Nanoemulsions in Drug Delivery: A Comprehensive Review of Formulation Strategies, Characterization, and Applications

Nanoemulsions are kinetically stable droplets of one liquid phase dispersed inside another immiscible phase, typically measuring approximately 100 nm. They exhibit numerous remarkable qualities, including a high surface area, excellent optical clarity, substantial stability, and adjustable rheology.[1] Water, oil, and an emulsifier are the usual ingredients of a nanoemulsion. Because it reduces the interfacial tension, or surface energy per unit area, between the oil and the surface, the use of an emulsifier is essential for the formation of tiny droplets. and the emulsion's water phases. Through steric hindrance and repulsive electrostatic interactions, the emulsifier also contributes to the stabilization of nanoemulsions.[2] Although proteins and lipids have also proven useful in the creation of nanoemulsions, surfactants are typically utilized as the emulsifier.[3] About 40% of newly discovered pharmacological compounds are weakly water soluble, and nano-sized carriers are acknowledged as effective drug delivery vehicles for these medications. Nanoemulsions have surfaced as one of the innovative strategies as possible substitute medication carriers.[4] Because of its composition and mechanism, this kind of surfactant-lipid-based formulation can interact with the body's natural barriers to facilitate medication absorption.Oil-in-water nanoemulsions may therefore be able to address the issue of such medications' poor solubility by enhancing bioavailability, boosting drug stability, and reducing adverse effects, offering a variety of uses.[5] The versatility of nanoemulsion is demonstrated by the range of manufacturing process choices available, including both high and low energy methods that allow for the creation of narrow-sized droplets via mechanical and spontaneous physicochemical mechanisms, respectively.[6] A variety of liquid lipids and surfactants can be combined to create nanoemulsions, which offer an inventive platform. This makes it possible for alternatives to offer the creation of novel drug delivery systems that are very applicable.[7] Numerous techniques are used to create nanoemulsions, and they can be broadly categorized as either high-energy or low-energy emulsification procedures, or a combination of the two. The use of mechanical devices that provide strong disruptive forces that split the water and oil phases to form oil droplets is a characteristic of high-energy techniques.[8] This technique makes use of sonication techniques, microfluidizers, and high-pressure homogenizers. On the other hand, low energy methods emulsify using the system's intrinsic chemical energy. This is accomplished by rerouting the formulation's inherent physicochemical characteristics of the surfactant, co-surfactants, and excipients.[9]

1.2 Nanoemulsion types:

Nanoemulsion is categorized into three categories based on its water and oil phases:

• Oil in water:

In this system, a nanoemulsion of oil in water consists of oil droplets from the internal phase that are dispersed within the external aqueous phase.

• Water in oil:

In this system, water is present in an oil nanoemulsion, where water droplets of the internal phase are dispersed within the external oil phase.

• Bi-continuous:

In this system, nanoemulsion consists of both oil and water droplets distributed throughout. The stability of the three types of nanoemulsion is achieved by incorporating an adequate amount of compatible surfactant and co-surfactant to minimize interfacial tension. The surfactant may be anionic, cationic, or nonionic. [10]

1.3 Component of Nanoemulsion:

• Aqueous phase
• Oil
• Co- Surfactant
• Surfactant

Aqueous phase:

The properties of the aqueous phase, including pH, ionic concentration, and electrolytes, influence the size and stability of nanoemulsions (NEs). Ringer’s solution, simulated gastric fluid (pH 1.2), simulated intestinal fluid (pH 6.8), phosphate buffered saline, and plain water can all serve as the aqueous phase for assessing the spontaneous nanoemulsification of NEs. Additionally, among the various characteristics of the aqueous phase, pH plays a crucial role in determining the behavior of NEs when a drug with pH-dependent solubility is introduced into the system. [11]

 Oil phase:

Oil in Water nanoemulsions is composed of 5%–20% oil or lipid as the disperse phase, and in some cases, this percentage can rise to as much as 70%. The lipids or oils utilized in nanoemulsions are typically selected based on the solubility of the drug. [12] This selection aids in the emulsification process, enhancing the solubility of water-insoluble drugs within the oil droplets. Furthermore, it enhances the absorption of oral medications by improving gastrointestinal drug permeation via the intestinal lymphatic system. In topical formulations, it acts as a penetration enhancer, facilitating the permeation of drugs through the skin.[13] Triglycerides such as tri-, di-, or mono-acylglycerols, vegetable oils, mineral oils, free fatty acids, etc., can be included in the oil phase of nanoemulsion formulations. Drug solubility is typically the basis for oil selection. When creating a nanoemulsion, oil phases with high drug loading are typically utilized.[14]

