Characterisation of the compaction response of reinforcement fabrics is an important component in the design of composite manufacturing processes. To standardise a best practice method, 22 international organisations participated in an exercise to assess the viability and reproducibility of the method discussed in this work. All participants were supplied with the same multiaxial E-glass fibre non-crimp fabric and instructed to measure the compaction stress as a function of the specimen thickness following a set of guidelines. The scatter in results between participants was quantified in terms of the coefficient of variation (CV). The CV of the maximum compaction stress determined at a target specimen thickness of 3 mm (for 10 fabric layers) was 42 % for dry specimens and 46 % for wet specimens, however this was influenced by scatter in the thickness values, which deviated from the target. The CV of the specimen thickness at a compaction stress of 105 Pa was 4 %. In addition, a power law model and a model based on bending of beams were fitted to the compaction curves. Both generally produced fits with high values of the coefficient of determination. The observed level of scatter is thought to be caused by issues with the implementation of the procedures and by variability in the specimen properties, as well as the very steep variation of the force/thickness curve at the required target. The guidelines used here aim to minimise inaccuracies in the test method and will be proposed as a test protocol for standardisation.

Where the trunk of a tree splits into co-dominant branches, wood fibres are highly interlocked. Such an arrangement of fibres has been shown to impart superior strength and toughness to this critical junction. Here, wavy patterns are 3D printed with a liquid crystal polymer (LCP) to evaluate the potential of wood-inspired localized adaptations to improve the robustness of junctions between orthotropic struts. The highly anisotropic, fibrillar microstructure of LCPs is harnessed by Fused Filament Fabrication, yielding Young's modulus and tensile strength reaching 30 GPa and 500 MPa respectively. However, unidirectional 3D-prints subjected to normal tensile stresses show weak interfaces, like in wood. To overcome this weakness, sinusoidal, helix and saw-tooth patterns are 3D-printed to create interlocking between layers. A trade-off is established between uniaxial tension and short-beam shear with increasing interlocking angle of the sinusoidal pattern. We find that the work associated with crack propagation in Mode I is increased three-fold compared to a unidirectional pattern, through extrinsic toughening. When applied to a more complex load case in a curved beam in four-point bending, helix-patterning at the junction zone increases the maximum load by 88 %. By locally controlling anisotropy via waviness, this method opens the possibility of improving toughness and transverse properties where the stress state is multi-axial without adding mass in future recyclable structural materials.

Design for composite material additive manufacturing is governed by multiple process variables that can be computationally expensive to optimize. This is especially true when considering discrete variables, such as the material type to be used, which lead to a lot of possible solutions that have to be evaluated. Here, we propose a workflow for optimizing topology and fiber placement of 3D volumetric structures based on mechanical performance under multiple load cases and environmental impact. An eco-informed material selection from a set fibers and polymers is followed by a methodology to optimize the manufacturing setting. By performing these two steps sequentially, the number of input parameter sets to be tested is reduced in a combinatorial scale, along with the computational cost. The framework can be easily extended by adapting the analyses and holds significant promise for the design of additive manufactured sustainable structures.

Ultrasonic wood welding is an ecofriendly method for rapidly joining wooden components in less than 2 s. However, this dynamic process results in low mechanical performance and poor durability under wet conditions. Inspired by natural wood's robust interlocking cellular structure, which leverages lignin fusion to enhance structural integrity, lignin fusion at wood interfaces is optimized, significantly improving lap shear strength and wet durability. These results demonstrate that enhanced lignin fusion at interfaces is crucial for obtaining strong wood joints by positioning lignin as a sustainable energy concentrator, promoting greener manufacturing of sustainable structures into complex shapes. The joints exhibit lap shear strengths and wet durability comparable to those achieved with water-based wood and epoxy adhesives, while also demonstrating conductivity which could be leveraged for multifunctional features such as strain sensing. The approach can be extended to other manufacturing methods, such as hot-pressing and continuous robotic manufacturing, emphasizing its potential for scalability and broad industrial adoption.

