Nanoemulsion Drug Delivery System: A Review

Nano-emulsion dosage forms have nano-sized droplets of disperse phase and are kinetically stable dosage form. Nano-emulsions are included under the category of new drug delivery system containing emulsified water in oil/oil in water system having mean globule size ranges from 10 nm to 1000 nm.  In the field of pharmacy, nano-emulsions play an essential role in the delivery of medication through various drug administration routes like parenteral, topical and oral route. Nano-emulsions are nano-sized emulsions which are used under high investigation as a drug carrier for enhancing the delivery of therapeutic agents. Nano-emulsions have enhanced functional properties as compared to standard emulsions. They are nowadays growing work for utilizing nano-sized particles in the research of pharmaceuticals, cosmetics and food products.  Mainly, intrigue has been creating simultaneously with higher emulsification techniques and mechanisms of stabilization. Nano-emulsions are formulated by both methods like high energy emulsification or low energy emulsification methods. Rapid energy emulsification technique includes high shear mixing, high-pressure homogenization or ultrasonication. In contrast, low energy emulsification technique includes the merit of the physicochemical characteristics of the system, which exploits phase transitions to obtained nano-emulsion. This review article is an effort to summarize comparative aspects like introduction, types, advantages, disadvantages, components, factors affecting, methods of preparations, methods of analysis of nano-emulsion and applications of nano-emulsion.


INTRODUCTION
The drugs moiety included in Biopharmaceutical Classi ication System Class II and Class IV has low aqueous solubility and remedy for the solubility problems are nanotechnology techniques. Nanotechnology techniques play an essential role at the molecular level and nano length scale size. To increase the dissolution rate of drug moiety, which directly leads to an increase in the bioavailability of drug depends on decrease drug particles into the nanoscale range. (Jain et al., 2013) The nanoemulsion dosage form is considered as thermodynamically stable dosage form consisting of a clear solution of two immiscible liquids like water and oil, stabilized by an interfacial ilm coat of surface-active agent molecules. Nano-emulsions are included under the category of novel drug delivery system containing emulsi ied water in oil/oil in water system having mean globule size ranges from 10 nm to 1000 nm. Due to nano-globules size of the disperse phase, nano-emulsions are different from emulsions. (Basha et al., 2013) Nano-emulsions are also called colloidal disperse system containing surfactant, co-surfactant, an oil phase and aqueous phase in right proportions. Nano-emulsions are formulated by both methods like high energy emulsi ication or low energy emulsi ication methods. (Kumar and Singh, 2012) Nanoemulsion is considered generally as safe by the FDA, which includes non-irritant and non-toxic. (Solè et al., 2010) Various types of oils like synthetics, semi-synthetics and natural oils are used in the manufacturing of nano-emulsions. Emulsion with the nanoscale size is prepared by using emulsifying agents that are mainly considered as secure and safe for human use. The kind of surfactant and its quantity in the aqueous phase are selected in such a manner to achieve better stability against coalescence. Nano-emulsions have better stability as compare to macroemulsion or microemulsion against locculation of disperse phase, creaming, sedimentation and coalescence. (Thiagarajan, 2011) Types of Nanoemulsions 1. Oil in Water Nanoemulsions: Oil phase considered as a disperse phase and aqueous phase is considered a continuous phase.
2. Water in Oil Nanoemulsions: Oil phase considered as a disperse phase, and the aqueous phase is considered a continuous phase.
3. Bi-continuous Nanoemulsions: Microdomains of oil and water are interspersed within the system.
In all these three types of nano-emulsions, the interface is made stabilized by a right proportion of surfactants and co-surfactants.
The signi icant difference between nano-emulsions and macroemulsions, Nanoemulsion dosage form, are considered as thermodynamically stable dosage form consisting of a clear solution of two immiscible liquids like water and oil, stabilized by an interfacial ilm coat of surface-active agent molecules.
Other differences in their appearance, i.e. nanoemulsions are transparent or translucent biphasic liquid dosage form. Nanoemulsion needs a large amount of energy for its preparation as compared to macroemulsion. (Date and Nagarsenker, 2008) Advantages of Nanoemulsions 1. Nanoemulsions dosage forms are considered as kinetically and thermodynamically stable dosage form against locculation of disperse phase, creaming, sedimentation and coalescence.
2. Nanoemulsion is a technique to increase the dissolution rate of drug moiety, which directly leads to an increase in the bioavailability of lipophilic drugs.
3. Nanoemulsion can be given by various routes like topical, transdermal, oral, parenteral etc.
4. Nano-emulsions can be used for administration of both lipophilic and hydrophilic drugs.
5. Nano-emulsions are capable of delivering peptides that are prone to enzymatic hydrolysis in the gastrointestinal tract.
6. Due to nano-size of a droplet of the disperse phase, which increases surface area and therefore enhances the availability of the drug in the systemic circulation.
8. Nano-emulsions also increase skin permeation of drugs.
2. Change in pH and temperature may affect the stability of nano-emulsions.
3. Due to Oswald ripening effect, instability of nano-emulsion may be observed.
4. Cost of nano-emulsion is more because of the size reduction of disperse phase globules. (Bhatt and Madhav, 2011)

