GB2032840A

GB2032840A - Hollow fibre forming a dialysis membrane with improved exchange surfaces

Info

Publication number: GB2032840A
Application number: GB7934085A
Authority: GB
Original assignee: Akzo NV
Current assignee: Akzo NV
Priority date: 1978-10-02
Filing date: 1979-10-02
Publication date: 1980-05-14
Also published as: GB2032840B; FR2437859A1; IT7950415A0; FR2437859B1; LU81734A1; IT1164713B; NL7907212A

Abstract

A hollow fibre composed of cellulose regenerated from cuprammonium solution for use as a dialysis membrane has a continuous cavity and a closed casing, the cross- sectional thickness of the fibre wall varying over the fibre perimeter in a continuous fashion. <IMAGE>

Description

SPECIFICATION Hollow fibre forming a dialysis membrane with improved exchange surfaces The present invention relates to a hollow fibre of cellulose, which has been regenerated from cuprammonium solutions, for use as a dialysis membrane, hollow fibre having a continuous cavity and a closed casing line.

A hollow fibre of regenerated cellulose, which has been regenerated from cuprammonium solutions, with a continuous cavity is known from German Patent Specification No. 736 321.

It is known from U.S.A. Patent Specification No.

32 38 877 that the hollow fibres which are produced according to the above mentioned German Patent Specification No. 736 321 are suitable as dialysis membranes and as membranes for reverse osmosis.

Hollow fibres of cellulose regenerated from cuprammonium solutions, which have a definite membrane structure and along the entire axis of the fibre have a constant wall thickness and an exactly circular cross-section are known from U.S.A. Patent Specification No. 38 88 771.

Moreover the hollow filaments are stretched out and have a high tensile strength. Although the greater strength obtainable by stretching the fibres would certainly be desirable, it is very disadvantageous to the swelling capability of the membrane and thus to the efficiency of the dialysis.

In dialysis, for example haemodialysis, it is necessary that the membrane walls be thoroughly washed by the dialysis fluid without any hindrance. If dialysis membranes, which are in the form of hollow fibres, packed together in bundles, are used, then with a bundle of several thousand hollow fibres which have an exactly circular crosssection and a fibre thickness of from approximately 500 to 1,000 per cm2 are liable to fit together and snugly along their entire length, in a manner similar to that of the glass plate effect of two plane parallel plates.

Because of this adhesion, access to the spaces between the hollow fibres for the dialyzate fluid is made difficult and the surfaces on which the hollow fibres stick together, remain unused for the material exchange, by which the efficiency of the hollow fibre module is diminished.

The efficiency of a dialysis membrane in haemodialysis depends on the ratio of exchange surface area to volume and on the concentration difference between the sides of the membrane wall. Up till now only hollow fibres with circular cross-section have been constantly used and cross-sections which deviated from the circular form have been considered detrimental.

An object of the present invention was to develop a dialysis membrane of cellulose, which has been regenerated from cuprammonium solutions, in the form of a hollow fibre with a continuous cavity and a closed casing line such that the efficiency of dialysis is increased through a better developed exchange surface.

According to the present invention there is provided a hollow fibre composed of cellulose regenerated from cuprammonium solution, for use as a dialysis membrane, the fibre having a continuous cavity and a closed casing, wherein on a section of the fibre perpendicular to the longitudinal axis thereof, the difference of the distance of the casing and of the cavity boundary respectively from the fibre centre as herein defined changes over the perimeter of the fibre.

By the term "fibre centre point" should be understood the centre point of a circumference drawn about the fibre cross-section. If the casing line forms a circle, then its centre point therefore coincides with the fibre centre point. With fibre cross-sections which deviate from the circular form and have an irregular form, the fibre circle centre point is considered as the circle centre point, which circumscribes the entire fibre cross-section.

An embodiment of the invention is characterised in that the curves forming the casing line have no centre point which is common to all the curves. Thus, cross-sectional forms are included whose casing lines are non circular.