In this study, a 3D-printed biomimetic overlapping curl structure inspired by spider silk molecular structure, containing sacrificial bonds and hidden lengths, is studied as a toughening mechanism for a bio-based epoxy. Experimental results of the fracture phenomena of the overlapping curl-reinforced bio-based epoxy identify three toughening mechanisms triggered by the overlapping curl: (1) crack re-initiation, (2) overlapping curl bridging, and (3) epoxy ligament. First, the integrated overlapping curl creates a void within the epoxy matrix. As the crack tip reaches the end of this void, the crack re-initiates. Then, as the hidden length of overlapping curl unfolds, it leads to a bridging effect in resisting crack growth. In addition, for the smallest hidden length, an epoxy ligament is formed due to crack branching, significantly improving the energy release rate. The epoxy fracture energy release rate increased by 13 %. The overall modest improvement is attributed to the large plastic dissipation energy of the epoxy and the relatively low overlapping curl load-capacity. However, when expanding the design space numerically, it was shown that as the failure load of the overlapping curl increases, the bridging effect increases progressively. The introduction of the bio-inspired overlapping curl structure into bio-based epoxy proves the concept of a toughening strategy for developing high-performance sustainable composite materials.

In the collaborative effort towards standardisation of out-of-plane permeability measurement, an international benchmarking exercise was carried out whereby 19 participants worldwide were instructed to measure the out-of-plane permeability following a number of strict guidelines, informed by the outcomes of the first international benchmarking exercise completed in 2021. This paper presents the results of the exercise and an assessment of the reproducibility of the data and the suitability of the proposed test method. The data returned were subjected to a number of statistical analysis methods, which showed that adherence to the test guidelines resulted in a high likelihood of a participant not being an outlier and therefore providing evidence that the test method proposed in this paper is a suitable way forward for a standardised test method.

Carbon fibre reinforced plastic (CFRP) exhibits complex optical behaviour due to its anisotropy and highly scattering surface. These optical characteristics pose significant challenges for the automated laser-based inspection systems used in CFRP manufacturing, as they lead to variations in light interaction with the material, affecting the accuracy and reliability of inspections. To investigate this complex optical behaviour, an inverse optical model based on the Multi-Gaussian method has been developed. Laser speckle patterns from the CFRP surface are decomposed into multiple Gaussian components to model the material's optical properties. A greedy optimisation algorithm is employed to estimate the optimal coefficients for the Gaussian sets, which are further refined by introducing negative amplitude Gaussian components. These enhancements improve the optimisation, resulting in a better correlation between the Multi-Gaussian model and actual laser speckle measurements.

Multi-material direct ink writing (DIW) of smart materials opens new possibilities for manufacturing complex-shaped structures with embedded sensing and actuation capabilities. In this study, DIW of UV-curable piezoelectric actuators is developed, which do not require high-temperature sintering, allowing direct integration with structural materials. Through particle size and ink rheology optimization, the highest d₃₃^*g₃₃ piezoelectric constant compared to other DIW fabricated piezo composites is achieved, enabling tunable actuation performance. This is used to fabricate ultrasound transducers by printing piezoelectric vibrating membranes along with their support structures made from a structural ink. The impact of transducer design and scaling up transducer dimensions on the resonance behavior to design millimeter-scale ultrasound transducers with desired out-of-plane displacement is explored. A significant increase in output pressure with increasing membrane dimensions is observed. Finally, a practical application is demonstrated by using the printed transducer for accurate proximity sensing using time of flight measurements. The scalability and flexibility of the reported DIW of piezo composites can open up new advancements in biomedical, human-computer interaction, and aerospace fields.

Organic polymer-based composite materials with favorable mechanical performance and functionalities are keystones to various modern industries; however, the environmental pollution stemming from their processing poses a great challenge. In this study, by finding an autonomous phase separating ability of fungal mycelium, a new material fabrication approach is introduced that leverages such biological metabolism-driven, mycelial growth-induced phase separation to bypass high-energy cost and labor-intensive synthetic methods. The resulting self-regenerative composites, featuring an entangled network structure of mycelium and assembled organic polymers, exhibit remarkable self-healing properties, being capable of reversing complete separation and restoring ≈90% of the original strength. These composites further show exceptional mechanical strength, with a high specific strength of 8.15 MPa g.cm⁻³, and low water absorption properties (≈33% after 15 days of immersion). This approach spearheads the development of state-of-the-art living composites, which directly utilize bioactive materials to “self-grow” into materials endowed with exceptional mechanical and functional properties.

