4D Printed Protein-AuNR Nanocomposites with Photothermal Shape Recovery.

4D printing is the 3D printing of objects that change chemically or physically in response to an external stimulus over time. Photothermally responsive shape memory materials are attractive for their ability to undergo remote activation. While photothermal methods using gold nanorods (AuNRs) have been used for shape recovery, 3D patterning of these materials into objects with complex geometries using degradable materials has not been addressed. Here, we report on the fabrication of 3D printed shape memory bioplastics with photo-activated shape recovery. Protein-based nanocomposites based on bovine serum albumin (BSA), poly (ethylene glycol) diacrylate and gold nanorods were developed for vat photopolymerization. These 3D printed bioplastics were mechanically deformed under high loads, and the proteins served as mechanoactive elements that unfolded in an energy-dissipating mechanism that prevented fracture of the thermoset. The bioplastic object maintained its metastable shape-programmed state under ambient conditions. Subsequently, up to 99% shape recovery was achieved within 1 min of irradiation with near-infrared light. Mechanical characterization and small angle X-ray scattering (SAXS) analysis suggest that the proteins mechanically unfold during the shape programming step and may refold during shape recovery. These composites are promising materials for the fabrication of biodegradable shape-morphing devices for robotics and medicine.

Hofmeister-Effect-Driven Hybrid Glycerogels for Perfect Wide-Temperature Shape Fixity and Shape Recovery in Soft Robotics Applications.

Shape memory gels have emerged as crucial elements in soft robotics, actuators, and biomedical devices; however, several problems persist, like the trade-off between shape fixity and shape recovery, and the limited temperature range for their application. This article introduces a new class of shape memory hybrid glycerogels (GGs) designed to address these limitations. The well-modulated internal structure of the GGs, facilitated by the Hofmeister salting-out effect, strategically incorporates a higher crystallite content, abundant crosslinking points, and a high elastic modulus. Unlike reported shape memory gels, the GG exhibits a perfect triple-step shape memory behavior in air with 100% shape fixity in a wide programming temperature range (75-135 °C) and simultaneously achieves 100% shape recoverability. The gel recovers its shape at -40 °C under near-infrared light across a wide programming temperature range (25-135 °C), showing unexpected initiation even at subzero temperatures. Inspired by the mechanics of composite structures, a method is proposed to integrate the GG seamlessly with a shape memory alloy, which further expands the temperature range that yields perfect shape memory properties. Finally, two light-controlled fluttering and crawling soft robot prototypes are engineered to illustrate the versatility and potential applications of the composite gel in soft robotics.

A self-elastic chitosan sponge integrating active and passive hemostatic mechanisms for effectively managing uncontrolled coagulopathic hemorrhage.

Developing a self-elastic sponge integrating active and passive hemostatic mechanisms for the effective management of uncontrolled coagulopathic hemorrhage remains a challenge. We here developed a chitosan-based sponge by integrating freeze-drying, chemical decoration of alkyl chains and phosphate groups, and physical loading of thrombin. The sponge exhibited high mechanical strength, self-elasticity, and rapid shape recovery. The sponge facilitated blood cell adhesion, aggregation, and activation through hydrophobic and electrostatic interactions, as well as accelerated blood clotting. The sponge exhibited higher efficacy than commercial gauze and gelatin sponge in managing uncontrolled hemorrhage from heparinized rat tail amputation, liver superficial injury, and liver perforating wound models. In addition, the sponge exhibited favorable biodegradability and biocompatibility. These findings revealed that the developed sponge holds great potential as a novel hemostat for effectively managing uncontrolled coagulopathic hemorrhage from superficial and perforating wounds.

Recent Advances in Polymer Nanocomposites: Unveiling the Frontier of Shape Memory and Self-Healing Properties-A Comprehensive Review.

