CA1180622A

CA1180622A - Coated wound dressing

Info

Publication number: CA1180622A
Application number: CA000404843A
Authority: CA
Inventors: Douglas G. Murray
Original assignee: Minister of National Defence of Canada
Current assignee: Minister of National Defence of Canada
Priority date: 1982-06-10
Filing date: 1982-06-10
Publication date: 1985-01-08

Abstract

ABSTRACT OF THE DISCLOSURE

The invention disclosed is a wound dressing comprising a blend of a water-soluble resin material such as poly(ethylene oxide) and polyethylen-imine, and gelatin, in the form of a substantially uniform coating of an aqueous mixture of the water-soluble resin material and gelatin which sets to a gel and dries to a continuous wound-adherent film in situ.

Description

Th:i.s inverlLion relates to surgicaL dressings and in particular to the covering of a wounfl, lesion or the lilce with an adherent film to effect protection therefor during thc healing f)roct?ss~ and in particular to a wo-md-adherent film formed in situ by apl)lying dll aqueous mi~ture of the film forming ingredients as a substantially uni:Eorm cc~at:i.ng onto a wound~ The coating sf?Ls to a gel and dries to f`orlll a [i :Im whi.ch adll~res to thte wtt~lnd.
A very large numl)er orf people are seriously burtlecl in clomestic ancl industrial accidents every year, ancl the number of thf~?se victi.ms which di.es in spite of intensive medical care is di.stressingly high. In Canada, the number of burn injuries requiring hosp:ital aclmission approaclles 2.~,000 per year. ln the United States, about 130,000 are hospitalized annually because of burns;
of these, 70,000 require intensive care at a cost e~ceeding $300,000,000., and 10,000 die.
The problem of burn treatment is even more acute in areas of the world in which there is armed con:Elict, since many of the weapons of modern warfare either directly or indirectly cause burns to both military personnel and civilians. In time of war the demands on medical facilities and supplies are very severe, and the mortality rate arnong burn victims is greatly increased.
In a third degree burn, the full thickness of the skin has been ~ destroyed. The complete absence of "skin" cells in the burnecl area means that a ne~" covering of skin will not spontaneously form there e~cept by the very slow proliferation of healt'rly cells at the edges of the burn. Onè treatment is to remove a thin sheet of healthy skin from the patient's own body and graft it on to the burned area. Only a partial thickness of skin is removed, so that the cells in the remaining layer of skin can regenerate a full thickness in the area from which the graft was 'rlarvtstecl. In cases of burns covering 50%
or rnore of the body surface area, this grafting procedure will be a lengthy process carried out in sevtral stages because of the timc requirecl for regener-ation of skin on tht? harvested site~.
f; In the interverling periocl betwet-n hospitalization and grafting, two ter,y sf?rio~ f~rot)lcrrls arc? cau~.~(l by Lhe~ abserlce of sk:in in the area of the ;)urn. Orle. of the rnc)st impc>rtlnt ftlnctions of normal skin is to restri.ct the i.oi~ of body "at-r by evaporat.ion. Tht~ drarnatic increase in water loss caused by destruction of the skin produces a la-rge rate of heat loss due to the cooling effect of evaporation. In order to maintain a normal body temperature, the metabolic rate must increase, and a rapid depletion of fat and protein reserves ensues. The other serious problem caused by the absence of skin is bacterial invasion. If this invasive infection is not restricted, the high bacteria population makes the wouncl unsuitable for skin grafting. The victim of a major burn who receives no treatment Eor this lnfection will ultimately die from it.
After removal of the layer oE deacl tissue (eScllar~ lying on top of the burned area, it is desirable to cover the wouncl with a clressing which wi]l control water loss and assist the body's own deEences in controlling bacterial proliferation until skin grafting can be carried out.
Two natural materials, pig skin and human skin from cadavers, are regarded as very effective burn dressings. These materials can, under favour-able circumstances, adhere to the wound very well and effectively control water loss and infection. The problem is that the body recognizes these bio-logical materials as foreign substances, and begins a cellular response to reject them. Because of this, these biological dressings must be replaced every 2 to ~ days. The other major drawback of these materials is their high cost. Cadaver skin at $80. and up per square foot, ancl pig skin at about $30.
per square foot, are so expensive that physicians can be reluctant to use the~
except in cases in which they are essential for survival of the patient.
The disadvantages associated with these biological materials has given rise to the preparation of a large n~ber of synthetic substitutes. The more effective of these have consisted of a foam, velour, or fibrous mat laminated to a synthetic resin film. Aclhesion to the wound occurs by growth of tissue into the interstices of the foam or Eibres, and the film controls loss of body water and prevents entry of bacteria. This basic approacll has some innerent drawbacks. Since the adhesion depends on ingrowth, some time must elapse before the dressing is firmly adherent to the wound. A more serious problem in many cases has been the tenacious adherence that eventually does ta~e place. Removal of the dressing can theil be a traumatic procedure which may produce excessive bleeding and leave fragments of the synthetic material in the wound. These fragments may delay the healing of the wound when skin grafting is carried out.
Two laminates of this type are on the marketO One of these, a laminate of microporous polypropylene and reticulated polyurethane foam is sold under the trademark EPIGARD by Parke, Davis and Co. For ~ variety of reasons, EPIGARD has not been widely accepted Eor clin;cal use by surgeons who treat burn patients. The other laminate is sold ~mder the trademark BIOBRANE by Hall-Woodroof Inc. It is a composi~e of a flexible nylon fabric and an ultra-thin silicone rubber membrane. The high cost of BIOBRANE is a very serious drawback which may limit its acceptance. Inspection of a commer-e9. ? sample of BIOBR~N~ has revealed a number of holes in the ultra thin mem-brane which are irregularly spaced and easily visible to the naked eye. These holes provide external bacteria with easy access to the wound.
A radica]ly different type of burn dressing has been developed by I.V. Yannas and J.F. Burke and co-worlcers in the United States. Their approach has been to produce a material which slowly b;odegrades at the wound surface and is assimilated by the body. During this biodegradation, the dressing not only restircts water loss and controls infection, but also acts as a tissue culture medium. Ultimately, migration of epidermal cells forms a new skin over the wound site. At the present state of development of their film, the healing of only relatively small wounds by this elegant process is possible. The Eilm can be used to advantage on large wounds, but skin grafting is still required at a later stage. A commercial form oÇ the film, when available, will probably be quite expensive.
In United States Patent No. 3,164,560 which issued 5 January 1965, John F. Suter teaches the use of a gelatin/polyalkylene oxide composition in the form of continuous films of improved extensibility and flexibility as compared to gelatin films and which have improved tensile strength and toughness as c~mpared to polyalkylene oxide films. The films are used, for example, in packaging and photographic supplies. There is no teaching or suggestion of use as wound d-ressings.

