Exchange structure for biomedical equipment

The structure has a core which may be a strand or braid, as well as a sheath surrounding the core and defining the exchange surface. Preferred applications are for biomedical equipment such as blood oxygenators and blood purification equipment.

FIELD OF THE INVENTION 
The invention relates to exchange structures. The invention has been 
developed with particular concern for its possible use, for example, in 
the field of biomedical equipment. 
In this context not only are mass-exchange structures currently utilized 
(such as those present in devices for the oxygenation of blood or for the 
purification of blood), but also heat-exchange structures (for example for 
the production of heat exchangers usable together with blood oxygenators). 
Substantially similar structures can be utilized in industrial fields for 
various purposes, for example, for purification processes based on a 
reverse osmosis mechanism 
A characteristic common to almost all these structures is that they provide 
for the arrangement, or flow, of at least two media (usually fluids) in at 
least locally facing positions so as to cause an exchange, usually of mass 
and/or heat, between these two media by various mechanisms. 
DESCRIPTION OF THE PRIOR ART 
For mass transfer or exchange structures, such as blood oxygenators, (it 
should however be specified that the invention is not intended to be 
limited in any way to this specific context or application), at least four 
different types of configurations have been proposed and utilized in the 
art: 
bubble oxygenators (see for example U.S. Pat. No. 3,915,650 and U.S. Pat. 
No. 4,428,934); 
film oxygenators (see for example U.S. Pat. No. 3,070,092 and the 
description contained in the introductory part of U.S. Pat. No. 
5,244,930); 
flat membrane oxygenators (see for example U.S. Pat. No. 4,698,207); and 
hollow fiber oxygenators (see for example EP-A-O 292 445). 
Film or bubble oxygenators have numerous elements which are critical in 
their operation, such as, for example, the problem of eliminating the 
bubbles of residual gas in the oxygenated blood before it is reintroduced 
into the patient's blood. 
Membrane oxygenators pose production problems, above all as regards the 
orientation of the membrane, which must be preserved as far as possible 
during use; this requirement imposes the need for extremely complex 
mechanical support structures. 
Hollow fiber oxygenators resolve many of the problems inherent in the other 
structures. They have, however, the disadvantage of becoming subject, 
during use, to phenomena of accumulation and condensation of water vapor 
and organic materials (typically proteins) which cause their operating 
efficiently to deteriorate gradually. This happens especially since the 
material of the fibers, originally hydrophobic, tends gradually to become 
hydrophilic. In general, also, so that they can be self-supporting without 
risk of collapse or rupture as a result of the pressure gradients applied 
to them in use, the fibers must have a wall thickness (typically of the 
order of 25-50 micron in the case of microporous fibers for oxygenation) 
which cannot be considered optimum for the exchange mechanism, which is 
usually improved or is made easier by smaller wall thicknesses. 
Considerations generally similar to those given above also apply to the 
other exchange structures mentioned initially, for example, for blood 
purification equipment (operating on various principles such as dialysis, 
haemofiltration, haemodiafiltration etc.) and/or heat exchangers. 
OBJECTS AND SUMMARY OF THE INVENTION 
The object of the invention is therefore to provide an exchange structure 
which, whilst in its essential parts resembles a conventional hollow fiber 
exchange structure, overcomes its disadvantages, particularly those 
discussed above, in a radical manner. 
According to the present invention, this object is achieved by an exchange 
structure having the characteristics specifically set out in the following 
claims.

DETAILED DESCRIPTION OF THE INVENTION 
As already mentioned above, the exchange structure according to the 
invention is, to a certain extent, like a fiber structure in the sense 
that it lends itself to being made in the form of an elongate filamentary 
body of indefinite length of which FIGS. 1, 3, 5 and 6 show any portion 
whilst FIGS. 2, 4 and 7 show a section taken on any diametrical plane (the 
longitudinal position in which this section is taken varies only the 
relative orientation of the elements considered). 
