Blood filter material

A filter material for filtering leucocytes from a fraction of or whole human blood, comprises a textile web having a thickness of between about 2 millimeters to about 12 millimeters and a bulk density of about 0.05 to 0.4 g/cm.sup.3. The web has a plurality of interlocked textile fibers with average deniers between about 0.05 and 0.75 and average lengths between about 3 millimeters and 15 millimeters. The textile fibers are distributed in the web to form a matrix thereof with spaces between adjacent interstices of interlocked fibers. A plurality of fibrillated particles of textile fiber material, having a surface area of between 5 and 100 square meters per gram are disposed within the spaces of the matrix. The weight ratio of the fibrillated particles to the textile fibers is between about 1:99 and 40:60. A plurality of glass fibers, having an average diameter of between 0.1 and 5 microns and being about 2% to 85% of the web, also form part of the matrix. A thermoplastic binder is disposed at least at cross-over portions of the matrix textile fibers and matrix glass fibers, with the amount of the binder being about 0.1% to 20% by weight of the web.

BACKGROUND OF THE INVENTION 
The prior application discloses a blood filter and method of filtration for 
removal of leucocytes from whole blood or blood fractions. The filter 
material is a shape-sustaining laid textile web having a thickness of at 
least about 1 millimeter and a bulk density of between about 0.05 and 0.4 
g/cm.sup.3. The web has a plurality of interlocked, textile fibers with 
average deniers between about 0.05 and 0.75 and average lengths between 
about 3 millimeters and 15 millimeters. The textile fibers are 
substantially uniformly distributed in the web so as to form a matrix of 
the fibers with spaces between interstices of the interlocked fibers. 
Within those spaces are disposed a plurality of fibrillated particles of 
polymeric material having a surface area of between about 5 and 60 square 
meters per gram. The fibrillated particles have a plurality of fine 
fibrils which are interlocked with adjacent textile fibers such that the 
fibrillated particles are not substantially displaceable from the web 
during filtration of blood. The weight ratio of fibrillated particles to 
textile fibers is between about 1:99 and 40:60. 
As can be appreciated, the textile fibers and the fibrillated particles 
must be so interlocked that significant amounts of fibers or fibrillated 
particles are not displaced from the filter material during filtering of 
blood, since displaced fibers or particles remain in the filtered blood. A 
significant amount of displaced fibers or fibrillated particles in the 
filtered blood could cause difficulties when the filtered blood is 
transfused into a human patient. 
As disclosed in that application, for efficient and effective depletion of 
leucocytes from blood passing through the filter material, both the fiber 
geometry and the surface area of the fibers are important, and that, very 
importantly, the surface area must be significantly greater than the usual 
prior art blood filters, since otherwise the degree of leucocyte removal 
is not sufficient. Further, since fiber geometry and surface area are 
important for leucocyte depletion, the depth (thickness) of the filter 
material is also important. Somewhat similarly, since the bulk density of 
the filter material and the denier of fibers affects fiber geometry and 
surface area, these are also important. 
However, to achieve the high surface area of the filter material required 
for effective leucocyte removal from blood, a critical component is that 
of the very high surface area fibrillated particles in the filter 
material. Ordinary textile fibers cannot provide such high surface areas 
to the filter material which surface area is required for high leucocyte 
removal. The fibrillated particles are somewhat elongated particles with 
an elongated central portion from which radiate a large number of fibrils. 
Generally speaking, a typical particle has an overall length of less than 
1000 microns, e.g. 5-300 microns, and a width and depth of from about 0.1 
to 50 microns, e.g. 0.1 to 5 microns. 
As can be appreciated, it is important to ensure that the very small 
fibrillated particles are not significantly displaced from the filter 
material during filtration of blood, and, as disclosed in that prior 
application, this is achieved by interlocking the fibrils of the 
fibrillated particles with adjacent matrix textile fibers. In addition, 
the depth (thickness) of the filter material, the bulk density thereof and 
the length of the matrix textile fibers (which affects the configuration 
of the laid matrix textile fibers) contribute to retaining the fibrillated 
particles. Further, especially at higher weight ratios of fibrillated 
particles to matrix textile fibers, e.g. 6:94 to 10:90, permanent securing 
of the fibrillated particles in the filter material may be improved by use 
of means for adhering the matrix textile fibers and fibrillated particles 
to each other, such as by tackifying adhesives and especially the use of 
sheath/core fibers for at least part of the matrix textile fibers, e.g. a 
sheath of low melt temperature polymer and a core of higher temperature 
polymer. When the matrix textile fibers are at least in part such 
sheath/core fibers, during usual processing of the filter material web, 
the sheath softens and causes bonding, upon cooling, between the matrix 
textile fibers themselves and fibrillated particles, especially the 
fibrils thereof. 
SUMMARY OF THE INVENTION 
While the filter materials of that prior application, as very briefly 
described above, are quite effective for leucocyte removal from blood or 
blood fractions, it has not been found that the overall performance of 
those filter materials may be further improved in certain regards by use 
of certain modifications thereof. In this regard, it has been found that 
the efficiency of the filter material (percent leucocyte removal per unit 
thickness of filter material) can be improved. This means that for a 
targeted leucocyte removal percentage, the present improved filter 
material can be of less thickness than the filter material of the prior 
application. This, in turn, means that the amount of blood or blood 
fractions retained in the filter material after filtration is completed is 
less than that of the filter material of the prior application. While this 
difference in the amount of retained blood is not an absolute large 
amount, the difference can be quite important, especially in certain 
filtrations of blood. 
Further, with the present invention, less fibers are released from the 
filter during an AAMI test (defined in Example 3) which is a very severe 
test, i.e. more severe than in practical use, but this well recognized 
test ensures a substantial safety factor for actual use of the filters. 
In the above regard, the present invention is based on several primary and 
several subsidiary discoveries. First of all, as a primary discovery, it 
was found that if the average sizes (diameter and length) of the matrix 
fibers are, generally speaking, less than the average sizes of the matrix 
fibers of the prior application, increased efficiency of the filter 
material occurs. As a subsidiary discovery, it was found that this is 
especially true when the matrix fibers are, at least in part, made of 
certain materials, especially glass. 