German publishecl patent application No. 2,914,~22 of l~ October 1979 in the name of Unitika Ltd., teaches a material for the healing of wounds which comprises blood coagulation factor XIII and a wide variety of polymers including gelatin poly(ethylene oxide) and polyethyleneimine, but not in combination~ Any materials of which blood coagulatlon Eactor ~III is a significant component would be expected to be very e~pensive.
~ pplLcant's co-pending Cana(lian application Serial No. 3c5,860, filed 14 September 1981, describes a wo~md adherent dressing, comprising a blencl of a water-soluble resin material and gelatin~ particularly in the form of a preformed ilm.
It has now been found by applicant that a similar ~ound-adherent film formed in situ by a coating process is superior in certain respects to a preformed film.
According to the invention, a wound dressing is provided, comprising a blend of a water-soluble resin material and gelatin, in the form of a substantially uniform coating of an aqueous mixture of the water soluble resin material and gelatin which sets to a gel and dries to a continuous wound-adherent film in situ.
It is important to the invention that the polymers used in the production of the film are water-soluble and the resulting film adheres to the wound without the need of additional securing devices. Also important is the ease of release of the film from the wound. By merely soaking the films adher-ing to the wounds in tap water or physiological saline for 15 minutes at the temperature of the wound surface, the adhesion is greatly diminished and the films become soft. They can then be removecl by gentle scraping. Preferably, the films should be sufficiently transparent to permit observation of the ~round and the possible appearance of purulence beneath tlle film.
In particular, combinations of gelatin and poly(ethylene oxide) (PEO);
and gelatin and polyethyleneimine (PEI) have been found to be especially effective.
~ ore specifically, an aqueous solution of l part gelatin of Bloom ~15() ancl 2 parts of "Polymin P" (equivalent to l part PEI as Polymin P is 6~22 supplied as a 50% aqueous solution) can be poured direct]y onto a debrided burn wound where it seLs -to a gel which dries to form an adherent f;lm. It is preEerable to have a more concentrated gelatin-PEI solution than that described in Example 2 of the principal disclosure for the preparation of a preformed film because (i) more concentrated solutions have increased viscosity and are less likely to flow away Erom the surface of a wound after applicntion, (ii) concentratecl solutions produce thicker dry films for a given thickness of liquid applied and (iii) l:hey dry faster~ Because gelatin an~ PEI form a strong complex, there is an upper limit to the concentration o aqueous gelatin and aqueous PEI solutions which can be mixed at a given temperature. For example, an aqueous solution containing 36~¢o w/w polymers, specifically 72 g of Polymin P (50% aqueous solution) mixed with a warm solution of 36 g of gelatin of Bloom ~150 in 90 ml of distilled water, resulted in a tough rubbery, translucent material which separated immediately from solution. In general, the upper limit of the solids concentration will in most cases be the maximum possible concentration permitted by the solubility of the complex of gelatin and PEI in water at the temperature of application i.e. about 30 C. Other things being equal, the higher the concentration the stronger the initially formed gel and the thicker the final dry film.
As the concentration of polymers is decreased, the dry film becomes thinner not only because the applied liquid film has less dissolved polymer, but also because the liquid film will be thinner because of reduced viscosity.
For gelatin of Bloom ~150 and Polymin P, it is contemplated that a total poly-mer concentration of 10% w/w would give too thin a film. For practical purposes, in the case of a single coating, 15% w¦w is contemplated as the lower limit for this pair of polymers.
E~ampl _ An aqueous gelatin-PEI (1:1) solution which has been used success-fully in trials on experimental wo~nds contains approximately 22% w/w of the polymers, i.eO PEI plus gelatin. It is prepared and applied as follows: 30 g of Polymin P were diluted with 40 ml of distilled water and warmed to about 35 C. 15 g of gelatin of ~loom ~150 (Davis Gelatin Co.) were dissolved in ``` ~L~8~62~

