Large bone defects with limited intrinsic regenerative potential represent a major surgical challenge and are associated with a high socio-economic burden and severe reduction in the quality of life. Tissue engineering approaches offer the possibility to induce new functional bone regeneration, with the biomimetic scaffold serving as a bridge to create a microenvironment that enables a regenerative niche at the site of damage. Magnetic nanoparticles have emerged as a potential tool in bone tissue engineering that leverages the inherent magnetism of magnetic nano particles in cellular microenvironments providing direction in enhancing the osteoinductive, osteoconductive and angiogenic properties in the design of scaffolds. There are conflicting opinions and reports on the role of MNPs on these scaffolds, such as the true role of magnetism, the application of external magnetic fields in combination with MNPs, remote delivery of biomechanical stimuli in-vivo and magnetically controlled cell retention or bioactive agent delivery in promoting osteogenesis and angiogenesis. In this review, we focus on the role of magnetic nanoparticles for bone-tissue-engineering applications in both disease modelling and treatment of injuries and disease. We highlight the materials-design pathway from implementation strategy through the selection of materials and fabrication methods to evaluation. We discuss the advances in this field and unmet needs, current challenges in the development of ideal materials for bone-tissue regeneration and emerging strategies in the field.

Second to autologous bone grafts are the calcium phosphate cements (CPCs) used as synthetic bone substitutes due to their chemical similarity to the mineral component of bone. Their ability to conform to complex bone defects and excellent osteoconductivity also render them excellent scaffolds for bone tissue engineering, although they do have their own limitations. Calcium phosphates can be divided into two main categories, namely apatite and brushite. Apatites exhibit low solubility, whereas, calcium phosphates that set to form brushite, are metastable, which degrade rapidly, but do subsequently form hydroxyapatite that retards the rate. In contrast dicalcium phosphate anhydrous (monetite) has a higher solubility than octacalcium phosphate and does not transform to an apatite; thus, it is able to continue to degrade with time. Herein, a new method was used via the addition of sodium chloride to β-tricalcium phosphate and monocalcium phosphate monohydrate to form micro- and macroporous monetite (DCPA). The X-ray diffraction and FTIR spectra confirmed the formation of monetite in the presence of both, 6.2 M NaCl solution or 60% of solid sodium chloride. The maximum compressive strength (σ_c = 12.3 ± 1.8 MPa) and the Young's modulus (E = 1.0 ± 0.1 GPa) of the monetite cements obtained were comparable to the upper limits of the values reported for cancellous bone and also higher than that reported by other routes used to form monetite. The porous cements analysed by microCT revealed an interconnected porosity with the preliminary in vitro biological evaluation indicating favourable osteoblast cell attachment and growth.