INTRODUCTION:

Colloidal drug delivery systems (CDDS) consist of lipid, natural or synthetic polymer particles, such as liposomes, solid lipid nanoparticles (SLN), poly D,L-lactide (PLA) or poly D, L-lactide-co-glycolide (PLGA) nanoparticles (NP), micelles, etc., encapsulating drugs, nucleic acids or plasmids. Following injection into the bloodstream, many studies have highlighted their rapid removal resulting from their interactions with the mononuclear phagocyte system (MPS)⁽¹⁾or with the complement system.

This premature elimination prevents from reaching the target by using mechanisms such as the accentuated microvascular permeability of the tumor⁽²⁾. Colloidal drug carrier systems such as micellar solutions, vesicle and liquid crystal dispersions, as well as nanoparticle dispersions consist of small particles of 10–400 nm diameter⁽³⁾. CDDS are recognized by opsonins such as the complement protein C3b, immunoglobulins G and M, fibronectin and apolipoproteins, or by specific or non-specific receptors present at the surface of the macrophage plasma membrane. Thus, the blood halflife could be accentuated by increasing the amount of injected CDDS, resulting from the fact that the endocytic capacity of macrophages and the amount of opsonins in the blood are limite⁽⁴⁾. Nevertheless, research has dealt with understanding the parameters influencing these interactions in order to escape immune system effects. Long-circulating CDDS, as well as CDDS of which the interest is to be captured by the MPS, have been formulated. Indeed, the latter constitute a line of attack in AIDS strategies⁽⁵⁾, or following infection of the macrophages by micro-organisms⁽⁶⁾. Otherwise, CDDS can also be used as adjuvants⁽⁷⁾.

Nanoemulsion:

Nanoemulsions are part colloidal dispersions of two immiscible liquids⁽⁸⁾. Nanoemulsions consist of fine oil-in-water dispersions, having droplets covering the size range of 100–600nm. Nanoemulsions, usually spherical, are a group of dispersed particles used for pharmaceuticals biomedical aids and vehicles that shows great promise for the future of cosmetics, diagnostics, drug therapies and biotechnologies⁽⁹⁾. Nanoemulsions do not form spontaneously; an external shear must be applied to rupture larger droplets into smaller ones. Compared to micro emulsion phases, relatively little is known about creating and controlling Nano-emulsions. This is primarily because extreme shear, well beyond the reach of ordinary mixing devices, must be applied to overcome the effects of surface tension to rupture the droplets into the nanoscale regime⁽¹⁰⁾.

Advantages of Nano emulsion ^{(11, 12, 13)}:

(1) It may be used as substitute for liposomes and vesicles.

(2) It improves the bioavailability of drug.

(3) It is non-toxic and non-irritant in nature.

(4) It has improved physical stability.

(5) Nanoemulsions have small-sized droplets having greater surface area providing greater absorption.

(6) It can be formulated in variety of formulations such as foams, creams, liquids, and sprays.

(7) It provides better uptake of oil-soluble supplements in cell culture technology.

(8) It helps to solubilize lipophilic drug.

(9) Helpful in taste masking.

(10) Less amount of energy is required.

Limitation of nanoemulsion⁽¹⁴⁾ :

Although this formulation provide great advantages as a delivery system for the consumers but sometimes the reduced size of droplets are responsible for the limited use of nanoemulsion formulation. Some limitations of nanoemulsion are as follows.

• The manufacturing of nanoemulsion formulation is an expensive process because size reduction of droplets is very difficult as it required a special kind of instruments and process methods. For example, homogenizer (instrument required for the nanoemulsion formulation) arrangement is an expensive process. Again microfludization and ultrasonication (manufacturing process) require high amount of financial support.

• Stability of nanoemulsion is quite unacceptable and creates a big problem during the storage of formulation for the longer time period. Ostwald ripening is the main factor associated with unacceptability of nanoemulsion formulations. This is due to the high rate of curvature of small droplet show greater solubility as compared to large drop with a low radius of curvature.

• Less availability of surfactant and co-surfactant required for the manufacturing of nanoemulsion is another factor which marks as a limitation to nanoemulsion manufacturing.

