Techniques for Formulation of Nanoemulsion Drug Delivery System: A Review.

Nanoemulsion drug delivery systems are advanced modes for delivering and improving the bioavailability of hydrophobic drugs and the drug which have high first pass metabolism. The nanoemulsion can be prepared by both high energy and low energy methods. High energy method includes high-pressure homogenization, microfluidization, and ultrasonication whereas low energy methods include the phase inversion emulsification method and the self-nanoemulsification method. Low energy methods should be preferred over high energy methods as these methods require less energy, so are more efficient and do not require any sophisticated instruments. However high energy methods are more favorable for food grade emulsion as they require lower quantities of surfactant than low energy methods. Techniques for formulation of nanoemulsion drug delivery system are overlapping in nature, especially in the case of low energy methods. In this review, we have classified different methods for formulation of nanoemulsion systems based on energy requirements, nature of phase inversion, and self-emulsification.

INTRODUCTION

Nanoemulsion drug delivery systems are a promising tool for delivering and improving the bioavailability of hydrophobic drugs and bioactive food components in the blood. The majority of drugs are hydrophobic (lipophilic) in nature, thus leads to low solubility and bioavailability problems (Karthik et al., 2017; Mu et al., 2013; Qian and McClements, 2011); the bioactive food components also show low bioavailabilities in conventional doses. Such drug and food component formulations have low oral bioavailability, uncertain absorption profiles, dose variations, wide intra and inter-subject variabilities, and increased the chance of food effect. Thus, these drugs and bioactive food components express poor therapeutic efficacy (Chatterjee et al., 2016; Dokania and Joshi, 2015; Karthik et al., 2017).

Currently, lipid-based formulations are a good choice for delivering drug and bioactive food components, which have low oral bioavailability and other formulation problems (Feeney et al., 2016). Nanoemulsion drug delivery systems are lipid-based formulation system which improve the solubility and bioavailability of hydrophobic drugs and bioactive food components (Belhaj et al., 2010; Gursoy and Benita, 2004; Mohsin et al., 2009; Yuan et al., 2008). Flavonoids (flavanols, flavones, flavanones, and isoflavones), nonflavonoids (hydroxybenzoic acids, stilbenes, and curcuminoids), and carotenoids (carotenes and xanthophylls) are food bioactive compounds that have been encapsulated successfully in nanoemulsions formulations (Donsì, 2018).

These nanoemulsion systems have high interfacial areas and stabilities, protect compounds from adverse environmental conditions and improve their stability (Madene et al., 2006). Nanoemulsion systems can be used for delivering drugs via transmucosal and transdermal routes. Therefore these systems can effectively improve bioavailability (Rehman et al., 2017).

NANOEMULSION

Nanoemulsion is defined as a colloidal dispersion of two immiscible liquids that is thermodynamically unstable. In nanoemulsion, one of the liquids forms the dispersed phase and other liquid forms the dispersing medium (McClements, 2012). Nanoemulsion comprises droplets with diameters ranging from 10~200 nm and each droplet has a protective coating of emulsifier molecules (Acosta, 2009; Cerpnjak et al., 2013; Gibaud and Attivi, 2012; Rehman et al., 2017).

Self-emulsifying formulation

Self-emulsifying formulations generally comprise of self-emulsifying drug delivery systems (SEDDS) and self-nanoemulsifying drug delivery systems (SNEDDS). SEDDS give coarse emulsion whereas SNEDDS provide nano-size emulsion. These systems are isotropic mixtures of an oil, surfactant, and co-surfactant. Upon in vivo dilution by the aqueous phase, these systems form emulsions (in case of SEDDS) or fine and optical clear nanoemulsions (in case of SNEDDS) under gentle agitation, experienced due to gastrointestinal tract (GIT) motility. SEDDS and SNEDDS are generally described as emulsion or nanoemulsion pre-concentrates because the emulsion or nanoemulsion is formed from dilution in aqueous media in vivo (Pouton, 2000; Pouton and Porter, 2008).