Shape memory and self-healing polymer nanocomposites have attracted considerable attention due to their modifiable properties and promising applications. The incorporation of nanomaterials (polypyrrole, carboxyl methyl cellulose, carbon nanotubes, titania nanotubes, graphene, graphene oxide, mesoporous silica) into these polymers has significantly enhanced their performance, opening up new avenues for diverse applications. The self-healing capability in polymer nanocomposites depends on several factors, including heat, quadruple hydrogen bonding, π-π stacking, Diels-Alder reactions, and metal-ligand coordination, which collectively govern the interactions within the composite materials. Among possible interactions, only quadruple hydrogen bonding between composite constituents has been shown to be effective in facilitating self-healing at approximately room temperature. Conversely, thermo-responsive self-healing and shape memory polymer nanocomposites require elevated temperatures to initiate the healing and recovery processes. Thermo-responsive (TRSMPs), light-actuated, magnetically actuated, and Electrically actuated Shape Memory Polymer Nanocomposite are discussed. This paper provides a comprehensive overview of the different types of interactions involved in SMP and SHP nanocomposites and examines their behavior at both room temperature and elevated temperature conditions, along with their biomedical applications. Among many applications of SMPs, special attention has been given to biomedical (drug delivery, orthodontics, tissue engineering, orthopedics, endovascular surgery), aerospace (hinges, space deployable structures, morphing aircrafts), textile (breathable fabrics, reinforced fabrics, self-healing electromagnetic interference shielding fabrics), sensor, electrical (triboelectric nanogenerators, information energy storage devices), electronic, paint and self-healing coating, and construction material (polymer cement composites) applications.

Synthesis and characterization of thermosensitive 2-hydroxypropyl-trimethylammonium chitin and its antibacterial sponge for noncompressible hemostasis and tissue regeneration.

Noncompressible hemorrhage is a leading cause of preventable death in battlefield/civilian trauma. The development of novel injectable and biodegradable hemostatic sponges, with rapid shape recovery and excellent antibacterial activity that can control hemorrhage in noncompressible bleeding sites and promote in situ tissue regeneration is still urgently needed. In this study, thermo/pH sensitive 2-hydroxypropyl-trimethylammonium chitins (QCHs) with low degree of quaternization substitution (DS: 0.07-0.23) and high degree of acetylation (DA: 0.91-0.94) were synthesized homogeneously for the first time. Their chemical compositions including DS and DA were characterized accurately by proton NMR for the first time. High strength QCH based sponges with good water/blood absorbency, rapid shape recovery and good antibacterial activity were prepared without using any crosslinkers but only due to their thermosensitive property, since they are soluble at low temperature but insoluble at high temperature. Compared with commercial products, the QCH sponges with cationic groups had the stronger pro-coagulant ability, better hemostatic effect in normal/heparinized liver perforation and femoral artery models in rats and porcine subclavian arteriovenous resection model. Moreover, the porous structure and biodegradability of the QCH sponges could promote in situ tissue regeneration. Overall, the QCH sponges show great clinical translational potential for noncompressible hemorrhage and tissue regeneration.

Heat-triggered shape recovery, EMI shielding and flame retardant: A novel cellulose/M(OH)(OCH3)@dopamine@Ag (M=Co, Ni) nanopaper for early fire alarm.

Fire alarm systems are essential for protecting lives and properties from fire hazards. However, most of the existing fire alarm nanopapers rely on the resistance reduction after heating, which requires direct contact with the flame. In this study, we present a novel fire alarm nanopaper (CMPA) based on heat-triggered shape recovery. The CMPA is composed of hydroxypropyl methyl cellulose (HPMC) as the matrix and 2D nanomaterials M(OH)(OCH3) as fillers. When the temperature of CMPA exceeded the glass transition, the thrice-folded CMPA-1.0 flattened in 30s and connected to the alarm circuit based on its conductive surface. According to the results, the CMPA-1.0 with a thickness of about 0.2 mm had an efficient electromagnetic shielding of 42.1 dB. Moreover, the CMPA-1.0 self-extinguished rapidly after being ignited with its original shape preserved. The peak heat release rate of CMPA-1.0 was 108.9 W/g, which was 61.9 % lower than that of HPMC. Furthermore, the thermal conductivity of CMPA-1.0 reached to 0.317 W m-1 K-1, which was 40.8 % higher than that of HPMC, reducing the heat accumulation effectively. This work shows that CMPA is an ideal material for sensitive and safe early fire alarm, and the strategy based on heat-triggered shape recovery is promising in fire alarm application.

