CA1100887A

CA1100887A - Hollow fiber dialysis

Info

Publication number: CA1100887A
Application number: CA264,841A
Authority: CA
Inventors: Kyu H. Lee
Original assignee: Extracorporeal Medical Specialties Inc
Current assignee: Extracorporeal Medical Specialties Inc
Priority date: 1975-11-05
Filing date: 1976-11-03
Publication date: 1981-05-12
Also published as: NL165058B; CA1106771A; AR215452A1; FR2330429A1; IT1063433B; SE7611942L; MX143979A; FR2330429B1; NL7612323A; SE434120B; BE847969A; CA1106770A; DE2650588A1; SE8100863L; BR7607355A; NL165058C; GB1569182A; JPS5258079A; JPS5616687B2; JPS5697458A

Abstract

HOLLOW FIBER DIALYSIS
Abstract of the Disclosure Improved dialyzer construction has elongated tubular casing with enlarged casing ends and longitudinally-extending partitioning that divides its interior into a plurality of generally parallel passageways containing hollow dialysis fibers, the casing ends containing di-alyzate flow manifolding and directing means for re-ceiving dialyzate from a supply thereof, directing it from one casing end to the other through one of the passageways around the fibers, and so to and fro through the successive passageways, finally directing the dialy-zate out through a discharge opening. The dialyzate flow can in each fiber-containing passageway be counter-current to the flow within the fibers of the liquid being dialyzed. A gas by-pass can be provided in the partitioning at one end to bleed out gas that tends to accumulate at the high point of the to-and-fro dialyzate travel.

Description

1100~8~

The present invention relates to dialysis, particularly dialysis of liquids such as blood.
There have been many suggestions for improving dialysis equip-ment especially of the artificial kidney type. A great many so-called coil-type artificial kidneys are in use in which the dialy-sis element is a tube many inches in diameter, but flattened and spirally wound with a spacer to separate the turns of the winding.
An artificial kidney so made is relatively large in size, and there have been suggestions for smaller constructions having bundles of thousands of hollow fibers each a very narrow tube that functions as a dialysis element, but this modified construction still presents problems. An example of hollow fiber apparatus is shown in U.S.
Patent 3,442,002, and a fiber which has recently come into use for hollow fiber dialyzers in the deacetylated cellulose acetate fiber referred to in U.S. Patent 3,545,209. These fibers are generally required to be kept wet with water at all times after the de-acetylation, in order to maintain their dialytic permeability.
This has led to the awkwardness of shipping and storing dialyzers based on such hollow fibers while the fibers are filled with water and with the water containing formaldehyde to keep it from develop-ing microbial growth. Before each use it then becomes necessary to flush out all the formaldehyde.
Thus, in accordance with the present teachings, a hollow fiber dialyzer is provided which has an elongated tubular casing which contains partitioning that divides its interior into a plurality of separate longitudinally-extending passageways with dialyzate flow directing means for receiving dialyzate from an external source and directing it to flow from one casing end to the other through one of the passageways and then back to the one casing end through another of the passageways and so to and fro lengthwise through the other passageways and finally out of the ; ~

casing, at least two of the passageways each having a longitudin-ally-extending bundle of elongated hollow blood dialysis fibers extending lengthwise therethrough with means connected to deliver blood to be dialyzed from an inlet to the fiber end of each of the bundles at the same end of the casing and to receive the blood from the other end of the hollow fibers for delivery to an outlet.
The dialyzer is characterized in that only some but not all of the passageways have bundles of fibers and the remaining passageways do not contain dialysis fibers, and the dialyzate flow directing means directs dialyzate to flow from one casing end to the other in alternating fashion through the passageways having bundles of fibers and the passageways which do not contain fibers so that the dialyzate flow through each of the passageways having bundles of fibers is always in the same lengthwise direction.
The invention will now be described in more detail, by way of example only, with reference to the accompanying drawings, in which:
Fig. 1 is an elevational view partly in section and partly broken away, of a hollow fiber dialyzer in accordance with the present invention;
Figs. 2, 3 and 4 are sectional views of the construction of Fig. 1, taken along the lines 2-2, 3-3 and 4-4, respectively;
Fig. 5 is a view similar to Fig. 1 of a modified hollow dial-yzer representative of the present invention;
Figs. 6, 7 and 8 are sectional views of the construction of Fig. 5 taken along the lines 6-6, 7-7 and 8-8 respectively;
Figs. 9, 10, 11 and 12 are views similar to Figs. 1, 2, 3 and 4 of a further moldified dialyzer typical of the present invention;
Fig. 13 is a sectional view of a cap suitable for use with the construction of Fig. 9; and Figs. 14, 15 and 16 are views similar to Figs. 1, 2 and 3 of yet another embodiment of the present invention.
The following examples illustrate very desirable manner of pre-paring hollow fiber dialyzers in accordance ~0()887 with th~ present application.