Components of Nanoemulsion
Various components of nano-emulsion are as follows, (Reza, 2011)

Oils
The choice of a correct oily or disperse phase is signi icant because it may affect the choice of another excipient of nano-emulsions, mainly just in case of oil in water type of nano-emulsions. Generally, the oil having the best solubilizing property can be referred to as a dispersed phase for the preparation of nano-emulsions. These provide to get increased drug-carrying within the nano-emulsions. Naturally developed oils and fats include a mixture of triglycerides that contain fatty acids of varying chain lengths and degrees of unsaturation. Triglycerides are divided as long-chain (>12 carbons), mediumchain (6 to 12 carbons) and short-chain (<5 carbons) and should be synthetically hydrogenated to minimize the degree of unsaturation, thereby conferring resistance to oxidative degradation. The choice of the oily phase is additionally a compromise between its capability to solubilize the drugs moiety and its capability to ease the preparation of nanoemulsion of required properties. Various oil phases are Myritol 318, modi ied vegetable oils, Captex 355, IPM, digestible or non-digestible oils and fats like hydrogenated as well as non-hydrogenated soybean oil, olive oil, oleic acid, palm oil, peanut oil, beeswax and sesame oil.

Surfactants
The surfactant should be able for micro emulsi ication of the oily or disperse phase and will also have the best solubilizing property for the aqueous insoluble drug moiety. The selection of the surfactants is very important for the nano-emulsion preparation. A surface-active agent having an HLB value less than ten-value are water insolubility like sorbitan monoesters and get water in oil type of nano-emulsion and surfactants having higher HLB value (>10) like polysorbate 80 are water-soluble and form oil in water type of nano-emulsion. The hydrophobic core increases the entrapment of drug, hence increasing its solubility. When the oil content is more, surfactant concentrates on the oil/water interface forming emulsions, and thus the drug is solubilized in the internal oil phase. On the other hand, when the oil content is less, minute oil entrapped surfactant globules are developed, which are referred to as nano-emulsions.
The surfactant utilized in nano-emulsion formation might be ionic or non-ionic surfactants aren't preferred due to its toxic effects. Many surfactants are mostly utilized like polysorbate 80, poloxamers and lecithins. Determination of surfactant concentration is crucial because a large quantity of it may act gastrointestinal irritation. The size of the globule and concentration of surface-active agents are interrelated. In some instances, greater the surfactant concentration may lead to smaller globules like within the case of a mixture of saturated C8-C10 polyglycolized glycerides. However, by enhancing the concentration of surfactant, the mean globule size also is increased.

