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.

A New Molecular Mechanism for Understanding the Actuated Strain of Dielectric Elastomers and Their Impacts

Dielectric elastomers (DEs) are a special material that deform responding to an electric field. The induced strain is known as actuated strain (AS). This phenomenon is totally different from electrostriction, for there is no crystal lattice in elastomers and the AS of DEs is much greater. The most accepted mechanism holds the view that the AS of DEs is induced by the Maxwell stress. According to this mechanism, materials exhibiting similar ratios of permittivity and Young's modulus should have similar ASs, while the experimental AS isn't relevant to the ideal value, contradicting this mechanism. The direction of uniaxial pre-strained DE's AS cannot be explained by this mechanism either. The electric field and DE are only regarded as a source of stress and a deformable body respectively in this mechanism, which ignores the interaction between those two. Recently, a new molecular mechanism for AS is proposed, in which the electric field first orient dipoles of chains, therefore the conformation of chains will be changed, finally leading to AS. With thermodynamical derivation and experiment, entropy-dominated elasticity is found to account for more during AS. This mechanism is systematically introduced in this perspective and presents current challenges and outlooks of DE.

Photothermal modulated dielectric elastomer actuator for resilient soft robots

Soft robots need to be resilient to extend their operation under unpredictable environments. While utilizing elastomers that are tough and healable is promising to achieve this, mechanical enhancements often lead to higher stiffness that deteriorates actuation strains. This work introduces liquid metal nanoparticles into carboxyl polyurethane elastomer to sensitize a dielectric elastomer actuator (DEA) with responsiveness to electric fields and NIR light. The nanocomposite can be healed under NIR illumination to retain high toughness (55 MJ m−3) and can be recycled at lower temperatures and shorter durations due to nanoparticle-elastomer interactions that minimize energy barriers. During co-stimulation, photothermal effects modulate the elastomer moduli to lower driving electric fields of DEAs. Bilayer configurations display synergistic actuation under co-stimulation to improve energy densities, and enable a DEA crawler to achieve longer strides. This work paves the way for a generation of soft robots that achieves both resilience and high actuation performance.

Structural evolution and dielectric properties of biaxially oriented polyethylene/multiwalled carbon nanotube composite films

In modern power systems, polymer nanocomposite films have been more frequently used as dielectric materials for capacitors. However, the low energy storage density limits its application in more areas. It is a very challenging task to prepare dielectric films with high dielectric constant (ε), high energy storage density, and low dielectric loss (tan δ) at the same time. In this study, one kind of BOPE (biaxially oriented polyethylene) nanocomposite dielectric films with a very small amount of MWCNTs (multiwalled carbon nanotubes) was reported. MWCNTs with a high aspect ratio were used for conductive filler, and the formation of more micro capacitors and interfacial polarization were caused by better dispersibility of MWCNTs in polyethylene matrix by biaxial stretching. The BOPE/MWCNT composite films with high-performance were successfully prepared by achieving the requirements of low content, high dielectric constant and high energy storage density while maintaining the advantages of low dielectric loss of the polymer matrix. The dielectric constant of the BOPE/MWCNT composite films increased from 2.26 to 4.68 with the drawing ratio was 4 × 4 and the content of MWCNTs was 0.6 wt% and the energy storage density was also increased from 1.01 J cm^-3 to 1.29 J cm^-3. From our work, the achievement of low thickness, high energy density and low loss at the same time implied BOPE/MWCNT composite films were promising materials for next-generation film capacitors.

Interface design for high energy density polymer nanocomposites

This review provides a detailed overview on the latest developments in the design and control of the interface in polymer based composite dielectrics for energy storage applications. The methods employed for interface design in composite systems are described for a variety of filler types and morphologies, along with novel approaches employed to build hierarchical interfaces for multi-scale control of properties. Efforts to achieve a close control of interfacial properties and geometry are then described, which includes the creation of either flexible or rigid polymer interfaces, the use of liquid crystals and developing ceramic and carbon-based interfaces with tailored electrical properties. The impact of the variety of interface structures on composite polarization and energy storage capability are described, along with an overview of existing models to understand the polarization mechanisms and quantitatively assess the potential benefits of different structures for energy storage. The applications and properties of such interface-controlled materials are then explored, along with an overview of existing challenges and practical limitations. Finally, a summary and future perspectives are provided to highlight future directions of research in this growing and important area.

