Variable-Stiffness and Deformable Link Using Shape-Memory Material and Jamming Transition Phenomenon

ABSTRACTIn the field of smart materials, an increasing demand is being observed for shape memory materials that exhibit advanced stimuli‐sensing and responsive capabilities. Two‐way shape memory polymers (2 W‐SMPs), as a novel type of smart shape memory material, offer numerous advantages over traditional one‐way shape memory polymers (1 W‐SMPs), particularly the fully reversible shape memory effect (SME), where the two‐way process (low‐temperature shape fixation and high‐temperature recovery to the original shape) occurs without the need for external energy input. These polymers are lightweight and exhibit remarkable deformation abilities, reversible changes in modulus, and multiple actuation modes. Additionally, they feature a high elastic modulus and increased response stress, making them particularly suitable and indispensable in fields ranging from medical devices and robotics to aerospace engineering and smart textiles. This review focuses on 2 W‐SMPs, detailing the underlying mechanisms of both 1 W‐SMPs and 2 W‐SMPs, as well as the various driving modes. We further discuss quasi‐2 W‐SMPs (with applied load force σ ≠ 0) and 2 W‐SMPs (with applied load force σ = 0) across different cross‐linking strategies. It also covers the broad applications of SMP materials and outlines the current challenges and potential avenues for future research.

Shape memory polymers (SMPs) possess the remarkable ability to recover their original shape after being severely deformed, triggered by the right stimulus. This chapter reviews recent progress in shape memory materials, focusing on traditional types like shape memory alloys (SMAs) and SMPs, as well as emerging shape memory hybrids, which enable customizable properties for specific applications in a “do-it-yourself” manner. SMPs, in particular, show great promise for morphing aircraft applications due to their temperature- and time-dependent mechanical properties. Tests on SMPs under monotonic loading in high shear demonstrate their ability to recover original shapes while resisting large loads. Advances in SMP composites (SMPCs), which incorporate functional materials and nanostructures, further enhance their deformation recovery, mechanical properties, and controllable actuation. SMPs have broad applications across aerospace, biomedical devices, flexible electronics, and more. This chapter also provides an in-depth analysis of the shape recovery mechanisms, multifunctionality, and potential future directions for SMP development.

Shape memory polymers (SMPs) are evolving intensively in smart materials research. SMPs can undergo large deformations, hold a temporary shape and then recover their original shape upon exposure to a particular external stimulus. The emerging development of fibre reinforced shape memory polymer composites (SMPCs) has improved SMPs’ mechanical properties to be comparable to modern fibre reinforced composites. Moreover, the inclusion of nanoparticles has enhanced the diversity in activation mechanisms and improved structural properties and durability.
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\nToday, the SMPCs are generally viewed as promising substances for engineered applications that have raised intensive interest among the scientists and engineers. Interestingly, stimulation by light enables unique and advanced shape memory effects (SMEs) in SMPCs that can fulfill the sophisticated demands of engineering. The ability to vary the light wavelength, intensity, periodicity, polarization and the irradiated position on the SMP component enables wavelength selective, reversible, sequential and multiple SMEs. Furthermore, light travel long distances in a vacuum, can be focussed onto a certain area, is safe to humans and, can be a control signal carrier and most notably, can be sent through optical fibres to extremely inaccessible areas.
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\nIncorporation of metallic, carbon and organic based photothermal fillers into a SMP matrix is a convenient proven method to prepare light activated shape memory polymer composites (LASMPCs). These photothermal fillers convert electromagnetic radiation into thermal energy that triggers the SMP matrix. The current study comprehensively reviewed the performances, mechanisms, challenges and limitations of the existing LASMPC systems and associated optical technologies. Predominantly this research was intended to develop reinforced LASMPC systems and allied optical technologies applicable for large scale structural engineering applications.
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\nIn this study, styrene and bisphenol A epoxy based thermosets and polylactic acid (PLA) based thermoplastic SMPs were used as polymer matrix materials. Carbon fibre, glass fibre, multiwalled carbon nanotube (MWCNT) and rare earth organic complexes of Nd(TTA)3Phen and Yb(TTA)3Phen were used as reinforcement and photothermal fillers. The samples were produced by means of vacuum bagging, mould casting and 4D printing. Morphological investigations were conducted to identify the microstructure and manufacturability of the LASMPCs. The samples’ mechanical and physical properties were tested by standard test methods for polymer matrix composites. Dynamic mechanical analysis was carried out to determine the thermomechanical characteristics. The photothermal effect was investigated through thermal imaging and associated shape memory behaviours were studied through shape fixing and recovery experiments. A finite element analysis was performed with ABAQUS to simulate the photothermal heating behaviour.
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\nThe current study established a material development framework for LASMPC materials and presented apt manufacturing methods for structural and large scale engineering applications. Different modes of light diffusion such as scattered low intensity light, focussed laser beams and light delivery through fibre optics were proved for structural LASMPCs. Sequential and selective wavelength activation of structural LASMPCs are presented. The FEA simulation provided additional understanding on light exposed LASMPCs’ 3-dimensional heating behaviour. Embedding a D-shaped optical fibre in an LASMPC created a single unit smart actuator, which was remotely activated by light. It was proven that flexible radiation shields can reduce the unanticipated shape recovery of LASMPCs components and protect the LASMPCs from polymer degradation due to radiation.
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\nThe improvements to their structural properties and innovative remote activation methodologies have justified LASMPCs’ aptness for space engineering applications. This study has proven its successful completion by three selected case studies of space applications. An LASMPC solar panel array model was sequentially deployed in four recovery steps. In a vacuum, a space habitat structure model was compressed to a third of its volume and then recovered its original shape. A 4D printed boom structure model was tested, proving customized LASMPC structures for sophisticated space equipment fabrication. The deployable LASMPC structures can reduce the occupied room in spacecraft. The improvements to the structural properties of LASMPCs while keeping the SME, understanding on thermomechanical and photothermal behaviours, remote actuation through optical fibres, radiation shields and the insight to space applications are the most significant novel outcomes of this study. This thesis opens up windows of opportunity for the scientific and engineering communities to provide innovative solutions to realize the sophisticated large scale engineering demands in aerospace and space applications and beyond.

