Shape Memory Actuators Research Articles

An intubation catheter integrated with flow sensors and smart actuators for characterizing airflow patterns in stenosed trachea: an objective guide for CAO management

Stenosis reduces the effective lumen area in the tracheal and bronchial segments of the airway anatomy. Loss in patency due to obstruction increases resistance to airflow; thus, severe narrowing is often associated with morbidity and mortality. Etiologies such as congenital tracheal stenosis, tracheomalacia, laryngeal and subglottic stenosis, atresia are few among the many pathologies causing major airway obstruction and respiratory distress. Diagnosis of such anomalies is usually based on clinical suspicion due to the non-specificity of the associated clinical symptoms. Visual assessment using conventional bronchoscopy or radiography images from CT scan for precisely locating obstruction site is highly subject to clinician’s expertise. Characterizing airflow patterns in stenosed airway calls for newer diagnostic tools that can effectively quantify changes in airflow due to construction sites. Our work presents a steerable intubation catheter that can quantitatively measure air velocity across various segments of the tracheobronchial tree. The catheter consists of a three-layer flexible printed circuit board integrated with micro-electro-mechanical system-based thermal flow sensors and a pair of sub-millimeter helical shape memory actuators. Flow distribution is measured in excised sheep tracheal tissues at 15, 30, 50, 65, and 80 l min−1 for normal and stenosed conditions. Even a 10% reduction in lumen area generated unique peaks corresponding to the obstruction site; thus, the catheter can locate stenosis at the precritical stage. For 50% tracheal obliteration, the sensor closest to stenosis showed a 2.4-fold increase in velocity when tested for reciprocating flows. Thus, flow rate scales quadratically with reducing cross-section area, contributing to increased airflow resistance.

Micrometer-sized electrically programmable shape-memory actuators for low-power microrobotics.

Shape-memory actuators allow machines ranging from robots to medical implants to hold their form without continuous power, a feature especially advantageous for situations where these devices are untethered and power is limited. Although previous work has demonstrated shape-memory actuators using polymers, alloys, and ceramics, the need for micrometer-scale electro-shape-memory actuators remains largely unmet, especially ones that can be driven by standard electronics (~1 volt). Here, we report on a new class of fast, high-curvature, low-voltage, reconfigurable, micrometer-scale shape-memory actuators. They function by the electrochemical oxidation/reduction of a platinum surface, creating a strain in the oxidized layer that causes bending. They bend to the smallest radius of curvature of any electrically controlled microactuator (~500 nanometers), are fast (<100-millisecond operation), and operate inside the electrochemical window of water, avoiding bubble generation associated with oxygen evolution. We demonstrate that these shape-memory actuators can be used to create basic electrically reconfigurable microscale robot elements including actuating surfaces, origami-based three-dimensional shapes, morphing metamaterials, and mechanical memory elements. Our shape-memory actuators have the potential to enable the realization of adaptive microscale structures, bio-implantable devices, and microscopic robots.

Good reactions for low-power shape-memory microactuators.

Microscale programmable shape-memory actuators based on reversible electrochemical reactions can provide exciting opportunities for microrobotics.

Numerical analysis of NiTi actuators with stress risers: The role of bias load and actuation temperature

NiTi wires and thin ribbons are used in shape memory actuators as active elements undergoing repeated thermal cycles. The NiTi actuators operate either under constant force (CF loading constraint) or they act against a variable force (VF loading constraint) from an elastic bias spring. Any stress riser represents a danger for potential actuator failure upon cycling. Although NiTi actuators always include stress risers either inherently in their design or as material flaws, their effect on actuation performance has, however, not been studied thoroughly yet. This issue is addressed in the present paper through numerical analysis of the effects of actuation load type (CF or VF loading constraint), maximum temperature, and stiffness of the bias springs (in VF loading constraint) on the local evolution of martensitic transformation (MT) and mechanical fields around the notch upon the cyclic thermomechanical loading of a thin NiTi shape memory notched ribbon. The analyses clearly show a strong amplification effect of the transformation strain on the stress concentration factor. The results reveal that the evolution of the forward MT upon cooling causes a sharp stress peak around the notch-tip the magnitude of which depends on the bias spring stiffness while upon heating, the stress at the notch-tip area relaxes due to the reverse MT of the surrounding bulk. Moreover, the simulations indicate that any overheating during the actuation is harmful to the notch-tip as the complete reverse MT at the notch-tip has stress rising effect while incomplete MT at the notch-tip has stress relaxing effect.

