Plastic blood bag system including a bag having a generally longitudinal
seal extending from the top of the bag and between two blood bag
ports downwardly to at least the lower half of the bag but not to
the bottom of the bag, thereby leaving the sole passageway within
the bag and between two ports located in the lower half of the bag.
The longitudinal seal terminates proximal to the central axis of
the bag with a spot weld to prevent tear. In addition, the cross-sectional
diameter of the channel increases as the channel nears the central
1. In a plastic blood bag system comprising a blood bag having
two major walls that meet at a top, a bottom, and two sides the
top having at least an inlet port and an outlet port, and the bottom
without a port, the improvement comprising:
A. a generally longitudinal seal of said walls extending from the
top of said bag and between said inlet port and said outlet port
to and along said bag bottom forming a blood outlet channel between
the seal and one of said sides and in communication with said outlet
port for expressing dense blood components from said bag bottom
through said outlet port and terminating at a passageway proximal
a central axis of said bag,
wherein said seal includes an upper portion positioned a predetermined
distance from and parallel to one of said sides, and a lower portion
angled away from said side and from said channel and along said
bag bottom, and
B. a spot weld at one terminal end of said longitudinal seal, said
spot weld positioned proximal said central axis.
2. The blood bag of claim 1 wherein said channel has a width D
that increases along said bag bottom.
3. The blood bag of claim 2 wherein said passageway has a diameter
D' that is greater than channel width D.
4. The blood bag of claim 2 wherein said longitudinal seal has
a width less than said channel width D.
5. The blood bag of claim 1 wherein said spot weld is substantially
round in shape.
6. The blood bag of claim 1 wherein the average distance from said
longitudinal seal along said bag side to said bag side is less than
20 percent of the total distance from that same side to said central
7. The blood bag of claim 1 wherein the top of said channel includes
a port in closed communication with another blood bag.
8. The blood bag of claim 7 wherein said port includes . a frangible
BACKGROUND OF THE INVENTION
This disclosure is concerned generally with the collection and
separation of whole blood into useful components and specifically
with a plastic blood bag system which permits a more dense centrifuged
component in the lower part of the bag to be expressed to and out
of the top of the bag.
2. Prior Art
Whole blood is commonly separated into its major components of
less dense plasma and more dense red blood cells (RBCs) by first
drawing the whole blood into a plastic bag known as a donor or primary
bag. The bag contents are then centrifuged under controlled conditions
to result in a lower, more dense portion of packed RBCs and an upper
less dense plasma portion, which may be rich in platelets (platelet
rich plasma or PRP).
The donor bag is typically connected via blood bag ports by plastic
tubing to one or more satellite bags to form a "closed"
system into which separated blood components (e.g., the PRP or RBCs)
may be expressed by external manipulation and valves for further
processing or use.
The above system for separating blood into its major components
has remained generally unchanged since the 1950's when plastic blood
bags were introduced commercially on a large scale.
In recent times, efforts have focused on preparing very specific
"components" from whole blood (or fractionated plasma)
so that if a patient needs a certain component (e.g., coagulation
factors, albumin, platelets, ISG, RBCs, etc.), only that specific
component can be administered. Although the system of this disclosure
may be used to prepare other blood products, it is especially useful
for preparing platelets.
The classical method of preparing platelet transfusion products
from whole blood consists of an initial centrifugation of whole
blood in a plastic blood bag at relatively low centrifugal force
to separate most of the PRP from the red cells. The PRP is then
commonly expressed into an attached satellite blood bag. This is
followed by centrifugation of the PRP in the satellite bag at relatively
high centrifugal force. This results in a lower sediment of platelets
and an upper platelet poor plasma (PPP). The sedimented platelets
are in the form of a pellet or "button" which is resuspended
in a small volume of the PPP donor plasma (50-60 ml) to give the
platelet concentrate (PC).
With good technique, about 2/3 of the platelets in a whole blood
collection unit (about 450 ml.+-.10%) are recovered in the platelet
concentrate. This is equivalent to about 8.times.10.sup.10 platelets
per concentrate. However, achieving this yield of platelets requires
strict attention to centrifugation protocols, frequent calibration
of the centrifuges, and operator diligence. The fact that the minimum
standard for platelet yield is only 5.5.times.10.sup.10 per concentrate
attests to the operator-dependent nature of this procedure.
