol-1. Finally, silanol end groups are quantitatively converted into aminopropyl (100%) or vinyl groups (92%), which are useful for cross-linking the polymers to elastic networks with improved structural control.

Stretchable alternating current electroluminescent (ACEL) devices have a bright future in wearable electronics and soft robotics. Still, their market application is hindered by high operating voltages. The voltage can be reduced by increasing the relative permittivity of the dielectric elastomer in the emissive layer. Here, a fluorine-free high-permittivity silicone elastomer functionalized with cyanopropyl side groups, specially designed for application in stretchable ACEL devices, is introduced. The polar silicone elastomer exhibits excellent mechanical properties and a dielectric permittivity four times higher than commercial PDMS. Light-emitting devices based on the polar elastomer reach 7.5 times higher maximum luminance at the same electric field than PDMS-based devices and turn on at a 50% lower electric field. Besides, the polar elastomer-based devices perform better than all materials tested in literature in achieving high luminance at low electric fields. Stretchable ACEL devices are built from the polar elastomer which shows bright and uniform light emission and can be operated up to 50% strain. The high-permittivity silicones are promising materials for stretchable ACEL devices and can help their breakthrough to market application by overcoming the drawback of high operating voltages. A high-permittivity, nitrile-functional silicone elastomer for application in stretchable electroluminescent devices is introduced. The polar elastomer emissive layer reaches significantly higher luminance and needs lower operating voltages than an emissive layer based on commercially available PDMS. Besides, the polar elastomer can be used to build bright and stretchable devices in a simple bottom-up procedure. image

Dielectric elastomer transducers (DET) are promising candidates for electrically-driven soft robotics. However, the high viscosity and low yield stress of DET formulations prohibit 3D printing, the most common manufacturing method for designer soft actuators. DET inks optimized for direct ink writing (DIW) produce elastomers with high stiffness and mechanical losses, diminishing the utility of DET actuators. To address the antagonistic nature of processing and performance constraints, principles of capillary suspensions are used to engineer DIW DET inks. By blending two immiscible polysiloxane liquids with a filler, a capillary ink suspension is obtained, in which the ink rheology can be tuned independently of the elastomer electromechanical properties. Rheometry is performed to measure and optimize processibility as a function of filler and secondary liquid fraction. Including polar polysiloxanes as the secondary liquid produces a printed elastomer exhibiting a four-fold permittivity increase over commercial polydimethylsiloxane. The characterization and multimaterial printing into layered DET devices demonstrates that the immiscible capillary suspension improves the processability of the inks and enhances the properties of the elastomers, enabling actuation of the devices at comparatively low voltages. It is anticipated that this formulation approach will allow soft robotics to harness the full potential of DETs. Capillary inks with suitable rheological profiles for direct ink writing are developed and used for 3D printing of dielectric elastomer transducers, which respond to a low electric field. The inks consist of three components: silica particles, polydimethylsiloxane, and a polar polysiloxane and allow solvent-free 3D printing into elastic dielectrics with a four-fold increase in the permittivity compared to polydimethylsiloxane elastomer.image

3D printing of dielectric elastomer transducers (DET) would significantly accelerate their application in soft robotics. Direct ink writing (DIW) of DET is limited by multiple factors, such as the need for a multi-material printing of dielectric and compliant electrodes and the relatively large thickness, high stiffness, and poor mechanical properties of elastomers. Increasing the permittivity of elastomers is the only tunable material parameter, which can reduce the actuation voltage, or increase the sensor signal, as the minimum thickness is fixed by the printing resolution. We present DIW printable high-permittivity polysiloxanes. Besides the high-permittivity further material parameters and interdependencies between ink requirements and final material performance are explored. The facile printing of these high-permittivity dielectrics with standard 3D printers is demonstrated. Lastly, the performance of various DIW printed DETs are presented.

