Shape Memory Actuators Research Articles

Catalyst-free synthesis of low-temperature thermally actuated shape memory polyurethanes with modified biobased plasticizers.

Recent years have seen research into developing specific application-based materials with particular components. Bio-based polyurethanes (PUs) with self-tightening effect through shape recovery at low temperature have been designed from sesame oil-based plasticizer (HSSO). Without using a catalyst, the produced plasticizer was used to create PU samples. In contrast, orcein-based PU has been created both with and without HSSO. The prepared samples have been analyzed through instrumental as well as chemical analyses for surface chemistry, thermal stability and morphology. The gel content and water absorption capacity of HSSO based PU samples has been observed to be 99.27% and 14.94%, respectively. Shape memory study of the PU samples revealed that HSSO-based PU showed fast shape recovery at 60 °C with shape recovery rate (R r) and shape fixing rate (R f) of 94.44% and 5%, respectively, in 150 seconds, whereas at 36 °C the sample showed 85% R r in 15 minutes with 93.1196 N force and 52.78% R r without force. Low-temperature thermal actuation and high water uptake highlight the prepared samples as suitable candidates for self-tightening structures in textile and biomedical fields.

Accurate sensorless displacement control based on the electrical resistance of the shape memory actuator

This paper aims to implement the controllable deformation of a structure using Shape Memory Alloys (SMA) actuators. A sensorless displacement estimation method is proposed. This method is tested on a prototype composed of a disc, beams, and SMA actuators. By measuring the variation of electrical resistivity in SMA springs, as a feedback signal in the closed-loop position control, the surface displacement is obtained without any external displacement sensor. The proposed method is validated by comparing the displacement values estimated by the electrical resistivity measurement with those measured by a laser sensor. The estimated displacement and the measured displacement follow the reference displacement with steady-state errors, respectively of 1.14% and 0.42%.

Magnetically Actuated Shape Memory Polymers for On-Demand Drug Delivery.

Repeated use of intravenous infusions to deliver drugs can cause nerve damage, pain, and infection. There is an unmet need for a drug delivery method that administers drugs on demand for prolonged use. Here, we developed magnetically responsive shape memory polymers (SMPs) to enhance control over drug release. Iron oxide magnetic nanoparticles (mnps) were synthesized and incorporated into previously developed SMPs to enable magnetically induced shape memory effects that can be activated remotely via the application of an alternating magnetic field. These materials were tested for their shape memory properties (dynamic mechanical analysis), cytocompatibility (3T3 fibroblast viability), and tunable drug delivery rates (UV–VIS to evaluate the release of incorporated doxorubicin, 6-mercaptopurine, and/or rhodamine). All polymer composites had >75% cytocompatibility over 72 h. Altering the polymer chemistry and mnp content provided methods to tune drug release. Namely, linear polymers with higher mnp content had faster drug release. Highly cross-linked polymer networks with lower mnp content slowed drug release. Shape memory properties and polymer/drug interactions provided additional variables to tune drug delivery rates. Polymers that were fixed in a strained secondary shape had a slower release rate compared with unstrained polymers, and hydrophobic drugs were released more slowly than hydrophilic drugs. Using these design principles, a single material with gradient chemistry and dual drug loading was synthesized, which provided a unique mechanism to deliver two drugs from a single scaffold with distinct delivery profiles. This system could be employed in future work to provide controlled release of selected drug combinations with enhanced control over release as compared with previous approaches.

Thermal actuation shape memory of ultra-high-molecular-weight polyethylene (UHMWPE) with molecular orientation

• Preparation of shape memory UHMWPE/HDPE samples with molecular orientation. • Cyclic tensile shape memory property of UHMWPE/HDPE samples. • Movement and evolution of chain segments in shape-changing process. Conventional compression-molding method cannot illustrate the effect of molecular orientation on shape memory property. Herein, high-density polyethylene (HDPE) is incorporated into ultra-high-molecular-weight polyethylene (UHMWPE) to prepare samples with molecular orientation by the single screw extrusion and traction machine. Interestingly, the shape recovery rate (R r ) of samples with the same tensile direction as molecular orientation decreases from 66.13% to 60.71% with the traction rate, while the R r of samples with tensile direction perpendicular to molecular orientation exhibits an opposite tendency in cyclic tensile shape memory tests. Meanwhile, the shape fixation rate of these two samples does not change significantly. In addition, the possible mechanism of thermal actuation shape memory of samples with molecular orientation is interpreted.

