BACKGROUND Mechanical ventricular assist device pumps appear to be a promising method of augmenting the blood pumping capacity of individuals with weak hearts. Various designs of these ventricular assist device pumps have demonstrated much promise in experimental use in animals and in limited use in humans. They can be used as left ventricular assist devices (LVAD's), right ventricular assist devices (RVAD's) or biventricular assist devices (BiVAD's). A description of these pumps and their typical operational characteristics is provided in an article entitled "In Vivo Evaluation of the Nimbus Axial Flow Ventricular Assist System, Criteria and Methods" (Antaki et al., ASAIO Journal 1993, pp. M231-M236). A common difficulty with these designs, however, is in their connection to the human vascular system. The transition from blood flow from the rigid metal pump to the living vascular system must be made smoothly and accomplished with biocompatible materials that provide adequate flexibility and allow for an effective and quickly made connection to the vascular system. Prior methods using tubing of porous polytetrafluoroethylene (PTFE) and polyethylene terephthalate (PET) fabric have not been able to make this transition smoothly and with adequate flexibility, allowing the accumulation of thrombus and release of emboli at the transition between the rigid pump and the connecting tubing and thereby compromising the effectiveness of the ventricular assist device. PET has been most commonly used as the tubing material. It is disadvantageous in that its use can result in erosion both of itself and of the surrounding tissue; further, it is also vulnerable to the spread of bacterial infection.

One such previous connector is available from TCI Heartmate (Woburn, MA). It comprises a length of corrugated PET tubing attached by swaging to a tubular fitting of sintered powdered titanium. The connector incorporates an abrupt step of about 0.5 mm between the PET tubing and the smaller inside diameter titanium fitting. Such a step is known to adversely affect the flow of blood through the assembly. Further, the connector is

retained to the pump by a removable securing ring which could be inadvertently removed by a surgeon SUMMARY OF THE INVENTION The present invention relates to a method of attaching a porous PTFE tube to a metal fitting wherein the liquid flow surfaces of the connection are substantially smooth with very little interference to liquid flow through the fitting and porous PTFE tube. More particularly, the invention relates to a porous PTFE tubing connector affixed to a rigid tubular inlet or outlet fitting on a mechanical ventricular assist device pump. As such, the connector allows for an effective connection of the pump to the living vascular system by providing a flexible and biocompatible length of tubing which allows for anastomosis to the vascular system by conventional means, while simultaneously providing for the smooth flow of blood from the ventricular assist device pump by virtue of the minimal interference to flow at the connection between the porous PTFE tubing and the metal ventricular assist device pump.

The connector is made by fitting an end of a tube of porous PTFE over the end of a rigid tubular fitting such that the ends of the fitting and tubing overlap for a short length (e.g., 2 cm, measured in an axial direction). The end of the rigid tubular fitting should be tapered to a thin section at the edge that provides the transition to the porous PTFE tubing at the interior flow surfaces of the connector. A mandrel is then placed into the tube and fitting assembly which fits snugly into the inside diameters of both of those components in the transition area. The end of the porous PTFE tubing that is fitted over the end of the rigid tubular fitting is secured to the fitting by wrapping a length of porous expanded PTFE film helically or circumferentially around the overlapping tubing and fitting; preferably an additional length of the tubing is also wrapped. The porous expanded PTFE film is a fibrillated film wrapped so that the fibrils are oriented circumferentially around the exterior of the porous PTFE tubing. The film is thermally bonded to the exterior of the tubing by applying heat in excess of the crystalline melt temperature of the PTFE; this heat also results in shrinkage of the circumferentially-wrapped film thereby providing a secure connection between the tubing and the fitting. This method can be used to avoid the creation of local areas of high stress which would result from the use of mechanical connection methods such as crimping. The rigid tubular fitting preferably incorporates surface irregularities such as annular grooves or ridges on its exterior surface which is overlapped by the porous PTFE tube; these surface irregularities further improve the integrity of the join between tubing and fitting. According to another alternative, the exterior surface of the rigid tubular fitting may be tapered so that the exterior surface of the fitting nearest the pump has a smaller outside diameter than a portion of the exterior surface of the

