APPARATUS FOR FORMING HOLLOW FIBERS AND SAID FIBERS

Abstract: A nozzle assembly for forming hollow fibers, particularly a "Z-fiber" having longitudinally oriented trabeculae with the fiber wall, is disclosed. The nozzle assembly (10) contains a replaceable nozzle cap (28), a replaceable seal-guide (26), and a retractable central bore (14) used to supply core material for the hollow fibers. The Z-fibers are particularly useful in enabling cells, solutes, or the like, to enter the fiber wall and move longitudinally into the center of the fiber wall.

Description: APPARATUS FOR FORMING HOLLOW FIBERS AND SAID FIBERS BACKGROUND OF THE INVENTION A hollow-fiber membrane is a hollow capillary tube in which the wall functions as a permeable, non-permeable, or semipermeable membrane depending upon the application. In many cases, hollow fibers are used as cylindrical membranes that permit selective exchange of materials across th- rails. They may also be used as containers to effect the cont oiled release of a specific material or as reactors to chemically modify a permeant as it diffuses through a chemically activat¬ ed hollow-fiber wall. Hollow fiber technology has recently become useful for the production of permselective, biocompat- ible immunoisolatory vehicles. These hollow fiber vehicles may contain materials such as living cells, proteins, or medic¬ aments. They are designed so that material within the hollow fibers can permeate through the walls of hollow fiber vehicle. In use, the hollow fiber vehicles are delivered to a site spe¬ cific in the body in need of the particular material present in the core of the hollow fiber. Specific applications for which such hollow fibers may be useful include restoration of insulin production and the treatment of neurotransmitter-defi- ciency diseases such as Parkinson's disease by delivery of a particular neurotransmitter to a site specific in the body. See, for example, U.S. Patent No. 4,892,538. No matter what the particular application of the hollow fiber vehicles, the morphology and thickness of the fiber mem¬ brane must be controlled to obtain the desired mechanical and transport properties suitable for the particular application. There are four conventional synthetic-fiber spinning methods that may be used to produce hollow-fiber membranes: (1) melt spinning; (2) dry spinning; (3) wet spinning; and (4) a combination of dry and wet spinning. In each of these meth¬ ods, a tubular cross-section of the hollow fiber is formed by delivering a spinning material such as a polymer, copolymer, cellulosic material, or the like, through a spinneret nozzle while simultaneously delivering a material to be housed in the core of the fiber. Spinneret nozzle assemblies exist in a va¬ riety designs such as those shown in U.S. Pat. Nos. 4,035,459; 4,127,625; 4,229,154; 4,322,381; 4,323,627; 4,342,711; 4,380,520; and 4,744,932. Some of the problems associated with these prior art nozzle assemblies and hollow fiber forming techniques include an inability to form fibers from highly viscous thermoplastic polymers due to nozzle clogging, an inability to adjust the nozzle assembly to produce varying membrane thicknesses and morphologies, an inability to change nozzle types or nozzle caps in the assembly so as to allow for the production of different types and shapes of hollow fibers, and an inability to produce Z-fibers having trabeculae oriented substantially longitudinally within the fiber wall and substantially inter¬ connected with one another within the fiber wall. Accordingly, it is an object of the present invention to produce a nozzle assembly that can be used with various ma¬ terials having different and widely varying viscosities. It is another object of the present invention to pro¬ vide a nozzle assembly of modular construction wherein parts of the assembly can be easily replaced and substituted for, so as to facilitate the production of hollow fibers of different sizes, shapes, membrane thicknesses, and surface morphologies. SUMMARY OF THE INVENTION Accordingly, one aspect of the present invention is directed to a nozzle assembly for use in making hollow fibers which may be non- permeable, semi-permeable, or permeable. The fibers may be empty or, when semi-permeable or permeable, filled with a material such as a medicament or a biological ma¬ terial such as living cells or proteins. The nozzle assembly generally comprises a housing which contains an elongated bore or tube extending through the housing and a second outer bore or tube disposed concentric to the inner base. The inner bore is moveable and may be retracted and extended relative to the stationary outer