Solid lipid nanoparticles are composed of high melting point lipid as a solid core coated by surfactants. The solid core allows the prolonged and controlled release of drugs and may protect incorporated drugs against chemical degradation. Lipid and surfactant chemical natures are important in drug loading capacity. This affect size distribution and physical stability ⁵ as well as improved bioavailability of poorly water soluble molecules. Disadvantages of SLNs includes Poor drug loading capacityand drug expulsion after polymeric transition during storage.⁶

Preparation methods of SLNs

Solid lipid nanoparticles system consists of spherical solid lipid particles in the nanometer ranges, which are dispersed in water or in aqueous surfactant solution. Generally, they are made of solid hydrophobic core having a monolayer of phospholipids coating. The solid core contains the drug dissolved or dispersed in the solid high melting fat matrix. The hydrophobic chains of phospholipids are embedded in the fat matrix. They have potential to carry lipophilic or hydrophilic drugs or diagnostics.

High pressure homogenization (HPH) technique

This technique is well established at large since fifties and still being used by the pharmaceutical industries. It has emerged as the most extensively used technique for the preparation of SLNs. It makes use of high pressure homogenizer which is accessible from several manufacturers. High pressure homogenizers push a liquid with high pressure (100-2000 bar) thorough a narrow gap of size of few microns. Previously this technique was used for manufacturing of nanoemulsions used for parenteral nutrition. In contrast to emulsions for parenteral nutrition which are normally stabilized by lecithin, the SLNs can be stabilized by other surfactants or polymers and their mixtures.

The two basic production methods for SLNs are as follows-

Hot homogenization technique.

Cold homogenization technique

For both techniques the drug is dispersed or solubilize in the lipids above their melting points ⁷

Hot homogenization technique

Hot homogenization is generally carried out at temperatures above the melting point of the lipid. A pre-emulsion of the drug loaded lipid melt and the aqueous emulsifier phase (same temperature) is obtained by high shear mixing device. The resultant product is hot o/w emulsion and the cooling of this emulsion leads to crystallization of the lipid and the formation of SLNs. Smaller particle sizes are obtained at higher processing temperatures because of lowered viscosity of the lipid phase. However, high temperature leads to the degradation rate of the drug and the carrier. Increasing the homogenization temperature or the number of cycles often results in an increase of the particle size due to high kinetic energy of the particles. Generally, 3-5 homogenization cycles at a pressure of 500-1500 bar are used.⁶

Cold homogenization technique .⁹

Cold homogenization has been developed to over-come the temperature related degradation problems, loss of drug into the aqueous phase and partitioning associated with hot homogenization method. Unpredictable polymeric transitions of the lipid due to complexity of the crystallization step of the nanoemulsion resulting in several modifications and/or super cooled melts. Here, drug is incorporated into melted lipid and the lipid melt is cooled rapidly using dry ice or liquid nitrogen. The solid material is ground by a mortar mill. The prepared lipid microparticles are then dispersed in a cold emulsifier solution at or below room temperature. The temperature should be regulated effectively to ensure the solid state of the lipid during homogenization. However, compared to hot homogenization, larger particle sizes and a broader size distribution are typical of cold homogenization samples.^{6, 8, 9}

Advantages

· Low capital cost.

· Demonstrated at lab scale.

Disadvantages

· Energy intensive process.

· Demonstrated at lab scale bimolecular damage.

· Polydisperse distributions.

· Unproven scalability

Ultrasonication/high speed homogenization

A. Probe ultrasonication

B. Bath ultrasonication

SLNs are also prepared by ultrasonication or high speed homogenization techniques. For smaller particle size combination of both ultrasonication and high speed homogenization is required. The advantages are reduced shear stress. However, this method suffers from problems such as broader size distribution ranging into micrometer range. Potential metal contaminations, physical instability like particle growth upon storage are other drawbacks associated with this technique.¹⁰

SLNs prepared through micro emulsion technique ⁸

This method is based on the dilution of micro emulsions. As micro-emulsions are two-phase systems composed of an inner and outer phase (e.g. o/w micro emulsions). They are made by stirring an optically transparent mixture at 65-70°C, which typically composed of a low melting fatty acid (e.g. stearic acid), an emulsifier (e.g. polysorbate 20), co-emulsifiers (e.g. butanol) and water. The hot microemulsion is dispersed in cold water (2-3°C) under stirring. SLN dispersion can be used as granulation fluid for transferring into solid product (tablets, pellets) by granulation process, but in case of low particle content too much of water needs to be removed. High-temperature gradients facilitate rapid lipid crystallization and prevent aggregation. Due to the dilution step; achievable lipid contents are considerably lower compared with the HPH based formulations

Advantages

Low mechanical energy input.