Co-surfactants
Various times, surfactant individually cannot decrease the oil-water interfacial or surface tension completely to get a nano-emulsion which necessitates the addition of a co-surfactant to bring about the interfacial tension on the brink of zero. Co-surfactants enter into the monolayer of surfactant, giving additional luidity to interfacial ilm and hence distracting the liquid crystalline phases developed when surfactant ilm is just too rigid. Generally, a lower HLB co-surfactant is employed with a higher HLB surfactant to switch or adjust the overall HLB value of the system. Similar to surfactant, the co-surfactant might not be able to form self-associated structures like micelles on its own. Hydrophilic co-surfactants mainly alcohols of intermediate chain length like pentanol, octanol and hexanol, which are known to scale back the oil/water interface and permit the rapid preparation of nano-emulsion.

Factors affecting the Formulation of Nanoemulsion
1. The emulsifying agent is the vital agent of any nano-emulsion. They ought to not form lyotropic liquid crystalline "micro-emulsions" phases. Systems including short-chain alkanes, water, surfactants and alcohols form the phases which are usually used with the co-surfactant.
2. The desired quantity is required to avoid Oswald ripening, and therefore the dispersed phase should be highly insoluble in the dispersion medium.
3. The existence of more amounts of surfaceactive agents develops a new surface area of nanoscale to be fast coated during emulsiication, thereby prohibiting induced coalescence. (Thakur et al., 2013)

METHODS OF FORMULATION OF NANOEMULSION
There are various methods available to prepare nanoemulions. A comparatively higher amount of energy is needed for the development of nanoemulsion than regular emulsions or other dosage forms. Various methods utilized for the formulation of nano-emulsion are:

Sonication technique
In the sonication technique, the size of the globules of a normal emulsion is made compact by using sonication mechanism. This method is utilized to develop a few quantities of batches of nanoemulsion. (Shah et al., 2010)

High-Pressure Homogenizer
In this technique, a huge amount of pressure is applied to the system containing aqueous or oil phase and co-surfactant or surfactant. The higher pressure is applied by using a homogenizer. Homogenization may sometimes cause certain problems like poor productivity, component deterioration, which are a result of the high-pressure application. By this technique, only oil/water type of nanoemulsion having concentration lower than 20% oily phase may be prepared. Higher viscosity cream nano-emulsion and thickness having a mean globule diameter lower than 200 nm couldn't be formulated. (Pratap et al., 2012)

Phase Inversion technique
This technique was based on the mechanism of changes of solubility of a surface-active agent such as polyoxyethylene with temperature. This surfaceactive agent is initially insoluble in lipids but changes into lipid-soluble by increasing temperature due to polymer chain dehydration. At a lower temperature, the surface-active agent monolayer has a better positive, spontaneous curvature producing oil swollen micellar solution phase. (Pey et al., 2006)

Production with high amplitude ultrasound
This technique is nowadays used as a substitute against high-pressure homogenization for small scale production of nano-emulsion. For Nanoemulsi ication, more shear forces are essential which are developed by ultrasonic cavitations. This develops violently and asymmetrically imploding vacuum bubbles and decreases molecule length to the nanometer scale. For small scale manufacturing of nano-emulsions, this technique is ef icaciously used. (Jafari et al., 2007)

Solvent Displacement technique
In this technique, the non-aqueous phase is mixed with various water-miscible organic solvents like ethanol, acetone, etc. The aqueous and organic phases are mixed with the help of emulsifying agents to develop nano-emulsion by using rapid diffusion of organic solvent. Vacuum evaporation technique is then utilized to evaporate the organic solvent from the mixture. (Narang et al., 2007)

Micro luidizatiion
A micro luidizer is a piece of equipment that includes a high-pressure positive displacement pump (200 to 500 PSI). This pump pressurizes the material through the interaction chamber, comprising of small channels referred to as microchannels. The material moves through the microchannels directly to an impingement area which brings about extremely ine particles of submicron extend. Aqueous and oily phase are combined and formed in an inline homogenizer to obtain a coarse emulsion. To obtain a resultant stable nano-emulsion, the coarse emulsion is processed further into a micro luidizer. (Savardekar and Bajaj, 2016)

Zeta potential
An instrument named Zeta PALS is utilized to calculate Zeta Potential. Zeta Potential is the electrokinetic potential difference on the surface of the globule in nano-emulsion. Surfactant develops surface charges; however, additionally, act as a mechanical barrier. Zeta potential develops electrical forces which are repulsive around oil globules and which produce restricts coalescence in some instances. The process of Zeta Potential is shown in Figure 1. (Gupta et al., 2010)

