Hollow fibre membrane having three dimensional texturing

Abstract

Hollow-fibre membrane having at least one first curl in the form of a first wave which is characterized by a first oscillation plane and a first wavelength and at least one second curl in the form of a second wave which is characterized by a second oscillation plane and a second wavelength, characterized in that the first oscillation plane and the second oscillation plane confine an angle with one another which is different from zero.

Claims

1. Hollow-fibre membrane having at least one first curl in the form of a first wave which is characterized by a first oscillation plane and a first wavelength and at least one second curl in the form of a second wave which is characterized by a second oscillation plane and a second wavelength, wherein the first oscillation plane and the second oscillation plane confine an angle with one another which is from 70° to 110°, said at least one first curl and said at least one second curl comprise periodic waveforms, and said at least one second curl is applied to said at least one first curl.

2. The hollow-fibre membrane according to claim 1, wherein the angle is from 85° to 95°.

3. The hollow-fibre membrane according to claim 1, wherein the angle is 90°.

4. The hollow-fibre membrane according to claim 1, wherein the first wavelength and the second wavelength are different from one another.

5. The hollow-fibre membrane according to claim 1, wherein the wavelength of the first wave is from 3 to 15 mm.

6. The hollow-fibre membrane according to claim 1, wherein the wavelength of the second wave is from 20 to 50 mm.

7. The hollow-fibre membrane according to claim 1, wherein the first wave has a first amplitude within the range from 0.2 to 0.6 mm.

8. The hollow-fibre membrane according to claim 1, wherein the second wave has a second amplitude within the range from 2.0 to 6.0 mm.

9. The hollow-fibre membrane according to claim 1, wherein the first wave and the second wave are sine-shaped.

10. The hollow-fibre membrane according to claim 1, wherein said hollow-fibre membrane has a demoulding force of from 3.4 to 10.0 N based on a hollow-fibre membrane bundle produced therefrom having 16,896 fibres, when the hollow-fibre membrane bundle is demoulded from a cylindrical shell having a diameter of 41.4 mm.

11. The hollow-fibre membrane according to claim 10, wherein the demoulding force is from 3.4 to 7.0 N.

12. A process for producing the hollow-fibre membrane of claim 1, said process comprising at least steps (a) and (b): (a) providing a hollow fibre having a first curl in the form of a first wave which is characterized by a first oscillation plane and a first wavelength; (b) applying a second curl to the hollow-fibre membrane provided in step (a), in the form of a second wave which is characterized by a second oscillation plane and a second wavelength; wherein the application in step (b) being effected such that, after application, the first oscillation plane and the second oscillation plane confine an angle with one another which is from 70° to 110°.

13. The process according to claim 12, wherein the first wavelength and the second wavelength are selected such that the first wavelength is shorter than the second wavelength.

14. The process according to claim 12, wherein the hollow-fibre membrane of step (a) is passed through at least two counter-rotating cog wheels in step (b), the rotation axes of the cog wheels not being aligned perpendicular to the first oscillation plane.

15. A bundle comprising a plurality of the hollow-fibre membrane as defined in claim 1.

16. A filter device for hemodialysis, comprising a housing and the bundle as defined in claim 15 and arranged in said housing.

17. A method to filter a fluid comprising passing the fluid through said filter device of claim 16 to obtain a filtered fluid.

18. The hollow-fibre membrane according to claim 1, wherein said at least one first curl and said at least one second curl consists of periodic waveforms.

Description

(1) FIG. 1 shows schematically a process for producing the hollow fibres according to the invention. Here, a hollow fibre having a first curl in the form of a first wave characterized by a first oscillation plane and a first wavelength is guided through at least two counter-rotating cog wheels (in the arrow direction), the rotation axes D of the cog wheels not being aligned perpendicular to the first oscillation plane, but parallel instead. In the course of this, a second curl characterized by a second oscillation plane and a second wavelength is applied to the hollow-fibre membrane. In this connection, the application in step (b) is effected such that, after application, the first oscillation plane and the second oscillation plane confine an angle which is different from zero. In FIG. 1, the angle is to be approximately 90°. The wavelengths are selected such that the first wavelength is shorter than the second wavelength.

(2) For the application of a curl, the hollow-fibre membrane can be provided in a form as has been described in the prior art, i.e. for example, plasticized by solvent. The waves can then be fixed by the solvent being evaporated.

