Patent Publication Number: US-2020276375-A1

Title: Gas Exchange Unit

Description:
The invention relates to a gas exchange unit and a method for producing a gas exchange unit. 
     BACKGROUND OF THE INVENTION 
     In patients with life-threatening lung conditions lung function can be maintained with an artificial lung (oxygenator or gas exchange unit) until the lung has recovered and natural lung function is reinstated. Extracorporeal circulation extracts the blood of the patient and returns the same following treatment in the oxygenator for this. 
     Most known oxygenators were developed as part of developing the heart-lung machine for use during heart surgery measures of a few hours and are mostly not optimised for long-term application. The application period that is part of respiratory support is however often clearly longer and can last a few weeks. During a long application period of a known oxygenator haemolysis and thrombus formation can for example occur due to a suboptimal flow path with high flow resistances and poorly flushed areas. The exchange capacity of such oxygenators also drops, so that these must be replaced during treatment, wherein complications and also blood loss or severe blood thinning may occur. 
     Conventional oxygenators for example consist of stacked hollow fibre mats arranged reciprocally at right angles to each other. In the edge area the mats are embedded four times with a casting compound by means of a spinning or centrifugal method, which results in a square cross-section surface, through which the blood flows. In order to distribute the blood across the fibre surface as evenly as possible perforated distribution plates are used with conventional oxygenators, which are installed in the inlet area and outlet area of the oxygenator. Such an oxygenator is for example from WO 2017/211460 A1. 
     SUMMARY OF THE INVENTION 
     It is the task of the present invention to improve the flow guidance in an oxygenator, so that the requirements of medium- to long-term respiratory support can be satisfied in a simple way. 
     The task is solved according to the invention by the characteristics of the independent claims. Further developments of the invention result from the subclaims. 
     According to the invention a gas exchange unit comprising hollow fibre mats is provided, wherein the gas exchange unit comprises an inlet that is positioned diagonally acentric. To distribute blood flow through the gas exchange unit as homogeneously as possible across the circular fibre surface, and here in particular to ensure adequate flow in the edge area, blood it rotated by means of the acentric diagonal inlet. The hollow fibre mats have a circular cross-section surface. The inlet of the gas exchange unit is arranged acentric in relation to the fibre layers of the hollow fibre mats in a further development. 
     As described below the measures according to the invention form a gas exchange unit where the distribution plates are omitted and fluid (in particular blood) flows evenly across the fibre surface formed from the hollow fibre mats with other means or measures. The gas exchange unit according to the invention has a longer application period or service life than known oxygenators. The probability of blood damage is also low with the gas exchange unit stated. 
     The hollow fibre mats of the gas exchange unit are formed from hollow fibres. The gas exchange unit can comprise one or more hollow fibre mats. The fibre orientation of a fibre mat can be arranged at an angle to the fibre orientation of a further fibre mat. Crossing the fibre mats can improve the gas exchange characteristics as well as blood guidance. 
     The gas exchange unit is also described as an oxygenator. An oxygenator is an apparatus where blood can be enriched with oxygen and carbon dioxide removed from the blood. The lung can thus be replaced or supported by means of the oxygenator in the short term as well as for longer periods. 
     The gas exchange unit envisages in one further development that the inlet is arranged on the facing side on a casing of the gas exchange unit. The arrangement of an inlet on the facing side of the gas exchange unit allows a homogeneous flow through the hollow fibre mats. The casing of the gas exchange unit has an inlet casing. 
     According to a further development of the gas exchange unit it is envisaged that the inlet is connected with the inlet casing, the surface of which has structures. Suitable structures on the inner, blood conveying surface of the inlet casing further distribute the flow as homogeneously as possible. The inlet is positioned diagonally here when an angle between a central axis of the inlet and a level that includes the inlet casing is larger than 0° and/or smaller than 90°. 
