Actuation Speed Research Articles

Actuation Performance and Versatility of Photothermally Driven Organic Crystals

AbstractPhotomechanical crystals exhibit mechanical motion upon light irradiation and may thus find applications as actuators. Over the last decades, many photomechanical organic crystals have been developed, commonly via photochemical reactions, particularly photoisomerization. However, photochemical crystal actuation is associated with several drawbacks, including a limited number of available crystals, slow actuation speed (<5 Hz), and narrow wavelength range (<550 nm). Such constraints have hindered the widespread use of crystals as actuation materials. In this minireview, we focus on crystal actuation by employing more universal physical phenomena (the photothermal effect and photothermally resonated natural vibration) and quantitatively evaluate actuation performance. Both mechanisms, particularly the latter, outperformed conventional photomechanical crystal activation in terms of both speed (maximum: 1,350 Hz) and the useful wavelength range (ultraviolet to near‐infrared). The oscillation frequencies of the crystals exceeded those of polymers, efficiently filling the gap between soft and hard materials. Both the photothermal effect and natural vibration can actuate any crystal that absorbs light. These two versatile physical actuation mechanisms could expand 40 years of research on photomechanical crystals—which had been based on photochemical reactions—from the realm of chemistry into engineering and lead to their practical applications in actuators and soft robots.

Actuation Performance and Versatility of Photothermally Driven Organic Crystals.

Photomechanical crystals exhibit mechanical motion upon light irradiation and may thus find applications as actuators. Over the last decades, many photomechanical organic crystals have been developed, commonly via photochemical reactions, particularly photoisomerization. However, photochemical crystal actuation is associated with several drawbacks, including a limited number of available crystals, slow actuation speed (<5 Hz), and narrow wavelength range (<550 nm). Such constraints have hindered the widespread use of crystals as actuation materials. In this minireview, we focus on crystal actuation by employing more universal physical phenomena (the photothermal effect and photothermally resonated natural vibration) and quantitatively evaluate actuation performance. Both mechanisms, particularly the latter, outperformed conventional photomechanical crystal activation in terms of both speed (maximum: 1,350 Hz) and the useful wavelength range (ultraviolet to near-infrared). The oscillation frequencies of the crystals exceeded those of polymers, efficiently filling the gap between soft and hard materials. Both the photothermal effect and natural vibration can actuate any crystal that absorbs light. These two versatile physical actuation mechanisms could expand 40 years of research on photomechanical crystals-which had been based on photochemical reactions-from the realm of chemistry into engineering and lead to their practical applications in actuators and soft robots.

Self-Repeatable snapping liquid–crystal-elastomer actuator

Liquid crystal elastomers (LCEs) show potential for use in sensors and robotics owing to their reversible stimuli response. However, current actuators have limitations such as low power and slow recovery. Incorporating snap-through transitions induced by mechanical instability into motion can enhance the speed and power of soft actuators. However, the snap-through transitions’ single-shot motion response without external assistance at complicates their application to continuous and self-repeatable actuation. This study introduces a promising and simple approach for creating a rapid and self-repeatedly regulated snapping motion in an LCE actuator by confining the LCE film in a frame. We explore the confinement effect of a freestanding pre-curved LCE film that smoothly changes its bending direction and shows anticlastic buckling. Interestingly, a self-repeating snapping motion is generated by attaching a rigid film onto the LCE film, thereby inducing synclastic deformation. Remarkably, the up-and-down bending snapping motion occurs at a very high speed (within 30 ms) and is self-regulated depending on the temperature of the bottom surface. By utilizing this thermally responsive snapping LCE actuator, we successfully demonstrate an organic-based novel fire alarm and a self-propelled soft-walk mechanism.

Optically Responsive Hydrogel with Rapid Deformation for Motion Regulation of Magnetic Actuators.

