Shape Memory Polymer Research Articles

Light-Driven Multidirectional Bending in Artificial Muscles.

Using light to drive polymer actuators can enable spatially selective complex motions, offering a wealth of opportunities for wireless control of soft robotics and active textiles. Here, the integration of photothermal components is reported into shape memory polymer actuators. The fabricated twist-coiled artificial muscles show on-command multidirectional bending, which can be controlled by both the illumination intensity, as well as the chirality, of the prepared artificial muscles. Importantly, the direction in which these artificial muscles bend does not depend on intrinsic material characteristics. Instead, this directionality is achieved by localized untwisting of the actuator, driven by selective irradiation. The reaction times of this bending system are significantly - at least two orders of magnitude - faster than heliotropic biological systems, with a response time up to one second. The programmability of the artificial muscles is further demonstrated for selective, reversible, and sustained actuation when integrated in butterfly-shaped textiles, along with the capacity to autonomously orient toward a light source. This functionality is maintained even on a rotating platform, with angular velocities of 6°/s, independent of the rotation direction. These attributes collectively represent a breakthrough in the field of artificial muscles, intended to adaptive shape-changing soft systems and biomimetic technologies.

Shape Memory Polymers in 4D Printing: Investigating Multi-Material Lattice Structures

This paper evaluates the design and fabrication of a thermoplastic polyurethane (TPU) shape memory polymer (SMP) using fused deposition modeling (FDM). The commercially available SMP filament was used to create parts capable of changing their shape following the application of an external heat stimulus. The characterization of thermal and viscoelastic properties of the SMP TPU revealed a proportional change in shape fixity and recovery with respect to heating and cooling rates, as well as a decreasing softening temperature with increasing shape memory history due to changes in the polymer microstructure. Inspired by the advancements in 3D and 4D printing, we investigated the feasibility of creating multi-material lattice structures using SMP and another thermoplastic with poor adhesion to TPU. A variety of interlocking lattice structures were evaluated by combining SMP with another thermoplastic that have poor adhesion with TPU. The tensile strength and failure modes of the fabricated multi-material parts were compared against homogenous SMP TPU specimens. It was found that the lattice interface failed first at approximately 41% of the ultimate strength of the homogenous part on average. The maximum recorded ultimate strength of the multi-material specimens reached 62% of SMP TPU’s ultimate strength. These characterizations can make 4D printing technology more accessible to common users and make it available for new markets.

Shape memory polymer lattice structures with programmable thermal recovery time

Shape memory polymer (SMP) lattice structures have garnered considerable attention due to their intrinsic capability for self-recovery and mechanical reconfiguration. The temporal path, encompassing aspects such as recovery time and deployment sequence, of the shape recovery process in SMP lattice structures holds paramount significance across various domains, including but not limited to medical devices and space deployable structures. Nonetheless, the programming of shape recovery time or deployment sequences in SMP lattice structures, particularly in scenarios devoid of external controllers, remains a challenge. In addressing these challenges, this study presents a novel class of SMP structures endowed with customizable thermal recovery times and programmable deployment sequences, leveraging the influence of structural geometry. Notably, the programmable recovery time and serialized deployment behavior of the proposed SMP lattice structure are achieved within a constant temperature environment, obviating the need for external time-varying stimuli. Finite element simulations and experimental validations corroborate that the proposed SMP structures can be programmed to exhibit recovery times spanning from mere seconds to several hundred seconds. Moreover, a three-stage sequential recovery behavior is attained without necessitating prior local configuration programming. Furthermore, exploiting the distinctive sequential reversibility inherent in a constant high-temperature environment, the designed lattice structure showcases the ability to transition to multiple distinct stable configurations by modulating the duration of high-temperature exposure. The proposed recovery time programmable SMP lattice structure thus presents a viable avenue for realizing intricate multistage controllable shape-shifting structures devoid of external control equipment.

