Abstract:
A method of sterilizing and drying a dialyzer or other device, wherein a vacuum drying phase is interrupted by interspersed pulses of air or other gas. The pulses lessen the evaporative cooling effect, to thereby reduce differential shrinkage that can structurally damage the dialyzer.

Description:
FIELD OF THE INVENTION  
     This invention relates to the field of dialyzers typically used in hemodialysis and related medical procedures, and in particular to a process for manufacturing a dialyzer with a strong and integral bond between a potting material holding dialysis fibers and a dialyzer housing. 
     BACKGROUND OF THE INVENTION  
     Patients with kidney disease suffer from the adverse effects of toxin build-up in their blood. Dialysis is a process which employs an artificial kidney to remove those toxins. In hemodialysis a dialyzer is used which contains a semi-permeable membrane dividing the dialyzer into two chambers. Blood is pumped through one chamber and a dialysis solution through the second. As the blood flows by the dialysis fluid, separated by the semipermeable membrane, blood impurities such as urea and creatinine diffuse through the semi-permeable membrane into the dialysis solution by the diffusion, convection and absorption. The electrolyte concentration of the dialysis fluid is set so as to maintain electrolytic balance within the patient. 
     The semi-permeable membrane is often a large number of microfibers encased in a chamber. The chamber is often a hollow cylinder open at both ends. Thousands of hollow semipermeable microfibers carry blood from one end to the opposite end so that blood flows through the microfibers in a first direction. Dialysate ports are also present on opposite ends of the chamber. One port carries dialysate into the chamber, the dialysate flows through the chamber in a countercurrent direction to the blood flow, and the other port carries the dialysate out of the chamber. The solute removal thus takes place across the semipermeable membrane that is the microfiber wall. This design produces a high surface area for solute removal in a relatively low volume device. 
     One significant challenge is to connect the microfiber interior channels to the blood lines, so that blood flows smoothly from the arterial blood line, into the microfiber interior channels where it can pass into and through the dialyzer chamber, and out the other end of the microfiber interior channels to the venous blood line. 
     This is done by potting the microfiber ends with a potting material. The potting material encases the microfiber ends to seal them from the chamber interior and to hold them in position within the chamber. The end result is a dialyzer in a generally cylindrical shape with a hollow interior chamber. Ports at each end of the chamber allow passage of dialysate therethrough. Within the chamber are the microfibers, positioned longitudinally so that their ends extend to the respective chamber ends. The blood passes through the lumens of the microfibers in the opposite direction from the direction of dialysate movement through the dialyzer chamber. The blood is thus separated from the dialyzer by the microfiber walls. At each end of the chamber, the microfiber lumens are open to the exterior of the dialyzer, which is fitted with the blood lines to carry blood between the dialyzer and the patient. The microfibers are held in place, and the dialyzer chamber is sealed from its ends, by the potting material. 
     This potting material is typically epoxy or urethane. It is injected into the dialysate ports on each end of the chamber, and the dialyzer is spun in a centrifuge. The centripetal force produced by the rotation in the centrifuge forces the potting material to each end, where it sets and hardens. 
     One of the manufacturing steps for a dialyzer, like many other medical devices that come into contact with body fluid, is sterilization. It is important that the dialyzer be free of pathogens that could migrate into the blood flowing through the microfiber lumens and thereby enter the patient&#39;s bloodstream. 
     A common method of sterilizing medical devices is by heated steam in an autoclave. Hot steam is introduced to kill pathogens to an acceptable level, and then the steam is removed and the device is allowed to cool. The removal of the steam is facilitated by a vacuum drying cycle; a vacuum is applied which draws off the remaining steam and any condensate. 
     It has been discovered that steam autoclaving as a method for sterilizing a dialyzer can affect the structural integrity of the dialyzer. Specifically, dialyzers subjected to steam autoclaving often show delamination between the potting material used to pot the microfibers in place and the dialyzer housing. The result may be a gap between the potting material or at least a structural discontinuity. 
     Investigation reveals that the reason for this delamination effect upon steam autoclaving may be two-fold. First, the potting material and the dialyzer housing typically have different coefficients of thermal expansion because they are made of different types of materials. The potting material is commonly a polyurethane, while the dialyzer housing is commonly a polycarbonate. The decrease in temperature at the end of autoclaving produces differential shrinkage between the two materials. This differential shrinkage produces strain at the interface between the materials which, at least in some cases, causes delamination or structural discontinuities. 
     Second, investigation suggests that the rate of cooling is different in the potting as compared to the dialyzer housing. This appears to be a result of evaporative cooling in the potting material. Autoclave steam condenses or collects to some extent on the surfaces of the dialyzer. Condensate not only collects but also is absorbed into the absorptive microfibers. Upon application of the drying vacuum, the condensate evaporates at different rates from the absorptive material of the microfibers compared to the nonabsorptive materials. The condensate evaporates fastest from the nonabsorptive material, since all the condensate is at the surface and can freely vaporize into the partial vacuum. In comparison, the condensate evaporates into the partial vacuum slower from the absorptive material of the microfibers, because some of the condensate must migrate from the interior of the material to the exterior surface where the molecules can evaporate. 
