Actuation Strain Research Articles

Biodegradable Electrohydraulic Soft Actuators

Biodegradable materials decompose and return to nature. This functionality can be applied to derive robotic systems that are environmentally friendly. This study presents a fully biodegradable soft actuator, which is one of the key elements in “green” soft robotics. The working of the actuator is based on an electrohydraulic principle, which is similar to that of hydraulically amplified self‐healing electrostatic actuators. The actuator developed in this study consists of a dielectric film made of polylactic acid (PLA) and polybutylene adipate‐co‐terephthalate (PBAT), with soybean oil as the dielectric liquid and electrodes made from a mixture of gelatin, glycerol, and sodium chloride (NaCl). The synthesized biodegradable electrode material exhibits a Young's modulus of 0.06 MPa and resistivity of 258 Ω·m when the mass fraction of NaCl relative to the amount of gelatin and glycerol is 10 wt%. The softness and resistivity of the electrode material results in actuation strain values of 3.2% (at 1 kV, corresponding to 1.2 kV mm−1) and 18.6% (at 10 kV, corresponding to 9.6 kV mm−1) for the linear‐type and circular‐type actuators, respectively. These values obtained for the biodegradable electrohydraulic soft actuators are comparable to those of nonbiodegradable actuators of the same type, representing the successful implementation of the concept.

Ultra-soft Organogel Artificial Muscles Exhibiting High Power Density, Large Stroke, Fast Response and Long-Term Durability in Air.

Polymeric gel-based artificial muscles exhibiting tissue-matched Young's modulus (10Pa-1MPa) promise to be core components in future soft machines with inherently safe human-machine interactions. However, the ability to simultaneously generate fast, large, high-power and long-lasting actuation in the open-air environment, has yet been demonstrated in this class of ultra-soft materials. Herein, to overcome this hurdle, we report the design and synthesis of a twisted and coiled liquid crystalline glycerol-organogel (TCLCG). Such material with a low Young's modulus of 133kPa can surpass the actuation performance of skeletal muscles in a variety of aspects, including actuation strain (66%), actuation rate (275%/s), power density (438kW/m3 ) and work capacity (105kJ/m3 ). Notably, its power density is 14 times higher than the record of state-of-the-art polymeric gels. No actuation performance degradation is detected in the TCLCG even after air exposure for 7 days, owing to the excellent water retention ability enabled by glycerol as co-solvent with water. Using TCLCG, we have successfully demonstrated mobile soft robots with extraordinary maneuverability in unstructured environments, including a crawler showing fast bidirectional locomotion (0.50mm/s) in a small-confined space, and a roller that can escape after deep burying in sand. This article is protected by copyright. All rights reserved.

Hybrid Fabrication of Prestrain-Locked Acrylic Dielectric Elastomer Thin Films and Multilayer Stacks.

Dielectric elastomers based on commercial acrylic dielectric elastomers (VHB adhesive films) are widely investigated for soft actuators due to their large electrically driven actuation strain and high work density. However, the VHB films require prestretching to overcome electromechanical instability, which adds fabrication complexity. In addition, their high viscoelasticity leads to a low response speed. Interpenetrated polymer networks (IPNs) are developed to lock the prestrain in VHB films, resulting in free-standing films that are capable of large-strain actuation. In this work, a prestrain-locked high-performance dielectric elastomer thin film (VHB-IPN-P) by introducing 1,6-hexanediol diacrylate to create an IPN in the VHB network and a plasticizer to enhance the actuation speed is reported. VHB-IPN-P based actuators exhibit stable actuation at 60% strain up to 10Hz and reach a peak energy density of 102 Jkg⁻1 . In addition, a hybrid process is also developed for the fabrication of multilayer stacks of VHB-IPN-P with strong inter-layer bonding and structural integrity. Four-layer stacks fabricated preserve the strain and energy density of single-layer VHB-IPN-P films but with linearly scaled force and work output.

Electro-thermal actuation in percolative ferroelectric polymer nanocomposites.

