Ankle-foot orthoses (AFOs) assist patients with gait impairment by correcting ankle and foot deformities, restoring mobility, reducing pain, and providing protection and immobilization. AFOs can beneficially manage various types of gait pathologies, including foot drop, crouch gait, equinus gait, and stiff knee gait. AFOs are produced in prefabricated or custom-made forms in various designs. The selection criteria for the fabrication of an AFO are the duration of usage, the amount of applied force, the degree of axial loading, the patient’s skin condition, and the cost. The accessibility of diverse materials in the past century has greatly advanced orthoses. The ideal orthotic materials must be light, stiff, and strong, and are made of plastics, metals, polymer-based composites, leather, or a hybrid of different materials. Deeper understanding of the materials employed in the fabrication of AFOs will lead to more advanced and efficient orthoses, which can improve patients’ ability to ambulate in the real world. The present review provides insight into the various materials utilized for the fabrication of AFOs and describes the benefits and challenges associated with these materials. An attempt has also been made to highlight typical gait pathologies and design concepts in response to these.

Gallium-based liquid metal (LM) is expected to be an ideal candidate for flexible electromagnetic interference (EMI) shielding materials in wearable electronics due to its excellent electrical conductivity and extraordinary fluidity. However, it is difficult to fabricate LM as a free-standing material due to its high surface tension and unmanageable fluidity, as well as its poor compatibility with other materials. Herein, we develop a processing strategy to fabricate free-standing LM-based films by vacuum filtration with the assistance of dough-like aramid nanofiber (ANF) putty. The ANF putty improves the compatibility of ANF and LM, and also greatly improves the stability, reliability and processability of ANF. Rational design of the structure enables the preparation of LM/ANF films with ANF as the nanobridge connecting LM micro/nanodroplets, and ANF-LM-ANF sandwich films with ANF as the shells, LM layer as the core. The LM/ANF film (tensile strength: 5.4 MPa) exhibit an ultrahigh EMI SE of up to 105.9 dB (8.0–12.4 GHz) at a thickness of 60 μm. While the ANF-LM-ANF sandwich film has higher mechanical strength (33.1 MPa), and also has good EMI shielding properties. The average EMI SE of the 10 μm-thick sandwich film exceeds 45 dB in the X-band.

This article presents a self-developed continuous fiber 3D printing device that incorporates a tilted printing axis. By positioning the print nozzle axis at an obtuse angle to the printing plane, the device effectively prevents the common issue of fiber damage during the 90° extrusion of continuous fibers. This improvement is substantiated through tensile strength experiments. Additionally, the pretreatment stage includes the twisting of continuous fibers to enhance friction along their length. This twisting process enables better retention of the matrix during impregnation, thereby improving impregnation performance. The cross-section of the printed sample is observed by scanning electron microscope, and the microscopic effects of different process parameters on the continuous fiber structure are analyzed. Consequently, a link from process parameters to the mechanical properties of continuous fiber 3D printed specimens is established. This link serves as a foundation for predicting and optimizing the properties of continuous fiber-reinforced materials.

Structures with enhanced mechanical properties and crashworthiness are competing materials in the transport industry to reduce structural weight, and auxetic materials are the ideal choice. Auxetic metamaterials are the class of cellular structures designed to possess negative Poisson’s ratio. The current study evaluates newly developed re-entrant diamond auxetic metamaterial’s in-plane and out-of-plane three-point bending performance. A series of experimental and numerical studies are performed to assess the quasi-static performance and crashworthiness characteristics of 3D-printed re-entrant diamond auxetic core and sandwich panels. The results are compared with regular re-entrant panels of the same unit cell size. The deformation and failure mechanism of both auxetic structures, core and sandwich, are discussed. Real-time displacement contours on the specimen surface are visualized using advanced image processing and finite element methods. The new re-entrant diamond auxetic members outperformed energy absorption characteristics in in-plane and out-plane directions. Compared to the counterpart, the new metamaterial has improved 88.33% and 29.24% in-plane energy absorption as a core and sandwich, respectively. A 13.51% and 13.35% increments in energy absorption as a core and sandwich are observed for re-entrant diamond structures compared to the regular re-entrant system in the out-of-plane direction.

