Molecular Strategies for Improved Dielectric Elastomer Electrical Breakdown Strengths

Towards high performance and durable soft tactile actuators

Soft actuators are gaining significant attention due to their ability to provide realistic tactile sensations in various applications. However, their soft nature makes them vulnerable to damage from external factors, limiting actuation stability and device lifespan. The susceptibility to damage becomes higher with these actuators often in direct contact with their surroundings to generate tactile feedback. Upon onset of damage, the stability or repeatability of the device will be undermined. Eventually, when complete failure occurs, these actuators are disposed of, accumulating waste and driving the consumption of natural resources. This emphasizes the need to enhance the durability of soft tactile actuators for continued operation. This review presents the principles of tactile feedback of actuators, followed by a discussion of the mechanisms, advancements, and challenges faced by soft tactile actuators to realize high actuation performance, categorized by their driving stimuli. Diverse approaches to achieve durability are evaluated, including self-healing, damage resistance, self-cleaning, and temperature stability for soft actuators. In these sections, current challenges and potential material designs are identified, paving the way for developing durable soft tactile actuators.

Largely Enhanced Service Life and Energy Harvesting Stability of Dielectric Elastomer Generator by Designing and Optimizing Compliance of Electrodes

Dielectric elastomer generator (DEG), which consists of a dielectric elastomer (DE) film sandwiched between two flexible electrodes (FEs), has the advantages of lightweight, high energy density, and high energy conversion efficiency, providing a simple and feasible solution for harvesting energy from human motion or nature. As crucial constituents of DEG, FEs are expected to possess excellent conductivity and compliance. Nevertheless, there is currently no quantitative characterization method for FE compliance. In addition, the impact mechanism of FE compliance on the energy harvesting performance and fatigue life of the DEG remains unclear. In this study, the dynamic mechanical property (DMP) was used to assess the compliance of FEs, and the quantitative characterization method of FE compliance was proposed. A series of silicone rubber electrodes (SREs) with different DMPs and compliance were designed and prepared, and the impact mechanism of FE compliance on the energy harvesting stability and fatigue life of the DEG was investigated. The results indicate that the key to achieving excellent FE compliance lies in reducing the difference in the magnitude of the complex modulus and phase angle between the FEs and DE, which can significantly reduce interfacial friction and extend the fatigue life of DEG. Benefiting from the enhanced FE compliance, the fatigue life and full-life energy density of the DEG device increase by 20.3 times and 26.4 times, respectively, compared with those of the commonly used carbon-based electrodes.

High Energy Harvesting Performances Silicone Elastomer via Filling Soft Dielectric with Stretching Deformability

Dielectric elastomer generators (DEGs) with high generated energy density and high conversion efficiency are of great interest. Among several dielectric elastomers (DEs), silicone elastomer filled with ceramic fillers have been extensively studied for their high elasticity, insulation, and permittivity. However, the stretched breakdown strength (E_bs ) of such composites decreases significantly under large strain, thus sharply reduces its energy harvesting performances. In this study, a polar rubber-based dielectric (GNBR) is synthetized and creatively used as "soft filler" for silicone elastomer. Benefiting from the deformability under stretching and its inherent strong interface bonding with silicone elastomer, this soft filler effectively avoids the formation of weak interface under large strain and reduces the local field strength of interface area. As expected, the composite filled with soft filler (GNBR/PMVS) shows enhanced E_bs of 2.8 times that of composite with traditional hard filler (TiO₂ /PMVS) under equibiaxial strain of 200%. As a result, GNBR/PMVS composite exhibits maximum energy density of 130.5 mJ g^-1 with up-to-date highest power conversion efficiency of reported DEG (44.5%). The findings will provide new insights in the rational design of DE composites characterized by high stretched breakdown strength for advanced energy harvesting system.

Crosslinking Methodology for Imidazole-Grafted Silicone Elastomers Allowing for Dielectric Elastomers Operated at Low Electrical Fields with High Strains

For improved actuation at low voltages of dielectric elastomers, a high dielectric permittivity has been targeted for several years but most successful methods then either increase the stiffness of the elastomer and/or introduce notable losses of both mechanical and dielectric nature. For polydimethylsiloxane (PDMS)-based elastomers, most high-permittivity moieties inhibit the sensitive platinum catalyst used in the addition curing scheme. In contrast to the classical addition curing pathway to prepare PDMS elastomers, here, an alternative strategy is reported to prepare PDMS elastomers via the crosslinking reaction between multifunctional imidazole-grafted PDMS with difunctional bis(1-ethylene-imidazole-3-ium) bromide ionic liquid (bis-IL). The prepared IL-elastomer entails uniformly dispersed IL and presents stable mechanical and dielectric properties due to the covalent nature of the crosslinking as opposed to previously reported physical mixing in of ILs. The relative permittivity was improved up to 200% by including the bis-IL in the elastomer, and Young's modulus was around 0.04 MPa. As a result of the excellent combination of properties, the dielectric actuator developed exhibits an area strain of 20% at 15 V/μm. The novel strategy to prepare PDMS elastomers provides a new paradigm for achieving high-performance dielectric elastomer actuators by a simple methodology.

