Shape Memory Polymer Actuators Research Articles

Insight into single-helix intelligent shape memory polymer cables: modeling and optimization under finite sliding contact

This pioneering study focuses on the finite element analysis (FEA) of thermomechanical properties of shape memory polymer (SMP) wire ropes and their components under both small- and finite-sliding contact deformation. To validate the FEA, we need to validate both geometric modeling and non-linear material behavior. Owing to intricate geometry, as well as excessive wire interactions in the structure, this part is studied by simulating a 1 × 37 steel wire rope and then comparing it with existing experimental data. To evaluate the response of non-linear material behavior, we employ the available numerical results to model the thermomechanical property of an SMP rectangular bar under a uniaxial test and then verify both constrained and unconstrained recovery behavior. After rigorous validation, two configurations of 1 × 7 and 1 × 27 SMP cables are modeled based on the thermo-visco-hyperelastic constitutive framework for acrylate polymer systems. Upon exerting an axially tensile load on these 1 × 7 and 1 × 27 SMP wire ropes, the response of force and shape recovery, as well as the normal and shear stress distributions, are measured under constrained and unconstrained conditions. For a deeper physical understanding, the influences of different temperature rates (5 and 1 °C min−1), inter-wire sliding frictional coefficient (0.1–0.6), and multiple-shape programming on the stress-strain-temperature relations of these SMP cables are also investigated. Furthermore, based on optimizing two cable factors of diameter and helix angle, and using the design of experiments method, the specific energy of a 1 × 6 SMP cable is maximized. Under different thermomechanical loadings, this study tries to cast light on the remarkable features and possible potential applications of these newly developed SMP actuators which may foster unparalleled advancements in various industries.

Biomimetic shape-morphing actuators with controlled bending deformation and photo-mediated 3D shape programming for flexible smart devices

Biomimetic soft actuators with programmable morphing behaviors and remote actuations have drawn significant attention in recent years. In this work, we present the network architecture of a four-arm crystalline shape memory polymer actuator via efficient thiol-acrylate coupling of polycaprolactone precursor bearing crosslinkable methacrylate end-groups with pentaerythritol, followed by post-approach surface chemistry of polydopamine (PDA) nanospheres coating. The excellent photothermal transfer efficiency of the coated PDA can enable rapid temperature increase over the transition temperature of polymeric matrix and optically drive the shape morphings. Combined with built-in internal stress, anisotropic polymer chain relaxation of the strained thin actuator film would generate asymmetric contractions and thus causes controlled bending deformations via surface directing of an 532 nm green laser. Beyond this, the dynamic nature of transesterification reaction also provides the actuator film with reprogrammability. Remarkably, the morphing behaviors of a flexible plane film into various soft robots including 3D airplane and chiral helix were achieved under a 300 mW/cm2 simulated light irradiation. The results presented in this study clearly provide a new method for designing actuators with spatiotemporally programmable morphing behaviors and will pave way for the construction of flexible smart devices.

Rapid and selective actuation of 3D-printed shape-memory composites via microwave heating

Three-dimensional (3D) printing allows the fabrication of complex shapes with high resolutions. However, the printed structures typically have fixed shapes and functions. Four-dimensional printing allows the shapes of 3D-printed structures to be transformed in response to external stimuli. Among the external stimuli, light has unique advantages for remote thermal actuation. However, light absorption in opaque structures occurs only near the sample surface; thus, actuation can be slow. Here, we propose and experimentally demonstrate the rapid and selective actuation of 3D-printed shape-memory polymer (SMP) composites using microwave heating. The SMP composite filaments are prepared using different amounts of graphite flakes. Microwave radiation can penetrate the entire printed structures and induce rapid heating. With sufficient graphite contents, the printed SMP composites are heated above their glass transition temperature within a few seconds. This leads to rapid thermal actuation of the 3D-printed SMP structures. Finally, dual-material 3D printing is demonstrated to induce selective microwave heating and control actuation motion. Our experiments and simulations indicate that microwave heating of SMP composites can be an effective method for the rapid and selective actuation of complex structures.

Heat-Induced Actuator Fibers: Starch-Containing Biopolyamide Composites for Functional Textiles.

