High-k, Ultrastretchable Self-Enclosed Ionic Liquid-Elastomer Composites for Soft Robotics and Flexible Electronics

Abstract

Soft robotics focuses on mimicking natural systems to produce dexterous motion. Dielectric elastomer actuators (DEAs) are an attractive option due to their large strains, high efficiencies, lightweight design, and integrability, but require high electric fields. Conventional approaches to improve DEA performance by incorporating solid fillers in the polymer matrices can increase the dielectric constant but to the detriment of mechanical properties. In the present work, we draw inspiration from soft and deformable human skin, enabled by its unique structure, which consists of a fluid-filled membrane, to create self-enclosed liquid filler (SELF)–polymer composites by mixing an ionic liquid into the elastomeric matrix. Unlike hydrogels and ionogels, the SELF–polymer composites are made from immiscible liquid fillers, selected based on interfacial interaction with the elastomer matrix, and exist as dispersed globular phases. This combination of structure and filler selection unlocks synergetic improvements in electromechanical propertiesdoubling of dielectric constant, 100 times decrease in Young’s modulus, and ∼5 times increase in stretchability. These composites show superior thermal stability to volatile losses, combined with excellent transparency. These ultrasoft high-k composites enable a significant improvement in the actuation performance of DEAslongitudinal strain (5 times) and areal strain (8 times)at low applied nominal electric fields (4 V/μm). They also enable high-sensitivity capacitive pressure sensors without the need of miniaturization and microstructuring. This class of self-enclosed ionic liquid polymer composites could impact the areas of soft robotics, shape morphing, flexible electronics, and optoelectronics.

Highly Sensitive Flexible Pressure Sensors Enabled by Mixing of Silicone Elastomer With Ionic Liquid-Grafted Silicone Oil

“…The test specimen was elongated uniaxially at 10 mm min −1 . Young’s modulus was obtained from the tangent of the stress-strain curves at 10% strain ( Liu et al, 2019 ; Ankit et al, 2020 ; Shi et al, 2021 ). Three measurements were taken for each sample and then averaged.…”

“…DRS testing was performed using a Novocontrol Alpha-A high-performance frequency analyzer (Novocontrol Technologies GmbH & Co, Germany) with an electrical field of about a 1 V mm −1 , in the frequency range of 10 −1 –10 6 Hz, at room temperature ( Yu and Skov, 2015 ; Liu et al, 2019 ; Ankit et al, 2020 ; Liu et al, 2020a ). Copper sheets were used as electrodes of functional oil and obtained elastomers to perform the DRS testing.…”

“…Liu et al( Liu et al, 2019 ) identified the most suitable IL (1-butyl-3-methylimidazolium hexafluoroantimonate) for platinum-catalyzed hydrosilylation out of several commonly used options, producing an optimized PDMS composite with a relative dielectric permittivity of 7.6. Methews and coworkers ( Ankit et al, 2020 ) reported a PDMS composite with a self-enclosed IL, indicating synergistically improved electromechanical properties—i.e., a twofold increase in dielectric constant and a 100-fold decrease in Young’s modulus. Shi et al (2021) prepared a PDMS composite with near-spherical IL droplets 0.5–1 μm in diameter which showed high relative dielectric permittivity (ε r ∼20) and good mechanical stretchability (>350% strain).…”

Improving Dielectric Constant of Polymers through Liquid Electrolyte Inclusion

“…[ 16,38–40 ] Just during the submission of the present manuscript, Methews and coworkers reported self‐enclosed IL‐elastomer composites with relatively high dielectric constant (around 7). [ 41 ] However, the sample was a little hazy, due to the light scattering of IL droplets within polymer matrix. Indeed, dielectric materials with high transparency, good stretchability, and high dielectric constant are of great interest and high demand for new applications in optical‐related areas, unfortunately, these properties still remain challenging to be achieved simultaneously.…”

Low‐Voltage Soft Actuators for Interactive Human–Machine Interfaces

“…[199] Generally, extrinsic enhancement approaches adopted include 1) the addition of high dielectric ceramic powders such as barium titanate (BaTiO 3 ) and titanium dioxide (TiO 2 ) [202,221,222] ; 2) conductive particles such as aluminum nanoparticles, polyaniline, and graphene [207,[223][224][225] ; 3) polymer blends [226] ; and more recently 4) liquid fillers such as liquid metal and ionic liquid that are mixed with the elastomer matrix. [227][228][229] While extrinsic enhancements have been successful to enhance the elastomer dielectric constant, they often come at a trade-off of either increased elastic modulus, higher dielectric loss, lower dielectric breakdowns and greater leakage currents. [197,200] Moreover, it is crucial to consider the filler size introduced as it limits the possible thicknesses that can be achieved.…”