Surgical dressing

An adhesive dressing for use on moist wounds which comprises a pressure sensitive adhesive-coated wound contacting first layer in which there are present holes capable of transmitting liquid water and a moisture vapor permeable, continuous film. The continuous film is attached to the first layer in such a way as to form a reservoir into which water can pass through the holes in the first layer and can evaporate from the reservoir through the continuous film. The adhesive coated first layer has a moisture vapor permeability of less than 300 gm.sup.-2 24 hr.sup.-1 at 37.degree. C. and 100% to 10% relative humidity difference and the continuous film has a moisture vapor permeability which is greater when in contact with liquid water than when not in contact with liquid water. Optionally, an intermediate water transmitting layer may be present between the first layer and the continuous film.

The present invention relates to adhesive, moisture vapour permeable 
surgical dressings for use on human bodies, for example in contact with 
moist wounds. In particular, this invention relates to a dressing 
comprising a continuous moisture vapour permeable film attached to the 
non-wound contacting surface of a first layer, said layer adapted to 
transmit water by having holes therein. The first layer carries on its 
wound contacting surface an adhesive layer capable of adhering the 
dressing to the skin. 
It is desirable to allow the wound to heal in its moist state, especially 
if covered with a layer of wound exudate as this state is believed to be 
capable of accelerating wound healing. The problems with moist wound 
healing when the wound is covered with a surgical dressing is that a 
"blister" of exudate can form under the dressing which is not only 
unsightly and uncomfortable but may also cause the dressing to leak, 
thereby defeating the aim of sterility. Such an excess of exudate 
therefore needs to be removed before a harmful blister forms. Normal 
methods of aspiration however, may also lead to wound infection. Finally, 
in order to preserve wound healing in a moist environment, it is desirable 
not to remove all the exudate as a "dry" wound and hence a slower healing 
wound would result. An attempt to overcome these disadvantages is 
described in European Patent Application No. 90564 in which the preferred 
dressings comprise an adhesive coated moisture vapour permeable, liquid 
water impervious first layer which contains perforations as a wound 
contacting layer and an imperforate, moisture vapour permeable, liquid 
water impermeable film attached to the first layer to form a sealed 
reservoir into which exudate from a wound may pass. The water in the 
exudate evaporates through the imperforate film thereby avoiding blister 
formation. Both films are required to have permeability in excess of 300 
gm.sup.-2 24 hr.sup.-1 when measured at 40.degree. C. and 80% relative 
humidity difference. We have found that as a result when no exudate is 
being released from the wound moisture can be lost from the skin and wound 
area at a rate that risks the wound drying out especially around the 
perforations and hence sticking to the dressing. Removal of the dressing 
when this occurs could cause damage to the newly formed tissue. 
Surprisingly it has been found that by using a combination of (i) a first 
layer which is formed from a low moisture vapour permeable sheet material 
but is adapted to transmit liquid water by the presence of holes and (ii) 
a continuous film of a moisture vapour permeable sheet material which has 
greater permeability when in contact with water than when not in contact 
with water, the disadvantages found in earlier dressings may be avoided or 
mitigated. 
The dressings of the present invention therefore mitigate the disadvantages 
of blister formation and wound adherence by providing a surgical dressing 
suitable for use on moist wounds on human bodies which comprises a 
continuous film having a moisture vapour permeability which increases as 
the amount of water with which the film is in contact increases and 
decreases as the amount of water decreases and a wound contacting first 
layer which is adapted to transmit water by the presence of holes which 
layer when uninterrupted has a low moisture vapour permeability this layer 
being attached to the continuous film so as to form a sealed portion or 
reservoir into which the wound exudate may pass. 
Accordingly the present invention provides an adhesive dressing for use on 
moist wounds which dressing comprises a pressure sensitive adhesive-coated 
first layer which has holes therethrough capable of transmitting liquid 
water and a moisture vapour permeable continuous film attached to the 
first layer thereby forming a reservoir into which water can pass and 
evaporate therefrom which dressing is characterised in that the adhesive 
coated first layer has an uninterrupted moisture vapour permeability of 
less than 300 gm.sup.-2 24 hr.sup.-1 and the moisture vapour permeable 
continuous film has a moisture vapour permeability which is greater when 
in contact with liquid water than when not in contact with liquid water. 