: A dialysi~ casing such as shown at 10 in Fig. 1 and molded or cemented together from polycarbonate or polystyrene resin, is first provided. This casing has an over-all length Or about 7-1/2 inches with its intermediate portion 12 having an internal diameter of 1-3/~ inches, each end 14, 16 being of enlarged bore having internal diameters about 1-7/8 inches.
The tubular length of the interior of the casing is divided into three individual passageways 2~, 22, 23 by an axial web 26 of three flanges, 31, 32 and 33. An inlet tube 36 opens into enlarged end 14, and a discharge tube 38 leads out from end 16.
Web 26 has ~low oontrol means at each end 14, 16 arranged 90 that fluid entering inlet 36 ~low~ upwardly through passaee-way 21 ~rom end 14 to end 16, then at end 16 moves rrom the top of passageway 21 to the top o~ passageway 22, then downwardly through passageway 22 to lower end 14 where it then transfers to the bottom Or passageway 23 along which it moves upwardly to end 16 for discharge through outlet 38. ~o efrect this flow control, flange 31 is arranged as a barrier seal againstthe outer wall o~ the oasing throughout the length of lower end 14 as w911 as throughout the length of the intermediate portion 12~ but not at the upper end 16. Instead at that upper end Mange 31 is cut out as shown at 40 to provide a flow-through space 41 that thus opens between the upper ends of passageways 21 and 22.
Flange 32 is simllarly shaped in an opposite sense so that at its lower portion it provides an edge 50 spaced rrom the lowest point o~ casing end 14 to provide a ~low-through space 51 between the lower ends of passageways 22 and 23.
-4 - .

