Transfer membrane assembly

A conduit assembly (23) for an artificial lung is formed from two sheets (30) of thermoplastic transfer membrane material which are sealed together along sinuous lines (24) to provide a unitary assembly of tubular conduits (26) each having longitudinally spaced hollows (27) which promote eddies in, and mixing of, blood when passed through the conduits with a pulsatile flow.

The invention is concerned with apparatus for effecting transfer of heat or 
mass between two fluids, of which at least one is usually a liquid, 
through a transfer membrane. Such apparatus is used in blood oxygenators, 
that is artificial lungs, and dialysers, such as artificial kidneys, in 
which case one fluid is blood, and the other is oxygen or dialysate. In 
practice the efficiency of the transfer across the membrane is limited by 
the extent to which the total volume of fluid can be brought into close 
proximity with the membrane. It is not sufficient to reduce the thickness 
of the fluid layers, by reducing the thickness of the conduits in which 
they flow, as this increases undesirably the pressure drop across the 
apparatus and leads to uneven perfusion and regions of stagnation, which, 
in the case of blood, provides a danger of thrombosis. 
We believe that the proper solution lies in so shaping the fluid flow 
conduits that significant mixing of the fluid occurs within the conduits. 
It has previously been proposed in British patent specification No. 
1,422,754 to provide an apparatus comprising a conduit for flow of one of 
the fluids at least partially defined by the membrane, a transverse 
dimension of the conduit varying in a regularly repeating manner along the 
length of the conduit, to provide a multiplicity of hollows in the 
membrane, the apparatus also comprising means communicating with the 
conduit for passing fluid through the conduit in pulsatile flow, the 
arrangement being such that pulsation of fluid past the hollows give rise 
in the hollows to rotational fluid flow having components of motion 
parallel and perpendicular to the general direction of flow in the conduit 
of the fluid. Such apparatus is hereinafter referred to as of the kind 
described. 
The conduit may be defined between two predominently planar surfaces, so 
that it has an elongate cross section transverse to the general direction 
of flow through the conduit, at least one of the surfaces then being 
provided by a membrane in which the requisite hollows are provided by 
furrowing at least one of the surfaces defining the conduit. 
Alternatively, the conduit may be tubular, that is essentially 
axisymmetric with its wall provided by a membrane. Tubular conduits have 
the advantage of being self supporting and do not require complex moulded 
support plates. However, they have not been cheap to produce in the 
numbers required in a typical apparatus. 
In accordance with the present invention, a conduit assembly for use in an 
apparatus of the kind described comprises two sheets of plastics material, 
at least one of which is a transfer membrane material, which are sealed 
together face to face along a number of pairs of sinuous lines extending 
generally in the same direction alongside one another with the undulations 
of the lines of each pair of lines out of phase with one another thereby 
providing, upon inflation of the passageways between the sheets and 
between each pair of lines, a tubular conduit with longitudinally and 
substantially regularly spaced hollows with intervening constrictions. 
In this manner there is readily provided a unitary construction consisting 
of a generally planar array of tubular conduits, with walls of transfer 
membrane material, and of the required internal configuration. 
If the undulations in the adjacent lines of each adjacent pair of lines are 
also out of phase with one another, a tubular conduit with the required 
internal configuration will also be provided between each adjacent pair of 
lines with the annular hollows in one conduit in lateral alignment with 
the constrictions in each adjacent conduit. This layout provides maximum 
saving of space and material. 
The unitary structure comprising the array of conduits may also incorporate 
fluid inlet and outlet manifolds by providing additional lines of sealing 
which define between the two sheets spaces communicating with respective 
ends of the conduits. 
Inlet and outlet passages to the respective manifold spaces may also be 
provided between the two sheets by means of further seal lines. However, a 
preferred way of leading fluid into or out of at least one of the 
manifolds is by the provision of a distributor which is sealed to and 
between the two sheets in the manifold, the distributor having a passage 
which opens, in use, through at least one of the sheets and into the 
respective manifold. In order to keep the interior of the assembly clean, 
prior to use, it may be manufactured with the distributor between 
imperforate sheets and a hole cut through at least one of the sheets in 
alignment with the passage in the distributor immediately prior to use. 
