Vented wound dressing

A vented wound dressing is disclosed comprising a thin conformable sheet material at least a portion of the surface area of which is intended for placement as a dressing over a wound, which portion carries a pressure-sensitive adhesive coating on one surface thereof for adhering the dressing to skin, the coating being applied to provide repeating areas of the sheet material containing no adhesive, at least a portion of the repeating areas of no adhesive having slits extending through the thickness thereof to permit transfer of wound fluids through the sheet material unimpeded by presence of adhesive material which can clog the slits and thereby inhibit fluid transfer therethrough.

BACKGROUND OF THE INVENTION 
It is most desirable in wound treatment to provide an adhesive dressing 
which will maintain the desired moist environment promoting healing while 
preventing scab formation; and also permitting removal of wound fluid 
which can build up to a pressure bubble beneath the dressing, thereby 
undermining the adhesive seal to the skin and thus increasing the 
possibility of the wound being contacted by ambient contaminants, 
including, of course, microorganisms which can cause infection. 
Seemingly, these two objectives are often at cross-purposes so that one of 
the two desired objectives is accomplished to the detriment of the other. 
Yet, dressings fulfilling both objectives are heretofore disclosed in the 
art. 
By way of illustration, reference is made to U.S. Pat. No. 4,541,426 issued 
to Webster whose discussion of the prior art as background to the 
invention is additionally worth mentioning. 
As stated in Col. 1 of the patent: 
It has long been a recognized problem that dressings are inclined to 
suffer from either or both the disadvantages that they sometimes tend to 
float away from a wound or else they sometimes tend to adhere to the wound 
surface. 
The first of these disadvantages generally occurs when the wound is one 
that produces large volumes of exudate. Generally the method of overcoming 
this problem is to provide the dressing with holes so that the exudate can 
escape and the dressing remain in contact with the wound. Certain attempts 
to achieve this end are disclosed in U.K. Pat. Nos. 778813, 1298011, 
1408345 and Patent Application Nos. 2061732 and 2074029. One successful 
dressing is Melolin (Trade mark available from T. J. Smith & Nephew Ltd., 
Hull, U.K.) which comprises a perforated synthetic polymer film and an 
absorbent cellulosic pad. The perforated film is placed next to the 
exuding wound, the exudate passes through the perforations and is absorbed 
by the pad. A more recent suggestion has been to use a perforated 
polytetrafluoroethylene film in an effort to minimise the risk of any 
adherency of the dressing to the wound. 
Alternatively dressings have been suggested which comprise a thin 
hydrophobic film laminated to a fibrous absorbent layer. The film contains 
a number of apertures in the form of slits. Such dressings are described 
in, for example, British Pat. Nos. 815,121 and 1,163,452 and U.S. Pat. No. 
3,602,220. However dressings of that type have not been found to be 
satisfactory because either the slits don not open or do not open wide 
enough to allow passage of exudate through the film to the absorbent. 
The second of the aforementioned disadvantages generally occurs when the 
wound has dried out due to lack of production of exudate. Generally the 
method of overcoming this problem is to provide the dressing with a 
continuous layer which retards the rate of loss of water. One effective 
method of achieving this end is described in British Pat. No. 1280631. 
However none of the known methods are free of disadvantages since what may 
be an excellent dressing for one kind of wound will be unsuitable for many 
other wounds since wounds differ greatly in their output of exudate. It 
has now been realised that, not only is there a need for a dressing which 
is suitable for use on a number of different wound types, there is also a 
need for a dressing which can better cope with the variation in rate of 
exudate production from a given wound. A dressing has now been discovered 
which allows passage of a greater amount of exudate from a wound which 
produces greater amounts of exudate and which aids in allowing the wound 
to remain a moist wound when it produces only smaller amounts of exudate 
so that it does not float away from the moist surface and has a reduced 
tendency to adhere to the wound. The new dressing has been found to aid in 
the re-epithelialisation of the wound. 
