Thermal Actuator Research Articles

Wet-Spinning Carbon Nanotube/Shape Memory Polymer Composite Fibers with High Actuation Stress and Predesigned Shape Change.

Actuators based on shape memory polymers and composites incorporating nanomaterial additives have been extensively studied; achieving both high output stress and precise shape change by low-cost, scalable methods is a long-term-desired yet challenging task. Here, conventional polymers (polyurea) and carbon nanotube (CNT) fillers are combined to fabricate reinforced composite fibers with exceptional actuation performance, by a wet-spinning method amenable for continuous production. It is found that a thermal-induced shrinkage step could obtain densified strong fibers, and the presence of CNTs effectively promotes the tensile orientation of polymer molecular chains, leading to much improved mechanical properties. Consequently, the CNT/ polyurea composite fibers exhibit stresses as high as 33MPa within 0.36 s during thermal actuation, and stresses up to 22MPa upon electrical stimulation enabled by the built-in conductive CNT networks. Utilizing the flexible thin fibers, various shape change behavior are also demonstrated including the conversion between different structures/curvatures, and recovery of predefined simple patterns. This high-performance composite fibers, capable of both thermal and electrical actuation and produced by low-cost materials and fabrication process, may find many potential applications in wearable devices, robotics, and biomedical areas.

Fast, variable stiffness-induced braided coiled artificial muscles

Biomimetic actuation technologies with high muscle strokes, cycle rates, and work capacities are necessary for robotic systems. We present a muscle type that operates based on changes in muscle stiffness caused by volume expansion. This muscle is created by coiling a mechanically strong braid, in which an elastomer hollow tube is adhesively attached inside. We show that the muscle reversibly contracts by 47.3% when driven by an oscillating input air pressure of 120 kilopascals at 10 Hz. It generates a maximum power density of 3.0 W/g and demonstrates a mechanical contractile efficiency of 74%. The muscle's low-pressure operation allowed for portable, thermal pneumatical actuation. Moreover, the muscle demonstrated bipolar actuation, wherein internal pressure leads to muscle length expansion if the initial muscle length is compressed and contraction if the muscle is not compressed. Modeling indicates that muscle expansion significantly alters its stiffness, which causes muscle actuation. We demonstrate the utility of BCMs for fast running and climbing robots.

Thermal Actuator Identification and Control for Thermomechanical Real-Time Cyber–Physical Testing

Thermal Actuator Identification and Control for Thermomechanical Real-Time Cyber–Physical Testing

Strategy for Compensation of Thermally Induced Displacements in Machine Structures Using Distributed Temperature Field Control

Thermal deformation is a major source of machining errors in modern machine tools. In addition to optimising the machine structure, correcting the axis position values in the numerical control is a common measure to reduce these errors. Another possibility is to directly influence the temperature field of the machine tool in the process, which requires a complex thermo-elastic modelling approach as well as appropriate thermal actuation and measurement capabilities. This paper presents a strategy for controlling the temperature field based on the eigenmodes of the thermal system. The various aspects of the concept are explained using a finite element model of an exemplary structural component. The basis is the modal analysis of the thermal system, which allows the temperature field to be described by independent discrete states. In addition to the placement of thermal sensors and actuators, this work focuses on the design of a suitable control approach. Transient simulation results are used to clearly demonstrate the performance of this method.

Fabrication and Characterization of Graphene-Mesoporous Carbon-Nickel-Poly(Vinyl Alcohol)-Coated Mandrel-Coiled TCPFLNR Artificial Muscle.

This study investigates the performance enhancement of mandrel-coiled twisted and coiled polymer fibers with a nichrome heater (TCPFLNR) by coating with a solution of graphene-mesoporous carbon-nickel-polyvinyl alcohol. The coating process involved a one-pot synthesis utilizing graphene powder, Ni nanoparticles, mesoporous carbon, and PVA as a binding agent. The coating was performed by manually shaking the TCPFLNR and the subsequent annealing process, which results in improved thermal conductivity and actuation behavior of the TCPFLNR. Experimental results on a 60 mm long actuator demonstrated significant enhancements in actuation displacement and actuation strain (20% to 42%) under various loads with an input current of 0.27 A/power 2.16 W. The blocked stress is ~10 MPa under this 2.16 W power input and the maximum strain is 48% at optimum load of 1.4 MPa. The observed actuation strain correlated directly with the input power. The coated TCPFLNR exhibited better thermal contacts, facilitating enhanced heat transfer, and reducing power consumption by 6% to 9% compared to non-coated actuators. It was found that the nanomaterial coating helps the TCP actuator to be reliable for more than 75,000 actuation cycles at 0.1 Hz in air due to improved thermal conductivity. These findings highlight the potential for further research to optimize electrothermally operated TCP actuators and unlock advancements in this field.

