Magnetic nanoparticles have attracted attention because of their current and potential usefulness as contrast agents for magnetic resonance imaging (MRI) or colloidal mediators for cancer magnetic hyperthermia. This review examines these in vivo applications through an understanding of the involved problems and the current and future possibilities for resolving them. A special emphasis is made on magnetic nanoparticle requirements from a physical viewpoint (e.g. relaxivity for MRI and specific absorption rate for hyperthermia), the factors affecting their biodistribution (e.g. size, surface hydrophobic/hydrophilic balance, etc.) and the solutions envisaged for enhancing their half-life in the blood compartment and targeting tumour cells.

Background Plasma and other body fluids contain membranous extracellular vesicles (EVs), which are considered to derive from activated or apoptotic cells. EVs participate in physiological and pathological processes and have potential applications in diagnostics or therapeutics. Knowledge on EVs is, however, limited, mainly due to their sub‐micrometer size and to intrinsic limitations in methods applied for their characterization. Objectives Our aim was to provide a comprehensive description of EVs from plasma of healthy subjects. Methods Cryo‐transmission electron microscopy combined with receptor‐specific gold labeling was used to reveal the morphology, size and phenotype of EVs. An original approach based on sedimentation on electron microscopy grids was developed for enumerating EVs. A correlation was performed between conventional flow cytometry and electron microscopy results. Results We show that platelet‐free plasma samples contain spherical EVs, 30 nm to 1 μm in diameter, tubular EVs, 1–5 μm long, and membrane fragments, 1–8 μm large. We show that only a minority of EVs expose the procoagulant lipid phosphatidylserine, in contrast to the classical theory of EV formation. In addition, the concentrations of the main EV sub‐populations are determined after sedimentation on EM grids. Finally, we show that conventional flow cytometry, the main method of EV characterization, detects only about 1% of them. Conclusion This study brings novel insights on EVs from normal plasma and provides a reference for further studies of EVs in disease situations.

Magnetic nanoparticles have attracted attention in modern medicine and pharmacology owing to their potential usefulness as contrast agents for MRI, as colloidal mediators for cancer magnetic hyperthermia or as active constituents of drug-delivery platforms. This review examines these in vivo applications through an understanding of the involved problems and the current and future possibilities for resolving them. A special emphasis is placed upon magnetic nanoparticle requirements from a physical viewpoint (e.g., relaxivity for MRI, specific absorption rate for hyperthermia and magnetic guidance), the factors affecting their biodistribution after intravenous injection (e.g., size and surface hydrophobic/hydrophilic balance) and the solutions envisaged for enhancing their half-life in the blood compartment and in targeting tumor cells.