INTRODUCTION:¹

Microbubbles are small spherical bubbles comprising of gas, they remain distinct from each other or separate from each other i.e. do not agglomerate, also they have their size range in micrometers usually 1-100 μm. Micro bubbles are miniature gas bubbles of less than 50 microns diameter in water. The micro bubbles, which mostly contain oxygen or air, can remain suspended in the water for an extended period. Gradually, the gas within the micro bubbles dissolves into the water and the bubbles disappear. In the medical field microbubbles have been used as diagnostic aids to scan the various organs of body and recently they are being proposed to be used as drug or gene carriers and also for treatment in cancer therapy. Microbubbles have been used in a variety of fields , these have been used to improve the fermentation of soil, used to increase the hydroponic plant growth, have been to used to increase the aquaculture productivity, these have been also used to improve the quality of water, used in sewage treatment. Biomedically microbubbles are defined as small spherical gas bubbles made up of phospholipids or biodegradable polymers, that are approximately the size of RBC’s and are used as diagnostic aids, as drug and gene carriers in combination with ultrasound.

Properties of Microbubbles:²

The ideal properties of microbubbles can be divided into two classes,

1) Functional Properties

2) Structural Properties

1) Functional Properties:

The functional properties are those which render them useful for performing their various functions these include,

a) Injectability: Since these microbubbles are to be injected into the body so as to exert their various actions they should

be injectable.

b) Ultrasound Scattering Efficiency: As these microbubbles act in combination with ultrasound they should have

ultrasound scattering efficiency.

c) Biocompatibility: Microbubbles interact with the vital organs of the body at cellular levels they should be

biocompatible.

2) Structural Properties:

These refer to the structure or the physical properties of the microbubbles, these are as follows,

a) Should have an average external diameter between the ranges of 1-10 μm, narrow size distribution so as to avoid complications when injected into the body.

b) Density and compressibility difference between themselves and the surrounding body tissues to create an acoustic impedance and to scatter ultrasound at a much higher intensity than the body tissues so as to be used as contrast agents.

c) Sufficient surface chemical properties to be modified for the attachment of various ligands to target them to specific tissues or organs.

d) Uniformity of shell thickness.

Components of Microbubbles:^3,4

Microbubbles basically comprise of three phases:

1) Innermost Gas Phase

2) Shell Material Enclosing the Gas Phase

3) Outermost Liquid or Aqueous Phase

In addition to this the formulation may also comprise of various other components.

1) Gas Phase:

The gas phase can be a single gas or a combination of gases can be used. Combination gases are used to cause differentials in partial pressure and to generate gas osmotic pressures which stabilize the bubbles. When a combination of gases is used two types of gases are involved one is the Primary Modifier Gas also known as first gas. Air is preferably used as primary modifier gas, sometimes nitrogen is also used as first gas. The vapor pressure of first gas is (760 - x ) mm of Hg , where x is the vapor pressure of the second gas. The other gas is Gas Osmotic Agent also known as second gas; it is preferably a gas that is less permeable through the bubble surface than the modifier gas. It is also preferable that the gas osmotic agent is less soluble in blood and serum. Gas osmotic agent is normally a gas at room temperature or liquid so long as it has a sufficient partial or vapor pressure at the temperature of use to provide the desired osmotic effect. Some examples of second gas are per fluorocarbons or sulfur hexafluoride.

2) Shell Material:

The shell material encapsulates the gas phase. It plays a major role in the mechanical properties of microbubble as well as diffusion of the gas out of the microbubble. The shell also acts a region for encapsulation of drug molecules also ligands can be attached to the shell membrane so as to achieve targeting of these microbubbles to the various organs or tissues. It accounts for the elasticity or compressibility of microbubbles. More elastic the shell material is more acoustic energy it can withstand before bursting or breaking up, this increases the residence time of these bubbles in body. More hydrophilic the shell material, more easily it is taken up by the body this decreases the residence time of these bubbles in the body. E.g.: The various types of shell materials that can be used are Proteins like albumin Carbohydrates like galactose Phospholipids like phosphotidylcholine, phosphotidylethanolamine etc. Biodegradable polymers like polyvinyl alcohol, polycaprolactone etc.

