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Materials for thermal management of buildings offer an attractive approach to reduce energy demands and carbon emissions in the infrastructure sector, but many of the state-of-the-art insulators are still expensive, flammable, or difficult to recycle. Here, a 3D printing process is developed and studied to create hierarchical porous ceramics for thermal insulation and passive cooling using recyclable and widely available clay as raw material. Inks comprising particle-stabilized foams are employed as a template for the generation of the hierarchical porosity. Using foams with optimized rheological properties, the printing parameters and sintering conditions required for the manufacturing of hierarchical porous ceramics via Direct Ink Writing are established. The sintering temperature is found to strongly affect the size distribution of micropores, thus controlling the mechanical, thermal, and evaporative cooling properties of sintered printed structures. By combining suspension- and foam-based inks in a multimaterial printing approach, inexpensive and recyclable clay-based bricks are manufactured with structural, thermal insulating, and passive cooling capabilities., Advanced Materials Technologies, 8 (4), ISSN:2365-709X

Iron is a cheap and environmentally friendly metal that is produced at the gigaton scale every year. While porous metals have long been used as filters in chemical processes and electrodes in functional devices, steel and iron-based structures with highly tunable porosity have only recently been explored as sustainable, lightweight materials for a variety of structural, biomedical, and energy-related applications. Here, an extrusion-based 3D printing process is reported to manufacture steel- and iron-based structures with ultrahigh porosity distributed over three hierarchical levels. Using particle-stabilized foams as printing inks, pores are generated at multiple length scales by controlling the print path, the concentration of air bubbles in the ink, and the thermal reduction and sintering process of the metal oxide precursor particles. This enables the manufacturing of iron and iron-based alloys with an elastic modulus above 300 MPa and a density below 1 g cm(-3) while keeping the mechanical efficiency expected for porous structures. The potential applications of such hierarchical porous iron and iron alloys are illustrated by printing structures with tunable chemical compositions that are ultra-lightweight, magnetoresponsive, and capable of spontaneously absorbing oil spills and of vaporizing liquids using low electric power., Advanced Materials Technologies, 8 (3), ISSN:2365-709X

Biological materials such as nacre have evolved microstructural design principles that result in outstanding mechanical properties. While nacre's design concepts have led to bio-inspired materials with enhanced fracture toughness, the microstructural features underlying the remarkable damping properties of this biological material have not yet been fully explored in synthetic composites. Here, we study the damping behavior of nacre-like composites containing mineral bridges and platelet asperities as nanoscale structural features within its brick-and-mortar architecture. Dynamic mechanical analysis was performed to experimentally elucidate the role of these features on the damping response of the nacre-like composites. By enhancing stress transfer between platelets and at the brick/mortar interface, mineral bridges and nano-asperities were found to improve the damping performance of the composite to levels that surpass many biological and man-made materials. Surprisingly, the improved properties are achieved without reaching the perfect organization of the biological counterparts. Our nacre-like composites display a loss modulus 2.4-fold higher than natural nacre and 1.4-fold more than highly dissipative natural fiber composites. These findings shed light on the role of nanoscale structural features on the dynamic mechanical properties of nacre and offer design concepts for the manufacturing of bio-inspired composites for high-performance damping applications., Proceedings of the National Academy of Sciences of the United States of America, 119 (31), ISSN:0027-8424, ISSN:1091-6490

The reflection of light from distributed microplatelets is an effective approach to creating color and controlling the optical properties in paints, security features, and optical filters. However, predictive tools for the design and manufacturing of such composite materials are limited due to the complex light-matter interactions that determine their optical response. Here, the optical reflectance of individual reflective microplatelets and of polymer-based composites containing these engineered platelets as an aligned, dispersed phase are experimentally studied and analytically calculated. Transfer-matrix calculations are used to interpret the effect of the platelet architecture, the number of platelets, and their size distribution on the experimentally measured reflectance of composites prepared using a previously established magnetic alignment technique. It is demonstrated that the reflectance of the composites can be understood as the averaged response of an array of Fabry-Perot resonators, in which the microplatelets act as semi-transparent flat reflectors and the polymer as cavity medium. By using an analytical model and computer simulations to describe the interaction of light with platelets embedded in a polymer matrix, this work provides useful tools for the design and fabrication of composites with tailored optical reflectance., Advanced Optical Materials, 11 (7), ISSN:2195-1071

