Mrunali R. Patel

The design and development of new drug delivery systems with the intention of enhancing the efficacy of existing drugs is an ongoing process in pharmaceutical research. It is necessary for a pharmaceutical solution to contain a therapeutic dose of the drug in a volume convenient for administration.­¹

Dispersions of oil and water are commonly employed in the pharmaceutical industry. 

These dispersions can be classified in three major categories: ²

1. Microemulsions
2. Micellar Solutions
3. Conventional Emulsions (or) Macroemulsions

(Figure 1. Structures of Micellar solution, Microemulsion and Emulsion)

Out of these three, Microemulsions have attracted much interest in recent years in terms of their drug delivery potential.

“Microemulsions are liquid dispersions of water and oil that are made homogenous, transparent (or translucent) and thermodynamically stable by the addition of relatively large amounts of a surfactant and a co-surfactant and having diameter of the droplets in the range of 100 – 1000 A (10 – 100 nm).”³

(Figure 2:- Microemulsion Structure)

They appear to represent a state intermediate between thermodynamically stable solubilized solutions i.e. micelles containing solubilized oils and ordinary emulsions which are relatively unstable.⁴

Three types:

1. O/W Microemulsion               
2. W/O Microemulsion
3. Bicontinuous Microemulsion

(Figure 3: O/W, W/O and Bicontinuous Microemulsions)

History Of Microemulsion

Microemulsions were not really recognized until the work of Hoar and Schulman in 1940, who generated a clear single phase solution by titrating a milky emulsion with hexanol.⁵

The term “microemulsion” was first used even later by Schulman et al. in 1959 to describe a multiphase system consisting of water, oil, surfactant and alcohol, which forms a transparent solution. ⁶

There has been much debate about the word “microemulsion” to describe such systems. Although not systematically used today, some prefer the names “micellar emulsion” or “swollen micelles”.  Microemulsions were probably discovered well before the studies of Schulman: Australian housewives have used since the beginning of last century water/eucalyptus oil/soap flake/white spirit mixtures to wash wool, and the first commercial microemulsions were probably the liquid waxes discovered by Rodawald in 1928. 

Interest in microemulsions really stepped up in the late 1970’s and early 1980’s when it was recognized that such systems could improve oil recovery and when oil prices reached levels where tertiary recovery methods became profit earning.⁷

Nowadays this is no longer the case, the other microemulsion applications are discovered, e.g., catalysts, preparation of submicron particles, solar energy conversion, liquid-liquid extraction (mineral, proteins, etc.).  Together with classical applications in detergency and lubrication, the field remains sufficiently important to continue to attract a number of scientists.  From the fundamental research point of view, a great deal of progress has been made in the last 20 years in understanding microemulsion properties.

Important Characteristics Of Microemulsions⁸

üParticle size < 200 nm

üThermodynamically stable

üOptically clear

üSurface area increased

üHigh solubilizing capabilities

Advantages Of Microemulsion Based Systems ⁹

Microemulsions exhibits several advantages as a drug delivery system :

1.Microemulsions are thermodynamically stable system and the stability allows self-emulsification of the system whose properties are not dependent on the process followed.

2.Microemulsions act as supersolvents of drug.  They can solubilize hydrophilic and lipophilic drugs including drugs that are relatively insoluble in both aqueous and hydrophobic solvents.  This is due to existence of microdomains of different polarity within the same single-phase solution.

3.The dispersed phase, lipophilic or hydrophilic (oil-in-water, O/W, or water-in-oil, W/O microemulsions) can behave as a potential reservoir of lipophilic or hydrophilic drugs, respectively.  The drug partitions between dispersed and continuous phase, and when the system comes into contact with a semi-permeable membrane, the drug can be transported through the barrier.  Drug release with pseudo-zero-order kinetics can be obtained, depending on the volume of the dispersed phase, the partition of the drug and the transport rate of the drug.

4.The mean diameter of droplets in microemulsions is below 0.22 mm; they can be sterilized by filtration.  The small size of droplet in microemulsions e.g. below 100 nm, yields very large interfacial area, from which the drug can quickly be released into external phase when absorption (in vitro or in vivo) takes place, maintaining the concentration in the external phase close to initial levels.