3) Aqueous or Liquid Phase:

The external, continuous liquid phase in which the bubble resides typically includes a surfactant or foaming agent. Surfactants suitable for use include any compound or composition that aids in the formation and maintenance of the bubble membrane by forming a layer at the interphase. The foaming agent or surfactant may comprise a single component or any combination of compounds, such as in the case of co surfactants. Also the persistence of microbubble in body is inversely proportional to La Place pressure which in turn is directly proportional to surface tension of bubble. In other words decrease in the surface tension acting on the bubble increases the persistence time of the bubble in the body. E.g.: Block copolymers of polyoxypropylene, polyoxyethylene, sugar esters, fatty alcohols, aliphatic amine oxides, hyaluronic acid esters and their salts, dodecyl poly (ethyleneoxy) ethanol, etc. Nonionic Surfactants: Polyoxyehylene polyoxypropylene copolymers Eg. Pluronic F-68, polyoxyethylene stearates, polyoxyethylene fatty alcohol ethers, polyoxyethylated sorbitan fatty acid esters, glycerol polyethylene glycol oxystearates, glycerol polyethylene glycol ricinoleate etc.Anionic Surfactants: Fatty acids having 12 -24 carbon atoms E.g. Sodium Oleate.

4) Other Components:

The various other components that may be incorporated in the formulation include osmotic agents, stabilizers, chelators, buffers, viscosity modulators, air solubility modifiers, salts and sugars can be added to fine tune the microbubble suspensions for maximum shelf life and contrast enhancement effectiveness. Such considerations as sterility, isotonicity and biocompatibility may govern the use of such conventional additives to injectable compositions.

Excipients:⁵

Surfactants are amphiphilic compounds containing both hydrophilic and lipophilic moieties. Due to their dual nature, surfactants tend to partition into the oil-air or oil-water interface to reduce the surface and interfacial tension and stabilize newly created interfaces. Surfactants can be derived from both chemically based (“chemical surfactants” or “synthetic surfactants”) and biologically based (“biosurfactants”) sources. Strictly speaking, a biosurfactant is a surfactant directly derived from a natural source (i.e., from a plant, animal or microorganism), but this term is often used in a broader sense to include surfactants synthesized from natural raw materials. Fatty acid esters of sugars and fatty acid esters or amides of amino acids are examples of the surfactants belonging to this category. Biosurfactants offer advantages over synthetic surfactants in terms of their derivation from renewable resources, low or non-toxicity, biodegradability, excellent surface activity, possible reuse through regeneration, high specificity, and effectiveness under extreme temperature and pH conditions. The major functions of biosurfactants include solubilization, emulsification, dispersion, wetting, foaming, and detergent capacity, as well as antimicrobial activity in some cases. Consequently, interest in biosurfactants is continuously increasing. Biosurfactants have been used in various industries alone or blended with other biosurfactants or synthetic surfactants to offer desired performance characteristics. In the food industry, biosurfactants provide multiple functions and act as emulsifying/foaming agents, stabilizers, antioxidant agents, and antiadhesives. Environmental and agricultural applications are major areas of biosurfactantutilization, where they play important roles in soil remediation, oil recovery, and plant pathogenelimination. Biosurfactants have also found applications in detergents, paints, coatings, cosmetics and pharmaceutics.

Type of Biosurfactants:

Biosurfactants can be classified by their chemical composition and their origin. In this review, biosurfactants are grouped into three categories of origin: microbially derived surfactants, animal-derived surfactants and plant-derived biosurfactants. Most biosurfactants are either anionic or neutral; only a few, such as those containing amine groups, are cationic.

a) Microbially Derived Surfactants:

