Oxygenator membrane

Details Oxygenator membrane Oxygenator membrane

1999-09-10

2002-01-15

Seidleck, James J. (Department: 1711)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Cellular products or processes of preparing a cellular...

C521S142000, C521S145000, C422S048000, C261S101000, C261S105000, C261SDIG002

Reexamination Certificate

active

06339108

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to an oxygenator membrane based on organically modified silicic-acid polycondensates and to a process for preparation thereof.
BACKGROUND OF THE INVENTION
Oxygenators for human blood are deployed, for example, during cardiac operations and for treatment of reversible pulmonary insufficiency and are intended temporarily to take over the natural function of the lungs. It is necessary to supply the blood sufficiently with oxygen and at the same time to remove the carbon dioxide formed as a result of metabolic processes within the body. The simplest process for oxygen enrichment in the blood uses so-called bubble oxygenators (BOs), which are simple and inexpensive in construction. In the BO, oxygen is blown into the blood, which flows through a column or specially shaped polymer pouch. Gas exchange occurs at the interface between blood and gas. The exchange of gas in this way unfortunately allows direct contact of gas with blood, and can induce thrombi formation. Such thrombi potentially cause embolism and adversely effect blood coagulation. Furthermore, thrombi formation at the interface of the gas bubbles drastically reduces the effective gas exchange surface area. In addition, it is impossible to prevent these microbubbles from entering the circulation. For these reasons, the patient is stressed physiologically. Accordingly, such oxygenators can be deployed only about 1 to 1.5 hours.
To prevent these problems, membrane oxygenators (MOs) are preferred, since they are better able to imitate natural lung function.
In the case of extracorporal membrane oxygenation (EMCO) it is customary to use hydrophobic membranes. A hydrophobic oxygenator is described, for example, in DE 3129064 A1, which proposes the use of hydrophobic membrane materials for oxygenation in the form, for example, of hollow fibers. In this case, blood is passed in an extracorporal circuit on one side of a hydrophobic membrane, while on the other side of the membrane oxygen is supplied in countercurrent, so that CO
2
/O
2
exchange is able to take place via the pores of the membrane.
Two types of hydrophobic membrane are commonly used: membranes consisting of a material that is hydrophobic per se, for example polypropylene, and membranes with surfaces that have been made hydrophobic with a hydrophobicizing agent, for example a silicone.
Hydrophobic membranes that comprise hydrophobic materials have comparatively large pores of from several 100 to 1000 nm. The membranes generally are in the form of several 1000 hollow filaments, resulting in an active surface area of up to 6 m
2
. In this case the blood flows either in the hollow filaments or else on the outside of the hollow filament, whereas the gas to be exchanged is passed by countercurrent on the opposite side. These membranes are commonly used in heart lung machines.
Hydrophobicized membranes consist of a thin layer of silicone on a porous support structure and are used for long-term EMCO treatment. Although hydrophobic membranes are more effective than siliconized membranes, since the diffusion through pores is more rapid, membranes comprising hydrophobic porous material have a significant disadvantage for long-term therapy. This disadvantage consists in the leakage of the membranes. Despite their hydrophobic structure the membrane pores fill with aqueous plasma constituents, leading to hydrophilicization or wetting of the membrane surface.
Prior art membrane material used for hollow fiber oxygenators generally is microporous polypropylene, and, in some cases polyethylene. Hollow polypropylene fibers having a defined, interconnecting porosity are obtained directly only by highly complex spinning processes (e.g., solution wet spinning) or by subsequent and thus additional process steps. When hollow fibers modified in this way are used for gas exchange, a risk exists for passage of the fluid phase. In the case of O
2
/CO
2
exchange in blood by an oxygenator, such pores are considerably hazardous. Under prolonged use, passage of blood through pores is a frequent complication. In addition, small gas bubbles can pass to the opposite side in such materials, forming microthrombi.
Very high gas permeation rates without pores are realizable only with highly specific polymer materials such as certain silicones or substituted polysilylpropynes. The high gas permeability is achieved, however, at the expense of extreme reduction in mechanical properties of the material. As permeability increases strength and modulus of elasticity decrease, i.e., the material becomes increasingly softer. Consequently, hollow fibers that combine sufficient mechanical stability with very low wall thickness and high gas permeability are not possible. Very different types of polymer are needed in combination with different production techniques in order to make suitable hollow fibers that have suitable permeability over a wide range.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide membranes for oxygenators, having permeability and flexibility that can be varied over a wide range and can be adapted to the requirements of a particular application. Another object is to provide membranes that should combine high mechanical stability with high gas permeability without risking passage of fluid phase. Another object is to provide membrane having high gas permeation values and which are self-supporting and toxicologically acceptable.
It is a further object of the present invention to provide a process to manufacture membranes for oxygenators having properties that can be varied over wide ranges. By simple variation of the process steps it should be possible to adapt the chemical and physical properties of the membrane, within wide ranges, to the requirements of the particular application. The process should be simple, rapid and inexpensive to carry out. By means of the process it should be possible to manufacture membranes which meet the above-mentioned requirements. Furthermore, the process should also be suitable for the continuous production of hollow fiber membranes and flat membranes. In addition, the surface modifications which are often necessary for various applications, for example, in order to avoid blood coagulation, in order to adjust the polarity, adsorption characteristics, etc., should be realizable both during the synthesis of the material, i.e., in situ, and also subsequently.
These objects are achieved by oxygenator membranes which are obtainable by processing a more or less viscous liquid or resin in accordance with conventional methods to form membranes, and, if desired, drying said membranes. The resultant membranes are then cured with thermal and/or radiation-induced and/or chemical induction.
One embodiment of the invention provides a process for producing an oxygenator membrane, comprising forming a membrane from a low-viscosity to resinous liquid produced by hydrolytic polycondensation of a material comprising at least one compound selected from the group consisting of:
a compound of formula I
 wherein
R=alkyl, alkenyl, aryl, alkylaryl or arylalkyl comprising between 1 to 15 carbon atoms, further optionally comprising an atom or group selected from the group consisting of oxygen atom, sulfur atom, ester, carbonyl, carboxyl, amido, and amino,
R
1
=alkylene, arylene, arylenealkylene or alkylenearylene comprising between 0 to 15 carbon atoms, further optionally comprising an atom or group selected from the group consisting of oxygen atom, sulfur atom, ester, carbonyl, carboxyl, amido, and amino,
R
2
=alkylene, arylene, arylenealkylene or alkylenearylene comprising between 0 to 15 carbon atoms, further optionally comprising an atom or group selected from the group consisting of oxygen atom, sulfur atom, ester, carbonyl, carboxyl, amido, and amino,
R
3
=hydrogen, R
2
—R
1
—R
4
—SiX
x
R
3−x
, carboxyl, alkyl, alkenyl, aryl, alkylaryl or arylalkyl comprising between 1 to 15 carbon atoms, further optionally comprising an atom or group selected from the group consisting o

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