fitting closer to the end of the fitting where the flow surface transitions to the porous PTFE tube. In another alternative, an adhesive such as a thermoplastic adhesive (e.g., fluorinated ethylene propylene) may be used between the overlapping surfaces of the rigid tubular fitting and the porous PTFE tube.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 describes a schematic view of a ventricular assist device pump with the graft-connector of the present invention, implanted in a human.

Figure 2 describes a perspective view of the connector of the present invention wherein the porous PTFE tube is fitted with exterior reinforcement in the form of a multiplicity of rings fitted to a portion of the exterior surface of the porous PTFE tube.

Figure 3 describes a perspective view of the connector of the present invention wherein the porous PTFE tube is fitted with exterior reinforcement in the form of a helically wound structure fitted to a portion of the exterior surface of the porous PTFE tube.

Figure 4 describes a perspective view of the connection between the porous PTFE tube and the rigid pump tubular fitting.

Figure 5 describes a longitudinal cross section of the connector of the present invention incorporating a helical wrapping of porous PTFE film.

Figure 5A describes a longitudinal cross section of the connector of the present invention incorporating a circumferential wrapping of porous PTFE film.

Figures 6 and 6A describe longitudinal cross sections of alternative connectors of the present invention wherein the rigid tubular fitting is provided with surface irregularities in the form of annular rings or grooves.

Figures 7 and 7A describe longitudinal cross sections of alternative connectors of the invention wherein the rigid tubular fitting is provided with an exterior surface having a taper or a step.

Figure 8 describes a longitudinal cross section of the invention wherein an adhesive is provided between the rigid tubular fitting and the porous PTFE tube.

Figures 9 and 9A describe enlarged longitudinal cross sections of the transition between the rigid tubular fitting and the tube, showing respectively the prior art and the present invention.

DETAILED DESCRIPTION OF THE INVENTION Figure 1 describes schematically the appearance of a ventricular assist device system as typically implanted in a human body to augment the pumping capacity of the

heart 10. Although a left ventricular assist device is shown, the schematic is intended to be representational of any ventricular device orientation. The inlet side 11 of mechanical ventricular assist device pump 14 is shown attached by sutures 1 3 to the left ventricle of the heart 10. The inlet side 11 is a porous PTFE tube 18 connected to pump 14 by a rigid tubular fitting 16, which collectively constitute the connection 20 of the present invention The outlet side 17 of pump 14 is attached to another length of porous PTFE tube 18. The opposite end of the porous PTFE tube 18 used for the outlet side 17 is attached to the ascending aorta 12 via a conventional anastomosis 19. Porous PTFE tubes 18 used for either the inlet side 11 or the outlet side 17 are preferably conventional vascular grafts provided with exterior structural reinforcement in the form of relatively rigid rings 21 or helical windings (spirals). Rigid tubular fitting 16 is typically made from a metal or a rigid plastic. Fitting 1 6 is conventionally a separate component which attaches to pump 14 although it may also be made integrally with the housing of pump 14. The lumen of fitting 16 may be of constant inside diameter or alternatively may be of variable (e.g., tapered) inside diameter.

Various alternative constructions of this system are sometimes used. For example, the inlet of the ventricular assist pump may be connected directly to the left ventricle of the heart without the use of an inlet tube. The outlet tube may be attached at its outlet end to the descending aorta rather than the ascending aorta.