Co- Surfactant:

These substances are incorporated into surfactants to enhance the interfacial film, as they effectively occupy structurally weaker areas [15]. Co-surfactants are employed when the surfactant does not sufficiently lower the interfacial tension between oil and water to create a stable nanoemulsion (NE). The co-surfactant interferes with the liquid crystalline phase by infiltrating the surfactant monolayer, thereby increasing fluidity.[16] Polyethylene glycol, propylene glycol, ethanol, transcutol-P (diethylene glycolmonoethyl ether), ethylene glycol, glycerin, and propanol are examples of co-surfactants or co-solvents that are frequently utilized in the creation of nanoemulsion systems.[17]

 Surfactant:

They are amphiphilic molecules that enhance the stability of nanoemulsions by preventing the aggregation of droplets and minimizing interfacial tension. Surfactants are easily adsorbed at the interface between oil and water, providing electrostatic, steric, or a combination of electro-steric stability. Lecithin (phosphatidylcholine) is the surfactant most commonly utilized in nanoemulsions, derived from soybean or egg yolk. In the market, surfactants such as bile salts (sodium deoxycholate) and cremophor EL (Polyoxyl-35 castor oil) have been employed. The adsorption of surfactants at an interface that encapsulates the dispersion medium and droplets, which reduces interfacial tension, is a prevalent technique for stabilizing nanoemulsions. [18]

Spans (sorbitan fatty acid esters), tweens (polyoxyethylene (POE) derivatives of sorbitan fatty acid esters), Cremophor EL (polyoxyl-35 castor oil), lauroyl macrogolglycerides, polysaccharides (gum and starch derivatives), phospholipids (egg, soy, or dairy lecithin), and amphiphilic proteins (whey protein isolate and caseinate).[19]

1.4 Advantages of Nanoemulsion:

• It enables site-specific drug delivery.

• Nanoemulsion possesses the ability to dissolve substantial amounts of hydrophobic substances.

• It has the capability to safeguard drugs from degradation, ensuring long-term stability, which contributes to the creation of an optimal drug delivery system.

• This may serve as an alternative to vesicles and liposomes.

• It is both non-irritating and non-toxic.

• These are utilized to enhance the bioavailability of drugs.

• It facilitates improved absorption due to the presence of small-sized droplets that offer a larger surface area.

• It can be formulated into various types of preparations, such as creams, liquids, foams, and sprays.

• This is also employed for taste masking.[20]

1.5 Disadvantages of Nanoemulsion:

• A significant concentration of surfactants and co-surfactants is necessary for stabilization.

• The stability is influenced by temperature and pH levels.

• Instability may arise from the Oswald ripening effect.[21]

• The surfactant and co-surfactant employed must be non-toxic.

• There is a low solubility capacity for materials with high melting points.[22]

1. Factors affecting the Formulation of Nanoemulsion:

• Proper composition is necessary to prevent Oswald ripening; the dispersed phase must be highly insoluble in the dispersed medium.

• The surfactant plays a crucial role in the Nanoemulsion. It is important that they do not create lyotropic liquid crystalline "microemulsion" phases. Typically, systems that include short-chain alkanes, alcohols, water, and surfactants form the phases that are commonly utilized alongside the co-surfactant.

• The presence of excess surfactants allows for the rapid coating of new nanoscale surface area during emulsification, thereby preventing induced coalescence.

• Significant shear must be applied to break down microscale droplets into nanoscale ones by exerting stress levels that exceed the Laplace pressure of the droplets, which ranges from 10 to 100 atm. Among various methods, ultrasonication is the most widely employed in laboratory settings. [23]

2. Formulation Techniques Of Nanoemulsion Drug Delivery Systems:

The methods utilized in the development of nanoemulsion drug delivery systems are varied and exhibit a significant level of overlap. We have categorized the various techniques for the preparation of nanoemulsion drug delivery systems based on energy requirements, the characteristics of phase inversion, and the process of self-emulsification.

