While magnetic nanomaterials are increasingly used as clinical agents for imaging and therapy, their use as a tool for tissue engineering opens up challenging perspectives that have yet been more rarely explored. To create thick, organized, purely cellular 3D artificial tissues, it is necessary to manipulate the component cells remotely to make them interact and self-organize. Magnetic forces acting at a distance are appealing candidates, provided the individual cells are first rendered magnetic by internalization of iron oxide nanoparticles.
We have first shown that magnetic compaction of mesenchymal stem cells (MSC) can yield a very thick cell layer rich in cartilage matrix with a high potential for integration. Our approach (patent “Methods for aggregation and differentiation of magnetized stem cells” FR2979634-A1; WO/2013/030393, 2013), consists of forming cartilage precursors between 0.5 and 1 mm thick from stem cells aggregated with miniaturized magnets, then merging them successively to obtain a layer of the same thickness (0.5-1 mm) with no size limit and a totally flexible geometry that can be adapted to fit any defect in patients’ native cartilage.
Figure 1 : magnetic cartilage engineering: Miniaturized magnets confine stem cells into cartilage building blocks and fuse these blocks into a larger structure rich in collagen. Fayol et al. Adv Mat, 2013.
Next, we combined the magnetic condensation technique with biodegradable polysaccharide scaffolds (collaboration Catherine Le Visage, LIOAD; Didier Letourneur, LVTS) containing lamellar pores of a size and organization that allowed cells to penetrate through the matrix, and we implemented dynamic culture conditions to ensure continuous diffusion of nutrients and gases, resulting in a successful attempt to achieve efficient chondrogenesis within the heart of a biomaterial (Luciani et al. Acta Biomateriala, 2016).
This 3D magnetic patterning ressembling the 3D bioprinting strategies can also be used to gain insights into tissular mechanics.
Figure 2 : We developed a novel magnetic technique to assembly millions of cells instantaneously, and to probe the establishment of 3D cell-cell adhesion at early stages. By testing a large number of cell types, we demonstrated that cell aggregates behave like complex materials, and we evidenced a transition from wet granulars to contractile structures, controlled by cell-cell interactions. This relation between the macroscopic behavior of a cellular aggregate and the component cells’ microscopic properties throws light on the material properties of cell assemblies. (Frasca et al. Soft Matter, 2014).
Figure 3 : To overcome the spheroid size barrier production, we developed a “magnetic molding” technique providing spherical aggregates with a diameter of between 0.3 and 2 mm. We managed to apply a magnetic “super-gravity” of more than 100g using an external magnet, thus providing a magnetic spheroid tensiometer. By combining these approaches we were able to observe that an elastic-capillary transition occurs at a critical diameter of around 0.4 mm. (Mazuel et al. Phys Rev Lett, 2016). Used together, the magnetic techniques tools for remote manipulation and subsequent investigation of tissular mechanical properties.
Construction of replacement tissues with a well-defined shape is still a challenge. We proposed a miniaturized bioengineering approach to build rod-shaped tissue structures from individual stem cells by applying magnetic forces at the cellular scale.
Figure 4 . In the process of tissue construction, we evidenced a spontaneous shape transition from tissue rods to sphere-like structures. This striking observation raises the intriguing question of how tissue shapes are constituted and grow. (Fayol et al. Integrative Biology, 2015).