5.Same microemulsions can carry both lipophilic and hydrophilic drugs.

6.Because of thermodynamic stability, microemulsions are easy to prepare and require no significant energy contribution during preparation.  Microemulsions have low viscosity compared to other emulsions.

7.The use of microemulsion as delivery systems can improve the efficacy of a drug, allowing the total dose to be reduced and thus minimizing side effects.

8.The formation of microemulsion is reversible.  They may become unstable at low or high temperature but when the temperature returns to the stability range, the microemulsion reforms.

Disadvantages Of Microemulsion Based Systems ¹

1.Use of a large concentration of surfactant and co-surfactant necessary for stabilizing the nanodroplets.

2.Limited solubilizing capacity for high-melting substances

3.The surfactant must be nontoxic for using pharmaceutical applications

4.Microemulsion stability is influenced by environmental parameters such as temperature and pH.  These parameters change upon microemulsion delivery to patients.

Comparison With Emulsions (Macroemulsions) ^10-15

Emulsions (Macroemulsions)	Microemulsions
Figure 4:   -  Emulsions	Figure 5:   -  Microemulsions
Emulsions consist of roughly spherical droplets of one phase dispersed into the other.	They constantly evolve between various structures ranging from droplet like swollen micelles to bicontinuous structure.
Droplet diameter: 1 – 20 mm.	10 – 100 nm.
Most emulsions are opaque (white) because bulk of their droplets is greater than wavelength of light and most oils have higher refractive indices than water.	Microemulsions are transparent or translucent as their droplet diameter are less than ¼ of the wavelength of light, they scatter little light.
Ordinary emulsion droplets, however small exist as individual entities until coalesance or ostwald ripening occurs.	Microemulsion droplet may disappear within a fraction of a second whilst another droplet forms spontaneously elsewhere in the system.
They may remain stable for long periods of time, will ultimately undergo phase separation on standing to attain a minimum in free energy. They are kinetically stable thermodynamically unstable.	More thermodynamically stable than macroemulsions and can have essentially infinite lifetime assuming no change in composition, temperature and pressure, and do not tend to separate.
They are lyophobic.	They are on the borderline between lyophobic and lyophilic colloids.
Require intense agitation for their formation.	Generally obtained by gentle mixing of ingredients.

Factors To Be Considered During Preparation Of Microemulsion ¹

Three important conditions:

Surfactants must be carefully chosen so that an ultra low interfacial tension (< 10^-3 mN/m) can be attained at the oil / water interface which is a prime requirement to produce microemulsions.

1.Concentration of surfactant must be high enough to provide the number of surfactant molecules needed to stabilize the microdroplets to be produced by an ultra low interfacial tension.

2.The interface must be flexible or fluid enough to promote the formation of microemulsions.

Theories Of Microemulsion Formation

Historically, three approaches have been used to explain microemulsion formation and stability. These are:

(i)Interfacial or mixed film theories ^6,16  

(ii)Solubilization theories ^17,18  

(iii)Thermodynamic treatments ^{19, 20,21}

An admittedly simplified thermodynamic rationalization is presented below.

Figure 6:   - Microemulsion Formation

The process of formation of oil droplets from a bulk oil phase is accompanied by an increase in the interfacial area, DA, and hence an interfacial energy DA_¡. ⁸ The entropy of dispersion of the droplets is equal to TDS and hence the free energy of formation of the system is given by the expression.

DG       =          DA_¡  -  TDS

With microemulsion, the interfacial tension is made sufficiently low that the interfacial energy becomes comparable to or even lower than the entropy of dispersion. ⁵

In this case, the free energy of formation of the system becomes zero or negative.  This explains the thermodynamic stability of microemulsions.

Thus, in microemulsion, the co-surfactant along with surfactant lower the intefacial tension to a very small even transient negative value

                                    Ü

At this value, interface would expand to form fine dispersed droplets.

                                    Ü

Adsorb more surfactant and surfactant / co-surfactant until their bulk condition is depleted enough to make interfacial tension positive again

                                    Ü

This process is known as “Spontaneous Emulsification” which forms the microemulsion. ²

Phase Behaviour 22

The phase behaviour of simple microemulsion systems composing oil, water and surfactant can be studied with the aid of ternary phase diagram (at fixed pressure and temperature) in which each corner of the diagram represents 100% concentration of the particular component.

