INTRODUCTION:

NEs are emulsions which have droplet sizes between 5-200 nm¹. Generally a typical NE is composed of oil, water and an emulsifier. The emulsifier is added to produce small sized droplets as it decreases the interfacial tension i.e., the surface energy per unit area between the oil and water phases of the emulsion². Due to its very minute droplet size, NE posses high stability as there are reduction in gravitation force to great extent. Moreover, its zigzag Brownian motion helps in cancelling gravity which results in stability against creaming or sedimentation during storage. Likewise, the active pharmaceutical ingredient encapsulated within NE droplets can be protected from oxidation, hydrolysis, and volatilization³.

They are effective delivery systems for cosmetic products, diagnostics purpose, therapeutic use and biotechnology field⁴. Addition to this, they also used as a novel delivery system in food industry, natural colors and flavors⁵. In many cases a co-surfactant or co-solvent is also used in addition to the surfactant, the oil phase and the water phase⁶.

Advantages of nanoemulsion over other dosage forms⁷:

1. NEs can be given by orally, parenterally, transdermally etc.

2. Both hydrophilic and lipophilic drugs can be can deliver by NEs⁸. It protects the drug from hydrolysis and oxidation due to encapsulation in oil-droplet. It also provides taste masking⁹.

3. Rapid and efficient penetration of the drug moiety.

4. Flocculation, aggregation, creaming and coalescence do not occur as NEs are thermodynamically and kinetically stable¹⁰.

5. NEs increase in the rate of absorption due to increased permeation which results in increased bioavailability^11,12.

6. It provides better uptake of oil-soluble supplements in cell culture technology¹³.

Disadvantages of nanoemulsion based systems¹⁴

1. Use of a large concentration of surfactant and co-surfactant necessary for stabilizing the nanodroplets.

2. Limited solubilizing capacity for high-melting substances.

3. The surfactant must be nontoxic for using pharmaceutical applications.

4. Nanoemulsion stability is influenced by environmental parameters such as temperature and pH.

5. These parameters change upon NE delivery to patients.

Formulation consideration of Nanoemulsion:

Three types of NEs are most likely to be formed depending on the composition¹⁵.

a) Oil in water NEs wherein oil droplets are dispersed in the continuos aqueous phase.

b) Water in oil NEs wherein water droplets are dispersed in the continuous oil phase.

c) Bi-continuous NEs where in microdomains of oil and water are interdispersed within the system.

In all three types of NEs, the interface is stabilized by a suitable surfactants and/or co-surfactants.

Components of nanoemulsion¹⁶:

Generally there are following components of NEs.

i. Oil/Lipid

ii. Surfactant/Co-surfactant

iii. Additives

a) Oil:

For the preparation of NEs,The commonly used oils in NEs preparation are IPM (Isopropyl Myristrate), Triacetin (Glyceryl triacetate), Sefsol 218 (Propylene glycol mono ethyl ether) etc. NEs prepared for eye, the viscosity must be between 2-3 centipose (keeping less than 5%). However, for all other medical uses, the amount of oil may be varied but generally is within 5-20% w/w. Sometimes a mixture of oils may be employed to facilitate drug solubility in the oil phase. A Medium chain triglyceride (MCT) is preferred over natural oil with long chain fatty acids. The Oils must be FDA approved GRAS (generally regarded as safe) certified which is to be used in nanoemulsion.

b) Surfactant:

The most frequently used surfactants in NEs formulation are phospholipids (generally from egg yolk sources), copolymers of polyoxyethylene-polyoxypropylene (poloxamer) and to a lesser extent acetylated monoglycerides. Other emulsifiers such as fatty acid esters of sorbitans (various types of Spans; ICI Americas) and polyoxyethylenesorbitans (various types of Tweens; ICI UK), are already approved by various pharmacopeias for parenteral administration and can therefore be considered for emulsion formulation design. Non-ionic surfactants are preferred over ionic surfactant due to lower toxicity. Non-ionic surfactant selection is done on the basis of HLB value given by Griffin in order to classify non ionic Surfactants. In O/W emulsion, non-ionic surfactants have HLB of 8-16 range¹⁷.

c) Additives:

For the the adjustment of pH and tonicity in NEs, additives are used in case of ocular and parentral administration. Usually glycerol is recommended as an isotonic agent in almost every parenteral emulsion while pH is adjusted with an aqueous solution of NaOH or HCl and generally adjusted to 7-8 for physiological compatibility and sustaining physical composition of nanoemulsion by minimizing fatty acid ester hydrolysis. To prevent oxidation, antioxidants or reducing agents such as tocopherols, feroxaminemesylate and ascorbic acid are used. Stabilizing agent like oleic acid or its sodium salt, are added to avoid phase separation¹⁸. Cholic acid, deoxycholic acid and their respective salts are used to enhance NE solubility.

