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International audience; Photopolymer-based additive manufacturing has received increasing attention in the field of acoustics over the past decade, specifically towards the design of tissue-mimicking phantoms and passive components for ultrasound imaging and therapy. While these applications rely on an accurate characterization of the longitudinal bulk properties of the materials, emerging applications involving periodic micro-architectured media also require the knowledge of the transverse bulk properties to achieve the desired acoustic behavior. However, a robust knowledge of these properties is still lacking for such attenuating materials. Here, we report on the longitudinal and transverse bulk properties, i.e., frequency-dependent phase velocities and attenuations, of photopolymer materials, which were characterized in the MHz regime using a double through-transmission method in oblique incidence. Samples were fabricated using two different printing technologies (stereolithography and polyjet) to assess the impact of two important factors of the manufacturing process: curing and material mixing. Overall, the experimentally observed dispersion and attenuation could be satisfactorily modeled using a power law attenuation to identify a reduced number of intrinsic ultrasound parameters. As a result, these parameters, and especially those reflecting transverse bulk properties, were shown to be very sensitive to slight variations of the manufacturing process.

The hardest and toughest tissues formed by living organisms are organic-mineral composites termed biominerals 1,2. When they are crystalline, their mesostructure includes the nano- and micro-scale crystallite size, shape, arrangement, and orientation. Mesostructures vary enormously across marine CaCO3 biominerals (aragonite, vaterite, calcite) because they result from divergent evolution: biominerals were formed long after organisms diverged from one another 3,4. Despite such diversity, CaCO3 marine biominerals share a convergent character: adjacent crystals are similarly oriented 5-32. The reason for such convergence is unclear. Here, we show with quantitative, precise measurements at the nanoscale that the slight misorientation is consistently between 1°-40° in diverse biominerals. Can this slight misorientation confer a desirable materials property and therefore an evolutionary advantage to the forming organisms? We test and confirm this hypothesis with nanoindentation in diverse biominerals, geologic aragonite, and in abiotic, slightly misoriented, synthetic spherulites. Molecular dynamics (MD) simulations of bicrystals reveal that aragonite, vaterite, calcite, exhibit toughness peaks when they are misoriented by 10°, 20°, 30°, respectively, demonstrating that slight misorientation alone increases crack deflection and therefore fracture toughness. Slight misorientation, along with other previously known and co-existing toughening mechanisms, was selected repeatedly and convergently, during the course of evolution, to postpone fracture and thus provide organisms with competitive advantage. We anticipate slight misorientation-toughening to be a starting point for more sophisticated materials synthesis and additive manufacturing in many fields. Compared to previously known toughening mechanisms, in fact, the advantages of slight misorientation are that it can and does occur in synthetic materials, it requires one material only and no specific top-down architecture, it is easily achieved by self-assembly of organic molecules (e.g. aspirin, chocolate), polymers, metals, and ceramics 29 well beyond biominerals.

International audience; Functional grading is a distinctive feature adopted by nature to improve the transition between tissues that present a strong mismatch in mechanical properties, a relevant example being the tendon-to-bone attachment. Recent progress in multi-material additive manufacturing now allows for the design and fabrication of bioinspired functionally graded soft-to-hard composites. Nevertheless, this emerging technology depends on several design variables, including both material and mechanistic ingredients, that are likely to affect the mechanical performance of such composites. In this paper, a model-based approach is developed to describe the interaction of ultrasound waves with homogeneous and heterogeneous additively manufactured samples, which respectively display a variation either of the material ingredients (e.g., ratio of the elementary constituents) or of their spatial arrangement (e.g., functional gradients, damage). Measurements are performed using longitudinal bulk waves, which are launched and detected using a linear transducer array. First, model is calibrated by exploiting the signals measured on the homogeneous samples, which allow identifying relationships between the model parameters and the material composition. Second, the model is validated by comparing the signals measured on the heterogeneous samples with those predicted numerically. Overall, the reported results pave the way for characterizing and optimizing multi-material systems that display complex bioinspired features.