A Review : Supercritical Fluid Extraction Technology

R.D.Gupta

This study includes information concerning supercritical fluid. The technology of supercritical fluid (SCF) and supercritical fluid extraction offers an opportunity to efficient and economically improve recovery, increase reproducibility, decrease use of halogenated solvents and provide cleaner extract to the measurement instrument.

Previously to bring about the easier dissolution of relatively insoluble material some physical action to the solvent like application of pressure or change in the temperature were found to be effective but to a certain extent. Apart from the key of using supercritical fluid as an efficient solvent at its critical point has been shown its applicability in various types processing such solvent for some specialized chemical reaction¹. SCF technology is making in-roads in several pharmaceutical industrial operations including crystallization, medium for particle design and engineering, particle size reduction, preparation of drug delivery systems, coating, and product sterilization. It has also been shown to be a viable option in the formulation of particulate drug delivery systems, such as micro particles and nanoparticles, liposomes, and inclusion complexes, which control drug delivery and/or enhance the drug stability².

Introduction:

Supercritical fluids: A General View: - 

Supercritical fluids technology has become an interdisciplinary field shared by chemical engineers, chemists, food scientists, agronomists, researchers in biotechnology and environmental control. Since then, applications of SCF technology have been performed in many areas.

At the critical temperature (Tc) and pressure (Pc), a substance’s liquid and vapor phases are indistinguishable. A substance whose temperature and pressure are simultaneously higher than at the critical point is referred to as a supercritical fluid (Figure 1). The typical operating temperature and pressures for supercritical fluids are 1.01(Tc) to 1.1(Tc) and 1.01(Pc) to 1.1(Pc), respectively³.

Fig 1: Typical diagram of supercritical region

The physical and thermal properties of SCFs fall between those of the pure liquid and gas. SCFs offer liquid-like densities, gas-like viscosities, gas-like compressibility properties and higher diffusivities than liquids. The properties of SCFs, such as polarity, viscosity, and diffusivity, can be altered several-fold by varying the operating temperature and/or pressure during the process. This flexibility is enabling the use of SCFs for various applications in the food and pharmaceutical industries (Table 1), with the drug delivery system design being a more recent addition.

Commonly used supercritical solvents include carbon dioxide, nitrous oxide, ethylene, propylene, propane, n-pentane, ethanol, ammonia, and water (Table 2).⁴ Of these, CO₂ is a widely used SCF in the pharmaceutical processing due to its unique properties. This review summarizes the general SCF techniques used for particle engineering, examples of drug delivery systems prepared with SCF processes, and factors influencing the characteristics of SCF products, and scale-up issues associated with SCF processes.

Table 1:  Critical condition for some solvent

Basic Techniques in Supercritical Fluid  (SCF ) Technology: -

1) Rapid Expansion of Supercritical Solutions: -

A supercritical solvent saturated with a solute of interest is allowed to expand at a very rapid rate, causing the precipitation of the solute. The rapid expansion/decompression is achieved by allowing into pass through a nozzle at supersonic speeds. This rapid expansion of supercritical solutions leads to super saturation of the solute in it and subsequent precipitation of solute particles with narrow particle size distributions. This process is also known as supercritical fluid nucleation (SFN). Figure 2 provides schematic view of the rapid expansion of supercritical solutions (RESS) process. The SF is pumped through a pre-heater into the vessel containing the solid solute at a particular temperature and pressure. The SF dissolves and gets saturated with the solute, and the resultant solution is introduced into a precipitation chamber by expansion through capillary or laser-drilled nozzle⁵. Typically, by altering the pressure, the precipitation unit is maintained at conditions where the solute has much lower solubility in the SF. During expansion or decompression phase, the density and solubilisingpower of the SF decreases dramatically, resulting in a high degree of solute super saturation and subsequent precipitation. The morphology and size distribution of the precipitated material is a function of its pre expansion concentration and expansion conditions. The pre-expansion concentration is dependent on the choice of SF, nature of solute, addition of cosolventsand operating pressure and temperature. The higher the pre-expansion concentration, the smaller the particles and narrower the particle size range.

