Engineering breast cancer tumor microenvironment: advanced fabrication for 3D <i>in vitro</i> models

The investigation of breast cancer initiation and progression has been significantly advanced by the development of three-dimensional (3D) in vitro models, which provide a more physiologically relevant representation of the tumor microenvironment (TME) compared to conventional two-dimensional cultures. Over the past decade, and particularly since 2020, a wide range of strategies has been developed to generate stable and functional 3D breast cancer models. This review provides a comprehensive overview of the most promising (bio)fabrication-based technologies developed for breast cancer modeling, critically discussing their applications, advantages, limitations, and future perspectives. Among current approaches, tumor spheroids have demonstrated considerable value due to their characteristic architecture, comprising a necrotic core surrounded by proliferative and quiescent cell layers, which partially mimics in vivo tumor organization. In parallel, organ-on-chip (OoC) systems have emerged as powerful platforms for drug screening and therapeutic testing, enabling dynamic culture conditions within microengineered and perfusable environments. However, spheroids lack an external extracellular matrix, while the compartmentalized nature of OoCs systems limits their ability to fully reproduce the structural and compositional complexity of the breast TME. To address these limitations, engineered 3D-printed scaffolds and constructs produced through biofabrication approaches have gained increasing attention. In particular, natural hydrogel-based systems offer high biocompatibility and tunable biochemical and mechanical properties, enabling the co-culture of malignant and healthy cells and supporting more predictive evaluations of anticancer therapies.