Optically and magnetically responsive soft actuators are gaining attention for their noncontact actuation, flexibility, and remote control capabilities. However, they face challenges in rapidly switching motion postures and modes, which limits their performance in complex environments. We developed bilayer hydrogel actuators based on poly(N-isopropylacrylamide) (PNIPAm) using an ice-templating method combined with free radical polymerization. This approach results in the formation of large, interconnected pores within the hydrogel. Under near-infrared light (27 W/cm2), the actuation speed of the actuator reached 38.5°/s, with complete recovery to the original shape 8 s after light cessation. In addition, the reversible changes in stiffness and volume enable the actuators to lock and dynamically adjust their magnetization curve, allowing for the decoupling of deformation and movement as well as the regulation of motion postures and modes. This work opens new pathways for multigait robots and shows promising applications in environmental monitoring and underwater exploration.

An asynchronous dual-foot inertial piezoelectric actuator with strong magnetic compatibility

To address the limitations in actuator selection within strong magnetic environments, this study introduces a novel inertial piezoelectric actuator with asynchronous dual-foot motion, which meets the need for high-precision actuation in strong magnetic field environments. The design principle is to use two driving feet that generate elliptical motion in an alternating method to achieve continuous linear motion of the actuator. This approach enhances load capacity, speed, and stepping characteristics. The design allows for better control of backward motion without modifying the driving waveform, thus improving the actuator's speed and ensuring consistent performance in both directions. A finite element model of the proposed actuator stator is developed and analyzed using commercial software to optimize its dimensional parameters. A prototype is fabricated, and experimental investigations are carried out. The results indicate that the dual asynchronous driving method significantly increases the speed of the actuator and the consistency of the motion by improving stepping characteristics. The prototype reached a maximum speed of 8.98 mm/s, supported a maximum load of 4.5 N, and achieved a displacement resolution of 19 nm. Finally, the strong magnetic field environmental test and MRI image quality test were conducted, and experimental results confirmed that the actuator has superior magnetic compatibility, highlighting its potential for precision applications in challenging environments. This research proposes new methods to enhance the retraction characteristics and output performance of inertial actuators, with their excellent magnetic field compatibility anticipated to be applied in future MRI-compatible injectors, puncture devices, or other actuating mechanisms.

A Review on Cushioning Characteristics in High-Speed Hydraulic Actuators

Abstract High speed actuators are an integral part of the fluid power driven systems and cylinder cushioning requirement in these actuators is one of the important aspects. Cushioning is imperative in any high-speed actuator as it not only increases its life but also gives operating comfort to the operator. Fluid power driven systems are very common in many defence and industrial applications due to their high force/torque and power density. This review paper brings out the available literature in the field of hydraulic cylinder cushioning. Literature, consisting mainly of journal articles, research papers and other media contents is analysed for the key parameters of research in this field. The classification of cushioning methodology by different researchers is projected. The methodologies of modelling and simulation work carried out by various authors are analysed. Thereafter the optimization studies done for the cushion piston design in various literature are discussed. Lastly, experimental validation strategies as employed in certain papers are also brought out. Experimental study, and validation of non-conventional cushioning methodology is discussed, which is not much evidenced in the literature.

Programming Hydrogen Bonds for Reversible Elastic-Plastic Phase Transition in a Conductive Stretchable Hydrogel Actuator with Rapid Ultra-High-Density Energy Conversion and Multiple Sensory Properties.

Smart hydrogels have recently garnered significant attention in the fields of actuators, human-machine interaction, and soft robotics. However, when constructing large-scale actuated systems, they usually exhibit limited actuation forces (≈2kPa) and actuation speeds. Drawing inspiration from hairspring energy conversion mechanism, an elasticity-plasticity-controllable composite hydrogel (PCTA) with robust contraction capabilities is developed. By precisely manipulating intermolecular and intramolecular hydrogen-bonding interactions, the material's elasticity and plasticity can be programmed to facilitate efficient energy storage and release. The proposed mechanism enables rapid generation of high contraction forces (900kPa) at ultra-high working densities (0.96MJm-3). Molecular dynamics simulations reveal that modifications in the number and nature of hydrogen bonds lead to a distinct elastic-plastic transition in hydrogels. Furthermore, the conductive PCTA hydrogel exhibits multimodal sensing capabilities including stretchable strain sensing with a wide sensing range (1-200%), fast response time (180ms), and excellent linearity of the output signal. Moreover, it demonstrates exceptional temperature and humidity sensing capabilities with high detection accuracy. The strong actuation power and real-time sensory feedback from the composite hydrogels are expected to inspire novel flexible driving materials and intelligent sensing systems.

