Schematic illustrations for designing fractal-like scaffolds. (a) Radial-gradient structure of a cross section of natural bone from cancellous bone to cortical bone. (b) Digital model of a natural snowflake. (c) Koch snowflake with 3 iterations. (d) Koch curves with 0, 1, 2, and 3 iterations, respectively (Figure S1
, Supporting Information). (e) 2D fractal-like tree curves with 0, 1, 2, and 3 iterations, respectively (Figure S2
, Supporting Information). (f) Digital model of adult femur based on micro-CT reconstruction. (g) Ideal hollow cylinder model obtained by circumscribing the ROI of the reconstructed model. (h) 2D fractal-like tree curves with 3 iterations. (i) Trimming the obtained 2D fractal-like tree curves. The yellow part was kept to building the porosity gradient in the radial direction. (j) 2D fractal-like layer curves obtained by a circular array of the trimmed curves. (k) Design of the 2D concentric ring layer curves for supporting the 2D fractal-like layer curves in the axial direction. (l) 3-iteration fractal-like 3D scaffolds obtained through a linear array of the 3D fractal-like layer (Layer I) and the 3D concentric ring layer (Layer II). (m) Fabrication G-code generation for controlling the printhead movement to deposit inks along the as-designed route.