110~)887 A batch of hollow cuprammonium regenerated celluloqe ~ibers having a wall thickness of about 12 microns plus or minus 2 microns and an internal diameter of about 200 microns plus or minus 50 microns is unspooled, pre~erably from a plurality of spools in parallel strands, cut to a length of about 9 inches and carefully cleaned. As generally supplied these ~ibers are made by extruding cuprammonium cellulose solution through an annular die into a regenerating bath while introducing a water-immi~cible liquid into the bore of the hollow extrudate. A typical water-insoluble liquid is isopropylmyristate. After regeneration is completed carerul washing with isopropanol removes such liquid. The interiors o~ the ribers can then be wet with a sortening agent such as glycerinef preferably leaving about 5% o~ the ~oftening agent 15 by weight Or the clean riber. Thi9 sortening is not essen-tial but helps guard the ribers against breakage or damage during subsequent handling, and does not detract rrom the erreotiveness by which the ribers are sealed into the casing 10.
A bundle o~ two to three thousand ribers so prepared i9 then inserted into one Or the passageways 21, 22, 23, and additional bundles in each o~ the remaining passageways. This insertion can be expedited by ~irst sliding over the bundle a tapered sleeve of polyethylene, then introducing the filled 25 sleeve, narrow end first, into one of the passageway~, and finally pulling the sleeve o~f the introduced bundle. At the narrow end of the taper the ~ibers are arranged to project from the sleeve so they can be gripped to help pull the sleeve of~ the other ends Or the fibers.
When all the passageways are rilled with ~ibers, the ll~)U887 potting can be started. At each end of the casing each bundleo~ fibers projects Q short distance. Each of these pro~ecting ends is dipped in melted carnauba wax which is then permitted to solidi~ after the carnauba wax has penetrated a very short distance into all of the individual fibers. Thc casing is then clamped longitudinally between potting heads connected to a pottlng ¢ompound container as illustrated in Fig. 19 of Patent 3,442~oo2, and centrifuged as also indicated in that patent ~hile the uncured liquid freshly mixed potting mixture is poured into the potting compound container. This mixture can be a polyurethane prepolymer resin with a chain extender, or an epoxy cement mixture as described in Patent 3,442,002, or a hardenable polysiloxane liquid or other settable resin.
When a hardenable polysiloxane liquid with a ~uring agent such as chlorplatinic aoid i9 u~ed, the ¢entrifuging is oonduoted at about 350 g while the mixture ls heated~ and after about 1/2 hour at 150F, the potting mixture is cured to the point that it no longer ~lows. The potting heads are then unolamped and removed, and the curing completed by holding the dialyzer in an air oven at 150F for two hours. After that the potting mixture is a cleanly cutting solid ~nd a sharp metal blade is used to cut the potting mixture flush with the open ends 14, 16 of the casing. This leaves the construction as illustrated in Fig. 1, the potting composition being shown at 56 and 57.- Covers 61, 62 each equipped with a ~low conneetion 64, 65 are then ~itted to the ca9ing ends 14, 16 as by welding or cementing~ although they can also be threaded in place if desired. The ¢onstruction ls then complete and only needs a ~lushing through to remove the water-soluble softening agent ~rom the inside of the hollow flbers -6_ llOQ887 before it i~ placed in ~ervice, The dialyzers can be stored either before or after washing out the sortening agent, without significantly a~fecting its dialysis properties, When the dialyzer is used it is generally held with end 16 up, a ~ouroe of dialyzate is connected to inlet 36, discharge 38 i8 connected to waste, and a supply of blood to be dialyzed connected to inlet 65 with a blood return to outlet 64, In use bubbles of air or other gases can ~orm in the dialyzate and tend to rise toward the upper end 16 o~ the dialyzer. ~o keep those bubbles from becoming trapped at the upper ends o~ passageways 21 and 22 and collecting there in an amount that could inter~ere with the dialysis~ a small hleed 59 is shown as provided at the upper end of web flange 32. For a ~lange with a wall thickness of 1/16 inch a round opening a~
little as 1/2 millimeter in diameter will enable the gas trapped at the above-mentioned ends to readily make its way into the upper end o~ pas~ageway 23 and out through disoharge opening 3~, without significantly reducing the e~ectiveness of the dialysis. The gas vent can even be made slightly smaller as for example 0.3 mm. in diameter. The optimum width of the vent i9 related to the thickness o~ the wall throu~h which it penetrates. For wall thicknesses greater than 1/16 inch the vent width is preferably a little larger than 1/2 millimeter.
A ~eature o~ the dialyzer construction of Fig. 1 is that such dialyzers are readily manufactured with more uniform dialysis er~ectiveness than corresponding dialyzers in which there is no partitioning and web 26 is completely omitted.
Notwithstanding the enlarged ends 14, 16 which ~erve as dialyzate manifolds that bring the dialyæate lnto direct contact with the outer layers of` fibers in the fiber bundles~
the dialyzate has a tendency to make its way through one end of the dialyzer to the other through the easiest path and thus find and establish a channel, even when the fibers are ~airly 5 well packed in place. Such channeling greatly reduces the effectiveness of the dialy~is particularly through the walls o~ those fibers that are ~ome distance laterally spaced from the channel. When this happens with a dialyzer contalning only a single dialyzate passageway, its e~iciency becomes so 10 poor that it genei~ly has to be di~cQrded~
Such ¢hanneling is more likely to t~ke place as the wall thicknes~ of the hollOw ~ibers diminishes and as the ~iber diameter decreases; these oau9e the ~ibers to be more flexible 90 that it is easier for the dialyzate to create a channel by 15 deflectlng the fibers. Wall thiaknesses o~ about 5 to about 20 microns are suitable ~or e~ecti~re use and thicknesses of` ~rom about 10 to about 1~ microns are preferred. Fibers with internal passageways not over about 500 microns wide, prefer-ably rangin~ from about 100 to about 300 microns in width, are 20 ~rery effective. Cuprammonium regenerated hollow fibers of thi~
type are relatively stiff, particularly when dry, and are acoordingly very easy to handle in the assembling of a bundle ror insertlon in a dialyzer~ and in the insertion it~elf.
In the construction of Fig. 1 a channeling-induced drop 25 in e~i¢iency of passageway 21 can also occur, but when that happens the dialyzate emerging from passageway 21 i9 less loaded with contaminants so that it becomes more ef~ective in its 9ubsequerlt passage throu~h pa~sageways 22 and 23~
In addition each of the passageways 21, 22 and 23 is 30 narrower than it would be without the web 26, and channeling 1~0887.
_9_ becomes less likely in narrower passageways. Also the total length o~ fibers contacted by the dialyzate in the construction of Fig. 1 is three times the len~th contacted i~ web 26 were omitted, and the efficiency loss throu~h channeling diminishes as such length increases.
Because o~ the more reproducible greater e~iciencies o~ the construction of Fig, 1, dialyzers having an operating length between potting seals 56, 57, Or only about 15 centi meters can be readily manufactured with the desired high qualities. This small b~lk is particularly desirable, desirable, although in general overall length~ o~ ~r~m about 6 to about 12 inches can be attractive for hospital use.