Component of nanoemulsion

Components of nanoemulsion systems include oils, lipids, surfactants, water-soluble co-solvents, and water. In the formulation of nanoemulsions, the oil phase may include triglycerides like tri-, di-, or mono-acylglycerols, vegetable oils, mineral oils, free fatty acids etc. (Gonçalves et al., 2018). Oil selection is generally based on the drug solubility. Oil phases which have high drug loading is generally used for development of nanoemulsion (Qadir et al., 2016). Common surfactants used in the nanoemulsion systems for drug delivery and food ingredients are spans (sorbitan fatty acid esters), tweens [polyoxyethylene (POE) derivatives of sorbitan fatty acid ester], Cremophor® EL (polyoxyl-35 castor oil), lauroyl macro-golglycerides (Gelucire® 44/14), polysaccharides (gum and starch derivatives), phospholipids (egg, soy, or dairy lecithin), and amphiphilic proteins (whey protein isolate and caseinate) (Komaiko and McClements, 2016; Singh et al., 2017).

Ultra-low negative interfacial tension is required for nanoemulsion formation. For this purpose, co-surfactants or co-solvents are used along with a surfactant. Co-surfactants or co-solvents that are generally used in formulation of nanoemulsion systems are polyethylene glycol, propylene glycol, ethanol, transcutol-P (diethylene glycol monoethyl ether), ethylene glycol, glycerin, and propanol (Khan et al., 2012; Singh et al., 2017).

FORMULATION TECHNIQUES OF NANOEMULSION DRUG DELIVERY SYSTEMS

The techniques employed in formulation of nanoemulsion drug delivery systems are diverse and show a large degree of overlapping. We have classified different methods for preparation of nanoemulsion drug delivery systems by energy requirements, nature of phase inversion and self-emulsification

High-pressure homogenization

Microfluidization

Ultrasonication

i) Phase inversion emulsification method

– Transitional phase inversion (TPI)

Phase inversion temperature (PIT)

Phase inversion composition (PIC)

– Catastrophic phase inversion (CPI)

Emulsion inversion point (EIP)

The self-nanoemulsification method

High energy methods

High energy methods are extensively used to formulate nanoemulsion (Mahdi Jafari et al., 2006). High mechanical energy is used that provide strong disruptive forces, which break up large droplets to nano-sized droplets and produce nanoemulsions with high kinetic energy. The disruptive forces are created by using mechanical devices such as ultrasonicators, microfluidizer, and high-pressure homogenizers (Gonçalves et al., 2018). By using high energy methods, we can achieve a greater control of particle size with a choice of formulation composition. High energy methods also provide controls for stability, rheology, and color of the emulsion (Graves et al., 2005; Gursoy and Benita, 2004). In case of food ingredients, high energy methods of nanoemulsion formulation have the advantage of reducing risk of spoilage and inactivation of food components without affecting food safety, and nutritional and sensory attributes (Gharibzahedi et al., 2019). High energy methods involve the following methods.

High-pressure homogenization

High-pressure homogenizers supply high energy and give homogeneous flow to generate smallest particle sizes. Therefore, high-pressure homogenizers are most widely used to prepare nanoemulsions. High-pressure homogenizers are used to create intensely disruptive forces that form nanoemulsions of extremely low particle size (up to 1 nm) (Rai et al., 2018). The coarse emulsion is then passed through a small orifice with high pressure (500 to 5,000 psi) (Fig. 1). Several forces, such as intense turbulence, hydraulic shear, and cavitation, are applied together during this process, to give nanoemulsions with very small droplet sizes (Floury et al., 2000; Schultz et al., 2004).

Fig. 1

High-pressure homogenization techniques (Wang, 2014).

The particle size of nanoemulsions produced by high-pressure homogenizers depend on sample composition, homogenizer type, and homogenizer operating conditions such as energy intensity, time, and temperature (Qian and McClements, 2011). Increasing the intensity of homogenization decreases the size of droplets of the nanoemulsions. In certain cases, such as when biopolymers are used as an emulsifier, intense homogenization can lead to an increase in particle size of resulting nanoemulsion. Hence small-molecule surfactants should be used as emulsifiers in high-pressure homogenizers as they are more effective than biopolymer for producing nanoemulsions (Azeem et al., 2009a; Azeem et al., 2009b; Jafari et al., 2008; Jafari et al., 2007). High-pressure homogenization is widely used to form food, pharmaceutical, and biotechnological ingredient nanoemulsions, as shown in Table 1 (Hsieh et al., 2012; Yuan et al., 2008).

