# Nanomaterials in Bone Regeneration
## 4 Nanoparticles
### Iron Oxide Nanoparticles  
Iron is a metal naturally occurring in the body, and therefore existing metabolic pathways allow the processing of remaining iron from nanoparticles. Iron oxide nanoparticles (Fe-NPs), mainly magnetite (Fe\({}_{3}\)O\({}_{4}\)) and maghemite (\(\gamma\)-Fe\({}_{2}\)O\({}_{3}\)), have excellent biocompatibility and unique magnetic properties. However, bare Fe-NPs may be toxic because their surfaces are chemically reactive, and they aggregate easily together [125].  
Biocompatibility can be increased by coating, which prevents the aggregation of nanoparticles. The coating can be organic or inorganic, and either one or more layers are made. Substances used for organic coating include citrate, dextran, polyethylene glycol (PEG), CS, polyethyleneimine, or phospholipids, and for inorganic coating, silica, gold, and calcium phosphate. The coating layers are further modified to apply drug molecules, proteins, and genetic materials for medical purposes. For in vivo use, an injectable complex is formed in which Fe-NPs are associated with cells or bioactive molecules [126].  
Fe-NPs are used as contrast agents, for iron replacement therapies, and for tumor therapies using local tissue hyperthermia. The magnetic properties of these particles can be used for optimal drug delivery to the required site, which minimizes side effects and improves the treatment of cancer and bone diseases. Fe-NPs and scaffolds containing these magnetic particles are used successfully in regenerative medicine, especially in bone tissue engineering and bone regeneration, because the magnetic field and magnetic responsive scaffolds promote bone repair and regeneration [127].  
Magnetic properties are crucial for the application of Fe-NPs. Nanoparticle-labeled cells can be observed and monitored in vivo when the labeled cells remain detectable by magnetic resonance imaging even after 7 days [128]. Therefore, thanks to the magnetic field of Fe-NPs, it is possible to study the behavior of cells, i.e., adhesion, proliferation, and differentiation. Stem cells labeled with magnetic nanoparticles can be used in bone regeneration therapy for magnetic targeting to targeted sites. The osteogenic potential of stem cells is important for further bone regeneration, wherein Fe-NPs are able to supply proteins to control the growth and differentiation of these cells. These magnetic particles, in combination with a pulsed magnetic field, have a positive effect on osteogenic differentiation, as demonstrated in an experiment with BM-MSCs, where the expression of Col1 was increased [129], and in an experiment with Fe-NPs coated with bovine serum albumin, was also found elevated ALP activity, calcium deposition, and expression of OC [130].  
Biopolymer scaffolds containing magnetic Fe-NPs show significant stimulation of cell adhesion and osteogenic differentiation in vitro and bone formation in vivo, which was confirmed by higher gene expression of key transcription factors for osteoblast (RUNX2 and Osx) and activity of ALP as well. Magnetic scaffolds are, therefore, useful for bone healing, especially in adjunctive therapy with external magnetic stimulation [131].