Actuating Shape Memory Polymer for Thermoresponsive Soft Robotic Gripper and Programmable Materials.

For soft robotics and programmable metamaterials, novel approaches are required enabling the design of highly integrated thermoresponsive actuating systems. In the concept presented here, the necessary functional component was obtained by polymer syntheses. First, poly(1,10-decylene adipate) diol (PDA) with a number average molecular weight Mn of 3290 g·mol−1 was synthesized from 1,10-decanediol and adipic acid. Afterward, the PDA was brought to reaction with 4,4′-diphenylmethane diisocyanate and 1,4-butanediol. The resulting polyester urethane (PEU) was processed to the filament, and samples were additively manufactured by fused-filament fabrication. After thermomechanical treatment, the PEU reliably actuated under stress-free conditions by expanding on cooling and shrinking on heating with a maximum thermoreversible strain of 16.1%. Actuation stabilized at 12.2%, as verified in a measurement comprising 100 heating-cooling cycles. By adding an actuator element to a gripper system, a hen’s egg could be picked up, safely transported and deposited. Finally, one actuator element each was built into two types of unit cells for programmable materials, thus enabling the design of temperature-dependent behavior. The approaches are expected to open up new opportunities, e.g., in the fields of soft robotics and shape morphing.

Ferroelastic-switching-driven large shear strain and piezoelectricity in a hybrid ferroelectric.

Materials that can produce large controllable strains are widely used in shape memory devices, actuators and sensors1,2, and great efforts have been made to improve the strain output3-6. Among them, ferroelastic transitions underpin giant reversible strains in electrically driven ferroelectrics or piezoelectrics and thermally or magnetically driven shape memory alloys7,8. However, large-strain ferroelastic switching in conventional ferroelectrics is very challenging, while magnetic and thermal controls are not desirable for practical applications. Here we demonstrate a large shear strain of up to 21.5% in a hybrid ferroelectric, C6H5N(CH3)3CdCl3, which is two orders of magnitude greater than that in conventional ferroelectric polymers and oxides. It is achieved by inorganic bond switching and facilitated by structural confinement of the large organic moieties, which prevents undesired 180° polarization switching. Furthermore, Br substitution can soften the bonds, allowing a sizable shear piezoelectric coefficient (d35 ≈ 4,830 pm V-1) at the Br-rich end of the solid solution, C6H5N(CH3)3CdBr3xCl3(1-x). The electromechanical properties of these compounds suggest their potential in lightweight and high-energy-density devices, and the strategy described here could inspire the development of next-generation piezoelectrics and electroactive materials based on hybrid ferroelectrics.

Shape-memory actuators based on dual-curing thiol-acrylate-epoxy thermosets

Shape-memory actuators based on dual-curing thiol-acrylate-epoxy thermosets

Actuators Displaying Unidirectional Movement

Shape memory polymer actuators have attracted great attention in the past few decades for developing soft robots, artificial muscles, and biomedical implants, among others. Herein, under a tutorial perspective, a series of shape memory actuators that display unidirectional movement upon exposure to specific external stimuli is explored. The mechanistic details associated with the movement of each system are also highlighted. In general, an overview of basic design principles, the chemical structure of molecular switches, the strategies used to achieve such controlled and unidirectional movement, as well as key experimental aspects, is provided. The most critical challenges of the existing devices that limit their practical applicability are also discussed. This is of utmost importance because such limitations must be overcome to fabricate micromotors that can generate complex movements, other than simple stretching/contraction or bending, to perform complex tasks.

