Hydrophobic diffusion membranes for blood having wettable surfaces

Hydrophobic diffusion membranes such as porous polypropylene may be rendered hydrophilic at their surfaces, without losing their valuable characteristics as diffusion membranes, by subjection to a corona discharge or other ionizing condition, preferably in air or a similar oxygen-containing atmosphere.

BACKGROUND OF THE INVENTION 
Membrane oxygenators for blood are presently being sold by Travenol 
Laboratories, Inc., Deerfield, Ill., which contain a microporous, 
hydrophobic diffusion membrane. The diffusion membrane is pressed between 
membrane support members, and provided with a pair of flow paths, one for 
blood along one side of the membrane and another for oxygen and respired 
gases along the other side of the membrane. 
The pores in the hydrophobic material are sufficiently small, compared with 
the thickness of the material, that blood cannot pass through the 
membrane. However, the pores provide improved permeability for gases 
through the membrane. Accordingly, oxygen, carbon dioxide, and water vapor 
are rapidly exchanged through the membrane. See U.S. Pat. Nos. 3,757,955 
and 3,927,980 for descriptions of the construction and use of oxygenators 
having hydrophobic membranes. 
Such oxygenators for blood made of porous, hydrophobic membrane have turned 
out to be a major step forward in the field of blood oxygenation, and are 
being used in open heart surgery and other medical procedures with 
significantly improved success over that which has gone before in the 
prior art. The devices of this invention exhibit excellent blood 
compatibility, permitting relatively long-term use, coupled with a high 
level of blood oxygenation. 
However, it has been considered desirable by some experts to use 
hydrophilic, blood-contacting membranes rather than hydrophobic membranes, 
for the reason that there is increased compatibility between hydrophilic 
membranes and the formed elements of the blood, as well as plasma 
portions, when compared with hydrophobic membranes. 
However, hydrophilic membranes may not be rendered porous without causing 
leakage of aqueous liquids across the membrane, and hydrophilic membranes 
generally exhibit far less capacity for oxygen and carbon dioxide 
transfer. 
In accordance with this invention, a hydrophobic membrane, preferably a 
porous, hydrophobic membrane, is provided with a hydrophilic outer 
surface. Accordingly, the resulting membrane can exhibit the desirable 
transfer characteristics of hydrophobic membranes, specifically the porous 
membranes, while at the same time presenting a hydrophilic surface to the 
blood, resulting in less platelet attachment and the like. 
Also, the membranes of this invention, and especially the porous membranes, 
can be used to process other aqueous liquids in the medical as well as 
other fields, as well as nonaqueous liquids having surface tensions 
similar to water, without leakage. 
DESCRIPTION OF THE INVENTION 
A diffusion device, typically for blood, may be assembled by overlaying 
membrane support means with a diffusion membrane comprising hydrophobic 
material, to form a diffusion device defining a first flow path for one 
fluid along one side of the diffusion membrane, and a second flow path for 
another fluid along the other side of said diffusion membrane. In 
accordance with this invention, prior to the overlaying of the support 
means by the membrane, one surface, typically the blood-contacting 
surface, of the diffusion membrane is subjected to ionizing atmosphere 
conditions, so as to increase the surface tension of the membrane surface 
to render it more hydrophilic. 
Typically, the ionizing conditions utilized herein are created by 
subjecting the membrane to a corona discharge in air. Corona discharge 
treatment of polyethylene film for other purposes is a well-known and 
conventional process. Apparatus for subjecting films to a corona discharge 
is sold by the Pillar Corporation, 7000 West Walker Street, Milwaukee, 
Wis. 
While the specific corona discharge conditions may vary in accordance with 
the nature of the diffusion membrane to be treated and other conditions, 
successful results have been obtained with a Pillar Solid State Corona 
Treater by generating a corona discharge field with 2,000 volt, 4 
Kilohertz alternating current, and passing the membrane material through 
the field. Porous, hydrophobic membranes having a pore size of no more 
than 5 microns, made of aliphatic hydrocarbons, are generally preferred. 
Conveniently, both sides of the membrane may be rendered hydrophilic, if 
desired. 
A specific membrane material subjected to the corona discharge field may be 
a polypropylene membrane having a thickness of 0.001 inch, and an 
effective pore size of 0.1 micron (Cellgard 2400, manufactured by the 
Celanese Corporation). By this processing technique, the polypropylene 
material described above, which normally has a surface tension of about 34 
dynes/cm. can be changed to a material having a surface tension of about 
60 dynes/cm. However, it may only be necessary to treat the surface only 
to an extent that the surface becomes wettable to the fluid which it will 
contact, i.e., the surface tension of the membrane may be raised only to 
just barely greater than that of the fluid, at the temperature and other 
conditions of the intended use. 
Typically, the process of this invention will be performed in the air, 
since the presence of oxygen appears to facilitate the process. 
Accordingly, it is generally preferred that the process be performed in 
essentially ambient pressures, and in an atmosphere having at least 10 
percent oxygen. However, some increase in the surface tension is noted 
when the diffusion membrane is subjected to corona discharge in, for 
example, a pure nitrogen atmosphere. 
While it is generally preferred to utilize diffusion membranes made of 
aliphatic hydrocarbon polymers, such as polyethylene and polypropylene, it 
is contemplated that other membranes may also be altered in their surface 
tension, such as silicone rubber film and polytetrafluoroethylene films. 
Also, copolymers of hydrocarbons such as ethylene and propylene, 
copolymerized with other units such as styrene (for stiffening the 
membranes), butadiene, and the like may be utilized if desired. 
The corona treatment process can be performed on a continuous basis, where 
each portion of the membrane is exposed to the corona field for only a 
fraction of a second, if desired, in accordance with the recommendation of 
the manufacturer of the particular corona discharge unit utilized. It is 
generally thought that for any particular membrane it may be possible to 
overtreat the membrane, so that the inner surfaces of the pores of the 
membrane are also rendered hydrophilic, resulting in an increased 
capability of the porous membranes to permit fluids from blood and the 
like to pass through the membrane. This is usually undesirable, and may be 
avoided by simply reducing the length of exposure of the membrane to the 
corona discharge field, or the intensity of the field. 
It is generally preferable to treat membranes in accordance with this 
invention to cause their surface tension to increase to at least 50 
dynes/cm., to obtain a significant increase in the hydrophilic 
characteristics of the membrane. 
It is also contemplated that other ionizing conditions may be used as well 
as corona discharge; for example oxygen ions and other ions may be 
generated by an electric arc in the vicinity of the membrane to be 
treated, or the membrane may be exposed to various forms of ionizing 
radiation.

