Note: Descriptions are shown in the official language in which they were submitted.
3~5
~IOUND DRESSIt~(i PIATERIAL
This invention relates to wound dressing materials, and more
specifically to a material which can be applied to many types of injury
or wound but is particularly useful as a temporary ~ound cover in the
therapeutic treatment of burns, varico,e ulcers, pressure areas and
other related injuries. The term "burns" covers thermal, chemical,
electrical and similarly inflicted wounds involving skin damaye or
destruction.
Burns require a unique combination of therapy and dressing when the
function of the skin is absent or impaired, because nutritious body
fluids and their essential components are continuously lost through the
wound, which in the case of a large area burn can cause dehydration of
t'ne pa~ien~ and in turn involve more serious complications such as lung
and kidney malfunctions, while the nor~al protection provided by the
skin from invading harmful bacteria and other toxic and noxious agents
is no longer available.
Many types of dressing material having been used heretofore. Absorbent
fibrous materials such as cotton gauze can cause excessive dehydration
and drying of the wound and thus become firmly adhered to the affected
area, so that dressing changes are painful and can cause further damage
to the wound. The natural healing process in which skin cells multiply
and migrate across the moist wound surface is delayed, because the
cells have to burrow deep under the dried area. Individual fibres can
alss become detached from the dressing and embedded in the wound and
~hereby impede healing.
To prevent adherence and maintain a moist wound environment, dressing
materials impregnated with greasy substances such as petroleum jelly
have been employed. These dressings require frequent changing to avoid
drying out, with consequent frequent exposure of the wound to airborne
bacteria, and the greasy substances provide a good environment for the
proliferation of bacteria so that wound infection is difficult to avoid,
' ' `~
~2~
-- 2 --
while the greasy substances can also be absorbed into the wound and
retard healing by ac~ing as foreign bodies.
Dressings made of polyethylene, polypropylene and polyamide films
eliminate adherence of the dressing to the wound and prevent particles
being embedded therein, but being non-porous they cannot absorb the
excess of liquid exudate which exudes from a burn and they can
therefore become painful to the patient unless changed frequently, with
consequent exposure of the wound to airborne bacteria. The
micro-climate under such film dressings, while favourable for healing,
is also ideal for such bacteria so that wound infection again becomes a
problem, while the fact that the condition of the wound is visible
through the film dressing is depressing to the patient. Other film
dressing materials made of gelatins, alginates and celluloses have been
des;gned to create the same favourable micro-climate in the wound by
dissolving or melting in the exudate and re-forming as part of the
eschar (scab). This increases wound dehydration and the films, when
mixed with the exudate in the eschar, become supportive of bacteria but
cannot be removed from th~ wound because they have become an inteyral
part of it. Even if infection is avoided, the dehydration effect
delays healing. Plastic spray-on dressings involve similar problems,
particularly encapsulation of dressing material deep in the wound.
Absorbent dressings of polyvinyl alcohol sponge and of polyurethane
foam with an open cell structure have been found to cause wound
dehydration and adherence of the dressing to the wound, while small
particles of the cells of the dressing material can become detached and
embedded in the wound, causing undesirable reactions. Also, passage of
the exudate right through the dressing can provide a nutrient path for
airborne bacteria back through the dressing into the wound. A
polyurethane foam dressing with an impermeable backing of polytetra-
fluoroethylene avoids the latter problem but still suffers from theproblem of detached particles.
Biological wound covers, mainly of porcine and cadaver skin, perform
~2~
-- 3 --
well in many respects but suffer from the problem that it is extremely
difficult to avoid leaving small fragments of dermis in the wound when
removing the dressing, producing subsequent foreign body reactions
which can cause rejection of autografts and later breakdown of healed
wounds, and causing production of antibodies which might involve
serious consequences at a later date.
In British Patent ~o. 1562244 I have described a wound dressing
material which is free from the disadvantages of the known materials
described above, and which comprises a sheet of a synthetic plastics
material which is permeable to water vapour and to air3 having on one
side a smooth glazed surface for application to a wound, a cellular
region behind the smooth surface, the cells in the cellular region
being closed cells, and a more dense region behind the other surface~
so that on contact with liquid exudate from a wound the dressing
material will absorb a limited amount of exudate but prevent it from
passing right through the dressing material.
