**1. Introduction**

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50 Application of Nanotechnology in Drug Delivery

7464-7472.

SLN are nanoparticles made of solid lipids with a photon correlation spectroscopy (PCS) mean diameter approximately between 50 and 1000 nm [1].

General ingredients include solid lipid(s), surfactant(s) and water. The term lipid is used here in a broad sense and includes triglycerides (e.g. tristearin), partial glycerides, fatty acids (e.g. stearic acid), steroids (e.g. cholesterol) and waxes (e.g. cetyl palmitate). All classes of surfac‐ tants (with respect to charge and molecular weight) have been used to stabilize the lipid dispersion [2].

SLN were developed in the early 1990s and up today they have been considered to be promising drug carrier systems, especially with the aim to give a sustained release profile to the incorporated active substances. In fact, compared to liquid lipid formulations, such as nanoemulsions, drug mobility is really lower in solid lipids than in liquid oils. Compared to polymeric nanoparticles, they show a better biocompatibility since they're made of lipids similar to physiological one, so the toxicity is reduced. Moreover SLN are physico-chemically stable and can be produced easily on a large industrial scale, and raw material and production costs are relatively low [1].

SLM have equivalent composition to SLN, but higher particle size (>1000 nm), meaning that their application domains and administration routes can be different [3].

© 2014 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

There are different techniques for the preparation of solid lipid nano-and microparticles: generally, the preparation of nano-and microparticles requires a dispersed system as precursor or template, otherwise particles are produced through the use of a particular instrumentation.

According to the character of the continuous and dispersed phase, emulsions can be direct (O/

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Emulsions can be used as precursors for solid lipid particles preparation since lipids, that are solid at room temperature, can be heated 5-10 °C above their melting point to obtain a liquid lipid that can be emulsified with water at the same temperature: the obtained hot emulsion is then cooled to room temperature and the droplets solidify in the form of solid lipid particles.

SLN can be obtained from hot O/W nanoemulsion (<1 μm droplet size), prepared through the so called hot homogenisation technique. Different methods can be used to achieve hot homogenisation: high pressure homogenisation, high shear homogenisation, ultrasound

SLM, instead, can be obtained from hot emulsions (>1 μm droplet size) by the so called melt

High pressure homogenisation (HPH) was applied for the first time in the nineties by Müller and Lucks [4] for SLN production and represents the main method established for them.

In the HPH technique the drug is dissolved in the lipid being melted at approximately 5-10 °C above its melting point. The drug-containing melt is dispersed under stirring in a hot aqueous surfactant solution of identical temperature, in order to obtain pre-emulsion, which is then homogenised using a piston-gap homogeniser; the produced hot O/W nanoemulsion is cooled

Practically, SLN can be derived from the emulsions for parenteral nutrition just by replacing the liquid lipid (oil) of the emulsion droplets by a solid lipid. In contrast to emulsions for parenteral nutrition which are normally stabilised only by lecithin, the SLN are stabilised also by other surfactants or polymers which can act as a suspending agent in order to avoid particle

This is a technique well established on the large scale for parenteral emulsions since the fifties and it is already available in the pharmaceutical industry. The production lines for parenteral emulsions are in most cases equipped with temperature control units because an increased temperature facilitates emulsion production, this means that existing production lines can be used for producing SLN by the hot homogenisation technique [1]. This technique allows the production of colloidal suspensions of SLN without solvents, which are generally used for polymeric nanoparticles, and this is a further advantage of this method from a toxicological

An important problem of SLN prepared through hot homogenisation is drug expulsion from nanoparticles, that can happen during lipid re-crystallisation on cooling of the hot nanoemul‐

dispersion technique: O/W or multiple W/O/W precursors can be used.

down to room temperature, the lipid re-crystallises and leads to SLN [1].

Owing to the size of the hot emulsion droplets, nano-or microparticles can be obtained.

W), reversed (W/O) and multiple (W/O/W and O/W/O).

homogenisation.

aggregation.

point of view.

**2.1. Hot homogenisation**

*2.1.1. High pressure homogenisation*

The most important precursor are emulsions (hot homogenisation technique, melt dispersion technique, PIT method, solvent evaporation-diffusion from emulsions), microemulsions (microemulsion dilution and microemulsion cooling techniques), micellar solutions (coacer‐ vation technique). Some preparation techniques are based on supercritical fluids. The most important techniques which involve the use of particular instrumentation are: membrane contactor technique, spray-drying, spray-congealing and electrospray.

In Table 1 a scheme of the main preparation techniques of solid lipid nano-and microparticles is reported.


**Table 1.** Preparation techniques of solid lipid nano-and microparticles

#### **2. Emulsion precursors**

The most important precursor used for the preparation of solid lipid particles is the emulsion.

Emulsion consists of a mixture of two immiscible liquids (eg. water – W-and oil-O), where one is dispersed in the other in the form of droplets, thanks to the use of emulsifying agent. According to the character of the continuous and dispersed phase, emulsions can be direct (O/ W), reversed (W/O) and multiple (W/O/W and O/W/O).