It will be appreciated that by employing a low moisture vapour permeable 
adhesive coated first layer, the dressings of this invention are radically 
different from those in the art which all attempt to employ highly 
moisture vapour permeable components. 
By low moisture vapour permeability is meant a moisture vapour vapour 
permeability of less than 300 gm.sup.-2 24 hr.sup.-1 when measured at 
37.degree. C. and 100% to 10% relative humidity difference. By an 
"uninterrupted moisture vapour permeability" is meant the moisture vapour 
permeability of the material in the absence of holes therethrough capable 
of transmitting liquid water, i.e. the moisture vapour permeability of the 
uninterrupted material. 
The dressings according to the invention have the advantage of allowing 
water to evaporate rapidly from the wound area in the presence of an 
excess of exudate in the environs of the wound but, as the amount of 
exudate diminishes, so does the rate of evaporation. The use of a low 
moisture vapour permeable layer in contact with the wound means that when 
non-exuding or not rapidly exuding the amount of exudate around the wound 
is enough to keep 10,000 moist without causing 24 of the dressing and will 
not cause the wound to dry out which may result in adherence of the 
dressing to the 4800 24 
The continuous film suitably has the following moisture vapour permeability 
(MVP) characteristics which are determinable by the Payne Cup Method (as 
described hereinafter): a moisture vapour permeability when in contact 
with water, that is when the Payne Cup is inverted, of not less than 8000 
gm.sup.-2 24 hr.sup.-1 measured at 37.degree. C. and 100% to 10% relative 
humidity difference, and is preferably greater than 10,000gm.sup.-2 
24hr.sup.-1 and a moisture vapour permeability when not in contact with 
water which is suitably not more than 4800gm.sup.-2 24hr.sup.-1 measured 
at 37.degree. C. and 100% to 10% relative humidity difference and is 
preferably less than 4000 gm.sup.-2 24 hr.sup.-1 and more preferably less 
than 3600 gm.sup.-2 24 hr.sup.-1. 
When the continuous film is attached to the first layer to form the 
dressing, the moisture vapour permeability of the dressing, as measured 
for example by means of an evaporimeter placed above the surface of the 
continuous film, when in contact with water or wound exudate will reflect 
the high value of the continuous film as liquid will pass through the 
openings in the first layer into the sealed portion and will be lost by 
evaporation through the continuous film. However, when the wound no longer 
produces exudate and the continuous film is not in contact with water the 
evaporation of moisture vapour will reflect the lower permeability of the 
combination of first layer and continuous film. The use of a low moisture 
vapour permeable adhesive coated first layer means the wound will not dry 
out and the advantageous moist conditions required for wound healing are 
not lost. 
Polymer materials which are suitable for use as continuous films by 
possessing the desired enhancement of "wet" moisture vapour permeability 
compared to "dry" moisture vapour permeability, are those containing 
chemical groups generally considered to be hydrophilic. Such groups 
include hydroxyl, ether, ester, carboxyl, amine, amide and carbonyl 
groups. Thus suitable materials include hydrophilic polyurethanes, 
cellulose derivatives, polyether-polyamides, polyamides, crosslinked 
polyvinyl alcohols and the like. 
It has been found that polyether polyurethanes are particularly suitable 
for use in the formation of such films. Favoured polyether polyurethanes 
are essentially free of reactive substituents such as hydroxy or carboxy 
groups. Such polyurethanes for use in this invention include random 
polymers containing units derived from diolic compounds and 
di-isocyanates. 