Polar group-modified polysiloxanes obtained by anionic ring-opening polymerization possess high dielectric permittivity and are of great interest for application in dielectric elastomer actuators (DEAs). A self-healing elastomer can be obtained by in situ polymerization and cross-linking using a cyclic siloxane monomer with polar side groups and a cross-linker consisting of multiple connected siloxane rings. In previous works, a non-polar cross-linker has been used, which requires the addition of a solvent for compatibilization with the polar monomer. In polymerization reactions of siloxanes, the addition of solvent leads to a more pronounced formation of cyclic by-products. These cycles impair the mechanical properties of the elastomer and cannot be removed after the reaction, as the material is already cross-linked. Therefore, in this work, we use a polar cross-linker that can be mixed with the polar monomer without adding solvent. Nitrile groups have been studied extensively for increasing the permittivity of the polysiloxane backbone. For a functionalization of 100%, a dielectric permittivity of ~18 was reached. In most cases, the nitrile group was attached to the siloxane backbone in the form of cyanopropyl groups. Still, the influence of the alkyl spacer on the material's dielectric and mechanical properties has not been studied. In this work, we synthesize cyanoalkyl-functional cyclic siloxanes with different lengths of the alkyl spacer and polymerize them solvent-free to high-permittivity polysiloxanes.

Soft strain sensors with high sensitivity and the ability to recover from damages are required in the emerging field of self‐healing soft robotics. Herein, printable supercapacitive strain sensors that can heal upon moderate heating (75 °C for 10 min) and exhibit a 30 times higher sensitivity than PDMS‐based sensors are developed. For the sensor's core layer and electrode, a nitrile‐functional polysiloxane that contains an active ionic initiator and can heal by siloxane equilibration at elevated temperatures is used. Supercapacitive strain sensors prepared from the elastomer are highly sensitive at low strains of 0–30%, enabled by the electric double‐layer formation of the ionic initiator. After healing, the sensors exhibit nearly unaltered performance in tensile testing. Due to the thermoreversible nature of the elastomer network, patterned core layers with different microstructures can be printed by direct ink writing. The capacitive sensors based on these microstructured films reach a higher sensitivity and linearity than those based on unstructured films. Finally, the sensor is integrated into a soft robotic finger and the sensor's ability to determine the bending angle is validated by motion capture. This technology can provide new opportunities to equip soft robotic devices with custom‐printed, healable strain sensors.

Stretchable electrodes are more suitable for dielectric elastomer transducers (DET) the closer the mechanical characteristics of the electrodes and elastomer are. Here, a solvent-free synthesis and processing of conductive composites with excellent electrical and mechanical properties for transducers are presented. The composites are prepared by in situ polymerization of cyclosiloxane monomers in the presence of graphene nanoplatelets. The low viscosity of the monomer allows for easy dispersion of the filler, eliminating the need for a solvent. After the polymerization, a cross-linking agent is added at room temperature, the composite is solvent-free screen-printed, and the cross-linking reaction is initiated by heating. The best material shows conductivity sigma = 8.2 S cm(-1), Young's modulus Y-10% = 167 kPa, and strain at break s = 305%. The electrode withstands large strains without delamination, shows no conductivity losses during repeated operation for 500 000 cycles, and has an excellent recovery of electrical properties upon being stretched at strains of up to 180%. Reliable prototype capacitive sensors and stack actuators are manufactured by screen-printing the conductive composite on the dielectric film. Stack actuators manufactured from dielectric and conductive materials that are synthesized solvent-free are demonstrated. The stack actuators even self-repair after a breakdown event.