Referring to the drawing, a roll 10 of porous, hydrophobic membrane 11 is 
provided, for example the porous polypropylene material described above. 
The roll of membrane material is unrolled to pass through a corona 
discharge device 12, which comprises a pair of electrodes 14, 16 with 
their facing surfaces being covered with an insulating material such as 
silicone resin or rubber, to create the corona field. 
The air in the space between electrodes 14, 16 ionizes during operation. A 
transformation of the surface of membrane 11 takes place, causing the 
normal surface tension for the polypropylene material used of about 35 
dynes/cm. to increase to about 60 dynes/cm., imparting hydrophilic 
characteristics to the membrane surface. 
Thereafter, the treated membrane passes to a cutting station 20, in which 
the membrane strip is cut into desired lengths of membrane 22. The lengths 
of membrane 22 are then laid over and against a membrane support sheet 24, 
which has been prescored and prefolded along fold lines 26 to form a 
plurality of sections 28. The specific details of the exact structure of 
the device being made may be as described in U.S. Pat. No. 3,757,955. 
Thereafter, membrane 22 and membrane support 24 may be folded together into 
a convoluted, pleated structure 30 to form a diffusion stack defining a 
series of flow channels 32 on one side of the stack for the passage of 
oxygen gas, and another series of flow channels 34 on the other side of 
membrane 22 for the passage of blood, as described in the U.S. patent 
mentioned above. Additional membrane support structures 36 may be added to 
the blood flow paths as desired. 
The entire stack 30 is then inserted in a suitable container, having 
manifold ports for the inlet and outlet of gas and blood. The container is 
sealed, and the device is sterilized in suitable manner for use. 
While the method of this invention finds particular utility in diffusion 
devices for blood, it is also contemplated that other types of diffusion 
devices may also be made in accordance with this invention. 
The above has been offered for illustrative purposes only, and is not for 
the purpose of limiting the invention of this application, which is as 
defined in the claims below.