Porous ceramics are demanded in a wide range of high-temperature, biological and energy-related applications, but may show conflicting properties or suffer from poor mechanical properties. Introducing pores at different length scales has been shown to be a promising design strategy to combine antagonistic performance parameters and reach high porosity without severely compromising strength. Here, a cost-effective and simple process to create strong, highly porous ceramics via direct ink writing of suspensions of hollow microspheres into cellular architectures with pores at three hierarchical levels is reported. X-ray diffraction, rheological measurements, scanning electron microscopy, and mechanical tests are conducted to thoroughly study the processing steps and morphology of the printed hierarchical porous ceramics. The presence of pores at multiple length scales increases significantly the mechanical strength of the porous structure, providing a useful platform for the manufacturing of lightweight ceramics from inexpensive and widely available feedstock materials., Advanced Materials Technologies, ISSN:2365-709X

Cells sense and respond to scaffold pore geometry and mechanical stimuli. Many fabrication methods used in bone tissue engineering render structures with poorly controlled pore geometries. Given that cell-scaffold interactions are complex, drawing a conclusion on how cells sense and respond to uncontrolled scaffold features under mechanical loading is difficult. Here, monodisperse templated scaffolds (MTSC) were fabricated and used as a well-defined porous scaffolds to study the effect of dynamic culture conditions on bone-like tissue formation. Human bone marrow derived stromal cells were cultured on MTSC or conventional salt-leached scaffolds (SLSC) for up to 7 weeks, either under static or dynamic conditions (wall shear stress (WSS) using spinner flask bioreactors). The influence of controlled spherical pore geometry of MTSC subjected to static or dynamic conditions on osteoblast cells differentiation, bone-like tissue formation, structure and distribution was investigated. WSS generated within the two idealized geometrical scaffold features was assessed. Distinct response to fluid flow in osteoblast cell differentiation were shown to be dependent on scaffold pore geometry. As revealed by collagen staining and micro-computed tomography images, dynamic conditions promoted a more regular extracellular matrix (ECM) formation and mineral distribution in both scaffold types compared to static conditions. The results showed that regulation of bone-related genes and the amount and the structure of mineralized ECM were dependent on scaffold pore geometry and the mechanical cues provided by the two different culture conditions. Under dynamic conditions, SLSC favored osteoblast cell differentiation and ECM formation, while MTSC enhanced ECM mineralization. The spherical pore shape in MTSC supported a more trabecular bone-like structure under dynamic conditions compared to MTSC statically cultured or to SLSC under either static or dynamic conditions. These results suggest that cell activity and bone-like tissue formation is driven not only by the pore geometry but also by the mechanical environment. This should be taken into account in the future design of complex scaffolds, which should favor cell differentiation while guiding the formation, structure and distribution of the engineered bone tissue. This could help to mimic the anatomical complexity of the bone tissue structure and to adapt to each bone defect needs.Impact statementAging of the human population leads to an increasing need for medical implants with high success rate. We provide evidence that cell activity and the amount and structure of bone-like tissue formation is dependent on the scaffold pore geometry and on the mechanical environment. Fabrication of complex scaffolds comprising concave and planar pore geometries might represent a promising direction towards the tunability and mimicry the structural complexity of the bone tissue. Moreover, the use of fabrication methods that allow a systematic fabrication of reproducible and geometrically controlled structures would simplify scaffold design optimization.