Accordingly, the patented invention is said to provide a dressing 
comprising a conformable film with aperatures therethrough characterized 
in that the film comprises a first layer laminated to a second layer, the 
first layer comprising a material which swells when in contact with water 
and the second layer comprising a material which when in contact with 
water does not swell or swells less than the first layer. According to the 
patentee the apertures are enlarged when in use on a wet surface and 
otherwise not enlarged, the enlarged openings permitting passage of water, 
e.g. wound exudate, the apertures when not enlarged preventing the wound 
from drying out; i.e. providing a moist environment. 
A particularly efficacious wound dressing permitting removal of wound 
fluids while maintaining a moist environment is described and claimed in 
copending application, Ser. No. 337,591 filed Apr. 13, 1989 and now U.S. 
Pat. No. 5,106,362. 
As discussed in the copending application, the dressing will comprise: a 
base sheet for contacting the skin, the base sheet having an opening for 
placement over a wound and adhesive means for securing the base sheet to 
the skin; and vent means for providing controlled leakage of fluid along a 
path from the wound through the opening of the base sheet, the vent means 
comprising cover means covering the opening, the cover means permitting 
passage of wound fluid therethrough while reducing evaporation through the 
opening and thereby helping to insure a moist environment when excess 
wound fluid is removed from the wound. 
The task of the present invention is to provide a wound dressing of 
simplified design and construction which permits removal of of wound 
fluids while at the same time maintaining a moist environment promoting 
wound healing. 
BRIEF DESCRIPTION OF THE INVENTION 
In accordance with the present invention it has been found quite 
unexpectedly, contrary to the teachings of the aforementioned Webster 
patent (to be discussed in more detail hereinafter) that the task may be 
solved in an elegant manner by providing a wound dressing comprising a 
thin conformable sheet material for placement over a wound, the sheet 
material carrying a pressure-sensitive adhesive layer on one surface 
thereof for adhering the dressing to the skin, the adhesive layer being 
applied so as to provide repeating areas of the sheet material containing 
no adhesive, at least a portion of the repeating areas free of adhesive 
having a slit extending through the thickness thereof to permit transfer 
of wound fluids through the sheet material unimpeded by the presence of 
adhesive.

DETAILED DESCRIPTION OF THE INVENTION 
As heretofore mentioned, it is most desirable to provide an adhesive 
dressing which will maintain a moist environment promoting healing while 
preventing scab formation; while at the same time permitting removal of 
excessive wound fluid which can cause a pressure build-up beneath the 
dressing to undermine the adhesive seal of the dressing to the skin and 
thereby increase the possibility of the wound being contacted with ambient 
contaminants. 
Applicant's aforementioned U.S. Pat. No. 5,106,362 is directed to a 
particularly efficacious wound dressing satisfying these objectives. Yet, 
it is still desirable to provide a wound dressing meeting the above-noted 
criteria which is of simplified design and thus of lower cost to 
manufacture. 
The aforementioned U.S. Pat. No. 4,541,426 of Webster is directed to what 
is described as a conformable apertured film which it is said (Col. 1) 
"allows passage of a greater amount of exudate . . . and which aids in 
allowing the wound to remain a moist wound when it produces only small 
amounts of exudate . . . ." 
This is said to be accomplished by providing a dressing having two layers 
laminated together, the first layer 1 (to be placed on the wound) being 
water-swellable, the second layer 2 being non-swellable. As described and 
seen clearly in the patent drawings, the laminar structure is provided 
with slits 4 extending through the thickness of the two layers. An 
absorbent pad 6 may be positioned over the non-swellable second layer 2 
and a layer of adhesive 5 may be provided along two opposed edges to 
adhere the dressing to the skin. 
The slitted dressing of Webster is said to be an improvement over prior art 
slitted dressings such as the dressings described and claimed in U.S. Pat. 