50 ml of distilled water at about 50C. This solution was added with stirring to the warm PEI solution. Stirring was continued for about 7 minutes while the mixture was kept in a bath at about 35C. The clear, light yellow solution formed a gel after it had cooled to near room temperature. This gel can be conveniently melted by warming it in a bath of water from a hot water tap (frequently arouncl 50C). A bath having a lower temperature, say 40C, may also be used, but the rate oE melting will be reduced. In any case the temper-ature of the liquid at the time of application shoulcl not be much higher than 30 C. In practice, a small amount of liquid is poured directly onto the wound and spread substantially uniformly over the surface of the wound and the healthyskin immediately surrounding the wound. A convenient technique is to simply spread the liquid with a hand covered with a sterile surgeon's glove, but a brush or other spreading device could also be used. If desired, a second coat of the liquefied gelatin-PEI gel can be applied on top of the first coat after it has dried.
Although the thickness of the dried film varies over the area of the wound because of the curvature and irregularity of the wound, the operable thickness range is about 0.05 to 0.30mm, preferably about 0.18mm~
Example 2 The same procedure is followed as in example 1. However in this case the amounts of polymers is 24% w/w, the Polymin P is diluted with 35ml distilled water and warmed to about 55 C, the gelatin is dissolvad in 45 ml of distilled water at about 50C, and stirring o the gelatin/PEI mixture is conducted in a bath at about 50 C.
Example 3 The same procedure is followed as in example 1. However, in this case, the amounts of Polymers is 18.8% w/w, the Polymin P is diluted with 35 ml of distilled water and warmed to abo~t 55C, the gelatin is of Bloom *275 and grade H4S and is dissolved in 45 ml of distilled water at about 50C. Moreover, when the gelatin/water solution was added with stirring to the warm PEI solu-~ion, a ~,teak gel formed. To this was added 35 ml of distilled water and the mixture was stirred at about 50C for 10 minutes, yieldlng a liquid which would flow readily.
Example 4 The same procedure is followed as in example 1. ~lowever, in this case the amounts of polymers is 19% w/w, 20 g of Polymin P were diluted with 35 ml of distilled water and warmed to about 55 C, 10 g of de-ioni~ed gelatin of Bloom ~300 (KIND and KNOX) were dissolved in 40 ml of distilled water at about 50C and stirring was continued for about 10 minutes while the mixture was kept at about 50C.
The embodiment of example 2 is preferred because the resulting increase in viscosity reduces the tendency of the applied liquid to flow away from the wound site. Although the viscosities of the formulations of examples 3 and 4 are adequate, they are somewhat inferior to that of example 2 in terms of convenience of application and produce thinner films.
The time required for the liquefied gel of the present formulation to set to a gel after application to the debrided burn wound depends on several factors. In our experience, provided that (i) the film of applied liquid is not too thick and is uniformly spread, (ii) bleeding of the wound has been arrested, and ~iii) the temperature of the liquid is about 30 C at the time of application, the liquid sets to a gel in about 20 minutes. This gel, which is not tacky, then dries to a clear, supple film which has excellent adhesion to the wound. Gelation and drying of the liquid film covering healthy skin near the edges of the wound is much faster because much less water passes through healthy skin than through a freshly debrided wound.
A frequently used experimental wound on domestic pigs has been a full thickness burn freshly excised to subcutaneous fat 5 to 7 days post burn.
When the gelatin-PEI liquid coatings described in examples 1 - 4 are spread on a wound of this type they dry to films which remain intact and adherent for generally 7 days or moreO One dressing formed by the application of two coats of the liquid embodied in example 1 lasted for several weeks. In no case has purulence been observed beneath the dressings during the period in which they remained intact and adherent.