Methods of preparation of nanoemulsions:

NEs can be prepared by using high and low energy methods. In high energy methods, mechanical devices are used to deliver required large amount of disruptive forces whereas in low energy methods, there is no need for an external force. In the preparation of NE, the stored energy of the system is utilized by altering parameters such as temperature, composition of the system by the method employed¹⁹.

High energy methods were only choice for researches in the initial studies and therefore high-energy stirring and ultrasonic emulsification were the most widely used methods²⁰. But because low energy methods are soft, non destructive and cause no damage to the encapsulated molecule, these methods are preferred more²¹.

High-Energy Emulsification Methods:

Generally, NEs are prepared by using high energy methods where mechanical energy input is applied through high pressure homogenizers, high-shear stirring, and ultrasound generators²². These mechanical devices facilitate strong forces that disrupt oil and water phases to form nanoemulsions. In high energy methods, input energy density is about 108-1010 W /kg²³.

High Pressure Homogenization:

It is the most widely used method for the production of NEs. In this method high pressure homogenizer or the piston homogenizer is used to produce NEs of particle size up to 1 nm. During the method, the macroemulsion is forced to pass through in a small orifice at an operating pressure between 500 to 5000 psi²⁴. Since several forces like hydraulic shear, intense turbulence and cavitation act together.

Extremely small droplet sized NEs are achieved. This process is repeated until the final product reaches the desired droplet size and polydispersity index²⁵. Lower polydispersity index (PDI) means higher uniformity of droplet size in NEs²⁶.

Advantage: High pressure homogenization can provide energy required the formulation homogeneous small sized particles in shortest time, it is rapid process.

Disadvantages: The formulation of small droplets require high amount of energy²⁷ which in turn increasing the temperature during high pressure homogenization cause deterioration of the components¹⁹. Thus thermolabile substance like proteins, enzymes may be destroyed^24,28 .

High-Shear Stirring:

This method includes high-energy mixers and rotor-stator systems for the preparation of nanoemulsions where droplet sizes of the internal phase can be significantly decreased by increasing the mixing intensity of these devices to get emulsions with the average droplet size less than 200-300 nm is rather difficult ^29.

Ultrasonic Emulsification:

In this method, premixed macroemulsion is agitated by vibrating solid surface at 29 kHz or larger frequencies generating enormous levels of highly localized turbulence causing micro implosions which disrupt large droplets into sub-micron size²⁶.

Advantage: by this process, it is possible to obtain emulsions with uniform droplet size at dilute concentrations³⁰.

Disadvantage: possibility of protein denaturation, polysaccharide depolymerization and lipid oxidation ^{31, 32.}

Microfluidization:

In this method, a device called microfluidizer is used which provides high pressures. During the process, high pressure forces the macroemulsion to go through to the interaction chamber and thus nanoemulsions with submicron ranged particles can be produced.

Advantage: Uniform NE production can be achieved by repeating the process many times and varying the operating pressure in order to get desired particle size^24,
25.

Low-Energy Emulsification Methods:

Nanomulsions can also be prepared by low-energy methods resulting in small size and more uniform droplets^22,33. Low-energy emulsification methods comprises of spontaneous nanoemulsification, Phase inversion methods and its components like Phase inversion temperature (PIT), Phase inversion composition (PIC)³².

Spontaneous Nanoemulsification:

In this method, the chemical energy is released upon dilution with the continuous phase which takes place usually at constant temperature without any phase transitions in the system during the emulsification process³³.

No special devices are required to produce nanoemulsions at room temperatures. It acts on interfacial tension, viscosity of interfacial and bulk, phase transition region, surfactant structure, and surfactant concentration¹⁹. The systems prepared by using this method in the pharmaceutical industry are usually referred as self-emulsifying drug-delivery systems (SEDDS) or self-nano-emulsifying drug-delivery systems (SNEDDS)³².

Phase Inversion Methods:

Due to phase transitions, chemical energies are produced which is used to produce nanoemulsions. The amount of energy needed is regulated by changing the composition at constant temperature (PIT) or by changing the temperature at constant composition (PIC)³⁴.

Evaluation:

NEs are not thermodynamically stable and because of that, their characteristics will depend on preparation method. Here some parameters are discussed which should be analyzed at the time of preparation of NEs.

1) Phase behavior study:

This study is a characterization and optimization of ingredients (surfactant, oil phase and aqueous phase). Generally the study is necessary in case of nanoemulsion formulation prepared by phase inversion temperature method and self-emulsification method in order to determine the phase of NE and dispersibility. The study is done by placing the different ingredients of NE by varying the concentration in glass ampoules and thoroughly homogenized at a certain temperature for a time until equilibrium and isotropic phase is identified by polarized light³⁵.

