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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.

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.

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

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

Magnetically driven soft actuators are unique because they are fast, remote-controlled, conformal to rigid objects, and safe to interact with humans. Despite these multiple functionalities, a broader utilization of such actuators is hindered by the high cost and equipment-intensive nature of currently available manufacturing processes. Herein, a simple fabrication route for magneto-responsive soft actuators is described using cost-effective and broadly available raw materials and equipment. The method utilizes castable silicone resins that are loaded with magnetic particles and subsequently magnetized under an external magnetic field. The experimental investigation of silicone-based composites prepared with particles of distinct chemistries, sizes, and morphologies enables the identification of the raw materials and magnetization conditions required for the process. This leads to functional soft actuators with programmable magnetic patterns that are capable of performing pick-and-place, lifting, catching, and moving tasks under the remote action of an external magnetic field. By removing manufacturing hurdles associated with costly raw materials and equipment, the proposed approach is expected to facilitate the design, implementation, and exploitation of the unique functionalities of magneto-controlled soft actuators in a wider number of applications., Advanced Intelligent Systems, 3 (8), ISSN:2640-4567

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.

Central airway obstruction is a life-threatening disorder causing a high physical and psychological burden to patients. Standard-of-care airway stents are silicone tubes, which provide immediate relief but are prone to migration. Thus, they require additional surgeries to be removed, which may cause tissue damage. Customized bioresorbable airway stents produced by 3D printing would be highly needed in the management of this disorder. However, biocompatible and biodegradable materials for 3D printing of elastic medical implants are still lacking. Here, we report dual-polymer photoinks for digital light 3D printing of customized and bioresorbable airway stents. These stents exhibit tunable elastomeric properties with suitable biodegradability. In vivo study in healthy rabbits confirmed biocompatibility and showed that the stents stayed in place for 7 weeks after which they became radiographically invisible. This work opens promising perspectives for the rapid manufacturing of the customized medical devices for which high precision, elasticity, and degradability are sought., Science Advances, 7 (6), ISSN:2375-2548

The brick-and-mortar architecture of biological nacre has inspired the development of synthetic composites with enhanced fracture toughness and multiple functionalities. While the use of metals as the “mortar” phase is an attractive option to maximize fracture toughness of bulk composites, non-mechanical functionalities potentially enabled by the presence of a metal in the structure remain relatively limited and unexplored. Using iron as the mortar phase, we develop and investigate nacre-like composites with high fracture toughness and stiffness combined with unique magnetic, electrical and thermal functionalities. Such metal-ceramic composites are prepared through the sol–gel deposition of iron-based coatings on alumina platelets and the magnetically-driven assembly of the pre-coated platelets into nacre-like architectures, followed by pressure-assisted densification at 1450 °C. With the help of state-of-the-art characterization techniques, we show that this processing route leads to lightweight inorganic structures that display outstanding fracture resistance, show noticeable magnetization and are amenable to fast induction heating. Materials with this set of properties might find use in transport, aerospace and robotic applications that require weight minimization combined with magnetic, electrical or thermal functionalities., Scientific Reports, 11 (1), ISSN:2045-2322

Fiber-reinforced polymers are widely used as lightweight materials in aircraft, automobiles, wind turbine blades, and sports products. Despite the beneficial weight reduction achieved in such applications, these composites often suffer from poor recyclability and limited geometries. 3D printing of liquid crystal polymers into complex-shaped all-fiber materials is a promising approach to tackle these issues and thus increase the sustainability of current lightweight structures. Here, we report a spin-printing technology for the manufacturing of recyclable and strong all-fiber lightweight materials. All-fiber architectures are created by combining thick print lines and thin spun fibers as reinforcing elements in bespoke orientations. Through controlled extrusion experiments and theoretical analyses, we systematically study the spinning process and establish criteria for the generation of thin fibers and laminates with unprecedented mechanical properties. The potential of the technology is further illustrated by creating complex structures with unique all-fiber architectures and mechanical performance., Advanced Functional Materials, 31 (52), ISSN:1616-3028, ISSN:1616-301X

