Abstract:
The invention relates to a filter comprising membranes made of hollow fibers (21) in which the hollow fibers ( 21 ) are arranged as a bundle ( 19 ) in a tubular housing ( 3 ). The housing ( 3 ) comprises an end cap ( 5 ) on each of its ends, whereby the hollow fiber bundle ( 19 ) is placed between the ends of the housing ( 3 ). The ends of the hollow fiber bundle ( 19 ) each comprise a support ring ( 40 ) that surrounds the same and are cast inside said support ring ( 40 ). The support ring ( 40 ) comprises axially protruding catches ( 42 ) and is displaceably mounted in the housing ( 3 ) in order to compensate for, in particular, axial contractions of the hollow fiber bundle ( 19 ) occurring during heat sterilization.

Description:
[0001]    The present invention relates to a filter comprising membranes made from hollow fibres, in which the hollow fibres are provided as a bundle arranged in a tubular housing. The housing is provided at each end with an end-cap, wherein the hollow-fibre bundle is arranged between the ends of the housing, a support-ring for each end of the hollow fibre bundle is arranged around each respective end of the hollow-fibre bundle, which ends are cast within a potting compound, and the ends of the hollow fibres are open.  
         TECHNICAL BACKGROUND  
         [0002]    Filters having hollow-fibre membranes are used i.a. in the area of dialysis for a wide variety of purposes. Such filters are used, for example, in haemodialysis, in which blood is directed into and through the inside of the semi-permeable walls of the hollow fibres while dialysis fluid is directed around the outside of the hollow fibres. Various convection and diffusion processes thereby take place through the walls of the hollow fibres, which processes serve to purify the blood and to remove excess fluid. Additionally, due to this, the electrolyte concentration in the blood can be conditioned, and buffers such as bicarbonate or acetate can be added to the blood.  
           [0003]    Filters of this type are also employed in so-called haemofiltration, in which a substitution fluid is added to the blood. According to this process, the blood is directed through the inside of the hollow fibres, although in this case no dialysis fluid is passed around the outside of the fibres. Here, in the filter, excess fluids, in particular water as well as waste products, are removed from the blood only by means of a pressure difference across the membrane, i.e. the semi-permeable walls of the hollow fibres. The substitution fluid can be added either before or after the filter.  
           [0004]    The above-mentioned filters can equally be used as so-called ultrafilters for the production of the substitution fluid. In this case, dialysis fluid can for example be directed through the inside of the hollow fibres and can, by means of a pressure difference, be filtered through the membrane or semi-permeable walls, whereby the dialysis fluid can be sterile-filtered as a result of the removal of endotoxins, bacteria and other contaminants.  
           [0005]    Further uses for the filters mentioned include haemodiafiltration; a combination of haemodialysis and haemofiltration, as well as plasmapheresis, in which the aqueous blood-plasma is filtered out of the blood and returned to the blood again after treatment. Such filters are also used in reverse osmosis.  
           [0006]    The aforementioned filters are usually so constructed that the hollow-fibres are arranged as a loose bundle collected together within a tubular housing. The housing is provided at each end with an end-cap, whereby the hollow-fibre bundle is arranged between the ends of the housing so that the end-caps cover the ends of the hollow-fibre bundle. A support ring is provided around each end of the hollow-fibre bundle, while the ends of the hollow fibres are embedded within a potting compound at the ends of the housing.  
           [0007]    The hollow fibres collected together in the hollow fibre bundle have ends which open into a hollow space located between the end cap and the end of the hollow-fibre bundle. It is therefore possible, with the appropriate arrangement of inlets and outlets, to provide, in a known manner, various forms of filter, such as the previously mentioned haemodialysis filters, haemofilters, haemodiafilters, ultrafilters etc..  
           [0008]    Examples of the previously mentioned filters are disclosed in EP-0 305 687, EP-0 355 325 and EP-0 525 317.  
           [0009]    Essentially, all the previously mentioned filters have certain features in common, namely that a first fluid is directed into and through the inside of the semi-permeable hollow fibres while a second fluid is present on the outside of the hollow fibres. This second fluid can either flow through the housing, past and around the hollow fibres, via appropriately located inlet and outlet means; or it can for example be removed from the first fluid and directed out of the housing via a suitable outlet, for example by means of a pressure differential across the hollow-fibre membrane.  