This study introduces the development of a thermally responsive shape-morphing fabric using low-melting-point polyamide shape memory actuators. To facilitate the blending of biomaterials, we report the synthesis and characterization of a biopolyamide with a relatively low melting point. Additionally, we present a straightforward and solvent-free method for the compatibilization of starch particles with the synthesized biopolyamide, aiming to enhance the sustainability of polyamide and customize the actuation temperature. Subsequently, homogeneous dispersion of up to 70 wt % compatibilized starch particles into the matrix is achieved. The resulting composites exhibit excellent mechanical properties comparable to those reported for soft and tough materials, making them well suited for textile integration. Furthermore, cyclic thermomechanical tests were conducted to evaluate the shape memory and shape recovery of both plain polyamide and composites. The results confirmed their remarkable shape recovery properties. To demonstrate the potential application of biocomposites in textiles, a heat-responsive fabric was created using thermoresponsive shape memory polymer actuators composed of a biocomposite containing 50 wt % compatibilized starch. This fabric demonstrates the ability to repeatedly undergo significant heat-induced deformations by opening and closing pores, thereby exposing hidden functionalities through heat stimulation. This innovative approach provides a convenient pathway for designing heat-responsive textiles, adding value to state-of-the-art smart textiles.

New Soft Actuating Way by Rapidly Temperature Change of Electrocaloric Effect.

Compared to traditional temperature control methods, the electrocaloric (EC) effect offers several advantages such as small size, rapid response, and environmental friendliness. However, current EC effects are generally used for the cooling area rather than heating. Here, poly(vinylidenefluorideter-trifluoroethylene-ter-chlorofluoroethylene) [P(VDF-TrFE-CFE)] film is combined with an electrothermal actuator (ETA) composed of polyethylene (PE) film and carbon nanotube (CNT) film. The heating and cooling process of the EC effect is used to help drive the ETA. The P(VDF-TrFE-CFE) film can produce a temperature change (ΔT) of 3.7 °C at 90 MV/m, and this process occurs within 0.1 s. With this ΔT, the composite film actuator can produce a deflection of 10°. In addition, due to the electrostrictive effect of P(VDF-TrFE-CFE), the composite film can also be used as an actuator. At 90 MV/m, the composite film actuator can produce a deflection over 240° within 0.05 s. Apart from other current driving modes for thermally responsive actuators, in this paper, a new type of soft actuating composite film by the temperature change of the EC effect is proposed. Except from ETAs, the EC effect can also have a wide application prospect in other thermally responsive actuators, including shape memory polymer actuators, shape memory alloy actuators, and so on.

Actuation of light-activated shape memory polymer laminated beams: theory and experiment

Actuation of light-activated shape memory polymer laminated beams: theory and experiment

Mode shifting shape memory polymer and hydrogel composite fiber actuators for soft robots

Soft robotics is a promising new field offering robot systems that mimic the versatility and complexity of movement and propulsion seen in natural organisms. Such systems require complex actuation including length expansion/contraction, bending and torsion. To date, such actuators have been configured individually. Here, it is shown that composite fibers formed from a shape memory polymer and a thermo-sensitive hydrogel could be configured into any one of the tensile, torsional and flexural actuators. Furthermore, the programmed mode of actuation could be thermally erased and the same fiber re-programmed into a different type of actuator. The fibers were prepared from poly(N-isopropylacrylamide) with polycaprolactone and the fiber composition was tuned to optimize both shape fixity and the degree of actuation. The fibers could be programmed by heating to 60 ℃ and then cooled under tensile, flexural or torsional load. The fiber was then conditioned by immersing in water at room temperature which induced swelling of the hydrogel phase and shape deformation. The pre-programmed shape of the fiber was then restored when the fiber was heated at 35 ℃ in water to de-swell the thermo-sensitive poly(N-isopropylacrylamide) hydrogel. Reversible actuation was observed through multiple cycles of heating and cooling. The optimized fibers generated torsional strokes of 10 turn/m; bending curvature changes of 0.4 mm−1 or tensile strokes of up to 92%. The composite fibers offer a convenient means for generating a variety of different actuation types for soft robotics.