According to a preferred embodiment of the invention, the points forming the casing line lie on an ellipse. Apart from the fact that, with the same volume of blood, the hollow fibres according to the invention provide a greatly enlarged exchange surface for dialysis and for this reason the efficiency of the metabolite exchange was substantially improved, a further improvement in the effectiveness of the dialysis results, due to the reduced thickness of the blood film in the dialysis membrane formed by a hollow fibre according to the invention. The know effect which occurs with circular dialysis fibres of adhesion between fibres has suprisingly not been observed, although such an effect should have been expected here to a greater extent, than that with circular hollow fibres. Apparently the fibres straighten out by chance and thereby escape a layered arrangement.

In a preferred embodiment, the points of the casing line lie on a kidney-shaped deformed ellipse. This form has proved to be particularly favourable, because the form of the hollow fibre has an outstanding stability together with an enlarged exchange surface.

The hollow fibres according to the invention do not in any way have to have a symmetrical shape.

A preferred embodiment of the invention is characterised in that the points forming the casing line lie on the circumference of a deformed square.

By the term "deformed square" should be understood a square whose respective opposite angles are at best approximately equal and whose sides are substantially curved.

The area of the cavity cross-section is preferably from 0.6 x 10-3 to 0.8mm2. With even larger cavity cross-sectional areas, the danger exists for a dialysis hollow fibre having a crosssection which deviates from the circular, that the hollow fibre will collapse thereby interrupting the passage of blood.

As known per se for dialysis hollow fibres having a circular cross-section, wall thickness is from 1 to 100 m, preferably from 5 to 50 #tm.

In order to achieve a good structural stability for the cross-sectional forms of the hollow fibres according to the invention, the wall thickness differs in neighbouring sections of the casing line.

Another embodiment of the invention is characterised in that the fibre section perpendicular to the fibre axis is formed such that the thickness of the wall of the hollow fibre increase along its circumference at least once in a continuous fashion up to a maximum thickness and decreases to a minimum thickness.

It was surprising to ascertain that the hollow fibre according to the invention, when compared with hollow fibre according to the prior art, having the same exchange surface area and using the same quantity of cellulose in the hollow fibre cross-section merely by distribution of the cellulose quantities over the cross-section in accordance with the invention, produced a clearly increased clearance for material-exchange metabolites. The thicker points of the hollow fibre membrane provide a stabilizing frame for the thinner membrane portions, without having to sacrifice their permeability.

The lumen can be developed in many forms, for example, circular, elliptic, triangular, square or even polygonal, wherein the corners are more or less rounded off, as determined by the production requirements. In a development of the invention, in the case of a multiple increasing and decreasing wall thickness, the areas with maximum or with minimum thickness, respectively occupy approximately the same lengths on the circumference. In particular through this, excellently stabilized hollow fibres are obtained with three, four and more cornered lumen developments. The resulting cross-section forms can be symmetrical or asymmetrical.

For the formation of a good structural stability, it is particularly advantageous that all those areas with maximum thickness or with minimum thickness are respectively equally thick.

In haemodialysis, hollow fibres with circular cross-sections are very often used. Here, dialysis hollow fibres are preferably formed such that the wall thickness of the hollow fibre increases and decreases, the fibre circumference and the cavity circumference being substantially circular and the centre points of these two circles being at a distance from each other. The distance between these centre points corresponds to from 20 to 80% of the difference between the radii of these two circles, and preferably from 40 to 60%.

With the known dialysis hollow fibres and in particular those whose cross-sectional form is characterised as being circular, the wall thickness is only reduced when the diameter of the hollow fibre is correspondingly decreased. Above a ratio of cavity diameter to wall thickness of about 1 0, and especially a ratio of 25, the hollow fibre is unstable and tends to lie out flat, which in haemodialysis leads to disruption of the passage of blood. In a development of the invention, the ratio of cavity diameter to minimum wall thickness is from 25 to 80. preferably from 40 to 60. The minimum thickness of the hollow fibres according to the invention is preferably from 3 to 9 yrm. The ratio of maximum thickness to minimum thickness of the hollow fibre wall is in a preferred embodiment of the invention from 2 to 6.