• High energy methods:

1. High-pressure homogenization.
2. Microfluidization.
3. Ultrasonication.

• Low energy methods:

1. Phase inversion emulsification method.
2. The self-nanoemulsification method. [ 24]

1. High energy methods:

High energy techniques are widely employed in the formulation of nanoemulsions. These methods utilize significant mechanical energy to generate powerful disruptive forces that break down large droplets into nano-sized droplets, resulting in nanoemulsions with elevated kinetic energy. The disruptive forces are produced through mechanical devices such as ultrasonicators, microfluidizers, and high-pressure homogenizers [25]. By employing high energy techniques, we can attain enhanced control over particle size while selecting the formulation composition. Additionally, high energy methods offer control over the stability, rheology, and color of the emulsion. The following methods are included in high energy techniques.[26]

High-pressure homogenization:

High-pressure homogenizers provide significant energy and ensure a uniform flow to produce the smallest particle sizes. Consequently, they are predominantly utilized for the preparation of nanoemulsions. These homogenizers generate highly disruptive forces that create nanoemulsions with extremely low particle sizes (down to 1 nm) [27]. The coarse emulsion is subsequently forced through a narrow orifice under high pressure (ranging from 500 to 5,000 psi). During this process, various forces, including intense turbulence, hydraulic shear, and cavitation, are simultaneously applied to yield nanoemulsions with very small droplet sizes.[28] The particle size of nanoemulsions generated by high-pressure homogenizers is influenced by factors such as sample composition, the type of homogenizer, and the operating conditions of the homogenizer, including energy intensity, duration, and temperature [29]. An increase in the intensity of homogenization results in a reduction of the droplet sizes of the nanoemulsions. In some instances, particularly when biopolymers are employed as emulsifiers, vigorous homogenization may cause an increase in the particle size of the resulting nanoemulsion. High-pressure homogenization is extensively applied in the formation of nanoemulsions for food, pharmaceutical, and biotechnological ingredients. [30]

Microfluidization:

Microfluidization is a micro-scale mixing technology that employs a device known as a microfluidizer. In this process, fluids are compelled to traverse through microchannels under high pressure, ranging from 500 to 20,000 psi. Microchannels are typically small channels that facilitate mixing at the micro level [31]. The aqueous and oil phases of a macroemulsion are combined and subsequently passed through the microfluidizer. The macroemulsion is directed through the microchannels under high pressure into the interaction chamber. Within the interaction chamber, two streams of macroemulsions collide at high velocities. This impact generates forces such as shearing, cavitation, and impact, resulting in the formation of stable nanoemulsions. Microfluidizers yield narrower and smaller particle size distributions of nanoemulsions compared to homogenizers [32]. Furthermore, microfluidizers can produce stable nanoemulsions with lower surfactant concentrations [33]. Techniques of microfluidization have been utilized to create nanoemulsions for food ingredients [34]. These methods generate food-grade nanoemulsions characterized by uniform droplet size distributions and enhanced stability [35].

Ultrasonication:

Ultrasonication surpasses other high-energy techniques regarding operational efficiency and cleanliness. In the process of ultrasonic emulsification, ultrasonic waves generate cavitation forces that transform macroemulsions into nanoemulsions. This technique employs ultrasonicators, which feature a probe that emits ultrasonic waves. By adjusting the input of ultrasonic energy and the duration of exposure, one can attain the desired particle size and stability of the nanoemulsion. In ultrasonic emulsification, the primary physical shear is delivered through the process of acoustic cavitation [36]. Cavitation refers to the formation and growth of microbubbles, followed by their collapse, which occurs due to pressure fluctuations from the acoustic wave. The collapse of these microbubbles induces significant turbulence, leading to the creation of nano-sized droplets [37]. When an oil and water system is subjected to ultrasound, cavitation forces are generated, providing additional energy for the formation of new interfaces, which results in the production of nano-sized emulsion droplets. Through the application of ultrasonication, it is possible to produce nanoemulsions without the use of surfactants [38]. A recent study indicated that the efficiency of ultrasonic emulsification is influenced by the intensity of ultrasonication, the duration of the process, and the characteristics of the surfactant used. Ultrasonication has been widely utilized for the production of nanoemulsions containing drugs and food ingredients. Nanoemulsions produced through food-grade ultrasonication exhibit enhanced stability, reduced droplet size, and require less energy input compared to other high-energy methods [39].

