Nanocellulose and multi-walled carbon nanotubes reinforced polyacrylamide/sodium alginate conductive hydrogel as flexible sensor.

Conductive hydrogels have been widely applied in human-computer interaction, tactile sensing, and sustainable green energy harvesting. Herein, a double cross-linked network composite hydrogel (MWCNTs/CNWs/PAM/SA) by constructing dual enhancers acting together with PAM/SA was constructed. By systematically optimizing the compositions, the hydrogel displayed features advantages of good mechanical adaptability, high conductivity sensitivity (GF = 5.65, 53 ms), low hysteresis (<11 %), and shape memory of water molecules and temperature. The nanocellulose crystals (CNWs) were bent and entangled with the backbone of the polyacrylamide/ sodium alginate (PAM/SA) hydrogel network, which effectively transferred the external mechanical forces to the entire physical and chemical cross-linking domains. Multi-walled carbon nanotubes (MWCNTs) were filled into the cross-linking network of the hydrogel to enhance the conductivity of the hydrogel effectively. Notably, hydrogels are designed as flexible tactile sensors that can accurately recognize and monitor electrical signals from different gesture movements and temperature changes. It was also assembled as a friction nanogenerator (TENG) that continuously generates a stable open circuit voltage (28 V) for self-powered small electronic devices. This research provides a new prospect for designing nanocellulose and MWCNTs reinforced conductive hydrogels via a facile method.

NIR responsive scaffold with multistep shape memory and photothermal-chemodynamic properties for complex tissue defects repair and antibacterial therapy.

Complex tissue damage accompanying with bacterial infection challenges healthcare systems globally. Conventional tissue engineering scaffolds normally generate secondary implantation trauma, mismatched regeneration and infection risks. Herein, we developed an easily implanted scaffold with multistep shape memory and photothermal-chemodynamic properties to exactly match repair requirements of each part from the tissue defect by adjusting its morphology as needed meanwhile inhibiting bacterial infection on demand. Specifically, a thermal-induced shape memory scaffold was prepared using hydroxyethyl methacrylate and polyethylene glycol diacrylate, which was further combined with the photothermal agent iron tannate (FeTA) to produce NIR light-induced shape memory property. By varying ingredients ratios in each segment, this scaffold could perform a stepwise recovery under different NIR periods. This process facilitated implantation after shape fixing to avoid trauma caused by conventional methods and gradually filled irregular defects under NIR to perform suitable tissue regeneration. Moreover, FeTA also catalyzed Fenton reaction at bacterial infections with abundant H2O2, which produced excess ROS for chemodynamic antibacterial therapy. As expected, bacteriostatic rate was further enhanced by additional photothermal therapy under NIR. The in vitro and vivo results showed that our scaffold was able to perform high efficacy in both antibiosis, inflammation reduction and wound healing acceleration, indicating a promising candidate for the regeneration of complex tissue damage with bacterial infection.

Micro-drilling on shape memory alloys-A review.

Technology like micro-hole drilling has progressed because of the machining industry's demand for high-precision micro components. Although micro drilling can be duplicated in theory, many challenges must be overcome before this becomes a reality. Micro-drilling with a thermal-mechanical interaction in the alloy is simulated in Abaqus. It looks at twist-bit modeling, material constitutive modeling, chip separation criteria, and element division. Focusing on work and tool electrode materials, hole specifications, micro hole types (through or blind holes), process parameters, performance measurements, and significant discoveries, this study presents a complete survey of the literature on spark methods for drilling microscopic holes. With an emphasis on spark erosion machining, drilling, and its modifications, this study seeks to aid researchers and academics by showcasing the potential of these methods to create extremely small holes. Microdrilling is critical for manufacturing miniaturized Nitinol components having applications in the medical field, robotics industry. Mechanical microdrilling offers a cost-effective and precise method over non-conventional ones. Thus, the model may identify the drilling performance's key process parameters. They can also help operators choose micro-drilling-optimal machining parameters. Modeling micro-drilling software helps in shaping an alloy.

Silk Fibroin-Based Multiple-Shape-Memory Organohydrogels.