Porous carbon ablators offer cost-effective thermal protection for aerospace vehicles during re-entry into planetary atmospheres. However, the exploration of more distant planets requires the development of ablators that are able to withstand stronger thermal radiation conditions. Here, we report the development of bio-inspired porous carbon insulators with pore sizes that are deliberately tuned to enhance heat-shielding performance by increasing scattering of high-temperature thermal radiation. Pore size intervals that promote scattering are first estimated using an established model for the radiative contribution to the thermal conductivity of porous insulators. On the basis of this theoretical analysis, we identify a polymer additive that enables the formation of pores in the desired size range through the polymerization-induced phase separation of a mixture of phenolic resin and ethylene glycol. Optical and electron microscopy, porosimetry and mechanical tests are used to characterize the structure and properties of porous insulators prepared with different resin formulations. Insulators with pore sizes in the optimal scattering range reduce laser-induced damage of the porous structures by up to 42%, thus offering a promising and simple route for the fabrication of carbon ablators for enhanced thermal protection at high temperatures., Carbon, 195, ISSN:0008-6223

The increasing use of lightweight composite materials in structural applications requires the development of new damage monitoring technologies to ensure their safe use and prevent accidents. Although several molecular strategies have been proposed to report damage in polymers through mechanochromic responses, these approaches have not yet been translated into lightweight bioinspired composites for load-bearing applications. Here, we report on the development of bioinspired laminates of alternating polymer and nacre-like layers that combine optical translucency, high fracture toughness, and damage-reporting capabilities. The composites signal damage via a fluorescence color change that arises from the force activation of mechanophore molecules embedded in the material's polymer phase. A quantitative correlation between the applied strain and the fluorescence intensity was successfully established. We demonstrate that optical imaging of mechanically loaded composites allows for the localized detection of damage prior to fracture. This fluorescence-based self-reporting mechanism offers a promising approach for the early detection of damage in lightweight structural composites and can serve as a useful tool for the analysis of fracture processes in bulk transparent materials.

Bacterial cellulose is an attractive resource for the manufacturing of sustainable materials, but it is usually challenging to shape it into elaborate three-dimensional structures. Here, we report a manufacturing platform for the creation of complex-shaped cellulose objects by printing inks loaded with bacteria into a silicone-based granular gel. The gel provides the viscoelastic behavior necessary to shape the bacteria-laden ink in three dimensions and the gas permeability required to sustain cellular growth and cellulose formation after the printing process. Using Gluconacetobacter xylinus as model cellulose-producing bacteria, we study the growth and the mechanical properties of cellulose fiber networks obtained upon incubation of the printed inks. Diffusion processes within the ink were found to control the growth of the cellulose structures, which display mechanical properties within the range expected for conventional hydrogels. By keeping the bacteria alive in the printed object, we produce living materials in complex geometries that are able to self-regenerate their cellulose fiber network after damage. Such living hydrogels represent an enticing development towards functional materials with autonomous self-healing and self-regenerating capabilities., Biomaterials Advances, 141, ISSN:2772-9508

Digital light processing (DLP) 3D printing is a promising technique for the rapid manufacturing of customized medical devices with high precision. To be successfully translated to a clinical setting, challenges in the development of suitable photopolymerizable materials have yet to be overcome. Besides biocompatibility, it is often desirable for the printed devices to be biodegradable, elastic, and with a therapeutic function. Here, a multifunctional DLP printed material system based on the composite of gold nanorods and polyester copolymer is reported. The material demonstrates robust near-infrared (NIR) responsiveness, allowing rapid and stable photothermal effect leading to the time-dependent cell death. NIR light-triggerable shape transformation is demonstrated, resulting in a facilitated insertion and expansion of DLP printed stent ex vivo. The proposed strategy opens a promising avenue for the design of multifunctional therapeutic devices based on nanoparticle-polymer composites.

Cu

Structural materials combining high stiffness and damping capabilities are in high demand for passive damping applications in vibration control, precision manufacturing, and resilient buildings. Ho...

We report a versatile method to form bacterial cellulose coatings through simple dip-coating of 3D objects in suspensions of cellulose-producing bacteria. The adhesion of cellulose-secreting bacteria on objects was promoted through surface roughness and chemistry. Immobilized bacteria secreted highly porous hydrogels with high water content directly from the surface of a variety of materials. The out-of-plane orientation of cellulose fibers present in this coating leads to high mechanical stability and energy dissipation with minimal cellulose concentration. The conformal, biocompatible, and lubricious nature of the in situ grown cellulose surfaces makes the coated 3D objects attractive for biomedical applications.