bore and the housing in a manner which pro¬ vides for precise and reproducible adjustment of the bore rela¬ tive to both the extrusion cap and the housing. The output end of the outer bore is designed to be a removable nozzle cap and to contain a removable seal-guide for the inner bore which can be interchanged with other caps and seal-guides so as to allow for the production of hollow fibers of different shapes and having different membrane thicknesses and morphologies. The inner bore may also be interchangeable with other inner bores to change the central lumen diameter or fiber wall mor¬ phology. The nozzle assembly of the present invention is par¬ ticularly useful in making hollow fibers which contain living cells or other biological materials. In addition, the present invention is directed to a nov¬ el fiber structure referred to herein as a "Z-fiber." A Z-fi¬ ber contains trabeculae within the fiber wall and the trabecu¬ lae (i) are oriented substantially longitudinally with respect to each other, (ii) are substantially interconnected with one another within the fiber wall, and (iii) some or all are con¬ tinuous with the outside of the fiber. In addition, a Z-fiber has a semipermeable inner skin which defines the fiber lumen and can be used to provide immunoisolatory function. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a plan view in cross section of a nozzle assembly of the present invention showing the inner bore fully extended. Fig. 2 is a plan view in cross section of a nozzle assembly of the present invention showing the inner bore retracted. Fig. 3a is a plan view in cross section of a nozzle cap for use with the nozzle of Fig. 1 except with a different noz- zle cap and seal-guide and further includes a partial plan view in cross-section of the bottom portion of a different seal-guide for the inner bore than in Fig. 1. Fig. 3b is an enlarged detail plan view in cross sec¬ tion of the outlet end of the nozzle cap and seal-guide of Fig. 3a. Fig. 4 is a series of three photographs of a single Z-fiber membrane at different degrees of enlargement, i.e. at about (i) 100 X, (ii) 200 X, and (iii) 500 X, to better show the trabeculae and their longitudinal relationship. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS As best shown in the drawings, a nozzle assembly 10 ac¬ cordingly to the present invention comprises a housing 12 made from a solvent resistant material. Within housing 12 is dis¬ posed an elongated hollow inner bore or tube 14. Also dispos¬ ed within the housing 12 is an outer bore 16 which is concen¬ tric to the inner bore 14 so as to define an annular space or channel 18. The inner bore 14 extends through and is fixedly attached to a threaded fitting 20 which is screwed into the housing 12 containing matching screw threads in the upper por¬ tion 22 of the outer bore 16. Material for inclusion within the core hollow fiber membrane may be fed into the nozzle as¬ sembly at the input end 23 of the inner bore 14. Inner bore 14 also passes through a seal 24 which prevents fluid present in the channel 18 from flowing upwards and out of the top of the housing. The seal 24 is held in place by the spring 46. In the lower portion of the housing, the inner bore 14 passes through a seal-guide 26 which is removably attached to housing 12 by compressive force generated by the attachment of a noz¬ zle cap 28. Seal-guide 26 maintains inner bore 14 in align¬ ment during extension and retraction and it contains one or more holes therein to direct fluid present in the channel 18 to enter the outer regions of channel 19. The nozzle cap 28 is removably attached by means of fastening means 30 such as a screw, bolt or the like to the housing 12. It is designed to extend the channel 18 to the exit end 36 of the nozzle assemb¬ ly. As shown, the contours of the bottom portion of seal- guide 26 and the nozzle cap 28 define the dimensions and path of channel 19. As shown in Figs. 1 and 2, the channel 19 just prior to its intersection with bore 14 is at about a 45° ang¬ le. The outer bore 18 is connected to a source of fluid for use in making the hollow fiber membrane by means of fitting 32 such as a conventional luer fitting. As shown, the fitting 32 contain a tubular inlet 34 through which fluid flows into the nozzle assembly and into the channel 16. A seal 33 provides for a fluid-tight fit preventing any flow of fluid outward through the fitting. The extension and retraction of inner bore 14 is accomplished by rotation of inner bore adjustment means 38 which is attached to threaded fitting 20 to form a bore adjustment assembly which screws into or out of matching screw threads of the outer