INTRODUCTION:

The first isolable organosilicon compound was synthesized in 1888 by Kipping¹. However, the era of silicone oil emulsions began in 1980’s when non-aqueous emulsions were synthesized from them^2,3andtheir use, as potential vehicles for toiletries gained momentum⁴. The development and availability of multiple silicone and other polymer based emulsifiers facilitated the application of these non-aqueous emulsions as vehicles for cosmetic ingredients and as systems for the delivery for a diverse variety of different products for personal care. Silicone oils, or in other words, siloxanes, comprise of a special class of polymers which may be of different types including those of hybrid inorganic or inorganic ones. Further they may be structurally linear or cyclic polymers with versatile applications in cosmetics^5,6, drug delivery systems⁷, textiles^8,9, home care products^10,11, printing ink formulations¹², semipermeable membranes¹³, etc. As a consequence of modern requirements in cosmetics and pharmaceutics, new approaches to drug delivery methods with greater bioavailability and distinctive formulations have evolved. Silicone oil nanoemulsions are one such type of formulations which have recently garnered a great deal of attention.

The exceptional properties of silicone oil has triggered its use in drug delivery systems and in a variety of cosmoceutical and pharmaceutical products.

Silicone oil, an important synthetic polymer, has a backbone of Si-O-Si, and a general formula of (R₂SiO)_n. Its exceptional flexibility is due to the Si-O-Si bonds, which show a torsional force constant of a very low value, thus making silicone oil available in variable viscosities ranging from 0.65 to 1,00,000 cst, with a T_g of 123°C¹⁴. Polydimethylsiloxanes (PDMS), one of the most common silicone derivatives exhibits high hydrophobicity due to the presence of gem methyl groups. The relatively high ionic Si-O bond imparts exceptional surface properties to the silicone oil. The surface tension with respect to these silicones shows a direct proportionality to molecular weight, which varies from about 16 mN/m for Me₃SiO-(Me₂SiO)_n - SiMe_3,

for n values of zero, to values between 20- 21 mN /m in case of silicones of medium and high molecular weight, with n values greater than ten¹⁵. Further, it was found that self emulsifying drug delivery systems enhance the stability of water soluble or lipophobic drugs¹⁶.

The present review focuses on the syntheses methods available for silicone oil emulsions along with their properties and applications in biomedical fields as well as in personal care products and pharmacology.

Types of emulsions and emulsification methods

Emulsification is a non-equilibrium process¹⁷ and the basic chemistry of emulsions^18-23apply to micro and nanoemulsion too. The main difference between micro and nanoemulsion is not based on composition but rather on thermodynamics^24-26. Microemulsion formation is a thermodynamically feasible process whereas the formation of nanoemulsion is not. Comparatively, microemulsions have lower free energies than the phase separated states, which explains the thermodynamic differences between the two. The kinetic stability can be elucidated by energy barriers and mass transport phenomena. Microemulsions cannot be stored for long time as they may degrade due to various environmental factors, but nanoemulsions, with energy barriers of > 20 kT (k is the Boltzmann constant and T is the absolute temperature referring to thermal energy of the system) can be stored for a longer time, indicating that the height of the energy barrier determines the kinetic stability of these emulsions²⁷. Nanoemulsions are prone to destabilization through various mechanisms which involve Ostwald’s ripening, flocculation, gravitational separation, etc. Hence their kinetic stabilities may be improved by incorporating micro structure emulsifiers, weight modifiers and ripening retarders into them²⁸. The preparative methods, properties and applications of nanoemulsions in general have been reviewed ^29-32.

Silicone oil emulsions have largely been prepared using conventional methods such as homogenization³³, ultrasonication³⁴ and mechanical stirring³⁵. The process of emulsification requires a high shear force to rupture the droplets followed by stabilization of the surfactant. Phase Inversion temperature (PIT) method³⁶ is a low flow emulsification method, where there is a change in interfacial properties of emulsion at the PIT. PIT generally involves the use of non-ionic surfactants. Either oil in water emulsions (O/W i.e., direct) or water in oil (W/O i.e., inverse) emulsions can be prepared by diverse emulsifications procedures. A rather simple and direct method to synthesise droplets of nano range involves high flow emulsification. In this procedure, a shear force is applied externally and this overcomes the viscous/ interfacial stress, which is present internally. This leads to the breaking of the larger droplets in the solution into smaller sized ones. Microfluidic homogenization involving high pressure^37-39and ultrasonic emulsification^40-42 are capable of producing emulsions down to diameters below 100nm^43-45.