No. 3,602,220 which are stated in Webster to be unsatisfactory because 
either the slits do no open or do not open wide enough to allow passage of 
exudate through the film to the absorbent. 
Applicant concurs with this assessment by Webster of the prior art slitted 
dressing. 
Such dressings do not of necessity contain an adhesive layer under the 
slits because such a layer would be a barrier to diffusion of exudate from 
the wound to the slitted sheet material and, moreover, would clog up the 
slits, thus acting in the nature of a dam precluding diffusion of the 
exudate therethrough. 
Yet, Applicant has discovered that for proper operation of the opening of 
the slits to permit passage of exudate, it is essential (absent some 
chemical or physical action as described in Webster) for there to be a 
build-up of fluid pressure beneath the slits to force them to open. Since 
there is no adhesive beneath the slits, the exudate will leak out 
laterally and, consequently, there can be little if any pressure build-up 
for efficient opening of the slits. 
Webster proposes in his patent that a dressing may be made having an 
adhesive coating on layer 1, the swellable layer. He states that "the 
adhesive may be applied over the entire surface of the first layer . . . " 
(Col. 5, line 62). He then goes on to state that "Alternatively the 
adhesive layer may be confined to a strip around the periphery of the 
dressing or to strips on two opposite edges of the dressing. This limited 
area of adhesive is not preferred." 
A dressing embodying the invention of the Webster patent has been marketed 
by the assignee, Smith & Nephew under the trademark "TransSite", which 
dressing is the one stated to be least preferred, i.e., the one having two 
adhesive strips on opposite edges of the dressing. It is believed that 
there are two factors precluding commercialization of the design stated to 
be preferred: (1) it is not possible to laminate an adhesive to the 
swellable film without delamination occurring when the film swells; and 
(2) the adhesive coating will tend to keep the slits from opening, since 
the adhesive will flow and "mend the slit after it is made. 
From what has been stated, it will be seen that while an adhesive coating 
beneath the openings of a slitted or perforated dressing is inoperable and 
thus contraindicated by the state of the art, Applicant has discovered 
that such an adhesive coating is nevertheless necessary (absent some other 
mechanism) to provide optimum transfer of the exudate through the openings 
of the dressing. 
Having determined the necessity of a pressure build-up induced by the 
presence of an adhesive layer retaining the dressing securely against the 
skin so as to preclude lateral leaking or diffusion of exudate, the 
essence of the present invention is the discovery that the paradox or 
seeming impossibility of having the adhesive to provide the requisite 
pressure build-up to optimize fluid transfer through the slits without 
inhibiting the transfer therethrough by the very presence of the adhesive 
can be obviated by providing the adhesive coating in such a way that there 
are repeated voids in the coating providing areas free of adhesive and 
then slitting the thin sheet material in these non-adhesive areas. 
Preferably the non-adhesive areas are arranged in a uniform geometric 
pattern, e.g. in a series of rows covering the entire surface area of at 
least that portion of the sheet material intended to cover the wound. 
The invention may best be understood by reference to the illustrative 
drawings taken in conjunction with the following detailed description. 
With reference to FIGS. 1 and 2, the novel vented wound dressing 10 of this 
invention will comprise a thin conformable sheet material 12, at least a 
portion of which is adapted for placement over a wound. At least the 
portion adapted for placement over a wound has a pressure-sensitive 
adhesive layer 14 on one surface thereof, the adhesive layer being applied 
to provide repeating spaced areas 16 free of adhesive. Preferably the 
non-adhesive areas 16 are arranged in a geometric pattern, e.g. in 
staggered rows as shown in the drawing. In any case, while the 
non-adhesive areas 16 are shown for purposes of illustration as being 
generally circular, the configuration is not critical and they may any 
desired shape, e.g. oval, rectangular, arcuate, etc. 