bore 16. The bore adjustment assembly rotates as a single unit and vertically moves inner bore 14 up or down, depending on the direction of rotation. A pin 42 is fixed on the inner bore adjustment means 38 as shown in Fig. 2 or (not shown) on the fitting 40 and extends there¬ between and causes both to rotate together. Although the fit¬ ting 40 rotates in conjunction with the adjustment means 38, it remains seated in the vertical direction due to tension ap¬ plied by the spring 44. The reproducibility of the extension or retraction between subsequent extrusions is improved by use of a measuring device such as a vernier scale affixed to the fitting 40. The extension and retraction may be performed man¬ ually or mechanically under computer control (not shown) . Fig. 1 shows a cross sectional plan view of the nozzle assembly with the inner bore 14 at an intermediate position be¬ tween full retraction and full extension. Fig. 2 shows the inner bore 14 at full retraction. All of the components of the nozzle assembly, including the housing, are made from organic solvent resistant materials such as stainless steel, glass, and Teflon ® , Delrin ® , or polypropylene polymers. The seals which must be compatible with and not deleteriously affected by the solvents utilized during the extrusion process are preferably made from Tef¬ lon ® polymers, but may also be made from materials such as polypropylene, nitrile polymers, or the like. In operation, a fluid used to form the hollow fiber mem¬ brane is forced by a pump (not shown) to flow into the outer bore 16 and channel 18 through fitting 32 and then into chan¬ nel 19 through one or more holes through seal-guide 26. The type of fluid employed will, of course, depend upon the specif¬ ic hollow fiber membrane that is desired. Suitable materials for use as the hollow fiber membrane include conventional mem¬ brane-forming materials such as thermoplastic polymers, thermo- setting polymers, gels, and hydrogels. Specific materials that may be used include cellulose, acrylic copolymers, poly- vinylidene fluoride, polyurethane isocyanates, alginates, poly- sulfones, polyvinyl alcohols, polyacrylonitriles, and mixtures thereof. Also, the material useful as core materials may be used to form the membranes. While the membrane forming materi¬ al may be a melt, it is preferably a solution. Suitable solv¬ ents for use in forming the solution include water soluble organic solvents such as dimethylacetamide, dimethyl ormamide, acetone, dimethylsulfoxide, N-methylpyrolidone, acetonitrile, and mixtures thereof, as well as other solvents such as hex- ane, diethylether, methylene chloride, and mixtures thereof. In most applications, although not in all, the hollow fibers formed using the device of the invention will contain a core material and/or a deliverable such as a medicament, a biological material such as living cells or proteins, or other similar materials dispersed therein. The core material and/or deliverable is generally in a fluid medium, generally water, to form a core material composition. The composition often includes one or more of crosslinking agents, gel formers, or viscosity enhancing substances. Other materials which may be present in the core material composition include such as Fi- coll, saline, ethanol, tissue culture media, serum, and methan- ol. The core material composition may also include biocompati- ble hydrogels, which may be used to immobilize cells or pro¬ vide growth substrata. Specific core materials may be anionic [alginate], cationic [collagen], amphoteric [poly(phosphaz- ene)s optionally having ionic side chains], or neutral [poly¬ ethylene oxide)s] . The alginate may be in liquid form or crosslinked after extrusion by allowing CaCl to diffuse in¬ to the liquid core. The core material composition is supplied through inner bore 14. The membrane material and the core ma¬ terial composition meet at or about the nozzle output end 36. When unfilled hollow fibers are to be formed, a fluid, e.g. water, a solvent, or a mixture thereof, is provided through inner bore 14. Thus the core material composition contains a fluid and optionally a core material, a deliverable, or both. By adjusting the position of the inner bore 14 relative to the nozzle output end 36, clogging of channels 18 and 19 especially in the nozzle cap portion can be controlled and sub¬ stantially eliminated. Clogging is a particular problem when the membrane forming material is a highly viscous or rapidly precipitating material such as a thermoplastic polymer in a so¬ lution. In certain instances during fiber membrane formation, it is desirable for the membrane forming material to