Characterization of 4D printed shape memory and conventional polymer blends for thermal responsive applications

Characterization of 4D printed shape memory and conventional polymer blends for thermal responsive applications

Thermo-mechanical properties of shape-recoverable structural composites via vacuum-assisted resin transfer molding process and in-situ polymerization of poly (tert-butyl acrylate-co-acrylic acid) copolymer

When selecting a polymer matrix to make shape memory polymer composites (SMPCs), it is crucial to consider high elastic modulus below the switching temperature (Tsw), a significant variation in the modulus above Tsw, and the ability to control Tsw. This research introduces shape-recoverable structural composites fabricated from poly (tert-butyl acrylate-co-acrylic acid) (PtBA-AA), which has a significant modulus variation before and after Tsw. Capillary numbers are assessed to minimize void formation at varying acrylic acid (AA) concentrations, which regulate the copolymer’s polarity and the thermo-mechanical properties. The glass transition temperature of PtBA-AA can be adjusted from 47.4 °C to 91.6 °C. Furthermore, the elastic modulus of SMPC increases from 13 GPa to 20 GPa, whereas the tensile strength increases from 526 MPa to 889 MPa. The maximum recovery strength measured 100.4 MPa at an AA molar ratio of 0.23, accompanied by a fixity of 89.1 % and a recovery ratio of 97.2 %.

Poly(ethylene terephthalate) glycol/carbon black composites for 4D printing

This research introduces PETG-CB (poly(ethylene terephthalate) glycol modified with carbon black) composites, a new class of shape memory polymers 4D printed using the Fused Deposition Modeling (FDM) method. Nanocomposites with varying concentrations of CB (0.5 %, 1 %, and 3 % by weight) are developed to enhance the functional performance of PETG in 4D printing applications. Comprehensive characterization at the micro- and macro-scale, including dynamic thermal mechanical analysis (DMTA), scanning electron microscopy (SEM), and mechanical testing, is employed to assess the viscoelastic behavior, microstructural integrity, and mechanical strength under thermal stimulation. Experimental results reveal that CB addition significantly alters the glass transition temperature and improves mechanical properties, with the 1 % CB composite demonstrating optimal tensile strength and enhanced shape memory effects. SEM analysis confirms a uniform distribution of CB particles, contributing to the improved mechanical properties and printability of the nanocomposites. The shape memory tests show excellent recovery rates above 97 %, with faster recovery observed in composites with higher CB content. These findings highlight the potential of PETG-CB composites in applications requiring rapid response and high mechanical performance, making them promising materials for future advancements in the 4D printing technology.

Stimuli-responsive materials for 4D Printing: Mechanical, Manufacturing, and Biomedical Applications

Four-dimensional printing (4DP) is an interesting field of study that integrates intelligent materials into three-dimensional printing (3DP). Using this method, objects that can gradually change form in response to environmental stimuli, such as moisture, electric or magnetic fields, UV light, temperature swings, pH variations, or alterations in the composition of ions, can be created. Intelligent materials, such as shape memory polymers, alloys, hydrogels, ceramics, and composites, which can react to external stimuli, are essential for 4DP. Unlike traditional 3D printing, the ability of 4D printed materials to undergo structural and/or property alterations expands their applications across industries such as aerospace, biomedical, soft robotics, engineering, and fashion, potentially revolutionizing manufacturing. This review provides an overview of current advancements in 4D printing and discusses both established and emerging smart materials with shape-changing capabilities and their responses to stimuli. It explores the differences between 4D and 3D printing technologies, traces the origins of 4D printing, and examines the materials involved, with emphasis on shape-memory polymers. This paper also highlights the applications of 4D printing in pharmaceuticals and biomedicine, particularly in dynamic polymers, and in the development of drugs for genetic diseases. In addition, it discusses progress in creating implantable biomedical devices and pharmaceutical medications. Despite being in its early stages, 4D printing technology has immense potential and promising exciting possibilities in the future.