While dissolution-precipitation, plastic deformation and fracture have been proposed to explain the compaction of carbonates in geological formations, the role of these mechanisms on the densification process of calcium carbonate nanoparticles in synthetic systems remains poorly understood. Here, we systematically investigate the effect of pH of the aqueous phase (1 ≤ pH ≤ 7), temperature (10 ≤ T ≤ 90 °C), and pressure (10 ≤ P ≤ 800 MPa) on the cold compaction of nanovaterite powder with water to shed light on the mechanisms underlying this unique densification. Compaction experiments reveal that the applied pressure plays a major role on the densification of vaterite nanopowder with water. Our experimental data thus suggest that plastic deformation or subcritical crack growth might be important densification mechanisms for vaterite nanoparticles. These findings provide a new perspective into the cold compaction of nanopowders with water and may open promising routes for the manufacturing of CO2-based structural materials at mild processing conditions.

The digital fabrication of oxide glasses by three-dimensional (3D) printing represents a major paradigm shift in the way glasses are designed and manufactured, opening opportunities to explore functionalities inaccessible by current technologies. The few enticing examples of 3D printed glasses are limited in their chemical compositions and suffer from the low resolution achievable with particle-based or molten glass technologies. Here, we report a digital light-processing 3D printing platform that exploits the photopolymerization-induced phase separation of hybrid resins to create glass parts with complex shapes, high spatial resolutions and multi-oxide chemical compositions. Analogously to conventional porous glass fabrication methods, we exploit phase separation phenomena to fabricate complex glass parts displaying light-controlled multiscale porosity and dense multicomponent transparent glasses with arbitrary geometry using a desktop printer. Because most functional properties of glasses emerge from their transparency and multicomponent nature, this 3D printing platform may be useful for distinct technologies, sciences and arts. ISSN:1476-1122 ISSN:1476-4660

Polysaccharides are attractive sustainable resources for the fabrication of advanced materials, but the assembly of these building blocks into complex-shaped structures combining the high strength ...

Fibre-reinforced polymer structures are often used when stiff lightweight materials are required, such as in aircraft, vehicles and biomedical implants. Despite their very high stiffness and strength1, such lightweight materials require energy- and labour-intensive fabrication processes2, exhibit typically brittle fracture and are difficult to shape and recycle3,4. This is in stark contrast to lightweight biological materials such as bone, silk and wood, which form by directed self-assembly into complex, hierarchically structured shapes with outstanding mechanical properties5–11, and are circularly integrated into the environment. Here we demonstrate a three-dimensional (3D) printing approach to generate recyclable lightweight structures with hierarchical architectures, complex geometries and unprecedented stiffness and toughness. Their features arise from the self-assembly of liquid-crystal polymer molecules into highly oriented domains during extrusion of the molten feedstock material. By orienting the molecular domains with the print path, we are able to reinforce the polymer structure according to the expected mechanical stresses, leading to stiffness, strength and toughness that outperform state-of-the-art 3D-printed polymers by an order of magnitude and are comparable with the highest-performance lightweight composites1,12. The ability to combine the top-down shaping freedom of 3D printing with bottom-up molecular control over polymer orientation opens up the possibility to freely design and realize structures without the typical restrictions of current manufacturing processes.

Developing strategies to reduce mass and heat transport limitations is one of the most important challenges in adsorption heat exchanger technology. Due to the strong coupling of mass and heat transport in these systems, it is difficult to determine the individual transport limitations quantitatively. In order to find an optimal design where heat and mass transport are balanced, a quantitative method that enables a direct comparison of these two transport phenomena is needed. In the present work, a novel experimental approach to discriminate between mass and heat transport is proposed based on the measurement of adsorbent temperature, heat exchanger surface temperature and vapor pressure. The methodology is applied to micro/mesoporous silica spheres arranged in a monolayer or bilayer loose grain configuration or in a monolayer configuration adhesively bonded to the substrate. While the monolayer configuration exhibits balanced heat and mass transport, we find that the bilayer and the thermally enhanced configurations are limited by heat and mass transport, respectively. The application of the proposed methodology to compare heat and mass transport limitations in other industrially-relevant adsorbent materials should greatly aid the design of more efficient adsorption heat exchangers for a wide range of applications.