Microbially derived surfactants are surface-active agents synthesized by bacteria, yeasts and fungi to facilitate growth on various substrates (e.g., sugars, oils, alkanes and wastes). They vary widelyin molecular weight. Low-molecular-weight biosurfactants include glycolipids, lipopeptides and phospholipids. They are effective in reducing surface and interfacial tension. High-molecular-weight biosurfactants are composed of polysaccharides, proteins, lipopolysaccharides, lipoproteins or complex mixtures of these biopolymers. They are more effective in stabilizing newly created surfaces. The increasing interest in microbially derived surfactants is based on their wide diversity in structure and function and on their low cost, which is enabled by their production from cheaper agro-based substrates and waste materials. However, their industrial application is limited by a lack of public acceptance of producer strains, low production yields, and the high purity necessary for food, cosmetic, and pharmaceutical applications, which result in higher costs. Consequently, they are mainly used in environmental applications. However, their high surface activity, biodegradability, low or non-toxicity, emulsifying and demulsifying ability, antimicrobial activity, and tolerance to wide ranges of pH, temperature and ionic strength make them promising for food, cosmetic and pharmaceutical applications.

b) Animal-Derived Surfactants:

Typical examples of biosurfactants derived from animal sources include lecithin, gelatin, casein, wool fat, cholesterol, and wax. They have a variety of uses because of their widely different chemical constitution. Lecithin is the only truly natural low-molecular-weight surfactant available for industrial application. It is a mixture of phospholipids that is a natural constituent of animals and plants. Animal-derived lecithin is usually produced from egg yolk and consists of zwitter ionic phosphatidylethanolamine (PE, 18.1%) and phosphatidylcholine (PC, 78.7%). Purified egg lecithin is mainly used as a pharmaceutical excipient for drug delivery and intravenous nutrition. Gelatin is a product obtained through the partial hydrolysis of collagen with dilute acid or base. The main sources of commercial gelatin are bovine skin and bones and pigskin. Increasing concern over bovine spongiform encephalopathy (BSE) has led to the development of fish gelatin alternatives. Gelatin is a high-molecular-weight polymer and had been used as a stabilizer, thickener, and texturizer in food and non-food applications. It is a relatively poor protein surfactant, but its emulsifying properties can be improved by enzyme-catalyzed attachment of hydrophobic side chains or by combination with other surface-active agents. Casein is a milk protein that accounts for ~80% of milk’s total protein content. It is a heterogeneous phosphoprotein that consists of four major fractions: αs1 (44%), αs2 (11%), β (32%), and κ (11%).Casein can be prepared by isoelectric precipitation or enzyme precipitation; its composition and functional properties depend on the method used. Caseins have relatively random and flexible structures in solution; some regions contain hydrophobic residues, and others contain polar and charged residues, and thus casein tends to form micelles in solution. Casein micelles (100–300 nm in diameter) consist of sub-micelles (10–20 nm) aggregated together. Caseins are widely used as emulsifiers, thickeners and gelling agents in various food products. Whey protein is a mixture of globular proteins containing β-lactoglobulin (55%), α-lactalbumin(24%), serum albumin (5%) and immunoglobulins (15%) and constitutes the remaining 20% of milk protein. Its emulsifying properties are strongly affected by pH, ionic strength, and temperature. To prepare stable whey protein emulsions, the pH must remain sufficiently far from the isoelectric point of the protein such that it can be stabilized through electrostatic repulsion. Egg albumin, bovine serum albumin, and human serum albumin are other widely used protein-based bio surfactants of animal origin. Other well-known and physiologically important animal-derived surfactants are bile acids and pulmonary surfactants. Pulmonary surfactant (PS) is a complex mixture of lipids and proteins that coats the interior surface of the vertebrate lung as a film. It consists of about 90% lipids (mainly phospholipids) and 8–10% protein. This proteolipidic material is synthesized by type II pneumocytes and follows a regulated exocytic pathway leading to secretion into the thin aqueous layer covering the alveoli. PS exists not only in alveoli but also in bronchioles and small airways. The composition of PS varies from one species to another, and even among individuals within the same species. Although phospholipids are the main surface-active components of PS, they require the participation of the proteins to function biophysically. The main function of PS is to maintain normal respiratory mechanics by reducing the surface tension at the alveolar air-liquid interface of lungs to avoid alveolar collapse at the end of expiration. The lack, deficiency or inactivation of PS causes severe respiratory disorders that can be lethal. Exogenous surfactant replacement therapy using either synthetic or modified natural PS extracted from bovine or porcine sources has been used to treat respiratory distress syndrome (RDS). Preclinical animal experiments and clinical practice suggest that animal-derived surfactants are superior to synthetic preparations. Difficulties in large-scale production, related to the high cost, suspension techniques, reproducibility and purity of natural surfactants, limit their clinical applications. Challenges in the production and utilization of animal-derived surfactants comprise the high cost of animal feedstock, variations in their emulsifying properties from batch to batch, customer concerns over BSE, religious restrictions, and strict government regulations. More plant-derived surfactants are expected to replace animal-derived surfactants.