Synthesis of High-Value Bio-Based Polyamide 12,36 Microcellular Foams with Excellent Dimensional Stability and Shape Recovery Properties.

The search for alternatives to petroleum-based thermoplastic polyamide elastomers (TPAEs) has recently drawn great interest. In this study, a bio-massed TPAE, PA12,36, was synthesized using 1,12-dodecanediamine (DDA) and fatty dimer acid (FDA, PripolTM1009) precursors via catalyst and solvent-free melt polycondensation. The molecular structure and molecular weight of the PA12,36 were characterized by 1H NMR, FTIR, and GPC. PA12,36 displayed a low melting temperature of 85.8 °C, an initial degradation temperature of 425 °C, and a glass-transition temperature of 30.4 °C, whereas it sustained satisfactory tensile strength (10.0 MPa) and superior strain at break (1378%). Furthermore, PA12,36 was foamed by supercritical CO2, and the cell size, cell density, and porosity were determined. The entangled long-chained FDA component generated a physically crosslinked network, which promoted the melt viscosity of PA12,36 against elongations of foam cell growth and increased foamability significantly. As a result, uniform structured cellular foams with a cell diameter of 15-24 µm and high cell density (1011 cells/cm3-1012 cells/cm3) were successfully achieved. The foaming window was widened from 76 to 81 °C, and the expansion ratio was increased from 4.8 to 9.6. Additionally, PA12,36 foam with a physically crosslinked structure presented a better creep shape recovery percentage (92-97.9%) and sturdier dimensional stability. This bio-based PA12,36 foam is a promising candidate to replace petroleum-based thermoplastic elastomer foams for engineering applications, particularly shoe soles.

Investigation of Shape Memory Polyurethane Properties in Cold Programming Process Towards Its Applications.

Thermoresponsive shape memory polymers (SMPs) with the remarkable ability to remember a temporary shape and recover their original one using temperature have been gaining more and more attention in a wide range of applications. Traditionally, SMPs are investigated using a method named often "hot-programming", since they are heated above their glass transition temperature (Tg) and after that, reshaped and cooled below Tg to achieve and fix the desired configuration. Upon reheating, these materials return to their original shape. However, the heating of SMPs above their Tg during a thermomechanical cycle to trigger a change in their shape creates a temperature gradient within the material structure and causes significant thermal expansion of the polymer sample resulting in a reduction in its shape recovery property. These phenomena, in turn, limit the application fields of SMPs, in which fast actuation, dimensional stability and low thermal expansion coefficient are crucial. This paper aims at a comprehensive experimental investigation of thermoplastic polyurethane shape memory polymer (PU-SMP) using the cold programming approach, in which the deformation of the SMP into the programmed shape is conducted at temperatures below Tg. The PU-SMP glass transition temperature equals approximately 65 °C. Structural, mechanical and thermomechanical characterization was performed, and the results on the identification of functional properties of PU-SMPs in quite a large strain range beyond yield limit were obtained. The average shape fixity ratio of the PU-SMP at room temperature programming was found to be approximately 90%, while the average shape fixity ratio at 45 °C (Tg - 20 °C) was approximately 97%. Whereas, the average shape recovery ratio was 93% at room temperature programming and it was equal to approximately 90% at 45 °C. However, the results obtained using the traditional method, the so-called hot programming at 65 °C, indicate a higher shape fixity value of 98%, but a lower shape recovery of 90%. Thus, the obtained results confirmed good shape memory properties of the PU-SMPs at a large strain range at various temperatures. Furthermore, the experiments conducted at both temperatures below Tg demonstrated that cold programming can be successfully applied to PU-SMPs with a relatively high Tg. Knowledge of the PU-SMP shape memory and shape fixity properties, estimated without risk of material degradation, caused by heating above Tg, makes them attractive for various applications, e.g., in electronic components, aircraft or aerospace structures.