           [0010]    A disadvantage of the previously mentioned filters can be seen in that during the usual heat-sterilisation process, for example using steam, shrinkage of the hollow fibres occurs, in particular with synthetic hollow fibres. These mainly axial shrinkages of the hollow fibres can lead to damage or in worse cases to leakage, especially when the extent of the shrinkage cannot be compensated for by the elasticity of the hollow fibres.  
           [0011]    It is clear, especially during the above-mentioned uses of these filters, that any leakage of the hollow fibre membrane should be avoided in all circumstances. A leakage in just a single fibre could lead to a serious risk for a patient.  
         Description of the Invention  
         [0012]    Against this background, the problem which the present invention seeks to solve is that of providing a filter having hollow-fibre membranes in which the hollow fibres are arranged as a bundle within a tubular housing and in which the housing is provided with an end-cap at each end thereof, wherein the hollow-fibre bundle is arranged between the ends of the housing, and wherein a support ring is arranged around each end of the hollow-fibre bundle, the ends of the hollow-fibres being embedded within a potting compound and wherein the ends of the hollow fibres are open, and furthermore, wherein the hollow fibres are not susceptible to damage resulting from axial shrinkage.  
           [0013]    This problem is solved by a filter of the aforementioned type, wherein the end of the hollow-fibre bundle is embedded substantially only within the support ring by means of the potting compound.  
           [0014]    A filter is thereby provided, which is substantially invulnerable to a heat or steam sterilization process. The term invulnerable as used herein is to be understood as meaning that the unavoidable shrinkage which occurs, in the present case in particular the axial shrinkage of the hollow fibres, does not lead to damage of the hollow fibres.  
           [0015]    This arises from the fact that the hollow-fibre bundle is attached only to the support ring and not to the housing, thus allowing a limited degree of movement of the hollow-fibre bundle together with the support ring within the housing. In this way, the shrinkage arising from heat or steam sterilization can be compensated, since the hollow-fibre bundle, or alternatively, the individual hollow fibres, is thereby capable of following the shrinkage and yielding in response to it. Leakages and damage to the hollow fibres can thus be effectively avoided.  
           [0016]    When, in accordance with a preferred embodiment, the support ring further comprises axially projecting lugs, which lugs are preferably arranged on the side of the support ring facing the centre of the housing, whereby the lugs are preferably formed to be flexible, the support ring is especially axially movable.  
           [0017]    The term flexible is here to be understood as meaning compliant in the sense that the lugs are formed so as to be compressible or foldable. When for example a corresponding compressive force is applied to the support ring in an axial direction, the lugs will be compressed or folded against a correspondingly formed shoulder on the housing and the support ring pushed axially in the direction of the compressive force until it again rests, with other parts or regions, against the shoulder on the housing.  
           [0018]    This provides the added significant advantage that axial shrinkages of the hollow-fibre bundle of the kind occurring, for example, during heat-sterilisation of in particular synthetic hollow fibres, can be compensated. To this end, during fabrication of the filter, the entire hollow-fibre bundle is axially compressed such that the embedded ends of the hollow-fibre bundle are shifted by a few millimeters towards each other. This has the effect that the individual hollow fibres in the hollow-fibre bundle and within the housing are relatively slack or loose, similar to a loose bundle of strings. The hollow fibres, embedded at either end within the potting compound, can thus undergo some axial contraction and thereby compensate for axial shrinkages arising for example from heat sterilization, without becoming damaged or being torn apart. For example, through an appropriate configuration of the flexible lugs, and by compressing each end of the hollow-fibre bundle by just two millimeters in a direction towards the centre of the housing, a compensation capability for the hollow fibres of up to four millimetres can be provided.  
           [0019]    The axially protruding, flexible lugs may of course be differently dimensioned, and they may for example be longer, so that a greater compression of the hollow-fibre bundle is possible by application of a compressive force on the support ring. The axially protruding lugs are thereby bent or compressed, thus allowing a movement of the support ring and with it of the embedded end of the hollow-fibre bundle in an axial direction, i.e. relative to the filter housing.  
           [0020]    Whilst it may seem redundant, it is nevertheless pointed out at this stage, that a compression of the hollow-fibre bundle is only possible when its ends are embedded within the respective support ring. Accordingly, the potting compound must lie only within the support ring, in order to avoid a connection between the support ring and the housing. The potting compound may be comprised of any suitable material such as, for example, polyurethane (PUR).  
