Nanotechnology-based drug delivery systems have increasingly been developed in the last decades and have significantly impacted the pharmaceutical sciences ^[1][2][3][1,2,3]. They play a critical role in delivering hydrophobic drugs, which comprise more than 40% of approved drugs [4]. The poor water solubility of these hydrophobic drugs is one of the major limiting steps that considerably affect drug release and bioavailability, which can be resolved using nanotechnology-based drug delivery systems. In addition, the incorporation of drugs in nanoparticles (NPs) increases drug stability, reduces enzyme degradation, prolongs circulation time, and improves the uptake of target cells, which, thereby, enhances the overall effectiveness and safety ^[5][6][7][5,6,7].

According to their chemical compositions and structure, nanotechnology-based drug delivery systems can be classified as inorganic NPs, polymeric NPs, and lipid-based NPs. Inorganic NPs are made up of inorganic materials, such as gold, silver, iron, and silica. Owing to some of the unique properties of the materials, these inorganic NPs may have distinct electrical, physical, optical, or magnetic features, such as the photothermal effects of gold NPs [8] or superparamagnetic properties of iron oxide NPs [9]. Inorganic NPs have good stability and are potential drug delivery systems in photothermal therapies, imaging, and diagnostics. However, as a result of their low water solubility and toxicity issues, they are not widely used in clinical applications ^[10][11][10,11]. Polymeric NPs are produced from a wide variety of natural or synthetic polymers. They include dendrimers, polymeric micelles, nanospheres, and polymersomes. Polymeric NPs can incorporate hydrophobic and hydrophilic drugs with different molecular weights, including small molecules, biological macromolecules, and proteins [12]. The advantages of polymeric NPs include biodegradability, biocompatibility, and co-delivery of different drugs. In addition, they can deliver drugs to targeted tissues with suitable surface modifications ^[13][14][15][13,14,15]. However, polymeric NPs have the disadvantages of toxicity and particle aggregation [5]. Lipid-based NPs have been widely investigated in the last decades. They have various advantages, such as biocompatibility, high bioavailability, high drug payloads, formulation simplicity, and self-assembly [5]. Lipid-based NPs include emulsions, liposomes, lipid NPs, and solid lipid nanoparticles (SLNs). They can be fabricated from biodegradable and non-toxic materials, making them ideal systems for clinical applications ^[16][17][16,17]. The second generation of SLNs is sometimes termed nanostructured lipid carriers (NLCs). However, SLNs can be used to mention both SLNs and NLCs.

SLNs were developed in the mid-1990s [18] and have been considered alternative systems to liposomes, emulsions, micelles, and polymeric NPs, due to various advantages. They are produced from physiologically biocompatible and biodegradable lipids as well as other materials that are generally recognized as safe (GRAS), and, therefore, they are safe nanotechnology-based drug delivery systems ^[19][20][19,20]. The solid matrices can protect the drugs incorporated in SLNs, and, thereby, the drug stability is efficiently improved [21]. Both hydrophilic and hydrophobic drugs can be encapsulated in SLNs with higher entrapment efficiencies than liposomes [22]. The drug release from SLNs can be controlled by altering the lipid components ^[23][24][23,24]. The surface of SLNs can be modified to target specific tissues and enhance stability [25]. SLNs can be produced using non-solvent techniques, such as high-pressure homogenization and high-speed stirring [26]. These advantages enable the wide applications of SLNs in oral, parenteral, transdermal, intranasal, ocular, and pulmonary drug delivery ^[27][28][27,28].

Considering the increasing interest in SLNs, in this entry, we provide an overview of SLNs. This entry is structured as follows: the primary features of SLNs, methods to prepare SLNs, recent applications of SLNs in drug delivery, and conclusions.