Organohydrogels (OHGs) are intriguing materials due to their unique composition of both hydrophilic and hydrophobic domains. This antagonistic nature endows the OHGs with several remarkable properties, making them highly versatile for various applications. We present here a simple and inexpensive approach to fabricate silk fibroin (SF)-based OHGs with multistage switching mechanics and viscoelasticity. The continuous hydrophilic phase of the OHG precursor consists of an aqueous SF solution, while the hydrophobic droplet phase consists of a crystallizable n-octadecyl acrylate (C18A) monomer and several long-chain saturated hydrocarbons (HCs) with various chain lengths between 14 and 32 carbon atoms, namely, n-tetradecane, n-octadecane, n-docosane, n-dotriacontane, and 1-docosanol. After the addition of a C18A/HC mixture containing Irgacure photoinitiator into the continuous aqueous SF phase under stirring, a stable oil-in-water emulsion was obtained, which was then photopolymerized at 23 ± 2 °C to obtain nonswelling OHGs with multiple-shape-memory behavior. By changing the chain length and mass proportion of HCs, a series of OHGs with tunable transition temperatures could be obtained, meeting various applications. OHGs containing dimer, trimer, and quadruple combinations of in situ-formed poly(C18A) and HC microinclusions exhibit effective triple- or quintuple-shape memory whose shape-recovery temperatures could be adjusted over a wide range, e.g., between 7 and 70 °C.

Reconfigurable origami with variable stiffness joints for adaptive robotic locomotion and grasping.

With its compactness and foldability, origami has recently been applied to robotic systems to enable versatile robots and mechanisms while maintaining a low weight and compact form. This work investigates how to generate different motions and shapes for origami by tuning its creases' stiffness on the fly. The stiffness tuning is realized by a composite material made by sandwiching a thermoplastic layer between two shape memory polymer layers. This enables the composite to act as a living hinge, whose stiffness can be actively controlled through Joule heating. To demonstrate our concept, we fabricate an origami module with four variable stiffness joints (VSJs), allowing it to have freely controlled crease stiffnesses across its surface. We characterize the origami module's versatile motion when heating different VSJs with different temperatures. We further use two origami modules to build a two-legged robot that can locomote on the ground with different locomotion gaits. The same robot is also used as an adaptive gripper for grasping tasks. Our work can potentially enable more versatile robotic systems made from origami as well as other mechanical systems with programmable properties (e.g. mechanical metamaterials).This article is part of the theme issue 'Origami/Kirigami-inspired structures: from fundamentals to applications'.

Microstructure and Mechanical Properties of NiTi Alloy Prepared by Double-Wire + Arc Additive Manufacturing Plus In Situ Heat Treatment.

In this study, NiTi shape memory alloy was prepared by double-wire + arc additive manufacturing plus in situ heat treatment using TA1 and ER-Ni welding wires as the raw materials. The results show that the microstructural evolution from the bottom to top is NiTi2 + NiTi → NiTi + Ni3Ti + Ni4Ti3 → NiTi + Ni4Ti3 + Ni3Ti2 + Ni3Ti + α-Ti. Complex thermal cycles led to the precipitation of Ni3Ti, which improves the hardness of the matrix (B2), and the average hardness value of the top region reaches 550.7 HV0.2. The fracture stress is 2075 ± 138.4 MPa and the fracture strain is 11.2 ± 1.27%. The sample shows 7.02% residual strain and 5.87% reversible strain after 15 cycles, and the stress hysteresis decreases with an increase in cyclic strain.

Characterization of Anisotropic Shape Memory Behavior of Thermoresponsive Components in 4D Printing.

Four-dimensional (4D) printing has emerged as a promising manufacturing technology in recent years and revolutionized products by adding shape-morphing capabilities when exposed to certain stimuli. Increasing research attention has been dedicated to studying the shape memory behaviors of the 4D fabricated structures. However, in-depth discussions on quantifying the influence of process parameters on shape fixity and recovery properties are limited, and the anisotropy induced by the layer-wise fabrication nature is significantly underreported. To further exploit the shape memory property of 4D printed structures, it is essential to investigate the process-induced anisotropic shape memory behaviors. In this study, the effects of critical process parameters on anisotropy in shape memory properties are mathematically quantified; meanwhile, the feasibility of tailoring the anisotropy of 4D printed parts is examined with joint consideration of total build time. Different scanning patterns are experimentally analyzed for their influence on anisotropic behaviors. It is found that the Triangle scanning pattern often leads to the best shape memory behaviors in different directions. The outcome of this study confirms the existence of anisotropy in both shape fixity and shape recovery ratios. In addition, the results also reveal that a smaller scanning angle tends to minimize the anisotropy and total fabrication time while ensuring satisfactory shape memory performance. Furthermore, layer thickness shows negligible effects on anisotropy, while the scanning angle and shape memory temperature suggest the opposite.

