Emulsion is defined as the even distribution of one liquid into another. This property of mixing of two liquids has been used in delivery of materials in drugs and cosmetics. Nanoemulsions are ultra fine emulsions with droplet size below 1 µ. They are transparent in nature due to the minute size and have good biophysical and sensorial properties. Nanoemulsions are biocompatible and biodegradable. They can be produced easily and used as carriers for lipophilic drugs which are prone to hydrolysis [17] . They enhance gastrointestinal absorption and reduce inter- and intra-subject variability for various drugs. Due to their very large interfacial area, they ex- hibit excellent drug release profile [18] . However, they have been found to be metastable and may be destabi- lized due to Ostwald ripening. To avoid these problems, a number of factors have to be considered during the synthesis process. These factors include selecting an appropriate composition, controlling the order of addition of components, applying the shear in a manner that will effectively rupture the droplets, and ensuring that the dis- persed phase molecules are insoluble in the continuous phase so that Ostwald ripening does not occur rapidly [19] .

Liposomes are vesicular structures with an aqueous core surrounded by a hydrophobic lipid bilayer, created by the extrusion of phospholipids [20] . They may be single layered or multi layered, varying in size from 15 nm to 100 nm or more. They have been found to be useful in cosmetics, drug delivery and food industries. Lipo- somes are used as drug carriers due to their ability to prevent degradation of drugs, reduce side effects and target drugs to site of action [21] . Liposomes work by attaching to the target cell or by its engulfment into the target cell, followed by the release of the drug encapsulated within to perform its function. For the attachment or en- gulfment to take place the modification of the surface properties of the liposome is essential. Surface modifica- tion is carried out based on the application of the liposome. They are used in transdermal drug delivery to poten- tiate skin permeation of drugs with high molecular weight and poor water solubility [22] . Liposomes have been used as carriers for gentamicin in order to reduce toxicity, as a means of delivery of drugs to the lungs by nebu- lization and in ocular delivery [23] [24] .

Transferosomes are a modified form of liposomes. They are more elastic than liposomes which make them a good vessel for transdermal drug delivery by deforming and passing through even narrow constrictions. The flexibility can be altered by changing the composition and mixing suitable surface active agents [25] . Transfero- somes possess an infrastructure consisting of hydrophobic and hydrophilic moieties together and as a result can accommodate drug molecules with wide range of solubility [26] . They are able to entrap a variety of drugs of varying molecular weights. They prevent the degradation of drugs and can be used for controlled drug delivery over a long period of time. Transferosomes also have their drawbacks like high cost, predisposition to oxidative degradation, chemical instability and concern regarding the purity of the phospholipids used [26] .

Dendrimers are unimolecular, monodisperse, micellar nanostructures, approximately 20 nm in size. They are globular having a central symmetric core from which arise highly branched structures. Drugs can be encapsu- lated within the core or attached to the surface [27] . Dendrimers provide controlled release from the inner core and the encapsulation of both hydrophobic and hydrophilic drugs [28] . Since they have multiple attachment sites a number of different types of molecules such as drugs, solubilizing groups, etc. can be attached to the surface in a defined manner as required. In this way, the surface can be modified according to the desired application.

Dendrimers have been extensively studied in the delivery of antitumor, antiviral, and antibacterial drugs, in intracellular drug delivery and gene delivery. They are also used in photodynamic therapy to detect tumors [29] . Variation in toxicity has been noted with the change in the functional groups attached. For example, it has been studied that primary amines are more toxic than secondary or tertiary amines [30] . The material of the core also affects the toxicity of the dendrimers. It is suggested that the aromatic interior of the dendrimer may cause hemolysis through hydrophobic membrane contact [31] .