Manufacture dependent differential biodegradation of 3D printed shape memory polymers

ABSTRACT In the field of tissue engineering, 3D printed shape memory polymers (SMPs) are drawing increased interest. Understanding how these 3D printed SMPs degrade is critical for their use in the clinic, as small changes in material properties can significantly change how they behave after in vivo implantation. Degradation of 3D printed acrylated poly(glycerol-dodecanedioate) (APGD) was examined via in vitro hydrolytic, enzymatic, and in vivo subcutaneous implantation assays. Three APGD manufacturing modalities were assessed to determine differences in degradation. Material extrusion samples showed significantly larger mass and volume loss at 2 months, compared to lasercut and vat photopolymerization samples, under both enzymatic and in vivo degradation. Critically, melt transition temperatures of degraded PGD increased over time in vitro, but not in vivo. Histology of tissue surrounding APGD implants showed no significant signs of inflammation compared to controls, providing a promising outlook for use of 3D printed APGD devices in the clinic.

Tuneable mechanical performance and reusability of 4D-printed heterogeneous metamaterials using shape memory biomass-derived polymer

ABSTRACT Herein, hybrid triply periodic minimal surfaces (TPMS) metamaterials were structurally designed and additively manufactured via 4D printing using a biomass-derived shape memory polymer. Two TPMS topologies including gyroid and diamond with homogeneous and graded structures were separately generated by adjusting the C parameter. The mechanical performance and deformation behaviour between the uniform and graded structures were compared, and the compressive shaping-recovery cycles were conducted. The results showed that the gyroid (C:0.7→0.4) and diamond (C:0.7→0.4) samples exhibited superior reusability to the uniform counterparts. Subsequently, heterogeneous metamaterials assembled by gyroid and diamond subsections were established and 4D-printed. It was observed that both the yielding and shape recovery were preferred occurring in the gyroid part, while the diamond subsection played a stronger supporting effect. Therefore, the compressive performance and deformation mode could be feasibly tuned by the hybridisation of TPMS, which broadened the functional and sustainable applications in energy absorption, intelligent protection and medical implants.

Shape memory behavior of polyethylene-foam-based nanocomposite for sustainable triboelectric nanogenerators

Durable self-powering electronic devices need high-performance triboelectric nanogenerators (TENGs) with shape memory capabilities. A novel porous shape memory polymer nanocomposite that includes polyethylene foam (PEF) with switching molecules and graphitic carbon nitride nanosheets is designed to create a robust, high-performance TENG. This nanocomposite shows outstanding thermal-actuated shape recovery capabilities by heating at 70 °C. Although the TENG initially excels in output performance (1840 V voltage, 79.66 mW.cm‐2 power density), prolonged harsh conditions cause nanocomposite deformation and performance decline. However, heating the nanocomposite to 70 °C fully restores the TENG’s original performance. Besides, the COMSOL Multiphysics simulation of TENG aligns with the experimental results. Furthermore, two TENG application scenarios are demonstrated: self-charging humidity sensing (where TENG’s output performance is humidity-sensitive and decreases as humidity rises) and powering electronic devices (where TENG turns on commercial LEDs, powers a smartwatch, and charges capacitors). Therefore, this TENG may open new possibilities for future self-powered electrical devices and sensors.

Mechanical and shape recovery characterization of conventional and shape memory polymer 4D printed sandwich composition specimens

Mechanical and shape recovery characterization of conventional and shape memory polymer 4D printed sandwich composition specimens

Advancing sustainable shape memory polymers through 4D printing of polylactic acid-polybutylene adipate terephthalate blends