Figs, 5, 6, 7 and 8 illustrate a modiried dialyzer 110 pursuant to the present invention. In this dialyzer there are three parallel dialyzer passageway~ along the lines Or Eigo 1 but the rlow Or dialyzate is arranged so that throughout its ~iber-conta¢ting path it moves on the outside o~ the individual ~ibers in a direction countercurrent to the ~low of blood or other medium being dialyzed within the fibers.
As in the construction o~ Fig. 1, dialyzer 110 has a central tubular section 112 with enlarged ends 114, 116 and with a partitioning web 126 inserted or molded in section 112.
Web 126 has rlanges 131, 132~ 133 similar to the three ~langes Or web 26~ and in addition also has two supplemental rlanges 134~ 135 that derine supplemental passageways 124, 125.
The bundles o~ hollow ribers are contained in passage-ways 121, 122, 123; pa~sageways 124, 125 being unrilled 80 that they provide paths ror the dialy2ste to ~low while out ..g_ 1~0~887 o~ contact with the fibers.
The flow of dialyzate is controlled by appropriate shaping of the web ~langes in the construction of Fig. 5 so that it enters and flows upwardly first through passage 121 then downwardly through passage 124 then back upwardly through passageway 122 returning this time to the bottom via passageway 125, and finally completing the dialysis by an upward travel through passageway 123 and discharge at outlet 138. For this result, the upper ends o~ webs 134 and 135 are spaced from the inside wall of casing end 116 and the lower ends o~ webs 131 and 132 are spaced from the inside sur~ace Or casing end 114, as more clearly illustrated in Figs. 7 and 8.
No gas vent is provided in the construction of Fig. 5 inasmuch as the dialyzate ~low rate is ~airly high in the very narrow return passageways 124, 125. Thu9 a flow rate o~ only about one ~oot a second is generally sufficient to sweep out gas bubbles that tend to ~orm. For slower rlow rates, as for example when the dialyzate is discarded a~ter a siDgle passage through the dialyzer and is not recirculated ~rom outlet 138 back to inlet 136, gas venting can be provided in the construc-tion o~ Fig. 5.
Gas venting can be eliminated where the dialyzate is treated to reduce gas evolution, as ~or example by boiling it under reduced pressure before it is introduoed into the dialyzer. This removes almost all of the dissolved gases~ and the maintenance o~ some pressure on the dialyzate as it i9 impelled through the dialyzer acts as an additional preventive to gas evolution.
The dialyzer casings o~ the present invention need not be circular in cross-section but can be oval, rectangular or l~OV887 triangular if de3ired, both in their external shape as well as ln the shape of the passageway~. Similarly, they do not have to be perfectly linear in longitudinal direction.

Figs, 9 throu~h 13 illustrate a dialyzer 210 according to the present invention which is generally triangular in cros~-section, particularly at its ends 214, 216. Those ends each have a mounting rib 217 which helps in positioning end connector covers 262. Moreover each rib 217 can be provided with a ridge 219 which need only be about 15 to about 20 mils high that helps in welding the cover in place aæ by sonic or ultrasonic vibration of the mounted cover against that ridge.
Upon vibration in this manner the ridge and the ridge-engaging portion of the cover fuse a~ a result of the frictional heating efrects of the vibration between them, and weld together making a very effective fluid-tight seal.
The construction and operation of Figs. 9 through 13 generally correæponds to that of Figs. 5 through 8, and similar portion~ such as partitloning web 226, passageways 221, 222 and 223 ~or reoeiving the hollow fibers, passageways 224 and 225 for dialyzate return, and inter-passage spacings 241, are similarl~ numbered~ However~ to better seal the blood or other dialyzand away ~rom undesired crevices and the like, covers 262 are eaoh provided with an internal sealing lip 263 shaped to engage the potting seal 257 outæide the fiber-containing zone.
The dialyzand is thus kept from penetrating into the crevice 265 between the internal surface of the cover and external æurface of the casing wall.