Towards Implantable Artificial Muscles: Epoxy Based Bilayer Thermal Actuators with Ambient Activation Temperatures

AbstractThe development of soft and biocompatible actuators is a current challenge in the fast‐growing field of soft robotics. A variety of shape‐memory and thermal actuators are currently being explored for application in these areas, especially to produce actuators that can operate at biocompatible temperatures for in vivo applications. Here we detail the synthesis of epoxy based materials, a class of materials not fully explored for soft actuators. By careful tuning of polymer composition by variation of monomer feed ratios, we show tuneable glass transition temperatures (Tgs) of these materials. Ambient temperature (30–60 °C) thermal actuators were then realised by casting of bilayers of epoxy resins with varying Tgs. Differences in thermal expansion when the Tg was reached lead to actuation in a specific direction. With optimisation, actuation in both the external‐heated and joule‐heated NiChrome‐epoxy actuators were able to occur at physiological temperatures (36–40 °C), previously unreported in the literature. For the joule‐heated NiChrome–epoxy actuators surface temperatures remained under 40 °C during actuation at low operating voltages (<2 V). Higher force and power output could be achieved in a repeatable fashion at higher driving voltages without loss of displacement, but also leading to higher surface temperatures. These materials and the actuation approach present an exciting opportunity to develop future implantable actuators to solve significant challenges related to quality of life and an ageing population.

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.

An integrated, multi-functional intrinsic responsive foldable protective layer for space station solar panels

The functionalized foldable protective layer proficiently alleviates the potential detriment endured by space station solar panels due to prolonged and uninterrupted exposure to the environment. Soft actuators demonstrate significant value in foldable protective layers due to their intrinsic response characteristics and programmability. However, current soft actuators still struggle to meet the requirements of work stability and scene applicability in the space environment. A PMT (Polydimethylsiloxane (PDMS)/Multi-walled carbon nanotubes (MWCNTs)-Thermoplastic polyimides (TPI)) soft actuator was proposed to address this issue in this paper. The PMT soft actuators exhibit strong interlayer interfacial adhesion, stable physical and chemical performance, and excellent mechanical performance. Notably, the PMT soft actuators can sense thermal, electric fields, infrared light, and mechanical force simultaneously with only one active layer. Furthermore, analytical models were calibrated to provide predictions, and the thermal and photoelectric actuation processes of PMT soft actuators with different structures and application scenes were predicted accurately. The experimental results validated the application value of PMT actuators as foldable protective layers for space station solar panels. The preparation, testing, application, and model construction of the PMT system provide new research ideas and development opportunities for the practical application of functional polymers in intelligent sensing and actuation systems.

A compound twisted and coiled actuators with payload-insensitive untwisting characteristics

Compound Twisted and Coiled Actuators (CTCAs) are promising candidates for artificial muscles due to their outstanding competences in producing large output forces and long strokes. However, as their thermal actuation performance depends on not only the driving temperature, but also the spandex deformation caused by the payload, it is rather difficult to obtain accurate actuation models for the conventional CTCAs and thus seriously restricts their control performance and application development. To tackle such difficulties, a CTCA with a novel strategic fabrication method is proposed in this work. Motivated by the kinetostatic performance of a stiffening spring with a high preload, the fiber draw ratio of the CTCA is significantly increased through pre-stretching the fiber during the fabrication process, which makes the thermal actuation performance of the CTCA insensitive to the external payload. The effectiveness of the proposed payload-insensitive characteristics of the CTCA is validated through experimental results, which shows that the CTCA has almost the same untwisting strain–temperature curves under different payloads. Consequently, a simplified yet accurate actuation model is obtained such that the displacement of the CTCA is mainly determined by the driving temperature within a meaningful range of payloads. Experiments are conducted to evaluate the accuracy of the actuation model and the model error is less than 7.6%. Such a simplified actuation model is implemented for the pose control of a 2-DOF CTCA-driven continuum robot joint module, in which the average steady-state error of the bending angle is less than 1.0∘.