Ultracentrifugal fractionation has been used for emulsion synthesis as this method improves the size distribution uniformity of Polydimethyl siloxanes - water system ¹⁷. It is an easy procedure which employs the variations seen in the rates of creaming between the droplets which are of different sizes, when they react to a buoyant force which arises because of the differences in density between oil and water. Nanoemulsion composite micro gels, a cross linkable nanoemulsion, can be synthesized by flow lithography with controlled droplet size, loading and stability⁴⁶. Photo polymerization of the oil in pregel nanoemulsion results in the formation of a chemically cross linked hydrogel in the continuous phase. A detailed account on flow lithography and droplet based methods is available in the literature⁴⁷.

Emulsions can be synthesised by dispersing droplets of one liquid dispersion in a second liquid medium. These are labelled as ternary / double emulsions or simply “emulsions of emulsions”. They form very complex dispersion systems and can of two types, viz., water-in-oil-in-water (W/O/W) or oil-in-water-in-oil (O/W/O). Traditionally, a two-step fabrication method has been used to prepare double emulsions of w/o/w type⁴⁸.Initially, a surfactant with a low Hydrophilic Lipophilic Balance (HLB) is employed under conditions of high flow. This surfactant is first dissolved in the oil employed in emulsion preparation and it stabilises the latter as per the requirements of Bancroft’s rule⁴⁹ leading to the formation of inverse emulsion. This is followed by the addition of W/O emulsion, to the water which already has the surfactant with high HLB incorporated into it. The emulsification is then carried out with a low flow rate, which avoids the process of destruction and coalescence of internal droplets, leading to the formation water in oil emulsion. Polymeric surfactants are mainly used for the stabilization of emulsions prepared by such methods as these tend to adsorb with a higher strength at the interface as compared to the emulsions which are of monomeric types. Stabilisation of the droplets can be achieved by using polymers which do so by mechanisms which involve both steric and depletion stabilisation⁵⁰. Polysiloxanes, which are structurally grafted with moieties of non-ionic poly (ethylene oxide) are very versatile due to their strong absorption at W/O interface, thus indicating that silicone based polymers are excellent with respect to their higher levels of emulsifying power^51,52. Further, silicone-derived polymers enhances the elasticity of the interfacial film due to their strong absorption to the O/W interface which explains the stability of these emulsions.

Double emulsions, which are monodisperse, have also been synthesised by the process of microfluidic flow focusing⁵³and those involving T-junctions⁵⁴. An O/W/O emulsion can be produced in a single step emulsification process involving the procedure of compound drop (surfactant solution in oil) which later turns to a double emulsion ⁵⁵. The compound drop is made with the help of a couple of syringes filled with surfactant solution and oil, with their ends held by a thin copper wire. The solution of the surfactant and the drops of oil are dropped from the syringes and they slide onto the wire and they terminate at the lowest point and finally merge to form the corresponding compound drop and bouncing on vibrated liquid surface. SDS solution favours the deformation of the oil/water interface at concentrations ten times to that of the Critical Micellar Concentration.

The preparation of monodisperse double-emulsion for viscous liquids had remained a challenge for several years. A method which involves biphasic flow of two liquids in a microcapillary device has now solved the problem⁵⁶. Double emulsion drops, with a core- shell geometry having an ultra-thin wall as a middle layer, is found to be suitable for both kinds of double emulsions viz, O/W/O or W/O/W. In spite of the small thickness of middle layer, they show high stability and biodegradable microcapsules of Polylactic acid, created by using this method, are potentially useful in drug delivery, cosmetics and also for encapsulation procedures.

Double emulsions due to their internal compartmentalization, offer several advantages over traditional O/W emulsions, as they can carry both polar or non-polar molecules, thus enhancing their applications in cosmetics and pharmaceutical products. Water in oil in water double emulsions have been synthesised in a nanoscale, by employing the process of high flow emulsification along with a diblock co polypeptide (DBC) amphiphile which is molecularly designed⁵⁷. Here, the control of H-bond presentation, in polypeptide segments, acts as a stabilizing factor. Multi compartment Poly (dimethylsiloxane) particles have been produced using (W/O/W) systems as templates and these can be used in controlled drug delivery and enzyme immobilization⁵⁸. These emulsions are prone to destabilization and are hence stabilized by a kinetic process which involves the addition of hydrophilic and hydrophobic surfactants. The mechanism of stabilisation possibly involves the surfactants encompassing the inner water molecules and oil globules, respectively⁵⁹. Diffusive transport studies of W/O/W emulsions have shown the requirement of osmotic regulators to maintain the concentration gradient between the inner and outer compartments of water phase ⁶⁰.