           [0021]    In addition, the axially protruding, flexible lugs may be accommodated on the housing in an appropriate manner instead of on the support ring. The axial movability of the support ring is thereby also enabled, allowing axial shrinkage of the hollow fibres and thereby also of the hollow-fibre bundle to be compensated.  
           [0022]    For the compensation of axial shrinkage, the hollow fibre bundle may be axially compressed during the fabrication process, as described above. This allows, so to speak, a compensation of axial shrinkage in advance, before its occurrence. Alternatively, the hollow fibre bundle is not compressed in advance, and instead the compensation of axial shrinkage of the hollow fibres may take place simultaneously with its occurrence. Accordingly, the flexibly formed lugs allow the contracting hollow fibres within the housing to thereby draw the axially movable support rings in a direction towards the centre of the housing.  
           [0023]    In order to seal the filter, or the housing, the end-cap is sealingly connected to the housing. The embodiment to be chosen for this feature depends upon the constructive arrangement of the filter. The sealing connection of the parts with one another may be achieved by the use of adhesive means, by welding, or by means of a seal between the respective parts. For example, elastic sealing rings or the like may be used for this.  
           [0024]    The sealing connection forming the seal between the first and the second fluid containing areas may also be achieved by use of adhesive, by welding or by means of appropriate seals such as, for example, O-rings.  
           [0025]    It should be noted at this point, that the previously mentioned axial movability as well as a certain radial movability of the support ring, i.e. its movability relative to the housing, is only possible when the support ring is not fixed to either the end-cap or to the housing or both, whether by adhesive or by welding means. Only then does a certain relative movability of the support ring and thereby of the hollow-fibre bundle remain within the finished filter.  
           [0026]    Finally, according to a preferred embodiment, the filter is advantageously provided with an inlet and an outlet for a first fluid, as well as at least one outlet for a second fluid. Preferably, the inlet for the first fluid is arranged on one end-cap, while the outlet for the first fluid is provided on the other end-cap, so that the first fluid may be readily directed into the housing, through the inside of the hollow-fibre bundle and out of the housing on the other side. The outlet for the second fluid can be accommodated on an end-cap or on the housing, depending upon the intended use of the filer.  
           [0027]    According to another preferred embodiment, the filter is provided with an inlet for the second fluid, which is preferably accommodated on an end-cap or on the housing.  
       
    
    
     DESCRIPTION OF THE DRAWINGS  
       [0028]    The invention will now be described in more detail in the context of a preferred embodiment with reference to the attached drawings. These show:  
         [0029]    [0029]FIG. 1-a partial-section of a filter according to the state of the art;  
         [0030]    [0030]FIG. 1A-an enlarged partial view of FIG. 1, showing a longitudinal section of the end of the hollow-fibre bundle with the support ring;  
         [0031]    [0031]FIG. 2-a longitudinal section through one half of a filter according to the present invention with a compressed hollow-fibre bundle;  
         [0032]    [0032]FIG. 3-an enlarged partial view of FIG. 2, showing the end of the hollow-fibre bundle with the support ring;  
         [0033]    [0033]FIG. 4-a longitudinal section though one half of a filter according to the invention, prior to the application of the potting compound and prior to compression;  
         [0034]    [0034]FIG. 5-an enlarged partial view of FIG. 4;  
         [0035]    [0035]FIG. 6-a longitudinal section through one half of a filter according to the invention after the application of the potting compound and after the axial compression;  
         [0036]    [0036]FIG. 7-an enlarged partial view of FIG. 4.  
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0037]    [0037]FIG. 1 depicts a partial longitudinal view of a filter  1  according to the state of the art. The filter  1  has a tubular filter housing  3 , which has an end-cap  5  attached at each end. Each end-cap  5  has an inlet  7  or outlet  9  for a first fluid, each of which is provided with an internal screw-thread  11 . The end-caps  5  are furthermore provided with an inlet  13  or outlet  15  for a second fluid, each of which is provided with an external screw-thread  17 . Within the tubular housing  3 , a hollow-fibre bundle  19  is arranged, which extends between the ends of the housing  3 . The ends of the hollow-fibre bundle  19  are surrounded by a support ring  23  and are embedded into the ends of the housing  3  by means of a potting compound  70 .  