     In a further development of the gas exchange unit it is envisaged that the structures are vane-shaped, bridge-shaped or arranged as cross members on the surface of the inlet casing. The structures, which are vane-shaped, bridge-shaped or arranged in as cross members, can distribute the flow across or the supply of the fibre surfaces in the gas exchange unit evenly. The flow is consistent in the inner as well as the outer area of the hollow fibre mats. 
     According to a further development of the gas exchange unit it is envisaged that the inlet has a cross-section surface that widens continuously in flow direction, so that the flow speed is reduced slowly and not suddenly. The ratio of dD/2dL&lt;1 between diameter (dD) and run length (dL) of the inlet is advantageous. (In other words, the ratio between the radius of the inlet and the run length is less than 1). This ratio changes across the run length. It can also change gradually in the area of the inlet. 
     According to a further development a diameter widening of the inlet is less than 45° across the run length. 
     According to a further development of the gas exchange unit it is envisaged that the cross-section surface of the inlet widens asymmetrically. The widening can be symmetrical in order to guide the flow in a targeted way and thus distribute it homogeneously. 
     In a further development of the gas exchange unit it is envisaged that the inlet has a cross-section surface that is variable across the length of the inlet. 
     In a further development of the gas exchange unit it is envisaged that the gas exchange unit has an outlet, the cross-section surface of which reduces in flow direction. The fluid, in particular blood, is accelerated through a continuous cross-section surface reduction. A ratio of less than 1 between the diameter widening (dD) and the run length (dL) is advantageous: dD/dL&lt;1 or dD/2dL. 
     According to another further development of the gas exchange unit it is envisaged that the gas exchange unit has an outlet comprising a deflection. To realise the most compact gas exchange unit possible the flow of the fluid, in particular blood, is deflected at an angle on the outlet side. The deflection angle is approximately 90° or between 70° and 90°. Secondary turbulence or flow separations of the fluid, in particular of the blood, may occur with such deflections. With a continuous reduction in the cross-section surface of the outlet the fluid is however accelerated, so that the formation of secondary turbulence or flow separations are counteracted or are almost prevented. 
     According to an independent idea of the invention the gas exchange unit or the oxygenator comprises a ventilation device, in particular arranged substantially centrally. If air is taken in, this is collected in the middle of the rotating flow due to the inlet geometry, formed of inlet and inlet casing, in particular in a cavity of the gas exchange unit. Air is aspirated by the ventilation device. The ventilation device is also an outlet for air bubbles that may collect at these points during operation. The ventilation devices can also be used for blood sampling whilst treating a patient. 
     To prevent the formation of blood coagulum (thrombi) it must be possible to flush all areas of the ventilation device or it must be prevented that blood lingers. 
     The ventilation device can be designed in such a way that it can optionally be realised in a first operating condition, under which a flushing of the ventilation device can be realised or it can be transferred into a second operating condition, in which a ventilation of the cavity of the gas exchange unit can be realised. Two technical functions can therefore advantageously be realised by means of the ventilation device, namely ventilation of the cavity and the flushing, and thus cleaning of the ventilation device. The ventilation device is preferably flushed after a ventilation process. It can thus be prevented that possible blood residues that may also flow out during a ventilation process remain in the ventilation device. Alternatively the ventilation device can optionally be transferred into the first operating condition, where no ventilation of the cavity of the gas exchange unit takes place, or into the second operating condition, where a ventilation of the cavity of the gas exchange unit takes place. With this design only one technical function, namely the ventilation of the cavity, can be realised with the ventilation device. 
     The gas exchange unit can have an underpres sure source that can be fluid-connected with the ventilation device. It can be ensured in a simple way by applying underpressure in the ventilation device by means of the underpressure source that the air located in the cavity can be vented from the same. 
     In a further development of the gas exchange unit it is envisaged that the ventilation device comprises a flexible membrane. The central area of the inlet is made of a flexible material, so that the same can be turned inside out for ventilation purposes, so that a larger volume results on the blood side for collecting the air bubbles. 