To further help with such sealing, the potting seal 257 can be arranged to project out a short distance 267, ~uch as ': :

1/~ inchj beyond the casing end.

Figs, 14, 15 and 16 illustrate a dialyzer 412 having a generally rectangular configuration both in its external aspect 5 as well as in its passageways. Such a configuration makes better use of space and can contain more fibers than other configurations having the same overall dimensions.
The construction and operation of this exempli~ication, as well as the numbering of its parts, is similar to that of Figs. 1 through 4, except that its covers 462 and cover engagement are like those of ~igs. 9 through 13 without the internal sealing lip. Internal sealing is provided in Fi~s~ 14 by having it~ end covers 462 tightly engage the outer margin of the potting seal. Al~o instead of having the fiber-containing passageways 421, 422 and 423 arrayed generally circumferentia~ly around casing 410, these passageways are arranged in a simple row all lying ln what can be considered the same thick plane.
The fiber bundles can be inserted in the dialyzer passageways without the help of a sleeve, particularly if the 20 walls of a casing end provide a gradual taper from their large internal bore down to the smaller bore of ¢entral portion 12 or 112. Alternatively the bundles can be sleeved and the sleeves left in the dialyzer in position around the bundles.
This alternative is particularly desirable when the sleeves 25 are of relatively thin wall section~ i.e. about 3 mils, so that they do not occupy much room.
~ he insertion of the fiber bundles is also made easier if this is done when the casings are hot. The heat expands the casing and thus provides a little more room for more )U887 readily sliding the bundles into place, after which the ca9ing cools down an~ tightly encloses the ~ibers, thus making ror added e~iciency~ Instead o~ an elongated sleeve to help the fiber însertion, a single narrow length o~ plastic or even wire oan be looped around a ~iber bundle adjacent one end, and tied or crimped against the fibers so as to provide a tail for the bundle. The bundl~ can then be pulled through a passageway by ~irst pa~sing the tail through the passageway and then pull-ing on the tail.
It is generally desirable to clean the hollow fibers ~or the dialysis as by washing or rinsing them with a readily volatilizable solvent~ particularly where the bores in the fibers contain a liquid which should not contaot the dialyzand or dialyzat~.
The advantages o~ partitioning are obtained when the dialyzer is partitioned to provide only two parallel dialyzing passageways. A very simple constructlon of this type has inlet and outlet tubes 36, 38 on opposite sldes of the upper casing 16 with a single central web extending longitudinally the entire length o~ the oasing but with an opening in its lower portion.
The partitioning can even be more subdivided than is shown in the drawings so as to provide 4 or ~ parallel dialysis passageways, but the use o~ more partitions takes away some of the space ~or fibers so that the bulk of the casing has to be increased to maintain the dialyzing effectiveness.
The partitioning o~ the present invention simpli~ies the mechanical handling in the manufacture o~ the dialyzer. The reduced width of the individual pa~sageways, e.g, one to three ' 110(~887 centimeters, as compared to an unpartitioned dialyzer, reduces the number of fibers per passageway and thus simpli~ies the preparation Or the individual bundles. By way of illustration, the task of preparing a 6000-fiber bundle for an unpartitioned dialyzer is more complex than that o~ preparing three 2000-fiber bundles for use in the dialyzer of Flg. 1 or Fig. 5 or Fig. 9.
The fiber-containing passageways can also be double tapered as illustrated at 211 in Fig. 9 so that they provide a constriction in their central portions. Such a constriction of about 1/2 to 1 millimeter helps grip the fibers and keep them from being deflected by the flow around them, thus reducing the tendency to channelling.
Another feature of the present invention i9 that the di~ferent compartments of the descrlbed dialyzers need not be used for the same function. One of the compartments can for example be used to hold an absorbent such as activated charcoal or the like, instead of fibers, 90 as to absorb impurities or other undesirable ingredients in the dialyzand. Different kinds of fibers can be used in different passageways to obtain different dialysis effeots on the dialyzand as it passes through the dialyzer. Indeed some of the passageways, such as passageway 124, can be fllled with absorbent for the purpose of treating the dialyzate as it moves through the dialyzer and better condition the dialyzate for its passage through the remaining fiber-containing passageways.

