Fig. 3

Orthogonally programmed stiffness and geometry in 3D-printed overhang structures. A beam was hold by two supporting rods, which were printed with the same geometry (diameter: 80 μm; height: 0.9 mm) but three different stiffness combinations, i.e. a stiff/stiff, b soft/stiff, and c soft/soft (stiff with UV dosage of 72 mJ cm⁻²; soft with UV dosage of 44 mJ cm⁻²). There was no visual difference among printed structures when first observed in PBS solution and ethanol due to buoyancy. When removed from solution and dried in air, the printed soft supporting rods collapsed. Only the stiff/stiff rod structure was strong enough to support the top beam without obvious bending and survived the drying process. Insets are schematic illustrations for functional support structures (light: stiff rod; dark: soft rod). d, e, and f show a printed warrior model with stiff body (UV dosage of 72 mJ cm⁻²) while a soft heart inside (UV dosage of 44 mJ cm⁻²). d A CAD model (light: stiff body; dark: soft heart) was modified from an open-source 3D object. e SEM image of printed warrior model. f Dark-field image shows apparent optical contrast between stiff body and soft heart. Scale bars are a, b, c 200 μm; e, f 500 μm