The ether units in such hydrophilic polyurethanes for use in this invention 
may be notionally derivable from ethylene diol and a propylene or butylene 
diol; that is they will contain CH.sub.2 CH.sub.2 O-- units and --CH.sub.2 
CH.sub.2 CH.sub.2 O--, --CH.sub.2 CH(CH.sub.3)O-- or --CH.sub.2 CH.sub.2 
CH.sub.2 CH.sub.2 O-- units. Preferably, the ether units in the 
polyurethane will contain --CH.sub.2 CH.sub.2 O and --CH.sub.2 
CH(CH.sub.3)O-- or (CH.sub.2).sub.4 O-- or mixtures thereof of which 
poly-- CH.sub.2 CH(CH.sub.3)O-- blocks are preferred. Desirably, the mole 
ratio of poly(ethylene glycol) to poly [(prop or but)ylene glycol)]-- 
derivable blocks present in the hydrophilic polyurethanes varies between 
1:1 to 1:30; preferably from 1:2 to 1:10; and more preferably from 1:2.5 
to 1:4. The molecular weights of these blocks is suitably from 600 to 6000 
and favourably from 900 to 4000, for example 1000 to 2000. 
Preferably, such hydrophilic polyurethanes for use in this invention will 
contain residues of aliphatic diols of up to 10 carbon atoms and more 
preferably up to 4 carbon atoms (of which ethane diol is preferred) as 
chain extenders wherein the mole ratio of diol to polyglycol used in the 
preparation of the polymer is from 3:1 to 1:4; preferably, 5:2 to 1:3; and 
more preferably from 2:1 to 1:2. 
The hydrophilic polyurethane should contain sufficient di-isocyanate 
residues to produce the water contents set forth above when the film is 
hydrated. 
The hydrophilic polyurethane for use in this invention may contain 
di-isocyanate residues which may be residues of aromatic or aliphatic 
di-isocyanates such as 4,4'-diphenylmethane di-isocyanate, toluene 
di-isocyanate, 1,6-hexamethylene di-isocyanate or the like. Favoured 
di-isocyanates for use in the hydrophylic polyurethane of this invention 
are 4,4'-dicyclohexylmethane di-isocyanate (which is preferred) and 
4,4'-diphenylmethyl di-isocyanate. 
Less preferably than using aliphatic diol chain extenders, the hydrophilic 
polyurethane may employ equivalent quantities of aliphatic diamine or 
aliphatic aminol chain extenders of which ethylene diamine is preferred. 
Similarly somewhat less preferably than using aliphatic diol chain 
extenders, the hydrophilic polyurethane may employ an aromatic diamine 
such as phenylenediamine, benzidine or diaminodiphenylmethane. 
Less preferably than using a mixture of poly(ethylene glycol) and poly 
[(prop or but)ylene glycol)] derived blocks, the hydrophilic polyurethane 
may employ poly(ethylene glycol) derived blocks alone together with a 
higher proportion of chain extender and di-isocyanate. 
More preferably, the hydrophilic polyurethane used in a dressing of this 
invention is essentially a single type of polymer (a product of the 
poymerisation of the same materials) although blends may be employed to 
form the hydrophilic polyurethane if desired. 
Further favoured materials are polyether-polyamide block copolymers whose 
preparation and properties have for example been described in British 
Patent No. 1473972, French Patents Nos. 1444437 and 2178205 and U.S. Pat. 
No. 3,839,243. A particularly apt polyether-polyamide block copolymer is 
known as Pebax 4011 RN 00 (available from ATO Chemical Products (UK) 
Ltd.). This polymer has a water content of 55% (approx) when hydrated and 
a `wet-MVP` of &gt;14000 gm.sup.-2 24 hr.sup.-1 and a `dry-MVP` of 4600 
gm.sup.-2 24 hr.sup.1 for a 70 micron thickness of film at 37.degree. C. 
and a relative humidity of 100-10%. 
A further suitable, though less preferred material is a plasticized 
regenerated cellulose film such as cellulose acetate. A suitable film is 
Rayophane 325P (available from British Sidac Ltd.). This film has a 
`wet-MVP` of &gt;14000 gm.sup.-2 24 hr.sup.-1 and a `dry-MVP` of 4100 
gm.sup.-2 24 hr.sup.-1 for a 30 micron film when measured at 37.degree. C. 
and 100-10% relative humidity. 
A further suitable, though less preferred material is a polyvinyl alcohol 
which has been cross-linked, usually by means of heat, to form an 
insoluble but water absorbent film. A suitable polyvinyl alcohol is 
available as Polyviol W28/20 (Trade Mark, available from Wacker-Chemie 
GmbH). This polymer may be cast into a film from aqueous solution, dried 
and cross-linked using heat for example by autoclaving. This film has a 
`wet-MVP` of &gt;13,000 gm.sup.- 2 24 hr.sup.-1 and a `dry-MVP` of 4800 
(approx) gm.sup.-2 24 hr.sup.-1 for a 37.5 micron film when measured at 
37.degree. C. and 100% to 10% relative humidity difference. 