     The gas exchange unit can have an adjustment element that serves for transferring the ventilation device into the first operating condition or into the second operating condition. Depending on the individual case the adjustment element can be designed differently, as is explained in more detail below. 
     The ventilation device can have a closure piston that is transferred into a first position for realising the first operating condition, or into a second position for realising the second operating condition. The closure piston can be moved linearly and/or rotatably mounted here, so that the same can be transferred from the first position into the second position or vice versa. In particular the closure piston can be mounted to rotate around its longitudinal axis. Alternatively or in addition the closure piston can be designed in such a way that a piston section is moved relative to another piston section for transferring the closure piston from the first position into the second position. 
     The closure piston can have a fluid line that is not fluid-connected with the cavity in the first position of the closure piston, and is in fluid-connection with the cavity in the second position of the closure piston. In addition the fluid line can be in fluid-connection with a supply line for supplying a flushing agent in the first position of the closure piston, and is not in fluid-connection with the supply line in the second position of the closure piston. In the second position the air located in the cavity can be vented from the cavity via the fluid line. With this design the closure piston can be rotatably mounted and the adjustment element can be designed in such a way that the closure piston is turned upon actuating the adjustment element. The adjustment element can preferably be connected with the closure piston in a torque-roof way. 
     With a further development a sealing ring of the ventilation device can be fitted to the closure piston. In addition the ventilation device can have a return member that is operatively connected with the closure piston in such a way that the return member pushes the closure piston out of the second position into the first position. The closure piston can be mounted in a linearly moveable way with this design. The return member can be a spring, in particular a pressure spring. A force, in particular a linear force, is applied to the closure piston by means of the adjustment element for transferring the closure piston from the first position into the second position. The adjustment element can for example be a syringe. 
     The closure piston can have a weak point in a further development, which is designed in such a way that it projects through the closure piston in the second position of the closure element. In the first position of the closure piston the closure piston can prevent an exit of air or blood from the cavity despite the weak point. The adjustment element can be a pipette that has at least one opening in its section that projects through the closure piston. Air located in the cavity can be vented from said cavity via the opening and the adjustment element here. The weak point of the closure piston can be realised with a cut in the closure piston. 
     To improve the gas exchange unit further it is envisaged in one further development that the hollow fibre mats are embedded in the gas exchange unit and have slanting transition from the embedded hollow fibre mats to adjacent components. The position of the transition from free fibres to the embedded fibres (potting level) can be subject to imprecise tolerances. The slanting transition with which the neighbouring, blood carrying components are equipped results in a smooth, impact-free, and therefore blood-friendly transition even for different positions of the potting level. The measure of the slanting transitions also strongly simplifies the manufacturing process of the gas exchange unit, as the tolerances lead to a lower quality loss over a wider range during manufacture. In a further development of the gas exchange unit it is envisaged that the inlet casing has stabilisers. In order to achieve a stability increase for the inlet casing during potting the inlet casing has stabilisers. These can be designed as bridges or cross members. The inlet geometry is therefore maintained during potting, where high temperatures and/or tension can be reached, and does not change substantially due to temperature changes. 
     The hollow fibres of known oxygenators are normally first closed at their ends and then embedded in a polyurethane adhesive. This potting step is carried out with a spinning method to prevent adhesion of the fibres in the area that later conveys blood due to capillary effects, and to enable a defined transition between the potting mass and free fibres. The fibres are then cut open with the cured potting mass from the outside transverse to the fibre direction in order to later enable a flow through the fibres with gas. The potting step normally takes place at both ends of the fibres and two potting processes therefore result with oxygenators with fibres arranged in parallel, or four potting steps with stacked fibre mats. 
     According to the invention a method for producing a gas exchange unit is offered. The method comprises: insertion of the casting mass for embedding the fibre ends, wherein this insertion takes place once, and forming a cylindrical cavity in the central area of the gas exchange unit. 