2) Gas Antisolvent Recrystallisation

It is a well-known phenomenon that a poor solvent of a particular solute can be added to the solution to precipitate the solute. This is called salting out and is widely used for crystallization purposes. However, disadvantages of this technique include poor control over the precipitated crystal morphology, size distribution and presence of residual solvents. Utilizing similar principle, the solubility of pharmaceutical compounds in supercritical solvents can be decreased by using SFs in gaseous form as antisolvents. It is possible to indurapid crystallization by introducing the antisolvent gas into a solution containing dissolved solute⁶. One of the requirements for this approach is that the carrier solvent and the SF antisolvent must beat least partially miscible. This process works in a semi batch mode, with the supercritical solvent introduced into an already existing stationary bulk liquid phase. This mode offers better control over the particle characteristics as governed by the rate of addition of the SF. However, the liquid phase cannot, in general, be completely removed, and requires additional processing steps before a dry product can be recovered.

 Fig 2: RESS Apparatus

Fig 3: Antisolvent precipitation Apparatus

3)  Precipitation with Compressed Fluid Antisolvent: -

The solute can be crystallized from a solution usingAntisolvents in two ways:

• Gas antisolvent rechrystallisation (GAS) method; or

• By spraying liquid into the SF antisolvent.

In the latter, the antisolvent rapidly diffuses into the liquid solvent, and the carrier liquid solvent a schematic view of the rapid expansion of supercritical solutions (RESS) process. The SF is pumped through a pre-heater into the vessel containing the solid solute at a particular temperature and pressure. The SF dissolves and gets saturated with the solute, and the resultant solution is introduced into a precipitation chamber by expansion through acapillaror laser-drilled nozzle. Typically, by altering the pressure, the precipitation unit is maintained at conditions where the solute has much lower solubility in the SF. During expansion or decompression phase, the density and solubilising power of the SF decreases dramatically, resulting in a high degree of solute super saturation and subsequent precipitation. The morphology and size distribution of the precipitated material is a function of its preexpansion concentration and expansion conditions⁷. The pre-expansion concentration is dependent on the choice of SF, nature of solute, addition of cosolvents and operating pressure and temperature. The higher the pre-expansion concentration, the smaller the particles and narrower the particle size range.

4) Impregnation or infusion of polymers with bioactive materials: -

Some gases cause swelling of polymers or drug carriers at high pressures. This swelling behavior can be exploited –for various such as control delivery of drugs. Sand has shown that substances such as fragrances, pest control agents, and pharmacologically active materials can be impregnated with a solid polymer, which is exposed to a supercritical fluid during the impregnated process. The polymers evaluated in this study included polypropylene, polyethylene, ethylene-vinyl acetate copolymer, and ethylene-ethyl acrylate copolymer and causes the migration of active material in to the polymer methods the diffusion of active material is increase significantly due to the swelling of polymer or drug carrier matrix when the pressure is reduced the SCF is driven out slowly resulting in the drug loaded polymer particles it has been found that the swelling is increase with increasing temperature at a constant pressure this approach can be utilize to develop novel control release dosage form to deposit thermo labile material in to the polymer⁸.

5) Solution enhanced Dispersion by Supercritical Fluid: -

This technique was developed at the University of Bradford to overcome some of the limitations of the RESS and GAS methods. The drug solution and the SF are introduced simultaneously into the arrangement causing rapid dispersion, mixing and Extraction of the drug solution solvent by SF leading to very high super saturation ratios. The temperature and pressure together with accurate metering of flow rates of drug solution and SFthrough a nozzle provide uniform condition for particle formation. This helps to control the particle size of the product and, by choosing an appropriate liquid solvent, it is possible to manipulate the particle morphology

Drug Delivery Applications Of Supercritical Fluids (SCFs ): -

1)Micro particles and Nanoparticles: - 

Drug and polymeric micro particles have been prepared using SCFs as solvents and antisolvents. Krukonis first used RESS to prepare 5- to 100-µm particles of an array of solutes including lovastatin, polyhydroxy-acids, and mevinolin. RESS process employing CO₂ was used to produce poly (lactic acid) (PLA) particles of lovastatin and naproxen. A GAS process was used to produce clonidine-PLA microparticles. In this process, PLA and clonidine were dissolved in methylene chloride, and the mixture was expanded by supercritical carbon dioxide to precipitate polymeric drug particles⁹.