c) Plant-Derived Biosurfactants:

Many surface-active compounds are derived from renewable plant resources. The European surfactant market in 2004 is estimated at 2.5 M metric tons, 25% of which are plant-derived. Here we focus on several important plant-derived surfactants that are widely used in industry or in bubble-related research and application. Saponins are a structurally diverse class of compounds widely distributed across the plant kingdom. They can be isolated from different plant parts (e.g., roots, stems, bark, leaves, seeds, and fruits).

The most significant sources of dietary saponins are the legumes: soybeans, chickpeas, mung beans, peanuts, broad beans, kidney beans and lentils; the saponin content in soybeans is 5–6%. Saponins are amphiphilic molecules in which sugars are linked to either a sterol or a triterpene non-polar group, by which they are classified. Saponins have emulsifying and foaming properties, pharmacological and medicinal properties, and antimicrobial and insecticidal activity and are used in beverages, confectionery, cosmetics and pharmaceutical products. As described above, lecithin is an important low-molecular-weight natural surfactant found in both animals and plants. Although lecithin can be extracted from animal sources, this process is too expensive for industrial applications. Instead, lecithin is predominantly manufactured from soybean oilseeds due to their abundance and low cost. Soy lecithin differs from egg lecithin in its phospholipid and fatty acid composition. The major phospholipids for soy lecithin are PC (29–46%), PE (21–34%) and phosphatidylinositol (PI, 13–21%). The concentration of total unsaturated fatty acids is much higher in soy lecithin than in egg lecithin; the egg variety contains less linoleic and nearly no linolenic acid but more long-chain polyunsaturated fatty acids than soy lecithin, which consists mainly of linoleic acid and a low amount of linolenic acid. Various methods have been employed to

improve the solubility and modify the functionality of lecithin. Soy lecithin and modified soy lecithin are widely used as emulsifiers, antioxidants, stabilizers, lubricants, wetting agents, and nutritional supplements. A variety of plants produce surface-active proteins. Soy protein is one of the most important plant-derived protein surfactants. Soybeans contain about 40% protein and 20% oil. Soy proteins are mainly globulins and can be classified into 2S, 7S, 11S, and 15S fractions. Soy proteins are available in three major forms that vary in protein content: soy flours, soy protein concentrates and soy protein isolates. Soy proteins have been used as nutritional and functional ingredients in every food category. They offer nutritional value and also affect the quality of food products. Because their properties as a surfactant are governed by factors such as solubility, hydrophobicity, molecular size and flexibility, and surface charge, many efforts have been devoted to improving the functional properties of soy proteins through chemical, physical and enzymatic modification.

The current increasing pressure on synthetic surfactants has stimulated the production of surfactants from renewable plant sources. Soybean oil is second largest source of vegetable oil and the second most consumed edible oil in the world. The growth in soybean oil production and the decline in dietary oil consumption due to health concerns have accelerated the development of non-food applications of soybean oil. Epoxidation is commonly used to modify soybean oil by converting its double bonds into more reactive epoxide or oxirane ring groups. The resultant epoxidized soybean oil (ESO) is a promising intermediate for the production of soybean oil–based surfactants. For example, it can be used to produce polyol surfactants through ring-opening hydrolysis and polysoap surfactants through ring-opening polymerization. The surface activity of polysoap surfactants is comparable to the reported activity of microbially derived surfactants and higher than those of some conventional synthetic surfactants; they can reduce the surface tension of Milli-Q water to a minimum

