**3. Structure of solid lipid nanoparticles**

As mentioned above, SLNs are composed of a lipid substrate surrounded by stabilizers and one or more active molecules incorporated into these particles.


#### **Table 2.**

*Surfactants used in the preparation of SLN.*


#### **Table 3.**

*Miscellaneous components used in the preparation of SLN.*

#### *Solid Lipid Nanoparticles (SLN) DOI: http://dx.doi.org/10.5772/intechopen.102536*

Thus, the structure of a particle consists of an outer layer that determines its surface properties and an inner layer or core material that determines factors such as the size, shape, and location of the active ingredient [32]. Therefore, how the lipid substrates stabilizers and active components that make up the colloidal lipid system are organized and interconnected determines their behavior as drug transporters and is strongly influenced, among other factors, and the crystalline behavior of lipids used.

It has been reported that under heat treatment or during storage, polymorphic transformations can occur, and the particles can change from spherical shapes to platelets. Platelet forms are associated with the stable polymorphic state β for the solid lipid of the matrix, and spheroidal or disc-shaped records are associated with the polymorphic state α [33].

In general, two structure models have been proposed for SLN, in which it is assumed that the particles are surrounded by the surfactant that forms the outer layer. The first is that the molten lipid droplets solidify during cooling, maintaining their spherical shape. On the other hand, the second model proposes that the cooling of molten lipids produces a flat laminar structure with surfaces structured by folds, edges, and steps that occur during the recrystallization process and where the lipid structure from polymorph α to polymorph β comes from stable [28].

#### **3.1 API localization within SLNs**

The location of the active molecule depends on the structural organization of the colloidal lipid systems; in this way, from the model corresponding to the basic spherical model for SLN, three general and commonly used classifications emerge: (i) active molecule is homogeneously distributed along the length of the particle structure (homogeneous matrix); (ii) the API is concentrated within the particle and is surrounded by the lipid matrix in a core-shell structure (enriched core); and (iii) the API is concentrated on the surface of the particle (enriched coat) (**Figure 2**).

In general, the higher temperature of the aqueous medium facilitates more excellent solubility of the drug-containing them. Considering that in most studies, the temperature used to prepare the granules is typically at least 10° above the melting point of solid lipids, this operating condition facilitates a substantial amount of drug in phase. Then during cooling, drug solubility decreases, the aqueous phase is supersaturated, and in theory, the active molecules should migrate to the lipid substrate due to their lipophilic properties.

The outer shell or enriched shell structure can be obtained when solid lipids have a high melting point and a high crystallization temperature, which leads them to crystallize before the active molecule during the nanoemulsion system O/W hot cooling process housing the active molecule on the surface of the particle. This concentrated outer shell layer shows an explosion effect on drug release, so this model is unsuitable for prolonged drug release [34].

Conversely, suppose the use of fat has a low melting point or crystallization point. In that case, high temperature may remain as a supercooled liquid or in a metastatic crystalline form where more particles are present at high temperatures. Active elements can only be contained in rotation. Instead, it can promote nucleation and thus be encapsulated within the seed (core-shell structure) or uniformly dispersed throughout the grain structure, mainly when homogenization is used at high cold pressure.

#### **Figure 2.**

*Structure of solid lipid nanoparticles. Models of drug incorporation of solid lipid nanoparticles. (a) Homogeneous matrix model; (b) rich core model; (c) enriched roof model.*

The homogeneous matrix model can be obtained using the cold homogenization method and by incorporating lipophilic active molecules into the SLNs with the hot homogenization method. This solidified structure relies on its solid state to provide a uniform drug distribution in SLN [35, 36].

In the core-shell or enriched core structure, the concentration of the active molecule is close to the saturation point in lipid fusion; when this highly concentrated lipid is cooled, the solubility of the active molecule in lipid fusion is reduced, and it is deposited in the center of the SLN, forming a nucleus enriched with the active molecule. This type of structure leads to a sustained release profile [37, 38].