(3) On the other hand, it is also possible to heat the cog wheels used for applying the curl, and so the hollow-fibre membrane is thermally deformed.

(4) FIG. 2 shows schematically the production of a hollow fibre provided in step (a), wherein the curl of said fibre is produced by a substantially linear hollow-fibre membrane being passed through two counter-rotating cog wheels (in the arrow direction). Here, the rotation axes are perpendicular to the oscillation plane of the formed curled hollow fibre having the first curl in the form of a first wave characterized by a first oscillation plane and a first wavelength.

(5) As depicted schematically in FIGS. 1 and 2, the wavelengths of the first curl and of the second curl, wherein that of the second curl must be greater than that of the first curl, can be set by the shape of the cogs of the cog wheels, the distance of adjacent cogs from one another and also the height of the cogs.

(6) The hollow fibres of the present invention can be processed to form a bundle according to known processes. Said known processes envisage, for example, the trimming of the hollow fibres to a certain length. The hollow fibres can then be shaped to form a bundle, with the ends of the fibres being cast with appropriate resins such as polyurethanes for fixation.

(7) The production of the hollow-fibre membrane filament per se can be carried out according to a known spinning process with phase inversion. Typical range parameters for a spinning process for a hollow-fibre membrane will be specified below. These conditions lead to the hollow-fibre membranes according to the present invention, but are not to be understood to be restrictive.

(8) All of the following “%” data are percent by weight data.

(9) For this purpose, a spinning dope consisting for example of from 16 to 18% polysulphone, from 3 to 6% polyvinylpyrrolidone and from 76 to 81% dimethylacetamide is prepared and the temperature is adjusted to from 30 to 60° C. The spinning dope is extruded through a corresponding annular die together with a precipitant consisting of 25-40% water and 60-75% dimethylacetamide. The spinning dope is in this case extruded through an annular gap together with the precipitant escaping through a central circular opening of the spinneret die. The annular gap can have a typical gap width of from 30 to 50 μm and an inner diameter of from 150 to 300 μm.

(10) The thus obtained spun filament can be guided through an air gap having a relative humidity value of from 40 to 100%, preferably from 80 to 100%, and a length of from 100 to 800 mm, preferably from 200 to 600 mm. Thereafter, the spun filament is introduced into a water precipitation bath adjusted to a temperature of from 60 to 80° C. for example and coagulated. The thus obtained hollow-fibre membrane is rinsed with water at temperatures of from 60° C. to 90° C. Thereafter, the hollow-fibre membrane is dried at temperatures of from 100 to 150° C. for from 1 to 10 minutes.

(11) The diameter of the thus obtained hollow-fibre membrane and the wall thickness of the membrane can be adjusted via the extrusion rate of the spinning dope and of the inner precipitant. Typical lumen widths of thus obtained hollow-fibre membranes are between 150 and 350 μm. Typical wall thicknesses of such hollow-fibre membranes can be from 30 to 50 μm.

(12) Following the described spinning process, the first curl and the second curl are impressed onto the hollow-fibre membrane by the appropriate curlers.

(13) Apparently, it is important that the first wave-stamping stiffens the wave-shaped hollow-fibre membrane in the oscillation plane of the wave to the extent that a turn-over of the wave-shaped hollow-fibre membrane takes place during the transition to the 2nd curler step.

(14) It has become apparent that the orientation of the two oscillation planes can depend on the wavelengths of the first wave and of the second wave in relation to one another. Therefore, in certain embodiments, a twist of the oscillation plane is also found which can accept angle values of from 70° to 110°.

(15) Therefore, in a fourth aspect, the invention provides a bundle comprising hollow fibres as defined in the first or second aspect or hollow fibres produced according to a process as defined in the third aspect.

(16) The hollow fibres of the present invention can be used for filtration purposes. To this end, they are typically arranged in a housing, preferably in the form of a bundle.

(17) More particularly, hollow-fibre bundles composed of fibres as defined in the first aspect exhibit a stronger pre-tension than conventional hollow-fibre membrane bundles consisting of straight or singly wave-stamped hollow fibres. The term “pre-tension” refers to a measure of a restoring force exhibited by a bundle when it is compressed. Fibres as defined in the first aspect strive to occupy a larger space in the hollow-fibre bundle than conventional straight or singly wavy fibres. With identical fibre number and identical housing dimensions, a higher force is therefore needed in order to demould the fibres in a cylindrical housing of a filter module than is the case with singly wave-stamped fibres. Here, demoulding is understood to mean the sliding out or pulling out of a hollow-fibre bundle from the cylindrical enclosure surrounding the hollow-fibre bundle, for example a dialyser housing or a cover film.