     The gas exchange unit according to the invention is produced from stacked hollow fibre mats. The casting mass for embedding the fibre ends is introduced during a step in a centrifuge with the method for producing a gas exchange unit, so that a cylindrical cavity results in the central area of the gas exchange unit, in which the hollow fibres come into contact with a fluid, in particular blood. The cylindrical cavity results in a homogeneous flow through the fibre mats. A requirement for this is a consistent facing side flow onto the cylindrical cavity. The selected construction shape realises a gas exchange unit with which production costs are low thanks to reducing manufacturing steps, as the fibres are embedded during one working step. With gas exchange units currently available on the market, embedding the fibres takes place during two or even four time-consuming steps in a centrifuge. 
     All components that come into contact with blood are also added in one working step with the described method. Further gluing is not necessary. 
     Contrary to this the homogeneous flow through the fibre mats takes place via distribution or diffusion plates with known oxygenators, which increase the flow resistance to various different degrees in a targeted way and distribute blood flow in this way. It is a disadvantage here that additional shear stresses and irritation can damage the blood. Thrombi can also be generated in the rear areas of the distribution plates where the flow is switched off, as wake spaces can be created there. Such plates can be omitted with the measures of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       Further details of the invention can be found in the embodiment examples, which are described with reference to the Figures below. Shown are: 
         FIG. 1 : an embodiment example of a gas exchange unit, 
         FIG. 2 : a further embodiment example of a gas exchange unit, 
         FIG. 3 : the inlet casing of  FIG. 2 , 
         FIG. 4 : an embodiment example of a surface of an inlet casing, 
         FIG. 5 : a cross-section view of an inlet casing, 
         FIG. 6 : an embodiment example of a ventilation device according to a first design, 
         FIG. 7 : a perspective illustration of a ventilation device according to a second design, 
         FIG. 8 : an enlarged illustration of a section illustrated in  FIG. 7 , wherein the ventilation device is in a first operating condition, 
         FIG. 9 : a section view of the section shown in  FIG. 8 , 
         FIG. 10 : an enlarged illustration of a section illustrated in  FIG. 7 , wherein the ventilation device is in a second operating condition, 
         FIG. 11 : a section illustration of the section shown in  FIG. 10 , 
         FIG. 12 : a perspective illustration of a ventilation device according to a third design, 
         FIG. 13 : an enlarged illustration of a section illustrated in  FIG. 12 , wherein the ventilation device is in a first operating condition, 
         FIG. 14 : a section illustration of the section shown in  FIG. 13 , 
         FIG. 15 : an enlarged illustration of a section illustrated in  FIG. 12 , wherein the ventilation device is in a second operating condition, 
         FIG. 16 : a section illustration of the section shown in  FIG. 15 , 
         FIG. 17 : a section illustration of a ventilation device according to fourth design, where the ventilation device is in a first operating condition, 
         FIG. 18 : a section illustration of the ventilation device according to the fourth design, where the ventilation device is in a second operating condition, 
         FIG. 19 : a section illustration of a ventilation device according to a fifth design, where the ventilation device is in a first operating condition, 
         FIG. 20 : a section illustration of a ventilation device according to the fifth design, where the ventilation device is in a second operating condition, 
         FIG. 21 : a section illustration of a ventilation device according to a sixth design, where the ventilation device is in a first operating condition, 
         FIG. 22 : a section illustration of a ventilation device according to a sixth design, where the ventilation device is in a second operating condition, 
         FIG. 23 : an embodiment example of an outlet casing, 
         FIG. 24 : a cross-section view of an outlet. 