         [0038]    The filter  1  functions as follows: connection tubes for a first fluid are connected to the inlet  7  and outlet  9  by means of the internal screw-thread  11 . Similarly, connection tubes for a second fluid are connected to the inlet  13  and to the outlet  15  by means of the external screw-thread  17  provided. A first fluid is then directed through the inlet  7  towards the end of the hollow-fibre bundle  19 . The ends of the hollow fibres  21 , which make up the hollow-fibre bundle  19 , are open and are comprised within the ends of the hollow-fibre bundle  19 , so that the first fluid can be directed inside the hollow fibres  21  and through to the other side of the filter  1 . There, the first fluid emerges from the end of the hollow-fibre bundle  19  and is conducted away through the outlet  9 .  
         [0039]    A second fluid is conducted into the housing  3  via the inlet  13 , where it then flows along the outside of the hollow fibres  21  of the hollow-fibre bundle  19  to the other end of the housing  3 , from where it is conducted away through the outlet  15 .  
         [0040]    The first fluid and the second fluid are conducted past one another in opposite directions and on different sides of the hollow-fibre membrane  21 , during which various known convection and diffusion processes take place through the walls of the hollow fibres.  
         [0041]    [0041]FIG. 1A depicts an enlarged partial view of FIG. 1 showing the end of the hollow-fibre bundle  19 . The hollow-fibre bundle  19  is surrounded at its end region by a support ring  23 , which support ring  23  has a first portion  25  and a second portion  29 . The first portion  25  is provided with a number of radial flanges  27  which connect the first portion  25  to the housing  3 . The flanges  27  are configured such that they support the support ring  23  both axially and radially. The peripheral first portion  25  is additionally provided with further radial flanges  31  by means of which it is connected with the second peripheral portion  29 . The second portion  29  surrounds the end portion of the hollow-fibre bundle  19 . This end-portion of the hollow-fibre bundle  19 , along with the support ring  23 , is potted within the end of the housing  3  by means of a potting compound  70 . The second portion  29  of the support ring  23  is thereby completely enclosed within the potting compound  70 .  
         [0042]    The end-cap  5  is placed on the housing  3  and is sealingly attached to the housing  3  by an adhesive  74 . The end-cap  5  is also fixed to the potting compound  70  by the same adhesive  74  and is in radial contact with the outside of the first portion  25  of the support ring  23 .  
         [0043]    A ring  72  of potting compound  70 , is disposed between the periphery of the hollow-fibre bundle  19  and the inside of the end-cap  5  and has a broad width. This arrangement prevents penetration of adhesive  74  into the hollow fibres  21 , the open ends of which are disposed in the end of the hollow-fibre bundle.  
         [0044]    [0044]FIG. 2 shows a longitudinal section of a filter  100  according to the invention, in which for the sake of simplicity only one half of the filter is shown and in which equivalent parts have been given the same reference numeral. The filter  100  has a tubular housing  3  and an end cap  5 , one of which is shown, attached to each end of the housing  3 . The end-cap  5  has an outlet  7  which comprises an internal screw-thread  11 . The housing  3  further comprises an outlet  15  for a second fluid, the said outlet  15  having an external screw-thread  17 . A hollow-fibre bundle  19  comprised of hollow fibres  21  is arranged within the housing  3 , the hollow-fibre bundle  19  having been compressed. The end of the hollow-fibre bundle  19  is potted into a support ring  40  by means of a potting compound  70 .  
         [0045]    The support ring  40  is provided at one end with a plurality of lugs  42 , which will be described more fully in the following description of the support ring  40 , along with the rim  44 , the flanges  46  and the steps  48  and  50 .  
         [0046]    It should be noted at this point, that an alternative arrangement of the inlets and outlets could also be provided, as is known to one skilled in the art. For example, the outlet  15  could equally be arranged on the end-cap  5 .  
         [0047]    [0047]FIG. 3 shows an enlarged partial view of FIG. 2, showing the end of the hollow-fibre bundle  19  potted into the support ring  40 . Also here, equivalent parts are given the same reference numeral.  