Modular multi-degree-of-freedom soft origami robots with reprogrammable electrothermal actuation

Soft robots often draw inspiration from nature to navigate different environments. Although the inching motion and crawling motion of caterpillars have been widely studied in the design of soft robots, the steering motion with local bending control remains challenging. To address this challenge, we explore modular origami units which constitute building blocks for mimicking the segmented caterpillar body. Based on this concept, we report a modular soft Kresling origami crawling robot enabled by electrothermal actuation. A compact and lightweight Kresling structure is designed, fabricated, and characterized with integrated thermal bimorph actuators consisting of liquid crystal elastomer and polyimide layers. With the modular design and reprogrammable actuation, a multiunit caterpillar-inspired soft robot composed of both active units and passive units is developed for bidirectional locomotion and steering locomotion with precise curvature control. We demonstrate the modular design of the Kresling origami robot with an active robotic module picking up cargo and assembling with another robotic module to achieve a steering function. The concept of modular soft robots can provide insight into future soft robots that can grow, repair, and enhance functionality.

Global joint position regulation of a class of torque-driven robot manipulators within actuators prescribed safe operating torque–speed area

This paper addresses the joint position regulation of a class of n degrees of freedom torque-driven robot manipulators under a novel practical motivated control objective: Simultaneous Regulation & Prescribed Safe Operating Area (SR&PSOA). More specifically, torque–speed characteristic safe limit functions associated to each actuator are taking into account to ensure that besides of global joint positioning, the requested torque control inputs to robot actuators remain inside user prescribed safe operating torque–speed areas, so avoiding thermal and physical actuator damages. A static smooth nonlinear controller is proposed to achieve global position regulation under prescribed speed-depending bound torque input. Numerical simulations and experimental results are included to illustrate the effectiveness of the proposed controller.

Design and analysis of novel microelectromechanical system based microgripper for manipulating microbiological species and micro objects

Design and analysis of a pair of novel microelectromechanical system (MEMS) microgrippers having single and multiple sets of jaws respectively, incorporated with displacement amplification mechanism and electrostatic force sensor is presented in this research work. The novelty of the proposed microgrippers is their unique and efficient designs, having the versatility of gripping ranges. The size of the microgripper is 3.2 × 3.7 mm2 in both cases and has one and three sets of jaws with gripping ranges of 30–64 µm, 175–200 µm, and 350–380 µm respectively. The three jaw sets are application-specific to the multicellular small organism, eukaryotic cells, and microcells, respectively. The microgrippers are being actuated by an optimized V-shaped thermal chevron actuator whose input is amplified by using a pantograph displacement amplification mechanism with an amplification factor of 54. The force applied to the gripping object is sensed by a rotary comb electrostatic force sensor. Special attention has been paid to the structural design, and detailed stress analysis has been discussed. This research work includes static, electrostatic, parametric, and electrothermal analysis carried out using Finite Element Method (FEM) techniques and analytical modeling. The results show that the operating voltage range of the devices is from 4 to 9.25 V, having a maximum operational temperature of 380 °C with a factor of safety of 1.026. The maximum capacitance change with the full operational range of microgrippers is 0.63 µf, having a capacitive sensitivity of 19.6 nf/µm. The microgrippers can be used as an instrument to grip not only the microbiological species but also can be beneficial to carry out manipulations at the micro-level like micro-assembly. The proposed microgripper has a substantial structural stability, has a high amplification factor and accommodates manipulation of a wide range of microorganism.

Highly efficient in crystallo energy transduction of light to work.

Various mechanical effects have been reported with molecular materials, yet organic crystals capable of multiple dynamic effects are rare, and at present, their performance is worse than some of the common actuators. Here, we report a confluence of different mechanical effects across three polymorphs of an organic crystal that can efficiently convert light into work. Upon photodimerization, acicular crystals of polymorph I display output work densities of about 0.06-3.94 kJ m-3, comparable to ceramic piezoelectric actuators. Prismatic crystals of the same form exhibit very high work densities of about 1.5-28.5 kJ m-3, values that are comparable to thermal actuators. Moreover, while crystals of polymorph II roll under the same conditions, crystals of polymorph III are not photochemically reactive; however, they are mechanically flexible. The results demonstrate that multiple and possibly combined mechanical effects can be anticipated even for a simple organic crystal.

