Note: Descriptions are shown in the official language in which they were submitted.
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HAEMOSTATI(_ WOUND DRESSING MATERIAL
This invention relates to a haemostatic wound dressing
material, and more particularly to a fibrous wound dressing
. material which i~; coated with an alginate. The invention
also provides wound dressings comprising such materials,
and methods for their manufacture.
Alginates have long been recognised as useful in wound
l0 dressings, because of their haemostatic properties. U.S.
Patent Specification No. 2512616 observes that alginates
exist in several physical forms, but indicates that fibrous
forms are preferred for use in surgical dressings.
Alginates woven into a gauze or in the form of loose wool
similar to absorbent cotton are particularly identified as
being useful.
British Patent Specification No. 629419 discloses a
haemostatic surgical dressing which is formed by
impregnating a cotton gauze or other fibrous material with
relatively large quantities of an insoluble alginate.
British Patent Specification 1329693 also discloses a
surgical dressing comprising an alginate as a haemostat.
In this case, the alginate is combined with a water-soluble
polymer such a;s polyvinylacetate/polyvinyl-pyrrolidine
copolymer, sodium carboxy methyl cellulose, polyethylene
oxide, a poly(ga:Lactose methacrylate), a poly(galactose
acrylate), a copolymer of methyl vinyl ether and malefic
anhydride , or allantoin polygalacturonic acid. Such a
water-soluble polymer is apparently used to allow the
alginate to be cast or spread in the form of a film, sheet
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or block of haemostatic material. This is said to have the
advantage that th~~ film, sheet or block slowly dissolves in
contact with a wound or burn to release the alginate which
is then free to exhibit its haemostatic properties.
British Patent Specification No. 1329693 further discloses
that the alginate/water-soluble polymer mixture may be cast
onto a PVC sheet or, in some examples, onto a sheet of
Polyweb Net 909 (Smith & Nephew). The latter is a wound-
release layer which is widely used in wound dressings.
The dressings proposed in the above-mentioned British
Patent specifications do not appear to be commercially
available. Currently, alginates are available for use in
wound treatment in two principal forms, namely fibrous wads
and powder sprays.
The commercially available alginate fibre wads generally
use relatively high amounts (typically greater than 100 g/mz
of wet spun fibres), and are consequently expensive.
Further expense is incurred if the wads are needled to
increase their into=grity.
Alginate powder sprays, such ~s are described in British
Patent Specification No. 1254534, represent a convenient
way of administering a haemostat rapidly to the surface of
a wound. However, the use of such sprays provides no
protection to a wound, and they cannot therefore be
regarded as providing an alternative to a conventional
wound dressing.
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According to the present invention, there is provided a
wound dressing material comprising a fibrous substrate
having a discontinuous coating of a pharmaceutically
acceptable alginate deposited on a surface thereof.
By applying the alginate coating directly to the surface of
a fibrous substrate, a very large surface area of alginate
in relation to its weight can be achieved. This is of
advantage in that, in contrast to the dressings disclosed
in British Patent Specification No. 1329693, the alginate
rapidly contacts and reacts with any blood from the wound
to which the dressing material is applied. Effective
haemostasis can therefore be obtained with relatively small
amounts of alginate. The alginate (or alginate
formulation) is preferably coated on the substrate at from
2 to 30 g/m2 dry weight, more preferably at from 5 g/m2 to
g/m2, and most preferably at from 8 g/mz to 12 g/mz.
Surprisingly, these levels of alginate also provide
20 significant wound--release properties, so that a separate
wound-release layer (such as a perforated plastics film or
plastics net of the type commonly used in wound dressings)
is not required.
Any material suitable for use as a wound-contacting
absorbent may be used as the substrate. Materials used as
wound-contacting absorbent layers can be woven, knitted or
non-woven structures, and may be composed of any suitable
fibre. Cotton, rayon, acrylic, polypropylene and polyester
fibres are examples of suitable fibre types and may be used
alone or in mixturE:s .
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The surface of the fibrous substrate may, if desired, be
rendered more amenable to wound-release, for example by
hot-calendering the surface fibres, especially with
thermofusible fibres. It will be appreciated, of course,
that such hot-calendering, if used, should not be to such a
degree that the :surface of the substrate completely loses
its fibrous character.
The preferred fabric for use in the wound dressing material
to of the present invention is 100% polyester Malifleece
within the weight range 100 g/m2 to 200 g/m2 (Ledatec
Limited). Polyester tends to resist wet-collapse more than
rayon, and hence makes a resilient pad which provides a
degree of protection by cushioning the wound.
The alginate coating may comprise any pharmaceutically
acceptable cationic alginate, such as sodium, calcium,
potassium and ammonium alginates or mixtures of these.
Sodium and calcium alginates and their mixtures are
preferred.