### **4. SLN preparation methods**

There are multiple methodologies for the preparation of SLN; the selected technique will depend on the physicochemical properties of the drug about the lipid matrix, the route of administration, among other parameters. The methods used for the preparation of SLN fall broadly into three main categories: (i) high-energy methods for dispersion of the lipid phase (such as high-pressure homogenization), (ii) low-energy methods where requires the precipitation of nanoparticles from homogeneous systems (such as microemulsions), and (iii) methods based on organic solvents.

#### **4.1 High energy methods**

High-energy-based methods are those that, in general, require the use of equipment capable of generating high shear forces, pressure distortions, or any other mechanism to achieve particle size reduction. In an emulsion, the mechanical energy required exceeds the interfacial energy by several orders of magnitude, thus requiring high energy to form submicron drops. The high-energy process uses intense mechanical force, resulting in large interfacial areas to form nanoscale emulsions [39]. The energy barrier for droplet fragmentation is highly dependent on the interfacial tension: the higher the interfacial tension, the more energy must be supplied to obtain tiny emulsion droplets. The interfacial energy and the curvature of the drop interface

#### *Solid Lipid Nanoparticles (SLN) DOI: http://dx.doi.org/10.5772/intechopen.102536*

give rise to a pressure difference between the inside of the drop and the outside, called Laplace pressure. This pressure will act as a resistance to whatever stress is applied, so the reductions are expected to break approximately at the point when the applied stress equals or exceeds the Laplace pressure. The nanoemulsion droplets have a greater thickness due to the adsorption layer of the emulsifier concerning the radius of the droplet, which makes them more stable towards coalescence [26, 38]. In general, the high-energy process is followed by two steps: (i) the deformation and disruption of macro droplets into the smallest droplets; (ii) the adsorption of surfactant at its interface (to ensure steric stabilization) [40].

#### *4.1.1 High-pressure homogenization (HPH)*

Depending on the temperature used for the production of SLN, this technique can be classified into hot homogenization and cold homogenization. The advantage associated with this method is that SLNs are obtained with small particle sizes and high entrapment efficiency. High-pressure homogenization, the molten lipid is pumped through a narrow space with a 500–5000 bar pressure at high speed. Generally, 5–10% lipid content is used, but up to 40% lipid content has also been investigated. Two general approaches to HPH are hot homogenization and cold homogenization [41–43].

### *4.1.1.1 Hot homogenization*

In the hot homogenization method, the procedure is carried out at temperatures above the melting temperature of the lipid. Here, the lipid and the drug are fused and combined with an aqueous surfactant at the same temperature. By using the high shear device, a hot pre-emulsion is formed. The hot colloidal emulsion droplets are recrystallized by cooling the emulsion to room temperature to generate the SLNs. In general, higher temperatures result in smaller particle sizes due to the decrease in the viscosity of the internal phase. However, high temperatures can also increase the rate of degradation of the drug and vehicle. In most cases, 3–5 homogenization cycles at 500–1500 bar are sufficient [41, 42]. Increasing the number of cycles or the homogenization pressure often increases particle size, inducing coalescence due to its high kinetic energy. Particle sizes < 500 nm can be obtained [42].

### *4.1.1.2 Cold homogenization*

In this technique, the lipid melt containing the drug is cooled. Solid lipids are ground into lipid microparticles. These lipid microparticles are dispersed in a cold surfactant solution producing pre-excitation. This hypothetical process is then homogenized at room temperature or below; gravity is strong enough to reak lipid microparticles directly into SLNs [41, 42]. The particle sizes achieved by this technique are generally in the range of 50–1000 nm [41].

#### *4.1.2 Ultrasonic/high-speed homogenization*

This technique is based on the use of ultrasound waves which create cavitation phenomena that include the formation, growth, and implosive collapse of microbubbles/cavities in the medium; It consists of very high temperatures up to 5000 K and pressures up to 1000 bar. The particle size of the SLNs in this technique depends on the stirring speed, the emulsion time, and the cooling temperature, reaching heights less than 100 nm [44].