Printed electronic circuits are beginning to attract commercial success in different area of applications that include low-cost wearables, biosensors, biomedical tags, packaging, e-textiles etc. However, the major part of the research in this domain has always been focused at developing high performance thin film transistors (TFTs), while the other essential circuit elements, that are required, for example, for low-loss conversion of the input power, have rarely been reported. In this regard, we demonstrate inkjet-printed amorphous oxide based diodes on glass, and flexible polyimide substrates with rectification ratio >104 and operation frequency up to 25 MHz, and 15 MHz, respectively. Next, using the printed diodes full-wave and double half-wave rectifiers have been fabricated to convert input AC signals to DC supply. In addition, we demonstrate wireless power transfer (WPT), where the input AC signal has been wirelessly transmitted from a distance of 3 cm, at 125 kHz. The demonstrated WPT technology can be suitable for invasive implantable devices and stand-alone systems in multiple mediums. Lastly, bending fatigue tests have been carried out with the printed diodes on flexible substrates, down to a bending radius of 2.5 mm to demonstrate tensile strain tolerance up to 2.5%.

The 3D bioprinting can controllably deposit bioink containing cells and fabricate complex bionic tissue structures in a fast and scalable way, which is expected to completely change the scenario of clinical organ transplantation. Bioprinting holds broad application prospect in tissue engineering, life sciences, and clinical medicine. In the process of 3D bioprinting, bioink, as the carrier of cells and bioactive substances, influences cell activity and accuracy of organ structure after printing. To better understand and design bioink, in this review, the concept, development, and basic composition of bioink are introduced, while focusing on the advantages and disadvantages of various biomaterials, and the use of common cells and biomolecules that constitute bioink. In addition, the properties and applications of various stimuli-responsive smart materials for 4D bioprinting are mentioned. The challenges and development trends of bioink are also summarized.

Intermetallic γ–TiAl based alloys are innovative structural materials dedicated to applications in the automotive and aeronautic industry. Especially their low density, high specific yield strength and excellent resistance against creep and oxidation make them a suitable choice for structural high-temperature components in combustion engines. However, further improvement of their properties and processability is required to conquer new application areas and facilitate cost-effective production. While the incorporation of additional alloying elements is a promising possibility, their effects on the phase transformations and phase equilibria need to be considered rigorously. In this context, this review provides a detailed survey of the research work on the influence of technically important alloying elements on the Ti–Al phase diagram. First, an introduction to the fundamentals of the phase transformations in γ-TiAl based alloys and the consequences of changed cooling conditions, relevant for example to the latest developments within the field of processing, is given. Afterwards, the alloying elements, categorized with respect to their stabilization effect, are discussed and their particularities highlighted. Topics covered include established ternary phase diagrams and isoplethal sections, the stabilization of additional phases as well as the influence of alloying elements on the microstructure of modern engineering intermetallic γ-TiAl based alloys.

TiC/Inconel 718 functionally gradient materials are prepared by direct energy deposition technology. The effect of TiC content on microstructure and mechanical properties of TiC/Inconel 718 functionally gradient materials is studied. With the increase of TiC content, the microhardness and carbide grain of the specimen are improved, and the density is reduced. The grain of the specimen changes from columnar dendrites to equiaxed crystal, and the equiaxed crystal size is decreased with the increase of TiC content. However, when TiC content is above 10 wt%, the number and size of the Laves phase, coarse TiC primary, and TiC secondary dendrite are increased which causes the generation of cracks. When the TiC content is above 5 wt%, the size of carbide and the number of cracked UMT increase and the impact toughness decreases. Therefore, the optimal maximum TiC content of TiC/Inconel 718 functionally gradient materials is 5 wt% when the laser power is 2200 W.