2. Low energy methods:

These techniques necessitate minimal energy for the creation of nanoemulsion systems. Low-energy emulsification techniques are more energy-efficient since they harness the internal chemical energy of the systems and only require gentle stirring for the production of nanoemulsions. Typically, low-energy emulsification methods include phase inversion emulsification and self-emulsification. [40]

Phase inversion emulsification method:

In this approach, the spontaneous curvature of the surfactant induces a phase transition throughout the emulsification process. Variations in the spontaneous curvature of the surfactant arise from alterations in parameters such as temperature and composition, among others. [41] There exist two categories of phase inversion emulsification techniques: TPI methods, which encompass PIT and PIC, and CPI methods, which include EIP. Transitional phase inversion occurs as a result of changes in the spontaneous curvature or affinity of the surfactant due to variations in parameters like temperature and composition [42]. Conversely, CPI transpires when the dispersed phase is continuously added until the dispersed phase droplets coalesce to create bi-continuous or lamellar structural phases. The term 'catastrophe' refers to a sudden alteration in the behavior of a system caused by changing conditions. For catastrophic phase inversion to take place, it is crucial that the surfactant is predominantly present in the dispersed phase, resulting in a high coalescence rate, which consequently leads to rapid phase inversion. During transitional phase inversion, the spontaneous curvature or surfactant affinity is modified, whereas in catastrophic phase inversion, the spontaneous curvature or surfactant affinity remains unchanged.[43]

Self-nanoemulsification method:

In the self-emulsification technique, the creation of nanoemulsions occurs without altering the spontaneous curvature of the surfactant. Molecules of surfactant and/or co-solvent swiftly migrate from the dispersed phase to the continuous phase, resulting in turbulence and the formation of nano-sized emulsion droplets. This self-emulsification technique is also known as the spontaneous emulsification method. Self-Nanoemulsifying Drug Delivery Systems (SNEDDS) are founded on the self-emulsification principle and typically include more hydrophilic surfactants or co-surfactants (co-solvents) along with a reduced lipid content [44]. SNEDDS can be characterized as an isotropic mixture comprising oil, surfactant, co-surfactant, and drug. Upon dilution with aqueous fluids in vivo, this mixture generates a fine and optically clear oil-in-water (O/W) nanoemulsion, facilitated by the gentle agitation from the digestive motility of the stomach and intestines. The two most frequently cited mechanisms for nanoemulsion formation from SNEDDS include the diffusion of the hydrophilic co-solvent or co-surfactant from the organic phase into the aqueous phase [45], and the generation of nanoemulsion negative free energy at transient negative or ultra-low interfacial tensions. Furthermore, SNEDDS are recognized as the most popular and promising method for the delivery of hydrophobic drugs with low bioavailability. They have also been utilized for the delivery of bioactive food components [46].

3. Characterization of nanoemulsion:

4.1 Zeta potential:

Zeta potential is assessed using an instrument referred to as Zeta PALS. This device is employed to evaluate the charge present on the surface of droplets in a nanoemulsion. Emulsifiers serve not only as a mechanical barrier but also contribute to the creation of surface charges. Zeta potential generates repulsive electrical forces between approaching oil droplets, thereby preventing coalescence. A more negative zeta potential correlates with a higher net charge of droplets, resulting in a more stable emulsion. Typically, zeta potential values below -30 mV signify a significant level of physical stability. The Malvern Zetasizer, which operates on the principle of dynamic light scattering, is utilized to measure zeta potential. [ 47]

4.2 Interfacial tension:

By assessing the interfacial tension, one can investigate the formation and properties of NEs. When a surfactant phase or middle phase NEs is in balance with either an aqueous or an oil phase, ultra-low interfacial tension values are associated with phase behavior. The remarkably low interfacial tension of NEs can be evaluated using a spinning-drop technique. [48]

4.3 Measurment of Droplet Size and Polydispersity Index:

The average particle size and polydispersity index are determined at 25°C through dynamic light scattering (DLS) utilizing a Malvern Zetasizer. The particle size is assessed using a disposable capillary cuvette fitted with electrodes. To prevent multiple scattering effects during the measurements, samples are diluted 100 times with double-distilled water just prior to measurement [49]. The droplet size and polydispersity index of the samples under investigation are acquired (in triplicate) by averaging 13 measurements taken at an angle of 173° [50].

4.4 Particle Size Analysis:

The dynamic light scattering (DLS) method is commonly employed for measuring particle size and its distribution in the context of nanoemulsion [51].

4.5 Refractive Index:

The refractive index of the nanoemulsion is assessed using an Abbe-type refractometer. Each sample's refractive index is measured three times, and the mean value along with the standard deviation (SD) is computed [52].