Recently, some transfusion services in Europe have begun to investigate
and in some cases employ an alternate method of platelet preparation,
specifically preparation from the "buffy coat" of centrifuged
whole blood. In this procedure the initial centrifugation of whole
blood is performed at relatively high centrifugal force to form
three portions: an upper layer of relatively cell-free plasma, an
intermediate "buffy coat" layer containing platelets and
leukocytes, and a lower layer of red cells.
The intermediate buffy coat is separated and mixed with either
a small volume of plasma (50-60 ml) or a synthetic medium. The mixture
is then centrifuged at low centrifugal force to separate platelet
concentrate (upper layer) from leukocytes (WBCs) and residual red
cells. Data suggest that platelets prepared in this fashion are
of improved quality, presumably because platelet activation that
would otherwise occur during the pelleting step of the PRP centrifugation
method is avoided.
The original work on buffy coat platelets was done at the Dutch
Red Cross. Referred to as the Amsterdam method, it employed a standard
quadruple multiple plastic bag system. After centrifugation of blood
and removal of plasma from the main bag, the buffy coat layer was
transferred to an empty connected satellite bag and then processed
to platelet concentrate. Using this method, Pietersz et al., Vox
Sang 1985; 49:81-85 found a mean of 7.2.times.10.sup.10 platelets
per concentrate; the volume of blood collected in this study was
about 500 Ml. Kretschmer et al., Infusionstherapie 1988; 15:232-239
found a mean of 6.3.times.10.sup.10 platelets per concentrate from
450 ml blood collections.
The Amsterdam method, while apparently giving respectable platelet
yields, was cumbersome and labor-intensive. The buffy coat transfer
step required the operator to massage the bag to prevent hang-up
of the "sticky" buffy coat layer. These manipulations
might influence platelet function and release of granulocyte enzymes.
There was also no way to control the volume of buffy coat removed.
Other efforts to improve blood separation procedures or at least
make it less burdensome are known. For example, U.S. Pat. No. 3911918
to Turner discloses a blood bag having an hour glass shape. That
bag has a top portion for plasma, a bottom portion for RBCs and
a middle portion for platelets and white blood cells. The hour glass
shape is said to help position clamping or sealing devices at the
juncture of the separated components after whole blood in the bag
is centrifuged. This system has not been used on any significant
commercial scale to date. See also U.S. Pat. No. 4857190 to Wada
et al. showing a blood bag having a continuous but smaller receptacle
adapted to help collect and define the interface of a centrifuged
In U.S. Pat. No. 4608178 to Johansson et al., there is disclosed
a "top/bottom" bag in which the upper and lower portions
of separated blood components can be simultaneously expressed from
a specially designed bag which leaves behind in the bag the intermediate
portion known as buffy coat. The expression of that system is controlled
by a pressure plate on the bag and sensors which monitor the position
of the intermediate layer such that it remains in the bag while
the upper plasma is expressed from a top part and the lower red
blood cells are expressed from a bottom part in the bag. Hence,
the name top/bottom bag. The sensors in that system assure the simultaneous
expression of the top and bottom components.
The above described systems are fairly recent and it is not clear
yet whether those systems will in time replace existing blood separation
systems based on the use of a relatively simple unmodified donor
However, the systems do offer new ways to separate WBCs from platelets
or to prepare platelets (contained in the intermediate or buffy
coat portion). The patent to Johansson et al. show how to do this
in a semi-automated manner. Hence, it potentially represents a semi-automated
way to prepare platelets.
In an effort to overcome problems associated with the Amsterdam
method, Johansson et al. developed the bag system with the top and
bottom drainage of the primary bag and a sensor device which allowed
partially automated blood separation. Kretschmer et al., cited above,
used that type of system to prepare platelet concentrates from buffy
coats and found a mean of 6.7.times.10.sup.10 platelets per unit.
In co-pending patent application Ser. No. 07/493024 filed in the
names of Carmen et al., there is disclosed another system for removing
the lower contents of a blood bag in a relatively simple way. That
system uses a tubular member which extends from an upper port into
the interior of the bag, terminating just above the bag bottom.
When pressure is applied to the bag, the lower contents of the bag
exit through the tubular member and the top of the bag.
