Functional gradients are material transitions that are found in nature and are known to result in materials with superior properties and multiple functionalities. The emerging multi-material 3D printing (=additive manufacturing, AM) techniques provide a powerful tool for the design and fabrication of bioinspired functionally graded materials (FGMs). In particular, the spatial distribution of materials can be controlled at the level of individual volumetric pixels (voxels i.e., cubes with side lengths of 20-40 μm), thereby ensuring accuracy, reliability, and reproducibility of the obtained properties and allowing for systematic studies of how various design variables affect the deformation and fracture behaviors of FGMs. Here, we designed, 3D printed, and mechanically tested tensile and notched FGMs specimens with step-wise (i.e., 5-, 10-, and 15-steps) and continuous (sigmoid and linear) gradients. The deformation and fracture mechanisms of these FGM composites were studied using digital image correlation, digital microscopy, and scanning electron microscopy. We further characterized the chemical composition and local mechanical properties of FGM composites using XPS and nanoindentation measurements, respectively. Tensile test specimens with a continuous gradient (i.e., linear) exhibited much higher Young's moduli (≈3-folds) and ultimate strengths (≈2-folds) but lower elongations (≈2-folds drop) as compared to those with stepwise gradients (i.e., 5-steps). Similarly, notched specimens with linear gradients exhibited 2-folds higher values of the stiffness and fracture stress, but 1.5-folds lower fracture strains as compared to those with 5-steps gradients. Although we found non-uniform highly concentrated strain distributions in all specimens, FGMs with linear gradients showed a smoother strain distribution and smaller crack blunting zones as compared to those with stepwise gradients. Our results imply that for stiffness and strength lineargradient perform better than abrupt hard-soft-hard specimens.

Living organisms have developed design principles, such as functional gradients (FGs), to interface hard materials with soft ones (e.g., bone and tendon). Mimicking such design principles can address the challenges faced when developing engineered constructs with soft-hard interfaces. To date, implementing these FG design principles has been primarily performed by varying the ratio of the hard phase to that of the soft phase. Such design approaches, however, lead to inaccurate mechanical properties within the transition zone. That is due to the highly nonlinear relationship between the material distribution at the microscale and the macroscale mechanical properties. Here, we 3D print micro-bricks from either a soft or a hard phase and study the nonlinear relationship between their arrangements within the transition zone and the resulting macroscale properties. We carry out experiments at the micro- and macroscales as well as finite element simulations at both scales. Based on the obtained results, we develop a co-continuous power-law model relating the arrangement of the micro-bricks to the local mechanical properties of the micro-brick composites. We then use this model to rationally design FGs at the individual micro-brick level and create two types of biomimetic soft-hard constructs, including a specimen modeling bone-ligament junctions in the knee and another modeling the nucleus pulposus-annulus fibrosus interface in intervertebral discs. We show that the implemented FGs drastically enhance the stiffness, strength, and toughness of both types of specimens as compared to non-graded designs. Furthermore, we hypothesize that our soft-hard FGs regulate the behavior of murine preosteoblasts and primary human bone marrow-derived mesenchymal stromal cells (hBMSCc). We culture those cells to confirm the effects of soft-hard interfaces on cell morphology as well as on regulating the expression of focal adhesion kinase, subcellular localization, and YAP nuclear translocation of hBMSCs. Taken together, our results pave the way for the rational design of soft-hard interfaces at the micro-brick level and (biomedical) applications of such designs.