Laser-Printed All-Carbon Responsive Material and Soft Robot.

Responsive materials and actuators are the basis for the development of various leading-edge technologies but have so far mostly been designed based on polymers, incurring key limitations related to sensitivity and environmental tolerance. This work reports a new responsive material, laser-printed carbon film (LPCF), produced via direct laser transformation of a liquid organic precursor and consists of graphitic and amorphous carbons. The high activity of amorphous carbon combined with the dual-gradient structure enables the LPCF to have a actuation speed of 9400°s-1 in response to the stimulus of organic vapor. LPCF exhibits a conductivity of 950Sm-1 and excellent resistance to various extreme environmental conditions, which are unachievable for polymer-based materials. Additionally, an LPCF-based all-carbon soft robot that can mimic the complex continuous backward somersaulting motions without manual intervention is constructed. The locomotion velocity of the robot reaches a value of 1.19BLs-1, which is almost one to two orders of magnitude faster than that of reported soft robots. This work not only offers a new paradigm for highly responsive materials but also provides a great design and engineering example for the next generation of biomimetic robots with life-like performance.

Thermal and light-driven soft actuators based on a conductive polypyrrole nanofibers integrated poly(N-isopropylacrylamide) hydrogel with intelligent response

The development of soft hydrogel actuators with outstanding mechanical properties, fast actuation speed, and available quantification of self-sensing actuation remains a challenging endeavor. In this work, dopamine-decorated polypyrrole nanofibers (DAPPy) were introduced into the polyethylene glycol diacrylate (PEGDA)-crosslinked poly(N-isopropyl acrylamide) network to generate a stretchable, NIR-responsive, and strain sensitive DAPPy/PNIPAM hydrogel layer. Besides, this active layer was combined with the passive ligninsulfonate sodium/polyacrylamide (LS/PAAM) to give DAPPy/PNIPAM//LS/PAAM bilayer hydrogel actuator, which exhibits ultrafast thermo-responsive actuation (19°/s) and underwater grasping and lifting performance. Moreover, the DAPPy/PNIPAM layer has excellent electrical conductivity (0.29 S/m) and thermal conversion ability (10.8 °C/min), which enable such a conductive hydrogel to act as a highly sensitive strain and temperature sensor with real-time resistance change in response to tensile strain (gauge factor up to 3.4), applied pressure, temperature, and remote NIR light irradiation. More importantly, the bilayer hydrogel actuator can integrate both actuation and self-sensing functions through the bending angle-surface temperature-relative resistance change relationship of the photothermal process. With excellent mechanical actuation and self-sensing ability, the resulting bilayer hydrogel showed a promising application potential as soft biomimetic actuating materials and soft intelligent actuators.

Comparative Experimental Study on the De-Icing Performance of Multiple Actuators

The issue of aircraft icing poses a substantial threat to flight safety. In order to investigate more efficient anti-icing and de-icing technologies, a comparative analysis was conducted on the de-icing characteristics of three types of actuator materials under varying conditions. Initially, experimental research was undertaken to analyze the temperature traits of three actuators under ice-free conditions. Three power densities were chosen for the experiment: 0.170 W/cm2, 0.727 W/cm2, and 1.427 W/cm2. The research findings revealed distinct characteristics: plasma actuators and resistance wire actuators exhibited a strip-like high-temperature region during operation, with well-defined boundaries between the high-temperature and low-temperature zones, whereas ceramic-based semiconductor actuators showcased a uniform high-temperature region. As energy consumption rose, the thermal equilibrium temperatures of all three types tended to converge, with resistance wire actuators operating at 1.427 W/cm2, showing the highest temperature rise rate at that power density. Subsequently, experimental research was carried out on the de-icing performance of three actuators under icing conditions at a specific power density. Following 120 s of de-icing, the ice layer covering the surface of the plasma actuator completely melted, forming a cavity. Conversely, the ice layer on the ceramic-based semiconductor actuator remained partially intact in a strip shape. Ice deposits were still visible on the surface of the resistance wire actuator. This observation highlights the remarkable de-icing speed of the plasma actuator. The propulsive force of plasma generated on the fluid inside the ice layer enhances heat transfer efficiency, thereby accelerating the de-icing process of the plasma actuator at the same power density. The analysis of the de-icing performance of these three novel types of actuators establishes a robust groundwork for exploring more effective aircraft de-icing methods. Furthermore, it furnishes theoretical underpinning for the advancement of composite anti-icing and de-icing strategies.