The interconversion between electrical and mechanical energies is pivotal to ferroelectrics to enable their applications in transducers, actuators and sensors. Ferroelectric polymers exhibit a giant electric-field-induced strain (>4.0%), markedly exceeding the actuation strain (≤1.7%) of piezoelectric ceramics and crystals. However, their normalized elastic energy densities remain orders of magnitude smaller than those of piezoelectric ceramics and crystals, severely limiting their practical applications in soft actuators. Here we report the use of electro-thermally induced ferroelectric phase transition in percolative ferroelectric polymer nanocomposites to achieve high strain performance in electric-field-driven actuation materials. We demonstrate a strain of over 8% and an output mechanical energy density of 11.3 J cm-3 at an electric field of 40 MV m-1 in the composite, outperforming the benchmark relaxor single-crystal ferroelectrics. This approach overcomes the trade-off between mechanical modulus and electro-strains in conventional piezoelectric polymer composites and opens up an avenue for high-performance ferroelectric actuators.

Ink-based transparent compliant electrode for direct coating on untreated hydrophobic PDMS surface

Polydimethylsiloxane (PDMS) is widely used as the substrate for wearable sensors and elastomeric actuators because of its good dielectric property, stretchability, thermal/chemical resistance, biocompatibility, and transparency. PDMS-based stretchable sensors and electroactive actuators need a coating of thin compliant electrodes. However, PDMS is hydrophobic with water contact angle above 120°. Such low surface energy of PDMS creates issues for uniform coating of ink-based electrodes and/or their adhesion. Similarly, metal thin-film or nanowire-based materials coated on PDMS easily peels off. Surface treatments like UV/plasma exposure can temporarily increase PDMS's surface energy. However, these surfaces return to its original state within hours leaving behind a brittle surface layer that cracks upon stretching. To address these issues, we formulated a viscous composite ink with PEDOT:PSS and PDMS that can easily be coated on untreated PDMS by blade casting, screen printing, and so on. These coatings are highly transparent, stretchable, electrically conductive and work as compliant electrodes. We fabricated transparent dielectric elastomer actuators by coating PEDOT:PSS/PDMS composite ink on Elastosil (PDMS) substrates. The actuation strain and breakdown-fields were slightly lower compared to DEA with conventional graphite electrodes. However, the additional self-clearing capability of PEDOT:PSS/PDMS electrodes enhanced robustness of the DEA in events of localized dielectric breakdown.

Four-Dimensional Printing of Temperature-Responsive Liquid Crystal Elastomers with Programmable Shape-Changing Behavior.

Liquid crystal elastomers (LCEs) are polymer networks that exhibit anisotropic liquid crystalline properties while maintaining the properties of elastomers, presenting reversible high-speed and large-scale actuation in response to external stimuli. Herein, we formulated a non-toxic, low-temperature liquid crystal (LC) ink for temperature-controlled direct ink writing 3D printing. The rheological properties of the LC ink were verified under different temperatures given the phase transition temperature of 63 °C measured by the DSC test. Afterwards, the effects of printing speed, printing temperature, and actuation temperature on the actuation strain of printed LCEs structures were investigated within adjustable ranges. In addition, it was demonstrated that the printing direction can modulate the LCEs to exhibit different actuation behaviors. Finally, by sequentially conforming structures and programming the printing parameters, it showed the deformation behavior of a variety of complex structures. By integrating with 4D printing and digital device architectures, this unique reversible deformation property will help LCEs presented here apply to mechanical actuators, smart surfaces, micro-robots, etc.

