TECHNICAL FIELD

The present invention concerns a novel process for making a fiber bundle. The illustrative embodiment concerns a novel process for making a hollow fiber mass transfer device bundle and also concerns a novel mass transfer device bundle.

BACKGROUND ART

In United States Patent No. 3,794,468, issued February 26, 1974, in the name of Ronald J. Leonard for "Mass Transfer Device Having a Wound Tubular Diffusion Membrane" and assigned to the assignee of the present invention, a mass transfer device is disclosed. The mass transfer device disclosed in that patent is made by winding a length of a hollow tubular conduit of semipermeable membrane about a core to form a wound bundle similar to packages of kite string, in which in¬ dividual adjacent windings in the same layer are generally parallel to each other, but individual adja¬ cent windings of the conduit in adjacent layers define an angle to each other. The device of that patent is assembled by winding a length of the hollow tubular conduit of semipermeable membrane about a core in a plane which defines an acute angle to the longitudinal axis and intersects both ends of the core. Si ultane-- ously, the core is rotated about its longitudinal axis, or the plane of winding is correspondingly rotated to the same effect, to laterally displace on the core each winding of conduit from its immediately preceding wind¬ ing. The resulting wound structure allows a low cost construction of a high performance mass transfer de- vice. The flow pattern is around the circumference of the cylindrical element formed by the winding process. However, as the flow goes around the circumference, it will encounter flow paths of different length. Addi¬ tionally, since the core is rotated at a constant rota- tional velocity and the angular velocity of the wind is constant, the void fraction of the resulting structure increases radially outwardly of the bundle. In other words, the void fraction near the outside of the bundle will be greater than the void fraction on the inside of the bundle. As used herein, the term "void fraction" connotes the ratio of space to space and fiber, whereby

a greater void fraction means more space.

We have discovered that performance of the mass transfer device is decreased if the void fraction near the outside of the bundle is substantially greater than the void fraction near the inside of the bundle. If the mass transfer device is an oxygenator, for example, having a void fraction on the outside that is substan¬ tially larger than the void fraction on the inside results in a much larger blood flow rate on the outside because there is more resistance on the inside. As the void fraction on the outside becomes smaller, there is less flow rate on the outside. However, since the flow path on the outside is inherently longer than the flow path on the inside, if the void fraction were equal throughout the bundle, the flow rate on the outside would not ^' be equal to the flow rate on the inside but the blood flow would be unequal because of the longer flow path on the outside.

We have discovered than an optimum hollow fiber oxygenator comprises a bundle in which the flow rate is substantially constant throughout the bundle, the blood outlet saturation is substantially constant throughout the bundle, and the void fraction increases slightly in the radial outward direction of the bundle. The pres- ent invention provides a novel process for obtaining this optimum device and also concerns a novel device having these optimum properties.

Although the illustrative embodiment of the inven¬ tion relates to a hollow fiber mass transfer device such as an oxygenator, it is to be understood that the present invention may be applicable to other devices using other fibers. For example, the present invention could be used with non-permeable tubing such as em¬ ployed in a heat exchanger; Further, the present invention could be used with an absorption filter, us¬ ing an absorbant filter material such as polyesters or

natural fibers. Other impregnated fibers could be em¬ ployed in connection with the present invention. DISCLOSURE OF THE INVENTION

In accordance with the present invention, a proc- ess is provided for making a fiber bundle. The process comprises the steps of providing a core having a longi¬ tudinal axis, winding a length of the fiber around the core from an arm in a plane that is at an acute angle with respect to the longitudinal axis of the core and is between opposite ends of the core, providing rela¬ tive rotation between the longitudinal axis of the core and the position of the winding arm, and, during wind¬ ing of the fiber around the core, changing the ratio of (a) the angular velocity of the wind and (b) the rela- tive rotational velocity between the longitudinal axis of the core and the position of the winding arm.

In the illustrative embodiment, the ratio-changing step comprises* the step of increasing the ratio during winding whereby the void fraction of radially-outward annuli of the wound bundle is less than the void frac¬ tion would be if the ratio remained constant.

In the illustrative embodiment, the outer 10 per¬ cent annulus of the bundle is wound at a ratio that is at least 5 percent greater than the ratio at which the inner 10 percent annulus is wound.

In the illustrative embodiment, the process is used for making a hollow fiber mass transfer device. The mass transfer device comprises a core with a hollow fiber bundle overlying the core and in symmetrical re- lationship to the longitudinal axis of the core.

