**5. Nanoparticles: controlling the spatiotemporal supplement of biological cues to iPSCs differentiation to neurons**

In addition to the provision of structural guidance using biomaterial-based scaffolds, exposing developing stem cells to molecular cues in a spatiotemporallycontrolled manner is another factor that is crucial to establish the mature neurons when attempting to recapitulate the native morphogen supply that is achieved during native spinal cord development. To this end, understanding the molecular supply mechanisms that determine spinal neuron cell fate is important. During spinal cord development, stem cells differentiate into each type of spinal neuron through the exquisite control of several morphogens and growth factors, which creates combinatorial gradients of factors releasing mainly from the notochord and paraxial mesoderm. However, *in vitro* differentiation schemes lack accuracy in terms of recreating these morphogen gradients, resulting in the formation of an often poorly defined population of neurons and glia at the end of the protocol. The lack of materials to function as a source of controlled morphogen release has been a major reason for this shortcoming in current protocols, exacerbated by our insufficient understanding of native morphogen supply scheme during spinal cord development. Therefore, there is an urgent need to develop material tools, with high cytocompatibility, to more closely mimic the exquisite biomolecule release schemes that exist *in vivo* and to do so in a tunable manner to be enable optimization of release kinetics for the various factors necessary to induce motor neuron differentiation (e.g., RA, SHH, SAG, etc.).

*Exploring the Potential for Biomaterials to Improve the Development of Spinal Motor Neurons… DOI: http://dx.doi.org/10.5772/intechopen.113275*

#### **Figure 6.**

*Characteristics of nanoparticles for use as morphogen carriers. Each class of nanoparticle has numerous advantages and disadvantages regarding cargo and delivery. (poly(lactic-*co*-glycolic acid): PLGA, Polyethyleminine: PEI, mesoporous silica nanoparticle: MSN, iron oxide nanoparticle: IONP).*

Nanoparticles are an attractive candidate to fulfill this requirement. Depending on the type of nanoparticle, its surface can be modified for active targeting, penetrating barriers, and releasing molecules through pH-responsive properties while also avoiding clearance by the immune system [68–71]. To improve neuronal differentiation, researchers have been developing many different types of nanoparticle carriers, such as DNA nanostructures, mesoporous silica nanoparticles (MSN), and polyethyleneimine (PEI) to effectively deliver neurogenic-inductive factors such as signaling ligands, DNA, siRNA, and mRNA into early-stage differentiating neuronal progenitors (**Figure 6**). In the context of supporting controlled release of morphogens, a study demonstrated that morphogen-loaded nanoparticles could be used as a source of controlled molecule release to provide pivotal molecules for neurogenesis, such as RA. Since RA has a very short half-life (14 minutes in PBS), it is difficult to maintain its active form to induce an effective role for a long-term period of culture *in vitro*. Using nanoparticles to release RA can facilitate the conversion of stem cells into neural cells by activating neural signaling pathways and minimizing cell cytotoxicity


*(Tetrahedral DNA nanostructure: TDN, Metal-organic framework: MOF, Mesoporous silica nanoparticle: MSN, Polyethyleminine: PEI).*

#### **Table 2.**

*Nanoparticle used in neuronal differentiation.*

(**Table 2**) [69, 73–76]. Such studies demonstrate that nanoparticles have a significant impact on neuronal differentiation from stem cell sources and with minimal effect on cell viability and proliferation. Nanoparticle modification can also provide an intracellular docking system for the simultaneous delivery of multiple factors to facilitate neuronal differentiation (**Figures 7**–**10**).

#### **Figure 7.**

*Application of nanoparticle for neuronal differentiation: Tetrahedral DNA nanostructure (TDN). The study investigated the effects of TDN on neural stem cells, demonstrating that TDNs promote self-renewal through the Wnt/β-catenin pathway and enhance neuronal differentiation by inhibiting the notch signaling pathway and suggesting their potential for nerve tissue regeneration. Because using TDN increase uptake of cells in stem cells [72].*

#### **Figure 8.**

*Application of nanoparticle for neuronal differentiation: Metal-organic framework (MOF). The study introduces a new platform, called SMENA, which utilizes MOF-embedded nanopit arrays to provide a stable and continuous supply of retinoic acid, promoting enhanced neuronal cell generation and demonstrating potential for various stem cell-based regenerative therapies [73].*

*Exploring the Potential for Biomaterials to Improve the Development of Spinal Motor Neurons… DOI: http://dx.doi.org/10.5772/intechopen.113275*

**Figure 9.**

*Applications of nanoparticle for neuronal differentiation: Mesoporous silica nanoparticle. This study used MSN as a delivery carrier of RA, which enabled rapid and high-quality neural differentiation of mouse embryonic stem cells, resulting in the successful derivation of stable neurite marker-expressing neural cells [74].*

#### **Figure 10.**

*Applications of nanoparticle for neuronal differentiation: Polyethyleminine (PEI). This study described the synthesis of functional RA-PEI complex nanoparticles with controlled release properties and their effectiveness in inducing neuronal differentiation of embryonic stem (ES) cells, suggesting their potential as a powerful tool for directing murine ES cell fate [75].*