Particles such as alumina, silica and titania are called as ceramic nanoparticles. These are porous, inorganic compounds which are biocompatible in nature. Because these particles can be easily engineered with the desired size and porosity, growing interest has recently emerged to utilize ceramic nanoparticles as drug vehicles [32] . These particles protect doped molecules (enzymes, drugs, etc.) against denaturation induced by external pH and temperature. Their ultra-low size (less than 50 nm) can help them evade the reticulo-endothelial system (RES) of the body [33] . Ceramic (inorganic) particles have been highlighted to provide increased mechanical strength, chemical stability, biocompatibility, and resistance to microbial attack as compared to their organic (polymeric) counterparts [33] . Ceramic nanoparticles have been used in research involving cancer therapy such as targeting cancerous cells in liver as well as in the delivery of insulin by oral administration. They are also effective in de- livering proteins and genes. However, these particles are not biodegradable and so there is a concern that they may accumulate in the body and cause harmful effects [34] .

Lipid nanocarriers, as the name suggests are nanoparticles with a lipid matrix. The different lipid based carri- ers include solid lipid nanoparticles (SLN), nanostructured lipid carriers (NLC) and lipid drug conjugate (LDC). These contain lipids, drugs and surfactants which act as stabilizers. The first generation of solid lipid nanopar- ticles (SLN) was developed at the beginning of the nineties as an alternative carrier system to emulsions, lipo- somes and polymeric nanoparticles. The main features of SLN with regard to parenteral application are the ex- cellent physical stability, protection of incorporated labile drugs from degradation, controlled drug release (fast or sustained) depending on the incorporation model, good tolerability and site specific targeting [35] . However, SLN have demerits such as high water content, drug expulsion after polymorphic transition on storage and in- sufficient loading capacity limited by the solubility of drug in lipid melt.

Nanostructured lipid carriers which are also solid at room temperature are produced by the blend of a solid li- pid with a liquid lipid. They were introduced to overcome the limitations of SLN. They show an increase in the payload and prevent drug expulsion. NLC have been used mainly for topical delivery and hence find application in cosmetics and dermatology [35] .

Lipophilic drugs are usually incorporated in SLN but due to partitioning effects during production, only high- ly potent hydrophilic drugs effective in low concentrations are incorporated in SLN. LDC allows the incorpora- tion of hydrophilic as well as lipophilic drugs and has a drug loading capacity of up to 33% [35] .

Polymeric nanoparticles are solid, colloidal particles which may have varying shapes such as spherical, branched or shell structures. They vary in size from 10 nm to 1000 nm. Their main advantage is their size which enables them to penetrate capillaries [15] . Drugs can be introduced into them by encapsulation, attachment and entrapment enabling a sustained drug release system. The extent of uptake by cells depends on the type of po- lymer used. Polymeric nanoparticles have been used for vaccines and cancer chemotherapeutics for more than 45 years. Some other applications of these nanoparticles include possible recognition of vascular endothelial dysfunction; oral delivery of insulin, brain drug targeting for neurodegenerative disorders such as Alzheimer’s disease, topical administration to enhance penetration and distribution in and across the skin barrier, and appli- cation as pH-sensitive nanoparticles to improve oral bioavailability of drugs such as cyclosporine A [32] . How- ever, the main concern of polymeric nanoparticles is the possible cytotoxic effect which may prove to be a major drawback.

Ethosomes are similar to liposomes but have high alcohol content. The ethanol from ethosomes’ composition plays the same role as the surfactant from the transferosomes, namely disorganizing the lipid bilayer and thus conferring a ten times higher deformability to the particles [36] [37] .

Polymeric micelles are composed of amphiphilic block copolymers. They have a hydrophobic inner core sur- rounded by a shell of hydrophilic polymers. They are able to reach targets that are not accessible by liposomes, but have a limited targeting ability due to the low drug loading and low drug incorporation stability.

Nanocapsules are hollow structures having a polymer membrane which encapsulates the drug in the cavity. Metallic nanoparticles, as the name suggests are made up of metals such as iron oxide, gold, silver and nickel. They are used for targeted drug delivery wherein their surface can be modified by changing the functional groups according to the application. Carbon nanomaterials include carbon nanotubes and fullerenes. They have been studied for various therapeutic applications; however, the toxicity of carbon nanotubes cannot be ignored.