Another object of the invention is to provide a process for the production of hollow fibres according to the invention for use as dialysis membranes. The process advantageously provides for the production of hollow fibres having semipermeable membranes of regenerated cellulose and having an exact cross-section from by means of a spinneret, these fibres having outstanding properties as semi-permeable membranes while not at the same time impairing its mechanical properties.

The process for the production of hollow fibres according to the invention is characterised in that a hollow fibre spinneret is immersed in an aqueous caustic soda lye and the ratio of drawingoff velocity of the hollow fibre at a first drawing-off roller to discharge velocity of a cellulosecuprammonium-solution issuing from the ring slit of the hollow fibre spinneret amounts to from 1.00 to 1.05, and the direction of the fiber run from the hollow fibre spinneret to the first drawing-off roller makes an acute angle with the axis of the hollow fibre spinneret openings.

With the use of hollow fibre spinnerets with irregular cross-sections, hollow fibres are obtained with exactly those cross-sections, with such slight deviations from the ideal form, as was not thought to be possible up till now.

If hollow fibre spinnerets are used for hollow fibres having eccentric cross-sections, then no decrease of the eccentricity is obtained, in contrast with the process according to the prior art. With regard to the eccentricity, it should be said that this can even be increased with the process according to the invention, even when the fibre is drawn-off such that an acute angle is formed between the fibre and the axis of the hollow fibre spinneret openings lies on the side of greater wall thickness. It would have been expected here, that a weakening of the eccentricity should have resulted.

It is in principle also possible with the present process to arrange the hollow fibre spinneret on the floor of a coagulation bath and to spin the fibre from above. Because of the great technical difficulties, which result with such an arrangement with the change of nozzle, with the sealing and starting of spinning, this embodiment is less significant for carrying out the present process, than the arrangement of the hollow fibre spinneret at the surface of the coagulation bath.

The hollow fibre spinneret is, by this process, preferably immersed to a depth of from 5 to 1 0 mm in the aqueous caustic soda lye. This immersion depth is just adequate to allow the fibre to coagulate fast enough, wherein the hollow fibre spinneret opening can still be clearly observed in the caustic soda lye which is coloured deep blue by the cuprammonium solution.

A first drawing-off roller is arranged in this process such that the spun hollow fibre, when is issues from the hollow fibre spinneret, is not carried vertically downwards, but is removed to such a distance that the direction of the fibre run from the hollow fibre spinneret to the first drawing-off roller makes an acute angle with the axis of the hollow fibre spinneret openings. This acute angle preferably comes to between 150 and 700.

In this process, the newly spun hollow fibre in coagulation bath is transported only under the application of very slight strains. The circumferential velocity of a second drawing- off roller, which is positioned after the first drawingoff roller, is preferably from 90 to 98% of the circumferential velocity of the first drawing-off roller. By this means a small shrinking of the newly spun hollow fibre is caused, while with the processes according to the prior art, the hollow fibre is stretched out immediately after leaving the spin neret.

Up till now it has been thought that for the production of hollow fibres, especially those which are used as semi-permeable membranes and which have diameters of from approximately 50 to 1,000 with wall thickness of from 10 to approximately 200,us, could only be produced with hollow fibre spinnerets whose dimensions are a multiple, for example, of from 10 to 50 times the hollow fibre dimensions. Within the scope of the invention, hollow fibre spinnerets are preferably used such that the dimensions of the ring slit of the hollow fibre spinneret are from 2.5 to 6 times the dimensions of the finished hollow fibre.

The cellulose content of the cellulosecuprammonium-solution does not generally deviate from the cellulose content of cellulosecuprammonium-spinning solutions which are usually used for the regeneration of cellulose.

However, the cellulose content is preferably from 6 to 10 % by weight with respect to the weight of the solution. The NaOH-content of the caustic soda lye can fluctuate with greater limits.