Comparative Analysis of Peltier Devices and Flexible Heater Strips for Enhancing Bandwidth in Thermo-Active Soft Actuators

Soft actuators are a new generation of robotic actuators designed for safer and more adaptable physical human-robot interaction, that can be triggered by various stimulating mechanisms, including pneumatic, electric, electromagnetic, light, magnetic, and thermal sources. Among the different types of soft actuators, thermoresponsive ones that utilize heat as the stimulus show great potential due to their ability to deliver a relatively high force-to-size ratio without the need for external air pumps, tethers, high voltage sources, or complex designs. However, a major drawback of such actuators is their limited bandwidth. Traditional methods rely on Joule heating for actuation, with the actuator deflating when the heat source is turned off and ambient temperature takes over. Recently, the Peltier mechanism has been introduced as an alternative approach for active heating and cooling. This research paper presents a comparative analysis of the Peltier and flexible heater mechanisms in terms of the bandwidth and energy consumption of phase-change thermo-active soft actuators. The study aims to assess the potential of Peltier-based actuation in addressing the bandwidth limitations observed in traditional soft actuators. The findings reveal that Peltier-based actuation can significantly improve actuation speed in thermoresponsive soft actuators. However, it is important to note that the performance of Peltier-based actuators decreases after a few cycles unless additional measures, such as the use of an external fan, are implemented. This increase in performance comes at the cost of higher energy consumption, which should be carefully considered in practical applications.

Development of Ni-Mn-Ga Enabled Micropumps for Hybrid Microdevices in Microelectromechanical Systems

This study selects a single crystalline Ni-Mn-Ga alloy by its exceptional actuator attributes, high actuation speed, precise position control, rapid response to external magnetic fields, and extended operational lifespan. Researchers venture into uncharted territory, aiming to harness the potential of Ni-Mn-Ga alloy to revolutionize micropump performance and refine fluid manipulation within miniature devices. The methodology at the heart of this endeavor involves the seamless integration of this specialized alloy with microdevice technology, giving rise to a set of unique pump components that substantially boost pump efficiency. Crucially, Ni-Mn-Ga is the chosen material for the active part of the micropump. At the same time, MEMS fabrication handles the passive elements, all facilitated by the 0.18 µm semiconductor technology and Sivalco TCAD simulation software. Computational simulations validate the alloy's suitability, impressively achieving an accumulated flow volume of 0.15 x 10e-4 µL in 10 microseconds. Beyond its scientific significance, this research bridges MEMS technology and magnetic-enabled smart materials, showcasing the remarkable capabilities of Ni-Mn-Ga alloy in significantly enhancing micropump performance. These innovative solutions promise to open doors to groundbreaking applications in microfluidic systems across many scientific and industrial domains.

Experimental study of hydraulic lifting platform based on multi-ring disc magnetorheological valve

To enhance the operational capabilities of a hydraulic lifting apparatus, a hydraulic lifting platform was engineered based on a multi-ring disc magnetorheological valve. The pressure drop characteristics of the multi-ring disc magnetorheological valve were determined through theoretical analysis, supplemented by magnetic field simulations to assess its performance. The effects of different currents on the stability and response time of the hydraulic lifting platform are studied, and the control accuracy of the hydraulic lifting platform position is verified. The results show that the saturation current of the magnetorheological valve is 2.5 A. The addition of current to the magnetorheological valve leads to a rapid increase in both actuator speed and acceleration, which then levels off. Similarly, the response time of the actuator decreases with an increasing current until it stabilizes. At a coil current of 1.5 A, the actuator achieves a speed of approximately 86 mm s−1, an acceleration of approximately 105 mm s−2, and a response time of approximately 7.15 s. Furthermore, the lifting position accuracy of the hydraulic lifting platform is ±1 mm.