Enhanced electromechanical behaviours of epoxidized natural rubber composites via constructing non-covalent and covalent bonds by using tannic acid and Si747

Dielectric elastomer actuators (DEAs) are attractive for use in energy harvesting devices and soft robotics. In this study, barium titanate (BaTiO3) nanoparticles (NPs) were modified by tannic acid (TA) and 3-mercapto-propyl-ethoxy-bis(tridecyl-pentaethoxy-siloxane) (Si747) (denoted as BaTiO3-TA-Si747) via non-covalent and covalent modification. Then the as-prepared BaTiO3-TA-Si747 were incorporated into the epoxidized natural rubber (ENR) matrix (denoted as BaTiO3-TA-Si747-ENR) to prepare ENR DEA, aiming at obtaining high actuated strains at a low electric field. The TA and Si747 exhibited the partial polymeric flexibility on the surface of BaTiO3 NPs, and Si747 participated in the cross-linking reaction of ENR, resulting in a low Young's modulus and a high sensitivity coefficient of the BaTiO3-TA-Si747-ENR composites. Furthermore, TA and Si747 enhanced the interfacial interaction and compatibility between BaTiO3 NPs and the ENR matrix, which decreased the dielectric loss of the ENR composites and increased the efficiency of the energy conversion. A high actuated strain of 14.18% at 57.2 kV/mm was demonstrated by the 10 phr BaTiO3-TA-Si747-ENR composite, whereas pure ENR exhibited a value of 10.42% at 51.4 kV/mm. The combination of non-covalent and covalent surface modification using TA and Si747 is highly efficient, and it is expected to be applied at a large scale in industrial production.

Sustainable Approach for the Synthesis of a Semicrystalline Polymer with a Reversible Shape-Memory Effect

Shape-memory polymers (SMPs) have demonstrated potential for use in automotive, biomedical, and aerospace industries. However, ensuring the sustainability of these materials remains a challenge. Herein, a sustainable approach to synthesize a semicrystalline polymer using biomass-derivable precursors via catalyst-free polyesterification is presented. The synthesized biodegradable polymer, poly(1,8-octanediol-co-1,12-dodecanedioate-co-citrate) (PODDC), exhibits excellent shape-memory properties, as evidenced by good shape fixity and shape recovery ratios of 98%, along with a large reversible actuation strain of 28%. Without the use of a catalyst, the mild polymerization enables the reconfiguration of the partially cured two-dimensional (2D) film to a three-dimensional (3D) geometric form in the middle process. This study appears to be a step forward in developing sustainable SMPs and a simple way for constructing a 3D structure of a permanent shape.

Polybutadiene Dielectric Elastomers with High Dielectric Constant and Ultralow Dielectric Loss via Tuning Grafted Polar Groups.

This work describes an easy and effective process for preparing a homogeneous polybutadiene (PB) dielectric elastomer with improved actuated strain by utilizing a photochemical thiol-ene click reaction. The carboxyl groups and various ester groups are used for grafting on the PB. Since the length of the alkyl chains of the ester groups will significantly influence the polarities of the carbonyl groups and hydrogen bonding effect, the influence of polarity and hydrogen bonding effect on the dielectric properties and mechanical properties of the modified polybutadienes is carefully discussed. Very interestingly, it is found that the increment of dielectric constant of PB modified by carboxyl groups is the lowest one compared to other modified PBs with the ester groups. Meanwhile, the modified PBs with the ester groups could achieve pretty low dielectric loss factor, and finally, a high dielectric constant (3.6), low dielectric loss factor (0.0005), and large actuated strain (≈25%) of the modified polybutadienes could be achieved by the modified PBs with butyl acrylate groups. This work provides a simple and effective method for the designing and synthesizing of a homogeneous high electromechanical performance dielectric elastomer with high dielectric constant and low dielectric loss.

Effect of water content on the constitutive response of a cellulose foam

Cellulose foams can undergo a large one-way actuation strain when taken from a dry, pre-compressed state to a damp state by the addition of water. In order to develop a predictive capability for the 3D actuation of a foam-based structure with a non-uniform water concentration, it is first necessary to understand the mechanical properties of the foam (including actuation strain) as a function of water content. Here, we report the anisotropic constitutive response of a cellulose foam as a function of water content. Over the range of strain rate and water content considered, the cellulose foam is adequately approximated by a visco-plastic, transversely isotropic, compressible constitutive model. Unloading from the plastic state by a stress drop equal to the initial yield stress leads to reversed plastic flow: this is an extreme form of kinematic hardening.