Application of Reversible Four-Dimensional Printing of Shape Memory Alloys and Shape Memory Polymers in Structural Engineering: A State-of-the-Art Review.

The rapid development and advancements in field of shape memory alloys (SMAA) has tremendously increased the progress in four-dimensional (4D) printing. The conventional 4D printing will require skilled manpower but utilization of reversibility aspect achieved using self adjusting external stimuli will eliminate the necessity of sophisticated devices and human intervention in 4D printing. The components created using reversible 4D printing can be reused after each recovery cycle that suits the current industry requirements. This review is divided into three sections: The first section starts with a detailed illustration of different mechanisms associated with SMAA and shape memory polymers SMPP along with an illustration of realistic 3D-printed SMAA and SMPP. The second section of this paper deals with the different methods of manufacture with the advantages and disadvantages of different types of SMAA. The third section deals with the mechanisms associated with SMPP, namely (1) Thermo-responsive mechanism, (2) Chemo-responsive mechanism, and (3) Photo-responsive mechanism along with a detailed insight into the aspect of repeatability and reversibility. The fourth section presents an exhaustive review of the application of SMAA and SMPP in civil engineering. The last section of this work throws light on the challenges faced in 4D reversible printing of SMAA and SMPP along with the potential solutions and presents directions for future research.

Versatile Keratin Fibrous Adsorbents with Rapid-Response Shape-Memory Features for Sustainable Water Remediation.

Biodegradable shape-memory polymers derived from protein substrates are attractive alternatives with strong potential for valorization, although their reconstruction remains a challenge due to the poor processability and inherent instability. Herein, based on Maillard reaction and immobilization, a feather keratin fibrous adsorbent featuring dual-response shape-memory is fabricated by co-spinning with pullulan, heating, and air-assisted spraying ZIF-8-NH2. Maillard reaction between the amino group of keratin and the carbonyl group of pullulan improves the mechanics and thermal performance of the adsorbent. ZIF-8-NH2 immobilization endows the adsorbent with outstanding multipollutant removal efficiency (over 90%), water stability, and photocatalytic degradation and sterilization performance. Furthermore, the adsorbent can be folded to 1/12 of its original size to save space for transportation and allow for rapid on-demand unfolding (12 s) upon exposure to water and ultraviolet irradiation to facilitate the adsorption and photocatalytic activity with a larger water contact area. This research provides new insight for further applications of keratin-based materials with rapid shape-memory features.

The effect of cobalt alloying on the phase transformation kinetics of Ni-Ti alloys.

This study investigates the influence of cobalt (Co) alloying addition and heat treatment temperature on the phase transformation behaviour controlling the superelasticity and shape memory effect (SME) of Nickel-Titanium (Ni-Ti) alloys, commonly known as nitinol. The microstructural evolution upon heat treatment conducted at a temperature ranging from 440 to 560 °C was thoroughly analyzed via Differential Scanning Calorimetry (DSC), X-ray Diffraction (XRD), and Scanning Electron Microscopy/Energy Dispersive Spectroscopy (SEM/EDS). Increase in heat treatment temperatures from 470 °C up to 530 °C led to the dissolution of particles present in as-received (cold-worked) condition. It was determined that Co addition into the Ni-Ti alloy system resulted in a change in the nucleation and growth kinetics of Ti-rich precipitates, leading to the formation of larger and fewer particles during processing. Both binary and ternary alloys showed a decrease in austenite finish temperature (Af) with increasing heat treatment temperatures, however, the rate of decrease was found to be higher for Co containing ternary alloys. This is linked with faster structural relaxation when Co is present and evidenced by lattice size variation during heat treatment. It is highlighted that heat treatment methodology needs to be tailored to the specific alloy composition for controlling superelasticity and SME via alloy design.

