Beyond traditional models: microfluidic technologies for engineering the human placenta in vitro

The human placenta is a highly specialized and dynamic organ that supports fetal development by regulating maternal-fetal exchange, endocrine activity, and immune tolerance throughout pregnancy. Despite its central role in maternal and fetal health, studying human placental physiology and pathology remains challenging due to ethical constraints, limited tissue accessibility, and the complexity of the maternal-fetal interface. Traditional <i>in vitro</i> models and animal systems have provided valuable insights but often fail to capture the dynamic, multicellular, and perfused nature of the human placenta. Microfluidic technologies have recently emerged as powerful tools for placental modeling <i>in vitro</i>. By integrating controlled fluid flow, three-dimensional architecture, and relevant placental cell types, placenta-on-chip platforms enable a more physiologically relevant reconstruction of the maternal and fetal compartments. These systems support the study of placental barrier function, nutrient and drug transport, endocrine signaling, immune interactions, and responses to pathological stimuli under defined and reproducible and tunable conditions. As the field rapidly expands, a comprehensive synthesis is needed to clarify how these systems complement or surpass existing models and to identify the remaining translational gaps. Reliable microphysiological systems that replicate placental "functionality-on-chip" are essential for global regulatory efforts. The need for clinical data to guide safe and effective use of medicines during pregnancy and breastfeeding has led to recommendations from the International Council for Harmonisation of Technical Requirements for Pharmaceuticals for Human Use to include pregnant and breastfeeding women in clinical trials. This creates an urgent push for early data collection on drug effects in pregnancy and breastfeeding using lab studies. This underscores the need to develop <i>in vitro</i> systems that reliably predict the effects of drugs and toxicants on the placenta. This review critically examines current placental models, from conventional two- and three-dimensional cultures and animal models to advanced microfluidic systems. We highlight how microfluidic placental models overcome key limitations of traditional approaches and discuss their applications in developmental biology, pharmacokinetics, toxicology, infection studies, and pregnancy-related disorders. Collectively, emerging evidence suggests that microfluidic placental models are central tools for mechanistic studies and preclinical testing, bridging the gap between reductionist systems and human physiology. Future progress will depend on improving model standardization, incorporating additional cellular complexity and immune components, and aligning microfluidic outputs with clinically relevant endpoints. Advancing these platforms toward greater physiological fidelity and interoperability with multi-organ systems will be critical for translating placental research into improved maternal-fetal health outcomes. Beyond summarizing recent technological advances, this review uniquely positions placenta-on-chip systems within the broader landscape of existing placental models and emerging regulatory needs, highlighting their translational potential for drug safety, developmental toxicology, infection biology, and pregnancy-related disorders.