A more detailed explanation of the invention is provided in the following description and claims, and is illustrated in the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a perspective view of a mass transfer device bundle, prior to its being enclosed within a

housing, constructed in accordance with the principles of the present invention.

Figure 2 is a diagrammatic cross-sectional view of a wound bundle. Figure 3 is a perspective view of a core and wind¬ ing mechanism.

Figure 4 is a perspective view of a core for the purpose of showing a winding layout.

DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENT

Referring to Figure 1, a core 10 having a bundle

12 formed by winding a hollow fiber around core 10 is illustrated. Figure 1 shows bundle 12 in a condition after the hollow fiber has been wound around the core 10, sealant bands 14 have been applied around the bun¬ dle, and the bundle has been cut transversely to form ends 16 and 18, as is disclosed in United States Patent No. 3,794,468. In Figure 1, bundle 12 is ^" formed of a hollow* fiber of semipermeable membrane that is wound about a core 10 to form a multi-layer winding of the fiber in which individual adjacent windings in the same layer are generally parallel to each other, but indi¬ vidual adjacent windings of the fiber in adjacent layers define an angle to each other. The ends of the core transverse to the fiber windings are then potted with a curable sealant. After cure of the sealant, the ends of the wound core are cut transversely through me¬ dial portions of the cured sealant to sever the fiber into a plurality of individual fibers, the resulting sections being secured together at their ends by por¬ tions of the cured sealant, and having their hollow bores open to the exterior. This structure is illus¬ trated in Figure 1. Thereafter, manifolds are applied to provide an oxygen flow path through the hollow bores of the fiber portions and a blood flow path between the exteriors of the fiber portions.

Figure 2 is a diagrammatic view of a cross-section of the bundle of Figure 1, with seven different annuli of the bundle being designated with the numbers 1 to 7.

Annulus 1 is the innermost annulus of the bundle while number 7 is the outermost annulus of the bundle.

In prior art constructions, the bundle of Figure 1 has been wound with a constant winding ratio. As used herein, the term "winding ratio" connotes the ratio of (a) the angular velocity of the wind and (b) the rela- tive rotational velocity between the longitudinal axis of the core and the position of the winding arm. With a constant winding ratio, the void fraction increases radially outwardly in the bundle. Thus referring to Figure 2, if the bundle is wound with a constant wind- ing ratio, annulus 2 will have a greater average void fraction than annulus 1, annulus 3 has a greater void fraction than annulus 2 , annulus 4 has a greater void fraction than annulus 3, etc. In addition, with a c _on- stant winding ratio the flow rate increases radially outwardly in the bundle. The blood saturation percent¬ age decreases outwardly in the bundle and the pressure drop remains substantially constant throughout the bun¬ dle.

The following chart shows the void fraction, flow rate, blood saturation percentage, and pressure drop with respect to the numbered annuli in Figure 2, with a constant winding ratio of 360:1.

ANNULUS WINDING VOID FLOW SATURATION PRESSURE

NO. RATIO FRACTION RATE PERCENT DROP

1 360:1 .5861 9.75 111.3 122

2 360:1 .6044 11.17 108.4 122

3 360:1 .6209 12.66 107.8 122

4 360:1 .6362 14.17 107.4 122

5 360:1 .6504 15.78 105.6 122

6 360:1 .6636 17.43 104.4 122

7 360:1 .6756 19.04 104.0 122

We have discovered that by increasing the winding ratio during winding of the bundle, a more effective mass transfer device can be provided.

In Figure 3, an apparatus is illustrated for wind- ing the bundle in accordance with the principles of the present invention. Referring to Figure 3, the appara¬ tus comprises a winding arm 20 that is driven by pulley 22 coupled through belt drives 24 and 26 which are con¬ nected to pulley 28 that is keyed to the shaft 30 of motor 32. Arm 20 is hollow and a hollow fiber 34 from a suitable source (not shown) extends through arm 20 and out its distal end 36.

