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Creating Nanoemulsions: An Overview

Written by Oisin Tierney

Making Oil and Water Mix

We all understand the concept that water and oil do not mix; yet without much effort, we can find numerous household examples of products where water and oil do mix through effective emulsification – mayonnaise, salad dressings, and lotions to name a few. A mixture of two immiscible liquids is called an emulsion and is made up of spherical droplets (dispersed phase), suspended in a liquid (continuous phase).

Generally, emulsions consist of oil suspended in water (o/w) but can also be found as water suspended in oil (w/o). A classic example of an o/w emulsion is milk, where the milk fat is dispersed in a continuous water phase, while an example of a w/o emulsion is butter, in which tiny droplets of water are dispersed uniformly throughout a continuous fat phase.

Most drugs in development are lipophilic in nature, resulting in low aqueous solubility and poor bioavailability (the rate and extent to which the drug reaches circulation). [1] Cannabinoids as active ingredients are no different, presenting several challenges for the cannabis formulator, especially when developing water-based beverages.

As cannabis consumers become more educated and demand more differentiated product types, a wide variety of formulation-enabling technologies have been adopted by the industry. The most prevalent are undoubtably nanoemulsions, which are kinetically stable emulsions with tiny suspended droplets with diameters between 20 nm and 400 nm. [2]

Nanoemulsions are particularly useful in the cannabis industry for creating infused beverages, but also offer the added benefit of improving cannabinoid bioavailability through particle size reduction and enhanced water-compatibility. [1] In many cases, transparent nanoemulsions can be developed as the dispersed oil droplets are on the order of 25% of the size of the smallest of visible light [3], meaning that they can be introduced into water without compromising optical clarity.

This article presents an overview of some of the important factors to consider when developing a nanoemulsion, including composition and methodology.

 

Components of a Nanoemulsion

Typically, successful nanoemulsions consist of three primary components: oil phase, aqueous phase, and emulsifier (Figure 1).

Figure 1. The primary components of a typical nanoemulsion

 

Oil Phase

The dispersed oil phase consists of the active, hydrophobic ingredient (i.e., cannabinoids) and a carrier oil. Although technically possible, developing a nanoemulsion in the absence of a carrier oil is not recommended. Because cannabis extracts are thick and highly viscous, the addition of a well-chosen carrier oil can reduce the viscosity of the oil phase, improving handling and processing capabilities during manufacturing. Triglycerides that are commonly used as carrier oils can also enhance absorption after ingestion, even promoting uptake through the lymphatic system to bypass first-pass metabolism by the liver.  [4] Furthermore, carrier oils can improve the stability of nanoemulsions by aiding in the reduction of Ostwald ripening, a prevalent destabilization pathway where small particles concede to larger particles.  [5]

 

Aqueous Phase

In the case of o/w formulations, the aqueous phase represents the major constituent (continuous phase) of a nanoemulsion. It is mainly made up of water, but can also include antioxidants, preservatives, flavors, vitamins, or sugars.[6] Of course, the presence of each additional ingredient will affect the performance (including stability and droplet size) of the nanoemulsion system, for better or for worse.

 

Emulsifier

Emulsifiers (belonging to the broader class of surfactants) are typically amphiphilic molecules that are added to stabilize nanoemulsions. Amphiphiles consist of a hydrophilic head and a hydrophobic tail which enables them to adsorb and remain at the oil/water interface, thus creating a defensive interfacial layer around oil droplets to prevent their subsequent accumulation (flocculation) and breakdown. Surfactants used in nanoemulsions can be natural or synthetic and commonly include lecithin and Tween 80 amongst others. In many cases, a combination of surfactants is deployed (including expensive proprietary blends) as well as co-surfactants that fortify the interfacial layer.

Surfactant selection and concentration influences taste, stability, droplet size, pharmacokinetics, and pharmacodynamics, and is a crucial consideration during development. [7] A smaller target droplet size will likely result in a higher concentration of surfactant. As the droplet size decreases, the total interfacial area increases, calling for more surfactant to effectively cover the entirety of the interfacial surface. Similarly, a higher concentration of carrier oil calls for additional surfactant to effectively coat the entirety of the oil phase. As such, careful selection of the oil/surfactant ratio in the context of your target droplet size is crucial when formulating emulsions.