Optimal shape morphing control of 4D printed shape memory polymer based on reinforcement learning

4D printing technology, as a new generation of Additive Manufacturing methods, enables printed objects to further change their shapes or other properties upon external stimuli. One main category of 4D printing research is 4D printed thermal Shape Memory Polymer (SMP). Its morphing process has large time delay, is nonlinear time variant, and susceptible to unpredictable disturbances. Reaching an arbitrary position with high precision is an active research question. This paper applies the Reinforcement Learning (RL) method to develop an optimal control method to perform closed loop control of the SMP actuation. Precise and prompt shape morphing is achieved compared with previous control methods using a PI controller. The training efforts of RL are further reduced by simplifying the optimal control policy using the structural property of the prior trained results. Customized protective visors against COVID-19 are fabricated using the proposed control method.

Harnessing ultra-high programmability and controllability for smart composite architecture using quadruple shape memory poly(aryl ether ketone)s

Multi-stimuli selective actuation of shape memory polymers (SMPs) with multi memory capabilities have attracted great attention in recent years, due to diverse active deformation application scenarios. Current multi-SMPs with low Tt, however, are faced with inherent barriers, such as low fixation and inaccurate control, because the shape retainability, controllability, and responsiveness of SMPs are highly determined by transition temperature (Tt). Herein, quadruple-shape memory poly(aryl ether ketone) (PAEK) with tunable high Tt and selective response characteristics were successfully fabricated for the first time, which especially harnessed ultra-high programmability and controllability. More significantly, the quadruple shape memory PAEK-based smart architectures such as “fingers” and “flower” were demonstrated to exhibit the controllable multi-stages actions. Utilizing a “man” as an example, we put forward different programming routes up to ten to make it perform complex pre-set deformation actions. The quadruple shape memory PAEK of high Tt possessed great utilizing potential in fields of soft robots, actuators, and deformable devices.

A review on progress in magnetic, microwave, ultrasonic responsive Shape-memory polymer composites

Shape memory polymers (SMPs) are the novel class of Smart materials where they memorize their shape/configuration and return to their original shape upon application of a variety of external stimuli. Triggering of SMPs can be done by various external stimuli such as heat, electricity, magnetism, light, water, microwaves, chemical solvents, pressure, and sound. Over decades, there has been a great deal of research for the different triggering of SMMs in both industrial and academic researchers in the automobile, aerospace, biomedical, electronics, and textiles. Heating is the conventional way of triggering the SMPs, but direct heating is not always practical in some circumstances because heating has both design and physical constraints. For example, in minimally invasive surgery, drug-releasing systems, artificial muscles, intravascular stents, self-stitching. The remote-controlled stimulus for SMPs has been a challenging task for controlling shape in real-time over decades. To overcome this, Indirect heating of SMMs has come into the field. Therefore it is essential to design a new triggering mechanism for the actuation of SMPs. Alternating magnetic field, light, microwave, and ultrasound would satisfy some of these constraints. In this report, the recent advances made in the development of indirect actuation of SMPs to recuperate their shape have been reviewed. Indirect heating mechanisms could be further split into Magnetic resistive Joule heating, Microwave, Ultrasound, Light (UV, infrared) SMPs. These new triggering mechanisms can enable us to design SMP for new targeted applications.

Switchable dry adhesive based on shape memory polymer with hemispherical indenters for transfer printing

Transfer printing based on switchable adhesive is essential for developing unconventional systems, including flexible electronics, stretchable electronics, and micro light-emitting diode (LED) displays. Here we report a design of switchable dry adhesive based on shape memory polymer (SMP) with hemispherical indenters, which offers a continuously tunable and reversible adhesion through the combination of the preloading effect and the thermal actuation of SMP. Experimental and numerical studies reveal the fundamental aspects of design, fabrication, and operation of the switchable dry adhesive. Demonstrations of this adhesive concept in transfer printing of flat objects (e.g., silicon wafers), three-dimensional (3D) objects (e.g., stainless steel balls), and rough objects (e.g., frosted glasses) in two-dimensional (2D) or 3D layouts illustrate its unusual manipulation capabilities in heterogeneous material integration applications.

A Mechanical Analysis of Chemically Stimulated Linear Shape Memory Polymer Actuation.