Emulsions can be used as precursors for solid lipid particles preparation since lipids, that are solid at room temperature, can be heated 5-10 °C above their melting point to obtain a liquid lipid that can be emulsified with water at the same temperature: the obtained hot emulsion is then cooled to room temperature and the droplets solidify in the form of solid lipid particles.

Owing to the size of the hot emulsion droplets, nano-or microparticles can be obtained.

SLN can be obtained from hot O/W nanoemulsion (<1 μm droplet size), prepared through the so called hot homogenisation technique. Different methods can be used to achieve hot homogenisation: high pressure homogenisation, high shear homogenisation, ultrasound homogenisation.

SLM, instead, can be obtained from hot emulsions (>1 μm droplet size) by the so called melt dispersion technique: O/W or multiple W/O/W precursors can be used.

#### **2.1. Hot homogenisation**

There are different techniques for the preparation of solid lipid nano-and microparticles: generally, the preparation of nano-and microparticles requires a dispersed system as precursor or template, otherwise particles are produced through the use of a particular instrumentation. The most important precursor are emulsions (hot homogenisation technique, melt dispersion technique, PIT method, solvent evaporation-diffusion from emulsions), microemulsions (microemulsion dilution and microemulsion cooling techniques), micellar solutions (coacer‐ vation technique). Some preparation techniques are based on supercritical fluids. The most important techniques which involve the use of particular instrumentation are: membrane

In Table 1 a scheme of the main preparation techniques of solid lipid nano-and microparticles

Hot homogenisation (high pressure

homogenisation, high shear homogenisation,

Membrane contactor technique SLN Cryogenic micronisation SLM Spray-drying SLM Electrospray SLN/SLM Spray-congealing SLM

SLN

SLN/SLM

**Precursor system Technique Nanoparticle type**

ultrasound homogenisation)

Emulsion Melt dispersion SLM Emulsion PIT method SLN Microemulsion Microemulsion dilution SLN Microemulsion Microemulsion cooling SLN Micellar solution Coacervation SLN Organic solvent solution Solvent injection SLN Organic solvent emulsion Solvent evaporation from emulsions SLN Organic solvent emulsion Solvent diffusion from emulsions SLN

contactor technique, spray-drying, spray-congealing and electrospray.

Gas-saturated solution/suspension Particles from gas-saturated solutions/

**Table 1.** Preparation techniques of solid lipid nano-and microparticles

**2. Emulsion precursors**

suspensions

Supercritical fluid emulsion Supercritical fluid extraction of emulsions SLN

The most important precursor used for the preparation of solid lipid particles is the emulsion. Emulsion consists of a mixture of two immiscible liquids (eg. water – W-and oil-O), where one is dispersed in the other in the form of droplets, thanks to the use of emulsifying agent.

is reported.

52 Application of Nanotechnology in Drug Delivery

Emulsion

#### *2.1.1. High pressure homogenisation*

High pressure homogenisation (HPH) was applied for the first time in the nineties by Müller and Lucks [4] for SLN production and represents the main method established for them.

In the HPH technique the drug is dissolved in the lipid being melted at approximately 5-10 °C above its melting point. The drug-containing melt is dispersed under stirring in a hot aqueous surfactant solution of identical temperature, in order to obtain pre-emulsion, which is then homogenised using a piston-gap homogeniser; the produced hot O/W nanoemulsion is cooled down to room temperature, the lipid re-crystallises and leads to SLN [1].

Practically, SLN can be derived from the emulsions for parenteral nutrition just by replacing the liquid lipid (oil) of the emulsion droplets by a solid lipid. In contrast to emulsions for parenteral nutrition which are normally stabilised only by lecithin, the SLN are stabilised also by other surfactants or polymers which can act as a suspending agent in order to avoid particle aggregation.

This is a technique well established on the large scale for parenteral emulsions since the fifties and it is already available in the pharmaceutical industry. The production lines for parenteral emulsions are in most cases equipped with temperature control units because an increased temperature facilitates emulsion production, this means that existing production lines can be used for producing SLN by the hot homogenisation technique [1]. This technique allows the production of colloidal suspensions of SLN without solvents, which are generally used for polymeric nanoparticles, and this is a further advantage of this method from a toxicological point of view.

An important problem of SLN prepared through hot homogenisation is drug expulsion from nanoparticles, that can happen during lipid re-crystallisation on cooling of the hot nanoemul‐ sion. Consequently, in order to improve drug loading within nanoparticles and to prevent drug leakage during storage, a new generation of lipid nanoparticles has been prepared [5,6]. NLC (nanostructured lipid carriers) are characterised by a solid lipid core consisting of a mixture of solid and liquid lipids: the resulting matrix of the lipid particles shows a melting point depression compared to the original solid lipid, but the matrix is still solid at body temperature. Depending on the method of production and on the lipid blend composition, different types of NLC are obtained: imperfect, amorphous and multiple type. In the imperfect type, lipid crystallisation is altered by small amounts of oils. In the amorphous type, the lipid matrix is solid but not crystalline (amorphous state): this can be achieved by mixing special lipids, e.g. hydroxyoctacosanyl hydroxystearate with isopropyl myristate. In the multiple type the solid lipid matrix contains tiny oil compartments: they are obtained by mixing a solid lipid with a higher amount of oil. The basic idea is that by giving a certain nanostructure to the lipid matrix, the payload for active compounds is increased and expulsion of the compound during storage is avoided [5,6].