3D Chiral Energy-Absorbing Structures with a High Deformation Recovery Ratio Fabricated via Selective Laser Melting of the NiTi Alloy.

Excellent energy-absorbing structures have been highly sought after in engineering applications to improve devices and personal safety. The ideal energy absorption mechanism should exhibit characteristics such as lightweight, high energy absorption capacity, and efficient reusability. To address this demand, a novel three-dimensional (3D) chiral lattice structure with compression-twist coupling deformation is fabricated by combining the left and right chiral units. The proposed structure was fabricated in NiTi shape memory alloys (SMAs) by using laser powder bed fusion technology. The compression experiment result indicates that the shape recovery ratio is as high as 94% even when the compression strain is over 80%. Additionally, the platform strain reaches as high as 66%, offering high-level specific energy absorption, i.e., 213.02 J/g. The obtained results are of great significance for basic research and engineering applications of energy-absorbing structures with high deformation recovery ratios.

Functionalized TMC and ε-CL elastomers with shape memory and self-healing properties.

Introduction: Smart elastomers, which possess self-healing and shape memory capabilities, have immense potential in the field of biomedical applications. Polycarbonates and polyesters have gained widespread interest due to their remarkable biocompatibility over the last century. Nevertheless, the lack of functional versatility in conventional polyesters and polycarbonates means that they fall short of meeting the ever-evolving demands of the future. Methods: This paper introduced a new smart elastomer, named mPEG43-b-(PMBC-co-PCL)n, developed from polyester and polycarbonate blends, that possessed shape memory and self-heal capabilities via a physical crosslinking system. Results: The material demonstrated a significant tensile strength of 0.38 MPa and a tensile ratio of 1155.6%, highlighting its favorable mechanical properties. In addition, a conspicuous shape retrieval rate of 93% was showcased within 32.5 seconds at 37°C. Remarkably, the affected area could be repaired proficiently with no irritation experienced during 6h at room temperature, which was indicative of an admirable repair percentage of 87.6%. Furthermore, these features could be precisely modified by altering the proportion of MBC and ε-CL to suit individual constraints. Discussion: This innovative elastomer with exceptional shape memory and self-heal capabilities provides a solid basis and promising potential for the development of self-contracting intelligent surgical sutures in the biomedical field.

Two-Pronged Approach: Synergistic Tuning of the Surface and Carbon Core to Achieve Yellow Emission in Lignin-Based Carbon Dots.

In this study, yellow emissive lignin-based carbon dots (Y-CDs) were successfully prepared through a synergistic approach to adjust its surface and carbon core states. The lignin was initially effectively oxidized and carboxymethylated to impart abundant -COOH onto the precursor, which eventually adjusts the surface state of the CDs. Subsequently, α-naphthol was employed during the solvothermal treatment of lignin with the aim of elevating the sp2 domain content in the CDs and, thus, adjusting its carbon core state. The obtained Y-CDs possessed abundant carboxyl groups and nanoscale spherical shape with an average diameter of 5.21 nm. Meanwhile, the energy gap of Y-CDs was 2.46 eV and the optimal emission wavelength was 561 nm under the excitation wavelength of 410 nm. Synergistic adjusting carbon core and surface of the Y-CDs would alter the surface charge distribution and promote the delocalization of π electrons, and thus lead to a red shifting with the emission wavelength of 154 nm. Furthermore, a shape memory film with excellent recovery performance and fluorescent properties was designed by embedding the Y-CDs into polyvinyl alcohol (PVA) polymer. The incorporation of Y-CDs could impart the film with considerable high-value applications in the fields of intelligent sensing, biomedicine, and tissue engineering.