Shape memory alloys (SMAs) have the disadvantage that cooling is difficult and the actuating speed during cooling is slow. To resolve this problem, shape memory material actuators that operate only with heating is required. SMAs are characterized by a low apparent Young's modulus below the transformation temperature and a strong shape recovery force above the reverse transformation temperature. Alternatively, shape memory polymers (SMPs) have two properties: shape fixability and shape recovery. The SMPs are hardened below the glass transition (Tg) temperature and the material is recovered to memorized shape above the Tg temperature. The other hand, 3D printer is a machine that can directly output a 3D-designed product designed by a computer in 3D, and molded materials such as polymer, resin, metal, and ceramics. In this research, we developed the SMC of SMA wire and SMP sheet using adhesive that develops actuates into two shapes only by heating.

Shape memory polymers (SMPs) have thermomechanical property drastically altering around the glass-transition temperature (Tg), temporary shape fixity and shape recovery property, and so on. To understand the mechanisms of such unique properties and design products including SMPs optimally, a mathematical model which is simple yet can represent these properties reasonably and accurately is necessary. Especially, it is important for the model to be able to consider the effect of temperature and strain history on the properties because the properties are considered to depend on not only the temperature and the strain themselves but also their history. Therefore, in this paper first the effect of the temperature and strain history on the shape fixity and shape recovery properties was examined. Then, a micromechanical model of SMP was developed to describe this effect. This model consisted of a lot of unit elements having four springs, two dash pots, one latch, and one thermal expansion element. Different Tg was assigned to each element because Tg depends on local condition inside of SMP and distributed in general. It was found from the experiment that the temperature and strain history applied to SMP when the shape is gradually frozen has an influence on the shape recovery process. With respect to the model, it was shown that the proposed micromechanical model can capture the thermomechanical behaviors of SMP and, especially, the temperature and strain histories dependence on the shape recovery process quite well.