Microstructure and Shape Memory Properties of Gas Tungsten Arc Welded Fe-17Mn-5Si-10Cr-4Ni-(V, C) Shape Memory Alloy.

In this study, microstructure, mechanical, and shape memory properties of the welded Fe-based shape memory alloy (Fe-SMA) plates with a nominal composition of Fe-17Mn-5Si-10Cr-4Ni-(V, C) (wt.%) by gas tungsten arc welding were investigated. The optimal heat input to ensure full penetration of the Fe-SMA plate with a thickness of 2 mm was found to be 0.12 kJ. The solidified grain morphology adjacent to the partially melted zone was columnar, whereas the equiaxed morphology emerged as solidification proceeded. The ultimate tensile decreased after welding owing to the much larger grain size of the fusion zone (FZ) and heat-affected zone (HAZ) than that of the base material (BM). Weldment showed lower pseudoelastic (PE) recovery strain and higher shape memory effect (SME) than those of the plate, which could be ascribed to the large grain size of the FZ and HAZ. Recovery stress (RS) slightly decreased after welding owing to lower mechanical properties of weldment. On the other hand, aging treatment significantly improved all PE recovery, SME, and RS via carbide precipitation. Digital image correlation analysis revealed that HAZ showed the lowest SME after heating and cooling, implying that the improved SME of FZ compensated for the low SME of the HAZ.

Experimental Investigation of the Impact of Loading Conditions on the Change in Thin NiTi Wire Resistance during Cyclic Stretching.

This paper presents the results of an experimental study designed to evaluate the effect of repeated stretching cycles on the electrical resistance change in a NiTi alloy wire. In particular, tests were carried out to determine the effect of the type of loading on resistance change in the investigated wires. Wires with a diameter of 100 µm were used in the research. The experiment was carried out on a dedicated test stand designed for this purpose. During the test, the samples were subjected to 40 identical tensile cycles. The electrical resistance, sample elongation, and tensile force during successive stretching cycles were measured. The conducted research demonstrated the impact of elongation and reorientation of the structure on the resistance change in NiTi alloy thin wires. The research included a comparison of the effect of two different types of loading on the electrical resistance change in the sample. During cyclic stretching of a NiTi alloy sample with constant displacement, a decrease in electrical resistance was observed after each successive stretching cycle. Alternatively, when stretching with a constant force, the value of electrical resistance increased. In both types of loads, the greatest change in resistance value was observed at the initial cycles.

Recent Development of Functional Bio-Based Epoxy Resins.

The development of epoxy resins is mainly dependent on non-renewable petroleum resources, commonly diglycidyl ether bisphenol A (DGEBA)-type epoxy monomers. Most raw materials of these thermoset resins are toxic to the health of human beings. To alleviate concerns about the environment and health, the design and synthesis of bio-based epoxy resins using biomass as raw materials have been widely studied in recent decades to replace petroleum-based epoxy resins. With the improvement in the requirements for the performance of bio-based epoxy resins, the design of bio-based epoxy resins with unique functions has attracted a lot of attention, and bio-based epoxy resins with flame-retardant, recyclable/degradable/reprocessable, antibacterial, and other functional bio-based epoxy resins have been developed to expand the applications of epoxy resins and improve their competitiveness. This review summarizes the research progress of functional bio-based epoxy resins in recent years. First, bio-based epoxy resins were classified according to their unique function, and synthesis strategies of functional bio-based epoxy resins were discussed, then the relationship between structure and performance was revealed to guide the synthesis of functional bio-based epoxy resins and stimulate the development of more types of functional bio-based epoxy resins. Finally, the challenges and opportunities in the development of functional bio-based epoxy resins are presented.

Role of Minor Phase Morphology on Mechanical and Shape-Memory Properties of Polylactide/Bio-Polyamide Nanocomposite.

In situ-generated nanofibrillar polymer-polymer composites are excellent candidates for the production of polymer materials, with high mechanical and SME properties. Their special feature is the high degree of dispersion of the in situ-generated nanofibers and the ability to form entangled nanofiber structures with high aspect ratios through an end-to-end coalescence process, which makes it possible to effectively reinforce the polymer matrix and, in many cases, increase its ductility. The substantial interfacial area, created by the in situ formed fiber/matrix morphology, significantly strengthens the interfacial interactions, which are crucial for shape fixation and shape recovery. Using the polylactide/bio-polyamide (PLA/PA) system as an example, it is shown that in situ PA fibrillation improves the mechanical and shape-memory properties of PLA. The modulus of elasticity increases by a factor of 1.4, the elongation at break increases by a factor of 30, and the shape-strain/fixity ratio and shape recovery increase from 80.2 to 97.4% and from 15.5 to 94.0%, respectively. The morphology of the minor PA phase is crucial. The best result is achieved when a physically entangled nanofibrous network is formed.