Theoretical stability

Disadvantages

Extremely sensitive to change.

Labor intensive formulation work.

Low nanoparticles concentrations

Supercritical Fluid technology

This is a novel technique recently applied for the production of SLNs. A fluid is termed supercritical when its pressure and temperature exceed their respective critical value. The ability of the fluid to dissolve compounds increases. This technology comprises of several processes for nanoparticles production such as rapid expansion of supercritical solution (RESS), particles from gas saturated solution (PGSS), aerosol solvent extraction solvent (ASES), supercritical fluid extraction of emulsions (SFEE). The advantages of this technique includes avoidance of the use of solvents, particles obtained as a dry powder, instead of suspensions, requires mild pressure and temperature conditions. Carbon dioxide solution is the good choice as a solvent for this method .^{6`, 11}

Solvent emulsification-evaporation technique

In solvent emulsification-evaporation method, the lipophilic material and hydrophobic drug were dissolved in a water immiscible organic solvent (e.g. cyclohexane, dichloromethane, toluene, chloroform) and then that is emulsified in an aqueous phase using high speed homogenizer. To improve the efficiency of fine emulsification, the coarse emulsion was immediately passed through the micro fluidizer. Thereafter, the organic solvent was evaporated by mechanical stirring at room temperature and reduced pressure (e.g. rotary evaporator) leaving lipid precipitates of SLNs .¹² Here the mean particle size depends on the concentration of lipid in organic phase. Very small particle size could be obtained with low lipid load (5%) related to organic solvent.

Solvent emulsification-diffusion technique

In solvent emulsification-diffusion technique, the solvent used (e.g. benzyl alcohol, butyl lactate, ethyl acetate, isopropyl acetate, methyl acetate) must be partially miscible with water and this technique can be carried out either in aqueous phase or in oil. Initially, both the solvent and water were mutually saturated in order to ensure the initial thermodynamic equilibrium of both liquid. When heating is required to solubilize the lipid, the saturation step was performed at that temperature. Then the lipid and drug were dissolved in water saturated solvent and this organic phase (internal phase) was emulsified with solvent saturated aqueous solution containing stabilizer (dispersed phase) using mechanical stirrer. After the formation of o/w emulsion, water (dilution medium) in typical ratio ranges from 1:5 to 1:10, were added to the system in order to allow solvent diffusion into the continuous phase, thus forming aggregation of the lipid in the nanoparticles. Here the both the phase were maintain at same elevated temperature and the diffusion step was performed either at room temperature or at the temperature under which the lipid was dissolved. Throughout the process constant stirring was maintained. Finally, the diffused solvent was eliminated by vacuum distillation or lyophilization.¹³

Double emulsion method

In double emulsion technique the drug (mainly hydrophilic drugs) was dissolved in aqueous solution, and then was emulsified in melted lipid. This primary was stabilized by stabilizer. Then this stabilized primary emulsion was dispersed in aqueous phase containing hydrophilic emulsifier. Thereafter, the double emulsion was stirred and was isolated by filtration. Double emulsion technique avoids the necessity to melt the lipid for the preparation of peptide-loaded lipid nanoparticles and the surface of the nanoparticles could be modified in order to sterically stabilize them by means of a lipid-PEG derivative. A major drawback of this is the formation of high percentage of micro particles.¹⁴

Spray drying method

It is an alternative technique to the lyophilization process. This recommends the use of lipid with melting point more than 70°C. The best results were obtained with SLN concentration of 1% in a solution of trehalose in water or 20% trehalose in ethanol-water mixture.⁸

Solvent injection technique

Here, the solid lipid is dissolved in water miscible solvent. The lipid solvent mixture is injected into stirred aqueous phase with or without surfactant. Finally, the dispersion filtered to remove excess lipid. Emulsion within the aqueous phase helps to produce lipid droplets at the site of injection and stabilize SLNs until solvent diffusion gets completed (Schubert et al., 2003) and (Mishra et al. 2010) prepared and evaluated SLNs using Solvent injection method for delivery of Hepatitis B surface antigen for vaccination using subcutaneous route.^15,
16