In the present work, we study the role of programming strain (50% and 100%), end loads (0, 0.5, 1.0, and 1.5 MPa), and chemical environments (acetone, ethanol, and water) on the exploitable stroke of linear shape memory polymer (SMP) actuators made from ESTANE ETE 75DT3 (SMP-E). Dynamic mechanical thermal analysis (DMTA) shows how the uptake of solvents results in a decrease in the glass temperature of the molecular switch component of SMP-E. A novel in situ technique allows studying chemically triggered shape recovery as a function of time. It is found that the velocity of actuation decreases in the order acetone > ethanol > water, while the exploitable strokes show the inverse tendency and increases in the order water > ethanol > acetone. The results are interpreted on the basis of the underlying chemical (how solvents affect thermophysical properties) and micromechanical processes (the phenomenological spring dashpot model of Lethersich type rationalizes the behavior). The study provides initial data which can be used for micromechanical modeling of chemically triggered actuation of SMPs. The results are discussed in the light of underlying chemical and mechanical elementary processes, and areas in need of further work are highlighted.

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.

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.

Shape-Memory Actuation of Individual Micro-/Nanofibers

Advances in the fabrication and characterization of polymeric nanomaterials has greatly advanced the miniaturization of soft actuators, creating materials capable of replicating the functional physical behavior previously limited to the macroscale. Here, we demonstrate how a reversible shape-memory polymer actuation can be generated in a single micro/nano object, where the shape change during actuation of an individual fiber can be dictated by programming using an AFM-based method. Electrospinning was used to prepare poly(e-caprolactone) micro-/nanofibers, which were fixed and crosslinked on a structured silicon wafer. The programming as well as the observation of recovery and reversible displacement of the fiber were performed by vertical three point bending, using an AFM testing platform introduced here. A plateau tip was utilized to improve the stability of the fiber contact and working distance, enabling larger deformations and greater rbSMPA performance. Values for the reversible elongation of erev = 3.4 ± 0.1% and 10.5 ± 0.1% were obtained for a single micro (d = 1.0 ± 0.2 μm) and nanofiber (d = 300 ± 100 nm) in cyclic testing between the temperatures 10 and 60 °C. The reversible actuation of the nanofiber was successfully characterized for 10 cycles. The demonstration and characterization of individual shape-memory nano and microfiber actuators represents an important step in the creation of miniaturized robotic devices capable of performing complex physical functions at the length scale of cells and structural component of the extracellular matrix.

Epoxy-based shape memory composite for space applications

Shape memory polymers are smart materials with an ability to recover their permanent shape from a temporary shape upon exposure to various external stimuli. In space applications, shape memory polymers can find their use as deployable devices due to their high strength-to-weight ratio and large deformability. The effect of vacuum environment in space on the deployment of shape memory polymers is an extremely important issue which is yet to be studied. As the energy budget in spacecraft is limited, means to increase the energetic efficiency of deployable devices need to be developed.In this work, shape memory polymer actuators based on carbon resistive heating fibers and epoxy matrix were developed for space applications. Their mechanical, thermal, and electrical properties, as well as their deployment kinetics at both ambient and vacuum conditions, were studied. A method for improvement of the deployment energetic efficiency of shape memory polymer actuators, based on aluminum coating for internal radiative heating, was introduced. An innovative technique, which provides motion sensing at the first stage of the shape memory polymer actuators deployment, was demonstrated. This technique uses the resistive heating characteristics of carbon fibers and in situ electrical resistance drop during deployment. Finally, the durability of the shape memory polymers actuators in the space environment was discussed.

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.

Temperature-controlled reversible pore size change of electrospun fibrous shape-memory polymer actuator based meshes

Fibrous membranes capable of dynamically responding to external stimuli are highly desirable in textiles and biomedical materials, where adaptive behavior is required to accommodate complex environmental changes. For example, the creation of fabrics with temperature-dependent moisture permeability or self-regulating membranes for air filtration is dependent on the development of materials that exhibit a reversible stimuli-responsive pore size change. Here, by imbuing covalently crosslinked poly(ε-caprolactone) (cPCL) fibrous meshes with a reversible bidirectional shape-memory polymer actuation (rbSMPA) we create a material capable of temperature-controlled changes in porosity. Cyclic thermomechanical testing was used to characterize the mechanical properties of the meshes, which were composed of randomly arranged microfibers with diameters of 2.3 ± 0.6 μm giving an average pore size of approx. 10 μm. When subjected to programming strains of εm = 300% and 100% reversible strain changes of εʹrev = 22% ± 1% and 6% ± 1% were measured, with switching temperature ranges of 10 °C–30 °C and 45 °C–60 °C for heating and cooling, respectively. The rbSMPA of cPCL fibrous meshes generated a microscale reversible pore size change of 11% ± 3% (an average of 1.5 ± 0.6 μm), as measured by scanning electron microscopy. The incorporation of a two-way shape-memory actuation capability into fibrous meshes is anticipated to advance the development and application of smart membrane materials, creating commercially viable textiles and devices with enhanced performance and novel functionality.