     Because the condensate evaporates slowest from the absorptive microfibers, and perhaps because the absorptive microfibers hold a relatively large amount of condensate, they stay moist the longest. At a point in the vacuum drying process, the nonabsorptive surfaces will be essentially dry while the absorptive microfibers are still moist. Continued application of the vacuum drying process beyond this point is necessary to dry the microfibers. At this stage, there is little or no evaporative cooling effect on the dry nonabsorptive surfaces, but there continues to be an evaporative cooling effect on the absorptive microfibers. 
     This differential evaporation produces differential cooling between the absorptive microfibers and the nonabsorptive materials. Early in the vacuum drying, when there is still condensate on the nonabsorptive materials, they experience relatively large rates of evaporation and consequently relatively rapid cooling. However, this cooling, while rapid, is very short-lived because the nonabsorptive materials have very little condensate on them to begin with. Later in the vacuum drying process, after the nonabsorptive material is essentially dry, the nonabsorptive material experiences relatively no evaporation and consequently relatively slow cooling. 
     In a typical scenario, the temperature gradients are as follows. The temperature of the entire dialyzer is high and uniform at the conclusion of the steam autoclaving. Upon application of the vacuum dryer process, the temperature of the nonabsorptive materials initially falls quickly to a temperature less than that of the absorptive microfiber due to rapid evaporation from those materials. Once the condensate is fully evaporated from the nonabsorptive materials, however, the temperature of the nonabsorptive materials stabilizes. Continued evaporation from the absorptive materials continues to cool them, so that their temperature becomes as low as the temperature of the nonabsorptive materials, and then even lower. 
     These temperature gradients due to differential evaporative cooling are mitigated slightly but not fully by other cooling mechanisms. For example, both the nonabsorptive materials and the absorptive materials cool by radiation. They also cool by conduction; that is, the surfaces lose heat to colliding molecules that pass by. This conductive cooling is of little effect, however, since it operates in a high vacuum; there simply are too few passing molecules. 
     The temperature difference in the absorptive microfibers, as described above, is reflected as well in the potting material into which they are potted. In other words, due to heat conduction through the potting material and the microfibers, the potting material tends to be warmer than the other nonabsorptive materials early in the vacuum drying. Later in the vacuum drying, the potting material tends to be cooler than the other nonabsorptive materials. This appears to be a significant reason for the differential cooling—and resulting differential shrinkage—between the dialyzer housing and the potting material during vacuum drying. 
     SUMMARY OF THE INVENTION 
     The present invention addresses the differential shrinkage between the dialyzer housing and the potting material. Rather than applying a continuous vacuum during the vacuum drying phase, the vacuum is interspersed with air or other gas at an atmospheric pressure. This has the effect of reducing the temperature differential between the dialyzer and the potting material in two ways. First, the air at atmospheric pressure has a much greater density of air molecules than are in the vacuum. These molecules produce conductive cooling in the nonabsorptive material of the dialyzer housing, thereby lessening the temperature differential between the dialyzer housing and the potting material. 
     Second, the atmospheric pressure lessens the evaporation from the microfibers, since a liquid evaporates into a higher pressure gas slower than it evaporates into a lower pressure gas. This interspersed atmospheric pressure thus reduces the rate of evaporation—and consequent evaporative cooling—from the microfibers at the same time that it increases the cooling of the dialyzer housing. 
     The reduced cooling rate in the microfibers produces a reduced cooling rate in the potting material into which their ends are embedded. The overall effect is a reduction in the temperature differential between the potting material and the dialyzer, so as to lessen the differential shrinkage. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a schematic representation of a dialyzer in accordance with the present invention. 
         FIG. 2  shows a graph of pressure in a vacuum dryer as a function of time in accordance with a preferred embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A preferred embodiment of a dialyzer  10  in accordance with the present invention is shown in  FIG. 1 . The dialyzer  10  includes a housing  12 , preferably having a generally cylindrical shape with a hollow interior which defines a dialysate chamber. The ends of the housing  12  have dialysate ports  23  and  25  in fluid communication with the chamber, so that dialysate can flow from one port  23 , into the interior chamber of the housing  12 , through the chamber, and out the other port  25 . 
     Extending longitudinally within the chamber of the housing are a set of microfibers  18 . The microfibers are held in place at their opposite ends by potting material  26  and  28 . The potting material is preferably polyurethane or epoxy. It is installed by injecting it in liquid form into the chamber through one or both dialysate ports,  23  and  25 , and then centrifuging the dialyzer  10  about an axis perpendicular to its longitudinal axis. The centripetal force produced by the centrifuge pushes the liquid potting material to the opposite ends of the chamber, where itf sets and hardens in the general configuration shown as  26  and  28 . The outcome of this procedure is a dialyzer  10  in which the microfiber lumens are completely isolated from the chamber, except via passage through the semipermeable membrane of the microfiber walls. 
     In a preferred embodiment, the dialyzer housing is constructed of a durable molded material such as a polycarbonate. In particular, polycarbonate is a suitable material. The potting material is a polyurethane or an epoxy. Many other materials are feasible. 
