Figure 1.3 Type of the nanostructured lipid carrier.

1.3.4 Nanoemulsion

      Nanoemulsion is an o/w type of emulsion with an average droplet diameter of 50–500 nm. The term “nanoemulsion” is used to define the dispersions of water and oil that are two immiscible liquids to form a thermodynamically stable and isotropically transparent system along with surface molecules involved in interfacial film formation. In addition, it should have an inner core of water or oil as an o/w or w/o emulsion. Nanoemulsion is composed of ingredients that are generally recognized as safe (GRAS) by FDA, approved surfactants for human use. The nanoemulsions consist of water-immiscible oil phase prepared under high shear pressure, or by mechanical extrusion system available throughout the world. Large-scale production of emulsion is easy. The use of nanoemulsion across various routes is favored due to their large surface area; thus, it is used for efficient drug delivery throughout the body [46]. Nanoemulsions are stable and have the ability to dissolve an increased amount of lipophilic drug along with certain vectors that prevent their enzyme degradation and hydrolysis [47, 48]. Reducing the size of droplets to nanoscale results in several fascinating physical properties such as visual transparency and peculiar elastic behavior. They are very promising in the non-material sector, as they are useful for the dispersion of deformable nanoscale droplets from fluid to highly solid and deformation of optical characteristic from opaque to nearly transparent [49].

      Preparation of nanoemulsion contains oil and aqueous phase along with drug as well as surfactants/co-surfactant and additives. The physical and chemical characteristics of these components play an important role in formulation stability and their performances. The choice of surfactant must also be taken into account as per the hydrophilic lipophilic balance (HLB) and critical factor. Strong HLB (8-18) surfactants are used in nanoemulsion preparation, while surfactant with low HLB (3 to 6) can be used in w/o nanoemulsion preparation. The right combination of high and low HLB surfactants results in the formation of stable nanoemulsion.

      The hybrid nanoemulsion preparation process combines low-energy emulsifying and high-energy emulsifying applications. Due to their drug solubilizing capacity in oil core without premature leakage, they are particularly preferred as the drug delivery system. The interactions between the lipid droplets on administration routes also reveal their targeting properties such as oral drug delivery, parental drug delivery, transdermal drug delivery, anticancer drug delivery, and vaccine drug delivery. Nanoemulsion can be used for both local and systematic targeting effectively, e.g., delivery through skin, lungs, brain, and ligand mediated drug targeting.

      1.3.5 SMEDDS, SEDDS, and SNEDDS

Various techniques are used to increase the oral bioavailability of poor soluble drugs [50-52]. As it gives high degree of patient tolerance, the oral route is the main route in the chronic treatment of various kinds of diseases. Nonetheless, 50% of drugs are mainly obstructed by oral delivery due to their high lipophilicity [53]. In recent years, various types of lipid-based carrier system such as self-micro emulsifying drug delivery system (SMEDDS), self-emulsifying drug delivery system (SEDDS), and self-nano emulsifying drug delivery system (SNEDDS) are the most promising approaches for improving bioavailability of drugs that are in insoluble lipophilic phase [54]. SMEDDS often provides a different feature. SMEDDS is defined as isotropic formulation of fine oil-in water (o/w) microemulsion formed by surfactants, co-surfactants, or drug and lipid mixtures, when combined with gentle stirring in aqueous media. These systems are important in improving oral bioavailability and are of primary interest to researchers, as a result of being a potential drug delivery through the incorporation of a wide range of drug molecules inside the vehicle. SMEEDS produce clear microemulsions of less than 50 nm of oil concentration and surfactant with HLB>12. SEDDS have been used to enhance the absorption of the drug via oral route [55-59]. Such formulations form fine oils rapidly in water emulsion or micro-emulsions when diluted in water [60], which are responsible for a negative free energy demand for forming emulsion [61]. Therefore, SEDDS are quickly dispersed throughout the GI tract and offer the agitation required for emulsification due to stomach and small intestine motility. SEDDS comprise the combination of oil, surfactant, and other chemicals and drugs. The selection of the lipid and the surfactant is done with their maximum ratio for optimum self-emulsifying property for the formulation [62-65]. In addition, it has been often shown that surfactant blending has superior emulsifying properties in comparison with the use of one single hydrophilicity–lipophilicity balance (HLB)-containing surfactant to attain the HLB quality, essential in emulsification process. The smooth mixing of aqueous media produces emulsion in the range of 10–100 nm with a droplet size. Droplets of emulsion were then spread into the gastrointestinal tract to meet the absorption point in SEDDS by emulsifying into the stomach [62]. SNEDDS is thermodynamically stable and the isotropic mixture of natural or synthetic oil, surfactant, and co-surfactant ability to form non-ionized (o/w) or (w/o) nanoemulsion dispersion under moderate agitation with particle diameter of 200 nm [66-67]. SNEDDS is important for oral absorption when formulating with medium-chain glyceride oils and non-ionic surfactants. In order to offer a large interfacial region between the oil and the aqueous phase, SNEDDs are of stable nanoemulsion. It enhances drug dissolution rate and increases drug formulation bioavailability. The formulation is usually incorporated into gelatin (soft/hard) or hydroxypropylmethyl cellulose capsules, which provides patient enforcement and is used commercially. The important factor in formulating liquid soft gelatin capsule is that the volume should be 1 g maximum [68].

Similarly, the differences between the three lipid-based carrier systems are given in Table 1.2 [69].

      1.3.6 Crystalline Mesophases

Most of the drugs present in the market have prevalent problems such as poor solubility, low bioavailability, drug development cost, and time taken into consideration for developing and formulating a novel drug delivery system. One of the primary methods of preparation of crystalline mesophases was selected based upon the shape, state, and form of drug molecules. Current procedures for preparing liquid crystalline mesophases have been extensively considered for over a couple of decades for improving solubility and controlled drug release rate. Crystalline mesophases are also categorized into liquid crystals, plastic crystals, and confirmatively dispersed crystals according to their transcription, orientation, and concordances that constitute a specific condensation state [70]. Disordering solids typically associated with amorphous substances have received a major focus on pharmaceutical products. The practical benefits of CMs including increased solubility in nanoparticles and greater stability for protein drugs obtained by mixing protein can be significant in amorphous condition. On the other hand, thermodynamically less stable amorphous compounds than the corresponding crystalline state can undergo physical and chemical modifications and therefore decrease the shelf life. Chemical reactivity levels of amorphous vs. crystalline materials are known for many structures, where the rates of degradation are significantly higher with amorphous materials [71-73]. CMs are mainly classified into lyotropic and thermotropic. Thermotropic liquid crystals show mesophase formation when temperature affects the transition, while the change in solvent in a mixture of components at a particular temperature forms lyotropic crystals. The availability of fluid crystals as medicines is a broad area for research. The demand for the application of drug delivery nanoparticles (cubosomes and hexosomes), in particular lyotropic crystal, has been high in the last few years. Drug delivery based on nanoparticles promises the choice of drugs to be effective, regulated, and targeted [74].

Table 1.2 Differences between SMEDDS, SEDDS, and SNEDDS.