A core 38 having hollow end extensions 40, 42 is positioned on a spindle 44 with the end extensions 40, 42 being coaxial with the spindle 44 and the axis of the cylindrical core 38. Spindle 44 is driven by a suitable driving means, including a variable speed mo¬ tor 42. The spindle and -hence the core axis are at an acute angle with respect to the plane of the wind of winding arm 20, preferably 2.5 . In this manner, with reference to Figure 3, when arm 20 is rotated, the wind will travel from point 1 which is in a plane on one side of extension 42 to point 2 which is in a plane on the opposite side of extension 42, to point 3 which is in a plane on the same side of extension 42 as point 2, and to point 4 which is in a plane on the same side of extension 42 as point 1. The resulting wind is illus¬ trated in more detail in Figure 4. This wind is contintued until an appropriately thick bundle is formed. The manner of winding is illustrated in Figure 3.

Referring to Figure 3, winding arm 20 moves in the direction of the arrow 46 about the axis of portion 48 of winding arm 20. The core rotates in the direction of arrow 50 about the axis of spindle 44 which, as stated above, is at an acute angle with respect to the

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direction of the wind and thus is not perpendicular to the axis of portion 48. In the illustrative embodi¬ ment, extensions 40, 42 have a .65 inch diameter and the core has a 1.975 inch outer diameter with a 9.95 inch length, as indicated in Figure 3. The winding continues until the bundle has an outer diameter of 2.626 inches, as indicated in Figure 2.

In the illustrative embodiment, the rotational ve¬ locity of spindle 44 decreases during the winding of the bundle, while the angular velocity of arm 20 re¬ mains constant and the position of the winding arm drive mechanism remains constant. However, there are various ways of changing the ratio of (a) the angular velocity of the wind and (b) the relative rotational velocity between the longitudinal axis of the core and the position of the winding arm. For example, the winding arm drive may move, in a planetary manner, about the core, with " a decreasing planetary motion during winding. This will increase the ratio of (a) the angular velocity of the wind and (b) the relative rotational velocity between the longitudinal axis of the core and the position of the winding arm. Addi¬ tionally, the rotational velocity of the core may be maintained constant while the position of the winding arm drive is maintained constant while the angular ve¬ locity of the wind is increased during the winding of the bundle. It can be seen that there are many varia¬ tions available for changing the ratio of (a) the angular velocity of the wind and (b) the relative rota- tional velocity between the longitudinal axis of the core and the position of the winding arm.

The following chart is correlated with Figure 2 and indicates the appropriate average winding ratios for the various annuli of the bundle, to achieve a con- stant void fraction throughout the bundle:

ANNULUS WINDING VOID ELCW SATURATION PRESSURE

NO. RATIO ^• FRACTION RATE PERCENT DROP

1 304:1 .65 16.2 97.4 99.2

2 319:1 .65 15.4 99.6 99.2

3 332:1 .65 14.8 101.8 99.2

4 346:1 .65 14.2 104.0 99.2

5 360:1 .65 13.6 106.3 99.2

6 375:1 .65 13.1 108.6 99.2

7 388:1 .65 12.7. 111.0 99.2

However, this bundle is not deemed optimum because the flow path lengths are more different than desired. A more ideal bundle has been developed in which the void fraction is not constant but is less variable throughout the bundle than would be if the winding ra¬ tio remained constant, the flow rate is substantially constant, and the blood saturation percentage is sub¬ stantially constant. This more ideal bundle is wound using the following average winding ratio for the cor¬ responding annuli of Figure 2.

ANNULUS WINDING VOID FLOW SATURATION PRESSURE NO. RATIO FRACTION RATE PERCENT DROP

1 333:1 .6175 14.3 103.9 99.9

2 338:1 .6291 14.3 103.9 99.9

3 342:1 .6400 14.3 103.9 99.9

4 346:1 .6501 14.2 103.9 99.9

5 351:1 .6595 14.3 103.9 99.9

6 355:1 .6682 14.3 103.9 99.9

7 359:1 .6765 . 14.3 103.9 99.9

It can be seen that a process has been disclosed for making a bundle in which the flow rate is substan¬ tially constant throughout the bundle, the blood outlet saturation is substantially constant throughout the bundle, and the void fraction increases slightly in the radially outward direction of the bundle. It is preferred that the outer ten percent annulus of the

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bundle be wound at a winding ratio that is at least five percent greater than the ratio at which the inner ten percent annulus is wound.

In the charts set forth above, the flow rate is in cubic centimeters per second and the pressure drop is in millimeters of mercury.

The invention is applicable to other devices using other fibers. Although illustrative embodiments of the invention have been shown and described, it is to be understood that various modifications and substitutions may be made by those skilled in the art without depart¬ ing from the novel spirit and scope of the present invention.