Formulation ingredients of Nanoemulsion:

Table No: 1 Ingredients of Nanoemulsion

Component	Examples
Oils	Castor oil, Corn oil, Coconut oil, Evening primrose oil, linseed oil, Mineral oil, olive oil, peanut oil
Emulgent	Natural lecithins from plant or animal source, phospholipids, castor oil Derivatives, polysorbates, sterylamine
Surfactant	Polysorbate20, Polysorbate80, Polyoxy 60, castor oil, Sorbitan monooleate, PEG300, Caprylic glyceride
Co- Surfactant	Ethanol, glycerine, PEG300, PEG400, Polyene glycol, Poloxamer
Tonicity modifiers	Glycerol, Sorbitol and xylitol
Additives	Lower alcohol (ethanol), propylene glycol, 1, 3-butylenes glycol, sugars such as butylenes glycol, sugars such as glucose, sucrose, fructose, and maltose
Antioxidants	Ascorbic acid and tocopherol

Method of Preparation:

Several methods have been suggested for the preparation of nanoemulsion. The basic objectives of the nanoemulsion preparation to achieve the droplet size range of 100-600 nm and another is to provide the stability condition. Formation of nanoemulsion system required a high amount of energy.This energy can be provided either by mechanical equipment or the chemical potential inherent within the component ⁽¹⁵⁾. Here some methods are discussed which are freely used for the nanoemulsion preparation.

1. Phase inversion method:

In this method fine dispersion is obtained by chemical energy resulting of phase transitions taking place through emulsification path. The adequate phase transitions are produced by varying the composition at constant temperature or by varying the temperature at constant composition, phase inversion temperature (PIT) method was introduced by Shinoda et al. based on the changes of solubility of polyoxyethylene- type surfactant with temperature. This surfactant becomes lipophilic with increase in temperature due to dehydration of polymer chain. But at low temperature, the surfactant monolayer has a large positive spontaneous curvature forming oil-swollen micellar solution phase ⁽¹⁶⁾.

2. Sonication method:

Sonication method is another best way to prepare nanoemulsion. In this method the droplet size of conventional emulsion or even microemulsion are reduced with the help of sonication mechanism. This method is not suitable for large batches only small batches of nanoemulsion can be prepared by this method⁽¹⁷⁾.

3. High pressure homogenizer:

This method is performed by applying a high pressure over the system having oil phase, aqueous phase and surfactant or co-surfactant. The pressure is applied with the help of a special equipment know as homogenizer. There are some problems which are associated with homogenizer such as poor productivity, component deterioration due to difficult mass production and generation of much heat. With this method only oil in water (o/w) liquid nanoemulsion of less than 20% oil phase can be prepared and cream nanoemulsion of high viscosity or hardness with a mean droplet diameter lower than 200 nm cannot be prepared ⁽¹⁸⁾.

Fig 1.Working of homogenizing valve

4. Micro fluidization:

Micro fluidization is a patented mixing technology, which makes use of a device called micro fluidizer. This device uses a high-pressure positive displacement pump (500-20000 psi), which forces the product through the interaction chamber, which consists of small channels called “microchannel.” The product flows through the microchannels on to an impingement area resulting in very fine particles of submicron range. The two solutions (aqueous phase and oily phase) are combined together and processed in an inline homogenizer to yield a coarse emulsion. The coarse emulsion is into a micro fluidizer where it is further processed to obtain a stable Nanoemulsion. The coarse emulsion is passed through the interaction chamber of the micro fluidizer repeatedly until desired particle size is obtained. The bulk emulsion is then filtered through a filter under nitrogen to remove large droplets resulting in a uniform Nanoemulsion⁽¹⁹⁾.

5. Spontaneous emulsification:

It involves three steps: (a) preparation of homogeneous organic solution consisting of oil and lipophilic surfactant in water miscible solvent and hydrophilic surfactant, (b) the organic phase is injected in aqueous phase under continuous magnetic stirring, o/w emulsion is formed, and (c) the aqueous phase is removed by evaporation under reduced pressure⁽²⁰⁾.

Factors to be considered during preparation of nanoemulsion:

1. Surfactant must be selected carefully such that an ultralow interfacial tension may be achieved which is a primary requirement to produce nanoemulsion.