As a second primary discovery, it was found that these smaller average 
matrix fibers could nonetheless adequately interlock with the fibrillated 
particles to prevent displacement of either the matrix fibers or 
fibrillated particles from the filter material during blood filtration 
when the filter material has therein an added thermo-softening binder. 
As a subsidiary discovery in the above regard, it was found that certain 
binders could be added to the web of matrix fibers and fibrillated 
particles, and the web could then be heated to uniformly distribute these 
binders and set the binder in such a manner as to interlock the smaller 
average matrix fibers and fibrillated particles so that no significant 
amount thereof is displaceable from the filter material during filtration 
of blood or blood fractions. 
As a primary discovery, with such binders, fibrillated particles of larger 
surface area may also be used and the efficiency of the filter material 
(and hence the required thickness of the filter material for a targeted 
leucocyte removal) could further be reduced. As a subsidiary discovery, 
smaller average matrix fibers may be used with the larger surface area 
fibrillated particles, when such binder is used, to provide very high 
efficient filter material. 
Thus, briefly stated, the present invention provides an improved filter 
material for filtering leucocytes from a fraction of or whole blood 
comprising a shape-sustaining laid textile web having a thickness of at 
least about 1 millimeter and a bulk density of between about 0.05 and 0.4 
g/cm.sup.3. The web has a plurality of interlocked textile fibers with 
average deniers between about 0.05 and 0.75 and average lengths between 
about 3 millimeters and 15 millimeters. The textile fibers are 
substantially uniformly distributed in the web so as to form a matrix of 
the textile fibers with spaces between adjacent interstices of interlocked 
fibers. A plurality of fibrillated particles of polymeric material having 
a surface area of at least five square meters per gram (but preferably 
less than 100 square meters per gram) are substantially disposed within 
the spaces of the matrix, and the weight ratio of the fibrillated 
particles to the textile fibers is between about 1:99 and 40:60. 
The web also has therein a plurality of glass fibers (forming at least a 
part of the matrix fibers) having an average diameter of between about 0.1 
and 5 microns and the amount of the glass fibers is about 2% to 85% by 
weight of the web. 
The web has added thereto a thermoplastic binder disposed at least at 
cross-over portions of the textile fibers and the glass fibers and the 
amount of the binder is about 0.1 to 10% by weight of the web. 
The binder is preferably applied to the formed web as an emulsion thereof.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION 
The present invention provides a filter material for filtering leucocytes 
from a fraction of or whole human blood. FIG. 1 shows such a filter made 
of such material. That filter, generally 1, is made from a filter material 
which is a shape-sustaining laid textile web. As shown in FIG. 1, the web 
has been cut in a circular configuration to form the filter, and the 
filter is suitable for loading into a cylindrical filter carrier. In this 
regard, the term "laid" is used in its ordinary technical sense. Thus, the 
web must be either air laid or wet laid, as opposed to, for example, 
needled, since it is in the laying process that the textile fibers, glass 
fibers and the fibrillated particles so interlock that the glass fibers 
and fibrillated particles are bound into the filter material. Needled 
textiles, for example, cannot provide such interlocking of the glass 
fibers and fibrillated particles with the textile fibers, and it is, 
therefore, necessary that the web be a laid web. 
The thickness T of that web must be at least 1 millimeter, most preferably 
at least 2 millimeters, and may be up to about 30 millimeters, or more. A 
filter depth, as opposed to a web depth, results from placing layers of 
filter webs one upon another, until the desired filter depth is achieved. 
Thus, if a filter depth of, e.g., 12 millimeters, is desired and the web 
has a depth of 2 millimeters, six such layers of web will be required. 
Thus, rather than producing thick webs which are more difficult to 
product, thinner, more easily produced webs are preferred, with an 
appropriate number of layers of web to achieve the desired filter 
thickness. 
However, for effective leucocyte depletion from a blood fraction or whole 
blood passing through the filter, there must be sufficient depth of the 
filter that the leucocytes have an opportunity to be significantly 
retained within the filter, both by the geometry of the fibers and by the 
surface area of the fibers and fibrillated particles. In this regard, a 1 
millimeter filter depth (1 millimeter thickness of the filter material) is 
considered to be the about the minimum effective depth, since, at this 
depth, about 70% of leucocytes will be depleted from the blood passing 
therethrough, and a 70% to 75% depletion is considered about the minimum 
depletion for effective filtration of leucocytes, although some depletion 
will occur with lesser depths. 
However, if the depth of the filter is about 2 millimeters, then the 
depletion percentage of leucocytes increases to about 80% to 85% or more. 
When the depth of the filter is about 6 millimeters, the percentage of 
depletion of the leucocytes increases to about 99%. When the depth of the 
filter is about 8 millimeters, the depletion of the leucocytes is at least 
99%. However, at about 15 millimeters depth, the pressure drop through the 
filter when filtering blood begins to significantly increase. That 
increase in pressure drop continues as the filter depth increases and the 
pressure drop becomes greater than that which would be desired for 
ordinary filtration of blood with filter depths above about 20 
millimeters. It is for these reasons that the filter depth is usually 
between about 2 and 20 millimeters, although greater than 20 millimeters 
and up to about 30 millimeters may be used in special cases where the 
pressure drop is not of concern and depths of only about 1 millimeter may 
be used where leucocyte filtration is not the primary concern. Filter 
depth is also important in regard to percentage of leucocyte depletion as 
a function of blood throughput. For example, if the thickness is too 
small, the filter may start with, for example, a 99% depletion for the 
first 200 mls of blood, but drop to 94% depletion for the next 200 mls of 
blood. 
The overall bulk density of the filter material must be between about 0.05 
and 0.4 g/cm.sup.3. As can be appreciated, the density of the filter 
material relates to the fiber geometry, and, as noted above, the fiber 
geometry plays an important role in both the depletion of leucocytes and 
in ensuring that the fibrillated particles are firmly and securely locked 
in the filter material. This density is the density of the laid web (on a 
dry basis) but before the binder is added thereto, and also does not 
include fillers or the like and does not include any purposeful 
compression of the web other than as described below. At densities 
significantly below 0.05, the amount of fibers in the filter material is 
simply not sufficient to ensure a large number of interstices between the 
fibers for securely locking the fibrillated particles into the filter 
material. At a density above about 0.4, the amount of fibers to ensure 
such locking of the fibrillated particles has been exceeded, and the 
additional density simply results in increased pressure drop, without any 
significant further benefit to the filter material. Therefore, the density 
of the laid web must be between about 0.05 and 0.4 g/cm.sup.3. 