One of the major challenges in 4D printed Shape Memory Polymers (SMPs) is their slow response to thermal stimuli. Synthetic carbon fillers have been introduced to address this issue; however, their use comes with environmental concerns. On the other hand, Polylactic Acid (PLA) is commonly used for 3D/4D printing of SMPs, but it requires softening to achieve large deformations. This research paper introduces a sustainable solution through blending PLA with Polybutylene Adipate Terephthalate (PBAT) that not only addresses these challenges but also possesses environmental eco-friendliness due to PBAT’s high biodegradability rate. PBAT with weight percentages of 15 %, 30 %, and 45 % is utilized to soften PLA, and their composites are successfully 4D printed. Mechanical properties, shape memory effects, morphology, and thermal behaviors are comprehensively investigated. The results show that blending PLA with 45 % PBAT weight results in excellent features such as 220 % elongation and 93 % shape recovery in just a few seconds. The other two PLA-PBAT blends achieve complete shape recovery in less than 8 s. The PLA-PBAT contains 15 % PBAT, in addition to high strength (40 MPa), has 17 % elongation. This, coupled with the low melting temperature of PBAT, drives the high shape recovery rate. Scanning electron microscopy images finally confirm the high printability of all three blends, with the PLA-PBAT composite containing 30 % PBAT exhibiting the best quality in layer interfaces and the least porosity.

Tuning Spatially Resolved Shape Memory Effects of Stimuli‐Responsive Macroporous Photonic Crystals

Stimuli‐responsive photonic crystals with patterned microstructures are of great interest in developing reconfigurable nano‐optical devices. Leveraging unconventional all‐room‐temperature shape memory efforts and spatially resolved photopolymerization, herein, a facile method for micropatterning stimuli‐responsive photonic crystals is reported. Macroporous shape memory polymer (SMP) photonic crystals fabricated by colloidal templating can be deformed by cold programming, triggering the disappearance of their original structural colors. Exposure of the deformed samples to UV light through a photomask selectively disables the shape memory capabilities in the UV‐exposed regions. Hidden micropatterns defined by the photomask can be revealed by exposing the colorless SMP films to ethanol vapor, which triggers the shape memory recovery of the “memorized” ordered microstructures and the corresponding structural colors. Extensive nanoindentation experiments indicate that the exposure to UV light increases the crosslinking density and enhances the elastic modulus and toughness of the exposed regions by a factor of ≈2.0 and ≈5.2, respectively. Due to the formation of these extra crosslinks in the deformed configuration, they prevent normal shape memory behavior where the strained polymer chains rearrange from the temporary to permanent configuration when triggered by an external stimulus. This simple micropatterning technology can enable multistimuli‐responsive reconfigurable nanophotonic devices and chromogenic anticounterfeiting labels.

Investigations into 4D printed PLA/graphite composite with thermal induced shape memory effect

PurposeThe paper aims to fabricate shape memory composites using polylactic acid (PLA) matrix and graphite. Shape memory polymers are a promising family of materials for biomedical applications because of their favourable mechanical properties, fast reactions and good biocompatibility. For most SMPs, however, achieving controllable sequential shape change is challenging.Design/methodology/approachIn the present work, 4D printing technology is used to fabricate shape memory composites using polylactic acid (PLA) matrix and graphite. A comparative study of pure PLA and graphite’s different weight % composition has been done.FindingsBy carefully managing the deformation state, PLA with graphite shape memory composites produced controllable sequential deformation with an amazing shape memory effect. Surface morphology, thermal properties, melt flow index and shape recovery tests have all been carried out to assess the qualities of manufactured samples.Originality/valueThis is a one-of-a-kind to fabricate shape memory composites using graphite and a PLA matrix. Thus, this research attempts to deliver the possible use of PLA/graphite composites fabricated using 4D printing in robotics and biomedical devices.Graphical

Smart wetting based on thermal-induced dual-scale surfaces

Surfaces wettability influences drag reduction in engineering applications, and surface structure largely determines the wetting properties. This study proposes a novel thermal shape memory polymer (TSMP) with dual-scale structure surface capable of reversible transformation via working temperature. A preparation method for TSMP with micro-nano dual-scale structure is also proposed to enhance hydrophobicity comparing with microstructure. Experimental results indicate TSMP micro-nanostructure surfaces achieve reversible conversion between hydrophilicity and hydrophobicity, TSMPs with micro-nano structure also exhibit good recovery. Simulation was established to understand tunable wettability and fluid flow mechanisms of TSMP reversible two-phase microstructures aiding design of TSMP microstructures with adjustable wettability.