The potting of the fiber ends can be accomplished with techniques other than that described above. Thus the preliminary dip of the fibers to plug their bores can be into melted resin-modified waxes or thermoplastic resins Or llOU887 compositions that harden to form thermosetting resins. ~he potting mixture itself can for example be used as a prel~minary dip of shallow depth~ ~ollowed by deeper potting. Also, by maintaining slightly higher press~e in unplugged ~iber bores as against the pressure over the potting mixture into which the unplugged fiber ends are dipped, the potting mixture is kept at a low level within those bores and the preliminary dip to plug those bore3 can be completely eliminated. ~he bores can alternatively be sealed by melting the fiber ends when they are of fusible nature, and in this way make a prior dip un-necessary.
While centrifugal force applied to the liquid potting mixture helps assure that such mixture thoroughly impregnates all crevices and pores around and between the fibers and in this way assures thorough sealing o~ the dialyzate chamber ~rom the dlalyzand gas pressure applied over the liquid potting composition during the potting, has a similar ef~ect. One end o~ a ~iber bundle can accordlngly be potted a~ a time, without the need ror the centrifugal potting apparatus.
Also the covers 61, 62 can be arranged to snap on over the potted ends o~ the dialyzer, as shown in Fig. 5 at 161, 162 ~or example. Such covers can be relatively ~lexible and the potted ends they snap over can be ritted with ridges as at 163 to help lock the snap-on covers in place.
The dialysis discussed above is to be distinguished from osmosis in that the dialysis uses fibers whose walls are extremely porous, much too porous for use in osmosis. This comparison is more clearly shown by the ~act that a reverse osmosis process desalinating brackish water ~or instance, requires membranes of relatively non-porous material such as ~ 87 polyvinyl chloride, as well RS the use of a driving pressure grsater than the osmotic pressure and as high as practicable.
An attempt to carry out such a reverse osmosis with the cupr-mmonium regenerated cellulose as described above, will merely cause the brackish water to rapidly filter through the regenerated cellulose fibers and emerge at the discharge face of the cellulose in substantially the same condition as it entered the entrance face.
The dialyzer construction of the present invention can also be used with the dialyzate passing through the bores of the hollow flbers and the dialyzand moving along the outside of the fibers, although this arrangement is not desirable where blood i9 the dialyzand. However with osmosis-type fibers, the structurQl arrangement of the present invention is suitable for osmotic processes such as reverse osmosis~ and in suoh use it is pre~erred to pass the fluid being treated around the hollow fibers so that the high pressures used on such flulds in reverse osmosis is applied to the exteriors of the fibers. Fiber failures are then not likely to cause leakage.
The apparatus of the present invention is also suitable for use in gas separation, again with an appropriate type of fiber, or in gas treatment of liquids as in the oxygenation o~ blood where silicone fibers are preferred.
Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is, therefore, to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A hollow fiber dialyzer having an elongated casing containing partitioning that divides its interior into a plurality of separate longitudinally- extending passageways and dialyzate flow directing means for receiving dialyzate from an external source and directing it to flow from one casing end to the other through one of the passageways and then back to said one casing end through another of the passageways and so to and fro lengthwise through the other passageways and finally out of the casing, at least two of the passageways in which dialyzate is directed to flow in the same lengthwise direction each having a longitudinally-extending bundle of elongated hollow blood dial-ysis fibers extending lengthwise through it, and means connected to deliver blood to be dialyzed from an inlet to the fiber ends of each of such bundles at the same end of the casing and to receive the blood from the other ends of the hollow fibers for delivery to an outlet, characterized in that a number of the long-itudinally-extending passageways are of relatively wide cross-section and the remaining passageways are of relatively narrow cross-section, each of the wide cross-section passageways having a bundle of fibers in it but the narrow cross-section passageways do not contain dialysis fibers, and the dialyzate flow directing means directs dialyzate to flow from one casing end to the other alternately through the wide cross-section and the narrow cross-section passageways so that the direction of dialyzate flow from one casing end to the other through each of the wide cross-section passageways having a bundle of fibers in it is opposite to the direction of blood flow through the hollow fibers from the blood inlet to the blood outlet.