     The ends of the dialyzer  10  are then cut or sawn off through the potting material  26  and  28  in order to expose the microfiber lumens to the exterior. Fittings (not shown) attach to each end of the dialyzer ends which connect to the blood lines, namely the venous line and arterial line (not shown). Fluid communication is thus established from an arterial line proximate the dialyzer outlet port  25 , into the microfiber lumens, through the microfiber lumens within the dialyzer chamber, out of the dialyzer  10  and into a venous line proximate the dialyzer inlet port  23 . This general design of dialyzers and method for manufacturing them is known in the art and is not further described here. 
     In operation, blood flows from a patient into the arterial line and then into the microfiber lumens at the dialyzer  10  proximate the dialysate outlet  25 . The blood then flows through the microfiber lumens and out the other end of the dialyzer  10  into a venous line, and back to the patient. Simultaneously, dialysate flows into the dialyzer chamber from the dialysate inlet port  23 , through the chamber in the direction opposite the direction of blood flow through the microfiber lumens, and out through the dialysate outlet port  25 . 
     As the blood flows in one direction through the microfiber lumens and the dialysate flows in the opposite direction in the chamber, blood impurities pass from the blood in the microfiber lumens to the dialysate by migrating through the semi-permeable membrane that is the microfiber wall. At the same time, desirable nutrients, drugs or other substances added to the dialysate will pass from the dialysate into the blood, also through the semipermeable membrane that is the microfiber wall. Appropriate dialysis treatment is accomplished by varying the duration of the treatment, the fluid flow, and the make-up of the dialysate in such a system, all in a manner generally familiar to those skilled in the field. 
     After the dialyzer  10  is manufactured and before it is used, it is sterilized in a steam autoclave device. Such a device ordinarily includes a sterilization chamber in which both the nature of the chamber gas, and its pressure, are controllable. The sterilization medium is usually steam. The steam is initially introduced in pulses in order to remove entrapped air from the dialyzer, and then the steam is allowed to reside in the chamber for a dwell time to destroy any remaining pathogens. 
     The steaming process is shown in the graph of  FIG. 2 , which depicts the chamber steam pressure as a function of time. It can be seen that the chamber pressure is initially at 0 bar, i.e., atmospheric pressure. An initial set of three steam pulses is then applied to remove the entrapped air. In each pulse, the steam pressure rises to approximately positive 0.7 bar, then is reduced to a partial vacuum of approximately negative 0.7 bar, and then is increased again to approximately positive 0.7 bar. This pulse is applied three times, as shown in  FIG. 2 , or such greater or lesser number of times as may be necessary to achieve substantially complete removal of air entrapped in the dialyzer. Each such pulse takes approximately eight minutes in a preferred embodiment. 
     The steam pressure is then increased at a relatively high level for a dwell time of approximately 15 minutes. The dwell pressure is about 1.1 bar in the preferred embodiment. During this phase, the steam is at a temperature sufficient to provide a 12 Fo level of sterilization. Other sterilization levels may be used for various applications, including sterilization levels of about 10 or 14 Fo. In a preferred embodiment, a sterilization level of 12 Fo is obtainable under a dwell time of 15 minutes if the steam is heated to approximately 121°. 
     The final portion of the vacuum drying process is to provide a series of vacuum pulses to remove the steam and any condensate from the dialyzer. Each vacuum pulse begins with the application of a negative pressure, to approximately negative 0.9 bar in a preferred embodiment. The pressure is then increased, by adding air or another inert gas slowly for about 2 minutes, and quicker for a period of less than a minute, to bring the pressure up to about negative 0.3 bar where it is maintained for about a minute. This cycle is repeated for a number of times until the dialyzer is essentially dry. In a preferred embodiment this typically requires about nine such cycles or “pulses.” As used in defining pressure levels in the claims herein, the word “approximately” means within 10 percent. 
     As described above, the application of a vacuum to the dialyzer after the steam autoclaving serves to remove residual gaseous and liquid water. If the vacuum were applied continuously, however, differential cooling, and consequent differential shrinkage, would result between the dialyzer housing and the potting material. This is because the potting material reserves the absorbent microfibers which are subjected to evaporative cooling long after the dialyzer housing has become dry. 
     Interspersing the vacuum with pulses of a lesser vacuum achieved by introducing air or some other gas reduces this differential cooling. 
     There are at least two reasons for this. One is that the air or other gas conductively cools the dialyzer housing by passing it on the way in and out of the chamber. The dialzyer housing cooling thus tends to “catch up” with the potting material cooling. The other reason is that the reduced vacuum during the period when the air or other gas is introduced lessens the evaporative cooling from the microfibers which are potted by the potting material. This is because moisture evaporates into a low vacuum quicker than into a high vacuum. 
     Although this process is described in the context of a dialyzer it will be appreciated that it is equally applicable to any other medical device or other device in which there are materials having differing coefficients of thermal expansion and differing absorptive properties, on which pathogens are destroyed by steam. Such devices may include, for example, blood filters, aphaeresis devices, and other extracorporeal devices such as line sets.