The determination of the fracture toughness of fiber-reinforced polymers (FRPs) exhibiting significant nonlinear material behavior requires the use of methods that take into account the contribution of nonlinear phenomena on the computation of the crack driving force. While the J-integral method has been standardized (ASTM E1820) and extensively used to quantify the fracture toughness of homogeneous nonlinear materials, its validity to unidirectional FRPs remains uncertain. This work addresses this issue by comparing different methods to quantify the energy dissipation during fracture of glass fiber-reinforced polymers with a nonlinear polyurethane matrix. To this end, the J-integral under mode I transverse intralaminar crack growth is evaluated for the first time by directly computing J versus load-line displacement curves using strain maps obtained around the crack tip with digital image correlation. This direct approach is then compared with 2 established data reduction methods and data obtained from virtual tests to determine the suitability of standardized procedures to quantify the fracture energy of such heterogeneous materials. The results show that the best agreement among the values obtained with the different experimental methods is reached when the initial crack length accounts for 40 to 50% of the specimen's width. Accounting for the experimental uncertainties, the procedure described in the standard ASTM E1820 is suitable to quantify the fracture energy of unidirectional laminates displaying nonlinear mechanical response.

More than just simple folding Origami involves folding two-dimensional sheets into complex three-dimensional objects. However, some shapes cannot be created using standard folds. Faber et al. studied the wing of an earwig, which can fold in ways not possible using origami and can alter its shape for flight. The authors replicated this ability by using a membrane that allows for deformations and variable stiffness. Prestretching generated energetically bistable origami patterns that exhibited passive self-folding behavior. Science , this issue p. 1386

Bituminous road pavements may suffer from cracking over the years due to repeated stresses. In this study, we compare the effect of different sizes and chemical compositions of magnetically-respons ...

Compartmentalized microcapsules are useful for the release of multiple cargos in medicine, agriculture, and advanced responsive materials. Although several encapsulation strategies that involve more than one cargo have been proposed, dual- or multicompartment capsules with high cargo loadings and sufficient mechanical stability are rarely reported. Here, we propose a single-step emulsification route for the preparation of strong dual-compartment capsules that can host the main cargo in their core in combination with another liquid cargo stored within their thick shell. Capsules are produced through the polymerization of the middle oil phase of water-oil-water double emulsions made by microfluidics. Compartmentalization results from the phase separation of monomers within the middle phase of the double emulsion. We investigate the effect of such phase separation process on the microstructure and mechanical properties of the capsules and eventually illustrate the potential of this approach by creating thermosensitive capsules with programmable bursting temperature. The large variety of possible mixtures of monomers and cargos that can be added in the oil and aqueous phases of the double emulsion templates makes this encapsulation approach a promising route for the fabrication of robust microcapsules for on-demand release of multiple cargos.

The active transport of cargo molecules within cells is essential for life. Developing synthetic strategies for cargo control in living or inanimate thermal systems could lead to powerful tools to manipulate chemical gradients at the microscale and thus drive processes out of equilibrium to realize work. Here we demonstrate a colloidal analog of the complex biological shuttles responsible for molecular trafficking in cells. Our colloidal shuttles consist of magneto-dielectric particles that are loaded with cargo particles or living cells through size-selective dielectrophoretic trapping using electrical fields. The loaded colloidal shuttle can be transported with magnetic field gradients before cargo is released at the target location by switching off the electrical field. Such spatiotemporal control over the distribution of chemically active cargo in a reversible fashion can be potentially exploited for fundamental biological research or for the development of novel technologies for advanced cell culturing, drug discovery and medical diagnosis., Nature Communications, 8, ISSN:2041-1723

Microcapsules with organic-inorganic hybrid shells can be used as functionally responsive delivery systems that are attractive for a broad range of applications. Hybrid-shell microcapsules have often been synthesized by the assembly of solid inorganic nanoparticles and polymers. Efforts to extend this approach to microfluidic emulsification have been hampered by problems with clogging and flow instabilities when utilizing dispersions of solid particles. In this work, hybrid shell microcapsules are synthesized through the reaction of liquid precursors, eliminating the use of solid dispersions. Our microfluidic water-oil-water emulsification technique also enables the preparation of hybrid-shell microcapsules with thicker and more robust shells compared to alternative techniques. By utilizing bridged-silane precursors to form the hybrid material, we demonstrate hybrid-shell microcapsules with independently tunable functional and mechanical/barrier properties. This independent tuning of physical and functional properties allows for the production of functional organic-inorganic hybrid shell microcapsules that can be tailored to meet the demands of a wide range of applications.

