Composites are often made heterogeneous in nature to fulfill the functional demands imposed by the environment, but remain difficult to fabricate synthetically due to the lack of adequate and easily accessible processing tools. We report on an additive manufacturing platform to fabricate complex-shaped parts exhibiting bio-inspired heterogeneous microstructures with locally tunable texture, composition and properties and unprecedentedly high volume fractions of inorganic phase (up to 100%). The technology combines an aqueous-based slip casting process with magnetically-directed particle assembly to create programmed microstructural designs using anisotropic stiff platelets in a ceramic, metal or polymer functional matrix. Using quantitative tools to control the casting kinetics and the temporal pattern of the applied magnetic fields, we demonstrate that this robust approach can be exploited to design and fabricate heterogeneous composites with thus far inaccessible microstructures. Proof-of-concept examples include bulk composites with periodic patterns of micro-reinforcement orientation and tooth-like bilayer parts with intricate shapes displaying site-specific composition and texture.Heterogeneity in composite materials is often encountered in the form of a nonuniform distribution of microstructural features, such as particles, pores, chemical species or living cells, throughout a macroscopic part. Tailored distribution of these 2 features can be an effective means to generate heterogeneous graded materials that combine antagonistic properties or that display functionalities that would not be accessible in uniform microstructures. The core-shell porous architecture of biological bones and engineered sandwiched foams is a typical example of mechanically functional structures that combine opposing properties like stiffness and low weight into a single component (1). The emergence of other unusual functionalities through heterogeneous design is also well illustrated, for example in optics, by the siliceous skeleton of marine sponges and by synthetic glass fibers, both of which display a chemical gradient to confine light within their high refractive index core and thus enable efficient transmission using silica glass as single base material (2).While several other examples of heterogeneous architectures exist among manmade materials, living organisms have taken this concept to unprecedented levels by designing heterogeneous composites with exquisite control over the local chemical composition and the texture at multiple length scales (3-5). The term texture is referred here to the orientation of building blocks in a particular direction at any given length scale of the material. Such microstructural design results from a long evolutionary process that has gradually crafted the material to satisfy specific demands of the surrounding environment (6). Remarkably, some universal structural motifs have emerged from this natural selection process and are conserved in numerous distinct living species across diff...

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.

Although strength and toughness are often mutually exclusive properties in manmade 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 2 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.

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Microplatelets and fibers that can be manipulated using external magnetic fields find potential applications as miniaturized probes, micromirrors in optical switches, remotely actuated micromixers and tunable reinforcements in composite materials. Controlling the surface chemistry of such microparticles is often crucial to enable full exploitation of their mechanical, optical and sensorial functions. Here, we report a simple and versatile procedure to directly magnetize and chemically modify the surface of inorganic microplatelets and polymer fibers of inherently non-magnetic compositions. As opposed to other magnetization approaches, the proposed non-aqueous sol-gel route enables the formation of a dense and homogeneous coating of superparamagnetic iron oxide nanoparticles (SPIONs) on the surface of the microparticles. Such coating provides a suitable platform for the direct chemical functionalization of the microparticles using catechol-based ligands displaying high affinity towards iron oxide surfaces. By adsorbing for example nitrodopamine palmitate (ND-PA) on the surface of hydrophilic magnetite-coated alumina platelets (Fe 3 O 4 @Al 2 O 3 ) we can render them sufficiently surface active to generate magnetically responsive Pickering emulsions. We also show that microplatelets and fibers coated with a uniform iron oxide layer can be easily manipulated using low magnetic fields despite their intrinsic non-magnetic nature. These examples illustrate the potential of the proposed approach in generating functional, magnetically responsive microprobes and building blocks for several emerging applications.