value less than 30 mN/m. The polysoap surfactant Palozengs (R-004) exhibits a unique aggregation behavior, forming small aggregates (pre-micelles) at very low concentrations. By changing polymerization and hydrolysis conditions, novel soybean oil–based surfactants with variable structure and functionality can be produced. With increasing health and environmental concerns over organic solvents, soybean oil–derived surfactants have been prepared using supercritical CO2, a more environmentally friendly solvent. Recent work has revealed that polymeric surfactants possess advantageous properties over those obtained by other methods. Previous work on nanoparticle drug delivery systems and biological hydrogels prepared with soybean oil-derived polysoap surfactants have suggested that soybean oil–derived surfactants have potential in a variety of applications due to their high surface activity, biodegradability, biocompatibility, and low cost.

Methods to prepare microbubbles:^5,6

The various methods that can be used for the preparation of these microbubbles include:

1) Cross Linking Polymerization.

2) Emulsion Solvent Evaporation.

3) Atomization and Reconstitution.

4) Sonication.

1) Cross Linking Polymerisation:

In this a polymeric solution is vigorously stirred, which results in the formation of a fine foam of the polymer which acts as a colloidal stabilizer as well as a bubble coating agent. The polymer is then cross linked, after cross linking microbubbles float on the surface of the mixture. Floating microbubbles are separated and extensively dialyzed against Milli Q water. E.g.: 2% aqueous solution of telechelic PVA is vigorously stirred at room temperature for 3 hrs at a pH of 2.5 by an Ultra Turrax T-25 at 8000 rpm equipped with a Teflon coated tip, fine foam of PVA is formed. The PVA is then cross linked at room temperature and at 5οC by adding HCl or H2SO4 as a catalyst, the cross linking reaction is stopped by neutralization of the mixture and microbubbles are then separated.

2) Emulsion Solvent Evaporation:

In this method two solutions are prepared, one is an aqueous solution containing an appropriate surfactant material which may be amphilic biopolymer such as gelatin, collagen, albumin or globulins. This becomes the outer continuous phase of the emulsion system. The second is made from the dissolution of a wall forming polymer in a mixture of two water immiscible organic liquids. One of the organic liquids is a relatively volatile solvent for the polymer and the other is relatively nonvolatile nonsolvent for the polymer. The polymer solution is added to the aqueous solution with agitation to form an emulsion. The emulsification step is carried out until the inner phase droplets are in the desired size spectrum. It is the droplet size that will determine the size of the microbubble. As solvents volatilizes, polymer conc. in the droplet increases to a point where it precipitates in the presence of the less volatile nonsolvent. This process forms a film of polymer at the surface of the emulsion droplet. As the process continues, an outer shell wall is formed which encapsulates an inner core of nonsolvent liquid. Once complete, the resulting microcapsules can then be retrieved, washed and formulated in a buffer system. Subsequent drying, preferably by freeze-drying, removes both the nonsolvent organic liquid core and the water to yield air filled hollow microbubbles.

3) Atomisation and Reconstitution:

A spray dried surfactant solution is formulated by atomizing a surfactant solution into a heated gas this results in formation of porous spheres of the surfactant solution with the primary modifier gas enclosed in it. These porous spheres are then packaged into a vial, the headspace of the vial is then filled with the second gas or gas osmotic agent. The vial is then sealed, at the time of use it is reconstituted with a sterile saline solution. Upon reconstitution the primary modifier gas diffuses out and the secondary gas diffuses in, resulting in size reduction. The microbubbles so formed remain suspended in the saline solution and are then administered to the patient.

4) Sonication: Sonication is preferred for formation of microbubbles, i.e. through an ultrasound transmitting septum or by penetrating a septum with an ultrasound probe including an ultrasonically vibrating hypodermic needle. Sonication can be accomplished in a number of ways, for eg. A vial containing a surfactant solution and gas in headspace of the vial can be sonicated through a thin membrane. Sonication can be done by contacting or even depressing the membrane with an ultrasonic probe or with a focused ultrasound “beam”. Once sonication is accomplished, the microbubble solution can be withdrawn from the vial and delivered to the patient. Sonication can also be done within a syringe with a low power ultrasonically vibrated aspirating assembly on the syringe.

