Electron paramagnetic resonance (EPR) meets drug delivery and biomaterials: a magnetic love story

This narrative review underscores the powerful role of Electron Paramagnetic Resonance (EPR), also known as Electron Spin Resonance (ESR), in characterizing drug delivery systems (DDSs). Using drugs or probes tagged with spin labels, EPR provides detailed insights into structural and dynamic properties, as well as the molecular microenvironment (including micro-viscosity, micro-polarity, and micro-pH) and enables real-time monitoring of drug release and degradation processes both in vitro and in vivo. In nanomedicine research, EPR can also serve as a quantitative tool to track the fate of DDSs doped with iron oxide particles that are used in theranostics. Beyond DDS characterization, EPR has contributed substantially to elucidating radical mechanisms within material matrices, notably in bone cements and dental resins used for restorative applications. Moreover, incorporating paramagnetic compounds into DDSs or biomaterials has broadened the scope of EPR applications, enabling precise measurements of oxygen and nitric oxide levels in complex biological environments. The incorporation of oxygen sensors into biocompatible matrices has also enabled the development of implantable resonators for measuring oxygen at substantial tissue depths. Incorporating oxygen sensors into cell therapy implantable devices or grafted tissues can serve as an indicator of both oxygenation and vascularization.