Slits 18 are shown to be provided within each of the spaces defined by the 
non-adhesive areas. However, as heretofore stated, it is within the scope 
of this invention to provide slitsin only a portion of the non-adhesive 
areas. While shown in FIG. 1 to be somewhat arcuate in shape, slits 18 may 
have other forms and, as shown for example in the aforementioned U.S. Pat. 
No. 4,541,426 of Webster, they may be linear or in the form of triangular 
flaps provided by slitting two sides of the triangle, so that the flaps 
remain attached by the third side. 
As heretofore mentioned, the novel dressings of this invention contemplate 
a sheet material at least a portion of which is adapted for placement on a 
wound, that portion having the described adhesive layer and slits. For 
purposes of illustration, in the embodiment illustrated in FIG. 1 the 
entire surface area of the dressing is adapted to cover a wound and the 
adhesive and the non-adhesive areas with the slits accordingly extend to 
the edges of sheet material 12 defining the periphery of the dressing. 
FIG. 3 illustrates a preferred embodiment wherein a per se known release 
sheet covers the adhesive layer prior to use in order to prevent premature 
unwanted contact of the adhesive-bearing surface. In the form illustrated 
in the drawing the release sheet consists of a pair of contiguous sheets 
20 which together cover the entire adhesive-bearing surface of sheet 
material 12. As seen, release sheets 12 may have free folded ends 22 for 
ease of gripping with the fingers to separate sheets 20 from the dressing. 
While FIGS. 1 and 2 illustrate the essential novel features of the present 
invention, i.e. the features upon which patentable novelty is herein 
predicated, the wound dressings will preferably also include an absorbent 
pad or other per se known equivalent fabric positioned on the non-adhesive 
bearing surface, i.e. above the dressing, to provide a reservoir for 
receiving and retaining wound fluids diffusing through the slits 18. 
Preferably, a cover sheet providing a bacterial barrier is situated over 
the fabric reservoir. Various materials and arrangements of elements 
providing this function are of course well known in the wound dressing art 
and consequently per se comprise no part of this invention. 
However, a particularly efficacious arrangement of elements providing a 
reservoir for receiving and retaining wound exudate contemplated for use 
in combination with the novel vented dressing of this invention is that 
described and claimed in Applicant's copending application, Ser. No. 
461,598 filed Jan. 5, 1990 and now U.S. Pat. No. 5,060,642. As disclosed 
therein, the wound dressing has a sealed absorbent fabric design and 
structure wherein an absorbent fabric providing a reservoir for retaining 
wound exudate is contained between a bottom liquid-permeable sheet 
material permitting the wicking or diffusion of wound exudate and an outer 
cover characterized as being a bacterial barrier, at least a portion of 
the outer cover also being air-permeable for permitting egress of air from 
the interstices or voids in the fabric reservoir to the ambient 
atmosphere. As stated therein, this removal or displacement of entrained 
air within the fabric reservoir is necessary to free these interstices to 
act as a sponge for retention of wound fluid diffusing thereto, thereby 
appreciably increasing the capacity of the reservoir for receiving wound 
fluids. 
FIG. 4 illustrates this preferred embodiment of the invention. 
As shown, an absorbent pad or other absorbent fabric 32 is seated on the 
surface of sheet 12 opposed from the adhesive-bearing surface. The 
absorbent fabric 32 is covered with a liquid- and bacteria-impermeable 
cover sheet 24. Cover sheet 24 and sheet 12 are sealed in liquid- and 
bacteria-tight relationship around their common periphery so that exudate 
cannot escape through the edges of the dressing, nor can any external 
contaminants, including bacteria enter into the dressing and then pass 
through the slits 18 in sheet material 12 and then to the underlying 
wound. 
The outer cover 24 is provided with one or more windows or openings 26 to 
permit egress of air from the interstices of the fabric reservoir 32 as 
well as the space 30 between the reservoir 32 and cover sheet 24. Each 
such window or opening is shown to be covered by an air-permeable 
bacterial barrier sheet 28 of slightly larger dimensions than the 
dimensions of opening 26. Each sheet 28 is sealed around its periphery to 
the edges of sheet 24 around opening 26 so as to prevent ingress of 
bacteria around the edges of the opening. 