4.6 Percent Drug Loading:

To ascertain the percentage of drug loading, a pre-weighed nanoemulsion is extracted by dissolving it in 25ml of an appropriate solvent. Subsequently, the extract is analyzed either spectrophotometrically or through HPLC in comparison to a standard drug solution. The drug content is quantified using the reverse phase HPLC method with various columns of suitable porosity [53].

4. Applications Of Nanoemulsions:

5.1 Nanoemulsions in drug delivery:

Nanoemulsions are employed in a range of drug delivery techniques, such as topical, ocular, intravenous, intramuscular, internasal, and oral administration. They leverage their lipophilic characteristics to solubilize water-insoluble medications and their adjustable charge and rheological properties to formulate aqueous solutions. Additionally, nanoemulsions provide benefits for hydrophobic drugs and are utilized as agents in ultrasound imaging [54].

5.2 Oral Delivery:

Lipids can serve as nanoemulsions to enhance the absorption of drugs in the gastrointestinal tract (GIT), especially for protein-based medications, by encapsulating them within lipids, which improves the overall absorption mechanism [55].

1. 3. Topical delivery:

Improving the penetration of drugs for topical use presents difficulties because of inadequate dispersibility and potential skin irritation. Nanoemulsions, including soybean lecithin, tween, and poloxamer, provide a synergistic effect of enhancing penetration while maintaining a concentration gradient.

5.4 Intravenous delivery:

Parenteral Nanoemulsions administer medications that exhibit reduced bioavailability and limited therapeutic indices. By coating or attaching a hydrophilic moiety, these formulations transform into stealth Nanoemulsions, which improve permeability and retention for targeted tumor therapy.

5.5 Targeted drug delivery:

Nanoemulsion technology has been extensively employed as a transdermal drug delivery system. The diminutive size of nanomaterials enables them to interact with a greater number of substances owing to their expansive surface area and efficient transport capabilities, while their surface drainage facilitates accumulation at the skin level [56].

5.6 Nanoemulsion in cosmetics:

Newer materials (NEs) are becoming increasingly significant for the controlled delivery of cosmetics and the optimized dispersion of active ingredients within skin layers.They are effective for transporting lipophilic compounds and facilitate skin penetration, thereby increasing the concentration of active ingredients.NEs also exhibit bioactive properties, which help to reduce trans-epidermal water loss and enhance the function of the skin barrier.Their acceptance in cosmetics is attributed to their absence of creaming, sedimentation, flocculation, and coalescence [57].TRI-K Industries and Kemira have introduced a novel nano-based gel, Kemira NanoGel, aimed at enhancing the efficacy of skincare products.The innovative NE Carrier system generates submicron emulsions from an oil-in-water concentrate, which minimizes trans-epidermal water loss and boosts skin production.This technology proves particularly beneficial in sun care, moisturizing, and anti-aging creams, while also providing a pleasant skin feel [58].

5.7 Nanoemulsion in cell culture technology:

Cell cultures serve as a medium for in vitro assays and the production of biological compounds. Oil-soluble substances have posed challenges for cellular absorption. New encapsulated substances (NEs) represent an innovative approach to delivering oil-soluble compounds to mammalian cell cultures. These transparent NEs, stabilized by phospholipids, exhibit high bioavailability, enhancing cell growth and vitality, and facilitating toxicity assessments of oil-soluble pharmaceuticals in cell cultures.

5.8 Nanoemulsion to improve the per-oral delivery of poorly soluble drugs:

Nanoemulsion is extensively utilized to enhance the solubility of drugs that are poorly soluble. Drugs classified as BCS class II and IV, which exhibit low solubility in water, encounter difficulties when formulated in traditional dosage forms. Nanoemulsions present a viable solution for enhancing solubility and therapeutic effectiveness [59].

CONCLUSION

Nanoemulsions represent a versatile and promising drug delivery system due to their nanosized droplets, high surface area, and enhanced stability. They significantly improve the solubility, absorption, and bioavailability of poorly water-soluble drugs, while also protecting them from degradation and minimizing adverse effects. With flexible formulation components and diverse preparation techniques, nanoemulsions can be tailored for various routes of administration, including oral, topical, intravenous, and targeted delivery. Their broad applications in pharmaceuticals, cosmetics, and biotechnology highlight their potential as an advanced platform for improving therapeutic performance and patient outcomes.