Evaluation of Shape Recovery Performance of Shape Memory Polymers with Carbon-Based Fillers.

This study focuses on enhancing the thermal properties and shape recovery performance of shape memory polymers (SMPs) through the application of carbon-based fillers. Single and mixed fillers were used to investigate their effects on the glass transition temperature (Tg), thermal conductivity, and shape recovery performance. The interaction among the three-dimensional (3D) structures of mixed fillers played a crucial role in enhancing the properties of the SMP. These interactions facilitated efficient heat transfer pathways and conserved strain energy. The application of mixed fillers resulted in substantial improvements, demonstrating a remarkable 290.37% increase in thermal conductivity for SMPCs containing 60 µm carbon fiber (CF) 10 wt% + graphite 20 wt% and a 60.99% reduction in shape recovery time for SMPCs containing CF 2.5 wt% + graphite 2.5 wt%. At a content of 15 wt%, a higher graphite content compared to CF improved the thermal conductivity by 37.42% and reduced the shape recovery time by 6.98%. The findings demonstrate that the application of mixed fillers, especially those with high graphite content, is effective in improving the thermal properties and shape recovery performance of SMPs. By using mixed fillers with high graphite content, the performance of the SMP showed significant improvement in situations where fast response times were required.

Influence of Infill Patterns on the Shape Memory Effect of Cold-Programmed Additively Manufactured PLA.

In four-dimensional additive manufacturing (4DAM), specific external stimuli are applied in conjunction with additive manufacturing technologies. This combination allows the development of tailored stimuli-responsive properties in various materials, structures, or components. For shape-changing functionalities, the programming step plays a crucial role in recovery after exposure to a stimulus. Furthermore, precise tuning of the 4DAM process parameters is essential to achieve shape-change specifications. Within this context, this study investigated how the structural arrangement of infill patterns (criss-cross and concentric) affects the shape memory effect (SME) of compression cold-programmed PLA under a thermal stimulus. The stress-strain curves reveal a higher yield stress for the criss-cross infill pattern. Interestingly, the shape recovery ratio shows a similar trend across both patterns at different displacements with shallower slopes compared to a higher shape fixity ratio. This suggests that the infill pattern primarily affects the mechanical strength (yield stress) and not the recovery. Finally, the recovery force increases proportionally with displacement. These findings suggest a consistent SME under the explored interval (15-45% compression) despite the infill pattern; however, the variations in the mechanical properties shown by the stress-strain curves appear more pronounced, particularly the yield stress.

Mechanical and Fatigue Properties of Welded Fe-Mn-Si Shape Memory Alloys.

This paper presents the experimental results of a study evaluating the mechanical and fatigue performance of welded Fe-Mn-Si SMA. For the experimental study, welded and welded-and-heat-treated Fe-Mn-Si SMA specimens were fabricated, and fatigue tests were performed at various stress amplitudes. In addition, direct tensile tests and recovery stress tests were also performed to evaluate the material properties of Fe-Mn-Si SMAs. The elastic modulus, yield strength, and tensile strength of the welded specimens were reduced by 35.4%, 12.1%, and 8.6%, respectively, compared to the values of the non-welded specimens. On the other hand, the elastic modulus, yield strength, and tensile strength of the welded-and-heat-treated Fe-Mn-Si SMA specimens were increased by 18.6%, 4.9%, and 1.3%, respectively, compared to the values of the welded specimens. Both welded and welded-and-heat-treated Fe-Mn-Si SMAs failed at lower cycles than the conventional Fe-Mn-Si SMAs at the same stress amplitude. High-cycle fatigue failure, characterized by cycles exceeding 104, typically occurs at relatively low stress levels within the elastic region, whereas low-cycle fatigue failure, generally occurring within cycles below 104, involves high stress levels that encompass both elastic and plastic deformation. Regardless of the welding condition, the stress amplitude at which Fe-Mn-Si SMA transitions from high-cycle to low-cycle failure exceeded the yield strength.