2. The hollow fiber dialyzer as in claim 1 wherein each of the wide cross-section passageways having a bundle of fibers has the fibers packed in it and the flow directing means contained in the casing further includes manifolding of enlarged cross-section for directing dialyzate into contact with the outer layers of fibers in each of said bundles as the dialyzate enters each wide cross-section passageway having a bundle of fibers to reduce un-desirable channeling of dialyzate.

3. The hollow fiber dialyzer as in claim 2 wherein the mani-folding of enlarged cross-section for each wide cross-section passageways having a bundle of fibers is defined by laterally enlarged casing ends.

4. The hollow fiber dialyzer as in claim 1 wherein each of the wide cross-section passageways having a bundle of fibers is generally circular in cross-section throughout its length between the dialyzate entrance and exit ends.

5. The hollow fiber dialyzer as in claim 4 wherein each of the narrow cross-section dialyzate passageways which do not con-tain dialysis fibers is generally circular in cross-section.

6. The hollow fiber dialyzer as in claim 1 wherein said casing has a substantially rectangular cross-section.

7. The hollow fiber dialyzer as in claim 6 wherein said plurality of separate longitudinally-extending passageways have a substantially rectangular cross-section.

8. The hollow fiber dialyzer as in claim 1 wherein there are three wide cross-section passageways having a bundle of fibers, and two narrow cross-section dialyzate passageways which do not contain dialysis fibers.

9. The hollow fiber dialyzer as in claim 8 wherein the three wide cross-section passageways having a bundle of fibers are arranged in closely-packed triangular relationship, and the two narrow cross-section dialyzate passageways which do not contain dialysis fibers are located adjacent the lateral peri-phery of the elongated casing.

10. The hollow fiber dialyzer as in claim 9 wherein the elongated casing is molded of a transparent resin material.

11. The hollow fiber dialyzer as in claim 10 wherein the resin is selected from the group consisting of polystyrene and poly carbonate.

12. A hollow fiber dialyzer having an elongated tubular casing containing partitioning that divides its interior into a plurality of separate longitudinally-extending passageways, dialyzate flow directing means for receiving dialyzate from an external source and directing it to flow from one casing end to the other through one of the passageways and then back to said one casing end through another of the passageways and so to and fro lengthwise through the other passageways and finally out of the casing, at least two of the passageways each having a longitudinally-extending bundle of elongated hollow blood dialysis fibers extending lengthwise therethrough, and means connected to deliver blood to be dialyzed from an inlet to the fiber ends of each of said bundles at the same end of the casing and to receive the blood from the other ends of the hollow fibers for delivery to an outlet, character-ized in that only some but not all of the passageways have bundles of fibers and the remaining passageways do not contain dialysis fibers, and the dialyzate flow directing means directs dialyzate to flow from one casing end to the other in alternating fashion through the passageways having bundles of fibers and the passageways which do not contain fibers so that the dialyzate flow through each of the passageways having bundles of fibers is always in the same lengthwise direction.

13. The hollow fiber dialyzer as in claim 12 wherein the passageways having bundles of fibers are of relatively wide cross-section and the passageways which do not contain fibers are of relatively narrow cross-section.

14. The hollow fiber dialyzer as in claim 13 wherein the lengthwise direction of dialyzate flow through the wide cross-section passageways having bundles of fibers is opposite to the direction of blood flow through the hollow fibers from the blood inlet to the blood outlet.

15. The hollow fiber dialyzer as in claim 12 wherein said casing has a substantially rectangular cross-section.

16. The hollow fiber dialyzer as in claim 15 wherein said plurality of separate longitudinally-extending passageways have a substantially rectangular cross-section.

17. The hollow fiber dialyzer as in claim 12 wherein the passageways having bundles of fibers are of relatively wide cross-section, and the remaining passageways do not contain dialysis fibers and have a cross-section which is circular and which also is sufficiently narrow to cause the flowing dialyzate to sweep out gas bubbles that may form in the dialyzate.

18. The hollow fiber dialyzer as in claim 17 wherein the passageways having bundles of fibers are generally circular in cross-section throughout their length between the dialyzate entrance and exit ends.