The fracture toughness of laminated composite materials is a very important property for damage-tolerant design, but remains poorly understood under transverse mechanical loading conditions. In this study, the transverse intralaminar fracture in unidirectional laminates with ductile polymer matrices is investigated. The presence of a ductile matrix calls for the use of concepts of nonlinear fracture mechanics for the accurate determination of the resistance of the composite to crack propagation. Unidirectional glass fiber-reinforced polymers with a polyurethane (PU) matrix are studied and compared to a benchmark epoxy (EP). The fracture toughness of these composites was evaluated through the determination of R-curves using the J-integral method, whereas the underlying fracture mechanisms were investigated through in situ testing in a scanning electron microscope. The results suggest stronger adhesion of the polyurethane matrix onto the glass fibers as compared to the epoxy-based polymer. This enables full exploitation of the ductile behavior of the PU matrix, ultimately endowing the laminate with outstanding fracture toughness. The investigated PU matrix has thus the potential to generate laminated composite parts with significantly higher damage tolerance than its epoxy counterpart.

Microcapsules for controlled chemical release and uptake are important in many industrial applications but are often difficult to produce with the desired combination of high mechanical strength and high shell permeability. Using water-oil-water double emulsions made in microfluidic devices as templates, we developed a processing route to obtain mechanically robust microcapsules exhibiting a porous shell structure with controlled permeability. The porous shell consists of a network of interconnected polymer particles that are formed upon phase separation within the oil phase of the double emulsion. Porosity is generated by an inert diluent incorporated in the oil phase. The use of undecanol and butanol as inert diluents allows for the preparation of microcapsules covering a wide range of shell-porosity and force-at-break values. We found that the amount and chemical nature of the diluent influence the shell porous structure by changing the mechanism of phase separation that occurs during polymerization. In a proof-of-concept experiment, we demonstrate that the mechanically robust microcapsules prepared through this simple approach can be utilized for the on-demand release of small molecules using a pH change as exemplary chemical trigger.

Cold sintering is a promising route towards the manufacturing of dense ceramics at mild processing conditions, but our poor understanding of the process has prevented the wider spread of this attractive densification approach. Using nanovaterite powders with well-defined multiscale morphology, we perform in-situ X-Ray tomography on compacts subjected to controlled mechanical load and quantify the multiscale deformation processes responsible for the water-assisted cold sintering of this powder with the help of instrumented indentation experiments at the micro- and nano-scale. Our results reveal the crucial effect of water in promoting the macroscopic densification process and highlight the dominant role of the nanoparticle network inside agglomerates in controlling the cold sintering of compacts at high mechanical loads. By providing new insights into the deformation processes responsible for the densification effect, this study can potentially guide the discovery of novel chemical compositions and particle morphologies that can be more easily densified through room-temperature cold sintering with water.

Porosity is an essential feature in a wide range of applications that combine light weight with high surface area and tunable density. Porous materials can be easily prepared with a vast variety of chemistries using the salt-leaching technique. However, this templating approach has so far been limited to the fabrication of structures with random porosity and relatively simple macroscopic shapes. Here, a technique is reported that combines the ease of salt leaching with the complex shaping possibilities given by additive manufacturing (AM). By tuning the composition of surfactant and solvent, the salt-based paste is rheologically engineered and printed via direct ink writing into grid-like structures displaying structured pores that span from the sub-millimeter to the macroscopic scale. As a proof of concept, dried and sintered NaCl templates are infiltrated with magnesium (Mg), which is typically highly challenging to process by conventional AM techniques due to its highly oxidative nature and high vapor pressure. Mg scaffolds with well-controlled, ordered porosity are obtained after salt removal. The tunable mechanical properties and the potential to be predictably bioresorbed by the human body make these Mg scaffolds attractive for biomedical implants and demonstrate the great potential of this additive technique.

