A team developed a conformal, ultrathin crystalline‑silicon Hall sensor array intended for early‑stage monitoring of three‑dimensional tumor tissues, with the work reported in npj Flexible Electronics. The device couples high‑sensitivity magnetic sensing with flexible form factors to track tumor growth and microenvironmental changes in vitro and in vivo models. Authors showcased the array’s ability to conform to complex tissue geometries and detect magnetic biomarkers or tagged cells with high spatial resolution. The technology could enable new minimally invasive monitoring tools for preclinical tumor models and potentially guide localized therapies or assess response to treatment when integrated with magnetic contrast agents.