         [0048]    The support ring  40  is provided, at its end facing the end-cap  5 , with an inwardly protruding shoulder  44  with flanges  46 . The rim  44  and the flanges  46  attached to it create a fitted connection between the support ring  40  and the potting compound  70 , in addition to the adhesive connection between the potting compound  70  and the support ring  40 . On its outside, the support ring  40  is provided with two steps  48  and  50 , which rest upon correspondingly shaped shoulders  52  and  54  on the housing  3 . At its lower end, the support ring has a plurality of flexible lugs  42 , one of which is illustrated here. The flexible lug  42  is bent and extends into a corresponding recess  58  in the housing  3 .  
         [0049]    According to the present embodiment, the end-cap  5  is sealingly connected to the housing  3  at a connection portion  64 , as shown here by means of welding, for example using ultrasound. A further possibility, from among several others, involves bonding, using suitable adhesive means known to the skilled person. The end-cap  5  is provided at its periphery with a recess  62 , in which a sealing ring  60  is accommodated. This sealing ring  60  delimits the peripheral region of free space formed between the end-cap  5  and the end of the hollow-fibre bundle  19 , such that only a narrow ring  72  of the potting compound is present in the aforementioned free space. This improves the flow characteristics of the first fluid which is conducted into the hollow space via the inlet  7  of the end-cap  5 , and from there passes into the open ended hollow fibres  21 , through which it flows to the opposite end of the filter housing  3 . The improved flow characteristics create i.a. a flushing of the first fluid through the peripheral region of the hollow space, i.e. the space above the ring  72  of potting compound, so that a build-up of deposits is prevented. When the first fluid is comprised of blood for example, as is foreseen in the embodiments mentioned in detail above, this prevents the blood from being stationary and thereby from clotting. This thereby greatly reduces the risk of blood clots and thus renders the above-mentioned examples of use or treatments considerably safer for the patient.  
         [0050]    In addition, the sealing ring  60  seals the region of the filter through which the first fluid flows from the region through which the second fluid flows.  
         [0051]    The filter  100  shown in FIG. 3 is a finished filter, i.e. ready for use. The hollow-fibre bundle  19  of the filter has been axially compressed, as can be readily seen from the bent flexible lug  42 , which has been forced into the recess  58  of the housing  3 . The function of the flexible lug  42 , as well as that of the compression operation of the hollow-fibre bundle  19 , will now be described in detail with reference to FIGS.  4 - 8 .  
         [0052]    [0052]FIG. 4 shows a longitudinal section of an end of the filter  100  prior to the application of the potting compound and prior to the axial compression of the filter bundle  19 . Here also, equivalent parts are given the same reference numeral.  
         [0053]    The hollow-fibre bundle  19 , placed within the filter housing  3 , is held in place vis-{fraction (a)}-vis a mould ring  80 . The mould ring  80  is attached to the housing  3  at one end, and at its other end, bears a moulding cover  82 . The support ring  40  rests with its flexible lugs  42  located upon a shoulder  56  on the housing. The dotted line  90  indicates the line along which the hollow-fibre bundle  19  is to be cut following the potting operation.  
         [0054]    For the sake of a better understanding, the description will now be continued with reference to FIG. 5, in which an enlarged partial view of FIG. 4 is depicted.  
         [0055]    As can be clearly seen from FIG. 5, the mould ring  80  has fastening means  84  at its end nearest the housing  3 , which means  84  engage with a correspondingly shaped projection on the housing  3  thereby securing the mould ring  80  to the housing  3 , together with the projection  86 , and the support ring  40 , as will be explained in detail. Furthermore, as can readily be seen, the mould ring  80  has fastening means  88  at its other end which cooperate with a correspondingly shaped portion of the moulding cover  82  to fix the moulding cover onto the mould ring.  
         [0056]    As is illustrated here, the flexible lug  42  of the support ring  40  is not deformed prior to the axial compression and potting operations, and is seated on a shoulder  56  on the housing. The free space between the step  48  of the support ring  40  and the shoulder  52  of the housing  3  is easily recognizable, as is the free space between the step  50  of the support ring  40  and the shoulder  54  of the housing  3 .  