A Voxel‐Based Approach for the Generation of Advanced Kinematics at the Microscale

In soft robotics, the ability to generate advanced kinematics is a necessary step toward any more sophisticated tasks such as microobject manipulation, locomotion, or configuration changes. To this end, herein, a modular voxel‐based methodology adaptable to any scale and with any soft transducer is presented. The methodology is implemented at the micrometer scale with a one‐step fabrication process. An innovative gray‐tone lithography method using the two‐photon polymerization of photosensitive poly(N‐isopropylacrylamide) hydrogel is developed to print the voxels. Bending, compression, and twisting voxels are designed, printed, and characterized. A voxel consists of an isotropically shrinking active material reinforced adequately with a passive pattern. Each elementary voxel deforms along one degree of freedom and is a building block for superstructures able of advanced kinematics. With a side length of 40 μm, the bending voxel achieves a bending angle of 25º or curvature of . The compression voxel reaches an actuation strain of 40%, and the twisting voxel bends up to 18º. Advanced kinematics are demonstrated by printing complex structures composed of multiple elementary voxels. Herein, a foundation toward soft microrobots capable of performing complex tasks is constituted.

Reconfigurable and Reprocessable Thermadapt Stress‐Free Two‐Way Shape Memory Polymers Based on a Dual Crosslinking Network with Outstanding Mechanical Properties

The reconfigurable and reprocessable thermadapt two‐way shape memory polymers (2W‐SMPs) are highly desirable for many advanced biomimetic applications, but the preparation of these polymers with economical raw materials and convenient approaches still remains a challenge. In this work, thermadapt 2W‐SMPs based on a dual crosslinking network are synthesized by linking polyethylene glycol with hydroxylated modified SBS via dynamic covalent bonds. Benefiting from the synergistic effect of combining a dynamic covalent network and a physical cross‐linking network, the thermadapt 2W‐SMPs exhibit a good balance of mechanical properties and favorable two‐way shape memory effect (2W‐SME) with the reversible actuation strain of 11.9%. The 2W‐SMPs can be reprocessed and transformed into complex 3D shapes after multiple folding processes due to the dynamic covalent exchange as well as the reversible physical crosslinking, and the obtained new samples still exhibit good 2W‐SMEs. Furthermore, the transition temperature of 2W‐SMPs is designed to be close to human body temperature, and it is manufactured into a biomimetic mimosa actuator that can undergo shape changes through finger contact and cooling. This work provides a new idea for the development of thermadapt 2W‐SMPs, and the obtained materials show great potential applications in intelligent actuators.

Liquid Crystal Elastomer Based Dexterous Artificial Motor Unit.

Despite the great advancement in designing diverse soft robots, they are not yet as dexterous as animals in many aspects. One challenge is that they still lack the compact design of an artificial motor unit with a great comprehensive performance that can be conveniently fabricated, although many recently developed artificial muscles have shown excellent properties in one or two aspects. Herein, an artificial motor unit is developed based on gold-coated ultrathin liquid crystal elastomer (LCE) film. Subject to a voltage, Joule heating generated by the gold film increases the temperature of the LCE film underneath and causes it to contract. Due to the small thermal inertial and electrically controlling method of the ultrathin LCE structure, its cyclic actuation speed is fast and controllable. It is shown that under electrical stimulation, the actuation strain of the LCE-based motor unit reaches 45%, the strain rate reaches 750%/s, and the output power density is as high as 1360Wkg-1 . It is further demonstrated that the LCE-based motor unit behaves like an actuator, a brake, or a nonlinear spring on demand, analogous to most animal muscles. Finally, as a proof-of-concept, multiple highly dexterous artificial neuromuscular systems are demonstrated using the LCE-based motor unit.