be in con¬ tact with the core material composition as long as possible within the nozzle assembly and before the two materials exit the nozzle since an extended time has been found beneficial in controlling fluid flow orientation within the nozzle and in producing specific wall dimensions and fiber wall morpholo¬ gies. As a result, it is often preferable to maintain the in¬ ner bore 14 in a substantially fully retracted position to per¬ mit prolonged contact before the two materials exit the nozzle assembly. Early and prolonged contact, however, causes clog¬ ging, especially in the nozzle cap because the fiber membrane forming material precipitates and hardens when it contacts the aqueous core material. This would normally slow the flow of material in channels 18 and 19 and cause clogging. The clog¬ ging, however, can be overcome with the device of the present invention by beginning the hollow fiber membrane forming pro¬ cess with the inner bore 14 extended so that the core material composition and the membrane forming material do not meet un¬ til they are both exiting the nozzle. Then the inner bore can be retracted, generally gradually, to increase the time during which the two materials are in contact before exiting the noz¬ zle. Normally once phase inversion, i.e. transition from li¬ quid to solid, begins at a specific flow rate, the flow will remain substantially constant even though there is prolonged contact between the materials. The flow rates of the two mate¬ rials are maintained by external pumps (not shown) . This tech¬ nique for avoiding clogging is preferred over prior art tech¬ niques which employ polymer solvents such as dimethylsulfoxide (DMSO) . In many applications, such as those involving the pro¬ duction of hollow fiber membranes for use within a human body to administer drugs or the like, solvents such as DMSO can not be used due to their toxicity. Another benefit of the device of the present invention is the use of the removable and replaceable nozzle cap 28 and seal-guide 26. By replacing the nozzle cap with other nozzle caps and/or seal-guide with other seal-guides the size and con¬ figuration of the annular channel can be altered near the out¬ put end of the assembly to change the resultant membrane. In addition, the direction of flow of the membrane forming materi¬ al can be altered. For example, as shown in Figs. 1 and 2, the direction of flow of the channel 18 in region 19 of the channel is at about a 45° angle in relation to the flow of ma¬ terial exiting inner bore 14. Figs. 3a and 3b, on the other hand, shows a cap 28 and seal-guide 26 wherein the direction of flow in the portion 21 of channel 19 is 90° to the flow of material exiting inner bore 14. Various other nozzle caps and seal-guides providing other flow angles may be employed. By changing the angle at which the membrane forming material con¬ tacts the core material composition, the morphology of the mem¬ brane wall can be changed. Also, by changing the diameter of channel 19, especially in the portion of the channel in the nozzle cap 28, and/or the height 21, the thickness of the re¬ sulting membrane wall can be altered. The nozzle cap of Figs. 3a and 3b further includes a sleeve 50 inside the nozzle cap housing 28 to narrow the diameter of channel 19, to facilitate the changing of orifice plate 52 to alter the opening size, or to affect the fiber size and/or morphology. The sleeve 50 al¬ so serves to transfer the force generated when the nozzle cap fastening means 30 are tightened into the housing 12. When the fastening means 30 are tightened, both the seal-guide 26 and a seal 54 located below the orifice plate 52 are compress¬ ed to attain an effective seal. The sleeve 50 terminates at orifice plate 52 which is generally held in place between sleeve 50 and seal 54 by compression, though it can be physic¬ ally attached to the sleeve 50. The orifice plate 52 can change the direction, i.e. angle, of the flow of the membrane material and the outer diameter of the membrane formed. The morphology of the fiber walls produced in anisotrop- ic membranes and the formation of trabeculae therein is con¬ trollable by the nozzle of this invention. The formation of trabeculae is, in part, a phase separation phenomenon. In gen¬ eral terms, the polymer used to create the fiber wall is the most non-polar component of the total system while the solvent in which the polymer is generally dissolved is more polar than the polymer and is water-soluble. During extrusion, water or the water component of the core material composition in the in¬ ner bore comes in contact with the polymer/solvent fiber form¬ ing material when the two