Most suitably the continuous film will be from 15 to 80 .mu.m thick, will 
more usually be from 20 to 60 .mu.m thick and will preferably be from 22 
to 50 .mu.m thick, for example 25, 30, 35 or 40 .mu.m thick. 
Suitably also the continuous film will be formed from a hydrophilic polymer 
which when hydrated contains up to 90% by weight of water, favourably 
contains 5 to 50% water, more favourably from 10 to 40% of water and 
preferably from 20 to 30% by weight of water, for example 25% by weight of 
water. 
A preferred hydrophilic polymer is a hydrophilic polyurethane which when 
hydrated contains from 10 to 40% by weight of water. 
Suitable hydrophilic polyurethanes are those which are described in 
European Patent Application No. 59035 which is incorporated herein by 
cross reference. A preferred hydrophilic polyurethane is therefore a 
linear polyether polyurethane which when hydrated contains from 20 to 30% 
by weight of water. 
The dressing of which the continuous film forms part should conform readily 
to the body area to which it is applied and should also be elastic. Such 
dressings have the advantage of adhering securely to the body by following 
the body contours and allowing the body to move without dislodging the 
dressing. Synthetic polymers are more conformable and elastic than natural 
polymers which tend to be stiff, inelastic and generally nonconformable. 
Synthetic polymers are therefore preferred for the continuous film. 
It is further preferred that the continuous film should be transparent so 
that the wound may be observed. The continuous film is self supporting, 
that it is coherent when wet or dry and can be used without recourse to 
additional support such as fabric, net or the like. 
In the dressings of the present invention the adhesive coated first layer 
when not possessing holes will have a low moisture vapour permeability, 
that is a moisture vapour permeability of less than 300 gm.sup.-2 24 
hr.sup.-1. Such dressings have been found to provide better conditions for 
healing of wounds and may be retained in place for extended periods in use 
which reduces the risk of infection or retraumatisation of the wound 
whilst it is healing. Clearly this low moisture vapour permeability for 
the adhesive coated first layer may be achieved by either coating a first 
layer of high moisture vapour permeability with a continuous layer of 
adhesive of low moisture vapour permeability or by coating a first layer 
of low moisture vapour permeability with an adhesive which may be in the 
form of a continuous, microporous or pattern spread coating which may have 
a high or low moisture vapour permeability. The relationship between the 
moist vapour permeabilities of an adhesive coated layer and its component 
layers being described hereinafter. Thus the skilled man would appreciate 
which adhesives and which polyme films could be combined to provide a 
first layer of the correct moisture vapour permeability. 
Material suitable for use as the first layer for use in dressings of this 
invention include elastic or non-elastic, conformable, natural or 
synthetic polymers. The sheet material of the first layer is adapted to 
allow transmission of water through the film, that is a water transmitting 
film by being perforated by means of holes. When used herein holes mean 
any shaped hole which is usually visible to the naked eye which passes 
through the film and its adhesive coat. Such holes include slits. 
Materials which are suitable for forming the first layer and which are 
permeable to moisture vapour include those which are described in our 
copending European Patent Application No. 107915 at page 15 lines 5 to 23 
and page 16 lines 6 to 16. A favoured first layer is polyurethane. The 
adhesive for use with such materials will give a low moisture vapour 
permeability to the adhesive coated material. Such adhesives suitable for 
use on skin are known in the art and include for example natural or 
synthetic rubber based adhesives. 
It is presently preferred to use for the first layer materials which 
themselves have a low uninterrupted moisture vapour permeability. 
Accordingly the present invention provides a moisture vapour permeable, 
adhesive surgical dressing comprising a conformable, liquid water 
impermeable, wound contacting first layer which is adapted to transmit 
liquid water by means of holes and which first layer has on its wound 
contacting surface a pressure sensitive adhesive layer suitable for 
adhering the dressing to the skin and a continuous, conformable, moisture 
vapour permeable film attached to the first layer forming a reservoir into 
which wound exudate passes and can evaporate therefrom is characterised in 
that the first layer has a low moisture vapour permeability and the 
continuous film has a moisture vapour permeability which is greater when 
in contact with water than when not in contact with water. 