Characterisation of Microbubbles:⁶

Once prepared these microbubbles are characterized as per the following parameters:

1) Microbubble Diameter and Size Distribution:

The average diameter as well as size distribution of these microbubbles can be determined by Laser light Scattering, Scanning Electron Microscopy, Transmission Electron Microscopy.

2) Shell Thickness:

Shell thickness is determined by coating the shell with a fluorescent dye like Red Nile, this is then determined by Fluorescent Microscopy against a dark background.

3) Microbubble Concentration:

The microbubble concentration is determined by counting the no. of microbubbles per ml by using the Coulter Counter Machine.

4) Air Content by densitometry:

The content of air encapsulated within the microbubbles in the suspension samples is measured by oscillation U-tube densitometry with a DMA-58. The instrument is calibrated with air and purified water prior to use. The density of the suspension is measured before and after elimination of encapsulated air. The complete removal of encapsulated air is achieved by 5 min high powered sonication in a sonicator. The air content is calculated as, Cair = ρ1 –ρ2/ρ2 *100 Where, Cair is air content (%v/v) ρ1 (g/ml) density before elimination of encapsulated air

ρ2 (g/ml) density after elimination of encapsulated air.

5) Ultrasound Reflectance Measurement:^7-9

Experimental set up consists of transducer, microbubble contained in a vessel consisting of metallic reflector and cellophane membrane, this vessel is in turn kept in another vessel containing water. The signals which are reflected are evaluated for the ultrasound reflecting capacity of these microbubbles.

Applications:^10-14

1. Ultrasound diagnosis and therapy and remediation or bio-remediation of contaminants, extensive research has been devoted to the use of microbubbles as ultrasound contrast agents for ultrasound diagnosis and therapy. These microbubbles can be stabilized by a surfactant, protein, lipid, polymer or a combination of these. The most commonly used microbubble contrast agents are albumin microbubbles. Their advantages include fragility when exposed to moderate energy ultrasound and ease of preparation.

2. A wide range of substances, such as drugs, DNA, and virus particles, can be bound to the shells of the microbubbles, making them potential delivery systems for drugs and genes.

3. Ultrasound-induced microbubble destruction thus provides a promising therapeutic approach for targeted treatments. As a wide variety of organic compounds and heavy metals are released into the environment by domestic and industrial effluents, the development of efficient and cost-effective remediation methods is required.

4. Numerous studies have demonstrated that surfactant-enhanced remediation is an effective method of treating a variety of contaminants. Surfactants are used to mobilize contaminants, readying them for remediation. They also promote the solubilization of water-insoluble contaminants by partitioning them into the hydrophobic core of micelles or lamellar structures at concentrations above the CMC. Ionic surfactants can be used to extract heavy metals through ion exchange, precipitation-dissolution, and counter-ion binding. However, the application of surfactant-enhanced remediation is hindered by the possibility of spreading of the contaminated zone and further contaminating ground water. The combination of foam technology and biosurfactants is a solution that has recently received increasing attention.

5. Rhamno lipid foam consisting of many tiny bubbles exhibits a higher efficiency in the removal of heavy metals compared with distilled water and surfactant solution. It was postulated that the metals were removed through the formation of complexes with the surfactants on the soil surface, which detached them from the soil and brought them into solution.

6. Anionic biosurfactants are effective in the removal of cationic metals owing to their high surface activity and the electrostatic metal―surfactant interaction. High pH conditions are likely to give better outcomes as a result of enhanced metal solubility and surfactant activity. The development of a protein-based foam as a carrier system for the delivery of microbes, nutrients, and oxygen to treat hydrocarbon-contaminated soil.

7. Foam stability increased with the concentration of protein hydrolysate. The addition of metal salts at relatively low concentrations greatly increased the quality and stability of the foam. Although the addition of viscosity modifiers (sodium alginate, carboxymethyl cellulose, and xanthan gum) retarded drainage, it has deleterious effects on foam generation and oxygen transfer. The inhibition of oxygen transfer due to increasing viscosity may result in anoxic conditions. Moreover, these viscosity modifiers may be consumed by the microbes as an alternative carbon source and may thus prevent or delay the biodegradation of hydrocarbon.