#### *4.1.3 Supercritical fluid-based method*

A supercritical fluid (SCF) increases the dissolving capacity of compounds, and fluid becomes a supercritical fluid when its temperature and pressure exceed its critical values. Supercritical fluid technology has the unique property of producing solids with a small size and irregular morphology. Supercritical fluids (SCF) have unique properties, such as high diffusivity, low viscosity, and high compressibility. Supercritical CO2 (SC-CO2) is the most common SCF because it is non-toxic, non-flammable, easy to obtain, and easily accessible. SLNs can be formulated by five main (SCF) methods: (i) rapid expansion of supercritical solutions (RESS), (ii) particles of gas-saturated solutions/ suspensions (PGSS), (iii) supercritical fluid extraction of emulsions (SFEE) [45, 46].

#### **4.2 Low energy methods**

Low energy methods do not consume significant energy to achieve particle size reduction, and some even proceed spontaneously. These inferior energy methods are based on the properties of the system and its complex interfacial hydrodynamic mechanisms. The chemical energy released during emulsification is believed to occur as a consequence of the change in spontaneous curvature of surfactant molecules from negative to positive (o/w) or from positive to negative (w/o) [47].

Low energy techniques are classified into thermal or isothermal methods. Emulsion formation due to temperature-dependent changes in surfactant properties is typical of thermal processes, while emulsion formation due to continuous temperature changes is typical of the isothermal method. Spontaneous emulsion (selfemulsifying) and inverting emulsions are included among the isothermal techniques. Spontaneous emulsion involves adding an oily surfactant mixture to the water, whereas, for emulsion phase inversion, the emulsion is formed when water is added to the oily surfactant mixture. An emulsion is formed through the phase inversion temperature method when the temperature of the oil, water, and surfactant mixture rapidly drops below the phase inversion temperature under continuous mixing. These methods have the advantage of producing tiny droplets without specialized equipment. Therefore they are cost-effective and easy to use [48]. The following variables influence the spontaneity of the emulsification process: structure of the surfactant, concentration and initial location, composition of the oil phase, addition of cosurfactant and non-aqueous solvent, and salinity and temperature [49].

#### *4.2.1 Microemulsion based method*

The IUPAC defines the microemulsion as a dispersion made of water, oil, and surfactant (s) anisotropic and thermodynamically stable system with a dispersed domain diameter ranging from approximately 1–100 nm, generally 10–50 nm [50].

#### *4.2.1.1 Hot microemulsion technique/microemulsion dilution technique*

In this method, the microemulsion forms spontaneously due to the high surfactant-lipid ratio. This method is simple and includes some common steps. Initially, the fat is melted and mixed with a hot surfactant solution. Light shaking is carried out until a microemulsion is formed. In the second step, the hot microemulsion is dispersed in a large amount of cold water (2–10°C) with moderate stirring [51]. Temperature gradients facilitate the rapid crystallization of lipids and prevent

#### *Solid Lipid Nanoparticles (SLN) DOI: http://dx.doi.org/10.5772/intechopen.102536*

aggregation. Due to the dilution step (1:25–1:50), achievable lipid contents are considerably lower than HPH-based formulations [52]. The particle size of the nanoparticles obtained varies from 50 to 800 nm [53].

#### *4.2.1.2 Microemulsion cooling technique*

The method consists of the preparation of an o/w microemulsion in which the emulsified wax melts at 37–55°C and the addition of water heated to the same temperature with minimal agitation to form a suspension homogenous milky white, after adding a specific amount of a pharmaceutically appropriate macromolecular surfactant to the water a stale and transparent microemulsion is produced in the form of a liquid matrix. This o/w microemulsion is further cooled to room temperature or 4°C to precipitate SLNs from it. This method is reproducible, simple, and easy to scale [54]. This method works at a moderate temperature, requires less time and obtains high drug entrapment. The particle size of the nanoparticles received varies from 50 to 300 nm [42, 54].

#### *4.2.2 Double emulsion method*

The double emulsion technique is one of the most used techniques to prepare nanoparticles encapsulated with hydrophilic active ingredients using stabilizers or surfactants. This method is also known as the multiple emulsion method, where it has three basic steps: (i) formation of the water-in-oil emulsion or reverse emulsion, (ii) addition of the W1/O emulsion in the aqueous surfactant solution to form a W1/O/W2 emulsion with continuous stirring (sonication or homogenization) and (iii) evaporation of the solvent or filtration of the multiple emulsion to form the nanoparticles [41].