However, it should preferably lie in the range of from 10 to 20% by weight, to guarantee a sufficiently fast formation of the Normancellulose, which introduces strain hardening to the hollow fibre.

In as much as the hollow fibre, which is produced according to the invention, is to be or has to be stretched, this stretching is advantageously carried out by passing the fibre through after treatment baths.

The hollow fibres according to the invention are produced by coagulation of a cellulosecuprammonium-solution extruded from a hollow fibre spinneret, in diluted causic soda lye, wherein the spinning solution is carried into the coagulation bath immediately after leaving the spinneret. For the formation of the continuous cavity, a hollow forming fluid is directed through a central bore in the known matter; examples of hollow forming fluids are halogenated hydrocarbons, hydrocarbons and ester, wherein isopropyl myristate has proved to be advantageous. It has also been shown however, that water and aqueous solutions, for example solutions of carboxymethyl cellulose salts are extremely suitable as hollow forming fluids.

The development of the hollow fibre form according to the invention is achieved through application of a hollow fibre spinneret slits arranged accordingly and a central bore for the hollow forming fluid. Thus, hollow fibres according to the invention having a cross-section such as that shown in Figure 4 of the accompanying drawings can be produced with a spinneret having an eccentric spinning slit and eccentrically arranged bore for the admission of the hollow forming fluid. For differently formed crosssections, spinnerets can be produced having suitable spinning slits and central bore arrangements, whose form is evident from the required cross-section.

For example, in order to achieve the desired elliptical shape, the ellipse on the spinneret is chosen with a larger ratio of major axis to minor axis, with respect to that of the ellipse which is to be the cross-section of the hollow fibre.

For other cross-sectional forms, hollow fibre spinnerets are used, whose spinning slits are arranged according to the required shape, for example as a slanting square. The width of the spinning slit is greater and smaller in certain sections, in so far as it is required that the wall thickness be different at neighbouring sections of the casing line.

It is also possible for the dialysis membrane forming the wall of the hollow fibre according to the invention to be composed of two or more layers, as is described in detail in the German Patent Applications P 26 27 858, P 27 05 735 and P 27 05 733, in order to increase its leak resistance or to obtain a surface layer which contains chemically modified cellulose, or to store adsorbents in the membrane wall.

Dialysis hollow fibres according to the invention are shown in Figures 1 to 6, wherein it is pointed out that the measurement ratios have been distorted in order to make clear the essential characteristics of the invention.

In Figure 7, a process for producing hollow fibres according to the invention is presented schematically.

Figure 1 shows a cross-section perpendicular to the fibre axis of the dialysis membrane according to the invention, wherein the points forming the casing line lines on an ellipse. Membrane wall B of hollow fibre A is composed of cellulose regenerated from cuprammonium solutions. The length of the major axis of the ellipse is indicated by a and the length of the minor axis of the ellipse byb.

Figure 2 shows a section perpendicular to the fibre axis of a hollow fibre A according to the invention, wherein the points forming the casing line lie on a kidney-shaped deformed ellipse.

Membrane wall B of the hollow fibre A is composed of cellulose, regenerated from cuprammonium solutions.

Figure 3 shows a section perpendicular to the fibre axis of a hollow fibre A according to the invention, wherein the points forming the casing line lie on a circumference of a deformed square.

Neighbouring sides of the square have a different thickness. Membrane wall B of the hollow fibre A is composed of cellulose, which has been regenerated from cuprammonium solutions.

Figure 4 shows a hollow fibre A, with a circular cavity and circular fibre cross-section. The centre points of both circles are arranged at a distance from each other, so that as is clearly visible, an area of maximum wall thickness passes in a continuous fashion into an area of minimum wall thickness. B indicates the hollow fibre wall which is composed of a regenerated cellulose.

Figure 5 shows a hollow fibre A having a triangular lumen, the corners of which are rounded off. Maximum and minimum thickness of the fibre wall B are arranged at approximately equal distances from each other, such that all three sections of maximum thickness or of minimum thickness are respectively of equal thickness. The hollow fibre wall is composed of regenerated cellulose.