Recent numerical studies have suggested the potential of substrates with streamwise-preferential permeability to reduce drag in turbulent boundary layers. Such a substrate is theorized to facilitate relaxation of the no-slip condition and thereby reduce the skin friction. So far, these beneficial effects have not been demonstrated experimentally yet and therefore the scope of this work is to present this concept in air flow where the substrate geometry satisfies the theoretical permeability requirements for an expected reduction in drag. For this, a three-dimensional-printed structure with anisotropic permeability (φxz=2.7, φxy=3.9) and small pores (s≈250μm), akin to an acoustic liner, was developed. The substrate was investigated using direct force measurements and 2D-2C PIV in the range of U∞≈5-35 ms-1, corresponding to frictional Reynolds numbers of Reτ≈430-1960. Results show an increase in drag of 0%<ΔCD<8% and, while contrasting the model predictions, this agrees with DNS data on structures with similar geometric properties when using the inverse wall-normal Forchheimer coefficient, or inertial permeability, as the equivalent roughness parameter. Hence the present results constitute the first experimental evidence that this is the governing property for the drag behavior of acoustic liners. The absence of the predicted beneficial flow modulation effects is attributed to the investigated substrate not strictly satisfying the theoretical framework assumptions on characteristic length scales. However, to expand beyond this structural limitation, we analytically derive that, for realistic, geometrically resolved cases, this length scale mismatch is unavoidable and thereby render it unfeasible to model the substrate as a continuum for the virtual-origin approach. We expect that translating the abstraction of substrates with streamwise-preferential permeability into physical realisations relevant for practical applications would result in structures very similar to riblets.

Spider silk is known for its excellent strength and fracture resistance properties due to its molecular design structure, characterized by sacrificial bonds and hidden lengths. These structures have inspired reinforcements of synthetic polymer materials to enhance toughness. In this study, we mimic these natural toughening mechanisms by designing and manufacturing 3D-printed polymeric structures incorporating overlapping curls consisting of coiling fiber with sacrificial bonds and hidden lengths. Utilizing the liquid rope coiling effect, we manufactured overlapping curls using three polymers: polylactic acid (PLA), liquid crystal polymer (LCP), and polyamide 6 (PA6). Uniaxial tensile tests were performed to characterize the mechanical properties of overlapping curl as a function of geometries, post-treatments, and material constitutive parameters. Our results show that single-sided overlapping curls can fully unfold while double-sided curls are prone to premature failure. Heat-pressure post-treatment was found to significantly increase the load-capacity of the sacrificial bonds by up to [Formula presented] due to increased contact area. However, the defects introduced in the fibre after the break of the sacrificial bonds, make the structure more susceptible to premature failure, limit the complete unfolding of the hidden length, and lead to a decrease up to [Formula presented] of the toughness. To guarantee the complete unfolding of the hidden lengths and improve the toughness, we demonstrate that selecting a polymer material with either high fracture strength (e.g., LCP, [Formula presented]) or high fracture strain (e.g., PA6, >2) is crucial, and increase toughness up to [Formula presented] and [Formula presented], respectively.

Since the inception of fibre-reinforced composite materials, they have been widely acknowledged for their unparalleled weight-to-performance ratio. Nonetheless, concerns are escalating regarding the environmental impact of these materials amidst global warming and pollution. This perspective explores a ground-breaking shift towards harnessing living organisms to produce composite materials. Living composites not only offer sustainable, carbon-capturing alternatives but also afford an unprecedented level of control over shape and anisotropy. Recent advancements in biology, particularly genetic engineering and sequencing, have provided extraordinary control over living organisms. Coupled with ever-evolving additive manufacturing techniques, these breakthroughs enable the construction of engineered living materials from the ground up. Here, we explore the key factors propelling the emergence of engineered living materials for structural applications and delves into the capabilities of living organisms that can be harnessed for creating functional materials, including harvesting energy, forming structures, sensing/adapting, growing and remodelling. Incorporating living organisms can revolutionise manufacturing for renewable and sustainable composite materials, unlocking previously unattainable functionalities.