An optimized complementary design based on trilayer graphene oxide and clay composites for three stimuli responsive actuator

To promote the practical development of actuators, it is urgent to develop new multi-responsive actuators. Asymmetric bilayer actuators were reported to be dual-responsive, which can be driven by humidity and light. However, it remains a great challenge to develop rational designs for fabricating high-performance multi-responsive actuators. Here, we propose an optimized complementary design for three stimuli responsive actuators that can respond to multiple organic vapors, humidity, and light. The actuator is designed to achieve complementarity through a trilayer construction with graphene oxide (GO) in the middle layer, and with montmorillonite (MMT) and biaxially-oriented polypropylene (BOPP) on both sides, utilizing their hygroscopicity, organic vapor absorption, light absorption and thermal expansion capabilities. The maximum bending curvatures of the actuator under humidity, organic vapor and light stimulation conditions are 6.60 cm-1, 4.12 cm-1 and 2.59 cm-1 respectively, which are better than that of mixed GO-MMT/BOPP bilayer actuators. To reveal the kinetics of actuation, the actuation speed, temperature and mass during the deformation process are studied, which behave in sync with the bending curvature. Finally, a wireless intelligent claw and an organic-vapor leakage alarm are demonstrated based on these actuators. This study will provide an enlightening reference for multi-responsive actuators and promote their practical development.

Knotted Artificial Muscles for Bio-Mimetic Actuation under Deepwater.

Muscles featuring high frequency and high stroke linear actuation are essential for animals to achieve superior maneuverability, agility, and environmental adaptability. Artificial muscles are yet to match their biological counterparts, due to inferior actuation speed, magnitude, mode, or adaptability. Inspired by the hierarchical structure of natural muscles, artificial muscles are created that are powerful, responsive, robust, and adaptable. The artificial muscles consist of knots braided from 3D printed liquid crystal elastomer fibers and thin heating threads. The unique hierarchical, braided knot structure offers amplified linear stroke, force rate, and damage-tolerance, as verified by both numerical simulations and experiments. In particular, the square knotted artificial muscle shows reliable cycles of actuation at 1Hz in 3000m depth underwater. Potential application is demonstrated by propelling a model boat. Looking ahead, the knotted artificial muscles can empower novel biomedical devices and soft robots to explore various environments, from inside human body to the mysterious deepsea.

Single-Material Solvent-Driven Polydimethylsiloxane Sponge Bending Actuators.

Soft robots mimic the agility of living organisms without rigid joints and muscles. Continuum bending (CB) is one type of motion living organisms can display. CB can be achieved using pneumatic, electroactive, or thermal actuators prepared by casting an active layer on a passive layer. The corresponding input actuates only the active layer in the assembly resulting in the bending of the structure. These two different layers must be laminated well during manufacturing. However, the formed bilayer can still delaminate later, and the detachment hampers the actuator's reversible, long-time use. An approach to creating a single material bending actuator was previously reported, for which spatial gradient swelling was used. This authentic approach allows a single material to be manufactured as a bending actuator, allowing easy access to such actuators without lamination. In this study, we show spatial porosity differences in the sponges of polydimethylsiloxane (PDMS) (a common material in soft robotics) can be used to create the required anisotropy for bending. The spongy polymers are manufactured through table sugar templates and actuated by (organic) solvent absorption/desorption. This enables some versatility in the mechanical properties, shape, actuation force, and actuation speed. The one-material system's straightforward production and seamless nature are advantageous for reversible and repetitive bending. This simple method can further be developed in hydrogels and polymers for soft robotics and functional materials.

Photothermal Actuation of Thick 3D-Printed Liquid Crystalline Elastomer Nanocomposites.