Elastomers with high dielectric permittivity that self‐heal after electric breakdown and mechanical damage are important in the emerging field of artificial muscles. Here, a one‐step process toward self‐healable, silicone‐based elastomers with large and tunable permittivity is reported. Anionic ring‐opening polymerization of cyanopropyl‐substituted cyclic siloxanes yields elastomers with polar side chains. The equilibrated product is composed of networks, linear chains, and cyclic compounds. The ratio between the components varies with temperature and allows realizing materials with largely different properties. The silanolate end groups remain active, which is the key to self‐healing. Elastomeric behavior is observed at room temperature, while viscous flow dominates at higher temperatures (typically 80 °C). The elasticity is essential for reversible actuation and the thermoreversible softening allows for self‐healing and recycling. The dielectric permittivity can be increased to a maximum value of 18.1 by varying the polar group content. Single‐layer actuators show 3.8% lateral actuation at 5.2 V µm –1 and self‐repair after a breakdown, while damaged ones can be recycled integrally. Stack actuators reach an actuation strain of 5.4 ± 0.2% at electric fields as low as 3.2 V µm –1 and are therefore promising for applications as artificial muscles in soft robotics.

The development of novel functional dielectric materials can open the doors to major technological innovations with societal impact. Stretchable capacitors transduce electrical into mechanical energy or vice-versa. Over the last 20 years, they have received significant interest from academia and industry. However, this technology still needs both improved dielectrics as well as conductive elastomers to achieve the desired low driving voltage and to realize devices with attractively high sensitivity. The currently most explored dielectric elastomers are polydimethylsiloxanes (PDMS). However, because of their low dielectric permittivity of only 3, the devices made of them require high voltages for operation. We synthesized polar polysiloxanes with different types and contents of polar groups, investigated their thermal and dielectric properties, and selected the most suitable groups to achieve the highest dielectric permittivity, yet sufficiently low glass transition temperature (Tg) to afford an excellent elastomer at room temperature after cross-linking. This research guided us to several promising polar polysiloxane elastomers modified with nitrile and nitroaniline groups, for which the properties were optimized. We reproducibly achieved dielectric elastomers with a dielectric permittivity of about 18. Some respond to a voltage as low as 200 V, while some give very large actuation and have a breakdown field reaching 100 V μm-1. By carefully selecting suitable synthetic chemistry, we could also achieve self-healable high permittivity elastomers. The materials can be processed into thin films by melt pressing. Stack actuators can be easily manufactured manually and give 5.4% actuation at an electric field as low as 3.2 V μm-1. Furthermore, the actuators can self-repair after a breakdown and be recycled after complete failure. A graphene nanoplatelets (GNPs) composite in PDMS as a conductive electrode was developed via in-situ polymerization. The synthesis and the processing by screen-printing were conducted solvent-free, making this composite the greenest electrode for this technology. This presentation gives an overview of recent research on improved materials for dielectric elastomer transducers (DETs) conducted at Empa. We are confident that our materials will impact fields including actuators, sensors, energy harvesting, artificial muscles, and soft robotics.

Dielectric elastomers (DEs) are key materials in actuators, sensors, energy harvesters, and stretchable electronics. These devices find applications in important emerging fields such as personalized medicine, renewable energy, and soft robotics. However, even after years of research, it is still a great challenge to achieve DEs with increased dielectric permittivity and fast recovery of initial shape when subjected to mechanical and electrical stress. Additionally, high dielectric permittivity elastomers that show reliable performance but disintegrate under normal environmental conditions are not known. Here, we show that polysiloxanes modified with amide groups give elastomers with a dielectric permittivity of 21, which is 7 times higher than regular silicone rubber, a strain at break that can reach 150%, and a mechanical loss factor tan delta below 0.05 at low frequencies. Actuators constructed from these elastomers respond to a low electric field of 6.2 V m(-1), giving reliable lateral actuation of 4% for more than 30 000 cycles at 5 Hz. One survived 450 000 cycles at 10 Hz and 3.6 V m(-1). The best actuator shows 10% lateral strain at 7.5 V m(-1). Capacitive sensors offer a more than a 6-fold increase in sensitivity compared to standard silicone elastomers. The disintegrated material can be re-cross-linked when heated to elevated temperatures. In the future, our material could be used as dielectric in transient actuators, sensors, security devices, and disposable electronic patches for health monitoring.