As can be easily appreciated, the density of a laid web of fibers depends 
on the method of measuring that density, since a thickness measurement 
must be made to calculate density and the thickness is open to 
interpretation. Ordinarily, with textile webs, the thickness is measured 
after a weight is placed on the web to provide a clear upper edge. This 
weight can vary with the particular filter material, and it is only 
necessary to have sufficient weight to provide a clear upper edge, but 
generally a weight of between about 0.5 to 1 lb. per square inch will 
provide such a clear upper edge. 
As seen in FIG. 2, which is a highly diagrammatic illustration of a portion 
of a section of the filter of FIG. 1, the filter material is comprised of 
a plurality of matrix textile fibers 5. In this regard, the present filter 
material provides high leucocyte depletion because the matrix textile 
fibers keep the fibrillated particles separated so that the high surface 
area thereof is not obscured or reduced due to coalescing or compacting. 
The average denier and length of the matrix textile fibers could result 
from a mixture of very low denier or short fibers and very high denier or 
long fibers, but this is not the intention, since this would not achieve 
the fiber geometry described above. Accordingly, to provide an acceptable 
average length and average denier, at least 60% of the matrix textile 
fibers will have lengths and deniers within about 3 to 15 millimeters and 
about 0.05 and 0.75 denier ranges, and preferably at least 70% and more 
preferably at least 80 to 85% will be within these ranges. 
As noted above, it is necessary to ensure that the matrix textile fibers 
are substantially uniformly distributed in the web, so that, 
correspondingly, the interstices are uniformly distributed and uniformly 
lock the matrix glass fibers and fibrillated particles in the filter 
material. In this regard, the matrix textile fibers must be so interlocked 
together that the matrix glass fibers and fibrillated particles are, in 
turn, so interlocked to the matrix textile fibers that the filter material 
can withstand a filtering blood pressure drop of at least two feet of head 
without substantial displacement of the matrix glass fibers or fibrillated 
particles from the web. 
As can be seen in FIG. 2, the matrix textile fibers 5 are substantially 
uniformly distributed through the web so that as to form a matrix of the 
textile fibers. That matrix has spaces 7 between adjacent interstices 6 of 
the interlocked matrix textile fibers 5 and the matrix glass fibers 8. 
Within these spaces, there are a plurality of fibrillated particles 10 of 
very high surface area, including the surface area of the fibrils 11 of 
the fibrillated particles 10 (see FIG. 3). Those fibrillated particles 10 
are disposed within spaces 7, as well as along and among the matrix 
textile fibers 5 and the matrix glass fibers 8, so as to provide the high 
overall fiber surface area of the filter material. In this regard, fiber 
surface area refers to the area of the surface of all of the matrix 
textile fibers 5 and matrix glass fibers 8 and the fibrillated particles 
10, including the fibrils 11 thereof, within the filter material. 
As a bench mark, conventional filters, even with very fine textile fibers, 
may have a total surface area of all fibers of perhaps 0.5 square meters 
or even perhaps about up to one square meter per gram, although, usually, 
the total surface area will be much less. Even with non-textile fibers, 
such as meltblown fibers used in prior art filters, surface areas of only 
about one square meter per gram can be obtained. In contrast, the total 
surface area of all fibers and fibrillated particles in the present 
invention will be at least one and one-half times that surface area and 
more usually at least two to three or four or five times that surface 
area, e.g. a total surface area of at least about 11/2 square meters per 
gram. 
As highly diagrammatically shown in FIG. 3, the fibrillated particles 10 
have a plurality of fine fibrils 11 which extend and radiate from some 
generally elongated central portion 12 of the fibrillated particles 10. 
Those fibrils, as diagrammatically shown in FIG. 2, interlock among the 
matrix textile fibers 5 and the matrix glass fibers 8, and particularly in 
the interstices 6 between the fibers. Thus, by wrapping the fibrils 11 
around the matrix textile fibers 5 and matrix glass fibers 8, and 
especially by the fibrils 11 being interlocked between matrix textile 
fibers 5 and matrix glass fibers 8 at interstices 6 thereof, the fibrils 
11 are securely interlocked with the matrix textile fibers 5 and matrix 
glass fibers 8 such that the fibrillated particles 10 are not 
substantially displaced from the filter material during filtration of 
blood. 
A typical fibrillated particle is an elongated particle, as shown in FIG. 3 
by the illustrated portion of a particle, with an elongated central 
proportion 12 and radiating fibrils 11. A length, width and depth of such 
a particle is, quite apparently, difficult to accurately measure or 
specify, but for understanding purposes, fibrillated particles have a 
general overall length of less than 1000 microns, e.g. 5 to 300 microns, 
but more usually somewhere about 5 to 50 microns. The width and depth vary 
considerably along the length of the central portion 12 and vary from 0.1 
micron or below to 50 microns, but, generally, the widths and depths are 
between about 0.1 and 5 microns, and more usually between 0.2 and 0.7 
micron. 
From the above dimensions, it will be appreciated that a particular 
fibrillated particle 10, as shown in FIG. 2, can be so long that it 
actually weaves in and out of spaces 7 and one fibrillated particle may 
serpentinely lie in a number of spaces 7, e.g. up to 100 of such spaces 7. 
Thus, such a fibrillated particle, and especially the fibrils 11 thereof, 
has a multitude of interstices 6 with which to interlock and a large 
number of matrix textile fibers 5 and matrix glass fibers 8 about which 
serpentinely wrap. This makes a very secure deployment of the fibrillated 
particles and, thus, ensures that the fibrillated particles will not be 
displaced during normal filtration of blood or a blood component. 
On the other hand, from the above dimensions, it will be appreciated that a 
particular fibrillated particle may essentially lie within a single space 
7 bounded by adjacent interstices, with the fibrils 11 wrapped around 
adjacent matrix fibers and locked between adjacent matrix fibers forming 
adjacent interstices. Also, a particular fibrillated particle may be of 
any intermediate size between the two sizes discussed above, and 
combinations of such locking of the fibrillated particles will occur. 