REFERENCES

1. Gupta A, Badruddoza AZ, Doyle PS. A general route for nanoemulsion synthesis using low-energy methods at constant temperature. Langmuir. 2017 Jul 18;33(28):7118-23.
2. T. Delmas, H. Piraux, A.-C. Couffin, I. Texier, F. Vinet, P. Poulin, M. E. Cates and J. Bibette, Langmuir, 2011, 27, 1683–1692.
3. Gupta A, Eral HB, Hatton TA, Doyle PS. Nanoemulsions: formation, properties and applications. Soft matter. 2016;12(11):2826-41.
4. Nishitani Yukuyama M, Tomiko Myiake Kato E, Lobenberg R, Araci Bou-Chacra N. Challenges and future prospects of nanoemulsion as a drug delivery system. Current pharmaceutical design. 2017 Jan 1;23(3):495-508.
5. Parveen R, Baboota S, Ali J, Ahuja A, Vasudev SS, Ahmad S. Oil based nanocarrier for improved oral delivery of silymarin: In vitro and in vivo studies. Int J Pharm 2011; 413(1-2): 245-53.
6. Yukuyama MN, Ghisleni DD, Pinto TJ, Bou-Chacra NA. Nanoe mulsion: process selection and application in cosmetics - a review. Int J Cosmet Sci 2016; 38(1): 13-24.
7. Lallemand F, Daull P, Benita S, Buggage R, Garrigue JS. Success fully improving ocular drug delivery using the cationic nanoemul sion, novasorb. J Drug Deliv 2012; 2012: 604204.
8. Ngan CL, Basri M, Lye FF, et al. Comparison of Box–Behnken and central composite designs in opti mization of fullerene loaded palm-based nano-emul sions for cosmeceutical application. Ind Crops Prod. 2014; 59:309–317.
9. Saberi AH, Fang Y, McClements DJ. Fabrication of vitamin E-enriched nanoemulsions: factors affecting particle size using spontaneous emulsification. J Colloid Interface Sci. 2013; 391:95–102.
10. M.R. Mangale (2015), Nanoemulsion as pharmaceutical overview. Int. J. Pharm. Sci. Rev. Res., (46): 244-252
11. Al-Hussaniy HA, ?lmajidi YQ, Oraibi AI, Alkarawi A?. Nanoemulsions as medicinal components in insoluble medicines. Pharmacia. 2023 Jul 27; 70:537-47
12. Jiang S-P, He S-N, Li Y-L, Feng D-L, Lu X-Y, Du Y-Z, et al Preparation and characteristics of lipid nanoemulsion formulations loaded with doxorubicin Int J Nanomed. 2013; 8:3141–50
13. Lawrence MJ, Rees GD. Microemulsion based media as a novel drug delivery system Adv Drug Deliv Rev. 2002;45:89–121
14. Qadir A, Faiyazuddin MD, Talib Hussain MD, Alshammari TM, Shakeel F. Critical steps and energetics involved in a successful development of a stable nanoemulsion. J Mol Liq. 2016. 214:718.
15. Mushtaq A, Wani SM, Malik AR, Gull A, Ramniwas S, Nayik GA, Ercisli S, Marc RA, Ullah R, Bari A (2023) Recent insights into nanoemulsions: Their preparation, properties and applications. Food Chemistry: X 18: 100684.
16. Wilson RJ, Li Y, Yang G, Zhao CX (2022) Nanoemulsions for drug delivery. Particuology 64: 85–97.
17. Khan AW, Kotta S, Ansari SH, Sharma RK, Ali J. Potentials and challenges in self-nanoemulsifying drug delivery systems. Expert Opin Drug Deliv. 2012. 9:1305-1317.
18.  Zhang T., Dong D., Lu D., Wang S., Wu B. Cremophor EL-Based Nanoemulsion Enhances Transcellular Permeation of Emodin through Glucuronidation Reduction in UGT1A1-Overexpressing MDCKII Cells. Journal of Pharmaceutics. 2016;501(1–2):190–198.
19. Komaiko JS, McClements DJ. Formation of food-grade nanoemulsions using low-energy preparation methods: a review of available methods. Compr Rev Food Sci Food Saf. 2016. 15:331-352.
20. Halnor VV, Pande VV, Borawake DD, Nagare HS. Nanoemulsion: A novel platform for drug delivery system. J Mat Sci Nanotechol. 2018;6(1):104.
21. Banker G., Lieberman H., Rieger M., Pharmaceutical dosage forms, Disperse systems, Marcel Dekker, 2002; 2:3: 339-40, 343-44.
22. Devarajan V, Ravichandran V (2011), Nanoemulsion as modified drug delivery tool. International journal of comprehensive pharmacy. (2): 1-6.
