the first step of the method of forming porous structures according to the present invention is to form a composite comprising at least a first material that is not soluble in supercritical fluid and a second material that is soluble in supercritical fluid . the second step of the method of the invention is to extract the scf - soluble second material from the composite to yield a porous structure , which may be a matrix or a particle . the composite can be formed using any technique known in the prior art . the composite can be in the form of composite particles and can be formed by processes such as , for example , spray drying , crystallization , precipitation , emulsion extraction and lyophilization . the amount of the scf - soluble second material present in the composite can be varied depending on the degree of porosity desired in the final product . suitable compounds for use as the scf - soluble second material include , but are not limited , lipids , waxes , polymers , sugar acetates , and fluorocarbons . in a first embodiment of the invention , porous particles are obtained by passing scf through a bed of composite particles comprising a first material that is not soluble in the scf and a second material that is soluble in the scf . the composite particles in this case can either be in a fluidized state or in the form of a packed bed depending on the scf flow rate . in a second embodiment of the invention , porous particles are formed by bubbling scf through a suspension of composite particles suspended in a non - scf soluble solvent . in some cases , surfactants may be employed to keep the composite particles suspended in the non - scf soluble solvent . in a third embodiment of the invention , porous matrixes are obtained by directly contacting a preformed composite matrix with scf . in this embodiment , the extraction step can be carried out using a high - pressure vessel of arbitrary or specific shape required to produce drug delivery devices in a desired form such as , for example , disks , films and pellets . with reference to fig1 , which shows a schematic representation of an apparatus 100 for carrying out the first embodiment of the invention , a high - pressure extraction vessel 101 , a scf pump 102 and a backpressure regulator 103 are interconnected . the high - pressure extraction vessel 101 is preferably tubular and defines an axis , and has first and second ends that are spaced axially apart . the high - pressure extraction vessel 101 has an inner surface that defines a chamber 104 . the extraction vessel has means , not shown , for accessing the interior of the vessel so as to charge the interior with a load material . the extraction vessel has a heating jacket 105 ( either circulating or electrical ) for heating the contents of the extraction vessel at the desired operating temperature . the load material consists of composite particles of preferably biologically active material , for example a drug , a pharmaceutical , or a therapeutic agent and a scf soluble material . alternatively , the load material can include composite particles consisting of for example , medicinal agents , proteins , peptides , sugars , pigments , toxins , insecticides , viral materials , diagnostic aids , agricultural chemicals , nutritional materials , alkaloids , alkyloids , animal and / or plant extracts , dyes , explosives , polymer precursors , cosmetics , antigens , enzymes , catalysts , nucleic acids , zeolites , polymer precursors , and combinations thereof . one of the materials in the vessel must be a scf soluble material . the load material can further include additional material ( s ), for example , a carrier , polymer , filler , disintegrant , binder , solubilizer , excipient , and combinations thereof along with a scf soluble material . a preferred load material is a combination of a biologically active material , a polymer and a scf soluble material . preferred polymers include , for example , a polysaccharide , polyester , polyether , polyanhydride , polyglycolide ( plga ), polylactic acid ( pla ), polycaprolactone ( pcl ), polyethylene glycol ( peg ), and polypeptide . preferred scf soluble materials include lipids , waxes and polymers and their combinations . the supercritical fluid pump 102 is preferably a p - 200 high - pressure reciprocating pump commercially available from thar technologies , inc . ( pittsburgh , pa ). suitable alternative pumps include diaphragm pumps and air - actuated pumps that provide a continuous flow of supercritical fluid . the high - pressure pump preferably comes factory - equipped with a burst - type rupture disc , manufactured by fike inc . ( blue springs , mo . ), which is plumbed into a pressure relief system . the supercritical fluid pump is in fluid communication with the high - pressure extraction vessel and supplies supercritical fluid to the extraction vessel . with reference to the supercritical fluid that the supercritical fluid pump supplies to the chamber , as noted