Responsive materials with birefringent optical properties have been exploited for the manipulation of light in several modern electronic devices. While electrical fields are often utilized to achieve optical modulation, magnetic stimuli may offer an enticing complementary approach for controlling and manipulating light remotely. Here, the synthesis and characterization of magnetically responsive birefringent microparticles with unusual magneto‐optical properties are reported. These functional microparticles are prepared via a microfluidic emulsification process, in which water‐based droplets are generated in a flow‐focusing device and stretched into anisotropic shapes before conversion into particles via photopolymerization. Birefringence properties are achieved by aligning cellulose nanocrystals within the microparticles during droplet stretching, whereas magnetic responsiveness results from the addition of superparamagnetic nanoparticles to the initial droplet template. When suspended in a fluid, the microparticles can be controllably manipulated via an external magnetic field to result in unique magneto‐optical coupling effects. Using a remotely actuated magnetic field coupled to a polarized optical microscope, these microparticles can be employed to convert magnetic into optical signals or to estimate the viscosity of the suspending fluid through magnetically driven microrheology. ISSN:1613-6810 ISSN:1613-6829

Multi-layered composites in nature often show functional properties that are determined by the specific orientation of inorganic building blocks within each layer. The shell of bivalve molluscs and the exoskeleton of crustaceans constitute prominent examples. An effective approach to artificially produce textured microstructures inspired by such complex composites is magnetically assisted slip casting (MASC). MASC is a colloidal process in which anisotropic particles are magnetically oriented at arbitrarily defined angles and collected at the surface of a porous mould to grow the material in an additive manner. Whereas a number of proof-of-concept studies have established the potential of the technique, the full design space available for MASC-fabricated structures, and the limits of the approach, have so far not been explored systematically. To fill this gap, we have studied both theoretically and experimentally the various torques that act on the particles at the different stages of the assembly process. We define the boundary conditions of the MASC process for magnetically responsive alumina platelets suspended in a low-viscosity aqueous suspension, considering the composition of the colloidal suspension and the dynamics of the particle alignment process under a rotating magnetic field. These findings lead to design guidelines for the fabrication of bio-inspired composites with customized multi-scale structures for a broad range of applications.

Colloidal processing of fine ceramic powders enables the production of complex shaped ceramics with unique micro and macro structures which are not possible to produce via conventional dry processing routes. Because of this enhanced structural control and shaping capabilities, colloidal processing has been exploited to produce ceramic components with ever increasing complexity and functionalities. In this review, we revisit some of the research efforts on this topic to highlight its relevance and growing importance for the advanced manufacturing of functional ceramics. Selected examples of colloidal systems with increasing level of complexity are discussed to showcase the wide range of structures that can be generated through wet processing approaches. The historical development and background knowledge pertaining to colloids and surface interactions is first briefly reviewed. The major colloidal shape forming and additive manufacturing processes that utilize colloidal pastes and inks are then reviewed, highlighting the control of suspension rheology needed in these techniques. Next, methodologies that combine suspended particles with a pore-forming phase are discussed as a means to produce porous ceramic materials. Further control over the interactions between anisotropic particles and their alignment in suspensions can be gained via externally applied fields (such as magnetic) to produce texturally aligned green bodies. This leads to bioinspired ceramics that can programmably morph into complex shaped objects upon sintering. Hierarchical porous structures with high mechanical efficiency are also shown as an example of the multiscale designs that can be generated through advanced colloidal processing. As drying of ceramic bodies is an inevitable consequence of wet colloidal processing, the current understanding of this critical processing step is reviewed. Finally, the gaps in knowledge in these fields are discussed to provide our perspective on where the field may support advances in ceramics in the future.

3D printing via direct ink writing (DIW) is a versatile additive manufacturing approach applicable to a variety of materials ranging from ceramics over composites to hydrogels. Due to the mild processing conditions compared to other additive manufacturing methods, DIW enables the incorporation of sensitive compounds such as proteins or drugs into the printed structure. Although emulsified oil-in-water systems are commonly used vehicles for such compounds in biomedical, pharmaceutical, and cosmetic applications, printing of such emulsions into architectured soft materials has not been fully exploited and would open new possibilities for the controlled delivery of sensitive compounds. Here, we 3D print concentrated emulsions into soft materials, whose multiphase architecture allows for site-specific incorporation of both hydrophobic and hydrophilic compounds into the same structure. As a model ink, concentrated emulsions stabilized by chitosan-modified silica nanoparticles are studied, because they are sufficiently stable against coalescence during the centrifugation step needed to create a bridging network of droplets. The resulting ink is ideal for 3D printing as it displays high yield stress, storage modulus and elastic recovery, through the formation of networks of droplets as well as of gelled silica nanoparticles in the presence of chitosan. To demonstrate possible architectures, we print biocompatible soft materials with tunable hierarchical porosity containing an encapsulated hydrophobic compound positioned in specific locations of the structure. The proposed emulsion-based ink system offers great flexibility in terms of 3D shaping and local compositional control, and can potentially help address current challenges involving the delivery of incompatible compounds in biomedical applications.

