## 3 Nanosized-Lipid-Based Drug Delivery Systems in Wound Healing
### Vesicular Systems  
#### 3.1.1 Conventional Liposomes in Wound Healing  
Liposomes are small spherical vesicles prepared based on natural phospholipids, such as lecithin, with a structure similar to most biological membranes' lipids [88] (see Figure 2a). The phosphatidylcholine represents another frequently used phospholipid, which is obtained from soybean or egg yolk. The physicochemical properties of liposomes, such as the lipid composition, particle size, membrane rigidity or elasticity, surface charge, number of lamellae and drug storing potential, determine their stability and drug delivery abilities [89]. Several methods of preparations of liposomes are represented by hand shaking, membrane extrusion, thin-film hydration, sonication or ethanol injection [90]. The stability of the liposomes is fragile, being influenced by several factors such as the composition of the lipid bilayer, the quantity of external water or the interaction between the drug and the bilayer. In addition, the shelf life of liposomes can be affected during flocculation or aggregation processes, modifying their size and stimulating the fusion of the vesicles. Lyophilization, or storage of the liposome in a dry state, is an effective technique to overcome the instability of the liposomes, specifically to develop stable boundaries between the vesicles [91,92]. A maximum drug loading capacity associated with prolonged storage would be around 15-20% [93]. The amphiphilic property of liposomes helps them to encapsulate both hydrosoluble and liposoluble drugs. Liposomal encapsulation also enhances the solubility and transdermal penetration for some hard-to-dissolve pharmacological agents [68]. Regarding the size of the liposomes, it modifies the capacity of penetration into the skin. Liposomes larger than 600 nm cannot deliver substances into the deep skin layers, while smaller vesicles can provide maximal penetrability [40]. According to their classification, multilamellar vesicles present a size range of 1-5 \(\upmu\). Large unilamellar vesicles have a size range between 100 and 250 nm, and small unilamellar vesicles show a size range between 20 and 100 nm [94]. The encapsulation efficiency (EE%) is an important quantitative parameter expressing the amount of the active agent enclosed in the nanosized-lipid-based systems. Liposomes present a typical drug delivery EE% less than 30% [95]. Another fundamental parameter is the zeta potential, which measures the electrical charge on the particle surface and provides information about particle stability. The surface charge of vesicles plays a crucial role in their capacity to penetrate deeper skin layers. It has been highlighted that cationic vesicles have a higher absorption rate across the skin layers than anionic liposomes [96]. Cationic liposomes can work as gene delivery systems by forming complexes with the negatively charged nucleic acids. Moreover, cationic lipid nanoparticles have been studied widely for small systemic interfering (siRNA) delivery and have shown promising results in topical applications. RNA interference (RNAi) can be a crucial tool for silence-targeted genes, but the poor availability of safe and effective delivery systems has limited its therapeutic use for cutaneous diseases. In this case, topical delivery is preferred over systemic delivery. Striving to manage this problem, scientists have explored nanosized-lipid-based drug delivery systems. For instance, [PERSON] et al. developed cationic liposomes and peptide-based ternary complexes to increase the transfection of Keap1 siRNA. This molecule stimulates antioxidant mechanisms and establishes tissue regeneration [97]. Human diabetic ulcers express a significantly reduced micro-RNA-132 compared with normal skin wounds. An effective wound closure, along with an increased proliferation of wound edge keratinocytes and reduced inflammation, has been revealed after local replenishment of microRNA-132 in the wounds of diabetic mice [98]. Furthermore, [PERSON] et al.