hereinabove and used herein “ supercritical fluid ” includes not only supercritical fluid , but also compressed gas and liquefied gas , and other materials suitable , for example , to form a melt as described herein . the supercritical fluid is preferably supercritical carbon dioxide (“ co 2 ”). suitable alternative fluids include , nitrous oxide , dimethylether , straight chain or branched c1 - c6 - alkanes , alkenes , fluoroform , chlorotrifluoromethane , chlorodiflueromethane , propylene , ammonia and combinations thereof . preferred alkanes include ethane , propane , butane , isopropane , and the like . the supercritical fluid is chosen generally with reference to the ability of the supercritical fluid to extract the scf soluble material from inside the composite particles for porous particle formation . disposed inside the extraction vessel is preferably a high - speed mixer 106 for mixing the contents of the extraction vessel intimately . the mixer apparatus includes a motor , a shaft extending from the motor through the second end of the mixing vessel and into the chamber , and a rotor disposed at a distal end of the shaft and located in the chamber . the mixing rate is controlled by the rotation speed and geometry ( type and diameter ) of the rotor . the rotor is preferably a propeller - shaped two - bladed mixer . additional , supplemental and alternative mixing methods include both static and moving mixing devices , such as baffles , rotors , turbines , shear - mixers , ultrasonic devices , and other devices or mechanisms used to mix the contents of the mixing assembly . pressure inside the extraction vessel is maintained using a backpressure regulator . the backpressure regulator 103 is preferably a 26 - 1700 - type regulator , which is commercially available from tescom , usa ( elk river , minn .). for safety the extraction vessel connected with a release valve not shown in figure . the release valve is preferably a model r3a ¼ ″ proportional pressure release valve , which is commercially available from swagelok , inc . ( solon , ohio ). the release valve is actuated by system pressure acting against a spring , and is capable of reseating . operation in accordance with the first embodiment of the invention using apparatus 100 is carried out using the following steps . first the composite particles containing the scf soluble second material is loaded into the extraction vessel . the thermostat controls the temperature of the extractor to a predetermined and equilibrated temperature . the supercritical fluid pump supplies supercritical fluid to the extractor chamber at the first end of the extractor at a predetermined constant pressure . the scf flows through a packed bed of composite particles or through a fluidized bed depending on its flow rate . the scf extracts the scf soluble second material from inside the composite particles leaving tiny pores in them . the rate of extraction of the scf soluble second material , the amount of this material , the pressure and temperature , all can be used to control the sizes of the pores formed . once the extraction process is complete , the scf flow is terminated and the extraction chamber is depressurized in order to collect the porous particles formed . all components of the apparatus 200 shown in fig2 , which is used to carry out the second embodiment of the invention , are similar to the one used in the first embodiment except that disposed inside apparatus 200 is a frit 107 that breaks the scf into a plurality of droplets inside the extraction chamber . operation in accordance with the second embodiment of the invention using apparatus 200 is carried out using the following steps . first the composite particles containing the scf soluble material is loaded into the extraction vessel in the form of a suspension in a non - scf soluble solvent such as , for example , water . surfactants are preferably used in the non - scf soluble phase in order to stabilize the composite particles . the thermostat controls the temperature of the extractor to a predetermined and equilibrated temperature . the supercritical fluid pump supplies supercritical fluid to the extractor chamber at the first end of the extractor through a frit at a predetermined constant pressure . the scf flows through a composite particle suspension in the form of fine droplets . the scf extracts the scf soluble second material from inside the composite particles leaving tiny pores in them . the rate of extraction of the scf soluble second material , the amount of this material , the pressure and temperature , all can be used to control the sizes of the pores formed . once the extraction process is complete , the scf flow is terminated and the extraction chamber is depressurized in order to collect the porous particle suspension . the components of the apparatus 300 shown in fig3 , which is used to carry out the third embodiment of the invention , consists of many components similar to the one used in the first and second