     
    
    
     DETAILED DESCRIPTION OF THE FIGURES 
     Examples of embodiments of the invention will be described below with reference to the enclosed drawings: 
       FIG. 1  shows an embodiment example of a gas exchange unit  1 , which comprises hollow fibre mats (not shown). The gas exchange unit  1  comprises an inlet  7 , arranged acentrically on an inlet casing  5 . The inlet  7  is positioned diagonally to the inlet casing  5 . The inlet  7  is arranged on the facing side of a casing of the gas exchange unit  1  that includes the inlet casing  5  and enables a homogeneous flow through the hollow fibre mats. The inlet casing  5  forms a facing side of the casing of the gas exchange unit  1 . The inlet casing  5  is covered by a casing part  3  of the gas exchange unit  1  in its edge area (compare for example  FIG. 1 ). 
       FIG. 2  shows a further embodiment example of a gas exchange unit  1 . The surface of the inlet casing  5  has structures  6  that additionally distributes the flow of blood flowing through as homogeneously as possible on the inner, blood-conveying surface of the inlet casing  5 . The inlet casing  5  can optionally have the stabilisers  8  shown in  FIG. 3 . The stabilisers  8  can be designed as bridges or cross members. With a potting, during which high temperatures and/or tensions can be reached, the inlet geometry of the inlet casing  5  is thus maintained and does not change substantially during temperature and/or tension changes. 
       FIG. 3  shows the inlet casing  5  from  FIG. 2 . The acentric diagonal arrangement of the inlet  7  is clearly recognisable. The structures  6  that distribute the flow homogeneously in the gas exchange unit  1  are designed as vane-shaped structures  6   a  and as bridge-shaped structures  6   b . The structures  6  can also be arranged as cross members on the surface of the inlet casing  5 . 
     These structures  6  mean that a flow through, or a supply of the fibre surfaces of the hollow fibre mats in the gas exchange unit  1  takes place consistently in the inner, central area of the hollow fibre mats as well as in the outer area, which comprises an edge area of the hollow fibre mats. 
       FIG. 4  shows an embodiment example of a structured surface of an inlet casing  5  in a top view. The different size of the vane-shaped structures  6   a  is recognisable. It guarantees a consistent, homogeneous flow through the gas exchange unit  1 . 
       FIG. 5  shows a cross-section view of an inlet casing  5 . The diagonal positioning of the inlet  7  as well as the acentric arrangement are also visible in this Figure. The inlet casing  5  includes the structures  6 . The hollow fibre mats  32  are embedded in the gas exchange unit  1  and are in contact with the inlet casing  5 . The inlet casing  5  has slanting transitions  9  in the contact area of the hollow fibre mats  32  with the inlet casing  5 . The position of the respective transition  9  from the free fibres to the embedded  10  fibres (potting level) can be subject to imprecise tolerances. 
     The slanting transitions  9  also result in a consistent, smooth, and therefore blood-friendly transition even with different positions of the potting level. 
       FIG. 6  shows an embodiment example of a ventilation device  15 . If air should be drawn in this will accumulate in the centre of the rotating flow due to the inlet geometry formed by inlet  7  and inlet casing  5 . The air is aspirated by the ventilation device  15  there. The ventilation device  15  is also an outlet for air bubbles. The ventilation device  15  comprises a ventilation pipe  19 . 
     The ventilation device  15  comprises a flexible membrane  17 . Part (A) of  FIG. 6  shows the membrane  17  in a condition without ventilation. During ventilation, shown in Part (B) of  FIG. 7 , the membrane  17  is turned inside out, so that an enlarged volume results on the blood side for collecting the air bubbles. 
       FIG. 7  shows a perspective illustration of a ventilation device  15  according to a second design. The ventilation device  15  is arranged next to the inlet  7 . The ventilation device  15  is also fluid-connected with an underpressure source  16  by means of a line  22 . The underpressure source  16  is designed as a syringe. 
       FIG. 8  shows an enlarged illustration of a section shown in  FIG. 7 . The gas exchange unit  1  has an adjustment element  21  in the form of a rotating lever, by means of which the ventilation device  15  can optionally be transferred into a first operating condition or into a second operating condition. The ventilation device is in the first operating condition in the design shown in  FIG. 8 . 