Table 1

Various active ingredients, loaded in nanoemulsion using different techniques

Techniques	Active ingredients
High-pressure homogenization	Quercetin (Karadag et al., 2013), peppermint oil (Liang et al., 2012), carotenoid (Salvia-Trujillo et al., 2015), peanut milk (Zaaboul et al., 2019), capsaicin (Akbas et al., 2018), primaquine (Singh and Vingkar, 2008), and paclitaxel (Yang et al., 2014).
Microfluidization	Essential oil (Salvia-Trujillo et al., 2015), D-limonene (Mahdi Jafari et al., 2006), fish oil (García-Márquez et al., 2017), curcumin (Kim et al., 2016), and β-carotene (Luo et al., 2017).
Ultrasonicator	Cinnamon oil (Sugumar et al., 2014), eucalyptus oil (Ghosh et al., 2013b), bovine serum, albumin (Tabibiazar et al., 2015), capsaicin (Akbas et al., 2018), aspirin (Tang et al., 2012), and artemether (Laxmi et al., 2015).
Phase inversion composition method	Hexadecane and oleic acid mixture (Maestro et al., 2008), lidocaine (Sadurní et al., 2005), and hydrogenated polyisobutene (Sonneville-Aubrun et al., 2009).
Phase inversion temperature method	Fisetin (Ragelle et al., 2012), isohexadecane (Izquierdo et al., 2004), minaral oil (Morales et al., 2006), cinnamon oil (Chuesiang et al., 2018), and lemon oil (Mashhadi et al., 2016).
Emulsion inversion point method	Curcumin (Borrin et al., 2016) and vitamin E (Mayer et al., 2013).
Self-nanoemulsification method	Glimepiride (Mohd et al., 2015), ibuprofen (Zhao et al., 2015), valsartan (Bandyopadhyay et al., 2015), and glibenclamide (Shakeel et al., 2013).

Microfluidization

Microfluidization is a mixing technology at micro size level that uses a device called microfluidizer. In microfludization, fluids are forced to pass through the microchannels under high-pressure (500~20,000 psi). Microchannels are generally micro size channels which allow mixing at micro size level (Qadir et al., 2016; Whitesides, 2006). The phases of macroemulsion (aqueous and oil phases) are mixed together and then passed through the microfluidizer. The macroemulsion is guided through the microchannels under high pressure towards the interaction chamber. In the interaction chamber, two streams of macroemulsions strike each other at high velocity. This collision creates forces like shearing, cavitation, and impact, which produce stable nanoemulsions (Fig. 2) (Jafari et al., 2007; Lovelyn and Attama, 2011; Maa and Hsu, 1999; Mahdi Jafari et al., 2006).

Fig. 2

Microfluidization techniques (Singh et al., 2017).

Microfluidizers produce narrower and smaller nanoemulsion particle size distributions of than homogenizers (Perrier-Cornet et al., 2005; Wooster et al., 2008). Also, microfluidizers produce stable nanoemulsions at low surfactant concentrations (Pinnamaneni et al., 2003). Microfluidization methods have been used to produce food ingredient nanoemulsions (Goh et al., 2015; Zhang et al., 2015). Microfluidization techniques produce food grade nanoemulsions with uniform droplet size distributions and greater stabilities (Villalobos-Castillejos et al., 2018).

Ultrasonication

Ultrasonication is better than other high energy methods in terms of operation and cleaning (Leong et al., 2009; Mahdi Jafari et al., 2006). In ultrasonic emulsifications, ultrasonic waves provide cavitation forces that break the macroemulsion to nanoemulsion. In this method, ultrasonicators are used, which consist of a probe that emits ultrasonic waves. By varying ultrasonic energy input and time, we can achieve the desired particle size and stability of the nanoemulsion. In ultrasonic emulsification, physical shear is mainly provided by the process of acoustic cavitation (Jayasooriya et al., 2004). Cavitation is the phenomenon of formation and growth of microbubbles and then collapse of microbubbles, which are caused by the pressure fluctuations of the acoustic wave (Fig. 3). The collapse of microbubbles cause intense turbulence that causes formation of nano-sized droplets (Canselier et al., 2002; Leong et al., 2009).

Fig. 3

Ultrasonication techniques (Singh et al., 2017).