Generally, pharmaceutical microemulsions contain additional components such as a cosurfactant and/or drug.  The co-surfactant is also amphiphilic with an affinity for both the oil and aqueous phases and partitions to an appreciable extent into the surfactant interfacial monolayer present at the oil-water interface. 

A large number of drug molecules are by themselves surface active and they are expected to influence phase behaviour.  For four or more components, pseudo ternary phase diagrams are used to study the phase behaviour.  In this diagram a corner will typically represent a binary mixture of two components such as surfactant/co-surfactant, water/drug or oil/drug.  The number of different phases present for a particular mixture can be visually assessed. 

Hypothetical Phase Regions of Microemulsion Systems¹

7:  - Hypothetical Phase Diagram

From above figure, we can see that,

ØWith high oil concentration surfactant forms reverse micelles capable of solubilizing water molecules in their hydrophilic interior.

ØContinued addition of water in this system may result in the formation of W/O microemulsion in which water exists as droplets surrounded and stabilized by interfacial layer of the surfactant / co-surfactant mixture.

ØAt a limiting water content, the isotropic clear region changes to a turbid, birefringent one.

ØUpon further dilution with water, a liquid crystalline region may be formed in which the water is sandwiched between surfactant double layers.

ØFinally, as amount of water increases, this lamellar structure will break down and water will form a continuous phase containing droplets of oil stabilized by a surfactant / co-surfactant (O/W microemulsions)

Characterization Of Microemulsions

Microemulsions have been characterized using a wide variety of techniques.  The characterization of microemulsions is a difficult task due to their complexity, variety of structures and components involved in these systems, as well as the limitations associated with each technique but such knowledge is essential for their successful commercial exploitation.  Therefore, complementary studies using a combination of techniques are usually required to obtain a comprehensive view of the physicochemical properties and structure of microemulsions.  At the macroscopic level viscosity, conductivity and dielectric methods provides useful information.

(A) Phase Behavior Studies²²

Phase behavior studies are essential for the study of surfactant system determined by using phase diagram that provide information on the boundaries of the different phases as a function of composition variables and temperatures, and, more important, structural organization can be also inferred.  Phase behaviour studies also allow comparison of the efficiency of different surfactants for a given application.  In the phase behaviour studies, simple measurement and equipments are required.  The boundaries of one-phase region can be assessed easily by visual observation of samples of known composition.  The main drawback is long equilibrium time required for multiphase region, especially if liquid crystalline phase is involved.

Other useful means and ways of representing the phase behaviour are to keep the concentration of one component or the ratio of two components constant.  As the number of components increases, the number of experiments needed to define the complete phase behaviour becomes extraordinary large and the representation of phase behaviour becomes extremely complex.  One approach to characterize these multicomponent systems is by means of pseudoternary diagrams that combine more than one component in the vertices of the ternary diagram.

(B) Scattering Techniques for Microemulsions Characterization

Small-angle X-ray scattering (SAXS), small-angle neutron scattering (SANS), and static as well as dynamic light scattering are widely applied techniques in the study of microemulsions. These methods are very valuable for obtaining quantitative informations on the size, shape and dynamics of the components.  The major drawback of this technique is the dilution of the sample required for the reduction of interparticular interaction.  This dilution can modify the structure and the composition of the pseudophases.  Nevertheless, successful determinations have been carried out using a dilution technique that maintains the identity of droplets. Small-angle X-ray scattering techniques have been used to obtain information on droplet size and shape. ^23-27

Static light scattering techniques have also been widely used to determine microemulsion droplet size and shape.  In these experiments the intensity of scattered light is generally measured at various angles and for different concentrations of microemulsion droplets. Dynamic light scattering, which is also referred as photon correlation spectroscopy (PCS), is used to analyse the fluctuations in the intensity of scattering by the droplets due to Brownian motion.  The self-correlation is measured that gives information on dynamics of the system. ^28-32