The particular materials employed for preparing sheets 24 28 and reservoir 
32 may be selected from those heretofore known in the art for providing 
their respective functions. Since such materials are well known and their 
selection will be a matter of choice within the expected judgement of the 
skilled worker in the light of the foregoing description, their selection 
per se accordingly comprises no part of this invention. 
Water-impermeable sheet 24 should as stated be impermeable to bacteria as 
well. It may, for example, be on the order of 0.5 to 1.0 mil thick and 
comprise a suitable polymeric material such as polyurethane, "Saran" 
(trademark of Dow Chemical), a polyolefin such as polyethlene or 
polyproplylene, a polyester such as polyethylene terephthalate, etc. In 
any event, sheet 24 should also be flexible and conformable. 
Bacterial barrier 28 may comprise any of the per se known bacterial barrier 
air filters such as NUCLEOPORE, MILLIPORE, GELLMAN, etc. 
Reservoir 32 may comprise any of the fabric materials heretofore employed 
for wound dressings to retain exudate, e.g. cotton, gauze sponges, 
absorbent pads such as those customarily used for abdominal surgery, and 
the like. If desired, they may additionally contain an antimicrobial agent 
such as chlorhexidine, although the use of such a reagent is not 
considered necessary. 
In the embodiment illustrated in FIGS. 1 and 2, the entire surface area of 
the dressing is adapted for placement over the wound and the slits (and 
adhesive/non-adhesive areas extend to the edges of the dressing. However, 
as previously stated, the present invention contemplates wound dressings 
wherein only a portion of the sheet material constituting the dressing is 
intended to be placed on a wound. 
FIG. 5 is directed to a particularly preferred aspect of this invention 
embodying the latter concept. 
As shown therein, a layer of pressure-sensitive adhesive 34 (which may be 
the same as or different from adhesive 14) is applied around the edges or 
periphery of sheet material 12, the adhesive 34 in turn defining the 
periphery of the portion of the dressing adapted for placement on the 
wound. Securing the dressing to the skin around its periphery in this 
manner serves to maintain the barrier function of the dressing against 
bacteria and other external contaminants as well as helping to insure that 
no wound exudate leaks out of the dressing laterally. 
As heretofore mentioned, sheet 12 is flexible so as to be conformable to 
the contour of the body part to which it is to be applied. It may, for 
example, be as thin as 0.5 mil or as thick as 5.0 mil, but is preferably 
on the order of 1.0 mil thick. Preferably it is an elastomer which is 
characterized as being non-swellable or only slightly swellable. Materials 
useful for preparing slitted sheet 12 are well known in the art and will 
be readily suggested to those skilled in the art in the light of the 
foregoing description. By way of example, useful materials will include 
polyurethane, copolyesters such as "HYTREL", polyvinyl chlorides, 
polyolefins, etc. 
The adhesive materials employed for preparing layer 14 and the peripheral 
coating 34 (FIG. 5) may likewise be any of the known so-called medical 
grade or hypoallergenic adhesives heretofore employed in securing 
dressings to the skin. Such known adhesives include the rubber-based, 
acrylic, vinyl ether and hydrocolloid pressure-sensitive adhesives. The 
adhesive may be applied to provide a layer of at least 1 mil thick, but 
preferably layers of adhesive at least 5 mils thick, e.g. on the order of 
5-10 mils, are contemplated. 
As was heretofore alluded to, the present invention is predicated upon the 
initial discovery that an adhesive layer is essential to provide to 
requisite pressure build-up to open the slits in a slitted dressing. In 
accordance with this invention, the adhesive-free areas containing the 
slits for transfer of wound exudate are isolated by adhesive coating 
forming a dam or barrier inhibiting lateral diffusion of the exudate so 
that it is instead directed upwardly through the slits provided in the 
dressing sheet. For optimum effectiveness, the ratio of surface area of 
the portion of sheet 12 adapted for placement on the wound which contains 
adhesive to the surface area which does not should be at least 1:1, i.e. 
at least 50% of the surface area of that portion should contain adhesive. 