The weight ratio of the fibrillated particles to the matrix textile fibers 
must be between about 1:99 and 40:60, and especially between about 5:95 
and 40:60, and preferably about 20:80. If that ratio is less than about 
3:97, the additional surface area supplied by the fibrillated particles is 
marginal for desired leucocyte filtration, and at below about 1:99, the 
surface area is simply not sufficient to achieve a minimum desired 
depletion of leucocytes, i.e. at least a 70% depletion. With increasing 
ratios of fibrillated particles to textile fibers, the depletion of 
leucocytes from blood will be correspondingly increased, such that at a 
ratio of about 5:95, the depletion percentage will be close to about 90%, 
and at about 10:90, the depletion will be about 99%. However, with 
increasing ratios, it will be appreciated that the number of matrix 
textile fibers, and the interstices formed thereby, will be 
correspondingly decreased, and there may not sufficient matrix textile 
fibers to keep the fibrillated particles separated. Therefore, high 
percentages of fibrillated particles results in lower average pore size 
and increased flow resistance without any increase in leucocyte depletion. 
Further, at a ratio of about 40:60, the number of matrix textile fibers in 
the filter material is decreased to the point where it is no longer 
reliable that most of the fibrillated particles will be separated, and it 
is for this reason that the ratio of fibrillated particles to matrix 
textile fibers should not exceed about 40:60, particularly should not 
exceed about 30:70, especially 20:80. 
All of the foregoing, of course, depends somewhat on the amount of matrix 
glass fibers 8 in the filter material, as well as the particular sizes of 
thin matrix glass fibers. As noted above, the amount of glass fibers can 
be as low as about 2% by weight of the filter material web, and quite 
obviously, at this low percentage, the above will remain essentially 
correct. However, as also noted above, the amount of matrix glass fibers 8 
in the filter material web may be as high as 85% and also, quite 
obviously, at this higher percent the above will not be correct, without 
correcting for the glass fiber content. Thus, especially at these higher 
percentages of matrix glass fibers 8, the ratio of fibrillated particles 
to matrix textile fibers could be reduced from 3:97, e.g. all the way down 
to the lowest limit of 1:99, and still provide sufficient surface area. 
However, it has been found that for the present improvement the above 
ratios may still be followed, although there is more permissible latitudes 
in connection therewith as a result of the presence of the matrix glass 
fibers. 
The glass fibers 8, as noted above, should have an average diameter of 
between about 0.1 and 5 microns. These are, of course, very fine 
non-textile fibers. Preferably, the average diameter will be between about 
0.3 and 2.0 microns and especially between about 0.5 and 1 micron. 
The glass of the fibers may be any conventional glass, such as E-glass, 
S-glass, borosilicate glass, etc. Further, many conventional ceramic 
fibers have essentially glass-like physical properties, as opposed to 
mainly ceramic properties. Thus, ceramic fibers (not based on silica) may 
be used when those ceramic fibers have glass fiber-like physical 
properties and are, therefore, intended in the definition of glass fibers. 
Irrespective of the type of glass, as is well known, glass fibers are, on 
a relative basis as compared with, for example, polymeric textile fibers, 
quite stiff. While this stiffness tends to result in a loftier structure 
and, hence, more depth filtration, this also means that it is difficult to 
ensure that the stiff glass fibers have been entwined sufficiently with 
the matrix textile fibers to ensure that the matrix glass fibers are 
interlocked therewith, especially with glass fibers in the higher diameter 
ranges, e.g. 4 or 5 microns, and especially since current manufactures of 
these fine glass fibers have considerable differences in the lengths 
thereof, e.g. the average length of these fibers can be between 0.3 and 3 
millimeters or even outside of these ranges. By selecting the glass 
fibers, e.g. with diameters between about 0.5 and 1 micron, which will 
have average lengths of between about 0.5 and 1 millimeter, e.g. 
especially about 0.65 microns in diameter, this difficulty of ensuring 
interlocking with the matrix textile fibers can be mitigated, but it 
cannot be avoided altogether. 
Also, while the use of sheath/core matrix fibers, as described more fully 
below, will also mitigate this problem, the problem cannot be altogether 
avoided thereby. It appears, in this regard, that the thermoplastic, and, 
hence, heat softenable, sheath, disposed on the relatively large matrix 
textile fibers (as compared with the size of the glass fibers) is not 
necessarily capable of fully adhering the much smaller and somewhat mobile 
matrix glass fibers. In addition, even at relatively low glass fiber 
percentages, e.g. 5% on a weight basis of the web, the number of these 
small glass fibers is quite large, and with this large number of 
relatively stiff, short glass fibers, complete securing by sheath/core 
matrix fibers is not ensured. 
In view of the above, it was found that to ensure the securing of the glass 
fibers into the web, a separate binder in the web was required. While 
theoretically this binder could be a thermosetting or thermoplastic 
binder, it was found that thermosetting binders were not satisfactory for 
a number of reasons, including the need for a catalyst which may not be 
compatible with blood filtering, longer reaction times for cross-linking 
and, hence, prolonged production times, generally uniform distribution 
thereof throughout the web instead of being concentrated at critical 
cross-over positions, as explained below, larger amounts to ensure binding 
of the glass fibers, and a somewhat stiff and boardy filter material. 
Hence, the binder must be a thermoplastic binder. 
In this latter regard, the binder may be any thermoplastic binder which has 
softening temperatures below those temperature which would adversely 
affect the matrix textile fibers, e.g. below about 400.degree. F., 
preferably below 350.degree. F., and especially below about 300.degree. F. 
or 250.degree. F. or even below 220.degree. F. In this regard, softening 
temperature is defined as that temperature at which the thermoplastic 
binder sufficiently softens so as to adhere to both the matrix textile 
fibers and the matrix glass fibers. However, it was found that certain 
thermoplastic binders have special advantages, in that the binders either 
have particularly good adherence to these fibers or tended to migrate to 
fiber cross-over positions, as explained below, or both. 
These thermoplastic binders are polyvinyl acetate, polyvinyl chloride, 
polyacrylics and acrylates, polyacrylonitrile, polybutadiene, 
polyethylene, polyisoprene, polyvinyl acetate ethylene, polyvinyl acetate 
acrylate and polystyrene-butadiene. It is also possible to use 
desolubilized gums such as polyvinyl alcohol and cellulose gum, but the 
desolubilized versions of these water-soluble gums are much more difficult 
to ensure sufficient desolubilization and are therefore not preferred. 