We are unaware of the use of longitudinal seals in any blood bag
system which are made to facilitate removal of the lower contents
of a blood bag through the top of the bag. In U.S. Pat. No. 4619650
to Wisdom, however, a blood bag with a different type of longitudinal
seal is shown. Although the central longitudinal seal of that bag
appears to leave a passageway between two halves of the bag at the
top (not the bottom) that bag would not allow the lower contents
of the bag to be removed from the top of the bag as in the present
system. More over, the purpose of the seal in that bag is to provide
a line along which the bag can be torn open for the efficient removal
(via machine) of frozen plasma.
Although the above system has been found useful and may be a practical
alternative to the top-bottom bag, we have now found what may be
an even more practical alternative which uses a novel blood bag
construction that offers surprising manufacturing and use advantages.
Details of the system are described below.
SUMMARY OF INVENTION
Our blood separation system comprises a main plastic blood bag
(a primary bag or a donor bag) in which separated components can
be expressed from the top of the bag and in an order chosen by the
The system includes a main or donor bag having a generally longitudinal
seal extending from the top of the bag and between two blood bag
ports downwardly to at least the lower half of the bag but not to
the bottom of the bag, thereby leaving an internal passageway between
the two ports that is located in the lower half of the bag. The
longitudinal seal angles inwardly at one end, terminating with a
spot weld proximal to the central axis of the bag. The channel,
formed by the longitudinal seal, also angles inwardly at the bottom
end and opens proximal to the central axis of the bag. The width
of the channel increases as it approaches the central axis. Thus,
taken in cross-section, the diameter of the channel at the top third
portion of the bag is smaller than the diameter of the channel at
the portion proximal to the central axis.
In one use, whole blood (or a component to be separated) is introduced
into the bag through one port via conventional methods. The whole
blood is then centrifuged to form an upper lighter portion and a
lower more dense portion. If desired, the denser portion can then
be removed from the lower part of the bag before removal of the
upper less dense component by applying pressure (can be manual or
automated) to the bag to push substantially all of the denser through
the internal passageway to one side of the longitudinal seal and
out of the bag through the port communicating with that side of
the longitudinal seal. That port must, of course, be opened for
In one embodiment, the longitudinal seal is relatively narrow (e.g.
less than 5 mm wide). Along one length of the bag, the seal runs
generally parallel and close to the one side. At the bottom end
of the seal, i.e., the end of the seal closest to the bottom of
the bag, the seal generally follows the contour of the bag, extending
proximal to the central axis. At the terminal end of the seal, a
spot weld is positioned to prevent the bag from tearing at the terminal
end of the seal.
The seal forms a channel, between the seal and the outer edge of
the bag. The channel, in combination with the passageway, provides
an outlet for the dense blood components from the bottom of the
bag through a port located at the other end of the channel and out
of the bag. The channel has one diameter along the length of the
bag, and an increasing diameter as it approaches the central axis.
The combination of the spot weld and increased channel opening diameter
at the passageway significantly reduce any vortexing, or Coriolis-type
effect that may occur at the passageway during blood component expression.
BRIEF DESCRIPTION OF THE FIGURES
FIG. 1 is a plan view of one preferred blood bag separation system.
FIG. 2 is a plan view of a plastic bag illustrating one embodiment
of the main bag of the invention.
FIG. 3 is a cross sectional view of the bag in FIG. 1 taken through
lines 3--3 of FIG. 2.
FIG. 3A is a cross sectional view of the bag in FIG. 4 taken through
lines 3A--3A of that figure.
FIG. 4 is a plan view of another embodiment of the plastic bag
of the invention.
Our preferred system for the separation of blood components comprises
a main donor bag having optical sensor-activated clamps or valves
associated with conventional tubings communicating with each of
the outlet ports of the bag. The outlet port for the more dense
component communicates with the internal passageway toward the bottom
of the bag interior. Preferably, the bag is made using simple, conventional
seals for the top, bottom, side and longitudinal seals.
In a preferred embodiment, the longitudinal seal is, on average,
within about 7 mm of the edge seal at one side of the bag (in a
flattened, empty bag). In that embodiment, the longitudinal seal
extends to within about 7 mm of the seal at the interior bottom
of the bag. When the bag is filled with fluid and balloons out,
the resulting passageway toward the bag bottom will have an average
diameter of less than about 7 mm. In that embodiment, the average
distance from the center of the longitudinal seal 3f along the side
of the bag to one side is less than 20%, preferably less than 5%,
of the total distance from that same side to the central axis A--A.