(C) Nuclear Magnetic Resonance Studies

The structure and dynamics of microemulsions can be studied by using nuclear magnetic resonance techniques. Self-diffusion measurements using different tracer techniques, generally radio labeling, supply information on the mobility of the components.  The Fourier transform pulsed-gradient spin-echo (FT-PGSE) technique uses the magnetic gradient on the samples and it allows simultaneous and rapid determination of the self-diffusion coefficients (in the range of 10^-9 to 10^-12 m²s^-1), of many components. ^33-36

(D) Interfacial Tension

The formation and the properties of microemulsion can be studied by measuring the interfacial tension.  Ultra low values of interfacial tension are correlated with phase behaviour, particularly the existence of surfactant phase or middle-phase microemulsions in equilibrium with aqueous and oil phases.  Spinning-drop apparatus can be used to measure the ultra low interfacial tension.  Interfacial tensions are derived form the measurement of the shape of a drop of the low-density phase, rotating it in cylindrical capillary filled with high-density phase. ⁹

(E) Viscosity Measurements

Viscosity measurements can indicate the presence of rod-like or worm-like reverse micelle. ^37,38  Viscosity measurements as a function of volume fraction have been used to determine the hydrodynamic radius of droplets, as well as interaction between droplets and deviations from spherical shape by fitting the results to appropriate models (e.g. for microemulsions showing Newtonian behaviour, Einstein’s equation for the relative viscosity can be used to calculate the hydrodynamic volume of the particles).

(F) Predicting Microemulsion Type

A well-known classification of microemulsions is that of Winsor who identified four general types of phase equilibria: ^39,40

Type – I The surfactant is preferentially soluble in water and oil-in-water (O/W) microemulsions form (Winsor I).  The surfactant-rich water phase coexists with the oil phase where surfactant is only present as monomers at small concentration.

Type – II  The surfactant is mainly in the oil phase and water-in-oil (W/O) microemulsions form.  The surfactant-rich oil phase coexists with the surfactant-poor aqueous phase   (Winsor II)

Type – III  A three-phase system where a surfactant-rich middle-phase coexists with both excess water and oil surfactant-poor phases (Winsor III or middle-phase microemulsion).

Type – IV A single-phase (isotropic) micellar solution, that forms upon addition of a sufficient quantity of amphiphile (surfactant plus alcohol).

(G) Simple tests used are

Dye Solubilization

A water soluble dye is solubilized within the aqueous phase of the W/O globule but is dispersible in the O/W globule.

A oil soluble dye is solubilized within the oil phase of the O/W globule but is dispersible in the W/O globule.

Dilutability Test

O/W microemulsions are dilutable with water whereas W/O are not and undergo phase inversion into O/W microemulsion.

Conductance Measurement

O/W microemulsion where the external phase is water are highly conducting whereas W/O are not, since water is the internal or dispersal phase.

To determine the nature of the continuous phase and to detect phase inversion phenomena, the electrical conductivity measurements are highly useful. 

A sharp increase in conductivity in certain W/O microemulsion systems was observed at low volume fractions and such behaviour was interpreted as an indication of a ‘percolative behaviour’ or exchange of ions between droplets before the formation of bicontinuous structures. ³⁷

Dielectric measurements are a powerful means of probing both structural and dynamic features of microemulsion systems.

(H) Electron Microscope Characterization⁴¹

Transmission Electron Microscopy (TEM) is the most important technique for the study of microstructures of microemulsions because it directly produces images at high resolution and it can capture any co-existent structure and micro-structural transitions.

There are two variations of the TEM technique for fluid samples.

1. The cryo-TEM analyses in which samples are directly visualized after fast freeze and freeze fructose in the cold microscope.
2. The Freeze Fracture TEM technique in which a replica of the specimen is images under RT conditions.