These binders may be applied to the web prior to or during formation 
thereof and as powders or solutions, but it has been found that very 
special advantages are provided when the binders are applied to the 
already formed web and as an emulsion. 
In this regard, as can be seen from FIG. 2, in order to ensure that the 
matrix glass fibers 8 are secured to the matrix textile fibers 5, it is 
only necessary to have the binder at cross-over positions 9. Any other 
disposition of the binder will not significantly improve the binding of 
the glass fibers to the textile fibers and will only serve to decrease the 
filtration throughput. Thus, if the binder could be arranged to be 
substantially disposed only at these cross-over positions, securing of the 
glass fibers is ensured and a minimum of stiffening binder may be used. 
It was found that if the binder, in emulsion form, is applied to the 
already formed web, the emulsion sized particles of the binder plastic in 
the emulsion tend to be more removed from the emulsion at these cross-over 
positions than at other places throughout the web. Thus, the emulsion form 
of the binder proved to be a substantially superior form thereof. While 
not bound by theory, as can be seen from FIG. 2, the cross-over positions 
constitute the closest physical restraints for removing binder particles 
from the emulsion. Hence, if the emulsion is in effect filtered through 
the web, the binder particles 4 will be preferentially removed from the 
emulsion at these cross-over positions and the retained binder particles 
are, therefore, in the preferred position in the web for maximum binding 
of the glass fibers and textile fibers with minimum amounts of binder. 
As can be appreciated from the above, this far-preferred filtering action 
of the binder particles cannot be reasonably achieved with a solution or 
melt of the binder, but only from a dispersion of the binder. Further, it 
could not be achieved when the binder is added before or during forming of 
the web; it can be achieved only after the web is formed. Also, while the 
dispersion could function even if not in emulsion form, quite obviously, 
an emulsion form will provide a more uniform dispersion and deposition at 
the cross-over positions. Hence, it is greatly preferred that the binder 
is in emulsion form and applied to the formed web. 
In this latter regard, the emulsion could be applied to the web by 
conventional padding or printing of the web with the emulsion, or even 
immersing the web in the emulsion, but it will also be apparent that the 
desired filtering action of the binder particles is far more sure when the 
emulsion is applied to only one side of the web, e.g. by spraying one side 
of the web, e.g. top side, with the emulsion and with a reduced pressure 
on the other side of the web. This means of applying the emulsion is also 
most convenient for use in a conventional web-forming process and 
apparatus, as discussed more fully below. 
In view of the above, certain of the above-described polymeric binders are 
preferred, since they can easily be formed into stable emulsions. Among 
these are polyvinyl acetates and acrylates, polyacrylics and acrylates, 
polybutadiene and polyisoprene, with polyacrylics and acrylates being more 
preferred. 
The emulsion may have solids contents between about 0.1% and 50% and still 
provide the required filtering action of the binder particles, especially 
between about 0.5% and 20%, e.g. between about 1% or 5% and 10%. This will 
also produce an add-on of binder particles to the formed web of about 0.1% 
to 10%, based on the weight of the web. At about 10% add-ons, the 
adherence of the glass fibers is well ensured and additional add-ons 
beyond this amount can cause decreases in pore sizes of the filter 
material, along with decreased throughput and increased pressure drop. 
Further, below that 10% add-ons, the flexibility of the finished filter 
material is not substantially reduced, so long as the binder polymer is 
flexible at room temperature and has a glass transition temperature of 
above 100.degree. F. At less than about 0.1% add-ons, the amount of binder 
is insufficient, although add-ons of about 0.25% or 1% are quite 
satisfactory. 
As noted above, a portion of the matrix textile fibers 5 may have a sheath 
13 and a core 14 (see FIG. 4). The sheath 13 will be of a low melt 
temperature polymer, and the core 14 will be of a higher melt temperature 
polymer. For example, the core 14 may be a polyester polymer, and the 
sheath 13 may be a low-melt olefin, such as polyethylene. When at least a 
portion of the matrix textile fibers of the filter material are the 
sheath/core fibers, the web of the filter material, when being processed, 
is subjected to temperatures such that the web has experienced 
temperatures sufficient to at least soften the polymer sheath 13 and cause 
at least some adherence of that softened sheath of the textile fibers 5 to 
other such fibers and to, at least, part of the matrix glass fibers and 
the fibrils of the fibrillated particles. As can be appreciated, this will 
cause a bonding of the matrix textile fibers together, to improve the 
strength of the interstices, and will cause bonding of the fibrils of the 
fibrillated particles to the sheath of the textile fibers. This will 
ensure better locking of the fibrillated particles in the filter material. 
While as little as about 1% of the matrix textile fibers may have the 
sheath thereon, and at least some improvement will be provided for 
securing the fibrillated particles in the filter material, generally, at 
least about 5% of the matrix textile fibers should be the sheath/core 
fibers. On the other hand, while a large percentage of the textile fibers 
could be the sheath/core fibers, this would render the resulting filter 
material rather stiff, which is not desired, simply for convenience of 
handling, and, therefore, it is preferred that the sheath/core textile 
fibers be no greater than about 30%. At percentages of about 30% or less, 
there is no significant deterioration in the handling qualities of the 
filter material. Thus, a preferred range for the sheath/core textile 
fibers is between about 5% and 30%. Within this range, the filter material 
so locks the fibrillated particles into the filter material that the 
filter material can withstand a filtering blood pressure drop of at least 
five feet of head without substantial displacement of the fibrillated 
particles from the web. 
The sheath/core fibers, if used, may have a denier of about 0.05 to 0.75 
denier, but if 30% or less of sheath fibers are used, that denier may be 
higher, e.g. up to about 3 or 4 denier. For example, when about 10% 
sheath/core fibers are used, conventional 2 denier sheath/core fibers may 
be used. 