Stabilization of the Cyclic Response of the Ni49.9Ti50.1 Shape Memory Actuators under Thermomechanical Loads

AbstractThis work is focused on the development of a novel strategy to achieve dimensional stability for 55NiTi actuators that operate at a relatively higher temperature; i.e., with an austenite fi...

Effect of Environmentally Friendly Oil on Ni-Ti Stent Wire Using Ultraprecision Magnetic Abrasive Finishing

Nickel-titanium (Ni-Ti) has been widely used to make shape-memory actuator wire for numerous medical industrial applications, with the result that it frequently comes into contact with the human body. High-quality and nontoxic surfaces of this material are therefore in high demand. We used a rotating magnetic field for an ultraprecision finishing of Ni-Ti stent wire biomaterials and evaluated the finishing technique’s efficacy with different processing oils. To create nontoxic Ni-Ti stent wire, the industrial processing oils that are generally used in the surface improvement process were exchanged for oils with low environmental impacts, and processed under rotating magnetic fields at different speeds and processing times. The processing performance of the different oils was compared and verified. The results show that ultraprecision magnetic abrasive finishing that uses olive and castor oil improves surface roughness by 66.67%, and 45.83%, respectively. SEM and energy-dispersive X-ray spectroscopy (EDX) analyses of the finished components (before and after processing) showed that the material composition of the Ni-Ti stent wire was not changed. Additionally, the magnetic abrasive tool composition was not found on the surface of the finished Ni-Ti stent wire. In conclusion, the environmentally friendly oil effectively improved the diameter of the Ni-Ti stent wire, demonstrating the utility of olive and castor oil in ultraprecision finishing of Ni-Ti stent wire biomaterials.

Modulation of Mesenchymal Stem Cell Migration using Programmable Polymer Sheet Actuators

Recruitment of mesenchymal stem cells (MSCs) to damaged tissue is a crucial step to modulate tissue regeneration. Here, the migration of human adipose-derived stem cells (hADSCs) responding to thermal and mechanical stimuli was investigated using programmable shape-memory polymer actuator (SMPA) sheets. Changing the temperature repetitively between 10 and 37 °C, the SMPA sheets are capable of reversibly changing between two different pre-defined shapes like an artificial muscle. Compared to non-actuating sheets, the cells cultured on the programmed actuating sheets presented a higher migration velocity (0.32 ± 0.1 vs. 0.57 ± 0.2 μm/min). These results could motivate the next scientific steps, for example, to investigate the MSCs pre-loaded in organoids towards their migration potential.

Reversible Actuation via Photoisomerization-Induced Melting of a Semicrystalline Poly(Azobenzene).

Photoisomerization of azobenzene in polymer matrices is a powerful method to convert photon energy into mechanical work. While most previous studies have focused on incorporating azobenzene within amorphous or liquid crystalline materials, the limited extents of molecular ordering and correspondingly modest enthalpy changes upon switching in such systems has limited the achievable energy densities. In this work, we introduce a semicrystalline main-chain poly(azobenzene), where photoisomerization is capable of reversibly triggering melting and recrystallization under essentially isothermal conditions. These materials can be drawn into aligned fibers, yielding optically driven two-way shape memory actuators capable of reversible bending.

A novel smart assistive knee brace incorporated with shape memory alloy wire actuator

The knee plays a significant role in locomotion and stability of the entire body through supporting the body weight and assisting the lower body kinematics during walking. However, the knee is at constant risk of becoming weakened due to disease, age, and accidents. One approach to treating weakened knee is wearing an assistive knee brace. To design a clinical knee brace, many factors such as weight and compliant mechanism should be considered. In this study, a novel smart assistive knee brace mechanism incorporated with wire actuators made of shape memory alloys is proposed to ameliorate the issues associated with weight and flexibility of existing brace designs. Unlike earlier studies, the proposed orthosis includes pressure sensor, shape memory actuator, and smart linkage. Furthermore, two distinct shape memory alloy actuator design concepts with improved stiffness are developed, and the best option is chosen systematically and prototyped. The novel mechanism proposed in this research overcomes the weight of the lower limb during swing phase using the combined shape memory alloy actuation and feed-forward controller design. As such, it can be used as a potential replacement to its conventional counterparts when the higher weight reduction as well as a flexible and controllable mechanism are simultaneously sought.