8~622 Some other gelatin/resin material liquid formulations which gel and then dry out on wo~mds e~cised to subcutaneous fat have been investigated, and they have been found inferior to the gelatin-PEI liquid. For example, a 31% w/w aqueous solution of gelatin of Bloom ~150 formed a film which was too stiff. ~ 26% w/w aqueous gelatin (Bloom A~50)-poly(ethylene oxide) ~molecular weight 15,000 - 20,000) (7:5) liquid mi~ture adjus~ed to pH 3 with phosphoric acid produced a supple film which disintegrated to a soft, weakly adherent, granular coating after several days. Due to the high pol~ner solids concentra-tions employed, the aforementioned mi~ture musC be shaken vigorously just before application. A 41% w/w aqueous solution of gelatin (Bloom ~150) - glycerol (6:1) gave a film which, like the coating of gelatin alone, was too stiff. A
50% w/w aqueous gelatin - glycerol (1:1) liquid set to a gel which eventually dried to form a pliable dressing, but the gel was still tacky 40 minutes after it had set.
The gelatin-PEI liquid is thus preferred and forms a wound dressin~
which is superior to the preformed gelatin-PEI film described in the principal disclosure. The superiority is almost certainly due to the ability of the liquid coating to completely cover the wound surface because it can flow into the crevices and depressions. This means that for a given wound area, the total area in contact with the dressing formed from the liquid will be greater than the area in contact with the preformed film. The increased area of contact will, other factors being equal, prod~lce improved adhesion. In addition, the air pockets between the film and the wo~md which occur in the case of the preformed film can become filled with liquid and may become pockets of infec-tion, although this has not been a problem to date.
The only apparent disadvantage of the gelatin-PEI liquid coating when compared to the preformed film is that it requires some time to gel and then dry on the wound. One method applicant has found for circumventing this is to apply a preformed gelatin-PEI film as described in co-pending Canadian application Serial No. 385,860, specifically of the preferred formulation, on top of the freshly applied gelatin-PEI liquid coating before it sets to a gel.