The fact that the dressing material is water vapour- and air-permeable,
but will not permit the liquid exudate to pass through it, assists in
maintaining a moist micro-climate favourable for healing in the
wound. The smooth glazed surface avoids the danger of detachment of
cell particles and assists in preventing adherence of the dressing to
the wound. The cellular region enables the dressing to absorb excess
of liquid exudate without drawing out so Much exudate that the wound
would become dried and the patient dehydra~ed. Dressings need not be
changed too frequently.
Although this wsund dressing material is very successful and is a
substantial improvement over the kno~n materials described above, in
certain circumstances it has a slight disadvantage in that it is not
! sufficiently pliable to conform closely to the ~ound area to be
protected and has to be maintained in position by application of
additional dressings and/or adhesive tapes.
~7~
- 4 -
I have now found an alternative wound dressing material which provides
all the advantages of my earlier wound dressing material, and which is,
in addition, exceedingly pliable and therefore will readily conform to
the surface of a wound to be dressed.
Accordingly the present invention provides a wound dressing material
comprising a sheet of a synthetic plastics ma~erial which is permeable
to water vapour and to air, having on one side a surface for
applica~ion to a wound, and immediately behind the said surface a
cellular region having a structure of collapsed, open cells which
intercommunicate with one another, and a more dense region behind the
cellular region, and an intermediate region in which there is a gradual
transition from the cellular region to the more dense region, so that
- on contact with liquid exudate from a wound, the dressing material will
absorb a limited amount of exudate into the cellular region but prevent
it from passing right through the dressing material.
The physical structure of a wound dressing material in accordance with
the present invention is illustrated in the accompanying drawing which
shows a photograph of a transverse cross-section of a material in
accordance with the invention magnified 85 times.
Preferably the synthe~ic plas~ics material is a polyurethane, in
particular the product of polymerisation of a polyoxyethylene polyol
with a polyisocyanate in the presence of a cross-linking agent or
catalyst which is reactive with the isocyanate groups, the
polymerisation being performed without any substantial amount of water
present.
The preferred catalyst is dimethylethanolamine. The cross-linking
agent preferably contains in its molecule two or more amine and/or
hydroxyl groups which are reactive wi~h the isocyanate groups.
In a preferred embodiment, the polyol is a polyoxyethylene diol having
a weight average molecular weight of approximately 3000. In a
~ Z~7~
~ 5
particularly preferred embodiment the wound dressiny material is
prepared from a formulation comprising the following composition by
weight;
Polyoxyethylene glycol (m.wt. 3000) lU0 parts
Dimethylethanolamine 0.1-1.0 parts
Silicone oil 1-10 par~s
Toluene di-isocyanate 25-50 parts
The invention also comprehends a method of making a wound dressing
material, comprising the steps of mixing a polyoxyethylene polyol with
a polyisocyana~e and, if desired, a surfac~ant with agitation to form
i an aerated creamy mix, incorporating a cross-linking agent or catalyst
in the mix, spreading the mix on to a smooth glazed release paper to
form a sheet of a predetermined uniform thickness, and allowing the mix
to polymerise at a temperature not higher than 25C, preferably below
10 C, with the exclusion of any substantial amount of water from the
mix during the said steps. Preferably the release paper is supported
on a flat surface, such as a sheet of glass, during the spreading and
polymerising steps. The spreading may be effected by means of at least
one spreader bar with a corrugated surface which is drawn over the
surface of the paper at a fixed distance above it. The curing is
preferably effected in a curing cabinet in which a forced draught of
refrigera~ed, anhydrous air is passed over the upper surface of the
sheet.
The present invention will now be illustrated by the Sollo~ing
Examples.
EXAMPLE 1
In a preferred embodiment of the invention, the dressing material is
formed by polymerisation of a polyethyleneglycol supplied by Union
Carbide under the Trade Name NYAX, and having an average molecular
weight of approximately 3000, with a toluene di-isocyanate supplied by
Lankro Chemicals and having an Index of 109.
. .
~L~ 3~
Thus ~00 parts by weight of ~YAX polyol are mixed with 1-1~ parts by
weight, preferably ~ parts by weight, of silicone oil (L-~2~; supplied
by Union Carbide) to form an aerated creamy mix at a temperature of
22-25C. Subsequently, 25-50 parts by weight, preferably ~ parts by
weight, of toluene di-isocyanate haviny an Index of 1~9 (supplied by
Lankro Chemicals~, are blended into the mix to form a cream. Then,
0.1 - 1.0, preferably 9.5 parts, of dimethylethanolamine (sold as
Propamine A by Lankro Chemicals) is mixed into the blended mix as
quickly as possible.