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.

Droplet evaporation on curved surfaces: transients of internal and external transport phenomena

We explore the transient evolution of thermo-fluid-dynamics of evaporating sessile droplets over curved substrates in the liquid and gaseous domains. A computational model using the Arbitrary Lagrangian-Eulerian framework is adopted. The governing equations in both liquid and gaseous domains are solved in a fully coupled manner, considering coupled effects of evaporative cooling and heat advection due to bulk fluid motion. This bulk motion in the liquid domain is caused by natural advection due to thermal actuations such as thermal Marangoni flow and buoyancy-driven convection. For the gaseous domain, the additional effects of solutal convection (due to vapour-concentration variation), Stefan flow and interfacial viscous stresses are also considered. To depict a generalized role of substrate curvature, both concave and convex surfaces with curvatures over a wide range are studied. The surface wettability effects are also explored by varying the true contact angle of the droplets. Computational predictions on evaporation rate and internal flow field are validated against experimental results from literature. The interplay of wetting state and substrate curvature is noted to substantially affect the evaporation process and its thermo-fluidics. The convex curvature significantly augments internal advection while the same is weakened over concave substrates due to altered mass loss rate. Consequently, the duration of multi-vortex Marangoni flows in the development stages of evaporation and the advection in the external gaseous domain is markedly different for different curvatures. Further, on superhydrophobic curved surfaces, the effects of re-distributed evaporative fluxes play a major role. In such cases, the reduced mass flux over a large interfacial area near the periphery expedites the stable state Marangoni and external flow features.

Deformable Joule heating electrode based on hybrid layers of silver nanowires and carbon nanotubes and its application in a refreshable multi-cell Braille display.

Stretchable electrodes are an essential component in soft actuator systems. In particular, Joule heating electrodes (JHEs) are required for thermal actuation systems. A highly stretchable, patternable, and low-voltage operating JHE based on hybrid layers of silver nanowires (AgNWs) and carbon nanotubes (CNTs) is reported. The conductive layers were applied on a locally pre-strained bistable electroactive polymer (BSEP) membrane to form a wrinkled conductive surface with a low resistance of 300 Ω/sq, and subsequently patterned to a serpentine trace by laser engraving. The resistance of the resulting electrode remains nearly unchanged up to ~80-90% area strain. By applying a voltage of 7 - 9 V to the electrode, the temperature of the BSEP membrane increased to more than 60 °C, well above the polymer's phase transition temperature of 46 °C, thereby lowering its modulus by a factor of 103. An electronic Braille device based on the JHEs on a BSEP membrane was assembled with a diaphragm chamber. The electrode was patterned into 3 × 2 individually addressable pixels according to the standard U.S. Braille cell format. Through Joule heating of the pixels and local expansion of the BSEP membrane using a small pneumatic pressure, the pixels deformed out of the plane by over 0.5 mm to display specific Braille letters. The Braille content can be refreshed for 20,000 cycles at the same operating voltage.

Numerical Simulation and Sensitivity Analysis Using RSM on Natural Convective Heat Exchanger Containing Hybrid Nanofluids

This work presents a numerical analysis for exploring heat transfer phenomena in an enclosed cavity using magnetohydrodynamics natural convection. Because of the numerous real-world applications of nanofluids and hybrid nanofluids in industrial and thermal engineering developments, hybrid nanofluids are used as fluid mediums in the fluid field. A hexagonal-shaped heat exchanger is taken with two circular surfaces along the middle part. The upright circular surface acts as a homogeneous heat source, while the lower circular surface functions as a heat sink. The remaining portions of the adjacent walls are thermally insulated. The copper (Cu) and titanium dioxide (TiO2) nanoparticles are suspended into water to make a hybrid nanofluid. For solving the corresponding governing equations, the weighted-residual finite element method is applied. To explain the major outcomes, isotherms, streamlines, and many others 2D and 3D contour plots are involved graphically with a physical explanation for different magnitudes of significant parameters: Rayleigh number 103≤Ra≤106, Hartmann number 0≤Ha≤100, and nanoparticle volume fraction 0≤ϕ≤0.06. The novelty of this work is to apply response surface methodology on the natural convective hybrid nanofluid model, to visualize 2D and 3D effects, and to study the sensitivity of independent parameters on response function. Due to the outstanding thermal properties of the hybrid nanofluid, the addition of Cu and TiO2 nanoparticles into H2O develops the heat transfer rate to 35.85% rather than base fluid. Moreover, a larger magnitude of Ra and the accumulation of mixture nanoparticles result in the thermal actuation of a hybrid nanofluid. With greater magnetic impact, an opposite response is exhibited.

