Plastic and nonmetallic article shaping or treating: processes – Pore forming in situ
Reexamination Certificate
2001-09-04
2002-04-23
Foelak, Morton (Department: 1711)
Plastic and nonmetallic article shaping or treating: processes
Pore forming in situ
C210S500360, C521S064000
Reexamination Certificate
active
06375876
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a process for producing a hydrophobic membrane using a thermally induced phase separation process, the membrane having a sponge-like, open-pored, microporous structure, and to the use of the membrane for gas exchange processes, in particular for oxygenation of blood.
2. Description of the Related Art
The invention relates to a process for producing a hydrophobic membrane using a thermally induced phase separation process in accordance with the preamble of Claim 1, the membrane having a sponge-like, open-pored, microporous structure, and to the use of the membrane for gas exchange processes, in particular for oxygenation of blood.
In a multitude of applications in the fields of chemistry, biochemistry, or medicine, the problem arises of separating gaseous components from liquids or adding such components to the liquids. For such gas exchange processes, there is increasing use of membranes that serve as a separation membrane between the respective liquid, from which a gaseous component is to be separated or to which a gaseous component is to be added, and a fluid that serves to absorb or release this gaseous component. The fluid in this case can be either a gas or a liquid containing the gas component to be exchanged or capable of absorbing it. Using such membranes, a large exchange surface can be provided for gas exchange and, if required, direct contact between the liquid and fluid can be avoided.
An important application of membrane-based gas exchange processes in the medical field is for oxygenators, also called artificial lungs. In these oxygenators, which are used in open-heart operations, for example, oxygenation of blood and removal of carbon dioxide from the blood take place. Generally, bundles of hollow-fiber membranes are used for such oxygenators. Venous blood flows in this case in the exterior space around the hollow-fiber membranes, while air, oxygen-enriched air, or even pure oxygen, i.e., a gas, is passed through the lumen of the hollow-fiber membranes. Via the membranes, there is contact between the blood and the gas, enabling transport of oxygen into the blood and simultaneously transport of carbon dioxide from the blood into the gas.
In order to provide the blood with sufficient oxygen and at the same time to remove carbon dioxide from the blood to a sufficient extent, the membranes must ensure a high degree of gas transport: a sufficient amount of oxygen must be transferred from the gas side of the membrane to the blood side and, conversely, a sufficient amount of carbon dioxide from the blood side of the membrane to the gas side, i.e., the gas flow or gas transfer rates, expressed as the gas volume transported per unit of time and membrane surface area from one membrane side to the other, must be high. A decisive influence on the transfer rates is exerted by the porosity of the membrane, since only in the case of sufficiently high porosity can adequate transfer rates be attained.
A number of oxygenators are in use that contain hollow-fiber membranes with open-pored, microporous structure. One way to produce this type of membrane for gas exchange, such as for oxygenation, is described in DE-A-28 33 493. Using the process in accordance with this specification, membranes can be produced from meltable thermoplastic polymers with up to 90% by volume of interconnected pores. The process is based on a thermally induced phase separation process with liquid-liquid phase separation. In this process, a homogeneous single-phase melt mixture is first prepared from the thermoplastic polymer and a compatible component that forms a binary system with the polymer, the system in the liquid state of aggregation having a range of full miscibility and a range with a miscibility gap, and this melt mixture is then extruded into a bath that is substantially chemically inert with respect to, i.e., does not substantially react chemically with, the polymer and has a temperature lower than the demixing temperature. In this way, a liquid-liquid phase separation is initiated and, on further cooling, the thermoplastic polymer solidified to form the membrane structure.
An improved process for producing such membranes, which permits specific adjustment of the pore volume, size, and wall, is disclosed in DE-A-32 05 289. In this process, 5-90% by weight of a polymer is dissolved, by heating to above the critical demixing temperature, in 10-95% by weight of a solvent system of first and second compounds, which are liquid and miscible with each other at the dissolving temperature, to form a homogeneous solution, whereby the employed mixture of polymer and the cited compounds has a miscibility gap in the liquid state of aggregation below the critical demixing temperature, the first compound is a solvent for the polymer, and the second compound increases the phase separation temperature of a solution consisting of the polymer and the first compound. The solution is given shape and, by cooling in a cooling medium consisting of the first compound or the employed solvent system, is brought to demixing and solidifying of the high-polymer-content phase, and the cited compounds are subsequently extracted.
The membranes disclosed in accordance with DE-A-28 33 493 or DE-A-32 05 289 have an open-pored, microporous structure and also open-pored, microporous surfaces. On the one hand, this has the result that gaseous substances, such as oxygen (O
2
) or carbon dioxide (CO
2
), can pass through the membrane relatively unrestricted and the transport of a gas takes place as a Knudsen flow, combined with relatively high transfer rates for gases or high gas flow rates through the membrane. Such membranes with gas flow rates for CO
2
exceeding 1 ml/(cm
2
*min*bar) and for O
2
at approximately the same level have gas flow rates that are sufficiently high for oxygenation of blood.
On the other hand, however, in extended-duration use of these membranes in blood oxygenation or generally in gas exchange processes with aqueous liquids, blood plasma or a portion of the liquid can penetrate into the membrane and, in the extreme case, exit on the gas side of the membrane, even if in these cases the membranes are produced from hydrophobic polymers, in particular polyolefins. This results in a drastic decrease in gas transfer rates. In the medical area of blood oxygenation, this is termed plasma breakthrough.
The plasma breakthrough time of such membranes, as producible in accordance with DE-A-28 33 493 or DE-A-32 05 289, is sufficient in most cases of conventional blood oxygenation to oxygenate a patient in a normal open-heart operation. However, the desire exists for membranes with higher plasma breakthrough times in order to attain higher levels of safety in extended-duration heart operations and to rule out the possibility of a plasma breakthrough that would require immediate replacement of the oxygenator. The aim, however, is also to be able to oxygenate premature infants or in general patients with temporarily restricted lung function long enough until the lung function is restored, i.e., to be able to conduct extended-duration oxygenation. A prerequisite for this is appropriately long plasma breakthrough times. A frequently demanded minimum value for the plasma breakthrough time in this connection is 20 hours.
From EP-A-299 381, hollow-fiber membranes for oxygenation are known that have plasma breakthrough times of more than 20 hours, i.e., there is no plasma break-through even under extended use. With the otherwise porous membrane with a cellular structure, this is attained by a barrier layer that has an average thickness, calculated from the oxygen and nitrogen flow, not exceeding 2 &mgr;m and is substantially impermeable to ethanol. The membrane is substantially free of open pores, i.e., pores that are open both to the outside and to the inside of the hollow-fiber membrane. According to the disclosed examples, the membranes in accordance with EP-A-299 381 have a porosity of at most 31% by volume, since at higher porosity values the p
Batzilla Thomas
Kessler Erich
Wechs Friedbert
Wiese Frank
Foelak Morton
Membrana GmbH
Oliff & Berridg,e PLC
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