MSSPA-GC: Multi-Scale Shape Prior Adaptation with 3D Graph Convolutions for Category-Level Object Pose Estimation.

Category-level object pose estimation aims to predict the 6D object pose and size of arbitrary objects from known categories. It remains a challenge due to the large intra-class shape variation. Recently, the introduction of the shape prior adaptation mechanism into the normalized canonical coordinates (i.e., NOCS) reconstruction process has been shown to be effective in mitigating the intra-class shape variation. However, existing shape prior adaptation methods simply map the observed point cloud to the normalized object space, and the extracted object descriptors are not sufficient for the perception of the object pose. As a result, they fail to predict the pose of objects with complex geometric structures (e.g., cameras). To this end, this paper proposes a novel shape prior adaption method named MSSPA-GC for category-level object pose estimation. Specifically, our main network takes the observed instance point cloud converted from the RGB-D image and the prior shape point cloud pre-trained on the object CAD models as inputs. Then, a novel 3D graph convolution network and a PointNet-like MLP network are designed to extract pose-aware object features and shape-aware object features from these two inputs, respectively. After that, the two-stream object features are aggregated through a multi-scale feature propagation mechanism to generate comprehensive 3D object descriptors that maintain both pose-sensitive geometric stability and intra-class shape consistency. Finally, by leveraging object descriptors aware of both object pose and shape when reconstructing the NOCS coordinates, our approach elegantly achieves state-of-the-art performance on the widely used REAL275 and CAMERA25 datasets using only 25% of the parameters compared with existing shape prior adaptation models. Moreover, our method also exhibits decent generalization ability on the unconstrained REDWOOD75 dataset.

Electrically and Thermally Triggered Three-Dimensional Graphene-Foam-Reinforced Shape Memory Epoxy Composites.

Shape memory polymer (SMP) epoxy composites have attracted significant attention due to their easy processing, lightweight nature, and ability to recover strain. However, their limited recovery rate and inferior mechanical properties have hindered their functional applications. This research explores the potential of three-dimensional (3D) graphene foam (GrF) as a highly efficient reinforcement for SMP epoxy composites. We demonstrated that the incorporation of a mere 0.13 wt.% GrF into mold-cast SMP epoxy leads to a 19% increase in the glass transition temperature (Tg). To elucidate the reinforcing mechanism, we fabricated and extensively analyzed composites with varying weight percentages of GrF. The GrF-based SMP epoxy composite exhibits a 57% increase in thermal conductivity, measuring 0.296 W mK-1 at 70 °C, due to the interconnected 3D graphene network within the matrix. Notably, this composite also demonstrates remarkable electrical conductivity, making it suitable for dual-triggering applications. The GrF-SMP epoxy composite achieves a maximum shape recovery ratio and a significant 23% improvement in the recovery rate, effectively addressing the issue of slow recovery associated with SMPs. We investigated the effect of switching temperatures on the shape recovery rate. We identified the optimal triggering temperature to initiate shape recovery for epoxy SMP and GrF-epoxy SMP as thermal energy equivalent to Tg + 20 °C. Additionally, we fabricated a bird-shaped composite using GrF reinforcement, which showcases self-healing capabilities through the crack opening and closure and serves as a tangible demonstration of the transformative potential of the composite. These GrF-epoxy SMP composites, responsive to stimuli, hold immense promise for diverse applications, such as mechanical systems, wearable sensors, morphing wings, foldable robots, and antennas.

Printing Parameters of Fused Filament Fabrication Affect Key Properties of Four-Dimensional Printed Shape-Memory Polymers.