SCF technology is now claimed to be useful in producing particles in the range of 5 to 2,000 nm. This patent covers a process that rapidly expands a solution of the compound and phospholipid surface modifiers in a liquefied-gas into an aqueous medium, which may contain the phospholipid. Expanding into an aqueous medium prevents particle agglomeration and particle growth, thereby producing particles of a narrow size distribution¹⁰. However, if the final product is a dry powder, this process requires an additional step to remove the aqueous phase. Intimate mixture under pressure of the polymer material with a core material before or after SCF salvation of the polymer, followed by an abrupt release of pressure, leads to an efficient solidification of the polymeric material around the core material. This technique was used to microencapsulate infectious Bursal Disease virus vaccine in a polycaprolactone (PCL) or a poly (lactic-co-glycolic acid) (PLGA) matrix.

2) Micro porous Foam: -

Using SCF technique, Hile et al prepared porous PLGA foams capable of releasing an angiogenic agent, basic fibroblast growth factor (bFGF), for tissue engineering applications. These foams sustained the release of the growth factor. In this technique, a homogenous water-in-oil emulsion consisting of an aqueous protein phase and an organic polymer solution was prepared first. This emulsion was filled in a longitudinally sectioned and easily separable stainless steel mold. The mold was then placed into a pressure cell and pressurized with CO₂ at 80 bars and 35°C. The pressure was maintained for 24 hours to saturate the polymer with CO₂ for the extraction of methylene chloride. Finally, the set-up was depressurized for 10 to12 seconds, creating micro porous foam.

3) Liposome: -

Liposomes are useful drug carriers in delivering conventional as well as macromolecular therapeutic agents. Conventional methods suffer from scale-up issues, especially for hydrophilic compounds. In addition, conventional methods require a high amount of toxic organic solvents. These problems can be overcome by using SCF processing. Fredereksen et al developed a laboratory scale method for preparation of small liposomes encapsulating a solution of FITC-dextran, a water-soluble compound using supercritical carbon dioxide as a solvent for lipids. In this method, phospholipid and cholesterol were dissolved in supercritical carbon dioxide in a high-pressure unit, and this phase was expanded with an aqueous solution containing FITC in a low-pressure unit. This method used 15 times less organic solvent to get the same encapsulation efficiency as conventional techniques. The length and inner diameter of the encapsulation capillary influenced the encapsulation volume, the encapsulation efficiency, and the average size of the liposomes. Using the SCF process, liposomes, designated as critical fluid liposomes (CFL), encapsulating hydrophobic drugs, such as taxoids, camptothecins, doxorubicin, vincristine, and cisplatin, were prepared. Also; stable paclitaxel liposomes with a size of 150 to 250 nm were obtained. Aphios Company’s patent (US Patent No. 5,776,486) on SuperFluidsTM CFL describes a method and apparatus useful for the nanoencapsulation of paclitaxel and campothecin in aqueous liposome formulations called TaxosomesTM and CamposomesTM, respectively. These formulations are claimed to be more effective against tumors in animals compared to commercial formulations¹⁰.

4) Inclusion complexes: -

 Inclusion complexes with cyclodextrins. For many nonpolar drugs, previously established inclusion complex preparation methods involved the use of organic solvents that were associated with high residual solvent concentration in the inclusion complexes. Earlier, cyclodextrins were used for the entrapment of volatile aromatic compounds after supercritical extraction. Based on this principle, Van Hees et al employed supercritical fluids for producing piroxicam and ß-cyclodextrin inclusion complexes. Inclusion complexes were obtained by exposing the physical mixture of piroxicam-ß-cyclodextrin (1:2.5 mol-mol) to supercritical CO₂ and depressurizing this mixture within 15 seconds. Greater than 98.5% of inclusion was achieved after 6 hours of contact with supercritical CO₂ at 15 MPa and 150°C.