The Shape Memory Properties and Actuation Performances of 4D Printing Poly (Ether-Ether-Ketone).

Shape-memory polymers (SMPs) have gradually emerged in the mechanism and biomedical fields and facilitate the upgrading of industrial mechanisms and the breakthrough of technical bottlenecks. However, most of the SMPs are infeasible in harsh environments, such as aerospace, due to the low glass transition temperature. There are still some works that remain in creating truly portable or non-contacting actuators that can match the performances and functions of traditional metal structures. Polyether-ether-ketone (PEEK) with a high glass transition temperature of 143 °C is endowed with outstanding high-temperature resistance and radiation-resistant properties and shape memory behavior. Thus, we explore the shape-memory properties and actuation performances of high-temperature PEEK in bending behaviors. The shape-recovery ratio, actuation speed and force under different programming conditions and structure parameters are summarized to complete the actuation capacities. Meanwhile, a metallic ball transported by shape-memory PEEK and deployed drag sail with thermo-responsive composite joints were shown to verify the potential in aerospace.

Production and performance of a low temperature shape-memory actuator based on twisted-coiled spring mechanics

Demand for new types of actuators continues to grow, and novel approaches have been made possible by the advent of new materials and fabrication strategies. Self-powered actuators have attracted significant attention owing to their ability to be driven by elements in ambient environments. This type of actuator can be used in flexible strain sensors, artificial muscles, soft robotics, and smart breathing textiles. However, petrochemical-based polymers are generally environmentally unfriendly and cause ecological problems. The use of biodegradable polymers is one of the preferred solutions to ecological problems. Polylactic acid is a biodegradable and biocompatible polymer with a high potential. In this study, nanoclay reinforced polylactic acid/thermoplastic polyurethane was used as a precursor. The yarn that was produced was highly twisted. The twisted yarn was then shaped into a coiled structure via mandrel annealing. An apparatus was designed to investigate the thermal actuation behavior of twisted-coiled yarn in an isometric state. The blocked force and free stroke were calculated in an isometric state by using linear material equations. The thermal actuation behavior of the twisted-coiled yarn was also studied in the isotonic state. This precursor exhibited a considerable two-way shape-memory effect in a twisted-coiled structure. It also showed a significant reversible contraction stroke within the low temperature range. The theoretical stroke was determined using two different models: the force–stroke equation and spring mechanics. The theoretical results were compared with the experimental results, which revealed acceptable agreement between the theoretical and experimental values.

Design and fabrication of fiber mesh actuators

Soft actuator performance can be tuned by chemistry or mechanical manipulation, but this adjustability is limited especially in view of their growing technological relevance. Inspired from textile engineering, we designed and fabricated fiber mesh actuators and introduced new features like anisotropic behavior and soft-tissue like elastic deformability. Design criteria for the meshes are the formation of fiber bundles, the angle between fiber bundles in different stacked layers and covalent crosslinks forming within and between fibers at their interfacial contact areas. Through crosslinking the interfiber bond strength increased from a bond transmitting neither axial nor rotational loads (pin joint) to a bond strength capable of both (welded joint). For non-linear elastic stiffening, stacked fiber bundles with four embracing fibers were created forming microstructural rhombus shapes. Loading the rhombus diagonally allowed generation of “soft tissue”-like mechanics. By adjustment of stacking angles, the point of strong increase in stress is tuned. While the highest stresses are observed in aligned and crosslinked fiber mats along the direction of the fiber, the strongest shape-memory actuation behavior is found in randomly oriented fiber mats. Fiber mesh actuators controlled by temperature are of high significance as soft robot skins and as for active patches supporting tissue regeneration.