An electric arc being deployed by a gas tungsten arc welding torch on top of an electron backscattered diffraction map, depicting the transition of the fusion zone to the base material. In the background an optical microscopy image of the same zone is displayed with detail of the base material and the fusion zone on the insets. Further information can be found in the article number 2300109 by Joao G. Lope, Joao P. Oliveira, and co-workers.

Smart Contact Lenses (SCL) incorporate a diversity of thin-film components and are also fabricated utilizing different thin films. They have been developed for the sensing and treatment of diseases such as intraocular pressure (IOP) and glaucoma treatment using the tear fluid. This report provides a review of how these thin films were processed to develop functional characteristics relevant for their application in smart contact lenses. Types of thin films used, as well as various applications were discussed from the historical developments leading to the invention of smart contact lenses to the state-of-the-art technologies related to thin films used in them. Finally, challenges with contact lenses and improvements to tackle these challenges using thin films were presented.

The surface properties of the AZ91D alloy are altered using surface mechanical attrition treatment (SMAT), a promising method of severe surface deformation, where the role of process parameters is crucial. In this study, specimens are SMATed using ≈3 and ≈10 m s−1 ball velocities (maintaining a constant percentage coverage). The SMATed specimens show higher twin density near the surface, which is reduced gradually, and twin thickness is increased with increasing depth. Further, high-velocity balls cause more twin density and better grain refinement (≈32 nm grain size at the surface). The higher ball velocity helps form a considerably thicker gradient layer (≈3500 μm) with higher hardness (≈1.98 GPa) and compressive residual stress (≈281 MPa) within a shorter SMAT duration (≈10 min). Ball velocity also influences nanomechanical properties such as nanohardness, creep resistance, strain rate sensitivity (SRS), etc. The non-SMATed alloy's SRS is about 0.037–0.040. The gradient microstructure affects SRS. The SRS value near the SMATed surface (where the reduced grain size plays a dominating role) is about 0.018–0.027; however, it drops suddenly to ≈0.01 (with a slight increase in depth), and subsequently, it rises with an increased distance in the SMATed layer (where twins play a dominating role).

The manufacturing of thin films with structured surfaces by large-scale rolling has distinct advantages over other techniques, such as lithography, due to scalability. However, it is not well understood or quantified how processing conditions can affect the microstructure at different physical scales. Hence, the objective of this investigation is to develop a validated computational model of the symmetric forward-roll coating process to understand, predict, and control the morphology of carbon nanotube (CNT)–polydimethylsiloxane (PDMS) pastes. The effects of the thin-film rheological properties and the roller gap on the ribbing behavior are investigated and a ribbing instability prediction model is formulated from experimental measurements and computational predictions. The CNT–PDMS thin-film system is modeled by a nonlinear implicit dynamic finite-element method that accounts for ribbing instabilities, large displacements, rolling contact, and material viscoelasticity. Dynamic mechanical analysis is used to obtain the viscoelastic properties of the CNT–PDMS paste for various CNT weight distributions. Furthermore, a Morris sensitivity analysis is conducted to obtain insights on the dominant predicted characteristics pertaining to the ribbing microstructure. Based on the sensitivity analysis, a critical ribbing aspect ratio is identified for the CNT–PDMS system corresponding to a critical roller gap.

Shaping of dense ceramics is difficult due to their inherent brittleness. Nano-grained ceramics like tetragonal zirconia (TZP) can be superplastically deformed and shaped at high temperatures owing to grain boundary sliding (GBS). Here, we demonstrate the enhanced plasticity of Gadolinium doped Ceria (GDC) ceramics under mild and strong AC electric current in terms of steady state creep rate under both compressive and tensile loading. A current density of 25 mA/mm2 and 200 mA/mm2 was used for the creep deformation. The creep rate increased by up to two orders of magnitude under electric current. The stress exponent remained unchanged for creep experiments at 1200 °C with and without electric current, suggesting a grain boundary sliding mechanism of plastic deformation in both cases. The field enhanced creep rate was attributed to the interaction of space charge layer and the electric field resulting in enhanced GBS. A higher current density resulted in enhanced ductility of GDC even with the Joule heating effect being compensated by reducing the furnace temperature.