Creating Nanoemulsions

Techniques for creating nanoemulsions are broadly classified into two categories: high-energy or low-energy methods.

Low-Energy Methods

Low-energy emulsification methods exploit the internal energy of the system to create small droplets. As a result, the input energy required can be achieved through gentle stirring during preparation. The two most widely used low-energy methods are self-emulsifications (SE) and emulsion phase inversion emulsification (EPI) (which includes phase inversion composition (PIC), and phase inversion temperature (PIT)). [8]

However, for food-grade nanoemulsions, low-energy methods are generally not considered. The lower energy input requires a higher concentration of surfactants, which can adversely affect the taste and safety of the final product. [9]

 

High-Energy Methods

Aside from the advantage of requiring a lower concentration of surfactants, high-energy methods also offer greater control over droplet size, and commercial manufacturing equipment is readily available for high-throughput production of nanoemulsions.

Generally, there are two steps involved in creating a nanoemulsion using high-energy methods (Figure 2). Firstly, the components of the system are mixed using a high-speed mixer or stirrer to form a coarse emulsion. Subsequently, the coarse emulsion is subjected to intense disruptive forces to promote a reduction in the dispersed droplet size. [8]

 

Figure 2. A high-level overview of the steps involved in creating a nanoemulsion using high energy methods.

 

There are several high energy mechanical devices available that provide the disruptive forces necessary for droplet size reduction, notably ultrasonicators, high pressure homogenizers, and microfluidizers, all of which are actively utilized in the cannabis industry.

 

High-Pressure Homogenization

High-pressure homogenizers make use of extreme hydraulic shear and turbulence to break apart macro-sized droplets in the coarse emulsion to a smaller size (Figure 3). During this process, the coarse emulsion is introduced into the first stage input chamber where it is subjected to high pressure by way of a piston pump. When the pressure reaches a pre-determined value, a narrow valve (typically 15-300 µm) opens downstream, and the coarse emulsion is forced through the slit and into a second chamber. [10] This process is repeated through multiple cycles until there is successful formation of nanoemulsions. The resultant particle size is dependant on the homogenization pressure, the number of cycles, and properties and concentrations of the oil phase and emulsifier.

 

Figure 3. Schematic of a high-pressure homogenizer [8]

 

Microfluidization

In microfluidization, the coarse emulsion is forced by way of a high-pressure displacement pump through an interaction chamber (Figure 4). Inside this interaction chamber, the sample moves through micro-channels that are approximately the diameter of a human hair. Pushing the sample through such narrow channels at extreme pressures of up to 30,000 psi causes the liquid streams to accelerate to velocities approaching 500 m/s. [11] These jet streams are ultimately subject to high disruption force as they collide with the chamber walls and themselves, in an area referred to as the zone of impaction. [12] Usually, the emulsion is subject to multiple cycles through the interaction chamber until the desired particle size is reached. A big upside of this method is its linearly scalability, making it an attractive option as you grow your business.

Figure 4. Reaction chamber of a microfluidizer

 

Ultrasonication

As the name suggests, ultrasonication makes use of ultrasonic vibration to produce high frequency sound waves in the range of 20-30 kHz. [7] Physical shear is provided through the phenomenon of cavitation, which is the formation and growth of vapor-filled microbubbles in the liquid caused by the pressure fluctuations of the sound wave. These microbubbles continue to grow until they eventually implode, generating shock waves that disperse through the surrounding liquid, pressurizing and breaking up the dispersed oil phase into nano-sized droplets (Figure 5). [13] Procedurally, the coarse emulsion is subjected to ultrasonication at different amplitudes and for varying periods of time until the desired particle size is achieved.

