Crystallization of ACP with timeTo probe the stability of the observed ACP in disordered phase with respect to the adjacent HA nanocrystals, we performed TEM analysis of the same region after a 30-day time lapse. During the 30 days, the TEM sample was stored at room temperature in a N2 environment. Figure S1a shows the initial arrangement of ACP (left) and HA nanocrystals (right), as well as the diffraction pattern inset showing the crystallinity of the HA. Figure S1b shows the same region after the 30 day incubation period; the HA nanocrystals on the right appear to have grown, confirmed by the increased (112), (211), (300) intensities in the diffraction pattern inset; we also observe the emergence of (002) reflections after one month, which further proves the formation of crystals. These results demonstrate suggest that biogenic ACP is a precursor for crystalline cHA formation in bone in the observed conditions, as similarly observed by Mahamid et al 1 . Figure S1. Time lapse micrographs of amorphous and nanocrystalline regions. Deprotonated mineral structure in disordered phase taken before (a) and after (b) a 30 day incubation at room temperature in a nitrogen environment. The electron diffraction pattern after the incubation (b, inset) shows an increase in crystallinity as evidenced by more intense HA reflections and the emergence of the (002) spots when compared to the initial diffraction pattern (a, inset). Size Dependent Model ConstructionWe propose that the yield strength, , decreases as the probability of having a flaw (i.e. a pore) on the pillar surface increases; that is, surface flaws serve as probabilistic stress concentrators, which initiate failure during compression. This is manifested most prevalently in the larger pillars, with diameters > 500nm. At these nano and micro length scales, it is reasonable to consider bone as a fiber-

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Natural materials exhibit emergent mechanical properties as a result of their nanoarchitected, nanocomposite structures with optimized hierarchy, anisotropy, and nanoporosity. Fabrication of such complex systems is currently challenging because high-quality three-dimensional (3D) nanoprinting is mostly limited to simple, homogeneous materials. We report a strategy for the rapid nanoprinting of complex structural nanocomposites using metal nanoclusters. These ultrasmall, quantum-confined nanoclusters function as highly sensitive two-photon activators and simultaneously serve as precursors for mechanical reinforcements and nanoscale porogens. Nanocomposites with complex 3D architectures are printed, as well as structures with tunable, hierarchical, and anisotropic nanoporosity. Nanocluster-polymer nanolattices exhibit high specific strength, energy absorption, deformability, and recoverability. This framework provides a generalizable, versatile approach for the use of photoactive nanomaterials in additive manufacturing of complex systems with emergent mechanical properties.

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Natural hard composites like human bone possess a combination of strength and toughness that exceeds that of their constituents and of many engineered composites. This augmentation is attributed to their complex hierarchical structure, spanning multiple length scales; in bone, characteristic dimensions range from nanoscale fibrils to microscale lamellae to mesoscale osteons and macroscale organs. The mechanical properties of bone have been studied, with the understanding that the isolated microstructure at micro-and nano-scales gives rise to superior strength compared to that of whole tissue, and the tissue possesses an amplified toughness relative to that of its nanoscale constituents. Nanoscale toughening mechanisms of bone are not adequately understood at sample dimensions that allow for isolating salient microstructural features, because of the challenge of performing fracture experiments on small-sized samples.We developed an in-situ three-point bend experimental methodology that probes site-specific fracture behavior of micron-sized specimens of hard material. Using this, we quantify crack initiation and growth toughness of human trabecular bone with sharp fatigue pre-cracks and blunt notches. Our findings indicate that bone with fatigue cracks is two times tougher than that with blunt cracks. In-situ data-correlated electron microscopy videos reveal this behavior arises from crack-bridging by nanoscale fibril structure. The results reveal a transition between fibril-bridging (~1µm) and crack deflection/twist (~500µm) as a function of length-scale, and quantitatively demonstrate hierarchy-induced toughening in a complex material. This versatile approach enables quantifying the relationship between toughness and microstructure in various complex material systems and provides direct insight for designing biomimetic composites.