An exchange structure according to the invention, generally indicated 1, 
comprises essentially two elements, both of indefinite length and 
therefore capable of being cut to any desired length or wound in any 
manner entirely similar to those utilized for fibers, that is to say: 
a core 2, 2', 2", 2"', which defines the general longitudinal development 
of the exchange structure 1, and 
a sheath or covering 3 of generally tubular form (although this 
configuration is not essential in itself which surrounds the core, 2, 2', 
2", 2"', more or less loosely, for example being fitted over the core 
itself. 
In this connection it should be stated that even though explicit reference 
will be made in the following description to an embodiment in which the 
core and the sheath are physically separate from one another, being 
constituted by two separate bodies, the invention also lends itself to 
being made with arrangements in which the core and the sheath are at least 
marginally connected together, for example because they are made 
simultaneously or almost simultaneously, for example by an extrusion, 
co-extrusion or like operation. 
Whatever specific constructional arrangement is adopted, the sheath 3 
defines an exchange surface between at least one first medium, which can 
flow within the space between the internal surfaces of the sheath 3 and 
the outer surface of the core 2, 2', 2", 2"', and at least one second 
medium which, in use, flows over the outer surface of the sheath 3 in a 
mechanism generally similar to that which regulates the functioning of 
hollow fiber or tune exchange structures. 
In the embodiment of FIGS. 1 and 2, the core 2 is formed by two filamentary 
or fiber elements (here exemplified in the form of solid fibers) 
preferably wound, at least locally, in a generally helical configuration. 
A substantially similar arrangement is adopted for the core 2' of the 
embodiment of FIGS. 3 and 4 in which there are three filamentary elements 
4 (here also shown in the form of solid fibers), preferably coupled in a 
generally stranded arrangement, also with a generally helical 
configuration in this case. 
In the embodiment of FIGS. 5 and 6, the core is constituted by a profiled 
body 2" or 2"' with a lobed cross-section, for example with four 
substantially identical lobes equiangularly spaced at 90.degree. about the 
longitudinal axis of the core. 
The difference between the embodiment of FIG. 5 and the embodiment of FIG. 
6 lies in the fact that, whilst the core 2" of FIG. 5 has a generally 
rectilinear form, the core 2"' of FIG. 6 is, so to say, twisted along its 
axis such that the lobes 5 are in a generally helical configuration. 
With regard to the embodiment illustrated in FIGS. 1, 3 and 6 it should be 
said that, in these drawings, the helical configuration of the core 2, 2', 
2", 2"' has been exaggerated) in order to illustrate the principal clearly 
(compared with the real situations currently considered preferable. 
A characteristic common to all the embodiments illustrated is the fact that 
the sheath 3 surrounds the core 2, 2", 2"' in such a way as to leave 
spaces 6 between the core and the inner face of the sheath 3. These spaces 
define longitudinal flow channels within the exchange structure 1 for one 
of the media which is involved in the exchange process (of mass or heat) 
in use of the structure. For example, in the case of a blood oxygenator 
device, in a haemofilter or a haemodialiser, the spaces 6 define the flow 
channels for the oxygen, the blood filtrate or the dialysis solution, 
whilst the blood which is oxygenated or purified flows over the exterior 
of the sheath 3. 
In embodiments such as those of FIGS. 1 to 4, the spaces 6 have a 
cross-section which is generally V-shaped with rounded sides (defined by 
the outer surfaces of the fibers 4). To these is added, in the 
arrangements of FIGS. 3 and 4, a central channel or space 6' of 
approximately triangular shape. Not being exposed to the sheath 3 and 
therefore not being usable in the exchange mechanism, this central space 
6' can be closed or utilized for other purposes (for example for the 
passage of guide wire). 
It goes without saying that the arrangement described in FIGS. 1 to 4 can 
readily be extrapolated to the production of further variants of the 
invention (not illustrated here) in which the core is formed by a strand 
or braid or four or more filamentary or fiber elements 4. 
The provision of a plurality of fibers 4 in a generally helical arrangement 
enables the outer surface of the sheath 3 also to have generally 
helically-ribbed shape if fitted tightly, or even slightly loosely around 
the core 2 or 2'. This particular conformation is advantageous in all 
those situation in which it is useful to have a degree of turbulence in 
the medium which flows over the outer surface of the exchange structure in 
order to achieve effective exchange: for example, in the cases cited 
above, blood which is to be oxygenated or purified. Similar, substantial 
turbulence, or at least non-stagnation, of the medium which flows within 
the exchange structure 1 is promoted by the generally rough form of the 
channels 6. 
