Characterization and compensation of distortions in complex 3D architected microstructures printed by two-photon polymerization

The emergence of three-dimensional (3D) printing, and specifically of the two-photon polymerization (2PP) technique, has rendered possible the fabrication of complex micrometric scale architected structures with applications in several fields from photonics to bio-scaffolds to microrobotics. However, even in ultra-low shrinkage resins, distortions still compromise the dimensional fidelity and, subsequently, the expected performance, of such 3D-printed structures. This study examines the sources of distortions arising in 2PP-fabricated structures, with the objective of eradicating them to improve the dimensional fidelity of the structures. In particular, a primary distortion—linked to the index mismatch between the resin and the immersion medium of the objective (air)—is shown to cause an overall stretching of a structure, while a secondary distortion—linked to a saturating voxel growth due to re-exposure during hatching and slicing—is shown to incur an additional thickness at the level of individual features within a structure. Furthermore, this work proposes a novel and practical method, first to readily assess the above-mentioned stretching factor and saturated voxel height from just two printed objects, and then to pre-compensate for the effects of distortions in the initial design of a structure. The overall methodology is shown to effectively reduce distortion factors from as high as 2.2 to nearly 1, for lattices and more complex structures such as triply periodic minimal surfaces. As a result, this work offers, for the first time, an easily implementable method for addressing a persistent problem that hampers printing fidelity, functionality, and performance of 3D structures fabricated by 2PP.