Figure 6 shows the cross-section of a dialysis membrane hollow fibre A according to the invention, with a square lumen, in which the wall thickness of the hollow fibre along its perimeter increases in a continuous fashion to a maximum thickness then decreases to a minimum thickness twice. The maximum thickness is reached at a point at which the continuous increase changes to a continuous decrease. The minimum thickness of this membrane is not limited to only one point, but remains constant to form an area of minimum wall thickness. Again the hollow fibre wall B is composed of regenerated cellulose.

The invention provides for a multiplicity of cross-sectional forms for the dialysis membrane of hollow fibres according to the invention. In Figures 1 to 6, only a few typical forms are presented. For example, corresponding cross-sections, whose lumens form pentagons or polygons, are also stabilized by having areas of maximum wall thickness such that a considerable proportion of the exchange surface can be formed with a clearly thinner wall thickness and which provides an improvement in the efficiency of the membrane in haemodialysis. Analogous to the description of Figure 6, the point of maximum wall thickness can also be extended to form an area of constant maximum wall thickness. The areas of constant wall thickness formed thereby should at maximum amount to less than half that of the perimeter.

Figure 7 shows an embodiment of a process for producing the hollow fibres. A cellulosecuprammonium-spinning solution 1 and a hollow forming fluid 2, for example isopropylmyristate or paraffin oil are introduced into a hollow fibre spinneret 3. This hollow fibre spinneret 3 is immersed in the aqueous caustic soda lye, which is conducted through a coagulation bath 4. The hollow fibre A which comes out of the hollow fibre spinneret 3 is diverted at a first drawing-off roller 6 and carried over a second drawing-off roller 7 to after treatment baths. The direction of transport of the fibre between the first drawing-off roller 6 and the hollow fibre spinneret 3 makes an acute angle with the axis of the hollow fibre spinneret openings. The after treatment baths are suitably constructed as tubs, of which two (8;14) are shown by way of example in Figure.In the after treatment baths diversion rollers 9 are arranged.

The driving rollers 10 11; 12 and 13 are run with increasing circumferential velocity, whereby the hollow fibre A is stretched out to the required length. The washed hollow fibre is carried over a last diversion roller onto a drier 15, is dried there and rolled up on a roller 16 to be rinsed.

In the following examples, the invention is described in more detail.

Example 1 - Production of an elliptic hollow fibre dialysis membrane according to the invention.

A cuprammonium-cellulose-solution having with a cellulose concentration of 8.9 percent by weight was forced through a hollow fibre spinneret, whose spinning slit was elliptical in form, the length of the major axis of which being 2.7 times that of the minor axis and whose inner bore was also elliptical in form having the same axis ratio. The width of the spinning slits of the hollow fibre spinneret used was constant throughout. The length of the major axis of the ellipse was 10 times the width of the spinning slits. As a hollow forming fluid, isopropylmyristate was pressed through the inner bore.

The spinneret is placed such that the outlet openings are 5 mm beneath the surface of 12.5 % NaOH in the regenerating bath.

The hollow fibre forming material issuing from the spinneret is carried at an angle of 400 to the axis of the hollow fibre spinneret opening to a first drawing-off roller situated in the regenerating bath and is carried over a second drawing-off roller after this roller. In this, the outflow velocity of the hollow fibre forming cellulose material is 30.9 m/min., the circumferential velocity of the first roller is 30.9 m/min. and the circumferential velocity of the second roller is 30.26 m/min.

Afterwards the hollow fibre is carried through the usual following baths to remove the copper. After a further caustic soda lye bath, there follows a water wash, a sulphuric acid wash, as well as further water washes. The reduction of the hollow fibre diameter to the desired dimensions results from stretching the fibre in the following baths.

The hollow fibre was dried in a cylindrical drier and while dry was wound on a cheese bobbin.

Hollow fibres with eliptical cross-sections were obtained. The major axis of the ellipse was 290 #m and minor axis was 1 60 #m. The uniform wall thickness over the perimeter was 1 7 us.