In an alternative embodiment, the longitudinal seal 3f essentially
follows the bag contour along the bottom to about the central axis
A--A of the bag 3 and is within about 7 mm of the bag edge seal
3j only along the length of one side of the bag. However, the approximately
bottom third of the seal 3f is an increasingly further distance
from the bottom edge seal 3j'. As the longitudinal seal 3f nears
the bag bottom, it angles inward and extends toward the central
axis A--A of the bag, generally following the line of the bag bottom.
The effect of this angular extension of the seal 3f is to increase
the width D of the channel along the bottom of the bag and reaching
a maximum width at the passageway 3d'. Preferably, the maximum width
of D' the channel at the passageway is within about 20 mm. In preferred
forms of the present invention the longitudinal seal 3f has a width
smaller than the channel width D.
One preferred system is a "triple" blood bag system consisting
of a primary or donor bag and two satellite bags pre-connected to
the primary bag by conventional PVC blood bag tubing and fittings.
The bags themselves may be made from conventional PVC plastic film
used to make blood bags (e.g., PVC plasticized with plasticizer
such as DEHP, TOTM, citrate esters and the like) or from other acceptable
plastic materials (e.g. polyolefins). The inner surfaces of the
major side walls that are sealed to form the bag may be frosted
or otherwise textured to prevent those inner surfaces from sticking
together when the bag is not filled with blood.
The primary bag 3 may be manufactured using conventional techniques
and modified to include a longitudinal seal 3f extending from between
two top ports 17a, 27a, toward the bottom of the bag as shown generally
as seal 3f in the Figures. The preferred bags and tubing of the
inventive system are essentially transparent.
FIG. 1 shows a "closed" triple multiple blood bag system
embodying the present invention and used in conjunction with electronic
fluid monitoring means. The main difference between the system of
FIG. 1 and that of co-pending patent application Ser. No. 07/493024
is in the middle bag, shown in more detail in FIGS. 2 and 3.
FIG. 2 shows one embodiment of the main bag 3 of FIG. 1 in more
detail. As can be seen in FIG. 2 the bag 3 has a generally conventional
appearance except for a longitudinal seal line 3f which extends
from the top of the bag, and between at least 2 ports, 17a and 27a,
downwardly to a position in the lower half of the bag but not to
the bag bottom. By not quite extending to the bottom of the bag,
a passageway 3d remains between the bulk of the bag interior 3g
and a channel 3e formed by the longitudinal seal 3f running along
and generally parallel to the left side of the bag. The only "closed"
communication between the upper ports 17a and 27a on either side
of the longitudinal seal 3f is passageway 3d. In a preferred embodiment
passageway 3d and the width of the channel 3e are as small as possible
but large enough to permit non-turbulent and unobstructed flow of
the lower bag contents out via tubing 17 when pressure is applied
to the bag (after centrifugation of the whole blood or other blood
FIG. 3 illustrates a cross-sectional view taken through lines 3--3
of FIG. 2 and shows the relatively small size and volume of the
channel 3e compared to the remaining bag volume 3g.
FIG. 4 illustrates an alternative embodiment of main bag 3 of the
present invention. In that illustrated embodiment the longitudinal
seal 3f' extends along the lower edge of the bag 3. The extended
longitudinal seal 3f' terminates at about the central axis A--A
in a spot weld 6.
FIG. 3A illustrates a cross-sectional view taken through lines
3A--3A of FIG. 4 and shows the larger channel diameter D' over the
channel 3e diameter shown in FIG. 3.
In systems without the spot weld 6 the seal 3f tends to tear open
when the bag 3 is full. The reliability of the seal 3f is improved
by the addition of the spot weld 6. The illustrated weld 6 is round,
preferably about 8 mm in diameter, however welds of other geometric
configurations, such as squared or oval, may be used.
In the illustrated embodiment of FIG. 4 the channel 3e has a width
D that remains relatively constant along the side edge 3j of the
main bag 3. In the lower approximately one-third of the channel
3e the width D of the channel 3e monotonically increases with respect
to the bottom bag edge 3j'. Thus, in the illustrated embodiment
the width of the passageway 3d' is larger than the channel width
The extended longitudinal seal 3f' in combination with the increased
passageway 3d' reduce the possibility of a Coriolis-type effect
occurring at the passageway 3d'. Such an effect is a vortexing that
essentially mixes the previously separated blood components as they
are moved through the passageway 3d' via channel 3e and out of the
After collection of whole blood via conventional needle assembly
3c and phlebotomy tubing 3a into a donor bag 3 such as that shown
in FIG. 1 the blood is centrifuged at relatively high centrifugal
force to form upper plasma component 9 intermediate buffy coat
component 13 and lower red cell component 11. The bag 3 is then
placed in a simple, conventional pressure-separator device (blood
bag expresser) consisting of a moving spring-loaded expresser plate
and a fixed plate.
