## 6 Delivery Systems for Transporting Cosmetic Ingredients  
Delivery systems are engineered technologies designed to transport active ingredients effectively. These systems enhance the permeation of active ingredients through the skin layers while modulating their concentration within the formulation and the skin. For cosmetic applications, a key objective is to confine the active ingredient to the superficial skin layers, thereby minimizing systemic absorption. Two critical characteristics of cosmetic delivery systems are burst release and sustained release. Burst release facilitates the rapid penetration of active molecules, while sustained release is crucial for mitigating irritation caused by high concentrations of active ingredients or for ensuring prolonged supply of the active ingredient to the skin. Various types of delivery systems are employed in cosmetic formulations, typically categorized into vesicular systems (e.g., liposomes, niosomes, transfersomes), emulsions (e.g., microemulsions, nanoemulsions), and particulate systems (e.g., microparticles, nanoparticles). Liposomes are spherical vesicles with a hydrophilic core encapsulated by at least one phospholipid bilayer. They can be tailored to different sizes and structures, including small unilamellar vesicles (SUV, 10-100 nm), large unilamellar vesicles (LUV, 100-3000 nm), multilamellar vesicles (MLV, >1000 nm), and multivesicular liposomes (MVL). Despite their hydrophilic core, liposomes can encapsulate hydrophobic, hydrophilic, and amphiphilic molecules. Their widespread use in research is attributed to their biocompatibility, biodegradability, low toxicity, ease of preparation, prolonged circulation time, and ability to extend product shelf life. However, the rapid release of poorly water-soluble drugs incorporated into the lipid bilayer limits their efficacy for hydrophobic molecules. Ethanol injection methods are commonly used to prepare liposomes, allowing scalability and control over vesicle size and encapsulation efficiency. Key parameters include lipid concentration, injection diameter, pressure, and aqueous phase flow rate. Niosomes are vesicles composed of nonionic surfactants, often combined with cholesterol or its derivatives. Cholesterol significantly influences entrapment efficiency, stability, storage duration, and release characteristics. Niosomes are easier to manufacture than liposomes due to the thermal and oxidative stability of surfactants. Transfersomes are deformable vesicles consisting of phospholipids and an edge activator, typically a single-chain surfactant with a high curvature radius. The edge activator reduces bilayer stiffness, enabling the vesicles to follow the hydration gradient and penetrate deeper into the skin. Ethosomes are phospholipid vesicles characterized by high ethanol content, which enhances their ability to deliver entrapped ingredients into deeper skin layers. Compared to liposomes, ethosomes exhibit smaller size, higher entrapment efficiency, and improved stability. They can enhance skin delivery under both occlusive and non-occlusive conditions, unlike transfersomes. Lipid nanoparticles, including solid lipid nanoparticles (SLNs) and nanostructured lipid carriers (NLCs), are oil-in-water (O/W) nanoemulsion systems where the liquid oil is replaced by solid lipids (SLNs) or a blend of solid and liquid lipids (NLCs). SLNs exhibit both burst and sustained release profiles, making them suitable for cosmetic applications. Their protective effects against UV radiation have led to their incorporation into sunscreen formulations. Compared to liposomes, lipid nanoparticles exhibit slower in vivo degradation, improved encapsulated molecule protection, controlled release, and cost-effective large-scale production. Nano- and microparticles are delivery systems designed to encapsulate active ingredients either within their core or distributed throughout their structural matrix.