8. The dissolved oxygen concentration in the foams generated with oxygen remained significantly higher than in the foams generated with air, supporting the hypothesis that biodegradation was enhanced by the sustained release of rate-limiting oxygen onto the n-hexadecane-contaminated soil and the associated improved oxygen transfer.

9. Microscopic observation has revealed bubbles densely covered with bacterial cells and significantly fewer bacteria in the aqueous phase of the foam lamellae, demonstrating the preferential sorption of the degrader at the bubble surface. The retention of bacteria within the foam is correlated to the cell surface hydrophobicity of the organisms used. Generally, proteins unfold and rearrange when they adsorb at the surface and expose their hydrophobic groups to the air phase. This process may facilitate the sorption of hydrophobic bacterial cells to the surfaces of bubbles. The protein hydrolysate used not only acts as a surfactant but also as a nitrogen-rich nutrient source. These suggest that potential of protein-based bioactive foams for the bioremediation of contaminated sites.

10. The microbubble dispersions prepared with the plant-derived surfactant described above can be used to remove hydrophobic organic compounds (HOCs) from soil, using hexachlorobenzene (HCB) as the model HOC. The recovery of HCB from soil columns using the biosurfactant-prepared microbubble dispersions was considerably higher than a simple water flood. The recovery of HCB increased with surfactant concentration due to increased solubility of HCB.

11. The use of a saponin-based microbubble suspension to enhance aerobic biodegradation of phenanthrene in a sand column. The gas content of the microbubble dispersion prepared with 2% saponin was about 40% addition of salt did not affect the properties of the microbubble dispersions, and the delivery of oxygen and phenanthrene-degrading bacteria were confirmed. Compared with saponin solution, the biodegradation of phenanthrene was improved; this effect could be further enhanced by repeated introduction of microbubble dispersions. The results revealed that microbubble dispersions prepared with plant-derived surfactants can be used as potential carriers of oxygen, pollutant-degrading microorganisms, and micronutrients to enhance aerobic biodegradation under oxygen-limiting environments.

12. Micro bubbles of concentrated oxygen containing about 2% ozone can be used to inactivate norovirus in shellfish and oysters. This norovirus is one of the major pathogens causing food poisoning in winter. This is a much more cost effective method compared to cultivating the oysters in sterile sea water and using chlorine-based germicide.

13. Another emergent usage of micro bubbles is in the areas of cancer treatment. Scientists are in the process of developing a method of diagnosing cancer lesions by injecting micro bubbles into the blood stream. During the ultrasonic scan for cancer lesions, the micro bubbles contract and expand rapidly due to the pressures produced by the ultrasonic beam. Groups of the micro bubbles at cancerous tumors will show up very visibly on ultrasonic scans to indicate the presence of cancerous cells.

14. Due to their large surface area volume ratio, micro bubbles can penetrate deeply into a surface for effective cleaning. This cleaning effect of micro bubbles is used in cleaning the inside of vegetables such as cabbage and radish sprout, as well as maintenance of freshness with vegetables in one particular vegetable processing center in Japan.

15. On a more personal level, the micro bubbles can penetrate deeply into skin for a good scrub without the need for any shampoo or soap. This skin treatment has been introduced with in Japan as well as shops specializing in bathing pets.

Suwa companies are also developing a small handy micro bubble generator which can be used at home. With all the product development going on, very soon, you may be able to purchase a micro bubble generator at your electronics store and relax in a micro bubble bath at home.

CONCLUSION:

From the above study it is conclude that many drugs can be administered through microbubble drug delivery system, as it can be prepared by various new development techniques. It plays vital role for administration of those drug which are difficult to administered by any other route. Compared to biosurfactant-stabilized emulsions, knowledge of biosurfactant stabilized microbubble dispersions is insufficient, which limits their application. Microbubbles destruction thus provides a promising therapeutic approach for targeted treatments. Advantages of microbubbles over other dosage form include fragility when exposed to moderate energy ultrasound and ease of preparation. A wide range of substances, such as drugs, DNA, and virus particles, can be bound to the shells of the microbubbles, making them potential delivery systems for drugs and genes.