embodiment . however the mixing device is usually not provided for extraction of the scf - soluble second compound from the polymer matrix . instead , the polymer matrix is placed inside the extraction vessel and scf is allowed to diffuse within the matrix to extract the scf - soluble compounds . operation in accordance with the third embodiment of the invention using apparatus 300 is carried out using the following steps . first the composite material in a preformed shaped is placed inside the extraction vessel . the vessel maybe of different size and shape compatible with the specific shape of the polymer matrix . the thermostat controls the temperature of the extractor to a predetermined and equilibrated temperature . the supercritical fluid pump supplies supercritical fluid to the extractor chamber at a predetermined constant pressure . the scf flows through the extraction camber and diffuses into the composite matrix . the scf extracts the scf soluble second material from inside the matrix thereby creating pores . the rate of extraction of the scf soluble second material , the amount of this material , the pressure and temperature , all can be used to control the sizes of the pores formed . once the extraction process is complete , the scf flow is terminated and the extraction chamber is depressurized in order to collect the porous matrix . the following examples are intended only to illustrate the invention and should not be construed as imposing limitations upon the claims . 3 . 0 grams of poly ( lactide / glycolide ) ( plga ) and 1 . 5 g of lipid ( gelucire 50 / 13 ) were added to 60 grams of dichloromethane ( dcm ) in order to form an organic solution . the organic solution was added into 160 grams of an aqueous solution of poly vinyl pyrolidone ( pvp , 2 % w / w ) in order to form a mixture . the mixture was dispersed or emulsified using a dispersator for about 2 minutes at 2000 rpm to form a crude o / w emulsion . the crude o / w emulsion was then homogenized using a homogenizer commercially available from microfluidics , inc . ( newton , mass .) at 5 , 000 psi pressure and three passes to form a final o / w emulsion . precipitation of particles was carried out by supercritical fluid extraction of the dcm from the final o / w emulsion using the apparatus described in example 1 ( a ) of chattopadhyay et al ., u . s . pub . app . 2004 / 0026319 a1 , which is hereby incorporated by reference in its entirety . the extraction conditions were 80 bar pressure and 35 ° c . temperature . the flow rate of supercritical co 2 employed was 30 g / min and the flow rate of final o / w emulsion was 3 ml / min . extraction of dcm from the emulsion resulted in precipitation of composite particles comprising plga and lipid , which were in the form of a pvp stabilized aqueous suspension . the pvp stabilized aqueous suspension of composite particles was loaded into an extraction chamber of an apparatus such as shown in fig2 . the extraction chamber temperature was maintained at 45 ° c . supercritical co 2 was made to flow through the extraction chamber and through the particle suspension at a 0 . 5 ml / min and 150 - bar pressure to extract the gelucire 50 / 13 lipid from the composite particles . after complete extraction of gelucire 50 / 13 from the composite particles , the supercritical co 2 flow was terminated and the extraction chamber was depressurized . the resulting aqueous suspension of porous particles thus formed was collected and taken for analysis . analysis of the particles was carried out using scanning electron microscopy ( sem ). fig4 is an sem micrograph of the composite particles before extraction of the lipid using supercritical co 2 . fig5 is an sem micrograph of the porous particles after the lipid had been extracted . fig4 and 5 illustrate the change in the surface morphology of the particles due to extraction of the scf soluble material and due to the introduction of pores . additional advantages and modifications will readily occur to those skilled in the art . therefore , the invention in its broader aspects is not limited to the specific details and illustrative examples shown and described herein . accordingly , various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents .
the present invention provides a method of producing porous structures , particles or matrixes , which may be comprised of one or a plurality of components , an apparatus for carrying out the method and particles formed in accordance with the method . the method is particularly suitable for producing porous composite or pure particles for pharmaceutical applications . in accordance with the method , a composite comprising a material such as a pharmaceutical , a biodegradable polymers and / or a biological agent is formed . the composite must further comprise a material that is soluble in supercritical fluid . extraction of the supercritical fluid soluble material produces porous structures , which may be in the form of particles or matrixes .