5) Solid Dispersions: -

SCF techniques can be applied to the preparation of solvent-free solid dispersion dosage forms to enhance the solubility of poorly soluble compounds. Traditional methods suffer from the use of mechanical forces and excess organic solvents. A solid dispersion of carbamazepine in polyethylene glycol 4000 (PEG4000) increased the rate and extent of dissolution of carbamazepine. In this method, a precipitation vessel was loaded with solution of carbamazepine and PEG4000 in acetone, which was expanded with supercritical CO₂ from the bottom of the vessel to obtain solvent-free particles.

6) Powders of Macromolecules: -

Processing conditions with supercritical CO2 are benign for processing macromolecules, such as peptides, proteins, and nucleic acids. Debenedetti0 et al used an antisolvent method to form microparticles of insulin and catalase. Protein solutions in hydroethanolic mixture (20:80) were allowed to enter a chamber cocurrently with supercritical CO₂. The SCF expanded and entrained the liquid solvent, precipitating sub micron protein particles. Because proteins and peptides are very polar in nature, techniques such as RESS cannot be used often. Also, widely used supercritical antisolvent processing methods expose proteins to potentially denaturing environments, including organic and supercritical nonaqueous solvents, high pressure, and shearing forces, which can unfold proteins, such as insulin, lysozyme, and trypsin, to various degrees. This led to the development of a method, wherein the use of the organic solvents is completely eliminated to sobtain fully active insulin particles of dimensions, 1.5-500 µm. In this invention, insulin was allowed to equilibrate with supercritical CO₂ for a predetermined time, and the contents were decompressed rapidly through a nozzle to obtain insulin powder. Plasmid DNA particles can also be prepared using SCFs. An aqueous buffer (pH 8) solution of 6.9 KB plasmid DNA and mannitol was dispersed in supercritical CO₂ and a polar organic solvent using a three-channel coaxial nozzle. The organic solvent acts as a precipitating agent and as a modifier, enabling nonpolar CO₂ to remove the water. The high dispersion in the jet at the nozzle outlet facilitated rapid formation of dry particles of small size. Upon reconstitution in water, this plasmid DNA recovered 80% of its original super coiled state. Such macromolecule powders can possibly be used for inhalation therapies.

7) Coating: -

SCFs can be used to coat the drug particles with a single or multiple layers of polymers or lipids. A novel SCF coating process that does not use organic solvents has been developed to coat solid particles (from 20 nm to 100 µm) with coating materials, such as lipids, biodegradable polyester, or polyanhydride polymers. An active substance in the form of a solid particle or an inert porous solid particle containing active substance can be coated using this approach. The coating is performed using a solution of a coating material in SCF, which is used at temperature and pressure conditions that do not solubilize the particles being coated.

8) Product Sterilization: -

 In addition to drug delivery system preparation, SCF technology can also be used for other purposes, such as product sterilization. It has been suggested that high-pressure CO₂ exhibits microbicidal activity by penetrating into the microbes, thereby lowering their internal pH to a lethal level. The use of supercritical CO₂ for sterilizing PLGA microspheres (1, 7, and 20 µm) is described in US Patent No. 6,149,864. The authors indicated that complete sterilization can be achieved with supercritical CO₂ in 30 minutes at 205 bar and 34°C.

9) Particulate Dosage Forms: -

Some gases at certain pressures cause swelling of polymers likpolypropylene, polyethylene, and ethylene-vinyl acetate co-polymer and ethylene ethyl acrylate copolymer or drug carriers, and allow migration of active material in polymer matrix to give diffusion-controlled drug delivery systems. This specific behavior can be exploited for various purposes replacing the traditional techniques like Spray-drying, solvent evaporation and freeze-drying. This approach can be utilized as a solvent-free approach to develop novel, controlled-release dosage forms and deposit thermo labile materials such as peptide drugs into the polymers.