The non-adhesive areas in theory need not be any larger than the slits to 
be provided therein. However, to provide optimum manufacturing tolerance 
for the slitting operation to be sure the slits do not at least in part 
inadvertently overlap into the adhesive area, it has been found that the 
non-adhesive areas should be at least on the order of about 1/8 inch wide 
in the directions of the slit. For example, if the non-adhesive areas are 
circular, they should have a 1/8 inch diameter; and if they are square the 
length and width should be on the order of 1/8 inch. In any case, one 
skilled in the art will understand that the minimal dimensions will depend 
primarily upon the preciseness of the manufacturing equipment to provide 
the slits accurately in the prescribed non-adhesive areas. 
In addition to permitting diffusion of wound exudate away from the wound 
and through the slits, it is essential that the wound dressings of this 
invention provide a barrier to evaporation of water (as distinguished from 
a barrier to removal of exudate) so as to keep the wound surface moist as 
excess exudate is removed. 
The benefits of maintaining a moist wound surface are of course well known 
and include faster reepithelialization, less pain and better cosmetic 
results. 
In order to do so, in accordance with the present invention the size and 
number of slits in the dressing should be such as to provide a dressing of 
the type known in the art as a moist healing wound dressing. In order to 
do so in accordance with this invention the size and number of slits 
should be such as to maintain a moisture vapor permeability or transition 
rate for the dressing of no greater than 1500 grams/meter/24 hours at 
37.degree. C. and 50% relative humidity. 
By way of illustration, successful results have been obtained by providing 
5-81/8" diameter non-adhesive areas per square inch of surface area to 
obtain the preferred moist dressing. 
The slits will optimally extend across the width or diameter of the 
non-adhesive areas. With 1/8" diameter circular areas, excellent results 
have been obtained with cross slits across the diameter of the circle. 
Many other slit designs may also be employed. For example, equal success 
has been obtained with 3/64 inch radius half circle slits in 1/8 inch 
diameter circles as well as S-shaped slits in 1/4 inch diameter areas. As 
shown in the drawings, however, the slits will have dimensions smaller 
than the non-adhesive areas where they are positioned so that each slit 
can be said to occupy only a portion of each non-adhesive area containing 
a slit. 
The vented dressing of this invention may be prepared in the following 
manner: 
(1) apply an adhesive layer of the desired thickness by calendering, 
casting, etc. between two release sheets of differential affinity; 
(2) punch holes of the desired configuration and spacing through the 
thickness of the resulting "sandwich"; 
(3) remove the release sheet of lesser affinity from the adhesive, leaving 
the other release sheet of greater affinity adhering to the opposed 
surface of the adhesive; 
(4) apply the free adhesive surface to the surface of the desired 
elastomeric sheet; 
(5) provide slits by cutting the sheet in areas where there is no adhesive, 
i.e. in those areas where holes had been punched through the adhesive 
"sandwich"; and 
(6) thereafter replace the release sheet having the holes punched through 
it with a new one free from holes, thereby providing a dressing as shown 
in FIG. 3. 
The further steps required to provide dressings such as shown in FIGS. 4 
and 5 will be readily apparent to those skilled in the art and need not be 
discussed in further detail. 
From the foregoing description, it will thus be appreciated that the 
present invention provides a wound dressing of simplified design which 
permits removal of excess wound exudate while at the same time providing 
the desired moist environment promoting proper healing. 
Since certain changes may be made without departing from the scope of the 
invention herein contemplated, it is intended that the foregoing 
description and accompanying drawings be taken as illustrative and not in 
a limiting sense.