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Calcium sulfate (CS) combines remarkable properties of biodegradability, biocompatibility, and osteoconductivity but its low strength limits the range of its applications in orthopaedic surgery. In this study we have addressed this limitation by optimizing the fabrication process for pure CS, and by using mechanical testing procedures which are relevant for load carrying, or structural bone grafts (flexural tests in hydrated condition). By optimizing the processing parameters (pressure during setting, CS powder to water ratio, saturated solution) we produced CS samples with the highest flexural strength ever reported in hydrated conditions. Once these optimal conditions are used, the addition of "reinforcing" inclusions in the material decreased its strength because these inclusions actually act as defects instead of reinforcements. In addition, the CS can be formed in precise shapes while maintaining optimal processing conditions and provided a strength similar to that of bone with the same dimensions. Dense and porous materials can be combined to duplicate the trabecular and cortical architecture of long bones, with only a small loss of overall strength. K E Y W O R D Sbiomineral, bone graft materials, calcium sulfate, material processing, strength

Integrating organic and inorganic components into hybrid materials is a promising pathway towards unique and useful combinations of stiffness, strength, and toughness. Nature provides superb examples of such materials: bone, teeth, and mollusk shells are made of stiff inorganic mineral inclusions bonded by more compliant proteins and polysaccharides that play a critical role by providing large deformations and energy absorption. Nanometer-thick polyelectrolyte multilayers (PEMs) provide an elegant approach to mimicking natural organic materials, but experimental data on their mechanical properties is scarce. In this work we have for the first time measured the shear performance and fracture toughness of PEM nanointerfaces that join two silicon substrates. Most of [a]

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