Enhancing Tensile Modulus of Polyurethane-Based Shape Memory Polymers for Wound Closure Applications through the Addition of Palm Oil.

Thermo-responsive, biocompatible polyurethane (PU) with shape memory properties is highly desirable for biomedical applications. An innovative approach to producing wound closure strips using shape memory polymers (SMPs) is of significant interest. In this work, PU composed of polycaprolactone (PCL) and 1,4-butanediol (BDO) was synthesized using two-step polymerization. Palm oil (PO) was added to PU for enhancing the Young's modulus of the PU beyond the set criterion of 130 MPa. It was found that PU had the ability to crystallize at room temperature and the segments of individual PCL and BDO polyurethanes crystallized separately. The crystalline domains and hard segment of PU greatly affected the tensile properties. The reduction of crystalline domains by the addition of PO and deformation at the higher melting temperature of the crystalline PCL polyurethane phase improved the shape fixity and shape recovery ratios. The new irreversible phase, raised from the permanent deformation upon stretching at the between melting temperature of the crystalline PCL and BDO polyurethanes of 70 °C, resulted in a decrease in shape fixity ratio after the first thermomechanical stretching-recovering cycles. The demonstration of PU as a wound closure strip showed its efficiency and potential until the surgical wound healed.

Nucleation, Development and Healing of Micro-Cracks in Shape Memory Polyurethane Subjected to Subsequent Tension Cycles.

Thermoresponsive shape memory polymers (SMPs) have garnered increasing interest for their exceptional ability to retain a temporary shape and recover the original configuration through temperature changes, making them promising in various applications. The SMP shape change and recovery that happen due to a combination of mechanical loading and appropriate temperatures are related to its particular microstructure. The deformation process leads to the formation and growth of micro-cracks in the SMP structure, whereas the subsequent heating over its glass transition temperature Tg leads to the recovery of its original shape and properties. These processes also affect the SMP microstructure. In addition to the observed macroscopic shape recovery, the healing of micro-crazes and micro-cracks that have nucleated and developed during the loading occurs. Therefore, our study delves into the microscopic aspect, specifically addressing the healing of micro-cracks in the cyclic loading process. The proposed research concerns a thermoplastic polyurethane shape memory polymer (PU-SMP) MM4520 with a Tg of 45 °C. The objective of the study is to investigate the effect of the number of tensile loading-unloading cycles and thermal shape recovery on the evolution of the PU-SMP microstructure. To this end, comprehensive research starting from structural characterization of the initial state and at various stages of the PU-SMP mechanical loading was conducted. Dynamic mechanical analysis (DMA), differential scanning calorimetry (DSC), wide-angle X-ray scattering (WAXS) and scanning electron microscopy (SEM) were used. Moreover, the shape memory behavior in the thermomechanical loading program was investigated. The obtained average shape fixity value was 99%, while the shape recovery was 92%, which confirmed good shape memory properties of the PU-SMP. Our findings reveal that even during a single loading-unloading tension cycle, crazes and cracks nucleate on the surface of the PU-SMP specimen, whereas the subsequent temperature-induced shape recovery process carried out at the temperature above Tg enables the healing of micro-cracks. Interestingly, the surface of the specimen after three and five loading-unloading cycles did not exhibit crazes and cracks, although some traces of cracks were visible. The traces disappeared after exposing the material to heating at Tg + 20 °C (65 °C) for 30 min. The crack closure phenomenon during deformation, even without heating over Tg, occurred within three and five subsequent cycles of loading-unloading. Notably, in the case of eight loading-unloading cycles, cracks appeared on the surface of the PU-SMP and were healed only after thermal recovery at the particular temperature over Tg. Upon reaching a critical number of cycles, the proper amount of energy required for crack propagation was attained, resulting in wide-open cracks on the material's surface. It is worth noting that WAXS analysis did not indicate strong signs of typical highly ordered structures in the PU-SMP specimens in their initial state and after the loading history; however, some orientation after the cyclic deformation was observed.