23. Kim YH, Ghanem AH, Mahmoud H, Higuchi WI. Short chain alkanols as transport enhancers for lipophilic and polar/ionic permeants in hairless mouse skin: mechanism(s) of action”, Int J Pharm. 1990; 80:17-31.
24. Mahdi Jafari S, He Y, Bhandari B. Nano-emulsion production by sonication and microfluidization–a comparison. Int J Food Prop. 2006; 9:475–485.
25. Gonçalves A, Nikmaram N, Roohinejad S, Estevinho BN, Rocha F, Greiner R, et al. Production, properties, and applications of solid self-emulsifying delivery systems (S-SEDS) in the food and pharmaceutical industries. Colloids Surf a Physicochem Eng Aspects. 2018; 538:108–126.
26. Graves S, Meleson K, Wilking J, Lin MY, Mason TG. Structure of concentrated nanoemulsions. J Chem Phys. 2005; 122:134703.
27. Rai VK, Mishra N, Yadav KS, Yadav NP. Nanoemulsion as pharmaceutical carrier for dermal and transdermal drug delivery: formulation development, stability issues, basic considerations and applications. J Control Release. 2018; 270:203–225.
28. Floury J, Desrumaux A, Lardières J. Effect of high-pressure homogenization on droplet size distributions and rheological properties of model oil-in-water emulsions. Innov Food Sci Emerg Technol. 2000; 1:127–134.
29. Qian C, McClements DJ. Formation of nanoemulsions stabilized by model food-grade emulsifiers using high-pressure homogenization: factors affecting particle size. Food Hydrocoll. 2011; 25:1000–1008.
30. Yuan Y, Gao Y, Zhao J, Mao L. Characterization and stability evaluation of β-carotene nanoemulsions prepared by high pressure homogenization under various emulsifying conditions. Food Res Int. 2008; 41:61–68.
31. Qadir A, Faiyazuddin MD, Talib Hussain MD, Alshammari TM, Shakeel F. Critical steps and energetics involved in a successful development of a stable nanoemulsion. J Mol Liq. 2016; 214:7–18. 
32.  Lovelyn C, Attama AA. Current state of nanoemulsions in drug delivery. J Biomater Nanobiotechnol. 2011; 2:626–639. 
33. Perrier-Cornet JM, Marie P, Gervais P. Comparison of emulsification efficiency of protein-stabilized oil-in-water emulsions using jet, high pressure and colloid mill homogenization. J Food Eng. 2005; 66:211–217.
34. Zhang J, Peppard TL, Reineccius GA. Preparation and characterization of nanoemulsions stabilized by food biopolymers using microfluidization. Flavour Fragrance J. 2015; 30:288–294.
35. Villalobos-Castillejos F, Granillo-Guerrero VG, Leyva-Daniel DE, Alamilla-Beltrán L, Gutiérrez-López GF, Monroy-Villagrana A, et al. Fabrication of nanoemulsions by microfluidization. In: Jafari SM, McClements DJ, editors. Nanoemulsions: Formulation, Applications, and Characterization. Academic Press; San Diego, CA, USA: 2018. pp. 207–232.
36. Jayasooriya SD, Bhandari BR, Torley P, D’Arcy BR. Effect of high-power ultrasound waves on properties of meat: a review. Int J Food Prop. 2004; 7:301–319.
37. Leong TSH, Wooster TJ, Kentish SE, Ashokkumar M. Minimising oil droplet size using ultrasonic emulsification. Ultrason Sonochem. 2009; 16:721–727.
38. Gaikwad SG, Pandit AB. Ultrasound emulsification: effect of ultrasonic and physicochemical properties on dispersed phase volume and droplet size. Ultrason Sonochem. 2008; 15:554–563.
39. Salvia-Trujillo L, Rojas-Graü MA, Soliva-Fortuny R, Martín-Belloso O. Impact of microfluidization or ultrasound processing on the antimicrobial activity against Escherichia coli of lemongrass oil-loaded nanoemulsions. Food Control. 2014; 37:292–297. doi: 10.1016/j.foodcont.2013.09.015. 
40. Solans C, Solé I. Nano-emulsions: formation by low-energy methods. Curr Opin Colloid Interface Sci. 2012; 17:246–254.