Extrusion-based (fused filament fabrication) three-dimensional (3D) printing of shape-memory polymers (SMPs) has the potential to rapidly produce highly customized smart-material parts. Yet, the effects of printing parameters on the shape-memory properties of printed SMPs remain poorly understood. To study the extent to which the 3D printing process affects the shape-memory properties of a printed SMP part, here temperature, extrusion rate multiplier, and fiber orientation were systematically varied, and their effect on shape-memory fixing and recovery ratios was evaluated. Fiber orientation, as determined by print path relative to the direction(s) of loading during shape-memory programming, was found to significantly impact the fixing ratio and the recovery ratio. Temperature and multiplier had little effect on either fixing ratio or recovery ratio. To facilitate the use of printed SMP parts in biomedical applications, a cell viability assay was performed on 3D-printed samples prepared using varied temperature and multiplier. Reduction in multiplier was found to increase cell viability. The results indicate that fiber orientation can critically impact the shape-memory functionality of 3D-printed SMP parts, and that multiplier can affect cytocompatibility of those parts. Thus, researchers and manufacturers employing SMPs in 3D-printed parts and devices could achieve improved part functionality if print paths are designed to align fiber direction with the axis(es) in which strain will be programmed and recovered and if the multiplier is optimized in biomedical applications in which a part will contact cells.

Chitosan Sponges with Instantaneous Shape Recovery and Multistrain Antibacterial Activity for Controlled Release of Plant-Derived Polyphenols.

Biomass-derived materials with multiple features are seldom reported so far. Herein, new chitosan (CS) sponges with complementary functions for point-of-use healthcare applications were prepared by glutaraldehyde (GA) cross-linking and tested for antibacterial activity, antioxidant properties, and controlled delivery of plant-derived polyphenols. Their structural, morphological, and mechanical properties were thoroughly assessed by Fourier-transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), and uniaxial compression measurements, respectively. The main features of sponges were modulated by varying the CS concentration, cross-linking ratio, and gelation conditions (either cryogelation or room-temperature gelation). They exhibited complete water-triggered shape recovery after compression, remarkable antibacterial properties against Gram-positive (Staphylococcus aureus (S. aureus), Listeria monocytogenes (L. monocytogenes)) and Gram-negative (Escherichia coli (E. coli), Salmonella typhimurium (S. typhimurium)) strains, as well as good radical scavenging activity. The release profile of a plant-derived polyphenol, namely curcumin (CCM), was investigated at 37 °C in simulated gastrointestinal media. It was found that CCM release was dependent on the composition and the preparation strategy of sponges. By linearly fitting the CCM kinetic release data from the CS sponges with the Korsmeyer-Peppas kinetic models, a pseudo-Fickian diffusion release mechanism was predicted.

Implantable and in-vivo shape-recoverable nanocellulose-hyaluronic acid composite hydrogel.

Hydrogels have been used as a filling material in medical cosmetology, but current injection hydrogels have poor shaping ability due to its fluidity, while the hydrogels with fixed shape are easy to cause large wound size, resulting in rarely used in clinical practice. An implantable and in-vivo shape-recoverable hyaluronic acid (HA) based hydrogel is developed for tissue filling. In this work, complexes were made by hydrogen bonding between two natural polysaccharides: HA and TEMPO-oxidation cellulose nano-fiber. The elastic modulus of the HA/TOCN physical crosslinking hydrogel was maintained at 2500 G' in Pa, while, when ethylene glycol diglycidyl ether was introduced in the hydrogel, the elastic modulus could reach 60,000 G' in Pa. The volume of shrunk hydrogel reduced 80 ± 6 % of initial state, importantly, it can recover the shape in vivo inducing by extracellular moisture environment. Facts have proved that these shape recovery hydrogels were non-toxic to mammalian cells.

Mechanical and Shape Recovery Characterization of MWCNTs/HNTs-Reinforced Thermal-Responsive Shape-Memory Polymer Nanocomposites.

Mechanical and shape recovery characteristics of thermal-responsive shape-memory polyurethane (SMPU) reinforced with two types of reinforcements, multiwalled carbon nanotubes (MWCNTs) and halloysite nanotubes (HNTs), were studied in the present research work. Three weight percentages of reinforcement (0, 0.5 and 1%) in the SMPU matrix were considered, and the required composite specimens were obtained through injection moulding. Tensile, flexural, impact and shape recovery behaviours were studied experimentally. Further, flexural tests were performed for multiple cycles to understand the specimens' flexural strength variation after shape recovery. The concentration of both reinforcements (MWCNTs and HNTs) considered in the present study significantly improved mechanical properties and shape recovery.