Materials which are suitable for forming the first layer and which have low 
moisture vapour permeability include polyolefin films, such as 
polyethylene, polybutadiene, polyolefin copolymers such as ethylene-vinyl 
acetate copolymers, polyisobutylene e.g. Oppanol (Trade Mark of BASF), 
styrene butadiene styrene block polymers e.g. Kratons (Trade Mark of Shell 
Chemicals Ltd.), polyesters and the like. A first preferred film is a 
polybutadiene. A more preferred film is made from a 
styrene-butadiene-styrene triblock copolymer. 
Suitably the first layer will have a thickness of up to 150 .mu.m, more 
suitably will be from 15 to 100 .mu.m thick, most suitably will be 20 to 
75 .mu.m thick and preferably 25 to 40 .mu.m thick, for example 25 .mu.m, 
30 .mu.m, 35 .mu.m or 40 .mu.m. 
Suitably a continuous sheet of the material which is adapted to form the 
first layer will have a moisture vapour permeability of less than 300 
gm.sup.-2 24 hr.sup.-1 and more suitably between 20 and 280 gm.sup.-2 24 
hr.sup.-1 and most suitably less than 100 gm.sup.-2 24 hr.sup.-1 when 
measured at 37.degree. C. and 100% to 10% relative humidity. For example 
some preferred films described hereinafter will have a moisture vapour 
permeability of between 25 and 90 gm.sup.-2 24 hr.sup.-1 and preferably 
between 40 and 80 gm.sup.-2 24 hr.sup.-1 A second group of preferred films 
such as the styrene-butadiene copolymers will have an uninterrupted 
moisture vapour permeability of 200 to 260 gm.sup.-2 24 hr.sup.-1. 
Suitably the first layer will be interrupted by means of perforations, for 
example circular holes. Such perforations will be capable of allowing the 
passage of liquid water and normally will be visible to the naked eye and 
may measure 0.1 to 2.5 mm, for example 1.5 mm in diameter. Usually the 
holes will be spaced 0.5 to 2.5 cm from each other and may be arranged in 
parallel rows or in staggered rows. 
Alternatively and preferably the first layer will be interrupted by means 
of slits. The slits may be from 0.3 to 1.5 cm in length and more suitably 
0.35 to 1.0 cm in length for example 0.4 cm, 0.5 cm and 0.6 cm and be 
spaced from 0.2 to 2.5 cm from each other and preferably 0.3 to 1.5 cm 
from each other. The slits may be straight, arcuate or in the form of two 
slits at right angles. It is preferred that the slits are arranged all in 
straight lines in parallel rows, for example 1 cm apart and with 0.2 cm 
between adjacent slits in any one row, in which case the slits are 
suitably 0.4 cm long. 
The continuous film is attached to the non-wound contacting side of the 
first layer so as to form a sealed portion or reservoir which is capable 
of holding wound exudate when in use. The attachment means includes heat 
sealing, ultrasonic welding, radio frequency welding or by means of an 
adhesive or adhesive tape depending on the nature of the polymers 
involved. It is preferred that the films are heat sealed together. The 
seal may be in the form of a border around the periphery of the reservoir 
portion but may also include seal lines within the border, for example to 
produce a quilted effect. 
In one embodiment therefore, the continuous film and first layer will be 
coextensive and sealed together at least around their edges. In a second 
embodiment the first layer will extend beyond the periphery of the 
continuous film. Most aptly the first layer will extend beyond the 
periphery on two opposite edges. The seal may therefore be formed around 
the edge of the continuous film so leaving a margin of the first layer 
which contains holes or may be arranged to be unperforated. In a third 
embodiment, though less preferred, the continuous film may extend beyond 
the periphery of the first layer in which case the first layer mey be 
sealed around its periphery to the continuous film. 