Film-ultrasound dispersion

Here, the lipid and the drug were put into suitable organic solutions, after decompression, rotation and evaporation of the organic solutions, a lipid film is formed, then the aqueous solution which includes the emulsions was added. Using the ultrasound with the probe to diffuser at last, the SLN with the little and uniform particle size is formed.⁸

Characterization of solid lipid nanoparticles (SLNs)

The methods for the characterization should be perceptive to the key parameters of the performance of SLNs. Several parameters which have to be considered in characterization include particle size, size distribution kinetics (zeta potential), degree of crystallintity and lipid modification (polymorphism), coexistence of additional colloidal structures (micelles, liposome, super cooled melts, drug nanoparticles), time scale of distribution processes and surface morphology.

Particle size and Zeta potential

Size of nanoparticles can be determined by several methods such as photon-correlation spectrometry (PCS), transmission electron microscopy (TEM), and scanning electron microscopy (SEM), SEM combined with energy-dispersive X-RAY spectrometry, scanned probe microscopy and fraunhofer diffraction. Among these, the most widely used techniques are PCS and electron microscopy methods. SEM and TEM are very useful in determining the shape and morphology of lipid nanoparticles and also allow determination of particle size and distribution. The particle size determination by photon correlation spectroscopy (PCS) detects size range of 3nm to 3μm and by laser diffraction in size range of 100 nm to 180 μm. Although PCS is a good tool to characterize nano-particles, but is capable for the detection of larger microparticles.¹⁷ Zeta potential measurement can be carried out using zeta potential analyzer or zeta meter. Before measurement, SLN dispersions are diluted 50-fold with the original dispersion preparation medium for size determination and zeta potential measurement. Higher value of zeta potential may lead to disaggregation of particles in the absence of other complicating factors such as steric stabilizers or hydrophilic surface appendages. Zeta potential measurements allow predictions about the storage stability of colloidal dispersions.^{6, 18}

Electron Microscopy

Electron Microscopy methods such as Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) are used to measure the overall shape and morphology of lipid nanoparticles. It permits the determination of particle size and distributions.SEM uses electrons transmitted from the surface of the sample while TEM uses electrons transmitted through the sample⁹

Atomic Force Microscopy (AFM)

It is an advanced microscopic technique which is applied as a new tool to image the original unchanged shape and surface properties of the particles. AFM measures the force acting between surface of the sample and the tip of the probe, when the probe is kept in close proximity to the sample which results in a spatial resolution of up to 0.01 nm for imaging.⁹

Degree of crystallinity

It can be measured by X-ray diffraction (powder X-ray diffraction) .The geometric scattering of radiation from crystal planes within a solid allow the presence or absence of the former to be determined thus permitting the degree of crystallinity to be assessed. Another method that is a little different from its implementation with bulk materials, DSC can be used to determine the nature and speciation of crystallinity within nanoparticles through the measurement of glass and melting point temperatures and their associated enthalpies.¹⁹

Acoustic methods

Another ensemble approach, acoustic spectroscopy, measures the attenuation of sound waves as a means of determining size through the fitting of physically relevant equations. In addition, the oscillating electric field generated by the movement of charged particles under the influence of acoustic energy can be detected to provide information on surface charge.⁶

Nuclear magnetic resonance (NMR)

NMR can be used to determine both the size and the qualitative nature of nanoparticles. The selectivity afforded by chemical shift complements the sensitivity to molecular mobility to provide information on the physicochemical status of components within the nanoparticle.⁶

Sterilization of SLN

For intravenous and ocular administration SLN must be sterile. The temperature reach during sterilization by autoclaving presumably causes a hot o/w micro emulsion to form in the autoclave, and probably alters the size of the hot nanoparticles. On subsequent slow cooling, the SLN reformed, but some nanodroplets may coalesce, producing larger SLN than the initial ones. SLN are washed before sterilization, amounts of surfactants and co surfactants present the hot systems are smaller, so that the nanodroplets may be not sufficiently stabilized.^{20, 21}

Storage stability

The physical stability of the SLNs during prolonged storage can be determined by monitoring changes in particle size, drug content, appearance and viscosity. This can also be done by thin layer chromatography.^{22, 23}

Pharmaceutical Applications of Solid Lipid Nanoparticles (SLNs)