Harvesting low-grade heat by coupling regenerative shape-memory actuator and piezoelectric generator

Low-grade thermal energy is ubiquitous in nature with different scales. Being concise in nature, solid-state energy conversion technologies based on ferroic materials experienced fast development over the years, yet their efficiencies are still not as competitive as the conventional Rankine cycle. To improve the efficiency of solid-state generators, this study proposed a novel composite device leveraging the strain coupling between the regenerative shape-memory actuator and the piezoelectric generator. Taking the realistic losses into account, our experimentally validated numerical model indicated that the new concept has 14.4% efficiency, equivalent to 57% second-law efficiency, when the length and cross-sectional area of the shape-memory actuator and the piezoelectric generator is properly matched. It was also found that relaxor ferroelectric materials are more efficient than the baseline PZT ceramics. The impact and design guideline of other parameters were also investigated, including the utilization factor, transmission factor, and transition temperature. The proposed concept provides a new path for harvesting the low-grade thermal energy, and could potentially be integrated with caloric cooling counterparts to provide power and refrigeration simultaneously as a next-generation co-generation and tri-generation technology.

Active Wearable Compression with Shape Memory Actuators for Treating Chronic Edema

Active Wearable Compression with Shape Memory Actuators for Treating Chronic Edema

Liquid Crystal Elastomer with Integrated Soft Thermoelectrics for Shape Memory Actuation and Energy Harvesting.

Liquid crystal elastomers (LCEs) have attracted tremendous interest as actuators for soft robotics due to their mechanical and shape memory properties. However, LCE actuators typically respond to thermal stimulation through active Joule heating and passive cooling, which make them difficult to control. In this work, LCEs are combined with soft, stretchable thermoelectrics to create transducers capable of electrically controlled actuation, active cooling, and thermal-to-electrical energy conversion. The thermoelectric layers are composed of semiconductors embedded within a 3D printed elastomer matrix and wired together with eutectic gallium-indium (EGaIn) liquid metal interconnects. This layer is covered on both sides with LCE, which alternately heats and cools to achieve cyclical bending actuation in response to voltage-controlled Peltier activation. Moreover, the thermoelectric layer can harvest energy from thermal gradients between the two LCE layers through the Seebeck effect, allowing for regenerative energy harvesting. As demonstrations, first, closed-loop control of the transducer is performed to rapidly track a changing actuator position. Second, a soft robotic walker that is capable of walking toward a heat source and harvesting energy is introduced. Lastly, phototropic-inspired autonomous deflection of the limbs toward a heat source is shown, demonstrating an additional method to increase energy recuperation efficiency for soft systems.

Heusler‐Based Cylindrical Micro‐ and Nanowires

Wire‐shaped materials are ideal form for rapid, cheap, and scalable production and application. Herein, the most recent results on Heusler‐based cylindrical micro‐ and nanowires are reviewed, emphasizing spintronic, magnetocaloric, and shape memory applications. Herein, how the Heusler alloy's composition can be tuned to achieve a material with repeatable magnetic and mechanical properties is showed. A wide range of possible Heusler compositions allows us to tune the structural transformation temperatures by alloying the original composition with a few percent of metalloid. Wire shape brings additional effect—well‐defined easy axis change due to a structural transformation that enhances the magnetocaloric effect at low‐magnetic fields. The unique wire's shape properties can be downscaled to nanoscale range by electrodeposition. It is showed that such a method allows the production of a highly ordered array of nanowires. As a result, the magnetocaloric effect of an array of nanowires can be one order higher than that obtained by other production methods, where the nanowires are oriented randomly. Finally, some possible applications of shape memory or magnetocaloric actuators and the possibility of implementing high spin polarization for the construction of spintronic devices are showed.

Rapidly NIR-responsive electrospun shape memory actuators with MXene/CNCs hybrids

A novel electrospun shape memory actuator featuring rapid near-infrared (NIR) responsiveness was fabricated by depositing Ti3C2Tx (MXene)/cellulose nanocrystals (CNCs) hybrids onto fibril mats. The CNCs significantly enhanced the mechanics and photothermal effect of the composite mats. The plateau temperature under the NIR exposure was up to 117 °C within 25 s. By optimizing hybrid compositions, the actuator was capable of lifting an object with the weight of 265 times by height of 80% extension within 10 s. The findings may greatly benefit exploration of electrospun intelligent polymers to actuate in remote manner.