Irradiation of an oil and water system by ultrasound causes cavitation forces and provide excess energy for new interface formations, forming nano-sized emulsion droplets. Through ultrasonication, nanoemulsions can be produced in the absence of surfactants (Gaikwad and Pandit, 2008; Jafari et al., 2007). In a recent study, it was shown that efficiency of ultrasonic emulsification is dependent on ultrasonication intensity, time, and nature of the surfactant (Landfester et al., 2004). Ultrasonication has been used extensively for producing nanoemulsions of drugs and food ingredients. Food grade ultrasonication nanoemulsion shows greater stability and smaller droplet size, and requires less energy input than other high energy method (Ghosh et al., 2013a; Salvia-Trujillo et al., 2014; Tiwari et al., 2006).

Low energy method

These methods require low energy for production of nanoemulsion systems. Low-energy emulsification methods are more energy efficient as these methods utilize internal chemical energy of the systems, and require only gentle stirring for production of the nanoemulsions (Solans and Solé, 2012). Low-energy emulsification methods generally involve phase inversion emulsification and self-emulsification. Generally low energy methods are not considered for formulation of food grade nanoemulsions as they require high concentration of surfactant, which adversely affect food formulation taste and safety (Komaiko and McClements, 2016).

Phase inversion emulsification method

In this method, spontaneous curvature of surfactant causes phase transition during the emulsification process. Changes in spontaneous curvature of the surfactant occur by changes in parameters like temperature, composition, etc. (Fig. 4) (Solè et al., 2010). There are two types of phase inversion emulsification methods: TPI methods, which involve PIT and PIC, and CPI methods, which involves EIP (Fig. 4).

Fig. 4

Phase inversion emulsification techniques (Solans and Solé, 2012). PIC, phase inversion composition; PIT, phase inversion temperature; EIP, emulsion inversion point; O/W, oil-in-water emulsion; W/O, water-in-oil emulsion.

Transitional phase inversion takes place due to the changes in spontaneous curvature or affinity of the surfactant due to changes in parameters like temperature and composition (Ishak and Annuar, 2016; Solans and Solé, 2012; Solè et al., 2010). However, CPI occurs when dispersed phase added continuously until the dispersed phase drops are aggregated with each other to form bi-continuous/lamellar structural phases (Ishak and Annuar, 2016). The catastrophe means a sudden change in the behavior of a system, due to changing conditions. For catastrophic phase inversion to occur, it is important that the surfactant is chiefly presented in the dispersed phase, thus the rate of coalescence is high, which leads to rapid phase inversion (Armanet and Hunkeler, 2007). During transitional phase inversion, spontaneous curvature or surfactant affinity is changed, whereas in catastrophic phase inversion spontaneous curvature or surfactant affinity does not change.

– PIT method

In the PIT method, surfactant spontaneous curvature is inversed by changing temperature. Nonionic surfactants, such as polyethoxylated surfactants, undergo dehydration of POE groups of polyethoxylated surfactant, which makes it more lipophilic and leads to changes in curvature of the surfactant. Thus, phase inversion occurs and nanoemulsion is produced (Moreira de Morais et al., 2006; Solans and Solé, 2012). In this method, oil, water, and nonionic surfactants are mixed at room temperature to form oil-in-water (O/W) emulsions. Then, as the temperature gradually increases, dehydration of surfactant POE groups occurs that makes surfactant more lipophilic and surfactant start showing a higher affinity towards the oily phase. This cause phase inversion from the initial O/W emulsion to water-in-oil (W/O) nanoemulsion through intermediate liquid crystalline or bi-continuous structures (e.g. lamellar phase). At hydrophile-lipophile balance (HLB) temperatures (an intermediate temperature) the non-ionic surfactant has zero curvature and shows a similar affinity to the aqueous and oily phases (Izquierdo et al., 2005; Izquierdo et al., 2004; Izquierdo et al., 2002). For efficient phase inversion, rapid cooling or heating of HLB (for obtaining O/W or W/O emulsions, respectively) is required. Rapid cooling or heating produces kinetically stable nanoemulsions (Solans and Solé, 2012).

– PIC method

The phase inversion composition or PIC method is similar to the PIT method; however, in PIC, phase inversion is achieved by changing the system composition rather than the system temperature (Sokolov, 2014).