       FIG. 9  shows a section illustration of the section illustrated in  FIG. 8 . As is clear from  FIG. 9  the ventilation device  15  has a closure piston  18 , which is connected with the adjustment element  21  in a torque-proof way. A fluid line  19  is located in the closure piston  18 . In a first position of the closure element  18  illustrated in  FIG. 8  the fluid line  19  is oriented in such a way that the fluid line  19  is not in fluid connection with the line  22 . A flushing of the ventilation device is not possible with this design. 
       FIG. 10  also shows an enlarged illustration of a section shown in  FIG. 7 . With the design illustrated in  FIG. 10  the adjustment element  21  has been rotated, so that the ventilation device  15  is in the second operating condition. 
       FIG. 11  shows a section drawing of the section shown in  FIG. 10 . With the second operating condition of the ventilation device  15  the closure piston  18  is arranged in a second position, where the fluid line  19  is in fluid connection with the cavity  23  of the gas exchange unit  1 . In addition the cavity  23  is in fluid connection with the line  22 , and thus with the underpressure source  16  illustrated in  FIG. 7 . With the second position of the closure piston  18  the air collected in the cavity  23  can be vented via fluid line  19  and the line  20 . 
       FIG. 12  shows a perspective illustration of a ventilation device according to a third design. The ventilation device  15  is in fluid connection with the underpressure source  16  via line  22 . The design illustrated in  FIG. 12  differs from the design illustrated in  FIG. 7  here in the design of the adjustment element  21 . It is clear from  FIG. 13  that the adjustment element  21  is designed as a rotary knob. 
       FIG. 14  shows a section illustration of the ventilation device  15  shown in  FIG. 13  and the adjustment element  21 . As is clear from  FIG. 14  no fluid connection exists between the line  22  and the cavity  23  in the first position of the adjustment element  18 . Air located in the cavity  23  can therefore not be vented in the first position of the closure piston  18 . The closure piston  18  is connected with the adjustment element  21  in a torque-proof way. 
       FIG. 15  shows an enlarged illustration of the ventilation device  15  shown in  FIG. 12  and the adjustment element  21 , wherein the ventilation device  15  is in the second operating condition. The adjustment element  21  is turned to transfer the ventilation device  15  from the first operating condition into the second operating condition. 
       FIG. 16  shows a section illustration of the ventilation device  15  shown in  FIG. 15  and the adjustment element  21 . A fluid connection exists between the cavity  23  and the line  22  in the second position of the closure piston  18 . The closure piston  18  is moved in a linear manner for transferring the closure piston  18  from the first position into the second position shown in  FIG. 16 . Air collected in cavity  23  can be vented to the underpres sure source  16  through a gap between the closure piston  18  and a ventilation device casing  31  and via line  22 . 
       FIG. 17  shows a section illustration of the ventilation device according to a fourth design. The closure piston  18  of the ventilation device  15  differs from the closure piston described above in that it has a weak point  25 . The weak point  25  equals a cut in the closure piston  18 . The weak point  25  is arranged in an area of the closure piston  18  here, which extends into the cavity  23 . A further difference consists of the closure piston  18  including a recess  26 . In  FIG. 17  the closure piston  18  is in the first position, so that air collected in the cavity  23  cannot be vented. 
       FIG. 18  shows a section illustration of the ventilation device  15  according to the fourth design, wherein the ventilation device  15  is in the second operating condition. As is clear from  FIG. 18  the adjustment element  21  passes through the closure piston  18 . In particular the adjustment element  21  passes through the same in the area of the weak point  25  of the closure piston  18 . As a consequence of this passing of the adjustment element  21  through the closure piston  18  a piston section  33  and another piston section  34  move away from each other. The adjustment element  21  is partly arranged in the recess  26  here. The adjustment element  21  has an opening  27  in its section that lies in the cavity  23 , via which air is vented from the cavity  23 . 
