3D printing is transforming traditional processing methods for applications ranging from tissue engineering to optics. To fulfill its maximum potential, 3D printing requires a robust technique for producing structures with precise 3D (x, y, and z) control of mechanical properties. Previous efforts to realize such spatial control of modulus within 3D‐printed parts have largely focused on low‐resolution (from mm to cm scale) multimaterial processes and grayscale approaches that spatially vary the modulus in the x–y plane and energy dose‐based (E = I0 texp) models that do not account for the resin's sublinear response to irradiation intensity. Here, a novel approach for through‐thickness (z) voxelated control of mechanical properties within a single‐material, monolithic part is demonstrated. Control over the local modulus is enabled by a predictive model that incorporates the material's nonreciprocal dose response. The model is validated by application of atomic force microscopy to map the through‐thickness modulus on multilayered 3D parts. Overall, both smooth gradations (30 MPa change over ≈75 μm) and sharp step changes (30 MPa change over ≈5 μm) in the modulus are realized in poly(ethylene glycol) diacrylate‐based 3D constructs, paving the way for advancements in tissue engineering, stimuli–responsive 4D printing, and graded metamaterials.

In high-resolution stereolithography, printed parts deviate significantly from the projected photomask because of the reaction complexity on the voxel scale. To better understand the reaction process, we have developed a technique to measure local photopolymerization rates using a nanocylinder-tipped atomic force microscope cantilever. The drag force experienced during cantilever oscillations can be correlated to the viscosity and extent of reaction. Fluid dynamics simulations show micrometer-scale measurement localization, and the resonance-based measurement allows submillisecond temporal resolution. In a thiol−ene resin exposed to patterned light, oligomer diffusion length scales are significant compared to the size of printed structures. Consequently, part resolution is dictated by the competition between polymerization and diffusion. Because of the radical polymerization mechanism, increasing the light intensity at a constant dose decreases the local conversion, even though the diffusion length decreases. In the case of printed test structures with features matching the measured diffusion lengths, increased light intensity causes increased geometric aberration from the projected mask. Overall, the results indicate a need for enhanced control over polymerization and diffusion to obtain dimensionally accurate and mechanically homogeneous parts.

Photo-tunable hydrogel mechanical heterogeneity using a single resin is presented here, informed by a predictive transport kinetics and swelling model.