The haemostatic effect of 100% sodium alginate coatings may
be increased by :substituting some of the sodium ions by
calcium ions. 7.2% calcium, based on the weight of sodium
alginate, is required for complete substitution of the
sodium ions by the calcium. However, suitable gels are
formed with much lower levels of ion exchange, for example
from 1 to 3% calcium by weight of alginate, and preferably
from 1.5 to 2.5% calcium by weight.
Since it is not desired to form a continuous film of
alginate on the :substrate, there is no need to include
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film-forming waf=er-soluble polymers in the alginate
coating. Indeed, it is preferred that the alginate coating
contain less than 40% by weight of other polymers, and more
preferably less than :L5% by weight. It is particularly
preferred that no more than 5% by weight of other polymers
be included in the alginate coatings, because their
presence may interfere with the haemostatic effect of the
alginate.
l0 In the absence of substantial quantities of a film-forming
polymer, alginates tend to be rather friable, and the
alginate coating in the wound dressing material of the
present invention therefore preferably also contains a
plasticiser such as ethylene .glycol, propylene glycol or
hexylene glycol, or an alkyl citrate or an appropriate
mixture. The preferred plasticiser is glycerol (propan-
1,2,3-triol). Th.e plasticiser may constitute from 0% to
80% by weight of t:he coating, and preferably from 10 to 70%
by weight. It is particularly preferred that the
plasticiser be present in the coating in an amount between
and 60% by weight.
The alginate coating may optionally incorporate other
additives such as antiseptics, analgesics or other
25 medicaments, preferably in amounts less than 5% by weight,
more preferably less than 2% by weight, and most preferably
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less than 1% by weight.
Other additives may also be desirable as processing aids.
For example, if a sodium/calcium alginate coating is
5 required, a viscous aqueous solution of sodium alginate can
be prevented from forming an immediate gel in the presence
of calcium ions by the inclusion of pH controlling materials
such as glucono lactone or adipic acid/sequestering agent
(e.g. sodium citrate) . Such materials are typically used in
amounts of from 1 to 10% by weight, and preferably from 2.5
to 7.5% by weight. A preservative may also be added to
increase resistance to microbial attack of the alginate.
Examples of suitable preservatives are Metasol*D3T (Merck),
Parasept*(methyl paraben) (Kaloma Chemical) and Bronopol*(2-
bromo-2-2-nit:ropropane-1,3 diol) (Boots Ltd.). Typical
ranges for the amount of preservative are from 0.1% to 5.0%
by weight, and preferably from 0.25 to 1.0% by weight.
When an alginate coating is applied to a fibrous substrate
in accordance with the invention, the individual fibres
which lie close to the surface of the substrate are at least
partly coated. Also, extremely thin films of alginate may
be formed between adjacent fibres near points of crossover.
A discrete film or layer of alginate on the surface of the
substrate is avoided, so that the permeability of the
substrate to gases (such as air and water vapour) is not
lost. Coating techniques which are suitable for achieving
such a distribution include knife-over-roll, Meyer rod,
screen printing and reverse roll coating. Reverse roll
coating is particularly suitable, because it allows
controlled amounts of material to be applied to a given area
of substrate, even with substrates having variable and
uneven surfaces.
The alginate is preferably applied to the substrate as a
viscous aqueous, or substantially aqueous, fluid. The
viscosity of the aqueous suspension of the alginate
composif:ion may be adjusted to suit the coating technique
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employed, but for reverse roll coating viscosities in the
range 2000 cp to 30,000 cp, and preferably from 3000 cp to
... 8000 cp are .appropriate. Control of the nip-gap and of the
casting ratio determines the amount of coating material
applied to the substrate, as is well known in the coating
art.
For reverse roll coating, the nip gap between the doctor
roll and the applicator roll is typically from 0.075 to 0.45
mm, and preferably from 0.10 to 0.30 mm, e.g. from 0.15 to
0.20 mm.
The casting ratio (i.e. the ratio of the applicator roll
peripheral speed to the backing roll peripheral speed) is
generally in the range 0.8:1 to 1.7:1, and more preferably
in the range 1:1 to 1.5:1, e.g. 1.2:1
Evaporation of the water phase may be achieved by any
suitable means, for example by passage through temperature-
controlled ovens. The oven is suitably at a temperature of
from 100 to 150°C, e.g, from 110 to 140°C. If desired, a
gradually increasing oven temperature may be employed.
Drying will typically take from 1 to 10 minutes, and more
usually from 2 to 5 minutes.
The alginate-coated wound dressing material of the invention
preferably has a gas permeability (as defined herein) which
is at least 50% of the gas permeability of the material
prior to coating with alginate. More preferably, the gas
permeability of the coated material is from 60% to 98% of
the permeability of the uncoated material, and most
preferably from 75% to 95%, eg. from 85% to 92.5%.
