## 3 Application of Nanotechnology in Stem-Cell Therapy
### Nanosubstrates for Large Scale Production of Stem Cells  
One of the major advantages of nanotechnology in stem-cell research is the preparing of the substrate for producing stem cells in large scales. Much effort has been focused mainly on the amplification of neural cells, which has significant impact on the development of the therapies for treatment of neurodegenerative diseases [21]. Having a mass culture of cells which can efficiently remain undifferentiated during mass production process, is one of the most essential prerequisites for cell transplantation procedures [50].  
Metallic nanoparticle (NPs) have a unique potential in terms of their application over a wide range of biomedicine, due to their specific physicochemical characteristics such as locating high energy atoms on their surface area. For example, in the transformation process of gold nanorods by using laser photofragmentation method, if the rate of photothermal heating increases, the internal energy of the lattice elevates and high-energy channels above that of melting open up. In this technique, using a nanosecond laser with longer pulse helps in opening up these channels, result in more photon absorption during the longer pulse, and as a consequence, would increase the lattice internal energy. There are various studies indicating the considerable influence of metallic nanoparticles (NPs) on proliferation and differentiation of different types of cells including stem cells. Intriguingly, there are various mechanisms which are involved in the proliferation and differentiation of stem cells via metallic NP-induced procedures, such as modulation of signaling pathways, generation of reactive oxygen species and adjustment of different transcription factors. Metallic NPs and their possible potential of toxicity, in vivo and in vitro have significant effects on stem-cell differentiation and proliferation [22]. Superparamagnetic iron oxide (SPIO) NPs are a type of IONPs (iron oxide NPs) that possess superparamagnetism properties which enable them to migrate to the injured site, so they can be a promising tool for regenerative disease therapy [51]. SPIO- (Ferucarbotran)NPs are able to promote the proliferation of human MSCs (hMSCs) via counteracting intracellular H\({}_{2}\)O\({}_{2}\) and improve the progression of the cell cycle through upregulation of the proteins related to cell cycle, such as cyclin D1, cyclin B and cyclin-dependent kinase 4. Therefore, SPIO-NPs can be used as a safe nanomaterial resource to enhance proliferation of stem cells [52].  
To maintain the pluripotency, iPSCs generally need to be cultured on the feeder layer cells. Due to the high biocompatibility at low concentration and 2D structure with ultralarge surface area, graphene (G) and graphene oxide (GO) have recently been developed as cell culture substrates. The culture of mouse iPSCs can be supported by G and GO which allow spontaneous differentiation of stem cells. These graphene structures induce discrete cell proliferation and differentiation properties (Figure 3). iPSCs culture on the G surface represent similar rate of cell adhesion and proliferation compare to glass surface, while GO surface exhibit faster rate of adherence and proliferation. Moreover, G have another advantage of maintaining the iPSCs in the undifferentiated stage while GO accelerates the differentiation [53].