Applications Of Microemulsions

• Pharmaceutical Applications⁴²

üParenteral delivery

üOral drug delivery

üTopical drug delivery

üOcular and pulmonary delivery

üMicroemulsions in biotechnology

Parenteral Delivery

Parenteral administration (especially via the intravenous route) of drugs with limited solubility is a major problem in industry because of the extremely low amount of drug actually delivered to a targeted site.  Microemulsion formulations have distinct advantages over macroemulsion systems when delivered parenterally because of the fine particle microemulsion is cleared more slowly than the coarse particle emulsion and, therefore, have a longer residence time in the body.  Both O/W and W/O microemulsion can be used for parenteral delivery.  The literature contains the details of the many microemulsion systems, few of these can be used for the parenteral delivery because the toxicity of the surfactant and parenteral use.  An alternative approach was taken by Von Corsewant and Thoren⁴³ in which C3-C4 alcohols were replaced with parenterally acceptable co-surfactants, polyethylene glycol (400) / polyethylene glycol (660) 12-hydroxystearate / ethanol, while maintaining a flexible surfactant film and spontaneous curvature near zero to obtain and almost balanced middle phase microemulsion.  The middle phase structure was preferred in this application, because it has been able to incorporate large volumes of oil and water with a minimal concentration of surfactant.

Oral Delivery

Microemulsion formulations offer the several benefits over conventional oral formulation for oral administration including increased absorption, improved clinical potency,and decreased drug toxicity.⁴⁴  Therefore, microemulsion have been reported to be ideal delivery of drugs such as steroids, hormones, diuretic and antibiotics. 

Pharmaceutical drugs of peptides and proteins are highly potent and specific in their physiological functions.  However, most are difficult to administer orally.  With on oral bioavailability in conventional (i.e. non-microemulsion based) formulation of less than 10%, they are usually not therapeutically active by oral administration.  Because of their low oral bioavailability, most protein drugs are only available as parenteral formulations.  However, peptide drugs have an extremely short biological half life when administered parenterally, so require multiple dosing.

A microemulsion formulation of cyclosporine, named Neoral^® has been introduced to replace Sandimmune^®, a crude oil-in-water emulsion of cyclosporine formulation.  Neoral^® is formulated with a finer dispersion, giving it a more rapid and predictable absorption and less inter and intra patient variability. ⁴⁵

Topical Delivery

Topical administration of drugs can have advantages over other methods for several reasons, one of which is the avoidance of hepatic first pass metabolism of the drug and related toxicity effects.  Another is the direct delivery and targetability of the drug to affected area of the skin or eyes. Both O/W and W/O microemulsions have been evaluated in a hairless mouse model for the delivery of prostaglandin E1. ⁴⁶ The microemulsions were based on oleic acid or Gelucire 44/14 as the oil phase and were stabilized by a mixture of Labrasol (C₈ and C₁₀ polyglycolysed glycerides) and Plurol Oleique CC 497 as surfactant. Although enhanced delivery rates were observed in the case of the o/w microemulsion, the authors concluded that the penetration rates were inadequate for practical use from either system. The use of lecithin/IPP/water microemulsion for the transdermal transport of indomethacin and diclofenac has also been reported. Fourier transform infra red (FTIR) spectroscopy and differential scanning calorimetry (DSC) showed the IPP organogel had disrupted the lipid organisation in human stratum corneum after a 1 day incubation.⁴⁷

The transdermal delivery of the hydrophilic drug diphenhydramine hydrochloride from a W/O microemulsion into excised human skin has also been investigated. The formulation was based on combinations of Tween 80 and Span 20 with IPM. However two additional formulations were tested containing cholesterol and oleic acid, respectively. Cholesterol increased drug penetration whereas oleic acid had no measurable effect, but the authors clearly demonstrated that penetration characteristics can be modulated by compositional selection. ⁴⁸

Ocular and Pulmonary Delivery

For the treatment of eye diseases, drugs are essentially delivered topically. O/W microemulsions have been investigated for ocular administration, to dissolve poorly soluble drugs, to increase absorption and to attain prolong release profile.

The microemulsions containing pilocarpine were formulated using lecithin, propylene glycol and PEG 200 as co-surfactant and IPM as the oil phase.  The formulations were of low viscosity with a refractive index lending to ophthalmologic applications. ⁴⁹

The formation of a water-in-HFA propellent microemulsion stabilized by fluorocarbon non-ionic surfactant and intended for pulmonary delivery has been described.

Microemulsions in Biotechnology

Many enzymatic and biocatalytic reactions are conducted in pure organic or aqua-organic media.  Biphasic media are also used for these types of reactions.  The use of pure apolar media causes the denaturation of biocatalysts.  The use of water-proof media is relatively advantageous.  Enzymes in low water content display and have –

1. Increased solubility in non-polar reactants
2. Possibility of shifting thermodynamic equilibria in favour of condensations
3. Improvement of thermal stability of the enzymes, enabling reactions to be carried out at higher temperatures.

