## 3 Graphene and Graphene-Related Materials for Biological Applications
### Graphene and Related Materials as Tool for Regenerative Medicine and Tissue Repair
[159] produced a highly elastic and electroconductive graphene /collagen cryogel for spinal cord regeneration. Firstly, the authors tailored the graphene surface with amine residues promoting an efficient cross-linking with collagen showing a conductivity of up to 3.8 \(\pm\) 0.2 mS/cm and a Young modulus of up to 347 kPa. The authors tested this composite for stem cell transplantation and neural tissue regeneration through the cryogelation approach by using the BM-MSCs cell line. They reported a good stem cell growth and expression enhancement of _CD90_ and _CD73_ gene upon electric stimulation of up to 100 mV/mm without the loss of stemness. Additionally, the authors observed an ATP secretion increment that could be helpful to favour neuronal regeneration and immunomodulation. Graphene-based cryogel promoted the neuronal differentiation of BM-MSCs with enhanced expression of MAP-2 kinase and \(\beta\)-tubulin III. In vitro observation proved a high indoleamine 2,3 dioxygenase activity under inflammatory conditions together with the proliferation of macrophages, which could aid in tissue repair. Cellular proliferation is the other relevant task to be faced. [PERSON] et al. [160] studied both GO and rGO as promoting agents for the cardiomyocyte and angiogenic differentiation capacity of human mesenchymal stem cells in vitro. The authors showed that, generally, those substrates fit the requirements for cellular proliferation with the exception of small GO flakes and highly reduced rGO, which decreased proliferation by inducing apoptosis. Nonetheless, this study proved that both GO and rGO did not alter stem cells' phenotype and cell cycle progression while they could tune their adhesive capabilities of these cells. These properties could be used in ocular regenerative medicine as reported by [PERSON] et al. [161] as a sound replacement for functionalized hydrogels [162]. [PERSON] et al. [163] realized poly(caprolactone) mixed with graphene for nerve tissue engineering through electrospinning. Nanofibrous composites showed good properties for the proliferation of rat stem cells with a preferential differentiation to dopaminergic neuronal cells with the increment of graphene percentage. Similarly, [PERSON] et al. [164] produced nanostructured scaffolds based on copolymers (poly(lactic acid)-poly(caprolactone)) containing GO, aiming to induce the aligned migration and accelerate the neuronal differentiation of neural stem cells. Authors proved the ability to re-establish the spatially oriented neural precursor cell connectivity of this material. [PERSON] et al. [165] used the highly conductive properties of graphene-based hydrogel for the realization of artificial muscles. The hydrogel was produced by incorporating GO into poly(acrylamide) using femtosecond laser ablation and a chemical reduction. Figure 4: Cellular guidance induced by magnetic GO fibres as described by [PERSON] et al. [158]. In vitro studies with C2C12 myoblasts revealed that the micropatterned graphene hydrogel induced the myogenesis and myotube alignment. In vivo implantation studies showed a good tissue compatibility. [PERSON] et al. [166] showed that osteoblast differentiation and gene expression could also be modulated by GO coating of titanium materials due to an upregulation of the expression of the bone matrix protein genes during late-stage osteoblast differentiation.