Role of surfactants

Stability of the silicone oil emulsions can be attained by employing surfactants which aid in decreasing the interfacial tension observed between the fluid and water junctions. The mechanism involves dispersion of droplets by preventing their coagulation and flocculation. Thorough studies on silicone oil emulsions have revealed non ionic surfactants to be more effective for emulsification than ionic ones for different applications. However in some cases, anionic surfactants have also been employed⁶¹. Development of new generation emulsions has extended the use of organic surfactants to silicone based emulsifiers, which invariably possess one or more hydrophilic groups (mostly poly oxyethylene). Moreover, multifunctional polymeric silicone emulsifiers absorb more strongly at the interface than the low molecular weight surfactants, due to their attachment at interface via several segments, thus leading to greater absorption energy. Lowering of the interfacial tension in case of aqueous systems may be a positive character for non-ionic surfactants, but their tolerance towards pH variations is very low and they hydrolyze to form long chain alcohols. The latter have been proved to be skin irritants, and hence this factor limits the use of non ionic surfactants for a wide range of pH.

Factors affecting Emulsification:

Emulsion stability can be influenced by various factors such as silicone oil viscosity, emulsification time, emulsifier concentration, etc ⁶². A new method based on automated stability dispersion analyser (DiStA 24), for studying the emulsion stability has been proposed ⁶³. As the viscosity of silicone oil increases, the droplet size also increases leading to destabilization of emulsions ^64-67. Investigations have shown that emulsions synthesised using oils of low viscosity have lower viscosities than prepared using high viscous oils⁶². Generally with increase in surfactant concentration, the droplet size decreases. Simultaneously, uniformity of emulsions can also be improved, leading to enhancement of emulsion stability⁶⁸. After reaching a certain point, further increase in surfactant concentration does not reduce the droplet size, leading to broad size distribution⁶⁹. During ultrasonication, an increased mixing time leads to decreased droplet size resulting in higher volume fractions and increased viscosity. This leads to enhanced emulsion stability. In contrast, emulsions synthesised using Span 60 emulsifiers, by mechanical stirring method with prolonged emulsification time, were of poor stability, due to high volume fraction, which often ultimately lead to droplets coagulation^70,71. Shear force is another critical parameter, affecting the emulsion stability, which can be varied with the time of mixing. With Span 60 emulsions, as the shear force increases, the size distribution becomes narrow, all within a fixed time duration. However, in case of CO - 530 emulsifier, the droplet diameter was found to be in agreement with Span 60 until it reached a critical point of shear force, after which the size of the droplets varied proportionally with time, leading to poor emulsion stability ⁷². High shear force and increased time for mixing may possibly result in higher rate of collisions among the droplets, which would culminate in broader size distribution of particles and deteriorated emulsion stability. This highlights the importance of using a shear force which is optimum and time which is conducive to enhance the stability of these emulsions.

Applications of Silicone oil based emulsions:

Silicone oil nanoemulsions are widely utilised in various fields which include cosmetology, ophthalmology and dermatology. They also find use in the treatment of gastrointestinal infections and discomfort and protein encapsulation (enzyme immobilization) procedures. Their applications as potential drug delivery systems at nano level are also important⁷³. Broadly these applications can be classified into two categories viz., personal care products and pharmacological / pharmaceutical preparations.

Cosmetic and Personal Care Products

With improvement in the standard of living, the demand for cosmetics and items of personal care has moved from away from traditional merchandise to products which add value to good grooming and social acceptance rather than wellness and health. This paradigm shift has galvanised the cosmetic scientists to research and develop formulations which are superior and very distinctive for consumer requirements. In the process, considerably large amounts of surfactants, stabilizers and electrolytic suspensions are being used in these formulations which may cause skin irritation, dry skin, inflammation and allergic reactions^74-79. In this scenario, silicone oil emulsions have come out as prospective systems to solve the limitations seen traditionally in age old formulations^80-82. Their uniform and size controllable properties, along with improved appearance, stability and good application performance have enhanced their usage. Coalescence and ostwald ripening are also inhibited in these systems as their size distribution is very narrow^83,84.