A Janus textile mimics leaf stoma for dynamic thermal regulation through a shape memory coating

The development of smart textiles with thermal/moisture regulation is crucial for outdoor activities and healthcare applications. However, incorporating dynamic adaptive thermal management into conventional fabrics to provide consistent comfort remains a significant challenge. Inspired by the ingenious stomata of plant leaves, a novel Janus-type cotton fabric is fabricated by covalently linking shape memory polymer (SMP) on the surfaces of cotton fibers via the combination of the “grafting from” strategy and the “mist polymerization” methodology. This unique coating induces yarns to dynamically twist in response to temperature, switching the pore apertures in the fabric and allowing for adaptive adjustment of both heat dissipation capability and moisture permeability. The resulting fabrics provide effective thermal insulation at low temperatures, while demonstrating an enhanced cooling effect at high temperatures. Meanwhile, the asymmetrical distribution of SMP across the fabric enables unidirectional water transport, improving sweat removal and cooling. Additionally, the excellent biocompatibility and fabric flexibility allow for comfortable garment fabrication. This work provides novel insights into the field of self-adaptive thermal regulation, demonstrating promising potential for smart textile applications.

Shape memory polymer composite hinges for solar sails

Smart hinges for self-deployable solar sails have been prototyped by using shape memory polymer composites (SMPCs). During hinge lamination, a shape memory (SM) epoxy resin is deposited between carbon fiber reinforced (CFR) plies, and consolidation is obtained by an out-of-autoclave (OOA) process. The smart hinges have been assembled with CFR booms to prototype a small self-deployable square sail. Polymeric supports, made by fused deposition modelling (FDM), have been added for correct assembly and to drive the recovery of the SMPC hinges. Flexible heaters have been used as heating elements for SMPC hinge activation. The sail consisted of a polyimide membrane which was connected with the composite booms by flexible joints. The solar sail prototype size has been limited to 260 × 260 mm2 for on-Earth testing, with 4 hinges at the middle length of the composite booms. Recovery tests have shown the ability of the smart hinges to overcome the sail weight and additional friction forces, and the small sail has been successfully deployed in laboratory. Full recovery has been achieved in less than 3 min under different configurations.

Application of Additive Manufacturing in the Development of Polymeric Bioresorbable Cardiovascular Stents: A Review

AbstractPolymeric vascular bioresorbable stents (BRSs) have been widely used for the treatment of coronary artery diseases. While additive manufacturing (AM) is changing the landscape of the healthcare sector by enabling the realization of patient‐specific stents with highly complex structures. There are, however, challenges associated with the use of polymeric BRS, particularly in‐stent restenosis (ISR), related to its poor mechanical properties. Therefore, the aim of this review is to provide an overview of recent advancements in the development of polymeric BRSs designed to meet both mechanical and biological requirements. First, biopolymers as well as shape memory polymers (SMPs) that are suitable for BRSs are highlighted and briefly described. Second, different types of designing structures of vascular stents in addition to introducing effective mechanical metamaterials, e.g., negative Poisson ratio (NPR) structures are addressed. Subsequently, AM methods currently being used to fabricate polymeric BRSs, are discussed and compared with conventional fabrication methods. Lastly, future directions for research are proposed in relation to existing challenges to the realization of a new generation of AM BRSs. Overall, this paper serves as a benchmark for future cardiovascular applications, especially in order to obtain clinically viable polymeric vascular stents by selecting suitable polymers, designs, and AM technologies.