Fig. 1: Coordination of two light sources in tomographic volumetric 3D printing grants a high degree of freedom to customize the spatial variation of mechanical properties inside a workpiece.

a The design, theoretical dose ratio (UV to visible light) and a photo of a 3 × 3 binary grid composite workpiece. The colors in the design pattern indicate the anticipated build-up of light doses of different wavelengths in the X–Y plane (parallel to the incident beams): blue—visible light; purple—UV light. The theoretical dose ratio was calculated using the inhibitor diffusion model. b Compressive modulus (green columns) of the nine zones (①–⑨). The mean values of the simulated dose ratio (purple line), averaged over each zone, is plotted on the right axis. The, respectively, colored error bars provide the standard deviation in the measured compressive modulus and dose ratio. Inset: the same geometry printed in single-color mode using visible light. c The design, theoretical dose ratio, and a photo of a gray sheet composite structure. d Compressive modulus along the diagonal (indicated by the purple arrow in the photo). Inset: mapping of surface stiffness over the entire square area (27 measurements). e Design, theoretical dose ratio and a photo of a radially graded composite sample. A circular crack appeared spontaneously. The two marked zones were further studied. f Bright-field optical image of the area f of the sample. A sharp contrast in sample transparency was observed 150 µm outside the circular crack (red dashed line, photo taken using a Nikon eclipse LV100ND optical microscope). g Zones of interest were identified on both sides of the crack. The spacing between the dashed lines is 150 µm (observed with the AFM embed camera). h, i Histograms of AFM nanoindentation measurements of zones h and i. Insets: 25 × 25 µm² a map of stiffness variation in the zone. V/μm measures the slope of the approach force curve and scales with local stiffness. Scalebars: a–c, e: 3 mm; f, g: 150 µm; h, i: 5 µm.