A Review of Electrically Driven Soft Actuators for Soft Robotics

In recent years, the field of soft robotics has gained much attention by virtue of its aptness to work in certain environments unsuitable for traditional rigid robotics. Along with the uprising field of soft robotics is the increased attention to soft actuators which provide soft machines the ability to move, manipulate, and deform actively. This article provides a focused review of various high-performance and novel electrically driven soft actuators due to their fast response, controllability, softness, and compactness. Furthermore, this review aims to act as a reference guide for building electrically driven soft machines. The focus of this paper lies on the actuation principle of each type of actuator, comprehensive performance comparison across different actuators, and up-to-date applications of each actuator. The range of actuators includes electro-static soft actuators, electro-thermal soft actuators, and electrically driven soft pumps.

From passive to emerging smart silicones

Amassing remarkable properties, silicones are practically indispensable in our everyday life. In most classic applications, they play a passive role in that they cover, seal, insulate, lubricate, water-proof, weather-proof etc. However, silicone science and engineering are highly innovative, seeking to develop new compounds and materials that meet market demands. Thus, the unusual properties of silicones, coupled with chemical group functionalization, has allowed silicones to gradually evolve from passive materials to active ones, meeting the concept of “smart materials”, which are able to respond to external stimuli. In such cases, the intrinsic properties of polysiloxanes are augmented by various chemical modifications aiming to attach reactive or functional groups, and/or by engineering through proper cross-linking pattern or loading with suitable fillers (ceramic, magnetic, highly dielectric or electrically conductive materials, biologically active, etc.), to add new capabilities and develop high value materials. The literature and own data reflecting the state-of-the art in the field of smart silicones, such as thermoplasticity, self-healing ability, surface activity, electromechanical activity and magnetostriction, thermo-, photo-, and piezoresponsivity are reviewed.

Adaptively reconstructing network of soft elastomers to increase strand rigidity: towards free-standing electro-actuation strain over 100

Soft biological tissues and muscles composed of semiflexible networks exhibit rapid strain-hardening behaviors to protect them from accidental rupture. In contrast, synthetic soft elastomers, usually featuring flexible networks, lack such behaviors, leading to a notorious issue when applying them to a promising artificial muscle technology (dielectric elastomer, DE), that is electromechanical instability (EMI) induced premature breakdown. We report that a facile thermomechanical training method can adaptively reconstruct the network of a soft triblock copolymer elastomer to transform its flexible network strands into semiflexible ones without extra chemical modifications and additives so that the electro-actuation performance is significantly enhanced by avoiding EMI. The free-standing actuators of trained elastomers exhibit a large stable electro-actuation strain and a high theoretical energy density (133%, 307 kJ m^-3 at 158.1 V μm^-1), and the capacity of actuating at low-temperature environments (-15 °C).

Novel polyrotaxane cross-linkers as a versatile platform for slide-ring silicone

Slide-ring elastomers have garnered a lot of interest for their potential use in dielectric elastomer actuators due to their intrinsically soft nature and high elasticity. However, the use of sliding cross-linkers has been constrained by their low miscibility with commonly used elastomer precursors and the specialized curing chemistries that are necessary for incorporating them into networks. Here, we have presented a method to produce vinyl functional polyrotaxane cross-linkers that are compatible with polysiloxanes and can be processed by industrially scalable methods. The sliding silicone films that were fabricated with these novel cross-linkers were highly extensible (>350%) and did not exhibit strain hardening even at high elongation. The composite films also retained the favorable dielectric properties of silicone elastomers such as the characteristic low dielectric loss. The modified polyrotaxanes present a robust platform for producing a new class of sliding silicone elastomers with well-defined networks structures.