Silicone oil is an important constituent of hair conditioners, and serves the purpose of preservation of moisture and also lubricates the hair^85-87. This oil has different characteristics compared to other counter organic oils. For example, thermodynamic stability of Si-O-Si bond, greater structural flexibility, large permeability, high lubricant property, resistance towards chemicals and UV light have made them unique as versatile conditioning agents in formulations relating to hair and skin^88,89. However, silicone oil may have a difficulty in getting absorbed directly onto the hair surface as it is very hydrophobic. To improve silicone deposition on hair, different polymers are being added into shampoos and conditioners⁹⁰. Employing nanoemulsions containing silicone oil, which are very stable, provides a potential solution to this problem. Addition of cationic conditioning polymer surfactants have resulted in deposition of silicone oil to hair and has solved the problem of shampoos functioning as both cleansing and conditioning agents ⁹¹. Emulsions of amino functional silicones and high molecular weight silicones are also widely used in hair care compositions to provide various aesthetic benefits ⁹². Current trends of synthesis and stabilization of these emulsions for hair care products has been reviewed ⁶².

Silicone emulsions for personal care is commercialised as antiperspirant formulations/ lotions/creams for skin care and UV protection ^93,94. They are largely W/O emulsions which incorporate all the best properties with respect to silicones and include very good spreading behaviour and film forming, glossiness and a dry non sticky feel⁵². Antiperspirants made from concentrated aqueous aluminium chlorohydrate, emulsified in cyclic dimethylsiloxanes is an important formulation which can be applied with ease. Siloxanes aid thorough application by lowering the stickiness of chlorohydrate and also evaporates without a feeling of a“chill effect”.

Biomedical and pharmaceutical applications

Silicone compounds, by themselves, can be used for various pharmacological applications and one such compound is simethicone, which is commonly employed for the treatment of gastrointestinal discomfort. Simethicone, an orally administered anti-foaming agent, comprises of a suspended mixture of poly dimethylsiloxane and hydrated silica gel. It is generally administered to reduce bloating in the stomach or intestines. Simethicone does not reduce or prevent gas formation in the digestive tract. However, it enhances the rate at which the gas leaves the body ⁹⁵. Silicone oil nanoemulsions have garnered attention as they retain the functionality of protein during encapsulation procedures, which in turn, bestows them with improved performance and stability.

Skin Care

Emulsions can be used for transdermal delivery of drugs and by adopting this method, the gastrointestinal tract absorption can be bypassed. This method thus improves the bioavailabilty of drug molecules ⁹⁶.Various cosmetic agents such as after shave lotions ⁹⁷, sun screen lotions^98-100etc.have been synthesized as emulsions of water in silicone oil. A method for barrier formulation, used in creams and lotions, comprising of silicone based emulsion has been reported ¹⁰¹. Emulsions prepared from silicone oils, in hydrogenated lecithin, were shown to be dermatologically active and the effect of lecithin on dermal safety of this emulsion has been thoroughly investigated, by the measurement of interfacial tension, using Ring method ¹⁰². Interfacial tension measurements for two systems viz., surfactant solution containing lecithin with paraffin oil and with silicone oil respectively, have revealed that with the raise in surfactant concentration, both systems show enhanced interfacial tension values. Further dynamic interfacial tension studies, of systems with Lecinol S 10 and Tween 60, at the same flow rate, have shown the formation of Lecinol S 10 and silicone oil nano emulsion system with small sizes of particles and very good stability. The conditions of formation have also been standardised¹⁰³. The silicone nano emulsion prepared from hydrogenated lecithin has shown zero value in the Patch Test and hence can be considered dermatologically safe. A health care active formulation of silicone oil emulsion, comprising of silicone gum/resin/PSA and ethylene oxide/propylene oxide copolymer has been patented and this can incorporate a wide range of molecules of therapeutic significance such as vitamins, retinols, anti microbial agents, plant extracts and anti parasitic agents ¹⁰⁴.

Silicone emulsions as encapsulating systems:

Generally, isolated proteins are prone to a denaturation process during which they undergo structural changes to lose their functionality. Methods to prevent such denaturation mechanisms have received great deal of attention and this has lead to the recognition of the significance of silicone oil emulsion systems for encapsulating proteins and organic molecules, which find applications in controlled and targeted drug delivery. But, all proteins are not amphiphilic in nature due to which they cannot absorb onto oil-water interfaces. Virus like droplets (VLDs), prepared by using silicone oil nanoemulsion for cowpea chlorotic mottle virus, is composed of an outer protein shell or capsid and inner RNA as genetic material. The capsid, composed of proteins, can be extracted in pure form by altering the pHand ionic strength¹⁰⁵ and can then be self assembled around stabilised silicone oil droplets of nanosize, anionically. This leads to the formation of virus like droplets (VLDs)¹⁰⁶ which are very much suitable for loading drug molecules.