Shape memory alloys (SMAs) or shape memory polymers (SMPs) enable soft actuators to achieve advanced adaptabilities applied in soft robotics. However, actuators that combine multiple shape memory materials struggle to achieve complex deformation effects and stiffness variations with effective control strategies. To achieve controllable, shape adaptation, and programmed deforming behavior, this study proposes an integrated control strategy for an SMA-SMP based programmable morphing structure used as an actuator in soft robotics. To achieve precise control over programming deformations and stiffness variation, a multi-target thermal sensing method (MTTSM) was proposed, integrated into an interaction-driven control framework. Based on MTTSM, the coordinated actuation between the SMA springs and the SMP structure is realized, enabling standby of preheating, stepping with programmed deformations, and dynamic stiffness changes. In addition, to achieve dynamic monitoring of deformed states, the co-training-based monitoring system is developed for collaboration, enabling the use of multisensor fusion for position estimation in the absence of end-effectors that can directly measure the deformed structure of the flexible body. In conclusion, the proposed integration strategy of MTTSM and the cotraining monitoring system offers a control solution for integrating multiple shape memory materials into morphing structures as smart actuators applied to soft robotic applications.

Morphing structures have the potential to significantly improve vehicle performance over existing fixed component designs. In this paper, we examine new composite material design approaches to provide combined high stiffness and large reversible deformation. These composites employ shape memory polymers (SMP) matrices combined with segmented metallic reinforcement to create materials with variable stiffness properties and reversible accommodation of relatively large strains. By adjusting the temperature of the sample, the storage modulus can be varied up to 200x. We demonstrate the segmented composite concept in prototype materials made using thermoplastic polyurethane SMP reinforced with interlocking segmented steel platelets. Measured storage moduli varied from 5-12 GPa, below SMP T_g, and 0.1-0.5 GPa above SMP T_g. The samples demonstrated more than 95% recovery from induced axial strains of 5% at 80°C. Viscoelastic effects are dominant in this regime and we investigate the rate dependence of strain recovery.

Shape memory polymer (SMP) systems exhibiting semicrystalline- elastomer blends, such as thermoplastic polyurethane and polylactic acid have been well studied, but their use in biomedical shape memory applications has been limited by their high activation temperature. SMPs are capable of deformation and recovery through the activation of an external stimuli, such as temperature. Critical criteria for SMPs used in biomedical applications is achieving a stimulus temperature close to 37°C while still experiencing sufficient shape recovery. A polymer’s glass transition temperature has been well defined as the SMP system’s activation temperature and therefore should be decreased to achieve a decreased activation temperature. In this work, a well-known, biocompatible plasticizer, polyethylene oxide (PEO), was added to thermoplastic polyurethane (TPU) – polylactic acid (PLA) SMP blends to observe the plasticizing effect on the structural, thermal, mechanical, and shape memory properties of the polymer blends. Additionally, the geometry of the fabricated SMP samples was tailored to further enhance the shape memory effect through a bowtie honeycomb structure. Our results suggest that the addition of PEO into the SMP system may be an effective method for decreasing the polymer’s glass transition temperature through the alteration of the polymer chain structure. With the addition of 30% PEO, the glass transition temperature of the TPU/PLA blend was successfully decreased from 62.4°C to 34.6°C while achieving 86.5% shape recovery when activated at 37°C, which is only a 5% decrease in shape recovery when activated at 50°C. These results suggest that the addition of a biocompatible plasticizer may overcome the limitation of employing temperature activated SMP systems in biomedical applications, and enhances the potential of these materials for reconfigurable structures, energy dissipation systems, and SHM in civil engineering applications.

Shape memory polymers (SMPs) are used in various industries due to their excellent variable stiffness behavior and innate response to external stimuli. The morphing wing is one of the technologies that employed the SMPs to reduce fuel consumption. This study proposes a novel morphing wing design using epoxy SMP as a wing skin, and a corrugated shape SMP composite (SMPC) as a wing core. The Dynamic mechanical analysis, bending and tensile tests were performed to determine the variable stiffness behavior of the skin, and the anisotropic behavior of the corrugated core. To evaluate the performance of the developed wing, the skin out-of-plane deformation is assessed, and the wing morphology is evaluated using the shape fixity and shape recovery tests. The results revealed that the skin out-of-plane deformation is within the acceptable limit for flight speed up to 0.16 Mach. Moreover, the wing shape recovery was excellent; the trailing edge of the wing is able to restore up to 99% of its bent position.