41. Solè I, Pey CM, Maestro Al, González C, Porras M, Solans C, et al. Nano-emulsions prepared by the phase inversion composition method: preparation variables and scale up. J Colloid Interface Sci. 2010; 344:417–423.
42. Ishak KA, Annuar MSM. Phase inversion of medium-chain-length poly-3-hydroxyalkanoates (mcl-PHA)-incorporated nanoemulsion: effects of mcl-PHA molecular weight and amount on its mechanism. Colloid Polym Sci. 2016; 294:1969–1981.
43. Armanet L, Hunkeler D. Phase inversion of polyacrylamide-based inverse-emulsions: influence of inverting-surfactant type and concentration. J Appl Polym Sci. 2007; 103:3567–3584.
44. Agrawal S, Giri TK, Tripathi DK, Ajazuddin Alexander A. A review on novel therapeutic strategies for the enhancement of solubility for hydrophobic drugs through lipid and surfactant based self micro emulsifying drug delivery system: a novel approach. Am J Drug Discovery Dev. 2012; 2:143–183.
45. Pouton CW. Lipid formulations for oral administration of drugs: non-emulsifying, self-emulsifying and ‘self-microemulsifying’ drug delivery systems. Eur J Pharm Sci. 2000;11: S93–S98.
46. Kheawfu K, Pikulkaew S, Rades T, Müllertz A, Okonogi S. Development and characterization of clove oil nanoemulsions and self-microemulsifying drug delivery systems. J Drug Delivery Sci Technol. 2018; 46:330–338.
47. Amin N, Das B. A review on formulation and characterization of nanoemulsion. International Journal of Current Pharmaceutical Research. 2019 Jul 15;11(4):1-5.
48. Azmi NAN, Elgharbawy AAM, Motlagh SR, Samsudin N, Salleh HM (2019) Review: Nanoemulsions: Factory for Food. Pharmaceuticaland Cosmetics Processes 7: 617.
49. Li Y, Zheng J, Xiao H, McClements DJ. Nanoemulsion based delivery systems for poorly water-soluble bioactive compounds: influence of formulation parameters on polymethoxyflavone crystallization. Food Hydrocolloids, 2012; 27:517–528.
50. Kovacevic A, Savic S, Vuleta G, Muller RH, Keck CM. Polyhydroxy surfactants for the formulation of lipid nanoparticles (SLN and NLC): effects on size, physical stability and particle matrix structure. Int J Pharm, 2011; 406:163–172. 49.
51. Gupta PK, Pandit JK, Kumar A, Swaroop P, Gupta S. Pharmaceutical nanotechnology novel nanoemulsion high energy emulsification preparation, evaluation and application. The Pharma Research, 2010; 3:117-138.
52. Baboota S, Shakeel F, Ahuja A, Ali J, Shafiq S. Design, development and evaluation of novel nanoemulsion formulations for transdermal potential of celecoxib. Acta Pharmaceutica, 2007; 57:315–332.
53. Sharma N, Mishra S, Sharma S, Deshpande RD, Sharma RK. Preparation and optimization of nanoemulsion for targeting drug delivery. International Journal of Drug Development and Research, 2013; 5:37-48.
54. Gupta A., et al. (2016). Nanoemulsions: formation, properties and applications. Soft Matter, 12(11): 2826– 2841.
55. Pathak, K., Pattnaik, S., & Swain, K. (2018). Application of Nanoemulsions in drug delivery. In Nanoemulsions, Elsevier, Pages 415–433.
56. Wu, Y., et al. (2013). The application of Nanoemulsion in dermatology: an overview. Journal of Drug Targeting, 21(4): 321–327.
57. Ahmad N., et al. (2019). Preparation of a novel curcumin Nanoemulsion by ultrasonication and its comparative effects in wound healing and the treatment of inflammation. RSC Advances, 9(35): 20192–20206.
58. Karthik, P., Ezhilarasi, P.N., & Anandharamakrishnan, C. (2017). Challenges associated in stability of food grade Nanoemulsions. Critical Reviews in Food Science and Nutrition, 57(7): 1435–50.
59. Thakur, A., Walia, M.K., & Kumar, S. (2013). Nanoemulsion in enhancement of bioavailability of poorly soluble drugs: a review. Pharmacophore, 4(1): 15–25..