Signaling pathways in fungi offer a profound avenue for harnessing cellular communication and have garnered considerable interest in biomaterial engineering. Fungi respond to environmental stimuli through intricate signaling networks involving biochemical and electrical pathways, yet deciphering these mechanisms remains a challenge. In this review, an overview of fungal biology and their signaling pathways is provided, which can be activated in response to external stimuli and direct fungal growth and orientation. By examining the hyphal structure and the pathways involved in fungal signaling, the current state of recording fungal electrophysiological signals as well as the landscape of fungal biomaterials is explored. Innovative applications are highlighted, from sustainable materials to biomonitoring systems, and an outlook on the future of harnessing fungi signaling in living composites is provided.

In addition to the well-documented resource efficiency and geometrical freedom, Digital Fabrication (DFAB) revolutionizes architecture by integrating functionalities into building elements, unlocking untapped potential from the micro- to the macroscales. This study uses binder-jet printed sand for a DFAB prototype—Fireplace2—tailored for indoor heating. Named after its traditional counterpart, Fireplace2 showcases DFAB’s prowess in crafting precise microclimates for heightened thermal comfort. Our research, tuning mechanical and thermal properties across micro and meso scales, illustrates DFAB's utility in architects' hands for crafting tailored microclimates. This approach manipulates the effective thermal conductivity and macroscale topology for stability against toppling (0.8 kN). A vertical infill porosity gradient establishes a surface temperature gradient, countering ventilation-induced thermal gradients. With a minimal operational temperature vertical gradient (+0.2°C), complying with international comfort standards (Predicted Mean Vote −0.23, People Dissatisfied 6%), Fireplace2 stands testament to DFAB’s microclimate-shaping capabilities despite challenges like foot-level ventilation. The study propels DFAB into a sustainable paradigm, aligning occupant comfort with environmental consciousness, thereby fostering more efficient and enjoyable indoor spaces.

This work explores a spatial printing method to fabricate continuous fiber-reinforced thermoplastic composites (CFRTPCs), which can achieve exceptional mechanical performance. For models giving complex 3D stress distribution under loads, typical planar-layer based fiber placement usually fails to provide sufficient reinforcement due to their orientations being constrained to planes. The effectiveness of fiber reinforcement could be maximized by using multi-axis additive manufacturing (MAAM) to better control the orientation of continuous fibers in 3D-printed composites. Here, we propose a computational approach to generate 3D toolpaths that satisfy two major reinforcement objectives: (1) following the maximal stress directions in critical regions and (2) connecting multiple load-bearing regions by continuous fibers. Principal stress lines are first extracted in an input solid model to identify critical regions. Curved layers aligned with maximal stresses in these critical regions are generated by computing an optimized scalar field and extracting its iso-surfaces. Then, topological analysis and operations are applied to each curved layer to generate a computational domain that preserves fiber continuity between load-bearing regions. Lastly, continuous fiber toolpaths aligned with maximal stresses are generated on each surface layer by computing an optimized scalar field and extracting its iso-curves. A hardware system with dual robotic arms is employed to conduct the physical MAAM tasks depositing polymer or fiber reinforced polymer composite materials by applying a force normal to the extrusion plane to aid consolidation. When comparing to planar-layer based printing results in tension, up to 644% failure load and 240% stiffness are observed on shapes fabricated by our spatial printing method. We demonstrate the versatility of our approach through various complex load cases which demonstrate their successful implementation of continuous fiber printing in 3D.

Carbon-based products are essential to society, yet producing them from fossil fuels is unsustainable. Microorganisms have the ability to take up electrons from solid electrodes and convert carbon dioxide (CO₂) to valuable carbon-based chemicals. However, higher productivities and energy efficiencies are needed to reach a viability that can make the technology transformative. Here, we show how a biofilm-based microbial porous cathode in a directed flow-through electrochemical system can continuously reduce CO₂ to even-chain C2–C6 carboxylic acids over 248 days. We demonstrate a threefold higher biofilm concentration, volumetric current density, and productivity compared with the state of the art. Most notably, the volumetric productivity (VP) resembles those achieved in laboratory-scale and industrial syngas (CO-H₂-CO₂) fermentation and chain elongation fermentation. This work highlights key design parameters for efficient electricity-driven microbial CO₂ reduction. There is need and room to improve the rates of electrode colonization and microbe-specific kinetics to scale up the technology.