(18) Hollow-fibre membrane bundles which have such increased pre-tension and which are cut into bundle segments according to the currently customary bundling processes exhibit the advantage of evenly-cut cut edges, unlike what is observed in the case of cut edges of singly wave-stamped hollow-fibre membrane bundles. This has an effect especially during the casting of the fibre ends with a casting compound. A typical process for casting the fibre ends of hollow-fibre bundles is, for example, described in DE 10 2006 021 066 A1. According thereto, a liquid polyurethane prepolymer is, for example, used in the casting of the fibre ends of a hollow-fibre membrane bundle. In such processes, the casting compound penetrates into the end region of the fibre bundle and, after curing of the prepolymer, fixes the fibre ends in their position. This leads to so-called “casting wedges”. A disadvantage is that a portion of the fibres is shifted from their position. This can lead to unusable filter modules later on in production. It was observed that the formation of such casting wedges decreases when the cut edge of the hollow-fibre membrane bundle can be produced more evenly.

(19) A further positive effect of the inventive double curl of the hollow-fibre membrane consists in a more even fibre distribution of the fibres within the fibre bundle. Owing to a more even spacing of the fibres, the fibres in the hollow-fibre bundle are less likely to merge to form so-called strands. In zones of said strands, individual fibres are not freely flushable under filtration conditions. Strand formation is then always associated with a reduction in the filtration efficiency of the filter module. In the present case, it was possible to achieve an improvement in filtration through the double curl according to the invention and through the more even fibre distribution associated therewith.

(20) Therefore, in a fifth aspect, the invention provides a filter device comprising a housing and a bundle as defined in the fourth aspect and arranged in said housing.

(21) Preferably, the filter device is a hollow-fibre dialyser for haemodialysis.

(22) Very generally, it is, however, possible for the hollow-fibre membrane as defined in the first aspect or second aspect or the hollow-fibre membrane produced according to a method as defined in the third aspect or the bundle as defined in the fourth aspect or the filter device as defined in the fifth aspect not to be used for the dialysis of blood, but for any fluid separation.

(23) Consequently, in a sixth aspect, the invention also provides for the use of a hollow-fibre membrane as defined in the first aspect or second aspect or produced as defined in the third aspect, or the use of a bundle as defined in the fourth aspect, or the use of a filter device as defined in the fifth aspect, for fluid separation.

EXAMPLES

(24) The measure of the filtration efficiency of dialysers that was used was the so-called clearance value, which was determined as described below. Clearance values of hollow-fibre bundles according to the invention were measured according to the requirements of the DIN EN ISO 8637 standard. This involved measuring both the input concentration and the output concentration of a particular guide substance during a simulated dialysis on a dialyser constructed from hollow-fibre bundles and calculating the clearance according to the formula:

(25) $\mathrm{Cl}=Q_B\left(1-\frac{C_{B,\mathrm{out}}}{C_{B,\mathrm{in}}}\right)+\frac{C_{B,\mathrm{out}}}{C_{B,\mathrm{in}}}$

(26) TABLE-US-00001 Cl Clearance [ml/min] Q.sub.B Flow rate on blood side [ml/min] Q.sub.F Filtrate flow [ml/min] C.sub.B, in Input concentration, blood side C.sub.B, out Output concentration, blood side

(27) Altogether, the clearance values were determined on 10 filter modules and the values obtained were averaged.

(28) The clearance measurement was carried out as follows: A dialyser was produced by moulding of the hollow-fibre bundle to be investigated, by end-side casting of the fibre ends in the housing of the dialyser. The end-side casting separated the dialyser into two flow spaces, a blood-side flow space, which comprises the cavities of the fibres, and a dialysate-side flow space, which comprises the space surrounding the fibres.

(29) The dialyser had an inlet port and outlet port on the blood side in order to introduce liquid into the fibre interior and to remove it at the other end of the fibres. Furthermore, the dialyser had an inlet port and an outlet port on the dialysate side in order to allow liquid to flow along the fibres on the dialysate side.

(30) To carry out the clearance measurement, the dialysate side was flushed with a 1% aqueous potassium chloride solution at 37° C. at a flow rate of 500 ml/min. The test liquid adjusted to a temperature of 37° C. flowed through the blood side at a flow rate of 300 ml/min.