The hollow fibre had a tensile strength of 24.103 cN/mm2, measured at 50 % air humidity and 230C, and an elongation of 23 %. The double refraction A n was 0.016.

Example 2 - Production of a dialysis hollow fibre according to the invention with an eccentric cross-section.

Analogous to Example 1, hollow fibres having eccentric cross-sections, as shown in Figure 4 were obtained. A cuprammonium-cellulosesolution with a cellulose concentration of 8.9 % by weight was pressed through a hollow fibre spinneret, as described above, but for a crosssection according to Figure 4. Isopropylmyristate as the hollow forming fluid was pressed through the inner bore. The extruded spinning solution passed into a regenerating bath immediately after leaving the spinneret. The regenerating bath contained 135 g/l sodium hydroxide, 6 g/l ammonia and 4 g/l copper. The temperature of the regenerating bath was 230C. After being converted into the Norman cellulose complex, the fibre produced was carried through the usual baths for after treatment. It was dried on a cylindrical drier and while dry, wound on a cheese bobbin.

The minimum wall thickness of the produced hollow fibres produced was 6 ##m, while the maximum wall thickness was 26,us. The inner diameter of the cavity was 21 5 ym.

Example 3 - Application to haemodialysis hollow fibres according to the invention having an eccentric cross-section.

Hollow fibre bundles were produced from the hollow fibres with eccentric cross-section. The hollow fibre bundles were installed in a hollow fibre test dialyzer and were examined under standard conditions.

The testing method is described in "Evaluation of Hemodialysers and Dialysis Membranes" Report of a study group for the artificial kidney chronic uremia program 1977; Elias Klein et al.

US-Department of Health, Education and Welfare.

Bathesda, Maryland 20014. Publication No. NIH 77-1294.

The dialysis hollow fibre according to the invention produced according to Example 2 were compared with commercial dialysis hollow fibres of cellulose regenerated from cuprammonnium solutions. The inner diameter of the commercial dialysis hollow fibres was 215 ztm, the wall thickness was 16,am. The solution flow was 200 ml/min.m2, the dialyzate flow was 500 ml/min.m2.

The ultra-filtration rate, with commercial hollow fibres was 3.0 ml/h.m2.mmHg; the urea clearance was 140 ml/min. By using dialysis hollow fibres according to the invention, the following was obtained: Ultrafiltration rate 4.0 ml/h.m2.mmHg.

Urea clearance 165 ml/min.

The fibres were stable in form and showed no tendency to lying flat. The cross-section surface areas of the commercial hollow fibres and of the hollow fibres according ta the invention were the same. Similar results were obtained with the dialysis hollow fibres according to the invention produced according to Example 1.

Claims

1. A hollow fibre composed of cellulose regenerated from cuprammonium solution, for use as a dialysis membrane, the fibre having a continuous cavity and a closed casing, wherein on a section of the fibre perpendicular to the longitudinal axis thereof, the difference of the distance of the casing and of the cavity boundary respectively from the fibre centre as herein defined changes over the perimeter of the fibre.

2. A fibre according to claim 1, wherein the curves forming the casing line have no common centre point in a section perpendicular to the fibre axis.

3. A fibre according to claims 1 or 2, wherein the casing line lies on an ellipse.

4. A fibre according to any one of claims 1 to 3, wherein the casing line lies on a kindney-shaped deformed ellipse.

5. A fibre according to any one of claims 1 to 3, wherein the casing line lies on a deformed square as herein defined.

6. A fibre according to any one of claims 1 to 5, wherein the area of the cavity cross-section is from 0.6 x 10-3 to 0.8 mm2.

7. A fibre according to any one of claims 1 to 6, wherein the wall thickness of the hollow fibre is from 1 to 100,us.

8. A fibre according to claim 7, wherein the wall thickness of the hollow fibre is from 5 to-50 fllm.

9. A fibre according to any one of claims 1 to 8, wherein the wall thickness has differing thickness in neighbouring sections of the casing line.