A Subspace-Inclusive Sampling Method for the Computational Design of Compositionally Graded Alloys

Abstract Compositionally graded alloys, a subclass of functionally graded materials (FGMs), utilize localized variations in composition with a single metal part to achieve higher performance than traditional single material parts. In previous work [Kirk, T., Galvan, E., Malak, R., and Arroyave, R., 2018, “Computational Design of Gradient Paths in Additively Manufactured Functionally Graded Materials,” J. Mech. Des., 140, p. 111410. 10.1115/1.4040816], the authors presented a computational design methodology that avoids common issues which limit a gradient alloy’s feasibility, such as deleterious phases, and optimizes for performance objectives. However, the previous methodology only samples the interior of a composition space, meaning designed gradients must include all elements in the space throughout the gradient. Because even small amounts of additional alloying elements can introduce new deleterious phases, this characteristic often neglects potentially simpler solutions to otherwise unsolvable problems and, consequently, discourages the addition of new elements to the state space. The present work improves upon the previous methodology by introducing a sampling method that includes subspaces with fewer elements in the design search. The new method samples within an artificially expanded form of the state space and projects samples outside the true region to the nearest true subspace. This method is evaluated first by observing the sample distribution in each subspace of a 3D, 4D, and 5D state space. Next, a parametric study in a synthetic 3D problem compares the performance of the new sampling scheme to the previous methodology. Lastly, the updated methodology is applied to design a gradient from stainless steel to equiatomic NiTi that has practical uses such as embedded shape memory actuation and for which the previous methodology fails to find a feasible path.

Recent Advances in Stimuli-Responsive DNA-Based Hydrogels.

Stimuli-responsive DNA-based hydrogels are attracting growing interest because of their smart responsiveness, excellent biocompatibility, regulated biodegradability, and programmable design properties. Integration of reconfigurable DNA architectures and switchable supramolecular moieties (as cross-linkers) in hydrogels by responding to external stimuli provides an ideal approach for the reversible tuning structural and mechanical properties of the hydrogels, which can be exploited in the development of intelligent DNA-based materials. This review highlights recent advances in the design of responsive pure DNA hydrogels, DNA-polymer hybrid hydrogels, and autonomous DNA-based hydrogels with transient behaviors. A variety of chemically and physically triggered DNA-based stimuli-responsive hydrogels and their versatile applications in biosensing, biocatalysis, cell culture and separation, drug delivery, shape memory, self-healing, and robotic actuators are summarized. Finally, we address the key challenges that the field will face in the coming years, and future prospects are identified.

Integrated Temperature and Position Sensors in a Shape-Memory Driven Soft Actuator for Closed-Loop Control

Soft actuators are a promising option for the advancing fields of human-machine interaction and dexterous robots in complex environments. Shape memory alloy wire actuators can be integrated into fiber rubber composites for highly deformable structures. For autonomous, closed-loop control of such systems, additional integrated sensors are necessary. In this work, a soft actuator is presented that incorporates fiber-based actuators and sensors to monitor both deformation and temperature. The soft actuator showed considerable deformation around two solid body joints, which was then compared to the sensor signals, and their correlation was analyzed. Both, the actuator as well as the sensor materials were processed by braiding and tailored fiber placement before molding with silicone rubber. Finally, the novel fiber-rubber composite material was used to implement closed-loop control of the actuator with a maximum error of 0.5°.

A novel smart composite: from self-powered sensors to multi-responsive shape memory actuators

A novel smart composite shows tunable functional applications from self-powered hydrogel sensors to multi-responsive shape memory actuators.