(31) For the measurement of sodium clearance, the test liquid used was a 154 mmol/l sodium chloride solution. For the measurement of vitamin B12 clearance, a 36.07 μmol/l test solution was used. After the respective liquids had flowed through both flow sides for 10 minutes, the concentration of the analyte at the output of the blood side and at the output of the dialysate side was determined.

(32) For the measurement of demoulding force, a fibre bundle driven into an HDPE film is used as a test body. In this case, the hollow-fibre membrane bundle inserted into the film assumes a cylindrical shape.

(33) The fibre bundle is then slid out of the film cover, such that the hollow-fibre bundle freely towers above the film cover by 2 cm. With the aid of an adhesive strip, the free bundle end is wrapped and fixed on a receiver unit of a tensile tester. The thus enclosed bundle end then has the same diameter as the bundle driven into the film. The fibre bundle prepared in this manner is positioned horizontally on a test bench. The film is fixed with appropriate holding devices.

(34) With the aid of the tensile tester, the fibre bundle was pulled out of the film shell. The pulling speed was 1 cm/sec. After 50% of the length of the fibre bundle had been pulled out of the film shell, the force value of the demoulding process was recorded on the tensile tester. The thus measured force value indicated the demoulding force of the fibre bundle.

Example 1

(35) A fibre bundle according to the invention with double curl was moulded into a commercially available F60S filter housing from Fresenius Medical Care. Specifications relating to fibre number, active membrane area, fibre diameter, wall thickness of the hollow-fibre membrane and length of the hollow-fibre membrane in the dialyser housing are shown in Table 1. The fibres of the hollow-fibre membrane bundle according to the invention had a first curl having a wavelength of 3 mm and a second curl having a wavelength of 30 mm. The oscillation planes of the two curls were in a 90° angle in relation to one another.

(36) The clearance values for sodium and vitamin B12 were determined according to the method described above. The sodium clearance was 253 ml/min. The vitamin B12 clearance was 135 ml/min.

Comparative Example 1

(37) Fibres of the comparative example were obtained according to the same spinning process as the fibres of the exemplary embodiment. Fibre dimensions and pore structure were therefore identical to those of the fibres of the exemplary embodiment. The fibres of the example were then provided with a single curl having a wavelength of 30 mm. The fibres were combined to form bundles and moulded and casted into a commercially available F60S filter housing from Fresenius Medical Care according to the same method as in example 1.

(38) The clearance values for sodium and vitamin B12 were determined according to the method described above. The sodium clearance was 238 ml/min. The vitamin B12 clearance was 127 ml/min.

(39) TABLE-US-00002 TABLE 1 Comparative Example 1 example 1 Housing inner diameter 39 mm 39 mm Number of fibres 9216 9216 Fibre inner diameter 200 μm 200 μm Wall thickness 40 μm 40 μm Fibre length in the filter module 227 mm 227 mm Active membrane area 1.31 m.sup.2 1.31 m.sup.2 1st curl wavelength 7 mm — Amplitude of the 1st wavelength 0.4 mm — 2nd curl wavelength 30 mm 30 mm Amplitude of the 2nd wavelength 3.5 mm 3.5 mm Sodium clearance 253 ml/min 238 ml/min Vit. B12 clearance 135 ml/min 127 ml/min

Example 2

(40) For the measurement of demoulding force, fibre bundles consisting of 16896 fibres having a length of 280 mm were produced. The fibre inner diameter of the fibres was 183 μm and the wall thickness of the fibres was 38 μm. The fibres were inserted into an HDPE film to form a cylindrical fibre bundle having a diameter of 41.4 mm. The demoulding force was determined according to the method described above. The demoulding force was measured on each of 30 fibre bundles with the inventive double curl having a first wavelength of 7 mm and an amplitude of 0.4 mm and a second wavelength of 30 mm with an amplitude of 3.5 mm.

(41) For comparison, the demoulding force was measured on 30 fibre bundles which differed from the fibre bundles according to the invention only in the nature of the curl. The fibres had a single curl having a wavelength of 30 mm and an amplitude of 3.5 mm.

(42) From 30 measurements, an averaged demoulding force of 4.2 N was found for the fibre bundles according to the invention. An averaged demoulding force of 3.4 N for fibre bundles was found for the fibre bundles with the singly curled fibres.