meet near the nozzle orifice. At this point, the water can diffuse into the polymer and the sol¬ vent/polymer/water mixture then partitions into solvent/poly¬ mer and solvent/water phases. The solvent/water phase creates a void in the polymer. The sum total of these voids results in a trabecular network within the fiber wall. Normally the trabeculae of hollow fibers are oriented perpendicular to the fiber wall as occurs when a 45° cap (as shown in Figs. 1 and 2) is used. A number of factors, such as the relative flow rates of the two materials, can be varied which can alter the orientation of the trabeculae. The pres¬ ent nozzle provides additional means for altering and control¬ ling the trabecular morphology. When a 90° cap and seal-guide (as in Fig. 3) are used, they direct the flow path of the poly¬ mer perpendicular to the flow from the inner bore so that, pre¬ sumably, void formation begins while flow is oriented in this direction. Then the sudden 90° change of direction distorts the shape of the incipient trabeculae to an angle oblique or parallel to the fiber wall, thereby forming fibers which are referred to herein as "Z-fibers". Z- ibers contain trabeculae within the fiber wall which trabeculae (i) are oriented substantially longitudinally with respect to each other, (ii) are substantially interconnected with one another within the fiber wall, and (iii) some or all are continuous with the outside of the fiber. In addition, a Z-fiber has a semipermeable inner skin which defines the fiber lumen and provides immunoisolatory function. Z-fibers are believed to be more beneficial than other fiber morphologies at inducing and enhancing capillary in-growth into the fiber walls when the fibers are used as implant devices in living animals. The Z-fiber morphology has not been found to be dependent upon the chemical composition of the membrane, the dimensions thereof, nor the composition flowing through the inner bore. The present nozzle also generally increases the uniform¬ ity of the resultant hollow fibers since traditional non-re¬ tractable coaxial nozzles tend to have substantial free play in the center bore which makes them sensitive to flow irregu¬ larities in the materials in the outer bore. This sensitivity is manifested by movement of the central bore which, during the extrusion process, leads to the production of irregular, asymmetric fibers. The seal-guide 26 in the present nozzle, due to its location close to the orifice of the inner bore, minimizes free play of the center bore and thereby leads to more uniform fiber preparation. Also, the seal-guide 26 reduces the need for frequent recentering of the inner bore and allows retraction and extension of the inner bore without change in bias and concentricity. The fibers prepared using the nozzle of the present in¬ vention may be non-permeable, permeable, or semipermeable. Preferably, they are semi-permeable fibers which are useful as or in filtration devices, bioreactors, or as cell encapsula¬ tion vehicles. EXAMPLE The following example demonstrates the formation of an unfilled Z-fiber. The nozzle assembly of Figure 3 was utilized wi._ a 90° cap and seal-guide. This assembly directs the flow path of the membrane-forming polymer perpendicular to the flow from the inner bore. The fiber membrane was prepared from a cast¬ ing solution of 12.5 % w/w poly(acrylonitrile-vinyl chloride) copolymer in dimethylsulfoxide. The inner bore contained dis¬ tilled water. The casting solution was pumped at a rate of 1 ml/min and the water at a rate of 5 ml/min. The nozzle was maintained at a distance of 7 cm above a precipitation bath containing 10 liters of distilled water at ambient temperature and pressure. The inner bore was positioned at a distance of about 0.5 to 3 mm, preferably about 1 mm, above the outer tip of the nozzle. The two solutions were then pumped concurrent¬ ly through the hollow fiber nozzle assembly and the resulting hollow fiber was collected in the precipitation bath. The resulting fiber exhibited the followed dimensions: 1,000-1,200 micron outer diameter and 80-100 micron membrane wall thickness. Figure 4 is a set of three scanning electron micrographs at various magnifications which show the longitud¬ inal trabecular orientation of the resultant Z-fiber. Although the resulting Z-fiber can be hand-loaded with cells for use in inducing and enhancing capillary in-growth into the fiber walls, an easier technique for accomplishing the same result is by replacing the distilled water pumped through the inner bore with cells in suitable aqueous growth media and crimping the fiber membrane shortly after commence- ment of its formation.