Sustainable wheat gluten foams with self-expansion and water/blood-triggered shape recovery.

A cheap and easily obtainable wheat gluten (WG) was used to fabricate bio-foams via a simple method of stirring, heating, and lyophilization. The foam possesses a 3D layered porous structure with interconnected channels, and the biofoam has excellent mechanical properties through glycerol plasticization and glutaraldehyde (GA) cross-linking. The water absorption and volume expansion rate can reach 793.67 ∼ 918.45% and 201.47 ∼ 239.53% respectively. In dry state, the foams had good compression resilience, and can basically recover its original shape after withstanding 60% compression strain for about 7 h. In wet state, they can withstand 10 cycles of compression test, and had good compressive resilience and durability; they also had fast liquid-triggered shape recovery performance, of which the foams can reabsorb liquid, expand, and recover its original shape within 40 seconds after withstanding 80% compression strain. In addition, The hemolysis rates of red blood cells treated with 1, 3, and 5 mg/mL of 14WG-20g-5GA foam suspension were 0.53 ± 0.12%, 2.12 ± 0.34%, and 3.97 ± 0.21%, respectively, all of which were below the permissible range for biological materials (<5%). The above-mentioned advantages made the sustainable foams be potentially useful for medical dressings, especially for the treatment of non-compressible haemorrhaging, which offered a new field of application for WG protein and its added value was also increased obviously.

A New Strategy for Achieving Shape Memory Effects in 4D Printed Two-Layer Composite Structures.

In this study, a new strategy and design for achieving a shape memory effect (SME) and 4D printed two-layer composite structures is unveiled, thanks to fused deposition modeling (FDM) biomaterial printing of commercial filaments, which do not have an SME. We used ABS and PCL as two well-known thermoplastics, and TPU as elastomer filaments that were printed in a two-layer structure. The thermoplastic layer plays the role of constraint for the elastomeric layer. A rubber-to-glass transition of the thermoplastic layer acts as a switching phenomenon that provides the capability of stabilizing the temporary shape, as well as storing the deformation stress for the subsequent recovery of the permanent shape by phase changing the thermoplastic layer in the opposite direction. The results show that ABS-TPU had fixity and recovery ratios above 90%. The PCL-TPU composite structure also demonstrated complete recovery, but its fixity was 77.42%. The difference in the SME of the two composite structures is related to the transition for each thermoplastic and programming temperature. Additionally, in the early cycles, the shape-memory performance decreased, and in the fourth and fifth cycles, it almost stabilized. The scanning electron microscopy (SEM) photographs illustrated superior interfacial bonding and part integrity in the case of multi-material 3D printing.

Influences of Crystallinity and Crosslinking Density on the Shape Recovery Force in Poly(ε-Caprolactone)-Based Shape-Memory Polymer Blends.

Shape-memory polymers (SMPs) show great potential in various emerging applications, such as artificial muscles, soft actuators, and biomedical devices, owing to their unique shape recovery-induced contraction force. However, the factors influencing this force remain unclear. Herein, we designed a simple polymer blending system using a series of tetra-branched poly(ε-caprolactone)-based SMPs with long and short branch-chain lengths that demonstrate decreased crystallinity and increased crosslinking density gradients. The resultant polymer blends possessed mechanical properties manipulable across a wide range in accordance with the crystallinity gradient, such as stretchability (50.5-1419.5%) and toughness (0.62-130.4 MJ m-3), while maintaining excellent shape-memory properties. The experimental results show that crosslinking density affected the shape recovery force, which correlates to the SMPs' energy storage capacity. Such a polymer blending system could provide new insights on how crystallinity and crosslinking density affect macroscopic thermal and mechanical properties as well as the shape recovery force of SMP networks, improving design capability for future applications.