The dressings of the invention in which the continuous film and first layer 
are coextensive will aptly have dimensions of from 8 cm.times.8 cm to 40 
cm.times.40 cm for example 8 cm.times.8 cm, 8 cm.times.12 cm, 10 
cm.times.10 cm, 20 cm.times.15 cm, 20 cm.times.30 cm, 40 cm.times.30 cm 
and 40 cm.times.40 cm. It is clear that the size of dressing will be 
chosen depending upon the size of the wound upon which it is to be used 
for example the sizes 8 cm.times.8 cm and 8 cm.times.12 cm will be used on 
small wounds while the larger sizes are suitable for donor sites. 
In those dressings of the invention in which there is a margin of either 
the continuous film or the first layer extending beyond the sealed portion 
this margin will be from 1 to 6 cm wide and preferably will be from 2 to 4 
cm wide. Smaller margins are usually found on smaller dressings, for 
example 2 cm while the larger margin is found on the large dressings for 
example 4 cm. This margin may be present in all four edges of the dressing 
but preferably is present only on two opposite sides. 
The adhesive employed in the dressings of this invention must be compatible 
with the wound, that is it must not adhere to it. Suitable adhesives 
include synthetic polymers or mixtures thereof. Such adhesives may be 
selected from those described in British Patent Specification No. 
1,280,631 and European Patent Application No. 35399. Suitable adhesives 
are formed from acrylate ester copolymers or polyvinyl ethyl ethers. If 
desired such adhesives may incorporate an antibacterial agent. 
A preferred pressure sensitive adhesive comprises a blend of high and low 
viscosity polyvinyl ethyl ethers in particular adhesive composition A 
disclosed in British Patent Specification No. 1280631. Other preferred 
pressure sensitive adhesives comprise copolymers of acrylate ester with 
acrylic acid for example as disclosed in European Patent Application No. 
35399 and in particular a copolymer of 47 parts by weight of butyl 
acrylate, 47 parts by weight of 2-ethylhexyl acrylate and 6 parts by 
weight of acrylic acid with an intrinsic viscosity of at least 1.9 dl/g 
polymerised in acetone according to the general method given in the above 
European Application. 
Suitably the adhesive is employed at a mass weight per unit area of 20 to 
80 gm.sup.-2, more suitably at 20 to 45 gm.sup.-2 and preferably at 25 to 
35 gm.sup.-2, for example 29 gm.sup.-2 or 32 gm.sup.-2. 
Suitably the adhesive layer is applied to the film of the first layer as a 
continuous layer prior to making the holes in the film, so that normally 
the adhesive layer will be interrupted during interruption of the film. 
Such adhesives which are applied continuously will have a moisture vapour 
permeability which is greater than 300 gm.sup.-2 24 hr.sup.-l and more 
preferably greater than 500 gm.sup.-2 24 hr.sup.-1 when measured at 
37.degree. C. and 100% to 10% relative humidity. 
Alternatively the adhesive layer may be in the form of a pattern spread or 
discontinuous spread adhesive layer using a conventional surgical adhesive 
prepared and spread by the method described in for example British Patent 
No. 819635. The adhesive may also be in the form of a porous or 
microporous layer. 
Similarly if the continuous film forming the moisture vapour permeable 
layer extends beyond the periphery of the first layer, this may carry an 
adhesive on the exposed margin, that is on the body contacting surface. 
The adhesive may be continuous or pattern spread or porous and formed from 
the polymers as hereinbefore described as being suitable for adhesion to 
the skin. 
In a further aspect of the present invention an intermediate layer is 
provided between the non-wound contacting surface of the first layer and 
the continuous layer. The intermediate layer will be water transmitting so 
as not to prevent the passage of water from the wound to the continuous 
film. The presence of the intermediate layer may in certain cases aid the 
manufacture of the dressing by preventing unwanted adherency of the 
continuous film to the first layer during, for example, the sterilisation 
process. The presence of the intermediate layer has further advantages in 
that it improves the handleability of the dressing and further slows down 
the rate of the evaporation of the moisture vapour from the surface of the 
continuous film which reduces the risk that the wound might dry out and 
stick to the dressing particularly around the holes in the first layer. 