Achieving high thermal conductivity and exceptional interfacial adhesion simultaneously in thermosensitive tactile recognition sensors poses a significant challenge. A copolymer, poly([[(butylamino)carbonyl]oxy]ethyl-ester)-co-polydimethylsiloxane (referred to as PP), is synthesized and subsequently complexed with alumina particles coated with liquid metal (LMAl2O3) to prepare a composite material called PP/LMAl2O3 with high thermal conductivity and strong interfacial adhesion to address this challenge. The best thermal conductivity (4.43 W m−1 K−1), electrical insulation (10−6–10−7 S m−1), and adhesion properties derived from hydrogen bonding (1316 N m−2) are obtained by adjusting the volume fraction of PP and LMAl2O3 in PP/LMAl2O3. PP/LMAl2O3 with high thermal conductivity and high interface adhesion can efficiently transfer heat between thermal flux sensors and the objects being sensed, reliably detecting small thermal flux variations and ensuring accurate thermal flux measurements. In this study, PP/LMAl2O3 is used to make up thermosensitive tactile sensor. Surprisingly, PP/LMAl2O3 demonstrates high thermal signal sensitivity for tactile recognition applications, allowing the smart thermosensitive tactile sensor system to distinguish unknown rock materials even in the dark. Overall, PP/LMAl2O3 may function as a fundamental material in thermosensitive tactile sensors for lithology identification.

Polyacrylonitrile-based carbon fibers (CFs) have demonstrated unparalleled physical and mechanical properties and are considered a promising material for aerospace, transportation, medical devices, and other applications. However, the current understanding of the heterogeneous structure of carbon fiber is still vague, and the relationship between the heterogeneous structure and the mechanical properties of carbon fiber is also unclear. In this review, the literature on the radial heterogeneous structure of CFs is summarized from four points of view: morphological structure, physical properties and chemical structures, mechanical properties, and structural models by combining different characterization tools such as nanoindentation, nanoscale infrared spectroscopy, and synchrotron wide-angle X-ray diffraction. First, the size and orientation of graphite microcrystalline, the degree of graphitization, density distribution, modulus distribution, chemical functional group distribution, and other physical and chemical properties of carbon fiber cross-sectional structures are discussed. Afterward, the relationship between the mechanical properties of carbon fiber and the heterogeneous structure is analyzed. Finally, the currently accepted models of the heterogeneous structure of carbon fibers are summarized and made some suggestions for research on the heterogeneous structure of carbon fibers.

This review aims to improve our understanding of the important factors which influence the susceptibility of thermomechanical controlled processed (TMCP) steels to sulfide stress cracking (SSC). Mechanisms involved in hydrogen embrittlement (HE) from three perspectives are focused on: the microstructure constituents of TMCP steels; environmental factors; and fracture mechanism of SSC. Microstructures are reviewed as they affect the diffusion and trapping of hydrogen that can reduce the resistance to fracture. Environmental factors discussed highlight that when exposed to an aqueous H2S environment, a sulfide layer can form and influence the ingress of hydrogen, and this is affected by pH, temperature, and H2S partial pressure. Fracture is influenced by the nature of the crack tip and the crack tip plastic zone during crack propagation, and hydrogen can significantly affect crack tip growth. This review provides a critical assessment of the interplay between these three factors and aims to provide understanding to enhance our engineering approaches to manage and mitigate against fracture of TMCP steels.