2. Concentration of surfactant must be high enough to stabilize the micro droplets to produce nanoemulsion.

3. The surfactant must be flexible or fluid enough to promote the formation of nanoemulsion.

Characterization of Nanoemulsion:

Different characterization parameters for Nanoemulsion include transmission electron microscopy, Nanoemulsion droplet size analysis, viscosity determination, refractive index, in vitro skin permeation studies, skin irritation test, in vivo efficacy study, thermodynamic stability studies, and surface characteristics.

The surface charge of the Nanoemulsion droplets has a marked effect on the stability of the emulsion system and the droplet in vivo disposition and clearance.

The inset shows microscopy image at a higher magnification. Nanoemulsion droplets were in the size range of 25-40 nm with some particle aggregates in the size range of 100-150 nm⁽²¹⁾.

Droplet size analysis:

Droplet size analysis of nanoemulsion is measured by a diffusion method using a light-scattering, particle size analyser counter, LS 230. It is also measured by correlation spectroscopy that analyses the fluctuation in scattering of light due to Brownian motion. Droplet size analysis of nanoemulsion can also be performed by transmission electron microscopy (TEM)^(22,23,24)

Viscosity determination:-

The viscosity of nanoemulsion is measured by using Brookfield-type rotary viscometer at different shear rates at different temperatures. Dilution test Dilution of a nanoemulsion either with oil or with water can reveal this type. The test is based on the fact that more of the continuous phase can be added into a nanoemulsion without causing the problem of its stability. Thus, an o/w nanoemulsion can be diluted with water and a w/o nanoemulsion can be diluted with oil⁽²⁵⁾.

Drug content :

Preweighed nanoemulsion is extracted by dissolving in a suitable solvent, extract is analysed by spectrophotometer or HPLC against standard solution of drug⁽²⁶⁾.

Polydispersity:

It indicates the uniformity of droplet size in nanoemulsion. The higher the value of polydispersity, lower will be uniformity of droplet size of nanoemulsion. It can be defined as the ratio of standard deviation to mean droplet size. It is measured by a spectrophotometer.

Dye test :

If a water-soluble dye is added in an o/w nanoemulsion the nanoemulsion takes up the colour uniformly. Conversely, if the emulsion is w/o type and the dye being soluble in water, the emulsion takes up the colour only in the dispersed phase and the emulsion is not uniformly coloured. This can be revealed immediately by microscopic examination of the emulsion.

Refractive index :

Refractive index of nanoemulsion is measured by Abbes refractometer.

pH :

The pH of nanoemulsion can be measured by pH meter.

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 Nanoemulsion⁽²⁷⁾.

Table No : 2 Marketed Preparation of Nanoemulsion

S No.	Brand	Drug	Dosage Form	Dose (mg)	Indication
1	Neoral	Cyclosporine	Soft gelatin capsule	25.100	Immunosuppresant
2	Norvir	Ritonavir	Soft gelatin capsule	100	HIV antiviral
3	Lipire	Fenoibrate	Hard gelatin capsule	200	Antihypertensive
4	Convule	Valporic acid	Soft gelatin capsule	100,200	Antiepileptic

Table No: 3 Some important patents related to Nanoemulsion's^(28,29,30)

S.no.	Patent name	Assignee	US Patent number
1	Method of Preventing and Treating Microbial Infections.	NanoBio Corporation (US).	6,506,803.
2	NE based on phosphoric acid fatty acid esters and its uses in the cosmetics, dermatological, pharmaceutical, and/or ophthalmological fields.	L’Oreal (Paris, FR).	:6,274,150.
3	NE based on ethylene oxide and propylene oxide block copolymers and its uses in the cosmetics, dermatological and/or ophthalmological fields.	L’Oreal (Paris, FR).	6,464,990.
4	NE of 5-aminolevulinic acid (6,559,183).	ASAT AG Applied Science and Technology (Zug, CH).	PCT/ EP99/08711.
5	NEs of poorly soluble pharmaceutical active ingredients and methods of making the same.		WO/2007/ 103294.

Application of Nanoemulsion:

Nanoemulsion has become a very attractive formulation for the delivery of pharmaceuticals. nanoemulsion also shows a good advantage in the field of cosmetics. The attraction of nanoemulsion formulation in pharmaceuticals and cosmetics is due to following reasons⁽³¹⁾.