Many polydimethylsiloxanes which are linear as well as cyclic in nature¹⁰⁷, are hydrophobic and hence denature a number of proteins ^108,109. However, these interactions with silicones (including with organic lipids e.g., in foods ^110,111) are not essentially harmful to the biomolecules. On the other hand, some of these interactions are highly beneficial and important. They serve to fulfill several applications in the cosmetic industry and also industries dealing with the manufacture of personal care products⁷⁵. Many proteins may not necessarily be influenced by contact with some types of silicone surfactants which have been modified hydrophilically. For example, polyalkylene oxide-modified silicone which has a comb like structure ¹⁰⁷(e.g., DC3225C) forms stable products. Lysozyme and R-chymotrypsin, when trapped in water-in silicone oil emulsions, retain their ability to process substrates for a nine day period and the levels are comparable to that of controls. In the presence of triethoxysilyl terminated silicone polymer, TES-PDMS, a modified silicone oil encapsulated albumin, retains the functionality in its water in silicone emulsions. Further it was found that both Human Serum Albumin (HSA) and TES-PDMS are important for formulating stable water-in-silicone oil emulsions¹¹². Fluorescent measurements have shown that no denaturation of HSA is seen in the initially formed, stable emulsion and confocal microscopic studies have revealed the preferential absorption at the oil/water interface.

Prefilled syringes are one of the promising storage and delivery methods which are generally used in parenteral administration of drugs /vaccines. They are very convenient for medicos and patients. However, if the proteins are exposed to silicone oil for protracted time inside such syringes there may be numerous problems. Systems made of silicone oil prepared in buffer emulsions of aqueous nature and model proteins, in formulations incorporating the surfactant, suitable salts like sodium chloride, or sucrose have been described¹¹³. The kinetic stability, effects of silicone oil on protein stability and effects of variables controlling protein formulations, on emulsion stability have been thoroughly investigated.

A systematic investigation of BSA encapsulation in polylactic-co-glycolic-acid (PLGA) has been carried out using the formation of a W/O/W emulsion followed by solvent removal. High yields of microsphere fabrication has been achieved by using silicon oil containing methylene chloride as a suspension medium instead of pure silicone oil, with minimal loss of polymer and protein drug (<2%)¹¹⁴. Microencapsulation methods for delivery of protein drugs have been reviewed¹¹⁵. These methods include solvent evaporation / extraction, phase separation (coacervation), spray drying, ionotropic gelation /polyelectrolyte complexation, interfacial polymerization and supercritical fluid precipitation. Each method has been described for its applications, advantages and limitations.

Proteins are currently being available in large quantities and hence finding applications as catalysts (enzymes) and drugs. Simultaneously newer strategies are being developed for their controlled delivery. Immobilization of enzymes such as Chymotrypsin and Lysozyme, by using silicone based surfactants, has been described¹⁰⁷and the retention of functionality conformed by spectrophotometric assays. The study on the adsorption of horseradish peroxidase (HRP) onto silicon wafers has also been reported ¹¹⁶. A detailed mechanism for stabilization of water in silicone oil emulsions by Peptide-Silicone Hybrid Polymers, named as active interfacial modifier (AIM), forms an interesting study where an independent third phase between immiscible two liquid phases has been formed ¹¹⁷. The presence of this third phase was confirmed by fluorescence probe method with Confocal Laser Scanning Microscopy. The synthesis involves reaction between a fluorescence probe (9-Chloromethylanthracene, 9-CMA) with carbonyl groups of silk peptides.

Emulsions formed from cyclic silicone oil and water, used in same proportions, and stabilized with the help of fumed silica nanoparticles can be inverted from oil-in-water (O/W) to water-in-oil (W/O) type. This can be achieved by merely increasing particle concentration ¹¹⁸. Inversion is restricted to systems with particles of intermediate hydrophobicity when dispersed in oil. However, emulsions which are formulated from the same particles, but initially dispersed in water remain O/W at all particle concentrations. Highly hydrophobic particles stabilize only W/O emulsions, and highly hydrophilic ones stabilize only O/W emulsions. These emulsions do not coalescence but creaming and / or sedimentation does occur as a function of time. With the help of rheology, TEM and light scattering experiments, it has been seen that particle dispersions in oil, at high concentrations, consist of connected aggregates in a gel whereas such aggregates remain discrete in aqueous dispersions. Functional silicone and proteins, in combination, lower the interfacial tension of water-in-silicone oil emulsions much more efficiently than the surfactants alone, which elucidates the synergistic interaction between the protein and silicones ¹¹⁹.