Colors enable interaction and communication between living species in a myriad of biological and artificial environments. While living organisms feature low-power mechanisms to dynamically control color in soft tissues, man-made color-changing devices remain predominantly rigid and energy intensive. Here, architectured composites that display striking color changes when stretched in selective directions under ambient light with minimum power input are reported. The orientation-dependent color change results from the rotation of reflective coated platelets that are embedded in a soft polymer matrix and pre-aligned in a well-defined architecture. The light reflected by the platelets generates structural color defined by the oxide coating on the platelet surface. By magnetically programming the initial orientation and spatial distribution of selected platelets within the soft matrix, composites with strain-modulated color-changing effects that cannot be achieved using state-of-the-art technologies are created. The proposed concept of strain-induced architectured color can be harnessed to develop low-power smart stretchable displays, tactile synthetic skins, and autonomous soft robotic devices that undergo fast and reversible color changes through the mechano-optic coupling programmed within their soft composite architecture., Advanced Materials, 34 (6), ISSN:0935-9648, ISSN:1521-4095

Biological living materials, such as animal bones and plant stems, are able to self-heal, regenerate, adapt and make decisions under environmental pressures. Despite recent successful efforts to imbue synthetic materials with some of these remarkable functionalities, many emerging properties of complex adaptive systems found in biology remain unexplored in engineered living materials. Here, we describe a three-dimensional printing approach that harnesses the emerging properties of fungal mycelia to create living complex materials that self-repair, regenerate and adapt to the environment while fulfilling an engineering function. Hydrogels loaded with the fungus Ganoderma lucidum are three-dimensionally printed into lattice architectures to enable mycelial growth in a balanced exploration and exploitation pattern that simultaneously promotes colonization of the gel and bridging of air gaps. To illustrate the potential of such mycelium-based living complex materials, we three-dimensionally print a robotic skin that is mechanically robust, self-cleaning and able to autonomously regenerate after damage. ISSN:1476-1122 ISSN:1476-4660

3D printing is a powerful manufacturing technology for shaping materials into complex structures. While the palette of printable materials continues to expand, the rheological and chemical requisites for printing are not always easy to fulfill. Here, a universal manufacturing platform is reported for shaping materials into intricate geometries without the need for their printability, but instead using light-based printed salt structures as leachable molds. The salt structures are printed using photocurable resins loaded with NaCl particles. The printing, debinding, and sintering steps involved in the process are systematically investigated to identify ink formulations enabling the preparation of crack-free salt templates. The experiments reveal that the formation of a load-bearing network of salt particles is essential to prevent cracking of the mold during the process. By infiltrating the sintered salt molds and leaching the template in water, complex-shaped architectures are created from diverse compositions such as biomedical silicone, chocolate, light metals, degradable elastomers, and fiber composites, thus demonstrating the universal, cost-effective, and sustainable nature of this new manufacturing platform.

Nanowires are often key ingredients of high-tech composite materials. The properties and performance of devices created using these, depend heavily on the structure and density of the embedded nanowires. Despite significant efforts, a process that can be adapted to different materials, compatible with current nanowire deposition methods, and that is able to control both variables simultaneously has not been achieved yet. In this work, we show that we can use low magnetic fields (80 mT) to manipulate nanowires by electrostatically coating them with super-paramagnetic iron oxide nanoparticles in an aqueous solution. Monolayers, multilayers, and hierarchical structures of oriented nanowires were achieved in a highly ordered manner using vacuum filtration for two types of nanowires: silver and gold-coated titanium dioxide nanowires. The produced films were embedded in an elastomer, and the strain-dependent electrical properties of the resulting composites were investigated. The orientation of the assembly with respect to the tensile strain heavily impacts the performance of the composites. Composites containing nanowires perpendicular to the strain direction exhibit an extremely low gauge factor. On the other hand, when nanowires are arranged parallel to the strain direction, the composites have a high gauge factor. The possibility to orient nanowires during the processing steps is not only interesting for the shown strain sensing application but also expected to be useful in many other areas of material science. ISSN:1936-0851 ISSN:1936-086X