As shown by Figures 2 and 3, the porous PTFE tube used to create connection 20 is preferably provided with exterior structural reinforcement in the form of a multiplicity of rings 21 or spirals 23. The use of such exterior structural reinforcing members prevents the tube 18 from being crushed or collapsed due to external pressures or due to a low pressure condition within the tube. The rings or spirals may be made from a biocompatible polymer which is preferably non-porous such as PTFE or fluorinated ethylene propylene (FEP); exterior structural reinforcement of this type is described, for example, by US Patent 5,556,426 to Papadiuk, et al. Vascular grafts of various inside diameters using rings as exterior structural reinforcement (which may optionally be removable rings) are commercially available as Ringed GORE-TEX Vascular Grafts from W.L. Gore and Associates, Inc.

(Flagstaff, Arizona). An alternative exterior structural reinforcement may be made integral to the porous PTFE tube in a manner taught by US Patent 4,550,447 to Seiler Jr. et al.

The manufacture of connection 20 is described by the perspective view of Figure 4 and the longitudinal cross sections of Figures 5-8 and 9A. A porous PTFE tube 1 8 having an inside diameter approximately equal to the inside diameter of pump fitting 16 is obtained.

Fitting 16 is provided with a means for retaining to pump 14 such as flange 49 which may be used in conjunction with a securing nut, not shown. A short length 47 of the exterior surface of fitting 16 may be left uncovered by tube 18 in order to provide room to remove the

securing nut to allow removal of connector 20 from pump 14.0ne end of tube 18 is forcibly fitted over the exterior surface of the outlet end of fitting 16, the end 46 of which is preferably provided with a tapered cross section as shown by Figure 5. The fitting of the tube may be more easily accomplished by removing any exterior reinforcing wrapping of porous PTFE film from the end of the tube 18; alternatively the end of the tube may be heated using heat less than 327 degrees C to make the tube more workable. Fitting may also be more easily effected by forcibly dilating the end of the tube a slight amount with a suitable tube dilating tool. Generally, however, the application of force alone is adequate.

After the tube 18 and fitting 1 6 have been joined with a suitable amount of overlap such as about 2 cm (measured in the axial direction), a cylindrical mandrel 43 (preferably stainless steel) of outside diameter approximately equal to the inside diameters of the tube 18 and fitting 16 is obtained and inserted into both of those components. The mandrel 43 (shown only in Figures 4 and 5) should fit into fitting 16 with a minimum of clearance. A thin film or tape of porous PTFE is then wrapped circumferentially around the exterior of the porous PTFE tube 18 in the joined region, as shown by helical wraps 41 of Figure 4. The film wrapping should extend toward the middle of the length of the tube for an additional distance beyond the overlapped region and must extend over any film wrapping originally provided with the porous PTFE tube (in order that any previously removed wrapping is replaced). Preferably, as shown by Figures 5 and 5A, the film wrapping should also extend over the end edge of tube 18 so that a small portion 45 of the film is applied to the exterior surface of the fitting 16. A suitable number of wraps must be used to provide a secure attachment. Alternatively to a helical wrapping, as shown by Figure 5A an entirely circumferential wrapping 51 (i.e., without a helical or axial component to the wrapping direction) of the porous PTFE film may be used if the film is of a suitable width in order that the entire area desired to be covered by the film is covered by that width. While the figure describes only three layers it is apparent that any suitable number of layers may be used.

The porous PTFE film for the wrapping may be made as taught by US Patents 3,953,566 and 4,187,390 to Gore; these patents are incorporated by reference herein. The film has a microstructure of fibrils oriented in the direction of the stretching performed during the manufacture of the film. When the film is cut into a narrow length (i.e., a tape) suitable for the described wrapping, these fibrils must be oriented parallel to the length of the tape.