2) Droplet size analysis:

Droplet size analysis of NE is measured by a diffusion method using a light-scattering, particle size analyzer counter, LS 230. It is also measured by correlation spectroscopy that analyzes the fluctuation in scattering of light due to Brownian motion. Droplet size analysis of NE can also be performed by transmission electron microscopy (TEM)^36,37.

3) Zeta potential:

Zeta potential is measured by an instrument known as Zeta PALS. It is used to measure the charge on the surface of droplet in NE³⁸.

4) Surface charge measurement:

Surface zeta potentialof nanoemulsion droplets should be measured with the help of mini electrode to predict the surface properties NE³⁷.

5) Drug content:

Preweighed NE is extracted by dissolving in a suitable solvent, extract is analyzed by spectrophotometer or HPLC against standard solution of drug^39,40.

6) Viscosity:

Viscosity should be measured to ensure the better delivery of formulation.

Applications of nanoemulsions:

1) Parenteral Delivery:

Due to requirement of droplet size lower than 1 micrometer, Parenteral (or Injectable) administration of NE is employed for a variety of purpose, mainly nutrition e.g. Fats, carbohydrates, vitamins etc. the nanoemulsions of natural oils (soyabean, sesame and olive oil) with the non-toxic surfactant³⁵.

2) Oral Delivery:

NEs have been proven ideal delivery of drugs such as steroids, hormones, diuretic and antibiotics due to increased absorption, improved clinical potency and decreased drug toxicity when given orally⁴¹.

3) Topical Delivery:

Topical administration of drugs can have advantages over other methods for several reasons, one of which is the avoidance of hepatic first pass metabolism of the drugs and related toxicity effects. Another is the direct delivery and targetability of the drug to affected area of the skin or eyes⁴².

4) Ocular Delivery:

For the treatment of eye diseases, drugs are essentially delivered topically. O/W NEs have been investigated for ocular administration, to dissolve poorly soluble drugs, to increase absorption and to attain prolong release profile⁴³.

5) In Cosmetic:

The aesthetic properties, i.e. low viscosity and transparent visual aspects of NE with droplet sizes below 200 nm and its high surface area allowing effective transport of the active ingredient to the skin make them especially attractive for their application in cosmetics. Moreover, NEs are acceptable in cosmetics because there is no inherent creaming, sedimentation, flocculation or coalescence^44,45.

6) Transdermal:

NE can be used for transdermal drug delivery due to increase the permeation through the skin and are also it is non-irritant^46,47.

7) In Biotechnology:

Many enzymatic and biocatalytic reactions are conducted in pure organic or aqua-organic media. Biphasic media are also used for these types of reactions. The use of water-proof media is relatively advantageous. Enzymes in low water content display and have –

a) Increased solubility in non-polar reactants.

b) Possibility of shifting thermodynamic equilibrium in favor of condensations.

c) Improvement of thermal stability of the enzymes, enabling reactions to be carried out at higher temperatures⁴⁸.

8) NE in treatment of various other disease conditions:

‘Pharmos’ (US-based company has developed the NE topical diclofenac cream as potential treatment for osteoarthritis (OA) pain. diclofenac is also being considered as treatment of soft tissue injury, sprain and strain⁴⁹.

9) Self nanoemulsifying drug delivery systems (SNEDDS) for oral delivery of protein drugs:

Formulation development, in-vitro transport study and in-vivo oral absorption study⁵⁰. An experimental design was adopted to develop SNEDDS for non-invasive delivery of protein drugs. Fluoroscent labeled beta-lactamase (FITC-BLM), a model protein was loaded into SNEDDS through solid dispersion technique⁵¹.

10) Nanoemulsion as Mucosal Vaccines:

NEs can be used for needle free immunization by delivering recombinant protein and inactivated organism to a mucosal surface as NE cause the protein surface to be adjuvanted and thus facilitates uptake by antigen-presenting cells⁵².

FUTURE PERSPECTIVES:

Since, NE right from emergence proved to be versatile and useful novel drug delivery system which is applicable for almost all routes of administration. Future perspectives of NEs are very promising in different fields of therapeutics, whether it is against development of vaccines or formulation against cancer along with the increasing application in development of cosmetics for hair or skin companies like Loreal is spending much on such innovative projects.

ACKNOWLEDGEMENT:

Authors are thankful to Vice Chancellor of Integral University, Lucknow Uttar Pradesh, India for valuable guidance (Manuscript Communication No. IU/R&D/2018-MCN000259).

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Received on 13.07.2018 Modified on 11.09.2018

Research J. Pharm. and Tech 2018; 11(11): 5191-5196.

DOI: 10.5958/0974-360X.2018.00948.4