Multicompartment poly (dimethylsiloxane) particles were synthesised initially by using templates of water-in-oil-in water (W/O/W) emulsions⁵⁵. Various silicone W/O/W emulsions have also been successfully synthesised with low viscosity silicone precursors. Fluorescein, a model hydrophilic molecule, has been chosen as encapsulating molecule and the technique of fluorescent optical microscopy has been used for real time monitoring of the formation of loaded multiple emulsions along with their cross-linking processes. Salt incorporation at low concentrations, in the inner aqueous phase, did not influence the emulsion behaviour. However, it was found that high concentrations of salt destabilises the emulsion.

Toxicology and biocompatibility:

The most widely used silicone, polydimethylsiloxanes, a synthetic polymer, is considered safe as no toxicity has been observed after its administration through different exposure routes which are typically used for drug administration. Silicones, are largely of high molecular weight and hence their absorption through the gastrointestinal tract is negligible. Their excretion is carried out without any structural modification. Their dermal absorption is also minimum¹²⁰. No mutagenic effects have reported through in vitro studies. Silicones, when administered repeatedly through the oral or cutaneous route have not found to exert any effect on different types of species. However inhalation of any types of aerosols, including those of silicones, may cause physical disturbances to the epithelial cell lining of the lungs and associated effects may be observed¹²¹. But in the case of Simethicone, mild diarrhea, nausea, regurgitation and vomiting are observed as the common side effects. Low molecular weight siloxanes are used in skin care products and antiperspirants, due to their volatile nature and dry skin feeling. None have these have been identified as toxic except cyclic siloxanes with formula, (SiMe₂O)_4,which showed a significant lowering of sites of implantation in uterus of female rats when are exposed to it¹²². However, this effect has not been not observed in case of humans. The next higher homologue of cyclosiloxane i.e., (SiMe₂O)₅, was shown not to have these effects¹²².

Usage of biosurfactant, however, reduces the pharmacological side effects such as inflammation, irritation and squalae bioaccumulation, which are generally imposed in the case of synthetic surfactants¹²³. These surfactants basically originate from either carbohydrate or amino acid sources and tend to undergo biodegradation easily. Glycosides, a class of carbohydrates, are found to be actively used in cosmetics¹²⁴. Microcrystals of cyclodextrin-oil inclusion complexes stabilizes O/W emulsions and resembles pickering emulsions¹²⁵. Cyclodetxtrin stabilized silicone oil emulsion resembles pickering emulsions as it involves cyclodextrin-oil inclusion complexes, synthesized by threading cylcodextrin from the aqueous phase, onto silicone oil from the emulsion drop surface. Silicones grafted with β-cyclodextrin undergoes spontaneous emulsification and enables the incorporation of greoseofulvin to form an inclusion complex. However, this is a difficult task owing to the low water solubility of greoseofulvin ¹²⁶.

The innocuous behavior of silicones highlight their versatile usage wherever there is a requirement of extended and long time contact with human body. This is particularly true in case of cosmetics, textile fabrics, food preservative containers and medical appendages and equipments. The low chemical reactivity displayed by silicones, along with low surface area and hydrophobic nature is important for serving these applications and contribute to their excellent biocompatibility ¹²².

CONCLUSION:

The exceptional properties of silicone oil allow it to be used in the formulation of micro and nanoemulsions which can be applied as excellent systems in fields of cosmetology, dermal care, enzyme immobilization and biomedical research. The interactions of proteins with silicone oil sometimes may cause their denaturation due to the hydrophobic nature of silicones. But in case of hydrophilically modified silicone nanoemulsions, the proteins get stabilized and retain their functionality. Synergistic interactions between protein and silicones have been investigated. The mechanisms of interactions have been studied and conformational studies of protein absorption have also been established by using Confocal Laser Scanning microscopy. Double emulsions with silicones have proved to be useful as encapsulating systems for several active pharmaceutical ingredients and medically important dyes.

ACKNOWLEDGEMENTS:

KHK thanks UGC-CSIR for providing a junior research fellowship. The authors thank National Aerospace Laboratories, Council of Scientific and Industrial Research, Bangalore and VIT University, Vellore for providing the facilities for the preparation of this review.

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Received on 08.04.2014 Modified on 10.05.2014

Research J. Pharm. and Tech. 7(5): May, 2014; Page 561-568