**4. Biocompatible magnetic materials: a new smart, multifunctional tool in nanomedicine**

The use of biomimetic scaffolds can be an effective approach for bone tissue regeneration, however the patients' metabolism plays an important role in the regulation of the kinetics and extent of new bone formation. Indeed, metabolic diseases, as well as degenerative conditions induced by aging, can seriously penalize new bone formation and fracture healing. In consideration of the ever increasing ageing of the world population, the occurrence of degenerative diseases is expected to steadily rise in the next decades, thus new therapeutic approaches are strongly required to boost and assist tissue regeneration in patients with reduced endogenous regenerative potential. Tissue engineering approaches and the use of drug delivery systems able to deliver growth factors are two main approaches for enhancing tissue regeneration. Particularly, a great effort is being dedicated to the development of scaffolds with the ability of controlled biochemical stimulation that should be delivered in temporo-spatially defined fashion [44].

Moreover, the endowment of the bone scaffolds with the channel-like structure of rattan resulted into anisotropic mechanical properties with values in the range of the trabecular bone, that reflect the complex bone response to directional loading. Preliminary biologic tests reported an outstanding affinity with cells, with complete coverage of the scaffolds by well spread cells (i.e. MG63 osteoblast-like cells) after 1 week and enhanced osteogenic ability compared to sintered HA scaffolds (**Figure 10**). Also, preliminary in vivo tests reported extensive bone formation and colonization in femoral bone defects, also showing good

The first results obtained with this new type of bone scaffolds are very promising for further development and application into more clinically-relevant models, for assessing the feasibility of regenerating long segmental bone parts. In this respect the exploitation of natural sources as models for generation of new hierarchically organized scaffolds can be considered as a completely new synthesis approach that may open to still unexplored applications in the

**4. Biocompatible magnetic materials: a new smart, multifunctional tool in**

The use of biomimetic scaffolds can be an effective approach for bone tissue regeneration, however the patients' metabolism plays an important role in the regulation of the kinetics and extent of new bone formation. Indeed, metabolic diseases, as well as degenerative conditions induced by aging, can seriously penalize new bone formation and fracture healing. In consideration of the ever increasing ageing of the world population, the occurrence of degenerative diseases is expected to steadily rise in the next decades, thus new therapeutic approaches are strongly required to boost and assist tissue regeneration in patients with reduced endogenous regenerative potential. Tissue engineering approaches and the use of drug delivery systems able to deliver growth factors are two main approaches for enhancing

morphological organization after one month from implantation [20].

140 Advanced Techniques in Bone Regeneration

**Figure 10.** MG63 cells morphology in contact with wood-derived HA scaffold.

incoming years.

**nanomedicine**

In this respect, recent advances in material science suggest that the use of weak magnetic fields is appealing as remote signalling for non-invasive controlling and *on demand* activation of biomedical devices in vivo [1]. The use of magnetic materials in nanomedicine is thus raising a steadily growing interest, as they can open to new personalized applications including cancer therapy by hyperthermia, magnetic resonance imaging, and other diagnostic approaches based on the guiding of such particles to specific targeted areas in vivo and their use as nanoprobes [45–52]. A serious drawback in the use of magnetic materials in nanomedicine is their long term cytotoxicity [53, 54]. Intense effort is therefore dedicated to engineering SPIONs (i.e. superparamagnetic iron oxide nanoparticles) with surface treatments to achieve enhanced biocompatibility and affinity with cells [55–57]. A significant advance can be the development of magnetic materials with intrinsic biocompatibility and resorbability. In this respect, it has been shown that the doping of the apatite lattice with Fe2+/Fe3+ ions in specific calcium sites yields a new phase with intrinsic paramagnetic behaviour (FeHA) [58]. By virtue of its chemical composition very close to the one of mineral bone, FeHA is characterised by excellent bio‐ compatibility, as also confirmed by in vitro studies revealing that FeHA nanoparticles do not reduce cell viability and at the same time enhance cell proliferation compared to undoped HA particles [59]. Moreover, a pilot animal study of bone repair (a rabbit critical bone defect model) demonstrated the in vivo biocompatibility and biodegradability of FeHA [59]. The achieve‐ ment of biocompatible nano-biomaterials with magnetic properties opens new perspectives in regenerative medicine. Particularly, the development of bone scaffolds with the ability of remote magnetic activation is now an emerging concept in regenerative medicine [60], since it has been demonstrated that weak magnetic or pulsed electromagnetic fields are effective in promoting bone fracture healing, spinal fusion, and bone ingrowth in various animal models [61–65]. However, the incorporation of FeHA phase into ceramic bone scaffolds is made difficult by the need of consolidating green ceramic bodies by high temperature treatments provoking lattice destabilization and loss of magnetic properties [59]. In this respect FeHA can also be synthesized by suitable modification of the biomineralization process to induce heterogeneous nucleation of FeHA nanophase on Type I collagen [66]. This method yielded biomimetic hybrid scaffolds with paramagnetic ability and mineralization extent that could be tailored from cartilage to bone-like level. The presence of a mineral phase with bone-like features and ability to be activated by remote magnetic signal make this new biomaterial very promising to boost regeneration of extended bone and osteochondral regions, even in patients with reduced endogenous regenerative potential [67–70].

Besides, the use of biocompatible magnetic materials can open to further, different approaches for enhanced bone regeneration. It is accepted that a key limiting factor in the regeneration of extended bone defects is the inability of cells to self-propagate in the inner part of the scaffold and to establish new bone and vascular tissue [20]. Recent progresses show that it is possible to locally guide the migration of magnetic nanoparticles and nanoparticle-labelled cells through the use of an externally applied magnetic field gradient [71]. In this respect FeHA nanoparticles can be easily incorporated into cells by endocytosis, thus obtaining "magnetic cells" without negatively affecting cell behavior (e.g. proliferation, morphology, differentia‐ tion). Through the application of an external magnetic field of low intensity, these cells can be guided within a scaffold, in order to have faster and more selective seeding for tissue engi‐ neering application (**Figure 11**).

**Figure 11.** Scheme of magnetic guiding enabling enhanced scaffold colonization.

Biocompatible magnetic media can also be associated to polymeric or hybrid carriers to achieve new smart drug delivery systems with the ability of magnetic activation [72–77].

Hollow micro- and nano-spheres with controlled size and magnetization level, made of polycaprolacton coated with adequate amounts of FeHA displayed dose-dependent biocom‐ patibility towards bone marrow mesenchymal stem cells, thus highlighting the positive effect of the mineralization extent on cell behaviour [78–80]. These carriers could be developed as magnetically-responsive drug delivery systems with activation and delivery kinetics modu‐ lated by phenomena of magnetoshaking or hyperthermia [81]. To explore these new ap‐ proaches for controlled drug delivery, careful investigation is needed to investigate the most suitable conditions, by means of intensity and frequency of alternated magnetic fields that shall provide the energy needed for the release of the linked bioactive molecules. Therefore, in the incoming years further development of this approach may represent a new tool enabling the release of different chemical species under defined temporo-spatial patterns, thus opening to more advanced and personalized therapies.