Fluid‐Driven High‐Performance Bionic Artificial Muscle with Adjustable Muscle Architecture

High‐performance artificial muscle is always the pursuit of researchers for robotics. Herein, a bionic artificial muscle is reported called “ExoMuscle” mimicking the sarcomere in skeletal muscle with a bio‐inspired structure to contract “myofilaments” enabling the artificial muscle to mimic the architecture of muscle such as parallel, fusiform, convergent, and pennation and beyond the performance of skeletal muscle. The reported actuators excel in various aspects compared with skeletal muscle including actuation stress (0.41–0.9 MPa), strain (50%), optimal length, velocity‐independence output, power density (10.94 kW kg−1), and efficiency (69.11%). With its own adjustable pennation architecture, it achieves variable actuation stress up to 0.9 MPa meanwhile maintaining high efficiency. Furthermore, ExoMuscle highly conforms to the anatomical complexity of the human body to cooperate with skeletal muscles closely opening the door for bio‐robotics, especially wearable robots.

Bioinspired Liquid Crystalline Spinning Enables Scalable Fabrication of High-Performing Fibrous Artificial Muscles.

Leveraging liquid crystal elastomers (LCEs) to realize scalable fabrication of high-performing fibrous artificial muscles is of particular interest because these active soft materials can provide large, reversible, programmable deformations upon environmental stimuli. High-performing fibrous LCEs require the used processing technology to enable shaping LCEs into micro-scale fine fibers as thin as possible while achieving macroscopic LC orientation, which however remains a daunting challenge. Here, a bioinspired spinning technology is reported that allows for continuous, high-speed production (fabrication speed up to 8400mh-1 ) of thin and aligned LCE microfibers combined with rapid deformation (actuation strain rate up to 810% s-1 ), powerful actuation (actuation stress up to 5.3MPa), high response frequency (50Hz), and long cycle life (250 000 cycles without obvious fatigue). Inspired by liquid crystalline spinning of spiders that takes advantage of multiple drawdowns to thin and align their dragline silks, internal drawdown via tapered-wall-induced-shearing and external drawdown via mechanical stretching are employed to shape LCEs into long, thin, aligned microfibers with the desirable actuation performances, which few processing technologies can achieve. This bioinspired processing technology capable of scalable production of high-performing fibrous LCEs would benefit the development of smart fabrics, intelligent wearable devices, humanoid robotics, and other areas.

Active Textile Fabrics from Weaving Liquid Crystalline Elastomer Filaments.

Active fabrics, responding autonomously to environmental changes, are the "Holy Grail" of the development of smart textiles. Liquid crystal elastomers (LCEs) promise to be the base materials for large-stroke reversible actuation. The mechanical behavior of LCEs matches almost exactly the human muscle. Yet, it has not beenpossible to produce filaments from LCEs that will be suitable for standard textile production methods, such as weaving. Based on the recent development of LCE fibers, here, the crafting of active fabrics incorporating LCE yarn, woven on a standard loom, giving control over the weave density and structure, is presented. Two types of LCE yarns (soft and stiff) and their incorporation into several weaving patterns are tested, and the "champions" identified: the twill pattern with stiffer LCE yarn that shows the greatest blocking force of 1-2Ncm-1 , and the weft rib pattern with over 10% reversible actuation strain on repeated heating cycles. Reversible 3D shape changes of active fabric utilize the circular weaving patterns that lead to cone shapes upon heating. The seamless combination of active LCE yarns into the rich portfolio of existing passive yarns can be transformative in creating new stimuli-responsive actuating textiles.

Poly(thioether) grafted Ti3C2Tx MXenes: New dielectric elastomer nanocomposites with high area strain at low driving voltage