Reliable Condensation Curing Silicone Elastomers with Tailorable Properties

The long-term stability of condensation curing silicone elastomers can be affected by many factors such as curing environment, cross-linker type and concentration, and catalyst concentration. Mechanically unstable silicone elastomers may lead to undesirable application failure or reduced lifetime. This study investigates the stability of different condensation curing silicone elastomer compositions. Elastomers are prepared via the reaction of telechelic silanol-terminated polydimethylsiloxane (HO-PDMS-OH) with trimethoxysilane-terminated polysiloxane ((MeO)3Si-PDMS-Si(OMe)3) and ethoxy-terminated octakis(dimethylsiloxy)-T8-silsesquioxane ((QMOEt)8), respectively. Two post-curing reactions are found to significantly affect both the stability of mechanical properties over time and final properties of the resulting elastomers: Namely, the condensation of dangling and/or unreacted polymer chains, and the reaction between cross-linker molecules. Findings from the stability study are then used to prepare reliable silicone elastomer coatings. Coating properties are tailored by varying the cross-linker molecular weight, type, and concentration. Finally, it is shown that, by proper choice of all three parameters, a coating with excellent scratch resistance and electrical breakdown strength can be produced even without an addition of fillers.

Insights into the Complex Prebreakdown Actuation of Silicone Elastomers and its Influence on Breakdown Behavior

Dielectric elastomer transducers can be applied in many different applications, but their current use is limited by either their electrical breakdown strength or by electromechanical instabilities in the case of soft elastomers. The breakdown process is never a single, simple process but rather-most likely-an ensemble of thermoelectric processes taking place in both elastomer and electrode materials, coupled with mechanical and potentially also chemical degradation. In this work, by using a high-speed camera, we follow silicone-based dielectric elastomers undergoing a ramp-up in voltage close to electrical breakdown strength, with differently constructed elastomers and electrodes. As such, we present experimental insights into the electromechanical processes immediately before the dielectric breakdown of elastomers and identify three different actuation mechanisms taking place prior to electrical breakdown, denoted prebreakdown actuation in the following. The prebreakdown actuation mechanisms observed herein include film thinning and stretching, as well as the formation of bubble- and ring-shaped structures from the elastomer surface, respectively. We furthermore present a theoretical explanation for the observed actuation mechanisms.

Enhanced High-Temperature Dielectric Properties of Poly(aryl ether sulfone)/BaTiO3 Nanocomposites via Constructing Chemical Crosslinked Networks

Heat-resistant and crosslinked polymers/ceramic composites have been prepared and investigated for enhancing high-temperature dielectric properties to adapt the development of advanced electric and electronic systems. Here, a series of crosslinkable heat-resistant poly(arylene ether sulfone)s (DPAES) with large dipole units of -SO₂ - are designed and synthesized as matrix, which are blended with BaTiO₃ (BT) nanoparticles to fabricate crosslinked polymer composites for boosting high-temperature dielectric properties. The results show that BT/c-DPAES possess great dielectric stability at measured frequency and temperature. Meanwhile, the discharged energy density and efficiency of BT/c-DPAES composites are higher than that of BT/DPAES at high temperatures, e.g., 10 vol% BT/c-DPAES has a discharged energy density of 1.7 J cm^-3 and efficiency of 73%, increasing by 42% and 128% in contrast to BT/DPAES, respectively. The enhanced high-temperature energy storage properties can be attributed to the construction of a crosslinked polymer network, reducing leakage current density of composites.

Compatibilization of porphyrins for use as high permittivity fillers in low voltage actuating silicone dielectric elastomers

Polysiloxanes represent, because of their unusual properties, a material with great potential for use in dielectric elastomers (DEs), a promising class of electroactive polymers. Currently, their application as actuators is limited by the need for high driving voltages, as a result of the low relative permittivity possessed by polysiloxanes (∼2–3). Reducing these voltages can be achieved to some degree by using high permittivity additives to improve the permittivity of the polysiloxane. However, modifying such additives so that they are compatible with, and can be dispersed within, polysiloxane elastomers remains challenging. For reliable actuation, full miscibility is key. In this work the porphyrin 5,10,15,20-(tetra-3-methoxyphenyl)porphyrin (TPMP) was investigated as a high permittivity additive. Its behaviour was compared to the analogue that was derivatized with bis(trimethylsiloxy)methylsilane groups using the Piers–Rubinsztajn reaction to improve compatability with silicone formulations. The derivatized porphyrin was dispersed in elastomers and their dielectric and mechanical properties were evaluated. It was discovered that only low levels of incorporation (1–10%) of the siliconized TPMP – much lower than the parent TPMP – were needed to elicit improvements in the permittivity and electromechanical actuation of the elastomers; actuation strains of up to 43% could be achieved using this method.

Derivatization of the porphyrin TPMP improves its dispersion in silicone dielectric elastomers, resulting in materials capable of achieving high actuations at low voltages.

How to tailor flexible silicone elastomers with mechanical integrity: a tutorial review

The tutorial aims to equip the beginners in silicone research with the knowledge to formulate recipes and process elastomer networks, targeting specific properties related to soft applications such as stretchable electronics without compromising the mechanical integrity of the elastomer.