Monolithic aerogels composed of crystalline nanoparticles enable photocatalysis in three dimensions, but they suffer from low mechanical stability and it is difficult to produce them with complex geometries. Here, an approach to control the geometry of the photocatalysts to optimize their photocatalytic performance by introducing carefully designed 3D printed polymeric scaffolds into the aerogel monoliths is reported. This allows to systematically study and improve fundamental parameters in gas phase photocatalysis, such as the gas flow through and the ultraviolet light penetration into the aerogel and to customize its geometric shape to a continuous gas flow reactor. Using photocatalytic methanol reforming as a model reaction, it is shown that the optimization of these parameters leads to an increase of the hydrogen production rate by a factor of three from 400 to 1200 µmol g−1 h−1. The rigid scaffolds also enhance the mechanical stability of the aerogels, lowering the number of rejects during synthesis and facilitating handling during operation. The combination of nanoparticle-based aerogels with 3D printed polymeric scaffolds opens up new opportunities to tailor the geometry of the photocatalysts for the photocatalytic reaction and for the reactor to maximize overall performance without necessarily changing the material composition. ISSN:1613-6810 ISSN:1613-6829

Porous structures offer an attractive approach to reduce the amount of natural resources used while maintaining relatively high mechanical efficiency. However, for some applications the drop in mechanical properties resulting from the introduction of porosity is too high, which has limited the broader utilization of porous materials in industry. Here, it is shown that steel monoliths can be designed to display high mechanical efficiency and reversible self-reinforcing properties when made with porous architectures with up to three hierarchical levels. Ultralight steel structures that can float on water and autonomously adapt their stiffness are manufactured by the thermal reduction and sintering of 3D printed foam templates. Using distinct mechanical testing techniques, image analysis, and finite element simulations, the mechanisms leading to the high mechanical efficiency and self-stiffening ability of the hierarchical porous monoliths are studied. The design and fabrication of mechanically stable porous monoliths using iron as a widely available natural resource is expected to contribute to the future development of functional materials with a more sustainable footprint., Advanced Materials, 35 (10), ISSN:0935-9648, ISSN:1521-4095

Living materials with embedded microorganisms can genetically encode attractive sensing, self-repairing, and responsive functionalities for applications in medicine, robotics, and infrastructure. While the synthetic toolbox for genetically engineering bacteria continues to expand, technologies to shape bacteria-laden living materials into complex 3D geometries are still rather limited. Here, it is shown that bacteria-laden hydrogels can be shaped into living materials with unusual architectures and functionalities using readily available light-based printing techniques. Bioluminescent and melanin-producing bacteria are used to create complex materials with autonomous chemical-sensing capabilities by harnessing the metabolic activity of wild-type and engineered microorganisms. The shaping freedom offered by printing technologies and the rich biochemical diversity available in bacteria provides ample design space for the creation and exploration of complex living materials with programmable functionalities for a broad range of applications., Advanced Materials, 35 (6), ISSN:0935-9648, ISSN:1521-4095

Industrial & Engineering Chemistry Research, 59 (15), ISSN:1520-5045, ISSN:0888-5885

Thermal insulators are crucial to reduce the high energy demands and greenhouse emissions in the construction sector. However, the fabrication of insulating materials that are cost-effective, fire ...

Pastes based on copper (Cu) nanoparticles (NPs) are promising electronic-packaging materials for the attachment of high-power devices. However, the rapid oxidation of nanostructured Cu requires the use of reducing agents during processing, which makes it less suitable for attaching large-area dies (> 4 mm2). Recently, the functionalization of Cu-NP surfaces with a mixture of amines prevented oxidation, allowing for sintering without the need for reducing agents. Here we investigate the sintering mechanisms involved during die attachment using pastes of passivated Cu NPs, with particular focus on the critical role of the carrier solvents. Using 1-nonanol or 1-decanol as solvents, we first demonstrate the absence of Cu-oxide phases in the pastes after fabrication and the stability of the resulting nanostructured copper for as much as 30 min in air. By measuring the evolution of the electrical characteristics of the paste during drying and sintering, we show that electrically conductive agglomerates form among the NPs between 141°C and 144°C, independent of the carrier solvent used. The carrier solvent was found to affect mainly the densification temperature of the copper agglomerates. Because they lead to uniform sintering of the material, Cu pastes based on solvents with a low boiling point and high vapor pressure are preferable for attaching dies with area greater than 25 mm2. We show that dies with an area as large as 100 mm2 can be attached using a Cu paste based on 1-nonanol. These pastes enables the formation of temperature-resistant bonding for high-power devices using a simple and cost-effective approach.