The sheath/core fiber, if used, may have a core of textile fiber material, 
and the sheath may be any low melting polymer. While a wide range of low 
melting polymer sheaths are known to the art, including methacrylates, 
vinyls and the like, polyolefin polymers, such as polyethylene or 
polypropylene, are preferred, since those polymers provide sheaths with 
very low softening points, and it is easy to soften those sheaths to 
provide the required adherence. The thickness of the sheath is essentially 
immaterial, since the only requirement of the sheath is that there be a 
sufficient layer of the sheath for adherence to adjacent fibers and 
fibrillated particles. Thus, the sheath can be from as little as 1% to 
50%, e.g. 5 to 30% or 10 to 20% of the core diameter. 
The fibrillated particles are particles of a synthetic or natural polymer, 
and a wide range of such polymers may be used, since it is not the 
particular polymer but the surface area thereof which is important. 
However, the polymers must be capable of substantial fibrillation, for the 
reasons expressed above. Thus, generally, the polymer will be a textile 
fiber material (polymer), since textile fiber materials, usually, are 
capable of being fibrillated to a high degree. Any of the textile fiber 
materials noted above may be used as the fibrillated particles. However, 
it is preferred that the fibrillated particles be particles of the textile 
fiber materials of polyester fiber material, acrylic fiber material, nylon 
fiber material, polyolefin fiber material and cellulosic fiber material, 
since these materials easily fibrillate to high degrees and provide 
fibrillated particles with a multitude of fibrils, as described above. 
These materials also provide fibrils which easily attach to and lock with 
the matrix textile fibers and/or the glass fibers, with or without 
sheath/core fibers. Further preferred is where the fibrillated particles 
are made of a textile fiber material which is predominantly a cellulosic 
fiber material, since fibrillated particles of that material provide a 
large number of fibrils, and it is especially preferred that the 
cellulosic fiber material be cellulose acetate, since a great number of 
fibrils are produced with that material, and the material has a natural 
hydrophilic nature and, thus, an affinity for leucocytes. 
It is not necessary to describe in detail the fibrillated particles, since 
these fibrillated particles are known to the art and are commercially 
available. A full description of such fibrillated particles may be found 
in U.S. Pat. No. 4,274,914 to Keith, et al, issued on Jun. 23, 1981. That 
patent describes, in detail, the method of manufacture of the fibrillated 
particles and the fibrillated particles themselves. Those fibrillated 
particles have been used in the art as binders, primarily, especially in 
filter papers, and as especially used for binding adsorbents, such as 
activated carbon powders, in non-woven media. Among other applications are 
combustible shell casings, specialty papers, speaker cones, and 
substitutions for asbestos or aramid fibers in friction materials. 
The aforementioned patent, the entire enclosure of which is incorporated 
herein by reference and relied upon for disclosure herein, also describes 
applications of the fibrillated particles to cigarette filters and face 
mask filters, where the fibrillated particles form those filters in 
combination with various fibers, particularly acetate and polyester 
fibers. Tobacco smoke filters are particularly described, and the use of 
the fibrillated particles in forming webs for cigarette-filter purposes is 
set forth in detail. Accordingly, while a detailed description of the 
fibrillated particles and the process for producing the present filters 
need not be set forth in this specification, a brief explanation is set 
forth below. 
Thus, very briefly, those fibrillated particles have overall lengths of 
less than about 1000 microns and overall widths of about 0.1 to 50 
microns, including the fibrils. They are three-dimensional particles, and 
the depth is approximately equal to the width. These particles are not 
fibers and cannot be spun into a yarn, i.e. are not textile fibers, e.g. 
of staple length. They may be best diagrammatically visualized as 
extremely small "duck down", in the sense of their physical appearance as 
viewed through a microscope. The fibrils of the particles are extremely 
small, e.g. generally less than 0.01 micron in diameter and in the order 
of 1 to 50 microns long. The fibrils radiate from an elongated central 
portion, but not in any organized fashion. 
In all of the above, the textile art terms are used in their common senses. 
Thus, the term "textile fiber material" is used in its common sense, i.e. 
that the material, e.g. a polymer, is capable of being formed into a fiber 
which can be processed by conventional textile machines into a textile 
material, either woven or non-woven. This, of course, also means that the 
fibers of the "textile fiber material" must be capable of interlocking 
among themselves or with other fibers, i.e. a length sufficient that the 
fibers may be interlocked together to form a yarn or capable of matting to 
form a non-woven textile or engaged by barbs of needles for producing a 
needled textile. More usually, this will require a "staple" length of the 
fibers, i.e. one which allows the fibers to be twisted into a yarn. Of 
course, the present glass fibers are, therefore, not of a "textile fiber 
material" in the sense that the present glass fibers are too small to be 
formed into, e.g., a yarn, and cannot be formed into a woven textile. 
The term "textile fiber", likewise means that the fiber is made of a 
"textile fiber material" and can likewise be formed into a textile, i.e. 
either woven or non-woven textile. This is opposed to "non-textile 
fibers", such as the present matrix glass fibers. These glass fibers have 
very smooth surfaces, are of small diameter and relatively short. Hence, 
they cannot, with usual textile processes, be spun into a yarn and hence 
cannot be formed into a woven textile. Neither can they be effectively 
needled into a non-woven textile, since the smooth surface and stiff 
character will not allow sufficient interlocking of the glass fibers to 
form a needled batt of any substantial strength. 
Accordingly, the present filter material is made of the larger matrix 
textile fibers, the small matrix glass fibers (non-textile fibers), the 
very fine fibrillated particles (non-textile particles) and the binder. 
The larger matrix textile fibers form major interstices, provide strength 
to the filter material and hold open the spaces between fibers to allow 
full use of the high surface area of the fibrillated particles. The 
smaller matrix glass fibers provide increased surface area to the filter 
material, as opposed to a filter material where the matrix fibers are all 
the larger matrix textile fibers, while not significantly decreasing the 
function of the larger matrix textile fibers. 
As shown in FIG. 5, the process, which is a known process and generally 
described in the above-noted patent, mixes the fibrillated particles, the 
matrix glass fibers and the matrix textile fibers in a beater box to 
provide a furnish thereof. While the weight percent of fibrillated 
particles and fibers to the water in the beater box can vary widely, for 
most applications of fibrillated particles to be incorporated in the 
fibers, the total solids content (fibrillated particles and fibers) should 
be somewhere between about one and five percent. Sufficient mixing in the 
beater box is conducted until a homogenous slurry of the fibers and 
particles is obtained. 