Hydrolytic Degradation of Actuators Based on Copolymer Networks From Oligo(ε-caprolactone) Dimethacrylate and n-Butyl Acrylate

ABSTRACTShape-memory polymer actuators often contain crystallizable polyester segments. Here, the influence of accelerated hydrolytic degradation on the actuation performance in copolymer networks based on oligo(ε-caprolactone) dimethacrylate (OCL) and n-butyl acrylate is studied. The semi-crystalline OCL was utilized as crosslinker with molecular weights of 2.3 and 15.2 kg∙mol−1 (ratio: 1:1 wt%) and n-butyl acrylate (25 wt% relative to OCL content) acted as softening agent creating the polymer main chain segments within the network architecture. The copolymer networks were programmed by 50% elongation and were degraded by means of alkaline hydrolysis utilizing sodium hydroxide solution (pH = 13). Experiments were performed in the range of the broad melting range of the actuators at 40 °C. The degradation of test specimen was monitored by the sample mass, which was reduced by 25 wt% within 105 d. As degradation products, fragments of OCL with molecular masses ranging from 400 to 50.000 g·mol-1 could be detected by NMR spectroscopy and GPC measurements. The cleavage of ester groups included in OCL segments resulted in a decrease of the melting temperature (Tm) related to the actuator domains (amorphous at the temperature of degradation) and simultaneously, the Tm associated to the skeleton domain was increased (semi-crystalline at the temperature of degradation).The alkaline hydrolysis decreased the polymer chain orientation of OCL domains until a random alignment of crystalline domains was obtained. This result was confirmed by cyclic thermomechanical actuation tests. The performance of directed movements decreased almost linearly as function of degradation time resulting in the loss of functionality when the orientation of polymer chains disappeared. Here, actuators were able to provide reversible movements until 91 d when the accelerated bulk degradation procedure using alkaline hydrolysis (pH = 13) was applied. Accordingly, a lifetime of more than one year can be guaranteed under physiological conditions (pH = 7.4) when, e.g., artificial muscles for biomimetic robots as potential application for these kind of shape-memory polymer actuators will be addressed.

Coupling of nonlinear shape memory polymer cantilever dynamics with focused ultrasound field

Research has found significant potential for ultrasound actuation of shape memory polymers (SMPs) in several fields such as biomedical and electronic devices among others. Example applications range from controlled drug delivery containers to soft robotics and flexible electronics located in otherwise inaccessible places or hazardous environments, where direct external heating is not possible. SMPs can be manipulated into any temporary shape and later recover to their stress-free permanent shape when triggered with external stimuli such as heat. Focused ultrasound (FU) has the ability to induce localized heating and activate multiple intermediate shapes and achieve complete shape recovery in the polymer, non-invasively and remotely. In addition, FU has a superior capability for temporal and spatial control of shape recovery by adjusting sample size, ultrasound frequency, exposure time and intensity as well as the position of ultrasound focusing. In this paper, indirect actuation of the thermally-induced shape-memory effect of SMPs by FU is studied theoretically and experimentally with a focus on the acoustic field, medium, geometric and material properties. The changes in thermomechanical properties, during FU actuation, are studied through dynamic mechanical analyzer tests. Using these properties, an analytical acoustic-thermo-elastic dynamic model is developed to predict the shape memory response of a SMP cantilever beam, considering acoustic and geometric nonlinearities. The governing equations of motion are derived using reduced order modeling and solved by perturbation techniques. Having obtained an analytical expression for the shape recovery of the beam as a function of acoustic parameters, experimental validations for a cantilever SMP beam exposed to FU are performed. The model has the ability to successfully estimate the variation in the amount of shape recovery due to the change in source frequency of the transducer and peak acoustic pressure field inside SMP domain without the need of analyzing any intermediary acoustic/thermal/elastic behavior.