Summary Synthetic implants made by traditional fabrication routes are not patient specific and rarely match the performance of their biological counterparts. We present an additive manufacturing approach for the digital fabrication of tissue-like aortic heart valves featuring customizable geometry and leaflet architectures that resemble those of native valve tissue. Using biocompatible silicones with tunable mechanical properties, heart valves were fabricated by combining spray and extrusion-based additive manufacturing processes. Computer simulations showed that bioinspired leaflet architectures strongly affect the stress distribution throughout the valves, minimizing stresses on the leaflet during a cardiac cycle. Our computational analysis was complemented by in vitro experiments in a pulse duplicator to demonstrate the outstanding hemodynamic performance of the printed heart valves under physiological pressure cycles. The ability to fabricate synthetic implants with tailored designs at multiple length scales is a key contribution toward the digital fabrication of functional implants that perform on par with native body parts.

Adsorption heat pumps offer a clean, zero-emission technology for universally applicable cooling or heating utilizing water as a refrigerant and waste or renewable heat as driving energy instead of electricity. Despite their attractive environmentally friendly prospects, the broader application of such classes of heat pumps has not yet been possible, mainly because of the low power density of adsorption heat exchangers and the corresponding large size and high cost of the adsorption heat pumps. We report an inexpensive route for the fabrication of zeolite coatings with high adsorption power density based on the bottom-up assembly of colloids directed by magnetic and capillary forces. Such an assembly process relies on the chaining of oil droplets under an external magnetic field during deposition of the coating, followed by the formation of a percolating network of bridged adsorbent particles upon drying. This results in vertically open channels and thermal bridges that facilitate directed mass and heat transport across the structured zeolite coating during sorption cycles. By reaching up to 3.3-fold higher performance than their unstructured counterparts using readily available zeolite as an adsorbent material, the architectured coatings produced through this facile, upscalable approach hold great potential for next-generation adsorption heat pumps.

The rate of water adsorption in SAPO-34 zeolite coatings for adsorption heat pumps has been shown to be limited by mass transfer. In the present contribution, uniformly spaced longitudinal channels of width 75 µm were introduced into SAPO-34 coatings and their effect on the heat and mass transport limitations during water sorption was explored. Different channel spacings at constant adsorbent mass per unit area were tested by means of temperature-swing adsorption measurements and thermal impedance analysis (TIA). The optimal ratio between the characteristic transport length (CTL) for heat transfer and the CTL for mass transfer was 6, which is in agreement with the TIA predictions and was validated experimentally. Geometries with CTL ratios greater or less than 6 exhibited higher thermal impedance and lower rates of water sorption. At the optimal CTL ratio, the water sorption rate was enhanced 2x at the same adsorbent mass per unit area when compared to unstructured SAPO-34 coatings. Compared to other adsorbent structures reported in literature, the structured coatings exhibit the highest power density and energy density at short cycle times. These findings may be used to improve efficiency and/or power in temperature-swing processes in which mass transfer in adsorbent coatings is rate-limiting. It is recommended to develop methods to structure coatings at the scales relevant for commercial adsorption heat pump modules, and further investigate the rate-limiting transport mechanisms in large adsorption heat exchanger modules.

Microcapsules made of synthetic polymers are used for the release of cargo in agriculture, food, and cosmetics but are often difficult to be degraded in the environment. To diminish the environmental impact of microcapsules, we use the biofilm-forming ability of bacteria to grow cellulose-based biodegradable microcapsules. The present work focuses on the design and optimization of self-grown bacterial cellulose capsules. In contrast to their conventionally attributed pathogenic role, bacteria and their self-secreted biofilms represent a multifunctional class of biomaterials. The bacterial strain used in this work