Following completion of the film wrapping the resulting assembly is placed into an oven and heated to a temperature greater than 327 degrees C for a suitable time to cause the porous PTFE film wrapping 41, 51 to thermally bond to the exterior surface of the porous PTFE tube 18. The entire assembly may be heated or merely the region having the film wrapping 41, 51. If the entire assembly is heated, the tube will need to be longitudinally restrained against the surface of the mandrel in order to prevent it from shrinking longitudinally during

heating. The application of this heat, in addition to causing the porous PTFE film 41, 51 to thermally bond to the exterior surface of porous PTFE tube 18, will also cause film 41, 51 to shrink in length, thereby forming a tight, constricting band about the joined region and thereby resulting in a strong join between tube 18 and fitting 16. This shrinking of the shrinkable polymeric component, film 41, 51, also helps to produce a smooth transition 31 between the inner flow surfaces 33 and 35 of tube 18 and fitting 16 by forcing the inner flow surface of 35 of porous PTFE tube 1 8 against the outer surface of mandrel 43 in the region of transition 31. The porosity of tube 18 also helps to make possible this smooth transition by allowing the material of the porous tube 18 to conform appropriately to substantially fill any gap that might otherwise exist at transition 31. Subsequent to heating and allowing to cool, mandrel 43 is removed from the interior of connector 20.

Because the heating step uses heat above 327 degrees C, it is also possible to use a porous PTFE tube 18 made as taught by US Patents 3,953,566 and 4,187,390 which has not yet been heated above that temperature (i.e., not yet sintered following expansion by stretching). Such a tube is softer and more easily fitted over the outlet end of fitting 16. The sintering of this tube may then be accomplished in the same heating step responsible for thermally bonding film 41, 51.

Figures 6a and 6A describe alternative embodiments wherein rigid fitting 16 is provided with exterior surface irregularities in the form of either annular grooves 52 or annular rings 53. The use of such grooves 52 or rings 53 is preferred in that they allow a yet stronger bond to be achieved. It is believed that grooves 52 are most preferred. The number, width and depth of the grooves or rings may be optimized experimentally.

Figure 7 describes an alternative wherein the rigid tubular fitting is provided with an exterior surface irregularity in the form of a taper 61 over its exterior surface wherein the portion 65 of the exterior surface nearer the pump has a smaller outside diameter than a portion 63 of the exterior surface closer to transition 31. Figure 7A describes a similar embodiment incorporating a rigid tubular fitting having a stepped exterior surface wherein the portion 65 of the exterior surface nearer to the pump also has a smaller diameter in comparison to a portion 67 of the exterior surface closer to transition 31. These embodiments are also anticipated to offer improved adhesion between the exterior surface of the rigid tubular fitting and the overlapping porous PTFE tube.

Figure 8 describes still another alternative embodiment wherein a layer of adhesive 71 which is preferably a thermoplastic adhesive is used between the exterior surface of the rigid tubular fitting 16 and the one end of the porous PTFE tube 18 which overlaps the exterior surface of the fitting. Particularly preferred adhesives are fluorinated ethylene propylene (FEP) and perfluorinated alkoxy resin adhesives (PFA). The adhesive may be applied, for example, in the form of a tape or film wrapped around the outer surface of the

rigid tubular fitting prior to placing the end of the porous PTFE tube over the rigid tubular fitting; alternatively, a short length of thin tubing (preferably a heat-shrinkable tubing) made from the desired adhesive material may be used. The thermoplastic adhesive then melts during the previously described process of heating the porous PTFE tubing, causing the adhesive to penetrate into the void spaces of the porous tubing to help adhere the tubing to the rigid metal fitting.

Figures 9 and 9A describe enlarged longitudinal cross sectional views of the transition 31 between the rigid tubular fitting 1 6 and tube 18. Figure 9 shows transition 31 of the prior art wherein fitting 1 6 and tube 18 may be secured to each other by typical means such as a clamping ring 91 wherein the transition is made with a significant step 32, which is responsible for disturbed blood flow with the likely accumulation of thrombus and increased risk of the release of emboli. Figure 9A describes the substantially smooth transition 31 of the connector of the present invention, which offers very little interference to flow through the fitting 16 and porous PTFE tube 18.