In PIC, one of the components such as water is added to a mixture, and oil-surfactant or oil is added to the water-surfactant mixture. POE type nonionic surfactants are generally used in PIC method to formulate nanoemulsions, although other types can also be used. When water is added slowly to the oil phase and as the volume of the water fraction increases, surfactant POE chain hydration occurs. The surfactant hydrophilic-lipophilic properties of the water phase will become balanced and spontaneous curvature of surfactant will change to zero, similar to at the HLB temperature in the PIT method. During this transition, a bi-continuous or lamellar structure is formed. When additional water is added the transition composition is exceeded, and the structures of the surfactant layer with zero curvature change to having high positive curvature. This change in curvature leads to phase inversion and causes nano-size droplet formation. Thus, changing the composition of the system causes phase inversion (Solans and Solé, 2012; Vandamme and Anton, 2010). Similarly, other composition parameters, such as the addition of salt and pH changes, also cause nano-size emulsion droplets by transitional phase inversion (Maestro et al., 2008; McClements and Rao, 2011; Sokolov, 2014).

– EIP method

In the EIP method, phase inversion occurrs through CPI mechanisms (Sokolov, 2014). The catastrophic phase inversion is induced by changing the fractioned volume of the dispersed phase rather than the surfactant properties (Fernandez et al., 2004; Ishak and Annuar, 2016; McClements and Rao, 2011). As the water phase is added to the oil-surfactant mix, the system starts acting as a W/O nanoemulsion. When increasing amounts of water is added to above a critical water content with continuous stirring, water droplets merge with each other and the phase inversion point is reached; this causes bi-continuous or lamellar structures to be formed. Further dilution with water causes phase inversion from a W/O to an O/W system through intermediate bi-continuous microemulsion. The sizes of the nanoemulsion droplets formed depend on the process variables, such as the rate of water addition and the stirring speed. For catastrophic phase inversion to occur, the surfactant should primarily present in the dispersed phase, so the rate of coalescence is high and rapid phase inversion occurs. Small molecule surfactants can be used in catastrophic phase inversion. These surfactants are able to stabilize both W/O emulsions and O/W emulsions (Armanet and Hunkeler, 2007; Fernandez et al., 2004; Sokolov, 2014). Initially in catastrophic phase inversion, the surfactant is mainly present in the dispersed phase, thus it behaves as an abnormal emulsion (unstable emulsion) which does not obey Bancroft’s rules. According to Bancroft’s rules, for a stable emulsion (normal emulsion) emulsifier should predominantly present in the continuous phase (Perazzo et al., 2015). Therefore, catastrophic phase inversion occurs from the abnormal emulsion to form a more stable normal emulsion.

Self-nanoemulsification method

In the self-emulsification method, nanoemulsion formation is achieved without changing the spontaneous curvature of the surfactant. Surfactant and/or co-solvent molecules rapidly diffuse from the dispersed phase to the continuous phase, which causes turbulence and creates nano-sized emulsion droplets. The self-emulsification method is also referred to as the spontaneous emulsification method (Solans et al., 2016; Solans and Solé, 2012; Solè et al., 2012). SNEDDS are based on the self-emulsification phenomenon and contain more hydrophilic surfactants or co-surfactants (co-solvents), and a lower lipid content (Agrawal et al., 2012). SNEDDS can be defined as isotropic mixture of an oil, surfactant, co-surfactant, and drug. When this mixture is diluted by aqueous fluids in vivo, it form fine and optical clear O/W nanoemulsion, aided by gentle agitation provided by digestive motility of the stomach and intestine (Fig. 5) (Bandyopadhyay et al., 2014; Khan et al., 2015).

Fig. 5

Self-nanoemulsification techniques (Alshamsan et al., 2018). SNEDDS, self-nanoemulsifying drug delivery system; GIT, gastrointestinal tract.

The two most commonly reported mechanisms of nanoemulsion formation from SNEDDS are diffusion of the hydrophilic co-solvent or co-surfactant from the organic phase into the aqueous phase (Pouton, 2000; Solans and Solé, 2012; Solè et al., 2012), and formation of nanoemulsion negative free energy at transient negative or ultra-low interfacial tensions (Agrawal et al., 2012; Gurram et al., 2015; Kohli et al., 2010). SNEDDS are also the most popular and promising tool for delivery of hydrophobic drugs with low bioavailability (Meena et al., 2012; Patel and Vavia, 2007; Patel et al., 2016; Suryawanshi and Kondawar, 2014). SNEDDS have also been used for delivery of bioactive food components (Kheawfu et al., 2018).

