## 3 Nanosized-Lipid-Based Drug Delivery Systems in Wound Healing
### Lipid Nanoparticles in Wound Healing
#### 3.2.2 Nanostructured Lipid Carriers in Wound Healing  
NCLs represent the second generation of lipid nanoparticles [41] and are produced by employing blends of solid and liquid lipids (oils), the combination thereof being solid at body temperature (see Figure 2e). To acquire the blends for the particle matrix, solid lipids are mixed with liquid lipids, ideally at a ratio of 70:30 up to a ratio of 99.9:0.1 [138]. Liquid lipid incorporation generates changes in the solid lipid structure, leading to a less ordered crystalline arrangement that averts drug leakage, supplies a high drug load and has great stability during storage [43, 142]. Moreover, these nanocarriers possess the ability to deliver both hydrophilic and lipophilic drugs. Hence, they have gained wide applicability in wound-healing treatment. The manufacturing process of NCLs is similar to that used for SLNs. It is noteworthy that they are easy to scale up through high-pressure homogenization, which is the first choice for the method of production [155]. In addition, the freeze-drying process has been investigated for several molecules encapsulated into NCL with satisfactory results in terms of stability [156, 157, 158, 159]. NCLs have a typical EE% of NCLs around 85%, a high drug loading capacity [160] and a mean particle size between 50 and 300 nm [161]. Given their small size, the contact of NCLs with the stratum corneum is enhanced, and a higher quantity of active agents penetrates through the skin. Thus, NLCs are adequate to enlarge the spectrum of drug delivery systems and surpass various disadvantages concerning conventional nanosized-lipid-based carriers. NLCs are viewed as an improved generation of SLNs due to several advantages: the increased loading capability of active substances, enhanced stability, reduced risk of drug extrusion and improved overall benefit/risk ratio. In terms of biotoxicity and the action mechanism, no significant differences have been found between SLNs and NCLs [162]. However, the incorporation of drugs in NCLs can be an innovative and suitable approach to overcome SLN formulation limitations. Thus, a comparison between SLN and NCL encapsulation of RHEGF showed an EE% higher in NCL-rhEGF than SLN-rhEGF. Nevertheless, there were no differences between the uptake capacity of SLN-rhEGF and that of NLC-rhEGF in the cell cultures being studied. Both doses of SLN-rhEGF and NLC-rhEGF achieved significantly higher wound area reduction than the empty nanoparticle control group. In this study, the bioactivity of the nanoparticle formulations was better in all of the study cell cultures than that of the free rhEGF [163]. Another in vivo and in vitro study compared the effect of SLNs and NLCs loaded with eucalyptus oil or rosemary oil. The oils encapsulated in SLNs showed poor bio-adhesion properties compared with the oils encapsulated in NCLs. This result was an effect of the olive oil presented in the NCL composition, known for its excellent proliferation improvement. Furthermore, eucalyptus oil encapsulated in NCLs, based on olive oil, achieved the best outcome in the proliferation and cell migration tests [52]. As was previously stated, LL37 is an antimicrobial agent produced by human skin which impacts the wound-healing process. In an interesting study, researchers encapsulated LL37 in NLCs to optimize its administration in terms of the dose, delivery pattern and lack of side effects. The particle size of NLCs is a critical factor in determining the rate of drug release and drug absorption. In this study, the obtained NLC formulation revealed the optimal physicochemical properties allowing for better drug release and absorption. Topical administration of LL37-loaded NCLs improved wound healing in terms of the wound closure rate, re-epithelialization and inflammation diminishment compared with free LL37 administration. LL37-loaded NCLs also showed significant antimicrobial effects [86]. Various studies have evaluated the potential of nanoparticles to enhance the bioavailability of natural antimicrobial agents such as EO.