The above also applies essentially to the embodiments shown in FIGS. 5 to 
7, especially for the embodiment of FIG. 6 in which the core 2"' has a 
generally helical form. 
The variants of FIGS. 5 to 7 have the further advantage that the profiles 
of the connector surfaces, indicted 6a, interconnecting the lobes 5 can be 
modified selectively. The surfaces 6a, together with the inner surfaces of 
the portions of the sheath 3 facing them define the cross-sectional 
profiles of the spaces or channels 6. 
Thus, for example, whilst the drawing of FIG. 7 shows surfaces 6a of 
generally arcuate shape, these surfaces could have an L or V shape or a 
polygonal or more complex shape. All this within the recognized preference 
for shapes in those regions of the lobes 5 intended to come into contact 
with the sheath which are generally rounded in order to support it: 
corners which are too sharp would in fact induce wear and possibly rupture 
of the sheath 3 itself. 
It will likewise be appreciated that, in all the embodiments illustrated, 
it is possible to make the channels 6 of such a shape that, in those 
regions in which the lobes 5 lie close to the sheath 3, corner regions 
with extremely small radii are formed. All this is for the purpose of 
causing, when liquids are present or flowing in the channels 6, capillary 
phenomena to occur which prevent the formation of gas-liquid fragmentation 
with the exchanger structure, according to the teachings provided in 
EP-A-O 521 430. 
As regards the choice of constituent materials the structure according to 
the invention is extremely flexible and can be varied according to 
specific fields of application. 
For example, for the production of mass-exchange structures (oxygenators, 
dialysers, structures for reverse osmosis, etc.), for the sheath 3 it is 
possible to use all materials conventionally utilized for the production 
of exchange membranes or fibers, for example, polymers, polyethylene, 
polypropylene, polyurethane, polysulphone, silicone, cellulose materials, 
etc., up to the use of metals in the case of heat-exchange structures, for 
example metals having a high thermal conductivity (copper, aluminum, 
steel, etc.) 
There is also the possibility, as far as the wall thickness of the sheath 3 
is concerned, of using extremely small values, for example, of the order 
of a few microns, which is optimum for the majority of the applications 
indicated above, and of making use of unified technologies (for example 
extrusion, spinning, co-extrusion, etc.) which do not impose particular 
overriding limitations as regards the dimensions of the structure. These 
can therefore be shown according to the specific requirements of use, for 
example as far as the overall section of the channels 6 is concerned. By 
way of example, structures of the type illustrated in the appended 
drawings can be made with diameters of the order of 150-400 microns, from 
which it will easily be understood how, in the drawings, and in particular 
in the sections of FIGS. 2, 4, and 7, the thickness of the sheath 3, which 
is a thin-walled body, is shown with dimensions which are greatly enlarged 
with respect to the real relative dimensions for reasons of clarity of 
illustration. 
The above is also true in essence as regards the production of the cores 2, 
2', 2", 2"'. Here, too, the choice of materials in just as wide, it being 
possible to range from very diverse types of polymer to metals (above all 
when it is desired to give the structure a certain longitudinal rigidity 
or strength, for example for the production of devices for 
catheterization). All this can be achieved by the manufacturer with the 
use of known manufacturing techniques, such as, typically, extrusion, 
co-extrusion of the sheath on the core, or fitting of heat-shrinkable 
sheaths and/or (in the case of structures such as those illustrated in 
FIGS. 1 and 3) spinning and winding techniques. As already mentioned, the 
sheath and the core may then consist equally well of two separate bodies 
or parts of a single body. This is regardless of whether they are made 
from the same material or are formed from different materials. 
Naturally, the principle of the invention remaining the same, the details 
of construction and the embodiments can be varied widely with respect to 
those described and illustrated, without thereby departing from the ambit 
of the present invention.