Edible solid particles constitute an attractive alternative to surfactants as stabilizers of food-grade emulsions for products requiring a long-term shelf life. Here, we report on a new approach to stabilize edible emulsions using silica nanoparticles modified by noncovalently bound chitosan oligomers. Electrostatic modification with chitosan increases the hydrophobicity of the silica nanoparticles and favors their adsorption at the oil-water interface. The interfacial adsorption of the chitosan-modified silica particles enables the preparation of oil-in-water emulsions with small droplet sizes of a few micrometers through high-pressure homogenization. This approach enables the stabilization of food-grade emulsions for more than 3 months. The emulsion structure and stability can be effectively tuned by controlling the extent of chitosan adsorption on the silica particles. Bulk and interfacial rheology are used to highlight the two stabilization mechanisms involved. Low chitosan concentration (1 wt % with respect to silica) leads to the formation of a viscoelastic film of particles adsorbed at the oil-water interface, enabling Pickering stabilization of the emulsion. By contrast, a network of agglomerated particles formed around the droplets is the predominant stabilization mechanism of the emulsions at higher chitosan content (5 wt % with respect to silica). These two pathways against droplet coalescence and coarsening open up different possibilities to engineer the long-term stabilization of emulsions for food applications.

Like many other natural materials, silk is hierarchically structured from the amino acid level up to the cocoon or spider web macroscopic structures. Despite being used industrially in a number of applications, hierarchically structured silk fibroin objects with a similar degree of architectural control as in natural structures have not been produced yet due to limitations in fabrication processes. In a combined top-down and bottom-up approach, we exploit the freedom in macroscopic design offered by 3D printing and the template-guided assembly of ink building blocks at the meso- and nanolevel to fabricate hierarchical silk porous materials with unprecedented structural control. Pores with tunable sizes in the range 40-350 μm are generated by adding sacrificial organic microparticles as templates to a silk fibroin-based ink. Commercially available wax particles or monodisperse polycaprolactone made by microfluidics can be used as microparticle templates. Since closed pores are generated after template removal, an ultrasonication treatment can optionally be used to achieve open porosity. Such pore templating particles can be further modified with nanoparticles to create a hierarchical template that results in porous structures with a defined nanotopography on the pore walls. The hierarchically porous silk structures obtained with this processing technique can potentially be utilized in various application fields from structural materials to thermal insulation to tissue engineering scaffolds.

Although strength and toughness are often mutually exclusive properties in man-made structural materials, nature is full of examples of composite materials that combine these properties in a remarkable way through sophisticated multiscale architectures. Understanding the contributions of the different constituents to the energy dissipating toughening mechanisms active in these natural materials is crucial for the development of strong artificial composites with a high resistance to fracture. Here, we systematically study the influence of the polymer properties on the mechanics of nacre-like composites containing an intermediate fraction of mineral phase (57 vol%). To this end, we infiltrate ceramic scaffolds prepared by magnetically assisted slip casting (MASC) with monomers that are subsequently cured to yield three drastically different polymers: (i) poly(lauryl methacrylate) (PLMA), a soft and weak elastomer; (ii) poly(methyl methacrylate) (PMMA), a strong, stiff and brittle thermoplastic; and (iii) polyether urethane diacrylate-co-poly(2-hydroxyethyl methacrylate) (PUA-PHEMA), a tough polymer of intermediate strength and stiffness. By combining our experimental data with finite element modeling, we find that stiffer polymers can increase the strength of the composite by reducing stress concentrations in the inorganic scaffold. Moreover, infiltrating the scaffolds with tough polymers leads to composites with high crack initiation toughness KIC. An organic phase with a minimum strength and toughness is also required to fully activate the mechanisms programmed within the ceramic structure for a rising R-curve behavior. Our results indicate that a high modulus of toughness is a key parameter for the selection of polymers leading to strong and tough bioinspired nacre-like composites.