Compared with the other high-energy methods listed as part of this article, ultrasonication requires the least amount of energy-input, and offers a more cost-effective approach to nanoemulsions, especially at smaller scales. Consideration must be given to the increased risk of contamination presented by introducing a probe into the sample, as well as the challenges associated with scaling up from bench-scale to high-throughput production. [7] However, commercial instrumentation is available that moves the sample from a reservoir through a ultrasonication chamber, allowing for a high-volume, continuous feed process.

 

Figure 5. Cavitation created by sonication, breaking oil droplets into smaller sizes [7]

 

A Strategic Approach

In conclusion, there are several important factors to consider when developing nanoemulsions that will affect their stability and physicochemical properties, including ingredient composition and methodology. The properties of your nanoemulsion should directly align with its intended application as well as the properties of your input cannabis material. Thus, specificity is key.

 

References:

[1] Williams HD, Trevaskis NL, Charman SA, et al. Strategies to address low drug solubility in discovery and development. Pharmacol Rev. 2013;65(1):315-499. [journal impact factor = 17.814; times cited = 804]

 

[2] 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. [journal impact factor = 7.727; times cited = 134]

 

[3] Anton N, Vandamme TF. Nano-emulsions and micro-emulsions: clarifications of the critical differences. Pharm Res. 2011;28(5):978-985. [journal impact factor = 3.424; times cited = 332]

 

[4] Nanjwade BK, Patel DJ, Udhani RA, Manvi FV. Functions of lipids for enhancement of oral bioavailability of poorly water-soluble drugs. Sci Pharm. 2011;79(4):705-727. [journal impact factor = 3.43; times cited = 82]

 

[5] Wooster TJ, Golding M, Sanguansri P. Impact of oil type on nanoemulsion formation and Ostwald ripening stability. Langmuir. 2008;24(22):12758-12765. [journal impact factor = 3.557; times cited = 514]

 

[6] Molet-Rodríguez A, Salvia-Trujillo L, Martín-Belloso O. Beverage Emulsions: Key Aspects of Their Formulation and Physicochemical Stability. Beverages. 2018; 4(3):70. [journal impact factor = N/A; times cited = 11]

 

[7] Singh Y, Meher JG, Raval K, et al. Nanoemulsion: Concepts, development and applications in drug delivery. J Control Release. 2017;252:28-49. [journal impact factor = 7.727; times cited = 297]

 

[8] Gupta A, Eral HB, Hatton TA, Doyle PS. Nanoemulsions: formation, properties and applications. Soft Matter. 2016;12(11):2826-2841. [journal impact factor = 3.140; times cited = 424]

 

[9] Kumar M, Bishnoi RS, Shukla AK, Jain CP. Techniques for formulation of nanoemulsion drug delivery system: A review. Prev Nutr Food Sci. 2019;24(3):225-234. [journal impact factor = 2.02; times cited = 26]

 

[10] Wang Y. Preparation of nano- and microemulsions using phase inversion and emulsion titration methods. Master’s thesis. Massey University, Auckland, New Zealand. 2014. [times cited = 4]

 

[11] Chomistek K.J., & Panagiotou T. Large scale nanomaterial production using microfluidizer high shear processing. MRS Online Proceedings Library. 2009;1209:301. [journal impact factor = N/A; times cited = 3]

 

[12] Maa YF, Hsu CC. Performance of sonication and microfluidization for liquid-liquid emulsification. Pharm Dev Technol. 1999;4(2):233-240. [journal impact factor = 2.347; times cited = 146]

 

[13] Taha A., Ahmed E., Ismaiel A., Ashokkumar M., Xu X., Pan S., & Hu H. Ultrasonic emulsification: An overview on the preparation of different emulsifiers-stabilized emulsions. Trends in Food Science & Technology. 2020;105:363-377. [journal impact factor = 11.077; times cited = 7]

 

Oisin Tierney holds a degree in Biomedical Science from the National University of Ireland, Galway and is an alumnus of the inaugural class of the Cannabis Applied Science postgraduate program at Loyalist College, Ontario where he currently serves as the Chair of the Program Advisory Committee. Since early 2017, he has worked extensively in both the Canadian and US cannabis markets, primarily offering scientific consulting services.

 

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