Claims: What is claimed is: 1. A nozzle assembly for use in producing hollow fi¬ ber membranes comprising a housing, an elongated first tubular means extending through the housing, the housing defining a second tubular means concentric with at least a portion of the first tubular means so as to form an annular channel between the two tubular means, said first tubular means being movable relative to said second tubular means so that in operation of the assembly it may be retracted and extended, the first and second tubular means each having input and output ends, the output end of the second tubular means comprising a cap which is detachable from the housing and which enables changing of the orientation of the annular channel relative to the first tubular means by the use of different caps. 2. The nozzle assembly of Claim 1, wherein said first and second tubular means are disposed relative to one another so that at their output ends the annular channel is perpendicu¬ lar to the first tubular means. 3. The nozzle assembly of Claim 1, wherein said first and second tubular means are disposed relative to one another so that at their output ends the annular channel is oriented at an about 45° angle to the first tubular means. 4. The nozzle assembly of Claim 1, wherein the first tubular means is a hollow tube. 5. The nozzle assembly of Claim 1, wherein the second tubular means is a cavity within the housing. 6. The nozzle assembly of Claim 1, wherein a guide means for guiding the first tubular means is disposed within the housing. 7. The nozzle assembly of Claim 6, wherein the first tubular means extends through the guide means. 8. The nozzle assembly of Claim 6, wherein the guide means is detachable from the housing. 9. In a method of forming a hollow fiber membrane by extrusion of a membrane-forming fluid material around a core material composition through a nozzle assembly having an inner bore for supplying the core material composition, the improve¬ ment comprising (i) beginning the extrusion with the nozzle positioned such that the core material composition and he mem¬ brane-forming fluid material do not first meet until tl y are both exiting the nozzle and then (ii) retracting the inner bore to increase the time during which the two materials are in contact within the nozzle assembly before exiting the noz¬ zle. 10. The method of Claim 9, wherein the retracting be¬ gins after phase inversion of the membrane-forming fluid mate¬ rial has commenced. 11. A hollow fiber membrane which comprises a fiber having (a) a semipermeable inner skin which defines a fiber lumen, (b) a fiber wall and (c) an outer surface, wherein the fiber wall contains trabeculae oriented substantially longi¬ tudinally therein and wherein at least some of the trabeculae are interconnected with one another within the fiber wall and at least some are continuous with the outer surface of the membrane. 12. The membrane of Claim 11, wherein there are living cells in the lumen.

Inventors: GENTILE FRANK T, LYSAGHT MICHAEL J, WARNER NICHOLAS F, GENTILE, FRANK, T., LYSAGHT, MICHAEL, J., WARNER, NICHOLAS, F.
Applicants: UNIV BROWN RES FOUND, BROWN UNIVERSITY RESEARCH FOUNDATION
Application year: 1992
Main classification: D01D5
IPC codes: A61M1, B01D69, D01D5, D01F6, G02B6