Thereafter, the furnish slurry is fed to a head box of an ordinary 
paper-making machine. Of course, if desired, intermediate stock tanks and 
mixers may be used, depending upon the particular machine and the schedule 
of processing thereon, all of which is well known in the art. Any of the 
conventional paper-making machines may be used, e.g. a rotating screen 
machine, a perforated drum machine, and the like, but a usual Fourdrinier 
machine is preferable, in view of the simplicity of operation of that 
machine and the control of the web thickness achieved thereby. 
In any event, the furnish slurry of fibrillated particles and fibers is fed 
from the head box to the former of the machine, whether it be the rotating 
perforated drum or rotating screen or the Fourdrinier machine, and that 
slurry of fibers and particles is dewatered, by vacuum, to form a wet web. 
After the vacuum is applied to the underside of the web, preferably, the 
vacuum is discontinued and then the binder emulsion is applied to the top 
side thereof and preferably the vacuum is thereafter reapplied. This 
application of the binder can be by way of a padding roll or a squeegy bar 
or doctor blade, but preferably it is simply sprayed onto the web with 
conventional spray jets, positioned so as to provide a relatively uniform 
wetting of the top side of the web by the sprayed emulsion. The 
conventional subsequently vacuum applied to the underside of the web sucks 
the emulsion through the web and filters the binder particles therefrom so 
as to deposit the binder particles predominantly at the cross-over 
positions, as explained above. No particular degree of vacuum is required, 
and, indeed, adequate filtering of the binder particles will take place 
with no vacuum or very high vacuum, e.g. -12 lbs. gauge. 
That wet web is then passed to a drier, which is usually a series of 
steam-heated cans, and is heated sufficiently to dry the web and set the 
binder. In the case of the use of sheath/core fibers, that drying 
temperature must be sufficient to ensure that the sheath of the 
sheath/core fibers is sufficiently softened to achieve the bonding, 
described above. That temperature, of course, will vary with the 
particular sheath, but, generally speaking, steam-heated cans with steam 
up to 1 to 2 atmospheres of superheat is more than sufficient to cause 
such adherence of the usual sheathed fibers, as well as set the binder. If 
a sheathed fiber is not used, then the steam-heated cans can be at almost 
any temperature, e.g. 120.degree. F. to 300.degree. F., consistent with 
setting the binder and drying the web to a relatively low moisture 
content, e.g. less than 10%, or usually less than 5% or 2% by weight 
moisture. From there, the dried web is collected in an ordinary collection 
mechanism, e.g. a roll collecting mechanism, and is then ready for cutting 
into desired filter shapes. 
The setting temperatures for the binder will vary substantially with the 
particular binder. The setting temperature must be high enough to cause 
the binder to tackify and adhere to the glass fibers. More preferably, the 
heat softened binder will be at a temperature high enough to at least 
partially move or flow to form configured binder pools, as shown in FIG. 
2. For most binders, especially the preferred binders, the temperature 
will be at least 150.degree. F., especially at least 200.degree. F. and as 
high as 250.degree. to 275.degree. F. 
As noted in the parent application, it was found that a modification of 
that process and the resulting filter material is of advantage. In this 
regard, it was found that the fibrillated particles have a tendency to 
agglomerate during the process until the furnish slurry is deposited on 
the former. In the finished filter material, such agglomeration can cause 
undesired reduction in surface area and, correspondingly, decreased 
leucocyte depletion. 
It has been found that such agglomeration can be avoided by incorporating 
into the furnish a small amount of small fibers. These small fibers are of 
less than staple size, i.e. non-textile fibers, and are used in amounts up 
to 10%. Amounts as low as 1%, however, are usually adequate. While these 
small fibers may be any natural or synthetic fibers, such as described 
above, the parent application points out that small fibers of ceramic and 
glass, i.e. microfibers, are preferred, e.g. small fibers usually having 
average diameters of about 0.1 to 2 microns. 
While the parent application pointed out that these small fibers are so 
interlocked with the fibrillated particles that they are not substantially 
displaceable from the filter material when filtering blood or a blood 
fraction, it has now been found that these small fibers are displaced more 
than had been appreciated. Further, as noted above, with sufficient of 
these fine fibers, an improved blood filtration is provided, so long as 
the filter material includes the above-described binder. Hence, the 
present glass fibers function both for the purposes described in the 
parent application and for the purposes described herein. 
As a means of avoiding any non-wetting and, hence, decreased filtration, in 
the filter material, the filter material may be treated with a small 
amount of FDA approved wetting agents, which are known to the art, e.g. 
the Tweens surface active agents. 
The filter material may be formed into a filter device in any of the usual 
manners of the art, and FIG. 6 shows an acceptable example thereof. As 
shown in FIG. 6, the filter material 1 may be sandwiched between a 
prefilter 15 and a subsequent filter 16. The prefilter 15 is used to 
remove large agglomerates and the like which often occur in stored blood, 
and this prefilter can be any of the usual prefilters known in the art for 
that purpose. These prefilters can be woven or non-woven textile materials 
or metal meshes or the like, and the particular prefilter can be chosen as 
desired. 
Similarly, the subsequent filter 16 may be simply a supporting subsequent 
filter, to give mechanical support to the present filter material and the 
prefilter. That subsequent filter may be, for example, a woven mesh or a 
wire mesh, simply to provide support for the pressure drop across the 
entire filter element (the prefilter, the present filter material and the 
subsequent filter). Here again, this subsequent filter can be as in the 
prior art, and no particular details are required in connection therewith. 
The filter assembly is held in a conventional housing 17 which has a 
conventional inlet 18 and a conventional outlet 19. The filter assembly is 
held in that housing in any convenient manner for mechanically locking the 
filter element into the housing, e.g. mechanical clamps. Such housing, 
along with the filter elements therein, is desirably disposable, and with 
the low cost of the present filter material, such a filter assembly in 
such a housing is disposable at a low expense. 
While FIG. 6 illustrates a convenient and conventional housing and 
arrangement of the filter elements, any of the other conventional housings 
and arrangements of filter elements known to the prior art may be used 
with the present filter material, since the present filter material is 
amenable to almost any desired configuration. 