         [0057]    The above-mentioned protrusion  86  of the mould ring  80  rests upon the step  48  of the support ring  40 . The support ring  40 , or its position within the housing  3 , is firstly thereby fixed. Secondly, the mould ring  80 , or its position in the housing  3 , is consequently also fixed. The step  48  of the support ring  40  limits the movement of the protrusion  86  and thereby also of the mould ring  80  in the direction towards the housing  3 , i.e. it limits the downward movement (FIG. 5), while the fastening means  84  of the mould ring  80  limit movement of the mould ring  80  in a direction away from the housing  3 , i.e. an upward movement (FIG. 5). At the same time, the protrusion  86  of the mould ring  80  limits the movement of the support ring  40  away from the housing  3 , while the flexible lug  42  which abuts the shoulder  56  on the housing  3 , limits the movement of the support ring  40  towards the interior of the housing  3 .  
         [0058]    In the condition illustrated in FIG. 5, the filter  100  is ready for the introduction of the potting compound for potting the end of the hollow-fibre bundle  19 , or the hollow fibres  21  arranged therein. For this purpose, the potting compound is usually introduced through the outlet  15  of the housing  3 , and is then pushed into the mould ring  80  and moulding cover  82  by centrifuge means, i.e. by rotation of the filter  100 , such that the hollow fibres  21  are cast together in order to seal the hollow-fibre bundle  19  at its ends.  
         [0059]    [0059]FIG. 6 depicts a longitudinal section view of an end of the filter  100  after the potting of the ends of the hollow fibres  21  with the potting compound  70  as well as after the axial compression of the entire hollow-fibre bundle  19 . Here again, equivalent parts are given the same reference numeral.  
         [0060]    From FIG. 6, it can easily be seen that the potting compound  70  casts the ends of the hollow fibres  21  together, so that a solid, cohesive end of the hollow-fibre bundle  19  is created. The potting compound  70  is distributed in the manner shown by virtue of the centrifugal force, while the support ring  40  together with the mould ring  80  and moulding cover  82 , define a mould for the potting compound. The potting compound  70  is thereby prevented from coming into contact with, and adhering to, the housing  3 . The relative movability between the support ring  40  and the housing  3 , necessary for the axial compression of the hollow-fibre bundle  19 , is thereby maintained, as is shown in FIG. 6. The compression operation causes the flexible lugs  42  to bend and to be pushed outwards, such that they are slid into the recess  58  in the housing  3 , thereby having enabled an axial movement of the support ring  40  along with the end of the hollow-fibre bundle  19 , which end is potted into the support ring.  
         [0061]    This can be more clearly seen from FIG. 7, which depicts an enlarged partial view of FIG. 6. Here also, equivalent parts are given the same reference numeral.  
         [0062]    The end of the hollow-fibre bundle  19  is pushed towards the center of the housing  3  by virtue of the compression operation. As a result of this, the flexible lugs, which abut the shoulder  56  of the housing  3 , are bent and forced into the recess  58  of the housing  3 . The hollow-fibre bundle  19  is shifted until such a time as the steps  48  and  50  abut the corresponding shoulders  52  and  54  of the housing  3 , as shown in FIG. 7. The configuration of the shoulders  52  and  54  on the housing  3  thus determines the maximum axial compressibility of the hollow-fibre bundle  19 . It is therefore necessary to arrange the shoulders  52  and  54  in a configuration corresponding to the desired axial compression of the hollow-fibre bundle  19 , while taking account of the material from which the hollow fibres  21  are comprised.  
         [0063]    The axial compression of the hollow-fibre bundle  19  is carried out by means of a compression force applied to the moulding cover  82  and/or to the mould ring  80 . The hollow-fibre bundle  19  is thereby longitudinally compressed, the flexible lugs  42  are bent and forced into the recess  58 , and the end of the hollow-fibre bundle  19  is partially pushed into the housing  3 . Since this is carried out simultaneously on both ends of the housing  3 , the hollow fibres  21  are thus relaxed by a few millimeters and thereby lie somewhat loose within the hollow-fibre bundle  19  and the housing  3 . This enables them to readily compensate shrinkage of a few millimeters without breaking or otherwise being damaged.  
         [0064]    As explained above at length, the relative movability between the support ring  40  or the hollow-fibre bundle  19  and the housing  3 , is enabled by virtue of the potting compound  70  being contained substantially only within the support ring  40 . This relative movability of the support ring  40  enables the compression of the hollow-fibre bundle  19 , which later allows a compensation of longitudinal shrinkage of the hollow fibres  21 . These shrinkages of the hollow fibres  21  are a consequence of the final heat-sterilisation of the finished filter  100 , and occur mainly in synthetic fibres.