Abstract:
Chemically cross-linked polymeric particles are formed using mechanical rather than chemical processes, facilitating production of small-diameter particles in a manner largely independent of the viscosity or density of the polymer. For example, an uncross-linked resin may be provided in particulate form, agglomerated, and compressed into a mass of a desired shape with a desired diameter, and subsequently cross-linked.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a continuation application of and claims priority to U.S. application Ser. No. 10/116,330, filed on Apr. 4, 2002. 
     
    
     TECHNICAL FIELD  
       [0002]     The invention relates generally to forming a chemically cross-linked particle and more particularly to forming a chemically cross-linked particle of a desired shape and diameter.  
       BACKGROUND INFORMATION  
       [0003]     Polymeric microspheres (i.e., microspheres formed at least in part from a polymer) are used in medical and industrial areas. These microspheres may be used as drug delivery agents, tissue bulking agents, tissue engineering agents, and embolization agents, for example. Accordingly, there are a variety of methods directed towards preparing polymeric microspheres. Typical methods include dispersion polymerization of the monomer, potentiometric dispersion of dissolved polymer within an emulsifying solution followed by solvent evaporation, electrostatically controlled extrusion, and injection of dissolved polymer into an emulsifying solution through a porous membrane followed by solvent evaporation.  
         [0004]     Additional methods of preparing polymeric microspheres include vibratory excitation of a laminar jet of monomeric material flowing in a continuous liquid medium containing a suitable suspending agent, irradiation of slowly thawing frozen monomer drops, emulsification and evaporation, emulsification and evaporation using a high shear air flow, and continuous injection of dissolved polymer into a flowing non-solvent through a needle oriented in parallel to the direction of flow of the non-solvent.  
       SUMMARY OF THE INVENTION  
       [0005]     The present invention facilitates production of microspheres having small diameters in a manner that is generally independent of viscosity and density. This is accomplished through the use of an uncross-linked polymer precursor in solid form, and a mechanical technique of compacting the precursor into a desired shape.  
         [0006]     Accordingly, in one aspect, the invention involves a method of forming a chemically cross-linked particle of a desired shape and diameter. The method includes providing an uncross-linked resin (e.g., polyvinyl alcohol) in particulate form, agglomerating the resin into a mass of a desired shape with a desired diameter, compressing the mass, and cross-linking the mass to thereby form the chemically cross-linked particle. An advantage of the present invention is the ability to avoid melting the resin in order to attain the desired shape. This is useful, for example, in connection with thermally unstable polymers.  
         [0007]     In one embodiment, the method further includes adding a binding agent (such as a starch or a sugar) to the resin and later removing the binding agent by exposing the particle to a solvent formulated to selectively dissolve the binding agent. The binding agent serves to hold the mass of uncross-linked resin particles together in the desired shape until the mass is cross-linked. In other embodiments, the binding agent comprises a polymer having a melting temperature lower than the melting temperature of the resin. In this way, the polymer becomes part of the chemically cross-linked particle.  
         [0008]     In another embodiment, the method further includes cross-linking the mass by exposing the mass to actinic energy such as an electron beam, ultraviolet radiation, or gamma radiation.  
         [0009]     In still another embodiment, the method further includes cross-linking the mass by exposing the particle to a gaseous cross-linking agent.  
         [0010]     In yet another embodiment, the method further includes agglomerating the resin into a mass in the shape of a sphere with a diameter of less than 600 microns.  
         [0011]     In another aspect, the invention involves a method of forming a chemically cross-linked particle of a desired shape and diameter. The method includes providing an uncross-linked resin in particulate form, adding a binding agent to the resin, and agglomerating the resin into a mass. The method further includes heating the mass to a temperature that is both above the melting point of the binding agent and below the melting point of the resin, compressing the mass into a desired shape with a desired diameter, and cooling the mass to a temperature below the melting point of the binding agent. The mass is then cross-linked to form the chemically cross-linked particle.  
         [0012]     In one embodiment, cross-linking the mass includes exposing the mass to actinic energy, such as an electron beam, ultraviolet radiation, or gamma radiation.  
         [0013]     In another embodiment, cross-linking the mass includes exposing the mass to a gaseous cross-linking agent.  
         [0014]     In still another embodiment, the method further includes removing the binding agent by heating the chemically cross-linked particle to a temperature above the melting point of the binding agent. The binding agent is thereby melted out of the chemically cross-linked particle.  
         [0015]     In yet another embodiment, the method further includes removing the binding agent by exposing the particle to a solvent formulated to selectively dissolve the binding agent.  
         [0016]     The foregoing and other objects, aspects, features, and advantages of the invention will become more apparent from the following description and from the claims. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]     In the drawings, like reference characters generally refer to the same parts throughout the different views. Also, the drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention.  
         [0018]      FIG. 1  is an illustrative flow diagram depicting the steps of forming a chemically cross-linked particle of a desired shape and diameter according to one embodiment of the invention.  
         [0019]      FIG. 2  is an illustrative flow diagram depicting the steps of forming a chemically cross-linked particle of a desired shape and diameter according to another embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0020]     Referring to  FIG. 1 , in one embodiment, the method of forming a chemically cross-linked particle of a desired shape and diameter is a mechanical process rather than a chemical process. First, an uncross-linked resin or polymer in particulate form is provided (Step  102 ). In one embodiment, the resin is a polyvinyl alcohol resin in particulate form having an average diameter of approximately 75 microns, such as a 99% hydrolyzed polyvinyl alcohol (e.g., product #341584 from Aldrich Chemical or Gohsenol NM-11 from Nippon Synthetic Chemical Industry Co.). In other embodiments, polymers such as polyvinyl acetate, vinyl polymers, polyamides, polyureas, polyurethranes, methacrylates, polyvinyl alcohols, or polymers having a pendant ester group that is easily cross-linked (or derivatives thereof) can be used. For many applications (e.g., embolics), the polymer is desirably hydrophilic.  