Bulk hierarchical porous ceramics with unprecedented strength-to-weight ratio and tunable pore sizes across three different length scales are printed by direct ink writing. Such an extrusion-based process relies on the formulation of inks in the form of particle-stabilized emulsions and foams that are sufficiently stable to resist coalescence during printing.

How scaffold porosity, pore diameter and geometry influence cellular behavior is-although heavily researched - merely understood, especially in 3D. This is mainly caused by a lack of suitable, reproducible scaffold fabrication methods, with processes such as gas foaming, lyophilization or particulate leaching still being the standard. Here we propose a method to generate highly porous silk fibroin scaffolds with monodisperse spherical pores, namely inverse opals, and study their effect on cell behavior. These silk fibroin inverse opal scaffolds were compared to salt-leached silk fibroin scaffolds in terms of human mesenchymal stem cell response upon osteogenic differentiation signals. While cell number remained similar on both scaffold types, extracellular matrix mineralization nearly doubled on the newly developed scaffolds, suggesting a positive effect on cell differentiation. By using the very same material with comparable average pore diameters, this increase in mineral content can be attributed to either the differences in pore diameter distribution or the pore geometry. Although the exact mechanisms leading to enhanced mineralization in inverse opals are not yet fully understood, our results indicate that control over pore geometry alone can have a major impact on the bioactivity of a scaffold toward stem cell differentiation into bone tissue. © 2016 The Authors Journal of Biomedical Materials Research Part B: Applied Biomaterials Published by Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 105B: 2074-2084, 2017.

Rough interfaces featuring nanoscale asperities are known to play a major role in the mechanics of nacre. Transferring this concept to artificial bioinspired composites requires a detailed understanding about the effect of the surface topography of reinforcing elements on the mechanical performance of such materials. To gain further insights into the effect of asperity size, hierarchy and coverage on the mechanics of nacre-inspired composites, we decorate alumina microplatelets with silica nanoparticles of selected sizes and use the resulting roughened platelets as reinforcing elements (15vol%) in a commercial epoxy matrix. For a single layer of silica nanoparticles on the platelet surface, increased ultimate strain and toughness are obtained with a large roughening particle size of 250nm. On the contrary, strength and stiffness are enhanced by decreasing the size of asperities using 22nm silica particles. By combining particles of two different sizes (100nm and 22nm) in a hierarchical fashion, we are able to improve stiffness and strength of platelet-reinforced polymers while maintaining high ultimate strain and toughness. Our results indicate that carefully designed hierarchically roughened interfaces lead to a more homogeneous stress distribution within the polymer matrix between the stiff reinforcing elements. By enabling the deformation of a larger fraction of the polymer matrix, this design concept improves the mechanical response of bioinspired composites and can possibly also be exploited to enhance the performance of conventional fiber-reinforced polymers.

Living organisms often combine soft and hard anisotropic building blocks to fabricate composite materials with complex microstructures and outstanding mechanical properties. An optimum design and assembly of the anisotropic components reinforces the material in specific directions and sites to best accommodate multidirectional external loads. Here, we fabricate composite films with periodic modulation of the soft-hard microstructure by simultaneously using electric and magnetic fields. We exploit forefront directed-assembly approaches to realize highly demanded material microstructural designs and showcase a unique example of how one can bridge colloidal sciences and composite technology to fabricate next-generation advanced structural materials. In the proof-of-concept experiments, electric fields are used to dictate the position of the anisotropic particles through dielectrophoresis, whereas a rotating magnetic field is used to control the orientation of the particles. By using such unprecedented control over the colloidal assembly process, we managed to fabricate ordered composite microstructures with up to 2.3-fold enhancement in wear resistance and unusual site-specific hardness that can be locally modulated by a factor of up to 2.5.

Chocolate consists of cocoa butter, cocoa particles, sugar, and additives, which together determine its taste and optical properties. The optical properties of chocolate play a vital role in consumer perception, representing type and quality in the blink of an eye. Here, we present a comprehensive analysis of the optical properties of white, milk, and dark chocolate, while demonstrating how to craft orange to blue chocolate via a thin food-grade coating. Using Mie theory, we show that chocolate can be treated as a turbid solid, where cocoa butter acts as a glass-like dielectric while all other ingredients contribute to its scattering and absorption. We expect the proposed coating to be easily adapted to other food surfaces to bring color to a broader range of edible products.