Oral administration

Solid lipid nanoparticles might be an interesting carrier system for per oral administration of poorly water soluble drugs with low per oral bioavailability. Oral administration of SLNs is possible as aqueous dispersion or in a traditional dosage form i.e. tablets, pellets, capsules or powders in sachets. The poor absorption of certain drugs can be related to their poor wettability, so that incorporation of drugs into solid lipid nanoparticles provides completely wettable carriers. The lipid particles undergo digestion similarly to food lipids. Due to high dispersivity of solid lipid nanoparticles, they exhibit a high specific surface area for enzymatic attack by intestinal lipases.²⁴ This enzymatic degradation of the lipids leads to release of incorporated drugs in molecularly dispersed form. The bile salts facilitate their solubilization in the intestine and subsequent absorption.

SLNs for Nasal Application ^{6, 25, 26}

Nasal administration was a promising alternative noninvasive route of drug administration due to fast absorption and rapid onset of drug action, avoiding degradation of labile drugs (such as peptides and proteins) in the GI tract and insufficient transport across epithelial cell layers. In order to improve drug absorption through the nasal mucosa, approaches such as formulation development and prodrug derivatization have been employed. SLN has been proposed as alternative transmucosal delivery systems of macromolecular therapeutic agents and diagnostics by various research groups. In a recent report, coating polymeric nanoparticles with PEG gave promising results as vaccine carriers.

SLNs as cosmeceuticals

The SLNs have been applied in the preparation of sunscreens and as an active carrier agent for molecular sunscreens and UV blockers. SLN and NLCs have proved to be controlled release innovative occlusive topical. Better localization has been achieved for vitamin A in upper layers of skin with glyceryl behenate SLNs compared to conventional formulations ²⁷

Solid lipid nanoparticles (SLNs) as targeting carriers

The extremely small particle size of solid lipid nanoparticles, which are less than 50 nm, might be beneficial with respect to drug targeting. Small carrier size generally favors reduced uptake by the reticuloendothelial system. Moreover after intravenously administration particles smaller than the fenestration of the endothelial wall i.e. below 150 nm might be able to leave the vascular compartment, through these fenestrae in the sinusoids of the liver spleen and bone marrow or at the location where the basal membrane of the endothelium is damaged; for example, at the site of inflammation or in tumor tissues. Drug targeting might also be possible by surface modification of solid lipid nanoparticles.²⁸

Application of SLNs in cancer chemotherapy

From the last two decades several chemotherapeutic agents have been encapsulated in SLNs and their in-vitro and in-vivo efficacy have been evaluated. Outcomes of these studies have been shown to improve the efficacy of chemotherapeutic drugs, simultaneously reduction in side effects associated with them. Improved stability of drugs, encapsulation of chemotherapeutic agents of diversified physicochemical properties, enhanced drug efficacy, improved pharmacokinetics and less in-vitro toxicity are the important features of SLN which make them a suitable carrier for delivering chemotherapeutic drugs. Several obstacles frequently encountered with anticancer compounds, such as normal tissue toxicity, poor specificity and stability and a high incidence of drug resistant tumor cells, are at least partially overcome by delivering them using SLN. The rapid removal of colloidal particles by the macrophages of the RES is a major obstacle to targeting tissues elsewhere in the body, such as bone marrow and solid tumors.⁸

CONCLUSION:

SLNs as colloidal drug carrier combines the advantage of polymeric nanoparticles, fat emulsions and liposome; due to various advantages, including feasibility of incorporation of lipophilic and hydrophilic drugs, improved physical stability, low cost, ease of scale-up, and manufacturing, SLNs are prepared by various advanced techniques. The site specific and sustained release effect of drug can better achieved by using SLNs. Nanoparticles have been used extensively for applications in drug discovery, drug delivery, and diagnostics and for many others in medical field. They are relatively novel drug delivery systems, having received primary attention from the early 1990s. Safety aspects and biodegradability reports revealed the SLN technology as a powerful tool which will serve and carve the niche among other conventional delivery systems for next coming decades. However, a further research for the validation on the toxicological profile, interaction mechanism of drugs, foreign bodies with the lipid matrix core and therapeutic safety at the cellular level is warranted in the long run for system suitability.

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Received on 25.02.2013

Modified on 20.03.2013

Accepted on 25.03.2013

Research Journal of Pharmaceutical Dosage Forms and Technology. 5(2): March- April, 2013, 56-61