Stability of nanoemulsion system

During storage, nanoemulsion may become turbid or phases of nanoemulsion may separate due to mechanisms of instability such as flocculation, sedimentation, coalescence, and Ostwald ripening (Karthik et al., 2017). Nanoemulsion system destabilization kinetics are very slow (several months), therefore nanoemulsion systems are kinetically stable (Rehman et al., 2017). Nanoemulsion systems produce smaller droplets sizes than conventional macro emulsions; thus, Brownian motion effects are much more dominate then gravitational forces and have greater gravitational separation stability. Flocculation and coalescence occur due to the attractive forces between the droplets, which are generally very low in nano-sized emulsion systems. Thus, nanoemulsion also shows much better stability towards the flocculation and coalescence (Qian and McClements, 2011). Ostwald ripening is another mechanism of nanoemulsion instability, which commonly occurs in food grade nanoemulsion containing essential oil and short chain triglycerides. Dairy based nanoemulsion is relatively stable towards Ostwald ripening due to the presence of insoluble long-chain triglyceride oils. Ostwald repining can be prevented by using greater hydrophobic oil during formulation (Karthik et al., 2017).

ADVANTAGES OF NANOEMULSION DRUG DELIVERY SYSTEM

Nanoemulsion drug delivery systems are effective in solubilizing active lipophilic compounds, and therefore have for several applications (Gutiérrez et al., 2008). The very small particle sizes of nanoemulsion drug delivery systems is a promising advantage over conventional emulsions; these systems therefore have optically transparent appearances, and show greater stability against droplet flocculation and coalescence (Qian and McClements, 2011; Solans et al., 2005; Tadros et al., 2004). Nanoemulsion drug delivery system has shown potential for effective systemic delivery of active components, such as food ingredients and lipophilic drugs, via oral, parenteral, ocular, and topical routes (Hu et al., 2004; McClements et al., 2007; de Oca-Ávalos et al., 2017; Sanguansri and Augustin, 2006; Solans et al., 2005; Weiss et al., 2008). O/W vitamin nanoemulsions and nutraceuticals facilitate solubilization of these hydrophobic bioactive food components in GIT, therefore increasing bioavailability (Komaiko and McClements, 2016).

Nanoencapsulation of food ingredients in nanoemulsion systems also increase the physio-chemical stability of bioactive compound. Moreover, nanoemulsion food component systems enhance delivery the bioactive compounds to the intradermal layers through diffusion, and are therefore helpful in developing herbal cosmetics (Karthik et al., 2017). The cosmetic applications of nanoemulsion systems derive from properties of system such as nano-sized droplets, low viscosity, and transparency (Solans et al., 2005; Sonneville-Aubrun et al., 2004; Thakur et al., 2012). The physio-chemical properties of nanoemulsion systems improve bioactivity of encapsulated components, and have wide application for delivery of antibiotics, anticancer agents, disinfectants, and antiseptics (Karthik et al., 2017). Thus, nanoemulsion drug delivery systems are promising methods for formulation of drugs, food ingredients, and cosmetic agents.

CONCLUSION

Nanoemulsion drug delivery systems effectively overcome the low bioavailability drawback associated with drugs and food components which are hydrophobic, and having high first pass metabolism. Methods for nanoemulsion drug delivery system formulation can be conveniently classified in a rigid fashion based on the energy required, nature of phase inversion and self-emulsification. High energy methods for nanoemulsion drug delivery system formulation have greater control over dispersion of particle sizes and are more flexible for the choice of composition. High energy methods have been used by researchers to improve delivery of drugs and bioactive food components. For providing high energy, sophisticated instruments are required; high energy methods are therefore more costly then low energy methods as these methods require low energy and are more efficient. High energy methods are more useful for delivery of nanoemulsions containing bioactive food components, as these methods require low concentrations of surfactant. For delivery of hydrophobic drugs with low bioavailability, SNEDDS are the most popular methods used by researchers. However, further research is required to uncover the potential of phase inversion emulsification methods in effective drug loading and delivery.