In the former case, lipophilic drug is dissolved into the melted lipids, which are emulsified with a hot surfactant solution through a high shear mixer: the obtained O/W emulsion is then

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In the latter case, a heated hydrophilic drug solution is emulsified with a melted lipid in order to form a primary W/O emulsion, which is then put in contact with an external aqueous phase at the same temperature; the resulting W/O/W multiple emulsion is then cooled to room

The PIT (phase inversion temperature) method is commonly used for the preparation of nanoemulsions. The PIT concept uses the specific ability of some polyethoxylated surfactants to modify their affinities for water and oil as a function of the temperature. In the PIT nanoe‐ mulsion preparation method, the use of such surfactant type leads to an emulsion inversion from O/W macroemulsion to a W/O emulsion when temperature is increased above the PIT, and to the formation of a O/W nanoemulsion when the temperature is next lowered below the

The PIT method has been used also for the preparation of LNC (lipid nanocapsules), nano‐ particles organised in an internal liquid or semiliquid oil core and an external lipid layer solid

Recently it has been adapted for the preparation of SLN. In this case two main components are used: an oil phase, constituted by solid lipids and non-ionic surfactants and an aqueous phase containing NaCl. The aqueous phase and the oil phase are separately heated at ~90° C (above the PIT); then the aqueous phase is added dropwise, at constant temperature and under agitation, to the oil phase, in order to obtain a W/O emulsion. The mixture is then cooled to room temperature under slow and continuous stirring. At the PIT, the turbid mixture becomes clear, then below the PIT an O/W nanoemulsion is formed, which turns in SLN below the lipid

Another precursor which can be used for SLN preparation is the microemulsion.

adding an alcohol as a cosurfactant, leading to a transparent, stable formulation.

Microemulsions are clear, thermodynamically stable, isotropic liquid mixtures of oil, water and surfactant, frequently in combination with a cosurfactant. The aqueous phase may contain salt(s) and/or other ingredients, and the "oil" may actually be a complex mixture of different

The concept of microemulsion was first introduced by Hoar and Schulman in 1943 [17]; they prepared the first microemulsion by dispersing oil in an aqueous surfactant solution and

The existence of this theoretical structure was later confirmed by the use of various technolo‐ gies, and today we can adopt the definition given by Attwood [18]: "a microemulsion is a

cooled to room temperature to obtain solidification of lipid microparticles [12].

temperature to form microparticles [13].

**2.3. PIT method**

PIT [14].

at room temperature [15].

melting point [16].

**3. Microemulsion templates**

hydrocarbons and olefins.

SLN prepared through hot homogenisation are not suitable for the encapsulation of hydro‐ philic drugs, since they would partition preferentially to the external aqueous phase of the hot nanoemulsion: to overcome this limitation lipid drug conjugate (LDC) nanoparticles were developed. In a typical process, an insoluble drug-lipid conjugate bulk is first prepared either by salt formation (e.g. with a fatty acid) or by covalent linking (e.g. esters or ethers). The obtained LDC is then processed as the lipid for nanoparticles preparation [7,8].

However, besides all its advantages and its versatility, HPH involves some critical process parameters like high temperatures, high pressures (cavitation forces), that may cause signifi‐ cant thermodynamic and mechanical stress for the resulting product: in particular this method is not suitable for thermo-sensitive drugs. Moreover, melted lipids are not good solvents for many drugs and this can cause a low drug loading in the SLN. For these reasons suitable alternative and easy handling production methods for lipid nanoparticles preparation have been widely investigated in last years [9].

#### *2.1.2. High shear homogenisation and ultrasound homogenisation*

High shear homogenisation and ultrasound homogenisation [10,11] are dispersing techniques which were initially used for the production of solid lipid nanodispersions. Both methods are widespread and easy to handle. However, dispersion quality is often compromised by the presence of microparticles. Furthermore metal contamination has to be considered if ultra‐ sounds are used [2]. In the case of high shear homogenisation, SLN can be obtained owing to the use of proper emulsifying agent, which allow the formation of the hot nanoemulsion under simple high shear mixing [10].

#### **2.2. Melt dispersion technique**

In the melt dispersion technique, SLM can be obtained either from O/W or multiple W/O/W emulsions, according respectively to the lipophilic or hydrophilic character of the drug.

In the former case, lipophilic drug is dissolved into the melted lipids, which are emulsified with a hot surfactant solution through a high shear mixer: the obtained O/W emulsion is then cooled to room temperature to obtain solidification of lipid microparticles [12].

In the latter case, a heated hydrophilic drug solution is emulsified with a melted lipid in order to form a primary W/O emulsion, which is then put in contact with an external aqueous phase at the same temperature; the resulting W/O/W multiple emulsion is then cooled to room temperature to form microparticles [13].