Dielectric elastomers (DEs) with high actuated strains at low electric fields have been a great challenge over the past few decades. In this work, we report the preparation of poly(thioether) grafted Ti3C2Tx MXene DE nanocomposites (MXeneK-FGE-BMI) with high area strain at low driving voltage (17.1 %@11.9 V/μm) by two steps: the pretreated MXene (MXeneK) initiated decarboxylative ring-opening polymerization (ROP) of furfuryl glycidyl ether (FGE) and carbonyl sulfide (COS), followed by the Diels-Alder (DA) crossslinking reaction between furan groups and bismaleimide (BMI). The grafting of poly(thioether) chains onto the MXeneK surfaces enables MXeneK well dispersed in the matrix, leading to the significant decrease of dielectric loss (tan δ, below 0.04@103Hz). Low content of MXeneK (below 1.0 wt%) resulted in high ε’ (∼12.3@103Hz) and low elastic modulus (Y, below 0.5 MPa). Therefore, the resultant DE nanocomposite present a high area strain of 17.1 % at a driving voltage of 11.9 V/μm, superior to the previous reports (0.7 %∼10 %@12 V/μm). This work provides a facile strategy to prepare DE nanocomposites with excellent electromechanical performances at low driving voltage.

Wavefront Control Strategies for Large Active Thin Shell Primaries with Unimorph Actuators

This paper presents various aspects of the wavefront control strategies for an ultra-lightweight composite reflector made of polymers for the large primary of a space telescope, and the shape control is made by a set of patterned unimorph strain actuators attached to the reflector. It starts with an analytical investigation of the mechanical behaviors of a strain-actuated curved shell, resulting in the accurate prediction of typical features, such as the damped wave deformation at the transition between electrodes and the limited morphing amplitude of a “print-through” actuation, which indicates that the curvature-induced rigidity deteriorates the performances of the forming accuracy of the active reflector and the morphing stroke of the actuators. The morphing capabilities are evaluated with both petal-like segmented and monolithic configured reflectors by numerical tests on forming target shapes of Zernike modes with various patternings of electrodes, and the structural dynamics are examined. Finally, a compound control strategy is proposed, which uses a deformable relay mirror to compensate for the residual surface error corrected partially by the active unimorph primary mirror, showing a great relaxation of the shape error budget of the thin-shell primary, especially for on-axis observation.

Viscoelastic Effects on the Nonlinear Oscillations of Hard-Magnetic Soft Actuators

AbstractThe hard-magnetic soft materials (HMSMs) belong to the magnetoactive category of smart polymers that undergo large actuation strain under an externally applied magnetic field and can sustain a high residual magnetic flux density. Because of these remarkable characteristics, HMSMs are promising candidates for the remotely controlled actuators. The magnetic actuation behavior of the hard-magnetic soft actuators (HMSAs) is considerably affected by the viscoelastic material behavior of HMSMs. In this article, we aim at developing an analytical dynamic model of a typical planar model of HMSAs concerning the viscoelasticity of HMSMs. A Zener rheological model in conjunction with an incompressible neo-Hookean model of hyperelasticity and Rayleigh dissipation function is employed for defining the constitutive behavior of the viscoelastic HMSA. The governing equations of dynamic motion are deduced by implementing the nonconservative form of the Euler–Lagrange equation. The established dynamic model is utilized for providing preliminary insights pertaining to the effect of the viscoelasticity on the nonlinear oscillations of the actuator. The phase–plane portraits, Poincaré maps, and the time–history response are plotted to investigate the stability, resonant behavior, and periodicity of the actuator. The results and inferences reported here should provide the initial step toward the design and the development of modern actuators for diverse futuristic applications in the medical and engineering fields.

Shape-memory effect in twisted ferroic nanocomposites

The shape recovery ability of shape-memory alloys vanishes below a critical size (~50 nm), which prevents their practical applications at the nanoscale. In contrast, ferroic materials, even when scaled down to dimensions of a few nanometers, exhibit actuation strain through domain switching, though the generated strain is modest (~1%). Here, we develop freestanding twisted architectures of nanoscale ferroic oxides showing shape-memory effect with a giant recoverable strain (>8%). The twisted geometrical design amplifies the strain generated during ferroelectric domain switching, which cannot be achieved in bulk ceramics or substrate-bonded thin films. The twisted ferroic nanocomposites allow us to overcome the size limitations in traditional shape-memory alloys and open new avenues in engineering large-stroke shape-memory materials for small-scale actuating devices such as nanorobots and artificial muscle fibrils.