Figure 9B is a further enlarged longitudinal cross section of transition 31 which provides a definition of what is meant by a substantially smooth transition between the fitting 16 and the tube 18. A substantially smooth transition 31 should have an offset (measurement 93) between tube 18 and the end of fitting 16 that is equal to or less than about 0.2 mm and preferably equal to or less than about 0.1 mm. The measurement 93 is made in a plane perpendicular to the longitudinal axis which intersects the end of fitting 16 at the luminal surface 33 as shown. The offset may be such that either the fitting 16 or the tube 1 8 extends furthest into the lumen at transition 31. Measurement 93 should be made by potting the lumen and the exterior of connection 20 with a machinable, transparent epoxy after which connection 20 is machined longitudinally through the longitudinal axis. This provides a suitable section which allows the offset (measurement 93) to be viewed and measured as described above using a suitable optical comparator.

EXAMPLE A titanium outlet fitting was obtained from a Nimbus AXIPUMP (Nimbus, Inc., Cordova, CA). This fitting was of about 26 mm length, 14 mm inside diameter, had a wall thickness of about 1.5 mm and an outer surface which tapered inward to the meet the inside diameter at the outlet end of the fitting. The fitting included a flange and had the general appearance of the fitting 16 of Figures 1-5b. Next, a GORE-TEX Vascular Graft was also obtained (part no. V14020,, W.L. Gore and Associates, Inc., Flagstaff, AZ). This graft was of 20 cm length and 14 mm inside diameter and was not provided with exterior structural reinforcement. One end of this graft was forcibly dilated a slight amount using a

diametrically-expanding hand tool for a length of about 2.5 cm ; this end of the graft was fitted over the outlet end of the Nimbus outlet fitting until an overlap of about 2 cm (measured parallel to the length of the tube) was achieved. A tubular stainless steel mandrel of 14 mm outside diameter was placed into the lumen of both the outlet fitting and the graft. A wrapping of porous PTFE film was then provided by a winding machine over the exterior of the end of the graft so that about 23 mm of the length of the graft was uniformly wrapped. About fifty wraps of film were used. The film used was of 2.5 cm width and 0.01 mm thickness and had a density of about 0.3 g/cc. The width of the film was placed over the end of the tube so that one 2.5 mm wide edge portion of the film overlapped the end edge of the porous PTFE tube and was therefore in contact with the outer surface of the rigid tubular fitting. The opposing edge of the film extended beyond the outlet edge of the outlet fitting by an additional 2.5 mm. The resulting assembly was then heated for about 25 minutes in a forced air oven enclosing a 15 cm length of the assembly; the oven was set at a temperature of about 380 degrees C. Following removal from the oven, the assembly was allowed to cool to about room temperature. The mandrel was then removed, completing the manufacture of the connector.

The integrity of the connection was then tested by exposing the lumen of the connector to water pressure at about room temperature. Five connectors made as described above were tested using increasing water pressure until failure occurred by either rupture of the graft or separation between the graft and the outlet fitting; mean failure value was 163.3 psi (1126 KPa) with a standard deviation of 6.7 psi (46 KPa).

An outlet connector made as described above except using a Ringed GORE-TEX Vascular Graft (Part no. R14030030L) was implanted with a Nimbus Version 2 axial flow pump in a calf for thirty days, after which it was retrieved and evaluated. The porous PTFE tubing remained tightly attached to the rigid tubular fitting. There did not appear to be any excess thrombus formation at the interface between the rigid tubular fitting and the porous PTFE tube. The neo-intima within the porous PTFE tube was typical for this material in calves, having a thickness of about 1.0 mm and a smooth, waxy appearance. The aortic anastomosis was smooth and widely patent.

While particular embodiments of the present invention have been illustrated and described herein, the present invention should not be limited to such illustrations and descriptions. It should be apparent that changes and modifications may be incorporated and embodied as part of the present invention within the scope of the following claims.