       FIG. 19  shows a section illustration of the ventilation device  15  according to a fifth design. The ventilation device  15  has a sealing ring  29  fitted on the closure piston  18 . In addition the ventilation device  15  has a return means  30  designed in such a way that it presses the closure piston  18  from the second position shown in  FIG. 20  into the first position shown in  FIG. 19 . The return element  30  supports itself on the closure piston  18  at one end, and on a ventilation device casing  31  at the other end. 
       FIG. 20  shows a section illustration of the ventilation device  15  according to the fifth design, where the ventilation device  15  is in the second operating condition. As is clear from  FIG. 20  the closure piston  18  is moved in a linear manner by an adjustment element  21  not shown here. The closure piston  18  is in particular pressed so far into the cavity  23  that a gap exists between the closure piston  18  and the ventilation device casing, through which air collected in the cavity  23  is vented. The air vented from the cavity flows through line  20  to the underpressure source  16 . The return element  30  is tensioned when the closure piston  18  is transferred into the second position. 
       FIG. 21  shows a section illustration of a ventilation device  15  according to a sixth design, where the ventilation device  15  is in a first operating condition, where a flushing of the ventilation device  15  takes place. With the first position of the closure piston  18  shown in  FIG. 21  the fluid line  19  of the closure piston  18  is in fluid connection with a supply line  20 . A flushing agent is added via the supply line  20 , which flows through the fluid line  19  as indicated by the arrows and exits via the closure piston  18 . No fluid connection exists between the fluid line  19  and the cavity  23  in the first operating condition. 
       FIG. 22  shows a section illustration of a ventilation device  15  according to a sixth design, where the ventilation device  15  is in a second operating condition, where the ventilation process takes place. In the second position of the closure piston  18  a fluid connection exists between the fluid line  19  and the cavity  23 . Air present in the cavity  23  can be vented via the fluid line  19  as shown by the arrows. 
       FIG. 23  shows an embodiment example of an outlet casing  11 . The outlet casing  11  has an outlet  12 . The cross-section surface of the outlet  12  reduces in flow direction as shown in the cross-section view of the outlet in  FIG. 22 . The inlet casing  5 , the outlet casing  11  and casing part  3  form a casing of the gas exchange unit  1 . 
     The outlet  12  has a deflection  13 . The deflection angle is 90°, but can also lie between 70° and 90°. The continuous reduction of the cross-section surface of the outlet  12  accelerates the fluid (blood), so that the formation of secondary turbulence or flow separations are effectively counteracted. 
     The outlet casing  11  optionally has stabilisers  8 , which can be designed as bridges or cross members. During potting, where high temperatures can be reached, the geometry of the outlet casing  11  is thus maintained and will not change substantially if temperatures change. 
       FIG. 24  shows a cross-section view of the outlet  12 . The cross-section surface  10  of the outlet  12  narrows continuously in flow direction of the fluid (blood) in the area of the change in main flow direction (deflection  13 ). The flow direction is indicated by an arrow. The narrowing can be asymmetrical. The cross-section surface  10  can be variable along the length of the outlet  12 . The fluid (blood) is accelerated by a continuous cross-section surface reduction on the outlet side of the gas exchange unit  1 . 
     It should be noted that the methods, devices and systems described in this document can be used on their own as well as in combination with other methods, devices and systems described in this document. All aspects of the methods, devices and systems described in this document can also be combined with each other in many ways. In particular the characteristics of the claims can be combined with each other in many ways. 
     The invention has been described in detail with reference to the drawings and the above description. This invention can however be realised in many different forms and should not be interpreted as limited to the embodiments illustrated here, instead these embodiments are provided to make this disclosure thorough and complete, and do not exhaust the scope of protection of the invention to its full extent for a person skilled in the art. The terminology used in the detailed description of the embodiments illustrated in the enclosed drawings are not intended to limit the invention. Identical reference numbers in the drawings refer to identical elements.