Silica-based glasses can be shaped into complex geometries using a variety of additive manufacturing technologies. While the three-dimensional printing of glasses opens unprecedented design opportunities, the development of up-scaled, reliable manufacturing processes is crucial for the broader dissemination of this technology. Here, we design and study phase-separating resins that enable light-based 3D printing of oxide glasses with high-aspect-ratio features and enhanced manufacturing yields. The effect of the resin composition on the microstructure, mechanical properties and delamination resistance of parts printed by digital light processing is investigated with the help of printing experiments, compression tests and electron microscopy analysis. The chemical composition and microstructure of the cured resins were found to strongly affect the stiffness, delamination resistance, and calcination behavior of printed parts. These findings provide useful guidelines to enhance the reliability and yield of the DLP printing process of multicomponent silica-based glasses.

Fused Filament Fabrication is a promising manufacturing technology for the circularity of space missions. Potential scenarios include in-orbit applications to maximize mission life and to support long-term exploration missions with in situ manufacturing and recycling. However, its adoption is restricted by the availability of engineering polymers displaying mechanical performance combined with resistance to space conditions. Here, a thermotropic Liquid Crystal Polymer (LCP) is reported as a candidate material with extrusion 3D printing. To expand its scope of applicability to structural parts for space applications, four different exposure conditions are studied: thermal cycling under vacuum, atomic oxygen, UV, and electron irradiations. While 1 MeV-electron irradiation leads to a green coloration due to annealable color centers, the mechanical performance is only slightly decreased in dynamic mode. It is also found that increased printing temperature improves transverse strength and resistance to thermal cycling with the trade-off of tensile stiffness and strength. Samples exposed to thermal cycling and the highest irradiation dose at lower printing temperatures still display a Young's modulus of 30 GPa and 503 MPa of tensile strength which is exceptionally high for a 3D-printed polymer. For the types of exposure studied, overall, the results indicate that LCP 3D-printed parts are well suited for space applications.

A novel approach is presented to 3D print vacuum–tight polymer components using liquid crystal polymers (LCPs). Vacuum–tight components are essential for gas storage and passive heat transfer, but traditional polymer 3D printing methods often suffer from poor interfaces between layers and high free volume, compromising vacuum integrity. By harnessing the unique properties of LCPs, including low free volume and low melt viscosity, highly ordered domains are achieved through nematic alignment of polymer chains. Critical gas–barrier properties are demonstrated, even in thin, single–print line–walled samples ranging from 0.8 to 1.6 mm. A 200 mm evacuated thermosiphon is successfully printed, which exhibits a thermal resistance of up to 2.18 K/W and an effective thermal conductivity of up to 28 W/mK at 60 °C. These values represent a significant increase compared to the base LCP material. Furthermore, the geometric freedom, enabled by 3D printing through the fabrication of complex–shaped thermosiphons, is showcased. The authors study highlights the potential of LCPs as high–performance materials for 3D printing vacuum–tight components with intricate geometries, opening new avenues for functional design. An application of integrating 3D printed thermosiphons as selective heat transfer components in building envelopes is presented, contributing to greenhouse gas emissions mitigation in the construction sector.

Anisotropic materials formed by living organisms possess remarkable mechanical properties due to their intricate microstructure and directional freedom. In contrast, human-made materials face challenges in achieving similar levels of directionality due to material and manufacturability constraints. To overcome these limitations, an approach using 3D printing of self-assembling thermotropic liquid crystal polymers (LCPs) is presented. Their high stiffness and strength is granted by nematic domains aligning during the extrusion process. Here, a remarkably wide range of Young's modulus from 3 to 40 GPa is obtained during by utilizing directionality of the nematic flow during the printing process. By determining a relationship between stiffness, nozzle diameter, and line width, a design space where shaping and mechanical performance can be combined is identified. The ability to print LCPs with on-the-fly width changes to accommodate arbitrary spatially varying directions is demonstrated. This unlocks the possibility to manufacture exquisite patterns inspired by fluid dynamics with steep curvature variations. Utilizing the synergy between this path-planning method and LCPs, functional objects with stiffness and curvature gradients can be 3D-printed, offering potential applications in lightweight sustainable structures embedding crack-mitigation strategies. This method also opens avenues for studying and replicating intricate patterns observed in nature, such as wood or turbulent flow using 3D printing.