Eliminating structural defects in large area mechanical metamaterials via hot lithography in large-area projection micro-stereolithography

Additive manufacturing has enabled the creation of new classes of architected meta-materials with exceptional structural and functional properties. Large-area projection micro-stereolithography (LAPµSL) has the potential for producing large volume metamaterials with millions of micro-scale unit cells and multiple orders of magnitude in length scales. Nevertheless, as part size grows with increasing number of unit cells, probability of finding embedded defects becomes significantly higher. Structural defects, such as internal cracks and non-uniformity embedded within networks of micro-scale unit cells, often remain undetected due to flawless external appearances. Herein, we elucidate the mechanisms underlying embedded structural defects during the free surface metamaterial printing. Our experimental investigations and theoretical calculations reveal a growing trend of surface tension effect at the interface between solidified pattern and uncured liquid. We found that this surface tension effect plays a dominant role in defect generation. We present a novel approach that uses in-situ Joule-heating to effectively minimize the free surface bulging phenomenon and enables the production of large, defect-free, low-density mechanical metamaterials. Our results are validated through in-situ non-contact resin surface profiling and X-ray computed tomography (XCT), confirming mechanical properties close to theoretical predictions. This work forms the basis of production of large volume metamaterials with millions of micro-scale features for advanced engineering applications.