Polydispersity
Polydispersity speci ies the uniformity of globules size within the preparation. It is the ratio of the standard deviation to mean droplet size. Polydispersity represents a direct relation between globule sizes in the preparation, i.e. more will be the polydispersity, less will be the uniformity of the globule size in the preparation. Malvern Zetasizer worked on dynamic light scattering mechanism and used to calculates polydispersity. (Suyal et al., 2018) Particle size analysis For analysis of particles distribution and particle size, dynamic light scattering (DLS) mechanism id utilized. This is the most suitable method to measure particle size and dispersion. Electron microscopic examination can also be done for this, but it is not used very frequently. (Pouton and Porter, 2008)

Percent of drug loading
By dissolving determined quantity of nanoemulsion in 25 ml of a suitable solvent. The collected extract is analyzed by High-Performance Liquid Chromatography (HPLC) with a reference solution of the drug. The content of the drug is then calculated by reverse-phase High-Performance Liquid Chromatography by utilizing various columns of suitable porosity. (Gursoy and Benita, 2004) Transmission electron microscopy (TEM) Transmission Electron Microscopy (TEM) is used for investigation structure and morphology of the nanoemulsion. (Chime et al., 2014) In-vitro drug release The In-vitro release investigation of nano-emulsion, including drug, may be completed by semipermeable membrane utilized in dissolution equipment. For this research, a round glass tube, 6 cm in length and 1.25 cm in radius, is connected rather than the basket and is irmly covered with the semi-permeable membrane. Nano-emulsion (drug-loaded) is kept in the round glass tube at the semi-permeable membrane surface. A round glass tube ought to dip in 100 ml buffer keeping the pH to permit the establishment of the sink situation and to preserve everlasting solubilization. The drug release investigation may be achieved for 24 hours at 32 • C. The stirring shaft ought to rotate at a speed of a 100 rpm. At predetermined time intervals (1,2,4,6,8,12,20,24 hours) aliquots of one millilitre of the delivery medium is withdrawn and diluted then iltered for evaluation and changed with an equivalent extent of the buffer medium for keeping up a regular volume. UV spectrometer is utilized for measurement of absorbance of the collected samples. (Nirmala et al., 2013)

APPLICATIONS OF NANO-EMULSIONS
Various Applications of Nano-emulsions are as follows, (Tamilvanan, 2004) 1. Nano-emulsions increase the solubility of poorly soluble drugs.
2. Nanoemulsions dispensing system eliminates the requirement of co-solvent, as well as encapsulating drug that would, in any other case, be irritants; thus, nano-emulsion can minimize irritation upon injection.
3. Nanoemulsions dispensing system decreases the toxicity of drugs.
4. Improving pharmacokinetic parameters for greater bene icial medication performance is an essential goal of drug transport studies in normal and for nano-emulsion speci ically, one speci ic parameter that will be referenced on various times is the zone below the concentration-time curve, abbreviated AUC.

CONCLUSION
Nanoemulsions, drug delivery system, offer several merits for effective drugs delivery, biological, or diagnostic agents. Nanoemulsion technology can protect labile drug, increase bioavailability, enhances drug solubility and control the release of the drug. In this review article, Nanotechnologybased drug delivery system, i.e. Nanoemulsion, has been presented with the efforts that they can serve as the building blocks for much more success in the ield. Stability of nano-emulsion preparation might be prevented by preventing various factors like type and concentration of surface-active agent as well as co-surfactant, type of oily phase, techniques utilized, procedure variables and excipients are added which are utilized over the interphases formulation of nano-emulsion. Recently, nano-emulsions with globules size of less than 100 nm have attracted signi icant attention because of their potential merits in pharmaceutical, biotechnology, cosmetics and food industries as a better drug delivery system due to their small globule size, clear, and higher kinetic stability. But still, there is a requirement to emphasize on the toxicological characterization of the formulated nano-emulsions, which may be a vast research ield in future.