Bioinspired Dynamic Matrix Based on Developable Structure of MXene‐Cellulose Nanofibers (CNF) Soft Actuators

AbstractInspired by natural organisms, soft actuators can convert environmental stimuli into mechanical deformation, making them indispensable for applications in a variety of fields such as soft robotics. MXene, displaying exceptional attributes in conductivity, thermal efficiency, good dispersibility etc., has emerged as a preferred material for high‐performance thermal actuators. However, single actuators struggle to achieve complex tasks realized by intelligent robotic systems. Herein, a bioinspired dynamic matrix is presented utilizing the developable structure of MXene‐cellulose nanofibers (CNF) soft actuators. The inclusion of CNF considerably bolsters the mechanical properties of MXene. The MXene‐CNF film possessed a higher working strain range (≈14%) than pure MXene film (≈2%). The designed developable structures complete the actuating movements. The sensing layer integration with the actuating layer led to an extremely low touch detection limit (0.3 kPa) and expedited actuating‐feedback due to the interaction between contact charging and electrostatic induction. A responsive dynamic matrix containing 3 × 3 soft actuators, completed with a close‐looped sensing‐feedback function, is developed similar to the behavior of the mimosa plant. This mimosa‐inspired dynamic matrix is capable of identifying the source of touch stimuli and providing immediate feedback. This research broadens the potential for enhancing adaptability and intelligence of soft robotic system.

Thermal and Near-Infrared Light-Responsive Hydrogel Actuators with Spatiotemporally Developed Polypyrrole Patterns.

Conjugated polymers are commonly adopted to develop electro- and photoresponsive materials due to their superior electronic conductivity and phototothermal convertibility. However, they are usually homogeneously polymerized within the network, which makes their functionalities challenging to spatiotemporally modulate. In this work, we report a convenient and extensible method to develop polypyrrole patterns in a thermally responsive sodium alginate/poly(N-isopropylacrylamide) hydrogel. The polypyrrole pattern is developed by spatial photoreduction of Fe3+ ions into Fe2+ ions and subsequently initiating oxidation polymerization of pyrrole by the residual Fe3+ ions. During this process, carboxylate groups coordinated with Fe3+ ions are also sacrificed in a gradient manner along the thickness direction, and the resulting concentration gradients of the carboxylate group endow the hydrogel with thermal-responsive actuation. The polymerized polypyrrole also renders the hydrogels' prominent temperature-rising behaviors upon NIR light irradiation. By designing the PPy pattern, hydrogels can exhibit versatile actuating behaviors and execute mechanical works such as lifting objects. This method is convenient and can be extended to develop other conjugated polymers in hydrogel systems for versatile applications.

Direct ink writing of polymer matrix composite with carbon for driving a flexible thermoelectric actuator of shape memory polymer

Carbon nanomaterials and polymer composites that possess excellent electrical and mechanical properties for soft robots have been investigated over decades in multidisciplinary research fields. As considering the incorporation of carbon into polymer composite, several major concerns include the electrical conductivity, elastic module, and manufacturing flexibility that can enable them to satisfy various functions such as actuation and sensing. In this paper, we report a comprehensive study on how to design and fabricate a soft gripper based on carbon polymer matrix composite and multilayer structure through multistep processes of 3D printing on paper. Using commercial conductive additives of graphene and carbon nanotube with polydimethylsiloxane (PDMS), a polymer matrix composite (PMC) was prepared for the ink of direct ink writing (DIW) with percolation conductivity. Additionally, a shape memory polymer (SMP) was utilized for fused deposition modeling (FDM) of a thermal actuator. Incorporating the DIW of PMC with the FDM of SMP on paper, a soft gripper with four fingers was designed and fabricated to show remarkable thermoelectrical actuation for grasping a 50 mm-diameter 10 g-weight spherical ball. In the future, it would provide an alternative approach for the fields of science and engineering, such as nanomaterials and nanotechnology, space applications, sustainable soft robots, and flexible autonomous systems.