         [0021]     A binding agent is then mixed with the resin particles (Step  104 ). The binding agent serves to hold the resin particles together before they are cross-linked. In some embodiments, the binding agent is a starch or a sugar (e.g., sucrose). In other embodiments, other materials such as alginates, polysaccharides, proteins, carrageenan, or vegetable gums, for example, can be used as binding agents. In still other embodiments, the binding agent can be a blend of one or more of the above synthetic or naturally occurring materials.  
         [0022]     After the uncross-linked resin is mixed with the binding agent, the resin particles are agglomerated into a mass of a desired shape with a desired diameter (Step  106 ). In one embodiment, the resin particles are forced into a mold (using conventional plastic injection molding techniques) conforming to the desired shape and diameter. In another embodiment, the resin particles are pressed into the desired shape and diameter using conventional compression equipment. In still another embodiment, a punch is used to punch the desired shape out of a solid sheet of the resin. In yet another embodiment, a combination of static electricity and mechanical vibration or agitation is applied to the uncross-linked resin to cause the uncross-linked resin to agglomerate. In another embodiment, the uncross-linked resin particles are agglomerated by being put into a suspension and rotated. Rotation forces the resin particles to collide with each other and form a mass that can thereafter be cross-linked. The size of the mass is selected by controlling the rate of rotation. As the rotation speed increases, so does the number of resin particle collisions. However, the forces acting to pull the agglomerated mass apart also increase. The final size of the mass is a function of rotation speed and the force acting to pull the mass apart.  
         [0023]     Preferably, the technique used to form the particle involves, or is followed by, some form of compression in order to ensure that the resin particles stay together in the desired shape, such as a sphere (Step  108 ). For example, molding can involve pneumatic, hydraulic, or other compression of the resin-filled mold form. Rotation generally provides adequate compression force.  
         [0024]     After the mass is compressed, it is cross-linked to form the chemically cross-linked particle (Step  110 ). In some embodiments, cross-linking the mass is accomplished by exposing the mass to actinic energy, such as an electron beam, ultraviolet radiation, or gamma radiation. In other embodiments, cross-linking the mass is accomplished by exposing the mass to a gaseous cross-linking agent such as formaldehyde, glutaraldehyde, or an acid, for example. Polyvinyl alcohol and other polymers can be cross-linked using any of these techniques.  
         [0025]     After the mass is chemically cross-linked and a chemically cross-linked particle is formed, the binding agent may be removed from the particle by exposing the particle to a solvent (Step  112 ) formulated to selectively dissolve the binding agent. For example, a polar solvent (e.g., water or alcohol) can be used to dissolve the binding agents discussed above.  
         [0026]     Referring to  FIG. 2 , in another embodiment, the binding agent is a polymer with a melting temperature that is lower than the melting temperature of the resin. First, an uncross-linked resin or polymer in particulate form is provided (Step  202 ). Next, a binding agent is added to the resin ( 204 ). After the uncross-linked resin is mixed with the binding agent, the resin particles are agglomerated into a mass of a desired shape with a desired diameter (Step  206 ).  
         [0027]     Exemplary binding agents useful in connection with this embodiment include Methocell methoylcellulose, hydroxypropyl methylcellulose, Ethocell Standard and Premium (organic solvent soluble) from Dow Chemical Co., Avicel PH-001 and Avicell PH-002 microcrystalline cellulose (water soluble) from Asahi Kasei Corp, potassium alginates, sodium alginates, or PEG 1400 (polyethylene glycol), for example. The agglomerated mass of binding agent and resin is heated to a temperature above the binding-agent melting point but below the resin melting point (Step  208 ). After the mass is heated, it is compressed (Step  210 ). Compression ensures that the resin particles stay together in the desired shape, such as a sphere, for example. After the mass is compressed, it is cooled (Step  212 ). Upon cooling, the binding agent resolidifies and the shape imparted to the mass remains “set.” 
         [0028]     After the mass is cooled, it is then cross-linked to form the chemically cross-linked particle (Step  214 ). The binding agent may remain in the particle during and following cross-linking of the resin. In some embodiments, cross-linking the mass is accomplished by exposing the mass to actinic energy, such as an electron beam, ultraviolet radiation, or gamma radiation. In other embodiments, cross-linking the mass is accomplished by exposing the mass to a gaseous cross-linking agent such as formaldehyde or glutaraldehyde, for example. After the mass is cross-linked, the resulting particle can be again heated to a temperature above the binding-agent melting point so that the binding agent can be melted out of the particle (Step  216 ). The binding agent may also be removed from the particle by exposing the particle to a solvent formulated to selectively dissolve the binding agent. For example, a polar solvent (e.g. water or alcohol) can be used to dissolve some of binding agents discussed above.  
         [0029]     Variations, modifications, and other implementations of what is described herein may occur to those of ordinary skill in the art without departing from the spirit and scope of the invention. Accordingly, the invention is not to be defined only by the preceding illustrative description.