The properties of conventional materials result from the arrangement of and the interaction between atoms at the nanoscale. Metamaterials have shifted this paradigm by offering property control through structural design at the mesoscale, thus broadening the design space beyond the limits of traditional materials. A family of mechanical metamaterials consisting of soft sheets featuring a patterned array of reconfigurable bistable domes is reported here. The domes in this metamaterial architecture can be reversibly inverted at the local scale to generate programmable multistable shapes and tunable mechanical responses at the global scale. By 3D printing a robotic gripper with energy-storing skin and a structure that can memorize and compute spatially-distributed mechanical signals, it is shown that these metamaterials are an attractive platform for novel mechanologic concepts and open new design opportunities for structures used in robotics, architecture, and biomedical applications., Advanced Science, 7 (22), ISSN:2198-3844

The complex behavior of highly deformable mechanical metamaterials can substantially enhance the performance of soft robots.

Materials combining optical transparency and mechanical strength are highly demanded for electronic displays, structural windows and in the arts, but the oxide-based glasses currently used in most of these applications suffer from brittle fracture and low crack tolerance. We report a simple approach to fabricate bulk transparent materials with a nacre-like architecture that can effectively arrest the propagation of cracks during fracture. Mechanical characterization shows that our glass-based composites exceed up to a factor of 3 the fracture toughness of common glasses, while keeping flexural strengths comparable to transparent polymers, silica- and soda-lime glasses. Due to the presence of stiff reinforcing platelets, the hardness of the obtained composites is an order of magnitude higher than that of transparent polymers. By implementing biological design principles into glass-based materials at the microscale, our approach opens a promising new avenue for the manufacturing of structural materials combining antagonistic functional properties., Nature Communications, 10 (1), ISSN:2041-1723

Electrodes for metal-ion batteries should combine high specific capacity with fast cycling-rate capability. Although the use of mesoporous particles is an attractive approach to reconciling these c...

The formation of droplets is ubiquitous in many natural and industrial processes and has reached an unprecedented level of control with the emergence of milli- and microfluidics. Although important insight into the mechanisms of droplet formation has been gained over the past decades, a sound understanding of the physics underlying this phenomenon and the effect of the fluid's flow and wetting properties on the droplet size and production rate is still missing, especially for the widely applied method of step emulsification. In this work, we elucidate the physical controls of microdroplet formation in step emulsification by using the wetting of fluidic channels as a tunable parameter to explore a broad set of emulsification conditions. With the help of high-speed measurements, we unequivocally show that the final droplet pinch-off is triggered by a Rayleigh-Plateau-type instability. The droplet size, however, is not determined by the Rayleigh-Plateau breakup, but by the initial wetting regime, where the fluid's contact angle plays a crucial role. We develop a physical theory for the wetting process, which closely describes our experimental measurements without invoking any free fit parameter. Our theory predicts the initiation of the Rayleigh-Plateau breakup and the transition from dripping to jetting as a function of the fluid's contact angle. Additionally, the theory solves the conundrum why there is a minimal contact angle of α = 2π/3 = 120° for which droplets can form.

The alignment of anisotropic particles during ink deposition directly affects the microstructure and properties of materials manufactured by extrusion-based 3D printing. Although particle alignment in diluted suspensions is well described by analytical and numerical models, the dynamics of particle orientation in the highly concentrated inks typically used for printing via direct ink writing (DIW) remains poorly understood. Using cellulose nanocrystals (CNCs) as model building blocks of increasing technological relevance, we study the dynamics of particle alignment under the shear stresses applied to concentrated inks during DIW. With the help of in situ polarization rheology, we find that the time period needed for particle alignment scales inversely with the applied shear rate and directly with the particle concentration. Such dependences can be quantitatively described by a simple scaling relation and qualitatively interpreted in terms of steric and hydrodynamic interactions between particles at high shear rates and particle concentrations. Our understanding of the alignment dynamics is then utilized to estimate the effect of shear stresses on the orientation of particles during the printing process. Finally, proof-of-concept experiments show that the combination of shear and extensional flow in 3D printing nozzles of different geometries provides an effective means to tune the orientation of CNCs from fully aligned to core–shell architectures. These findings offer powerful quantitative guidelines for the digital manufacturing of composite materials with programmed particle orientations and properties. ISSN:1936-0851 ISSN:1936-086X