In use, one or a stack of the unitary assemblies of tubular conduits will 
be provided within a housing and connected to appropriate pumping 
arrangements so that one fluid can be pumped through the conduits in 
parallel with a pulsatile flow and the other fluid will be pumped through 
the housing around the assembly or assemblies in contact with the transfer 
membrane material. The second fluid will preferably flow in countercurrent 
or crosscurrent to the first fluid through the housing. When a stack of 
the assemblies are used and the assembly manifolds have the distributors 
referred to, the passages of the distributors of the corresponding 
assembly manifolds may be sealed to one another to provide a continuous 
passage leading to or from all the assemblies. 
The plastics material from which each of the two sheets of the conduit 
assembly is made will be of a suitable hygienic grade when the assembly is 
for use in a medical application. The plastics material will usually be a 
thermoplastic material, i.e. one which softens upon being heated and then 
resets upon cooling, such as polypropylene, polyethylene or 
polytetrafluoroethylene (PTFE). The seal lines may then be produced by 
heat sealing, for example by direct application of a hot die or by an 
ultrasonic welding tool. Alternatively a solvent weld may be possible. 
In order to inflate, and preferably set, the walls of the tubular conduits 
in their desired configuration, the conduits may be inflated by being 
filled with a fluid, preferably a liquid, under a pressure sufficient to 
stretch the sheet material just beyond its elastic limit. After this 
treatment the tubular conduits adopt a cross section throughout which is 
substantially circular. It may be possible to carry out the inflation step 
more quickly using heated air under pressure. 
In order that the conduits are set in a shape in which the transition 
between the annular constrictions and hollows is smooth, the conduits are 
preferably longitudinally stretched, for example by being clamped in a 
frame, or indeed in the final housing in which they are to be used, during 
the setting. Some appropriate sheet materials are produced with the 
facility for stretching more easily in one direction than in the 
perpendicular direction. It is desirable to use such material with the 
direction of easier stretching extending substantially parallel to the 
lengths of the tubular conduits.

The illustrated artificial lung has a rectangular housing formed by a cover 
10 and base 11 which are of similar shape and which are secured together 
at their corners by screws 12 which pass through the holes 13 in the cover 
and screw into tapped holes 14 in the base. A sealing gasket 15 is 
interposed between the edges of the cover 10 and base 11 to seal a chamber 
within the housing. The inner surfaces of the cover and base are stepped 
at 16 so that the chamber within the housing consists of a central portion 
17 of greater depth, and, at each end, portions 18 of lesser depth. 
The cover 10 is provided centrally at each end with a larger opening 19 to 
which external hoses 20 are coupled, and, at diagonally opposite 
positions, smaller openings 21 to which external hoses 22 are fitted. 
The housing contains a conduit assembly 23 in the shape of an elongate 
octagon. The assembly is formed by two sheets 30 of microporous 
polytetrafluoroethylene membrane material of a kind sold under the trade 
name Gore-tex which are heat sealed together along a number of generally 
parallel but sinuous lines 24 and along a peripheral octagonal line 25. 
The lines 24 define between them, and between the two membrane sheets a 
side by side array of tubular conduits 26 which, when the conduits are 
expanded, are of circular section and repeatedly increasing and decreasing 
diameter, thereby providing a series of longitudinally spaced hollows 27 
separated by constrictions 28. It will be appreciated that the hollows and 
constrictions of adjacent conduits 26 are out of phase with one another so 
that they nest together and are joined along the sinuous lines 24 which 
remain substantially coplanar. The maximum diameter of each conduit 26, in 
each of its hollows, is substantially 5 mm. and the pitch between adjacent 
hollows is a similar dimension. 
