*3.3. Gene Delivery*

The rapid development of nanotechnology made available novel DNA and RNA delivery systems for gene therapy (Figure 25) that can be used instead of viral vectors [487]. Gene therapy collectively refers to therapies aimed at manipulating gene expression in living organisms by supplying exogenous DNA or RNA that is incorporated or not incorporated to cure or prevent diseases [488]. There is great incentive to work towards safer and targeted viral vectors and to engineer more effective non-viral systems that can achieve secure, effective gene therapy in humans because of the enormous potential for gene therapies to influence medicine.

**Figure 25.** Fundamental steps of gene delivery by nanocarriers (orange spheres). (Adapted with permission from ©2016 Royal Society of Chemistry [489]).

There are a number of nanocarriers used for gene delivery (Figure 25) [489] applications which are based on lipids [490,491], liposomes [492–494], dendrimers [495], polymers [496,497], graphene [498,499], carbon nanotubes (CNTs) [500,501], mesoporous silica [502], gold nanoparticles [503,504], magnetic nanoparticles [505,506], and other types of inorganic nanoparticles.

The number of clinical trials for cancer [507], liver disease [508], hemophilia [509], and bone regeneration [510–512] is continuously increasing due to the promising opportunity to correct gene disorders. Nanomaterials are used for gene delivery because of their small size and superior stability [513]. Before the use of these nanomaterials, surfaces need to be functionalized with small biomolecules or polymers to adapt their physiochemical properties such as hydrophobicity, charge density, and binding affinity [514,515]. Factors including molecular weight, biodegradability, rigidity, charge density, pKa value, solubility, crystallinity, and hydrophobicity ensure effective and safe gene delivery [516,517].

Surface-modified graphene oxide through cationic polymers such as polyethylenimine (PEI) provides a large surface area for the encapsulation of DNA molecules [518]. DNA/drug molecules attached graphene oxide conjugated provide an outstanding platform for the immobilization of nucleotides on its surface [519]. Mesoporous silica nanospheres (MSNs) and functionalized single-walled carbon nanotubes (SWCNT) represent an excellent gene delivery system [520]. In Figure 26, a potential route is recorded for the use of dendrimers as vectors of gene delivery. As plasmid DNA penetrates the cell membrane, it makes (in vitro) a complex between dendrimer and DNA (called dendriplex). This complex is transported through the blood system to the specific cell. Finally, the DNA moves through the cytoplasm to reach the nucleus for gene expression in series [521,522].

**Figure 26.** Schematic diagram for a possible route in the use of dendrimers as gene delivery vectors.