Rapid Synthesis of Elastomers and Thermosets with Tunable Thermomechanical Properties.

Rapid, solvent-free synthesis of poly(1,4-butadiene) in ambient conditions is demonstrated by frontal ring-opening metathesis polymerization (FROMP) of 1,5-cyclooctadiene (COD). Furthermore, cross-linked copolymers with a wide range of tunable properties are readily prepared by FROMP of mixtures of COD and dicyclopentadiene (DCPD). Specifically, glass transition temperature and tensile modulus are varied from -90 to 114 °C and 3.1 MPa to 1.9 GPa, respectively, by controlling the comonomer ratio. Copolymers with subambient glass transition temperature exhibit robust elastomeric behavior, with the ability to repeatedly recover from large elastic deformations. As a demonstration of the capability of this manufacturing strategy, gradient materials are fabricated in less than a minute with spatially controlled properties for multistage shape memory actuation. This simple yet powerful manufacturing strategy enables rapid synthesis of copolymers ranging from elastomers to thermosets with precise control over thermomechanical properties.

Programing polyurethane with rational surface-modified graphene platelets for shape memory actuators and dielectric elastomer generators

The filler network in smart nanocomposites is capable of dictating the strain actuation and electric generation behaviour in shape-memory and dielectric-elastomer systems, respectively. Noteworthy, the dissipation parameters, such as loss modulus and dielectric loss, are the critical criteria for designing an ideal smart polymeric composite. For manipulating dissipation parameters, we developed three types of nanofiller, including (I) simple graphene-oxide (GO), (II) reduced graphene-oxide (rGO), and (III) noncovalent-factionalized graphene with (polyamine-anchored)-perylene-bisimide (XGO). After fabrication of poly-urethane (PU) nanocomposites at various filler loading, the robust correlation between microstructure, electrical and mechanical, including static and dynamic (linear and nonlinear viscoelasticity), properties of nanocomposites has been deduced. These smart polymeric nanocomposites demonstrate the capability of shape-memory actuating as well as harvesting the electric energy from mechanical work. First and foremost, the harvested-energy-density of PU was meaningfully improved when blended with XGO. A composite-film containing 5 wt% XGO with the harvested energy density of 2.97 mJ/cm3 was achieved, which is about 6.7 times superior to that of pristine PU. Furthermore, the shape-memory recovery ratio of functionalized 2 wt%-nanocomposite significantly improved from 86.2% for pure PU to 93.4% for the XGO sample. The observations in this work strongly suggest compositing is an auspicious-way to provide proper shape-memory actuator and better dielectric-elastomer candidates for forthcoming practical generators.

Fast Triggered Controllable Electrically Actuated Shape Memory Epoxy: Graphene oxide Nanocomposites

Shape memory polymers (SMPs) are the new class of smart fascinating polymer materials with different potential applications. This research work aims to present systematic investigations of electrically actuated shape memory (EASM) properties at different electric voltage of epoxy polymer filled with 0.1 to 0.4 wt. % of Graphene (GO) to develop SMPs nanocomposites. EASM effect and surface morphology functional analysis of composites were observed after EASM characterization by Field Emission-Scanning Electron Microscopy (FE-SEM) and Fourier Transform Infrared Spectroscopy (FT-IR). 0.4 wt. % addition of GO showed highest shape recovery at 180 S, 140 S and 95 S for 40 V, 60 V and 80 V respectively. These results suggest that GO could be promising materials for electrically actuated shape memory applications.