The creamed mixture is then poured on to a smooth glazed release paper
which is tightly stretched over a long table having a flat surface,
such as a sheet of smooth plate glass, to ensure absence of wrinkles,
folds, or ridges. The creamy mix is then spread over the paper to form
a layer of uniform thickness, usually 0.5 mm thick.
The coated paper is then placed on trays which are transferred to a
curing cabinet through which a current of dry, cooled air is circulated
while curing of the mix takes place.
The mixing and polymerisation processes are performed in conditions
which are as anhydrous as possible. Thus, the mixiny of the reactants
is performed in conditions of minimum relative humidity and the air
passing into the curing cabinet is dried by passing it through silica
gel. The temperature in the curing cabinet is preferably maintained
below lO~C, or even lower by refrigerating the air ~hich enters into
the curing cabinet.
Under these conditions it is found that the volume of the creamy mix
begins to increase about five minutes after the catalyst Propamine A
has been added. As the mix cures it continues to rise for about a
further 30 minutes to 1 hour. Thereafter, the layer begins to
"defla~e" and after a further 3 hours it has fallen to a level
comparable with its ori3inal thickness.
~ fLY~3
- 7 --
EXA~PLE 2
A wound dressing material in accordance with the present invention is
prepared in the same manner as described in Example 1 and using the
same formulation as described in Example 1 but incorporating
additionally the following ingredients.-
Stannous octoate (supplied by Durham Raw 1 - 1~ parts by weight
Materials under the Trade Namepreferably 4 parts by weiyht
NEVOCURE)
Trichlorofluoromethane5 - 20 parts by weight,
preferably 9 parts by weight
It is believed that the process used to prepare the wound dressing
material of the present invention causes the initial formation of a
"foamed" region in the layer of polymerising materials, which foamed
region consists of open cells which communicate with one another. On
completion of curing, a non-rigid structure is obtained which
"collapses" as air escapes from the intercommunicating cells through
the outer surface of the cured polymer.
Curiny and subsequent "collapse" of the material is usually completed
in approximately four hours. The dressing material, still with its
release paper backing, is cut into the required shapes and sizes for a
medical/surgical wound cover by means of a band knife or roller press
and is wrapped in a siliconised tissue and packaged in foil pouches
which are sealed and gamma-irradiated to ensure sterile conditions.
A typical transverse cross-section of a sheet of wound dressing
- material in accordance with the present invention is illustrated in the
/ accompanying drawing which is a photograph of ~5 times magnification.
The photograph shows a first region A of the collapsed, open cells
which intercommunicate with one another, and the more dense region C
behind the cellular region, with an intermediate region B in which
there is a gradual transition from the cellular region A to the more
dense region C. In use, the lower surface of cellular region A as
~ 73
illustrated is applied to the ~ound to be healed.
Tests have shown tha~ the polymerized dressing material is free from
~oxic cyanate or amin~ residues and is highly effective as a wound
cover in the treatment of burns. On application to a burn, it conforms
to the contour of the wound and absorbs a limited amount of the liquid
exudate, while swelling slightly. I~ i 5 believed that this swelling on
contac~ with liquid causes the cells to open to receive liquid.
In addi~ion to the advantages mentioned above, the dressing matPrial
has the specific advantages of resembling skin in texture, of
conforming eas;ly and readily to any anatomical contour, and of causing
no pain in application. It also has thermal insulation qualities whicn
enable it to maintain an optimum temperature in the wound to promote
healing. It is permea~le to water vapour and air, so as to permit the
passage of gases through it, but being impermeable to liquids it
prevents drying and dehydration in or around the wound area. It is non-
adherent to wounds, making for painless dressing changes. It does not
distort or impede X-ray examination. It is unaffected by contact with
antiseptics, and under slight pressure it can act as a haemostat. It
will not support bacterial life or growth and it produces no loose
fibres or particles which could become embedded or encapsulated in a
wound. It requires no soaking before use as with bilogical wound
covers, thus saving valuable nursing time, and it can be applied by any
trained nurse without specialised training. The risk of infec~ion is
reduced because of the relatively infrequent dressing changes required,
and the similarity of skin in texture has a good psychological effect
on the patient. The production of a correct micro-clima~e in the wound
increases the speed of healing.