Ø In Pharmaceutical Industries: -

1) Medium for Crystallization: -

To generate high purity polymorphs, even with some morphological viz. high degree of Enantiomeric enrichment. SF technology appears to be a potential modality. Moreover, size and shape of the polymorph can be manipulated by controlling temperature and/or pressure during processing while degree of crystallization can be improved by manipulating the rate of crystallization & high degree of crystallinity. Better candidate in metered dose inhaler compared with conventionally crystallized and micronized drug.

2) Solubilization of pharmaceuticals: -

RESS technology has been used. Most of pharmaceutical compounds below 60 c and 300 bars showed a considerable higher solubility. In many a process of solubilization of polar or non-volatile compounds a limited solubility in SC CO2 is fails to form a homogenous solution under practical conditions. To aid the solubilization in such cases the CO2-philic solubilizers are being developed which rather the SC CO2 insoluble substances and make them solubilize in SC CO2.

3) Extraction And Purification: -

Supercritical fluid extraction technique could be utilized to separate impurities mainly organic complexes from the pharmaceuticals. Methods developed by Zoel are now widely used in industry as in caffeine production & Isolation of Taxol from the bark of the Taxus brevifolia in which SC CO2 is used. Purification via SCF technology gives a better alternative to all conventional purification methods as it is almost automated, quick, high yielding, SCF methods are also reported for the extraction of bryostatins, natural products, production of fat free products.

4) Medium For Polymerization And Polymer Processing: -

Supercritical fluids mainly SC CO2 is rapidly becoming an alternative solvent for polymerization. Solubility plays a very important role in the synthesis of polymers.            

Mainly two processes used

1) Step growth: SC CO2 has been reported very yielding in the production of polycarbonates, polymides, polyesters, polypyrrols, polyphenoxides and silica gels.

2) Chain growth: free radical polymerization of styrenics, armlets and methacrylates, cationic polymerization of isobutylene.

Supercritical CO2 in polymerization is increased plasticization because of CO2. The highly plasticized state of polymers is also results in increased polymerization rates by the enhanced diffusion of monomer into the polymer¹¹.

5) As A Supercritical Bio-catalyst: -

Randolph et al primarily found the enzyme alkaline phosphates active in a batch reaction system yhat employed SC CO2 as solvent. In the comparison SC CO2 as the adverse effect of pressure was less profound in case of compressed propane and ethane. Nakamura et al studied the acidolysis of trioline with stearic acid in SC CO2 by using Lipase as a bio-catalyst¹².

6) Micronization Of Pharmaceuticals: -

The RESS process has been shown to be capable of forming micron-sized particals. Krukonics, first extensively studied RESS in micronization of a wide variety of materials, including pharmaceuticals, biologicals, and polymers¹³. He produced uniform submicron powder of estradiol. Loth and Hemgesberg studied the micronization of phenacetin by RESS and compared with jet-milled phenacetin. The main limitation of RESS is the inability to process those materials which are insoluble or very less soluble in the SCF.So for this materials the SAS process has been successfully used to produced micron sized particles like insuline, bovine liver catalase, lysozyme, trypsin, methylprednisolone and hydrocortisone acetate. Insuline were in two crystalline forms; spheroidal (smaller than 1 micron) and needle (5 micron). ASES process has been studied for the preparation of a range of steroids for pulmonary delivery.

Conclusion: -

The special properties of SEFs bring certain advantages to chemical separation technique. Several applications have been fully developed and commercialized which include food and flavouring, pharmaceutical industry, inviormental protection for volatile and lipid soluble compounds, extraction of high value oils, extraction of natural aromas, recovery of aromas form fruits, meat and fish, isolaltion of lipid soluble compounds. Also in order to understand the mechanism of SFE, modling consideration of SCFs and main criteria for SCF techniques are focused in this review.

References: -

1) Kompella UB, Koushik K. Preparation of drug delivery systems using supercritical fluid technology. Crit Rev Therapeut Drug Car Sys. 2001; 18(2): 173-199.

2)  Hile DD, Amirpour ML, Akgerman A, Pishko MV. Active growth factory delivery from poly (D,L-lactide-co-glycolide) foams prepared in supercritical CO2. J Controlled Rel. 2000; 66(2-3): 177-185.

