ナノ粒子は、ポリマーと無機製剤とその両方の組み合わせに基づいて、先進的な生物医学研究のツールを提供することがあります。病気の早期発見のための診断テストのように、標的薬物送達システムとして、イメージングと創薬のためのツールとして、それは多くの生物医学アプリケーションで使うことができます。
Nanoparticles used in biomedical applications include liposomes, polymeric micelles, block ionomer complexes, dendrimers, inorganic and polymeric nanoparticles, nanorods and quantum dots. All have been tested pre-clinically or clinically for targeted drug and gene delivery and as agents to enhance diagnostic imaging output like in MRI. Properties present only on the nanoscale level, like the increased intensity of fluorescent light emission of semiconductor crystals (quantum dots) or switchable magnetic properties of superparamagenetic nanoparticles (SPIONs), make these materials unique and useful for applications in the biomedical field of medical imaging and cell tracking. Other nanoparticles like water-soluble synthetic polymers (dendrimers) were tested in pre-clinical models for the delivery of drugs, genes, and as imaging agents showing a rich versatility for tailoring their binding properties to several requirements, among them facilitation of cellular uptake of drugs (e.g. cancer drugs).
Magnetic materials encompass a wide variety of materials and are classified in terms of their magnetic properties and their uses. They are classified by their susceptibility to magnetic fields into diamagnetic materials with weak repulsion from an external magnetic field (negative susceptibility), paramagnetic materials showing small and positive susceptibility, and ferromagnetic materials which exhibit a large and positive susceptibility to magnetic fields and are known as magnets in the daily life (“horseshoe magnets”). In the first two categories the magnetic properties do not persist if the external magnetic field is removed, while for ferromagnetic materials, which exhibit strong attraction to magnetic fields, these properties are stable even after removal of the external field.
If a sufficiently large magnetic field is applied, the spins within the material align with the field. The maximum value of magnetization achieved in this state is called the saturation magnetization, MS. As the magnitude of the field decreases, spins cease to be aligned with the field and the total magnetization decreases. In ferromagnets, a residual magnetic moment remains at zero field. The value of the magnetization at zero field is called the remanent magnetization, MR. The magnitude of the field that must be applied in the negative direction to bring the magnetization of the sample back to zero is called the coercive field. Changes in magnetization of a material occur via activation over an energy barrier. If the number of atoms per particles is decreasing, the interaction energy (exchange energy) could reach values as low as the thermal energy kBT at room temperature.
This leads to a spontaneous random orientation of the magnetic spin inside the particles, or in other words, the remanence magnetisation as well as the coercitivity will be zero. This means no hysteresis and therefore paramagnetic behaviour. Such a behaviour could be observed at sizes < 20 nm for iron oxide γ Fe2O3; Maghemite) or at 3 nm for pure iron. The saturation magnetisation reaches values of 90% of the bulk ferromagnetic material. This lower value is explained by the magnetically inactive first atomic layer at the surface of the particles. This layer, also existing in bulk material, has a non-negligible volume in small particles. The superparamagnetic behaviour is characterized by a typical relaxation time τ; the time which the systems need to achieve zero magnetisation after an external magnetic field is switched off:
where τ0 is the characteristic time (10-9 s), K is the anisotropy energy (20’000 J/m3 for iron oxide) and V the volume of the particle: κB is the Boltzmann constant, T is the temperature.
Another effect which occurs only in such small sized crystallites is the presence of energy absorption in superparamagnetic particles due to Néel relaxation. If the magnetic domains, fixed within the particles, are directed to an alternating external magnetic field, the magnetic dipole moments of the nanosized crystals have to be reoriented very quickly depending on the frequency and magnetic strength of the applied magnetic field, the size of the particles as well as the environmental temperature. The loss power of the particles is used to heat the surrounding environment, e.g. a tissue.
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