Bactericidal surfaces are in high demand for biomedical as well as industrial applications. In this paper, we have investigated if black diamond is useful for this application. Black diamond is derived from black silicon, a silicon surface structured into nano-sized needles. These needles can pierce through bacteria on contact. Black diamond is obtained by coating black silicon with a thin diamond film rendering the nanostructures more robust. We compared the bactericidal and anti-bacterial properties of fluorine-terminated and hydrogen-terminated black diamonds with those for black silicon and for flat surfaces of diamond (on silicon) with the same terminations. In this study we evaluated the ability to repel and kill Gram-positive S. aureus and S epidermidis, which have a thicker cell wall and are more mechanically robust than the bacteria that have been studied before. We evaluated the short-term 1 h initial adhesion as well as long-term 24 h biofilm formation. We found that the number of bacteria that initially adhered to the fluorine-terminated black diamond surface was significantly reduced and had the highest dead bacterial ratio compared to fluorine-terminated flat diamond, black silicon surfaces and hydrogen-terminated diamond surfaces, respectively. Biofilm formation after 24 h showed that while all surfaces outperform glass over the long-term (24 h), diamond-coated surfaces with both fluorine and hydrogen termination have a significant inhibiting biofilm formation effect. In conclusion, fluorinated and hydrogenated diamond-coated surfaces with and without nano-needles have repelling, bactericidal and biofilm-inhibiting effects on Gram-positive bacterial strains and are promising antimicrobial surfaces.

This study investigates the mechanical and piezoresistive self-sensing performance of additive manufacturing-enabled 2D nanocomposite lattices under monotonic and cyclic tensile loading. Lattice structures comprising hexagonal, chiral, triangular, and re-entrant unit cell topologies are realized via Digital Light Processing (DLP) using an acrylic photocurable resin filled with carbon nanotubes (CNTs). The results reveal that the piezoresistive sensitivity of re-entrant and triangular lattices is nearly insensitive to changes in the relative density. In contrast, the gauge factors of the hexagonal and chiral lattices rose by 300 and 500%, respectively, with an increase in relative density from 20 to 40%, which can be ascribed to their bend-dominated behaviour, causing an increase in surface strains in the lattice struts with increasing relative density for an imposed macroscopic strain. The measured stress vs. strain responses compare well with nonlinear finite element results. Under strain-controlled cyclic loading, the electrical resistance of the 2D lattices is found to decline over time due to re-orientation of the CNTs in the surrounding viscoelastic polymer matrix. The findings provide valuable insights into the interrelations between sensing performance, cell architecture and relative density of the lattices, and offer guidelines for the design of architected strain sensors and self-sensing lightweight structures.

Soft robotic actuators can be designed to achieve complex and tailored motions while simultaneously leveraging their compliance to interact with complex and often delicate environments. Mechanical metamaterials reveal a route to customizable deformations, force exertion, and mechanical energy efficiency attainable by careful arrangement of local geometric features. Herein, modular soft robotic actuators are developed from soft elastomers and flexible thermoplastic sheets of various unit cell designs. The efforts are focused on center-symmetric perforated sheets, which are formed into flexible cylindrical skins that surround the soft inflatable actuators. The results demonstrate the influence of perforation geometry on the spatial stiffness of the reinforcement structure and the proposed actuators’ response through several investigations. It is demonstrated that the free-boundary displacement, maximal force exertion, and mechanical energy efficiency of extensile actuators are dependent on a change of deformation mode in the mesostructure. The spatial stiffness concept is extended to develop soft robotic actuators that can bend, twist, and perform hybrid motions, such as simultaneous bending and twisting. Multisegment soft robotic arms are also developed from the aforementioned actuators. Investigations in this study provide a step toward the development of highly customizable and programmable soft robotic actuators for various applications.

Temperature-independent properties are critical for high-temperature thin-film strain gauges (TFSGs). In this study, by controlling the electron scattering and tunneling effects in the TiB2/SiCN composites, the environmental interference of temperature fluctuations was successfully eliminated, and a temperature-independent TFSG was fabricated. The effects of pyrolysis temperature and TiB2 content on the microstructural evolution and electrical properties of the ceramic films were studied. The temperature insensitivity was mainly attributed to the balance between the intrasheet resistance with a positive temperature coefficient of resistance (TCR) and the intersheet resistance with a negative TCR. This composite showed nearly constant resistance values over an ultra-wide temperature range of 300–700 °C, with less than 0.05% deviation of the normalized resistance and TCR values as low as 1.6 ppm/°C. In addition, the TiB2/SiCN films exhibited stable piezoresistive responses, with a gauge factor of 4.28, and the temperature-independent strain response in the high temperature range was verified.