Numerical design of shape memory polymer composites with temperature-responsive SMA fillers

Shape memory polymers (SMP) are a class of stimuli-responsive materials that are able to respond to external stimulus such as heat by altering their shape. Bio-compatible SMPs have a number of advantages over static materials and are being studied extensively for biomedical and clinical applications (such as tissue stents and scaffolds). A previous study has demonstrated that the bio-compatible polymer blend of polylactic acid (PLA)/ thermoplastic polyurethane (TPU) (50/50 and 70/30) exhibit good shape memory properties. In this study, the mechanical and thermo-mechanical (shape memory) properties of TPU/PLA SMP blends were characterized; the compositions studied were 80/20, 65/35, and 50/50 TPU/PLA. In addition, porous TPU/PLA SMP blends were fabricated with a gas-foaming technique; and the morphology of the porous structure of these SMPs foams were characterized with scanning electron microscopy (SEM). The TPU/PLA bio-compatible SMP blend was fabricated with melt-blending and compression molding. The glass transition temperature (Tg) of the SMP blends was determined with a differential scanning calorimeter (DSC). The mechanical properties studied were the stress-strain behavior, tensile strength, and elastic modulus; and the thermomechanical (or shape memory) properties studied were the shape fixity rate (Rf), shape recovery rate (Rr), response time, and the effect of recovery temperature on Rr. The porous 80/20 PLA/TPU SMP blend was found to have the highest tensile strength, toughness and percentage extension, as well as the lowest density and uniform pore structure in the micron and submicron scale. The porous 80/20 TPU/PLA SMP blend may be further developed for specific biomedical and clinical applications where a combination of tensile strength, toughness, and low density are required.

Ultra-high stretchability and shape fixation rate shape memory polyurethanes based on cyclic polytetrahydrofuran molecular rings

Chapter 14 reviews shape memory polymers (SMPs). Shape memory polymers belong to the family of shape memory materials (SMMs), which can be deformed into a predetermined shape under some imposed specific conditions such as temperature, electric or magnetic field, as well as strain and stress. These shapes can be relaxed back to their original field-free shapes under thermal, electrical, magnetic, strain, stress, temperature, laser, or environmental stimuli. These transformations are essentially due to the elastic energy stored in SMMs during initial deformation. As a member of SMMs, SMPs are stimuli-sensitive polymers. Shape memory polymers normally use either heat or laser light energy as a stimulant to change shape. The thermally-induced shape memory effect can be observed by irradiation with infrared light, exposure to alternating magnetic fields, application of an electric field or immersion in water.

Multifunctional shape memory composites composed of shape memory polymers and shape memory alloys exhibit superior shape memory properties; therefore, it is of great interest to researchers to model their thermo-mechanical behavior numerically. Although a number of constitutive models of shape memory alloys and shape memory polymers have been developed, very few models have been developed for shape memory composites and validated with experimental data. In this study, we first review separately constitutive models of shape memory alloys and shape memory polymers developed in previous studies. Both models were validated with thermo-mechanical tests conducted on a shape memory alloy fiber and shape memory polymer, respectively. A constitutive model for the shape memory composite was then developed utilizing the homogenization scheme. Shape memory composites containing 0.5% or 1% of the shape memory alloy fiber volume content embedded in the shape memory polymer matrix were fabricated. Thermo-mechanical tests were carried out on these shape memory composites to validate the proposed constitutive model. The experimental results showed that the proposed shape memory composite model was able to predict the general trend of the thermo-mechanical behavior of the shape memory composites.

This chapter investigates the tribological behavior of shape memory material (SMM) thin films, a critical aspect of their successful implementation in diverse applications such as microelectromechanical systems (MEMS), biomedical devices, and smart coatings. SMMs, encompassing shape memory alloys (SMAs) like NiTi and shape memory polymers (SMPs), exhibit unique shape recovery capabilities, enabling innovative functionalities. The chapter begins by reviewing common fabrication techniques for SMM thin films, including PVD, CVD, sol-gel methods, electrospinning, and spin coating. Subsequently, it discusses key tribological testing and characterization methods, encompassing friction and wear tests (e.g., pin-on-disk, ball-on-disk, and scratch tests) and characterization techniques. The core of the chapter delves into the tribological properties of SMM films, analyzing factors influencing friction (e.g., surface roughness, and phase transformations) and wear (e.g., hardness, and elastic recovery). Strategies for mitigating wear and friction, such as solid lubrication, self-lubricating coatings, and surface texturing, are explored. This chapter includes the critical role of wear resistance in MEMS devices, the importance of biocompatibility and minimal wear debris in biomedical implants, and the potential of SMM-based smart coatings for adaptive friction control in various industrial settings.