A 3D‐Printed Vascular‐Like Perfusion Tumor‐on‐a‐Chip Recapitulates High‐Grade Breast Cancer With in Vivo‐Relevant Drug Resistance

Tumor progression and drug resistance in breast cancer are fundamentally governed by a complex microenvironment, characterized by extreme cell densities and associated nutrient/oxygen gradients. Conventional in vitro models often fail to replicate these features due to mass transport limitations in non-perfused systems. Here, we present a next-generation tumor-on-a-chip platform engineered via high-resolution projection micro-stereolithography (PµSL) to sustain exceptionally high-density breast cancer tissues (up to 400 million cells/mL) under continuous perfusion. By integrating a vascular-inspired perfusion architecture with precision-engineered micropores (∼7 µm), our system enhances nutrient and drug transport compared with traditional non-perfused 3D cultures. This high-density microenvironment drives density-dependent molecular reprogramming of ERα, Ki67, and CTSL2, closely mirroring the gene expression signatures observed in high-grade patient tumors. Notably, the drug resistance to Elacestrant observed in our high-density tissues uniquely aligns with clinically reported Cmax concentrations, effectively bridging the gap between in vitro screening and in vivo outcomes. This platform provides a physiologically relevant tool for studying aggressive tumor biology and evaluating personalized therapeutic strategies with high predictive power.