At each end of the conduit assembly, the peripheral seal line 25 defines, 
between the membrane sheets, and in communication with the adjacent ends 
of the conduits, inlet and outlet manifolds 29 of rhombic shape. Within 
each manifold is positioned a rigid annular distributor 31 made from a 
plastics material such as polycarbonate, heat sealed to the inner faces of 
the two membrane sheets. The distributor has a series of radial ports 32 
opening into the respective manifold from a central passageway 33 through 
the distributor. On one face the distributor has a projecting annular 
flange 34, in alignment with the passage 33 and extending through a hole 
in the upper membrane sheet of the conduit assembly into the adjacent 
passage 19. The extension of these flanges 34 into the two openings 19 
locates the conduit assembly in the housing and the fixing is completed by 
the distributors 31 with the membrane sheets above and below, being 
clamped between the cover 10 and base 11, thereby determining the narrower 
depth of the chamber portions 18. 
In use the hose 20 at the left hand end of the housing as shown in FIGS. 1 
and 2 will be connected to a blood supply which is pumped to the 
artificial lung with a pulsatile flow, for example by means of a 
unidirectional roller pump in series with a reciprocatory pump. Similarly 
the hose 20 at the right hand end as seen in FIGS. 1 and 2 will be 
connected to the return blood circuit. Blood will thus flow in through the 
left hand hose 20, through the left hand distributor 31, and the inlet 
manifold 29, in parallel through the conduits 26, into the outlet manifold 
29 and through the outlet distributor 31 and the right hand outlet hose 
20. The blood flows with a mean flow velocity through the conduits 26 but 
the superimposed reciprocatory component causes the blood alternately to 
accelerate and decelerate and this sets up eddies B in the hollows 27, as 
shown in FIG. 4. These promote intimate mixing of the blood and contact 
between the blood and the transfer membranes. 
At the same time oxygen is pumped in a steady stream through the oxygen 
inlet hose 22 at the right hand end of the housing as seen in FIGS. 1 and 
2, essentially diagonally through the housing in contact with the outside 
of the conduit assembly 23, and out of the oxygen outlet hose 22 at the 
left hand end of the housing. The oxygen O thus flows partly in 
crosscurrent and partly in countercurrent to the blood and good transfer 
occurs through the membrane walls of the conduit assembly 23. 
In vitro tests of the lung have been carried out using sheep blood. The 
blood was pumped at a mean flow rate of 75 ml. per min. with a 
superimposed reciprocatory component using a reciprocatory pump with a 
stroke volume of 1.3 ml. and a pulse frequency of 200 per min. Oxygen 
transfer rates reached 200 ml./min.m.sup.2. The surface area of the 
artificial lung was 106 cm.sup.2. 
The same artificial lung was tested in a live sheep, in veno-venous bypass 
using cannulae in the external jugular veins. At a mean blood flow rate of 
75 ml/min. the lung raised the oxygen saturation of the blood from 63% at 
the inlet to 83% at the outlet, achieving oxygen transfer rates of 226 
ml/min.m.sup.2. 
Clearly the depth of the housing could be increased to accommodate two or 
more of the conduit assemblies stacked together, in which case the annular 
flanges 34 on the distributors of the lower assemblies would project up 
through holes in the lower membrane sheets of the assemblies above and 
project into and be sealed within the lower ends of the passages 33 of the 
assemblies above. Single supply and return lines of blood would then be 
provided for all the assemblies in the stack. 
The illustrated conduit assembly is produced by cutting two sheets to the 
appropriate elongate octagonal shape, and placing them face to face on a 
heat sealing die formed by a number of upstanding metal strips 
corresponding in shape to the lines 24 and 25. The metal strips are 
embedded in rubber or asbestos substrate and connected into an electrical 
heating circuit. The sheets are then covered by a soft pad to press the 
sandwiched sheets down against the die and heating current is passed 
through the metal strips in turn. Each pair of sinuous lines require 
sealing for about 2 to 3 seconds utilizing a power of 170 watts. After all 
the lines of heat sealing have been set, the assembly is pressurized with 
water at about 16.degree. C. and a superatmospheric pressure of about 15 
psi. This is sufficient to inflate the conduits 26 and indeed stretch the 
membrane sheet material just beyond its elastic limit so that, upon 
subsequent depressurisation, the conduits maintain their tubular shape 
with smooth transition between the hollows and constrictions. During the 
inflation step, the assembly is stretched parallel to the lengths of the 
conduits 26 either on a frame or possibly simply by locating the assembly 
in the illustrated housing by means of the distributors 33 and the 
clamping pressure in the region of the distributors.