Many enzymes,including lipases, esterases, dehydrogenases and oxidases often function in the cells in microenvironments that are hydrophobic in nature.  In biological systems many enzymes operate at the interface between hydrophobic and hydrophilic domains and these usually interfaces are stabilized by polar lipids and other natural amphiphiles.  Enzymatic catalysis in microemulsions has been used for a variety of reactions, such as synthesis of esters, peptides and sugar acetals transesterification; various hydrolysis reactions and steroid transformation.  The most widely used class of enzymes in microemulsion-based reactions is of lipases. ⁴²

• Other Applications⁵⁰

üMicroemulsion in enhanced oil recovery

üMicroemulsions as fuels

üMicroemulsions as lubricants, cutting oils and corrosion inhibitors

üMicroemulsions as coatings and textile finishing

üMicroemulsions in detergency

üMicroemulsions in cosmetics

üMicroemulsions in agrochemicals

üMicroemulsions in food

üMicroemulsions in environmental remediation and detoxification

üMicroporous media synthesis (microemulsion gel technique)

üMicroemulsions in analytical applications

üMicroemulsions as liquid membranes

üNovel crystalline colloidal arrays as chemical sensor materials

Conclusion

To date microemulsions have been shown to be able to protect labile drug, control drug release, increase drug solubility, increase bioavailability and reduce patient variability. Furthermore, it has proven possible to formulate preparations suitable for most routes of administration. There is still however a considerable amount of fundamental work characterizing the physico-chemical behaviour of microemulsions that needs to be performed before they can live up to their potential as multipurpose drug delivery vehicles. Recently, several research papers have been published for the improvement of drug delivery, but still there is a need to emphasis on its characterization part including in vitro evaluation. Besides this, research papers shows higher percentage of surfactant (much higher than CMC level) used for the formation of microemulsion, irrespective of different routes of administration, but there is a lack of toxicological evaluation of the prepared microemulsion, which can be a broad research area in future.

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About Authors:

Mrunali R. Patel is currently working as lecturer, Department of Pharmaceutics & Pharmaceutical technology, Indukaka Ipcowala College of Pharmacy, New Vallabh Vidyanagar, Gujarat; India. She completed her B.Pharm and M.Pharm from A R College of Pharmacy, Vallabh Vidyanagar, Gujarat; India. Her area of interest in research includes New Drug Delivery systems.
Contact: rashmru@gmail.com

Mr. Rashmin B. Patel is currently working as lecturer, department of Pharmaceutical Chemistry, A R College of Pharmacy, Vallabh Vidyanagar, Gujarat; India. He completed his B.Pharm and M.Pharm from A R College of Pharmacy, Vallabh Vidyanagar, Gujarat; India. His area of interest in research includes New Drug Delivery systems & Pharmaceutical Analysis

Dr Jolly R Parikh is currently working as Assistant Professor and Head, Department of Pharmaceutics & Pharmaceutical Technology, at the A R College of Pharmacy, Vallabh Vidyanagar, Gujarat; India. She is the Chairman Board of studies in Pharmaceutics & Pharmaceutical Technology at the Sardar Patel University, VallabhVidyanagar. She has about 20 years of teaching experience. She is currently guiding M.Pharm & PhD students in the research area of New Drug Delivery systems.

Dr Kashyap K. Bhatt is currently working as Professor and Principal, Indukaka Ipcowala College of Pharmacy, New Vallabh Vidyanagar, Gujarat; India. He has about 23 years of teaching experience. He is currently guiding M.Pharm & PhD students in the research area of New Drug Delivery systems, Pharmaceutical Analysis and Pharmaceutical Chemistry.

Mr. Aliasgar J Kundawala is currently working as lecturer, Department of Pharmaceutics & Pharmaceutical technology, Indukaka Ipcowala College of Pharmacy, New Vallabh Vidyanagar, Gujarat; India. He completed his B.Pharm from A.R.A. college of Pharmacy, Dhule. Maharastra and M.Pharm from Al-Ameen college of Pharmacy. Bangalore. Karanataka. His area of interest in research includes New Drug Delivery systems.