10. A fibre according to any one of claims 1 to 9, wherein the hollow fibre comprises two or more self adhesive permeable layers of regenerated cellulose, which stick together firmly.

11. A fibre according to claim 10, wherein one layer of the hollow fibre contains 1 to 95 % by weight of adsorbents.

12. A fibre according to claim 1, wherein on the fibre section perpendicular to the longitudinal axis thereof the wall thickness of the hollow fibre increases along its perimeter in a continuous fashion up to a maximum thickness and decreases to a minimum thickness at least once.

13. A fibre according to claim 12, wherein with a manifold increase and decrease in wall thickness, the areas of maximum thickness and the areas of minimum thickness respectively occupy approximately equal lengths of the fibre perimeter.

14. A fibre according to claim 12 or 13, wherein those areas having maximum thickness, or having minimum thickness, are respectively of equal thickness.

15. A fibre according to claim 12, wherein the wall thickness of the hollow fibre increases and decreases such that the fibre perimeter and the cavity perimeter are substantially circular, the centre points of these two circles being at a distance from each other.

16. A fibre according to claim 15, wherein the distance between the centre points is equal to from 20 to 80 % of the difference in radii of both circles.

1 7. A fibre according to any one of claims 12 to 1 6, wherein the ratio of average cavity diameter to the minimum wall thickness is from 25 to 80.

18. A fibre according to any one of claims 12 to 1 7, wherein the minimum thickness is from 3 to 9 gtm.

19. A fibre according to any one of claims 12 to 18, wherein the ratio of the maximum wall thickness to the wall minimum thickness is from 2 to 6.

20. A hollow fibre of cellulose regenerated from cuprammonium solution for use as a dialysis membrane when formed with reference to the examples.

21. A hollow fibre of cellulose regenerated from cuprammonium solutions for use as a dialysis membrane substantially as herein described with reference to any of Figures 4 to 6 with or without reference to any of Figures 1 to 3 of the accompanying drawings.

22. A process for spinning hollow fibres, according to any one of claims 1 to 21, comprising forcing a cellulose cuprammonium solution through a ring slit of a hollow fibre spinneret in aqueous caustic soda lye, and forcing a hollow forming fluid through the inner bore of the hollow fibre spinneret wherein the hollow fibre spinneret is immersed in the aqueous caustic soda lye, and the ratio of drawing-off velocity of the hollow fibre at a first drawing-off roller to discharge velocity of the cellulose-cuprammonium -solution from the ring slit of the hollow fibre spinneret is from 1.00 to 1.05, and angle formed between the direction of the fibre run from the hollow fibre spinneret to the first drawing-off roller and the axis of the hollow fibre spinneret opening is acute.

23. A process according to claim 22, wherein the hollow fibre spinneret is immersed to a depth of from 5 to 10 mm in the aqueous caustic soda lye.

24. A process according to claim 22 or 23, wherein the angle formed between the direction of fibre run from the hollow fibre spinneret to the first drawing-off roller and the axis of the hollow fibre spinneret openings is in the range of from 150 to 700.

25. A process according to any one of claims 22 to 24, wherein the circumferential velocity of a second drawing-off roller, which is arranged downstream of the first drawing-off roller with respect to the direction of the fibre run, is from 90 to 98 % of the circumferential velocity of the first drawing-off roller.

26. A process according to any one of claims 22 to 25 wherein the dimensions of the ring slit of the hollow fibre spinneret is from 2.5 to 6 times the dimensions of the finished hollow fibre.

27. A process according to any one of claims 22 to 26, wherein the cellulose content of the cuprammonium solution is from 6 to 10 % by weight and the NaOH-content of the aqueous caustic soda lye is from 10 to 20 % by weight of the respective solutions.

28. A process according to any one of claims 22 to 27 wherein the hollow fibre formed is after treated.

29. A process according to claim 28 wherein the hollow fibre is stretched while passing through after treatment baths.

30. A process for the production of hollow fibres according to any one of claims 1 to 21, substantially as herein described with reference to Figure 7 of the accompanying drawings.