REFERENCES:

1) Rajesh Patel;Microbubble : An ultrasound contrast agent in molecular imaging, Pharma Times, May 2008 ;Vol. 40 ; Pg:15.

2) Deepika Maliwal; Microbubbles Contrast Agents Using Ultrasound; Research Journal of Pharmacy and Technology; Vol. 1;issue 03; July-Sept. 2008.

3) Eniola A.O. and Hammer D.A.; In vitro characterization of leukocyte mimetic for targeting therapeutics to the endothelium using two receptors; Biomaterials; 2005; Vol.26; Pg: 7136-44.

4) Eniola A.O., Willcox P.J. and Hammer D.A.; Interplay between rolling and firm adhesion elucidated with a cell-free system engineered with two distinct receptor-ligand pairs; Biophys. J.; 2003; Pg: 85; 2720-31.

5) Yiyao Liu, Hirakazu Miyoshi; Encapsulated Ultrasound microbubbles: Therapeutic application in drug/ gene delivery ;Journal of Controlled Release ; Vol. 114;2006;Pg: 89.

6) Bjerknes K., Sontaum P. C.; Preparation of polymeric microbubbles : formulation studies and product characterisation. International Journal of Pharmaceutics; Vol.158; 1997;Pg 129.

7) Bloch, S.H.; Short, R.E.; Ferrara, K.W.; Wisner, E.R. The effect of size on the acoustic response of polymer-shelled contrast agents. Ultrasound Med. Biol. 2005, Pg: 31, 439–444.

8) Bekeredjian, R.; Grayburn, P.A.; Shohet, R.V. Use of ultrasound contrast agents for gene or drug delivery in cardiovascular medicine. J. Am. Coll. Cardiol. 2005, Pg: 45, 329–335.

9) Teupe, C.; Richter, S.; Fisslthaler, B.; Randriamboavonjy, V.; Ihling, C.; Fleming, I.; Busse, R.; Zeiher, A.M.; Dimmeler, S. Vascular gene transfer of phosphomimetic endothelial nitric oxide synthase (S1177D) using ultrasound-enhanced destruction of plasmid-loaded microbubbles improves vasoreactivity. Circulation 2010, Pg: 105, 1104–1109.

10) Wang, S.; Mulligan, C.N. An evaluation of surfactant foam technology in remediation of contaminated soil. Chemosphere 2004, Pg: 57, 1079–1089. Mulligan, C.N.; Wang, S. Remediation of a heavy metal-contaminated soil by a rhamnolipid foam. Eng. Geol. 2006, Pg: 75–81.

11) Ripley, M.B.; Harrison, A.B.; Betts, W.B.; Dart, R.K.; Wilson, A.J. Enhanced degradation of a model oil compound in soil using a liquid foam-microbe formulation. Environ. Sci. Technol. 2000, Pg: 34, 489–496.

12) Ripley, M.B.; Harrison, A.B.; Betts, W.B.; Dart, R.K. Mechanisms for enhanced biodegradation of petroleum hydrocarbons by a microbe-colonized gas-liquid foam. J. Appl. Microbiol. 2002, Pg: 92, 22–31.

13) Choi, Y.J.; Kim, Y.J.; Nam, K. Enhancement of aerobic biodegradation in an oxygen-limiting environment using a saponin-based microbubble suspension. Environ. Pollut. 2009, Pg:157, 2197–2202.

14) Park, J.Y.; Choi, Y.J.; Moon, S.; Shin, D.Y.; Nam, K. Microbubble suspension as a carrier of oxygen and acclimated bacteria for phenanthrene biodegradation. J. Hazard. Mater. 2009, Pg:163, 761–767.

Received on 17.10.2011 Modified on 28.10.2011

Research J. Pharm. and Tech. 5(1): Jan. 2012; Page 27-33