Novel Melamine/o-Phthalaldehyde Covalent Organic Frameworks Nanosheets: Enhancement Flame Retardant and Mechanical Performances of Thermoplastic Polyurethanes

Covalent organic frameworks (COFs) nanosheets prepared from condensation reaction between melamine and o-phthalaldehyde are first prepared through ball milling and then incorporated into thermoplastic polyurethanes (TPU) by solution mixing. Transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared (FTIR) spectrometer are applied to characterize COFs nanosheets. It is observed apparently from TEM image that COFs nanosheets are obtained. Successful preparation of COFs nanosheets is proved further by vanishment of typical diffraction peak of COFs at around 23.5° in COFs nanosheets XRD pattern, appearance of quadrant and semicircle stretching of the s-triazine ring at 1568 and 1469 cm^-1 in FTIR spectra and N═C bond at 389.5 eV in N_1s high-resolution XPS spectra of COFs nanosheets. The thermal property, combustion behavior and mechanical performance of TPU naoncomposites are also investigated. Incorporation of COFs nanosheets into TPU contributes to char forming of TPU under nitrogen atmosphere and 14.3% decrease of peak heat release rate of TPU. Besides, the elongation at break, Young's modulus, and fracture strength of TPU nanocomposites increase sharply compared with that of neat one.

Dielectric Elastomer Generator with Improved Energy Density and Conversion Efficiency Based on Polyurethane Composites

Dielectric elastomer generators (DEGs), which follow the physics of variable capacitors and harvest electric energy from mechanical work, have attracted intensive attention over the past decade. The lack of ideal dielectric elastomers, after nearly two decades of research, has become the bottleneck for DEGs' practical applications. Here, we fabricated a series of polyurethane-based ternary composites and estimated their potential as DEGs to harvest electric energy for the first time. Thermoplastic polyurethane (PU) with high relative permittivity (∼8) was chosen as the elastic matrix. Barium titanate (BT) nanoparticles and dibutyl phthalate (DBP) plasticizers, which were selected to improve the permittivity and mechanical properties, respectively, were blended into the PU matrix. As compared to pristine PU, the resultant ternary composite films fabricated through a solution casting approach showed enhanced permittivity, remarkably reduced elastic modulus, and relatively good electrical breakdown strength, dielectric loss, and strain at break. Most importantly, the harvested energy density of PU was significantly enhanced when blended with BT and DBP. A composite film containing 25 phr of BT and 60 phr of DBP with the harvested energy density of 1.71 mJ/cm³ was achieved, which is about 4 times greater than that of pure PU and 8 times greater than that of VHB adhesives. Remarkably improved conversion efficiency of mechano-electric energy was also obtained via cofilling BT and DBP into PU. The results shown in this work strongly suggest compositing is a very promising way to provide better dielectric elastomer candidates for forthcoming practical DEGs.

Remarkably improved electromechanical actuation of polyurethane enabled by blending with silicone rubber

Electromechanical actuation strain of polyurethane is enlarged ten times by constructing a bicontinuous structure with silicone rubber.

Dopamine and silane functionalized barium titanate with improved electromechanical properties for silicone dielectric elastomers

A combination of bio-inspired poly(dopamine) deposition and silane grafting is proposed to functionalize bariumtitanate to improve the electromechanical properties of silicone dielectric elastomers.

Enhanced actuated strain of titanium dioxide/nitrile-butadiene rubber composite by the biomimetic method

An excellent dielectric elastomer was fabricated by incorporating titanium dioxide nano-particles into nitrile-butadiene rubber by a simple dopamine biomimetic method.