In one preferred embodiment, the separation system includes two
on-off tubing clamps, 19 and 31 one on tubing 17 and one on tubing
27 activated by a sensor such as a photocell shown generally as
box 16. The lower RBCs pass through port 17 while the upper plasma
passes through port 27. Other ports (not shown) may be added as
needed to the donor bag in place or in addition to bag access port
15. Passage of the bag contents through the attached tubings may
be controlled using conventional frangible valves 3h (see FIG. 2).
An example of one such frangible valve is described in detail in
U.S. Pat. No. 4586928 to Barnes et al.
At the start of the plasma expression, the valve 3h communicating
with tubing 27 is open and the corresponding valve 3h for tubing
17 is closed. Plasma is expressed into bag 35 by pressure on bag
3 until red cells from the buffy coat 13 are first seen or detected
in tubing 27 by a photocell sensor positioned on the tubing 16
at which time clamp 31 on tubing 27 closes in response to an electrical
signal on wire line 25 from the sensor and the clamp 19 on tubing
17 opens in response to a signal on similar line 25. Red cells are
then expressed via passage way 3d through the top of bag 3 into,
for example, a second satellite bag 21 containing a conventional
red cell preservation solution such as AS-3 or the like (not shown).
The expression continues until only a volume of about 50 to 60 ml,
i.e., the buffy coat 13 remains in bag 3. This volume may be set,
if desired, by a simple stop (not shown) between the expresser plate
and the back plate against which the bag 3 is pressed in an otherwise
conventional blood bag expresser. Optical sensors may be positioned
at convenient places along lines 27 or 17 as needed to activate
valves 31 and 19.
For test purposes, whole blood was collected into a bag containing
a conventional anticoagulant and or preservative solution (CP2D/AS-3),
then transferred into the "channel bag" of this disclosure.
See bag 3 of FIG. 1. In practical use, however, the whole blood
would be collected directly into the donor bag 3 via phlebotomy
needle assembly 3a.
The blood unit was then centrifuged at about 3000Xg (2977Xg) for
9 minutes. Using a manual plasma expressor the upper plasma was
expressed via tubing 17 into an empty bag 35. Red cells were then
expressed through passageway 3d (see FIG. 2) and the channel (see
3e of FIG. 2) into additive bag 21. Expression of plasma was stopped
when red cell layer was about 1 cm from the top of the bag. Expression
of red cells was stopped when the pressure plate of plasma expressor
reached a predetermined mark. At this point about 60 g of buffy
coat (BC) remained in primary bag. About 60 g of plasma from bag
35 was expressed back to the BC in bag 3.
The BC in bag 3 was incubated overnight at about 22.degree. C.
on a platelet agitator to break up platelet aggregates. The next
morning the BC was transferred into another separation bag. An elongated
separating bag as shown in U.S. Pat. No. 4892537 to Carmen et
al. for neocyte separations was used. This bag can be connected
directly via tubing to an added port at the top of bag 3 for a truly
closed system. The reconstituted BC (about 60 g BC in 60 g plasma)
was centrifuged at 454Xg for 7 minutes.
The top layer of platelet concentrate was then expressed into an
attached TOTM plasticized PVC blood bag (see U.S. Pat. No. 4280497
to Warner et al.) for storage study.
Platelet counts were performed on all fractions using either an
electronic cell counter (plasma and platelet concentrate) or a manual
method. Data are summarized in the table below.