For certain filtering applications, it is desirable to choose among various 
properties of the filter material for achieving a desired end result. With 
the present filter material, choices of properties of filtration are 
easily obtained. Thus, for example, where the filtration is primarily 
intended to remove larger particles in the blood, with only a minimum 
depletion of leucocytes, then the filter material may have a relatively 
low ratio of fibrillated particles to textile fibers. On the other hand, 
where a high depletion of leucocytes is required, but the rate of 
throughput of the blood through the filter is not particularly critical, 
then a relatively high ratio of the fibrillated particles to textile 
fibers may be used. 
Alternatively, such changes in surface area of the filter material may be 
achieved by choosing the surface area of the fibrillated particles. Thus, 
one could achieve such a filter material, as described above, by choosing 
fibrillated particles with a relatively low surface area, e.g. 10 square 
meters per gram, or, with the same ratio of fibrillated particles to 
textile materials, a filter material of high surface area could be 
achieved by choosing fibrillated particles with a relatively high surface 
area, e.g. 30 or 50 or 70 square meters per gram. 
The preferred method of achieving different properties of the filter 
material is that of using fibrillated particles with a relatively high 
surface area, e.g. between about 10 and 60 or more square meters per gram, 
and adjusting the ratio of fibrillated particles to textile fibers. 
However, with the present glass fibers, fibrillated particles having 
surface areas up to 100 square meters per gram may be used and still 
retain those particles in the filter material. 
The invention will now be illustrated by the following examples, where all 
percentages and parts are by weight, unless otherwise noted, which is the 
case of the foregoing specification and claims. 
EXAMPLE 1 
Preparation of Prefilter 
3 denier and 6 denier polyester fibers (Dacron Type 54) were carded on 
separate cards. The carded webs were crossed lapped separately onto a 
moving conveyor to form a mat with discrete layers of 3 and 6 denier 
fibers. The mat was needled with a conventional needled-punching machine. 
The needled mat was hot calendared (350.degree. F.). The 3 denier side of 
the mat was glazed with a hot (500.degree.-550.degree. F.) knife. This 
material is Lydall style #CW140 which is a commercial filter material for 
use in a cardiometry reservoir. 
Preparation of Filter Material 
A wet laid non-woven was produced in the following manner: 70% Teijin 0.5 
denier polyethylene terepthalate (PET) fibers of about 6 to 7 millimeters 
average length, 10% of Code 106 microglass fibers (Schuller--formerly 
Manville) having an average diameter of about 0.65 microns, an average 
length of about 3 millimeters and an average surface area of about 2.4 
square meters per gram, 10% Chisso EKC 2 denier core 
(polypropylene)/sheath (polyolefin copolymer) fibers of about 5 to 9 
millimeters average length and 10% Hoescht-Celanese cellulose acetate 
"fibrets" (fibrillated particles of textile fiber material) were weighed 
with adjustments made for the moisture content of the materials. Thus, in 
this example, most of the matrix fibers are the textile fibers as opposed 
to the glass fibers. This furnish was placed in a commercial web-forming 
machine at about 5% in water. The furnish slurry was stirred at high speed 
for 2 minutes to achieve good dispersion. This slurry was discharged from 
the dump box to a moving screen. Dewatering by vacuum (-5 psig) resulted 
in the forming of a web on top of a screen wire. An acrylic polymer 
emulsion of about 48% solids (Rohm & Haas HA8) was diluted to about 0.25% 
solids and sprayed on the top of the dewatered web to wet the web to 
excess of saturation. Further dewatering was achieved by using the same 
vacuum. The web was dried at 250.degree. F. on steam-heated cans to 
produce the present filter material. The thickness of the filter material 
was approximately 2 millimeters, and the add-ons of binder was about 5%. 
Testing 
The dried web and prefilter were die cut to 2.574 inch circles. Five of 
these were inserted into a test filter rig similar to that shown in FIG. 
6, where the prefilter and filter material are clamped together to form a 
filter media. The filter media was oriented in the vertical direction and 
the blood entered at the bottom of the filter media and exited at the top 
(opposite to that shown in FIG. 6). The media was oriented so that blood 
first encountered the prefilter material (3.0 denier side first). 
The rig was connected with tubing to a unit of human packed red cells about 
10 days old. 80 gms of packed cells were passed through the filter. The 
leucocyte level of the filtered blood was reduced by about 99%. 
EXAMPLE 2 
The same prefilter of Example 1 was used, but the filtering material was 
made of 85% of the glass fibers, 10% of the sheath/core fibers and 5% of 
the fibrillated particles of Example 1. Otherwise, the filter material, 
including the binder, and add-ons were the same as in Example 1. Thus, in 
this filter material most of the matrix textile fibers were replaced by 
the matrix glass fibers. Two layers of the filter material and two layers 
of the prefilter (about 4.5 millimeters thickness in total) were placed in 
the test rig of Example 1, and the test described therein was carried out. 
A 99.9% reduction in leucocyte content was achieved, and the two layers of 
filter material retained only two-fifths of the blood retained by Example 
1. 
Thus, as this example shows, the glass fibers may replace a large portion 
of the textile fibers of the filter material, and these higher amounts of 
glass fibers provide filters with excellent efficiency at low thicknesses. 
EXAMPLE 3 
The same prefilter of Example 1 was used, but the filter material was made 
of 75% of the PET fibers, 5% of the fibrets, 10% of the microglass fibers 
and 10% of the core/sheath fibers. Otherwise, Example 1 was repeated. 
This filter was tested according to the American Association of Medical 
Instrumentation (AAMI) standard ANSI/AAMI BF-7-1989, Section 4.2.3.1. 
A control filter material was also made according to this example, but the 
acrylic polymer emulsion binder was not applied thereto, and this filter 
material was also tested according to the above-noted standard. 
However, in both tests, only a single layer of the filter material was 
used, as opposed to the multiple layers of Example 1, and the test rig 
without any filter material therein was tested according to that standard 
to determine ambient filter contamination. 
The ambient filter contamination for the empty test rig had a fiber count 
of 8; the filter material of this example (having the present binder) had 
a fiber count of 10; and the control filter material (not having the 
present binder) had a fiber count of greater than 1200. The difference 
between a fiber count of 8 and 10 is not significant, but a fiber count of 
1200 is high. 
As can be seen from the examples, therefore, the present invention 
significantly improves the efficiency of the filter material, while at the 
same time reduces the amount of fibers displaced therefrom to 
substantially zero.