TABLE 1.

Desired requirements for lipid nanoparticles for siRNA delivery.

	Factors	Requirements
1	Sizes	Smaller size of gene carriers can be beneficial in penetrating tissues and in the cellular uptake. Smaller than 100 nm: necessary to access hepatocytes; 100–200 nm: accumulate in the tumor site through the enhanced permeability and retention (EPR) effect (Huang and Liu, 2011; Xia et al., 2015)
2	Zeta potential	Reduced nonspecific interactions; good particle stability; enhanced cellular uptake and retention; and sensitivity to environmental factors or triggers like pH and temperature (Gilbile et al., 2019)
3	PEGylation	Short PEG (e.g., PEG1K or smaller) cannot notably reduce the adsorption of proteins and extend the circulation time efficiently. Long PEG (e.g., PEG5K or larger) reduces the cellular uptake and the endosomal escape of liposomes. Commonly used PEG2K: ＜5%, coverage ＜ 100%, mushroom-like; 5–15%, full coverage, mushroom-like and brush-like; ＞15%, full coverage, brush-like (Xia et al., 2015)
4	Target ligand	The type, density, and orientation of the target ligand are key factors for targeting efficiency. Further modification with targeting ligands can increase gene silence efficiency but does not significantly affect the pharmacokinetics or biodistribution profiles of the nanoparticles. High amount of targeting ligands may reduce the effect of PEG on the surface of lipoplexes (Huang and Liu, 2011; Ge et al., 2015; Xia et al., 2015)
5	Endosome escaping	PEG removable: diorthoester, hydrazone linker (low pH), disulfide (reducing agents), and peptide (enzyme). Incorporating pH sensitive cores, such as CaP, cationic polymer. (Huang and Liu, 2011). Cell penetrating peptide or pore forming peptide (Gabizon and Papahadjopoulas, 1992; Song et al., 2005; Tseng et al., 2009; Dominska and Dykxhoorn, 2010; Gao and Huang, 2013)