The liquid can then gel, and dry by transmission of water vapour through the 6~:2 preformed film above it. Preformed gelatin-PEI films of thickness 0.10 and 0.17 mm have been used successfully in combination with the gelatin-PEI liquid.
It is expec~ed that films thinner than 0.10 mm will become excessively soft and tacky when placed on the freshly applied coating of liquid and consequently will offer no advantage over the use oE the liquid alone.
When the coating of liquid beneath the preformed film has dried, it has good adhesion to the preformed Eilm, and the combinatlon nf the two adher-ent films serves as the wound dressing until removal of the dressing is desir-able. On occasion it has been possible to pull the outer layer of preormed film away from the inner layer o dried gel which remained adherent to the wound.
It is felt that, in addition to a preformed gelatin-PEI film, o~her resin film materials which are highly permeable to water vapour might serve as a useful outer covering for the freshly applied coating of gelatin-PEI

Q

liquid. Other preformed films which could be employed include Celgard~ a microporous polypropylene material made by Celanese, and fine nylon mesh mater-ial as employed in commercial stockings. These film materials could remain in place when the liquid had dried if their physical properties were suitable, or, depending upon their adhesion, they could be peeled away.
It is contemplated that the limits of weight ratios in the gelatin-PEI liquid which will still give an outstanding wound dressing is (gelatin-PEI) 7:3 to 3:7; compared to ratios for the preformed gelatin-PEI fi~m in the prin-cipal disclosure of 9:1 to 3:7. It is also contemplated that the different types of gelatin and PEI which can be used are gelatin of Bloom~f-8o-35o and PEI of molecular weight 10,000 to 100,000.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY OR
PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A wound dressing, comprising a blend of a water-soluble polyethyl-enimine material and gelatin, in the form of a substantially uniform coating of an aqueous mixture of the water-soluble polyethylenimine material and gelatin which sets to a gel and dries to a continuous wound-adherent film in situ.

2. A wound dressing according to claim 1, wherein the aqueous mixture contains about 24% w/w of polyethylenimine plus gelatin.

3. A wound dressing according to claim 1 or 2, wherein the blend comprises gelatin and polyethylenimine in a weight ratio of 7:3 to 3:7.

4. A wound dressing according to claim 2, wherein the blend comprises gelatin and polyethylenimine in a weight ratio of 1:1.

5. A wound dressing according to claim 1 2 or 4, wherein the gelatin has a Bloom # of 80 - 350 and the polyethylenimine has a molecular weight of 10,000 to 100,000.

6. A wound dressing, according to claim 1, 2 or 4, additionally compris-ing a preformed film of a suitable water-vapour permeable resin material cover-ing the freshly applied coating.

7. A wound dressing, according to claim 1, 2 or 4, additionally comprising a preformed film of a blend of gelatin/polyethylenimine of a thickness of about 0.10 to 0.17 mm covering the freshly applied coating, wherein the weight ratio of gelatin: polyethylenimine is about 1:1.

8. A wound dressing according to claim 1, 2 or 4, wherein the thickness of the film is in the range of 0.05 to 0.30 mm.

9. A wound dressing according to claim 1, 2 or 4, wherein the thickness of the film is about 0.18 mm.

10. A wound dressing according to claim 1, 2 or 4, wherein the gelatin has a Bloom # of about 150 and the polyethylenimine is POLYMIN P?.

11. A burn wound dressing, comprising a blend of water-soluble polyethylenimine of a molecular weight of 10,000 to 100,000 and gelatin of a Bloom # of 80 to 350, in the form of a substantially uniform coating of an aqueous mixture of the water-soluble polyethylenimine and gelatin which sets to a gel and dries to a continuous wound-adherent film in situ, wherein the blend comprises gelatin and polyethylenimine in a weight ration of 7:3 to 3:7.

12. A burn wound dressing according to claim 11, wherein the thickness of the film is in the range of 0.05 to 0.3 mm.

13. A burn wound dressing according to claim 12, wherein the gelatin has a Bloom # of about 150.

14. A burn wound dressing according to claim 11, 12 or 13, additionally comprising a preformed film of a blend of gelatin/polyethylenimine of thick-ness of about 0.10 to 0.17 mm covering the freshly applied coating, wherein the weight ratio of gelatin: polyethylenimine is about 1:1.