## 3 Graphene and Graphene-Related Materials for Biological Applications
### Graphene and Related Materials as Tool for Regenerative Medicine and Tissue Repair
Magnesium alloys are ideal materials for cardiovascular stents, but they are prone to corrosion [149]. The authors proved that a chitosan/heparinized GO multilayer coating enhanced the corrosion resistance by reducing the haemolysis rate and platelet adhesion and promoting the adhesion and proliferation of endothelial cells. [PERSON] et al. [150] used a similar approach by producing GO/magnesium nanohybrids for enhancing the osteoinductive capability of 3D-printed calcium phosphate-based scaffolds. This material aimed to be highly compatible with dental pulp stem cells. Authors firstly modified rGO by combining it with gelatine and mixed it with phosphates. Then, an enhancement of mechanical features, cell proliferation, and differentiation is achieved. Furthermore, they observed an up-regulation of bone-related genes and proteins COL-I, RUNX2, OCN. [PERSON] et al. [151] confirmed the suitability of graphene and magnesium for the production of orthopaedic hard-tissue replacement applications. By only a 0.1 wt.% of graphene nanoplates mixed with magnesium and zirconium, authors observed an improvement of wear resistance by 92% at 200 \(\upmu\)N and a reduction of both wear depth and coefficient of friction of the alloy. [PERSON] et al. [152] reported a step forward in the use of graphene and its related composites for tissue engineering applications. The authors produced a spongy Nb-Ta-xMn alloy (x: 2, 4, and 6 wt.%) coated with graphene/Ti alloy, reducing the cytotoxicity and improving the biocompatibility upon that of uncoated material. Moreover, in this case, graphene coating improved the cell adhesion and proliferation without compromising the cell morphology. [PERSON] et al. [153] went in deep in the topic by studying the osteoinmunomodulatory role to enhance osteogenesis of titanium coated with GO. The authors evaluated the ability to stimulate the expression of osteogenic genes and extracellular matrix mineralization in human mesenchymal stromal cells. They found that GO-coated surfaces could induce the polarization of macrophages and expression of inflammatory cytokines via the Toll-like receptor pathway. In vivo, GO-coated titanium activated macrophages and induced mild inflammation and a pro-osteogenic environment, with a slight increase in the levels of proinflammatory cytokines. In the meanwhile, an improved expression of TGF-\(\beta\)1 and oncostatin M genes is verified. [PERSON] et al. [154] proposed an interesting application for sintered rGO flakes and liquid rGO/titania material as a synthetic keratoprosthesis skirt for end-stage corneal disorders. These authors observed an improvement of mechanical and chemical properties together with a better wear and corrosion resistance for a loading of 1 wt.%. In vivo studies were carried out by implanting to rabbit corneas without observing any immune and inflammatory responses. Regenerative medicine is another field where graphene-based materials have released their full potential [155; 156]. The adherence of a cell on graphene is a critical property as shown by [PERSON] et al. [157]. By studying the adherence and proliferation of the SH-SYSY neuron cell line on graphene foams, authors proved the viability of this approach in more complex procedures. [PERSON] et al. [158] produced an iron-oxide-based, magnetic, electrospun, short nanofibre-wrapped GO for a guiding cellular behaviour as shown in Figure 4.  
As shown in Figure 4, the authors produced short GO magnetic fibres by electrospinning techniques achieving materials with diameters of up to 300 nm and a length of up to 80 \(\upmu\)m and a magnetization saturation of up to 50.33 emu/g. These fibres were able to induce an extremely tight adhesion with cells due to the functionalized GO. The observations were particularly promising for tissues manipulation by using external magnetic fields to selectively move specific cellular groups by tailoring the GO surface. Furthermore, the creation of controlled cellular aggregates could be a step forward in specific site repairing. [PERSON] et al.