Cosmetics:

Nanoemulsion is used as vehicle for controlled delivery and as effective transport vehicle. It will reduce Trans-epidermal water level. Kemira Nano gel-nanoemulsion based carrier system is a patented system for cosmetic purpose it will enhance skin production and penetration of API. Apart from that it will also provide good skin feel⁽³²⁾. Topical administration itself has many advantages and by combining it with nanoemulsion this formulatory may impart the better way of drug delivery system. It can bypass the hepatic first pass metabolism of the drug and related toxicity effects^(33;34). One can achieve the targeted delivery by compilation delivery site and this formulation is helpful for this purpose⁽³⁴⁾. The nanoemulsion with the topical antimicrobial agent is helpful for the fungal infection on internal body part^(34;
35).

Antimicrobial Nanoemulsion:

These are O/W Type nanoemulsions with size range from 200-600 nm and are stabilized by surfactants and alcohol. They are having broad spectrum of activity against bacteria and viruses even spores. Fuse with lipid containing organisms due to charge interaction⁽³⁶⁾.

Prophylectic in bio-terrorism attack:

Based on its antimicrobial property research has begun to use it in the prophylactic treatment. Research has made to use it for anthrax and ebola. It has also been tested by US army on gangrene and Clostridium botulinum⁽³⁷⁾.

Non-toxic disinfectant cleaner:

A Nano-emulsion based non-toxic disinfectant cleaner has been developed by envirosystem Inc. It has broad spectrum of activity and kills virus, bacteria, spores, fungi and tuberculosis bacilli. It requires no warning labels. It doesn’t irritate eyes. Parachlorometaxylenol (PCMX) is one kind of such NE marketed as EcoTruTM ⁽³⁸⁾.

Nanoemulsions as a mucosal vaccines:

NEs are being used to deliver either recombinant proteins or inactivated organisms to a mucosal surface to produce an immune response. The first applications, an influenza vaccine and an HIV vaccine, can proceed to clinical trials. The NE causes proteins applied to the mucosal surface to be adjuvanted and it facilitates uptake by antigen-presenting cells. This results in a significant systemic and mucosal immune response that involves the production of specific IgG and IgA antibody as well as cellular immunity^{(39,40 )}.

Nanoemulsions in cell culture technology⁽⁴¹⁾ :

Cell cultures are used for in vitro assays or to produce biological compounds, such as antibodies or recombinant proteins. To optimize cell growth, the culture medium can be supplemented with a number of defined molecules or with blood serum. Up to now, it has been very difficult to supplement the media with oil-soluble substances that are available to the cells, and only small amounts of these lipophilic compounds could be absorbed by the cells. NEs are a new method for the delivery of oil-soluble substances to mammalian cell cultures. The delivery system is based on a NE, which is stabilized by phospholipids. These NEs are transparent and can be passed through 0.1 mm filters for sterilization. NE droplets are easily taken up by the cells. The encapsulated oil-soluble substances therefore have a high bioavailability to cells in culture. The advantages of using NEs in cell culture technology are better uptake of oil-soluble supplements in cell cultures; improve growth and vitality of cultured cells, and allowance of toxicity studies of oil-soluble drugs in cell cultures.

CONCLUSION:

Nanoemulsions have gained popularity over the past decade because of their exceptional properties such as high surface area, transparent appearance, robust stability and tunable rheology. The most important application of nanoemulsion is for masking the disagreeable taste of oily liquids. It may also protect the drugs, which are susceptible to hydrolysis and oxidation. Nowadays, nanoemulsions are used for targeted drug delivery. Nanoemulsion can also provide prolonged action of the medicaments. Overall all nanoemulsion formulation may be considered as effective, safe and with increased bioavailability. It is expected that further research and development will be carried out in the future regarding nanoemulsion. Due to their small size, molecules sitting at the interface of nanoemulsions experience higher curvature which will greatly influence self-assembly at the interface. Nanoemulsions can also serve as a model system to enhance understanding of colloidal assembly and rheology of complex emulsion systems.

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Received on 25.04.2018 Modified on 19.05.2018

Res. J. Pharm. Dosage Form. & Tech. 2018; 10(4): 253-258.

DOI: 10.5958/0975-4377.2018.00037.X