## 3 Magneto-Responsive Techniques for Cartilage Regeneration  
With the development of biotechnology, other promising techniques have been developed such as magnetic cell manipulation, achieved by the synergy between magnetic objects and magnetic field [8; 9]. In general, there are four techniques based on the magnetic cell manipulation for the tissue engineering approaches. The first one includes the magnetic field-based guiding of the cells to the targeted site, which permits to the relatively small number of cells (i.e., magnetic mesenchymal stem cells) to accumulate at the level of the defect site and promote cartilage regeneration [10; 11]. The second one is based on the enhancement of the seeding ability of cells within the scaffold, which permits the cells to migrate symmetrically and to promote the cell condensation, providing a suitable environment for cell proliferation and differentiation [12]. The third technique regards the formation of magnetic scaffolds, where the magnetic force is used to assemble the 3D structure to mimic the native tissue [13; 14]. This technique based on magnetic patterning, works across a range of materials (e.g., hydrogels) and diamagnetic objects (e.g., living cells, drug delivering microspheres, etc.), characterized by differential magnetic susceptibility, with the potential to predictably position these objects in 3D materials, in response to brief magnetic field application. It confers several advantages, including remote control ability, sufficient cell density and cell adhesion enhancement, permitting one to achieve a very good grade of engineered tissue biomimicry [8; 9]. [PERSON] et al. [15], demonstrated that a naturally diamagnetic objects, comprising living cells, can be predictably positioned throughout the 3D hydrogel. In this study, the magnetic susceptibility of the latter was enhanced by the addition of magnetic contrast agent (gadodianide). After the cells achieved the required position, by the brief exposure to magnetic field, they were 'locked in' by a photo-crosslinking method. Afterwards, the magnetic contrast agent was washed out of the hydrogel to not interfere with the long-term cell viability. In the study, the authors applied this method to engineer cartilage constructs with a depth-dependent cellularity, mirroring that of the native tissue. The fourth strategy is based on guiding cell assembly into sheet-like structures to stack layer-by-layer, used for the formation of scaffold-free 3D cell culture. In this way, magnetic-labeled cells can be guided to a targeted location and form 3D arrangements in a convenient microenvironment to mimic tissue properties without the use of scaffolds [16; 17].  
In addition, it has been widely demonstrated that mechanical forces acting as an additional tool to mimic the in vivo environment are also applied to improve cartilage reconstruction as suggested above [7; 18]. Magnetic nanoparticles represent the excellent candidates to apply remote magnetic-induced mechanical stimulation. [PERSON] et al. [19] used magnetic MSCs to enhance their seeding density and condensation into the scaffolds subjected to dynamic bioreactor. The results showed that MSC differentiation was markedly improved. [PERSON] et al. [20] exposed magnetic nanoparticle-labeled MSCs to static magnetic field and magnet-derived shear stress, demonstrating higher chondrogenic differentiation efficiency and no hypertrophic effects. Further, [PERSON] et al., [21] demonstrated that the multifunctional hyaluronic acid-graft-amphiphilic gelatin microcapsules, loaded with the superparamagnetic iron oxide nanoparticles and chondrocytes, subjected to static magnetic field and magnet-derived shear stress, were able to stimulate chondrogenesis and fabricate cartilage tissue-mimetic pellets.