Liquid crystalline elastomers (LCEs) are stimuli-responsive materials that transduce an input energy into a mechanical response. LCE composites prepared with photothermal agents, such as nanoinclusions, are a means to realize wireless, remote, and local control of deformation with light. Amongst photothermal agents, gold nanorods (AuNRs) are highly efficient converters when the irradiation wavelength matches the longitudinal surface plasmon resonance (LSPR) of the AuNRs. However, AuNR aggregation broadens the LSPR which also reduces photothermal efficiency. Here, the surface chemistry of AuNRs is engineered via a well-controlled two-step ligand exchange with a monofunctional poly(ethylene glycol) (PEG) thiol that greatly improves the dispersion of AuNRs in LCEs. Accordingly, LCE-AuNR nanocomposites with very low PEG-AuNR content (0.01wt%) prepared by 3D printing are shown to be highly efficient photothermal actuators with rapid response (>60%strains-1) upon irradiation with near-infrared (NIR; 808nm) light. Because of the excellent dispersion of PEG-AuNR within the LCE, unabsorbed NIR light transmits through the nanocomposites and can actuate a series of samples. Further, the dispersion also allows for the optical deformation of millimeter-thick 3D printed structures without sacrificing actuation speed. The realization of well-dispersed nanoinclusions to maximize the stimulus-response of LCEs can benefit functional implementation in soft robotics or medical devices.

2-DOF Woven Tube Plane Surface Soft Actuator Using Extensional Pneumatic Artificial Muscle

Soft actuators, designed for fragile item conveyance and navigation in complex environments, have garnered recent attention. This study proposes a cost-effective soft actuator, created by weaving tubes into twill patterns, capable of transportation and movement. The actuator achieves this by inducing traveling waves on its upper and lower surfaces through sequential pressurization of tubes. Notably, its fabrication does not require specialized molds, contributing to cost efficiency. The single actuator generates traveling waves with two degrees of freedom. Conventional silicone tube-based actuators demonstrate slow transport speeds (3.5 mm/s). To address this, this study replaced silicone tubes with pneumatic artificial muscles, enhancing overall body deformation and actuator speed. Experiments involving both extensional and contractional artificial muscles demonstrated that soft actuators with extensional artificial muscles significantly improved transportation and movement speed to 8.0 mm/s.

Scaling effects of manufacturing processes and actuation sources on control of remotely powered micro actuators

Over the past decade, remotely powered micro actuators have gained increased attention for biomedical and environmental remediation applications, owing to their ability to access confined regions and the nonintrusive nature of control. Recent studies focus on improving the functionality and versatility of micro actuators through the development of new fabrication and actuation techniques. However, there is a possibility that a limited understanding of the scaling impact of various physical principles governing design and control has affected the successful implementation of such devices in practical scenarios. Thus, the main focus of this review is to evaluate the most widely utilized manufacturing methods and remote actuation sources in light of various characteristics such as resolution, productivity, shape complexity, actuation speed, actuation mode, operating medium, and so on. State-of-the-art developments in each type of manufacturing and actuation are introduced and delineated. Finally, the limitations of current devices are reviewed, and the future direction to enable the full potential of this field is provided.

Multistable States and Snap-Through Instabilities in an Interconnected Dielectric Elastomer Actuators System

Abstract Snap-through instabilities in single and interconnected dielectric elastomer actuators have demonstrated their potential in enabling many functionalities such as large deformation, high-speed actuation, and enhanced flowrates in fluid pumps. However, the nonlinear nature of dielectric elastomers and the complex interplay of mechanics in interconnected inflated actuators, make the modeling of such systems challenging. In this paper, we present a methodology to analytically model the instabilities in a system of three interconnected homogeneous spherical dielectric elastomer actuators through graphical and numerical approaches. The simulation results reveal the presence of a locus of initial stable states that the interconnected actuators can achieve at zero voltage. In specific loading conditions, the system exhibits multiple stable states which can be cyclically transitioned between by selectively applying voltage to specific actuators. In other conditions, the system may undergo two successive instabilities when the voltage applied to a single actuator in the system is increased monotonically. These results retrieve existing experimental results theoretically for the first time and identify a new behavior of cascading instabilities in inflated dielectric elastomer actuators. We hope this work will pave the way for programmable design of multistable systems to unravel new capabilities in dielectric elastomer actuators and soft robotics.