The term "ga;s permeability" refers to the permeability of
a wound dre:~sing material to cyclohexane vapour, which
(unlike water vapour) is inert to the alginate and will not
cause the alginate to swell. The test method is as follows:
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ml of cyclohexane~ is placed in a 4 em diameter
cylindrical test cell, which is then closed using a layer
of the test material. The cell is placed in a fume
cupboard at 25° a.nd the rate of loss of cyclohexane vapour
5 is measured by w<~ighing the cell periodically (eg. hourly
for 5 hrs). Permeability may conveniently be expressed in
terms of grams of cyclohexane lost/m2/24 hrs.
Typically, the wound dressing materials of the present
l0 invention have a gas permeability of from 2500 to 20000
g/m2/24 hrs, and more preferably from 4000 to 15000 g/m2/24
hrs. Particularly preferred are dressing materials having
a gas permeability of from 5000 to 10000 g/mz/24 hrs.
The wound dressing materials of the present invention are
suitable for use :in a variety of forms. For example, they
may be used either alone (being secured in place by
bandaging, adhesiz~e tape, or any other suitable means), or
they may be formed into composite dressings. Indeed, the
material of the present invention can advantageously be
used in any of the circumstances in which absorbent wound
dressing pads are conventionally used. They are
particularly suitable for use as post-operative dressings.
Composite dressings inc:Luding the dressing material of the
present invention will. generally comprise a fibrous pad
having an alginatE~ coating on one surface, and a backing
material secured to the opposed surface. The backing
material may be porous or non-porous, but materials which
3o are impermeable to water but permeable to water vapour are
particularly preferred. Such materials include, for
example, cast polyurethane films. Alternatively, the
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backing material may be a perforated plastics film, such as
those conventionally used in first-aid dressings.
Wound dressings comprising the wound dressing material of
the present invention will generally be supplied in sterile
form, contained in a bacteria-proof envelope. Such
envelopes may be of any conventional form, such as a pouch
formed from two superimposed layers of plastics film, heat-
sealed around the_Lr periphery.
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Sterilisation may be achieved by any conventional means,
such as by autoclaving, gamma-irradiation and ethylene
oxide treatment.
The invention is further described by reference to the
following example.
EXAMPLE 1
An alginate composition having the following formulation
2o was prepared by simple mixing of the listed components:
%w/w
Sodium alginate 1.96
Calcium orthophosphate 0.17
Glucono lactone 0.17
Glycerol 1.34
Bronopol 0.02
Water 96.34
The ratio of calcium ions to sodium ions in this
formulation is 46.~5/53.4.
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The composition was reverse roll coated onto the surface of
Malifleece P/175/15 fabric, which is a 100°s polyester
fabric of weight 175 g/m2. The coating conditions were
chosen to produce a coating weight of 273 g/m2, which after
drying in an oven at a temperature increasing from 110°C to
130°C for 5 minutes, yielded a dry coating weight of 10
g/m2.
Scanning electron microscopy showed the alginate coating to
l0 form extremely thin films between individual fibres, with
very little effect on the overall porosity of the fabric.
This can clearly be seen from Figures 1, 2 and 3, which are
micrographs at magnifications of 13X, 31X and 67X,
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respectively.
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The coated material was found to be highly effective as a
wound dressing. Not only was the small amount of alginate
found to give effective haemostasis, but it also
significantly improved release of the dressing from the
wound, as compared with uncoated Malifleece fabric.
EXAMPLE 2
The composition described in Example 1 above was applied to
two samples of 150 g/m2 polyester Malifleece fabric at a
rate of 10 ~g/mZ. In the first case, the alginate was
applied to the smooth surface of the fabric, while in the
second case t:he alginate was applied to the opposite, less
dense, surface. The gas permeability of the two samples and
of the uncoated Malifleece fabric was measured using the
test described above. The results were as follows:
S<~mple Gas Permeability
1 0400 g/m2/24 hr
uncoated fabric
fabric coated on smooth surface 7600 g/m2/24 hr
fabric coatec! on less dense surface 7800 g/m2/24 hr
EXAMPLE 3
Example 2 above was repeated, using a knitted hydrophobic
polyester fabric. The coating conditions were selected to
achieve a coating rate of 30 g/m2 on the smooth surface, but
the same coating conditions gave a coating rate of 25 g/m2
when applied to the raised surface. It is thought that the
different structure of the raised surface simply results in
a lower pick-up of the alginate solution from the applicator
roll.
Sample Gas Permeability
uncoated fabric 9800 g/m2/24 hr
fabric coated on smooth surface 5400 g/m2/24 hr
fabric coated on raised surface 4900 g/m2/24 hY
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This example illustrates that a coating weight as high as
30 g/m2 can :be applied to a knitted hydrophobic polyester
fabric while retaining 50% of the gas permeability of the
fabric.
It will be understood that the present invention has been
described above purely by way of example, and many
variations will be possible within the scope of the
invention.
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