Multifunctional Spun Yarns and Textiles from Nickel‐Titanium Microfilaments

AbstractImprovements in multifunctional materials have led researchers to reinvigorate traditional textile structures by integrating emerging material technologies to offer novel solutions to diverse industries. However, there exist few multifunctional materials capable of being produced with micrometer diameters that can be spun into yarns for textile manufacturing, limiting the wearability and tunability of multifunctional textiles. Here, the creation of nickel‐titanium (NiTi) smart material‐based yarns is enabled by the availability of small diameter (≤10 µm) NiTi filaments that can survive the yarn spinning manufacturing process. NiTi microfilament yarns exhibit traditional superelastic and shape memory properties, and afford additional improvements to mechanical performance such as a tunable structural stiffness, plateau strength, and actuation contractions through the introduction of controllable geometric parameters—yarn count, surface twist angle, and manufacturing strains. This work concludes in a densely knitted, closed‐form textile with shape memory actuation and superelastic recovery. NiTi microfilament yarns and textiles have promising impacts in actuating and energy‐absorbing technologies for medical, robotics, aerospace, and defense applications.

A Design Approach for a Valve Based on a Magnetic Shape Memory Actuator

With the motivation to meet the increasing requirements on fast-switching pneumatic valves, Magnetic Shape Memory alloys (MSM) are considered as a promising alternative to conventional actuators. Regarding the desired strokes, forces and actuation frequencies MSM alloys meet the requirements best among other solid-state transducers for valve applications. However, the hysteresis in the magneto-mechanical behavior is challenging during the sizing process of a component based on an MSM actuator. It has proved to be valuable to apply a system simulation including a lumped element model of the MSM element in the early design phase. In this contribution, a design approach for the valve based on an MSM actuator is shown. A system network model was used to design a functional prototype of the valve to achieve the specifications defined within a requirements analysis. These specifications were derived from characteristic features of conventional established fast-switching pneumatic valve. The analysis of the subsystems was done to exploit the deviations and their influences on the system behavior. Furthermore, the experimental research of the functional prototype of the valve was carried out to check for functionality and to evaluate whether the specifications were met. A nominal volume flow of 120 Nl/min was reached at a supply Voltage U of 48;{text{V}} and current i of 7.6;{text{A}} which exceeds the aim of 100 Nl/min. By means of an optimization of the actuators geometry the response time could be reduced to less than 1;{text{ms}}. The desired maximum modular dimension of 9 mm was not achieved by the functional prototype due to the available production process, but would be feasible with series production.

Shape-Memory Actuation in Aligned Zirconia Nanofibers for Artificial Muscle Applications at Elevated Temperatures

Artificial muscle is one of the key technologies to accelerate the development of robotics, automation, and artificial-intelligence-embedded systems. This work aims to develop shape-memory ceramic ...

Actuators Based on Oligo[(ε-caprolactone)-co-glycolide] with Accelerated Hydrolytic Degradation

ABSTRACTPolyester-based shape-memory polymer actuators are multifunctional materials providing reversible macroscopic shape shifts as well as hydrolytic degradability. Here, the function-function interdependencies (between shape shifts and degradation behaviour) will determine actuation performance and its life time.In this work, glycolide units were incorporated in poly(ε-caprolactone) based actuator materials in order to achieve an accelerated hydrolytic degradation and to explore the function-function relationship. Three different oligo[(ε-caprolactone)-co-glycolide] copolymers (OCGs) with similar molecular weights (10.5 ± 0.5 kg∙mol−1) including a glycolide content of 8, 16, and 26 mol% (ratio 1:1:1 wt%) terminated with methacrylated moieties were crosslinked. The obtained actuators provided a broad melting transition in the range from 27 to 44 °C. The hydrolytic degradation of programmed OCG actuators (200% of elongation) resulted in a reduction of sample mass to 51 wt% within 21 days at pH = 7.4 and 40 °C. Degradation results in a decrease of Tm associated to the actuating units and increasing Tm associated to the skeleton forming units. The actuation capability decreased almost linear as function of time. After 11 days of hydrolytic degradation the shape-memory functionality was lost. Accordingly, a fast degradation behaviour as required, e.g., for actuator materials intended as implant material can be realized.