TABLE ______________________________________ Platelets from Buffy
Coat Data on Channel Bag (n = 9) Platelets Platelets % of Whole
Leukocytes Fraction No. .times. 10.sup.10 Blood No. .times. 10.sup.8
______________________________________ Whole blood 13.6 .+-. 4.2
100 Plasma 1.4 .+-. 0.6 10.3 Red Cells 0.3 .+-. 0.4 2.2 12.4 .+-.
7.7 Platelet Concentrate 8.1 .+-. 2.4 59.5 0.2 .+-. 0.2 Residual
Buffy Coat 3.0 .+-. 1.2 22.1 ______________________________________
The main bag of this disclosure provides manufacturing advantages
since it can be made using conventional blood bag manufacturing
methods. For example, many conventional blood bags are made by simply
sealing the edges along the perimeter of two sheets of plasticized
PVC film. The seal can be accomplished using conventional heat or
radio frequency (RF) plastic sealing devices. Chemical solvent sealing,
though possible, is less preferred because of possible seal failure
The longitudinal seal for both embodiments of the bag of this disclosure
was made using an RF sealing device and this method of sealing is
preferred for bags made from plasticized PVC. Bags made from other
plastics (e.g., polyolefins) could be sealed, for example, using
a simple heat seal.
The main bag 3 of this disclosure may be used for a gross separation
of whole blood into plasma (PRP) and RBCs or, preferably, a finer
separation of a blood component (e.g., the separation of platelets
from the buffy coat portion of blood) as described above.
The sequence of component removal may be varied to suit a particular
need or the availability of expressors (machines designed to press
a bag to squeeze out a given component).
For example, in our platelet preparation steps using the main bag
3 of this disclosure, we now prefer to express the less dense plasma
portion from the top of the bag before the RBCs are expressed. This
is done to reduce the chance of inadvertently getting some of the
platelets from the buffy coat into the channel 3e from where it
would be difficult to recover those platelets. By thus first expressing
the upper platelet poor plasma, the adjoining buffy coat is kept
as far away from the passageway 3d as possible.
Thus, in our preferred method, we first express the plasma from
the main bag 3. This leaves only the upper buffy coat and the lower
RBCs in the bag. It should be understood that the upper buffy coat
will contain some residual RBCs but, in general, there is a clearly
visible interface between the buffy coat portion and the lower RBCs.
Since the volume of buffy coat portion in a typical unit of blood
is about 50-60 ml, and since this amount is typically reconstituted
with an equal amount of plasma (60 ml), this total volume (120 ml
or about 120 g) can be used to determine how much of the RBCs should
be expressed to leave behind only the buffy coat portion.
For example, we have found that when a conventional blood bag expressor
is used, either a V-expressor or a two-parallel plate expressor,
the expressor can be marked to show a distance that the expressor
plates should be apart to result in a remaining volume of about
120 ml (or about 120 g). After the position of the mark, which may
be a simple pencil mark, is set with about 120 ml of a fluid (e.g.,
water) the mark can then be used as a guide for future expressions
of the red blood cells.
In a preferred method for preparing platelets from whole blood,
a 50-50 mixture of buffy coat and plasma is made by transferring
about 60 ml (or about 60 g) of plasma from the plasma collection
bag 35 back to the main bag 3. This mixture is then incubated overnight
at room temperature with gentle agitation to break up platelet aggregates.
Incubation can be in the original main bag having the longitudinal
seal or in another bag to which it is expressed, preferably under
closed conditions. Ideally any such other bag is preconnected via
tubing to the main valve and includes an externally manipulated
valve to control timing of the transfer.
We have found that the elongated bag of U.S. Pat. No. 4892537
to Carmen et al. provides especially good platelet from buffy coat
separations. That bag has a length to width ration of at least 2
to 1 and a tapering and portion that expands to form a funnel-like
guide for directing a separated component from the bag after centrifugation.
In our application, the less dense platelets form the top portion
after centrifugation of the buffy coat in the elongated bag. Those
platelets are then expressed out for storage into yet another preconnected
bag, preferably a bag suitable for such storage. Exemplary suitable
bags include a polyolefin bag or a plasticized PVC bag, either having
been made from plastic having a relatively high gas transmissivity,
helpful for platelet storage.
One such PVC bag is the TOTM plasticized bag of U.S. Pat. No. 4280497
to Warner et al. The bag described in that patent had wall thicknesses
in the range of about 0.01 to 0.20. The walls had a CO.sub.2 transmission
of at last about 4000 ml/meter.sup.2 /day and an O.sup.2 transmission
of at least about 600 ml/meter.sup.2 /day. These rates would be
considered relatively high gas transmissivity as the expression
is used here.
Given the above disclosure it is thought variations will now occur
to those skilled in the art. Accordingly, the above examples should
be construed as illustrative and the scope of the invention disclosed
herein should be limited only by the following claims.