Non-woven shape-memory polymer blend actuators

The hierarchical design approach provides various opportunities to adjust the structural performance of polymer materials. Electrospinning processing techniques give access to molecular orientation as a design parameter, which we consider here in view of the shape-memory actuation performance. The aim of this work is to investigate how the reversible strain varepsilon^{prime}_{{{text{rev}}}} can be affected by a morphology change from a bulk material to an electrospun mesh. varepsilon^{prime}_{{{text{rev}}}} could be increased from 5.5 ± 0.5% to 15 ± 1.8% for a blend from a multiblock copolymer with poly(ε-caprolactone) (PCL) and poly(L-lactide) (PLLA) segments with oligo(D-lactide) (ODLA). This study demonstrates an effective design approach for enhancing soft actuator performance, which can be broadly applied in soft robotics and medicine.Graphic abstract

Magnetically Levitated Linear Drive Using an Active Gravity Compensation Based on Hybrid Shape Memory Actuators

This article presents a magnetically levitated linear drive for precision engineering with integrated active gravity compensation. The gravity compensation combines permanent magnets with novel hybrid shape memory actuators. The magnetically levitated system uses a homopolar linear drive as well as nine reluctance actuators to levitate a passive armature with no mechanical connection between the stator and armature. The mechanical design is based on geometrically decoupled horizontal and vertical guidance axes. A decoupled Cartesian control scheme is developed, which compensates for the actuator and system nonlinearities in order to achieve high bandwidths. The active gravity compensation adjusts the air gap between the permanent magnets and armature to the required gravity compensation force. This helps to keep the dissipated power and temperature increase in the magnetically levitated drive low (and thus, system precision high). To avoid the generation of particles in clean room applications, the actuators positioning the permanent magnets are solely based on solid-state effects: thermal shape memory alloys actively generate motion, while passive magnetic shape memory alloy elements realize a multistable behavior based on their internal twinning force. The resulting actuator is compact, energy efficient, and abrasion-free. The implementation of the gravity compensation in the levitated linear drive is verified and the functionality of the system is demonstrated in experiments.

Analytical model of shape memory alloy embedded smart beam, under actuated condition

Vibration characteristics of actuated Shape Memory Alloy (SMA) embedded smart composite beam are studied and it is extended to reduce the impact of resonance in cantilever beams. Smart composite beam with SMA embedded at neutral layer is studied for its vibrational characteristics under martensite and austenite conditions. The smart beam is developed as a analytical model incorporating the change in Young's modulus and damping factor under martensite and austenite conditions of SMA. The variation of natural frequency and damping are evaluated and verified with experimentation at different volume fraction of SMA. The thermo elastic nature of SMA and GFRP incorporated in the numerical model depicts the shift in natural frequency of 10% and reduction in magnification factor of 50% under actuation conditions. The frequency response of the smart beam depicts the capability of SMA in active vibration control and improvement of the structural health of composite beam. The thermo mechanical analytical model derived can be utilized to optimize the volume fraction of SMA to be embedded. The study can be extended to optimize actuation current to minimize the effect of resonance.

Synthesis and Properties of Multistimuli Responsive Shape Memory Polyurethane Bioinspired from α‐Keratin Hair

AbstractHerein, a multistimuli responsive shape memory polyurethane (multiresponsive SMPU) containing disulfide bonds (–S–S–), bioinspired from α‐keratin hair component, was synthesized using a two‐step reaction and activated via four‐varied stimuli, including water, heat, redox agent, and UV‐light. First, the prepolymer was prepared by the reaction between the comonomers of hexamethylene diisocyanate and polycaprolactone‐2000, and once ended, 1,4‐butylene glycol and 3‐mercapto‐1,2‐propanediol (C3H8O2S) have been appended to the prepolymer to initiate the chain‐extension reaction. The chemical structure and properties of the multi‐responsive SMPU were decoded by FT‐IR, XRD, DSC, and Raman spectra. The correlation between microstructure, dynamic‐mechanical‐analysis, and shape‐memory properties of the multi‐responsive SMPU were systemically discussed. As a primary clue, the opening/closing of H‐bonds and disulfide bonds approved that they dictated the shape memory performance. The clear‐cut results clarified that the multiresponsive SMPU endowed composition‐dependent stress relaxation, and its reversible variation of storage modulus was examined by changing temperature. Owing to such molecular switches, the strip pieces were able to represent a multistimuli responsive shape memory effect —induced by water, heat, redox agent (NaHSO3‐H2O2 solutions), and UV‐light. This research paves a feasible road to prepare a novel multistimuli responsive shape memory actuator with high potential features in flexible sensor and robotic applications.