#### *4.2.3 Phase inversion method by temperature (PIT)*

The temperature-induced phase inversion technique involves performing, using heating and cooling cycles, successive phase inversions of an O/W emulsion to a W/O emulsion until finally an emulsion is obtained. O/W correlation, where each inversion conducts to reduce the size of the droplet so that nanoparticles can be obtained with a final dilution step in cold water [36, 37]. This method brings SLNs of 30–100 nm in diameter.

#### *4.2.4 Membrane contactor method*

In this method, the lipid phase is pressed through the membrane's pores and allows the formation of tiny droplets at the outlets of the pores, which are carried away by the circulating water. SLNs are obtained after cooling the preparation to room temperature [55]. This method can achieve particle sizes of 100–200 nm.

#### *4.2.5 Coacervation technique*

The coacervation method is based on the precipitation of free fatty acids from their micelles in the presence of a surfactant. In this process, a fatty acid salt is uniformly dispersed in the stabilizer solution. The mixture was heated to the Krafft point of the fatty acid salt and stirred continuously until a clear solution was obtained. Subsequently, an ethanolic solution of the API is slowly added under continuous stirring to get a single phase. Then, a coacervation agent or an acidifying solution is

added to obtain the nanoparticle suspension [54]. The particle size of SLN depends on the concentration of the micellar solution and the degree of polymer used for stabilization and can produce a particle size of 260–500 nm [56].

#### *4.2.6 Organic solvent free dual emulsion/melt dispersion technique*

In this technique, the lipid phase is heated above its melting point and dispersed in water using a low HLB surfactant. Again, an aqueous solution of surfactant with a high HLB content is added to the emulsion without thus forming; this double w/o/w emulsion is poured into cold water with gentle agitation to promote the formation of SLN [57].

### **4.3 Organic solvent approaches**

#### *4.3.1 Solvent evaporation emulsion method*

The solvent evaporation emulsion (SEE) method has three basic steps for preparing nanoparticles. In step (I), lipid material is added to a known volume of organic solvent (immiscible in water) and suitably mixed to produce a clear homogeneous lipid solution. In step (II), the solution prepared above is added to the correct volume of a hot aqueous solution containing surfactant above the melting point of lipids to form a thick emulsion using a high-speed homogenizer. The nanoemulsion is then obtained in step (III) using a high-pressure homogenizer, which converts the coarse emulsion into a nanoemulsion due to the high pressure. Nanodispersion is formed after evaporation of the organic solvent since the lipid material will precipitate in the water. The lipids precipitated in an aqueous medium are separated by filtration through the sintered disk filter funnel. The nanoparticles prepared by this strategy are nano-sized, not flocculated (single entity), and have a high trapping efficiency [58]. This method can achieve 30–500 nm particle sizes.

#### *4.3.2 Solvent emulsion and diffusion method*

The method is based on the organic phase's initial saturation and thermodynamic equilibrium with a stabilizer containing an aqueous phase. The drug is dissolved with the help of a homogenizer in the saturated solution formed; in the next step, an o/w emulsion is produced by dispersing it in an aqueous solution with an emulsifier. Diffusion of the solvent into water is facilitated by adding more water in an appropriate ratio to the emulsion under moderate magnetic stirring, which leads to nanoprecipitation and the formation of SLNs. Particles with an average diameter of 30,100 nm can be obtained with this technique [59].

#### *4.3.3 Solvent injection technique (or solvent displacement)*

In this technique, the lipid and the active ingredient are dissolved in a watermiscible solvent. The mixture is then dispersed in an aqueous solution of a surfactant with gentle mechanical agitation, producing a suspension of lipid nanoparticles; the solvent is subsequently removed. The particle size depends on the speed of the distribution process. Higher speed makes smaller particles. More lipophilic solvents give larger particles that can become a problem. The strategy offers advantages, for example, low temperatures, low cutting pressure, easy handling, and a fast

production process without really advanced equipment (for example, high-weight homogenizer) [60]. The particle size of the nanoparticles obtained varies from 100 to 500 nm [53].