Proof of Concept of a New Revision Procedure for Ceramic Inlays of Acetabular Cups Using a Shape-Memory Alloy Actuator System.

The revision of ceramic inlays of acetabular cups is a challenging surgical procedure. The mechanical impact during the inlay extraction process can damage the ceramic or metal cup rim. To avoid these risks, a concept for a new revision procedure was developed. It is based on an actuator system, which allows a non-destructive release of the ceramic inlay. To integrate the actuator system, different design concepts of acetabular cup components were investigated, and an actuator based on shape-memory alloy (SMA) wires was developed. The process chain for the actuator, starting from nickel-titanium wires manufactured into the actuator geometry by laser welding and thermo-mechanical treatment for the shape setting process up to the functionality evaluation of the actuator system, was implemented on a laboratory scale. The new revision procedure is based on a phase transformation of the SMA wire actuator, which was obtained through two methods-applying an electrical current by an instrument and rinsing the wire with heated water. The phase transformation of the actuator resulted in a contraction between 3.2% and 4.3% compared to its length after pre-stretching and was able to release the ceramic inlay from the cup. Therefore, the developed actuator design and process chain is a proof of concept towards a new revision procedure for modular acetabular cups.

NIR-Responsive Deployable and Self-Fitting 4D-Printed Bone Tissue Scaffold.

The treatment of irregular-shaped and critical-sized bone defects poses a clinical challenge. Deployable, self-fitting tissue scaffolds that can be implanted by minimally invasive procedures are a promising solution. Toward this, we fabricated NIR-responsive and programmable polylactide-co-trimethylene carbonate (PLMC) scaffolds nanoengineered with polydopamine nanoparticles (PDA) by extrusion-based three-dimensional (3D) printing. The 3D-printed scaffolds demonstrated excellent (>99%), fast (under 30 s), and tunable shape recovery under NIR irradiation. PLMC-PDA composites demonstrated significantly higher osteogenic potential in vitro as revealed by the significantly enhanced alkaline phosphatase (ALP) secretion and mineral deposition in contrast to neat PLMC. Intraoperative deployability and in vivo bone regeneration ability of PLMC-PDA composites were demonstrated, using self-fitting scaffolds in critical-sized cranial bone defects in rabbits. The 3D-printed scaffolds were deformed into compact shapes that could self-fit into the defect shape intraoperatively under low power intensity (0.76 W cm-2) NIR. At 6 and 12 weeks postsurgical implantation, near-complete bone regeneration was observed in PLMC-PDA composites, unlike neat PLMC through microcomputed tomography (micro-CT) analysis. The potential clinical utility of the 3D-printed composites to secure complex defects was confirmed through self-fitting of the scaffolds into irregular defects in ex vivo models of rabbit tibia, mandible, and tooth models. Taken together, the composite scaffolds fabricated here offer an innovative strategy for minimally invasive deployment to fit irregular and complex tissue defects for bone tissue regeneration.

Mechanically adaptive and deployable intracortical probes enable long-term neural electrophysiological recordings.

Flexible intracortical probes offer important opportunities for stable neural interfaces by reducing chronic immune responses, but their advances usually come with challenges of difficult implantation and limited recording span. Here, we reported a mechanically adaptive and deployable intracortical probe, which features a foldable fishbone-like structural design with branching electrodes on a temperature-responsive shape memory polymer (SMP) substrate. Leveraging the temperature-triggered soft-rigid phase transition and shape memory characteristic of SMP, this probe design enables direct insertion into brain tissue with minimal footprint in a folded configuration while automatically softening to reduce mechanical mismatches with brain tissue and deploying electrodes to a broader recording span under physiological conditions. Experimental and numerical studies on the material softening and structural folding-deploying behaviors provide insights into the design, fabrication, and operation of the intracortical probes. The chronically implanted neural probe in the rat cortex demonstrates that the proposed neural probe can reliably detect and track individual units for months with stable impedance and signal amplitude during long-term implantation. The work provides a tool for stable neural activity recording and creates engineering opportunities in basic neuroscience and clinical applications.