3) Frederiksen L, Anton K, Hoogevest PV, Keller HR, Leuenberger H. Preparation of liposomes encapsulating water-soluble compounds using supercritical carbon dioxide. J Pharm Sci. 1997; 86(8): 921-928.

4) Van Hees T, Piel G, Evrard B, Otte X, Thunus T, Delattre L. Application of supercritical carbon dioxide for the preparation of a piroxicam-beta-cyclodextrin inclusion compound. Pharm Res. 1999; 16(12): 1864-1870.

5) Moneghini M, Kikic I, Voinovich D, Perissutti B, Filipovic-Grcic J. Processing of carbamazepine-PEG 4000 solid dispersions with supercritical carbon dioxide: preparation, characterization, and in vitro dissolution. Int J Pharm. 2001; 222(1): 129-138.

6) Winters MA, Debenedetti PG, Carey J, Sparks HG, Sane SU, Przbycien TM. Long-term and high temperature storage of supercritically processed micro particulate protein powders. Pharm Res. 1997; 14(10): 1370-1378.

7) Tservistas M, Levy MS, Lo-Yim MYA, O’Kennedy RD, York P, Humphery GO, Hoare M. The formation of plasmid DNA loaded pharmaceutical powders using supercritical fluid technology. Biotech Bioeng. 2000; 72(1): 12-18.

8)  Steckel H, Thies J, Muller BW. Micronizing of steroids for pulmonary delivery by supercritical carbon dioxide. Int J Pharm. 1997; 152(1): 99-110.

9)  Bodmeier R, Wang H, Dixon DJ, Mawson S, Johnston KP. Polymeric microspheres prepared by spraying into compressed carbon dixoide. Pharm Res. 1995; 12(8): 1211-1217.

10)  Palakodaty S, York P. Phase behavioral effects on particle formation processes using supercritical fluids. Pharm Res. 1999;16 (7):976-985.

11) A.Rawat, V. Jaitely, P. Kanaujia, V. Mishra and P. Singh. Supercritical Fluid Technology perspectives and industrial application. The Eastern Pharmacist. 1994; 12: 35-43.

12) Mc Hugh, M. A. and Krukonis, V.J. Supercritical Fluid Extraction: Principles and Practice, 1994, 2^nd ed, ButterworthHeineman, Sloneham.

13) Super Critical Fluid Chromatography : Fundamentals and Applications

About Authors:

R.D.Gupta

M. Pharm, Lecturer, Dept. of Pharmaceutics, P.B. No.6, Mhow-Neemuch Road, Mandsaur, 458001 (India) Mob: 09329635883, Fax: 07422-255504,
E-mail: rd24@rediffmail.com

V. K.Chatap

M. Pharm, Lecturer, Dept. of Pharmaceutics, B.R.Nahata College of Pharmacy (BRNSS Contract Research Center)
P.B. No.6, Mhow-Neemuch Road, Mandsaur, 458001 (India)
Mob: 09300933017, Fax: 07422-255504, E-mail:Chatap@rediffmail.com

Dr.V.B.Gupta

Professor & Director, B R Nahata College of Pharmacy,(Additional Designation: Director BRNSS Contract Research Center)
P B No. 6, Mhow-Neemuch Road, MANDSAUR (MP) 458001
Tel: 07422-255734, Fax: 07422-255504, Mobile: 09826774144, E-mail: vbgupta@hotmail.com 
Web: http://www.brncop.org/bio-data.pdf

D. K. Sharma

M. Pharm. (Ph.D), Reader & Head, Dept. of Pharmaceutics, B.R. Nahata College of Pharmacy (BRNSS Contract Research Center)
P.B. No.6, Mhow-Neemuch Road, Mandsaur, 458001 (India) , Mob: 09827374668
E-mail: dineshsharma1973@rediffmail.com

S.D. Parial

M. Pharm. (Ph.D), Reader, B.R. Nahata College of Pharmacy (BRNSS Contract Research Center)
P.B. No.6, Mhow-Neemuch Road, Mandsaur, 458001 (India); mob: 09424532372
E-mail: parialcognosy@rediffmail.com