The intermediate layer also appears to encourage flow of exudate through 
the holes by means of a wicking action and removes any risk of the first 
layer and continuous film adhering to each other in use which may prevent 
operation of the dressing. The intermediate layer may also carry a 
medicament which is released to the wound area in use. Suitably the 
medicament will be an antimicrobial agent, for example chlorhexidine or 
its salts or povidone iodine. 
Materials suitable for forming the intermediate layer include woven and 
non-woven fabrics, nets, perforated films, hydrogels or hydrophilic 
polymers and the like which are water permeable. Aptly the intermediate 
layer is a non-woven fabric or a perforated film or an integral net. 
Preferably the layer is a non-woven fabric. Generally suitable non-woven 
fabrics will be formed from hydrophobic polymers such as polyolefins. 
Preferred non-woven fabrics include a spun bonded polypropylene fabric 
known as Novelin (Trade Mark, available from J. W. Suominen). In the 
manufacture of the dressings a piece of the non-woven fabric may be placed 
over the perforated area of the first layer, the continuous film placed on 
top of the non-woven fabric and all three layers sealed together around 
their edges or the continuous film may be simply only sealed to the first 
layer, thereby trapping the intermediate layer between the two. 
Films, which when perforated, are suitable for use as an intermediate layer 
include polyolefin films and polyester films such as Melinex (Trade Mark, 
available from I.C.I. plc). Aptly these intermediate layers are perforated 
in a similar manner to the first layer as described hereinbefore, that is 
they are perforated with holes or slits. Surprisingly it has been found 
that it is advantageous to maintaining the moistness of the wound and to 
the progress of wound healing if the holes or slits in the first layer are 
not in register with the holes or slits in the intermediate layer. 
Aptly when an intermediate layer is present, this layer is also transparent 
so that the progress of wound healing may be observed. However, in the 
case where the layer is a non-woven fabric the fabric may not be 
transparent, so the centre of this layer may be removed prior to sealing 
between the continuous film and the first layer so that the wound may 
still be observed. 
When the intermediate layer of water transmitting material is in the form 
of a polymeric film then it will have a thickness similar to that used for 
the interrupted film that is up to 150 microns. If the intermediate layer 
is a non-woven or woven fabric then the layer tends to be thicker. 
Thus in a further aspect the present invention provides an adhesive 
dressing suitable for use on moist wounds which dressing comprises a 
pressure sensitive adhesive-coated first layer which has holes therethrogh 
capable of transmitting liquid water and a moisture vapour permeable 
continuous film attached to the first layer thereby forming a reservoir 
into which water can pass and evaporate therefrom which dressing is 
characterised in that the adhesive coated first layer has a moisture 
vapour permeability of less than 300 gm.sup.-2 24 hr.sup.-1 and the 
moisture vapour permeable continuous film has a moisture vapour 
permeability which is greater when in contact with liquid water than when 
not in contact with liquid water and that there is a further water 
transmitting intermediate layer present between the first layer and the 
continuous film. 
The materials which may comprise the continuous film and first layer are as 
hereinbefore described. 
Optionally the first layer may also incorporate or may have attached to its 
surface remote from the continuous film a water-absorbing material such as 
a hydrogel such as Spenco (Trade Mark) or a hydrophilic foam such as Hypol 
(Trade Mark) foam. The presence of such a material does not interfere with 
the escape of excess water but provides a reservoir of exudate which 
remains. 
When first put over the wound, the dressing is dry so the holes are 
relatively insignificant as the major area of the wound is covered by the 
first layer and the holes are themselves covered by the continuous (e.g. 
hydrophilic polyurethane) film. As the amount of wound exudate increases, 
tending to blister-formation, the exudate seeps through the holes, 
hydrating the hydrophilic polyurethane, the MVP of which increases so the 
water evaporates. Once the "blister" has subsided, the MVP of the 
continuous film decreases, but the wound still remains moist because most 
of its area is covered by the less permeable first layer. 
Therefore in a further aspect the present invention provides a method of 
dressing an exuding wound on an animal body comprising placing over the 
wound a dressing of the present invention and adhering the dressing to the 
body and allowing the wound exudate to pass through the holes of the first 
layer into a reservoir and allowing moisture vapour from said exudate to 
escape by transmission through the continuous film. 
In a preferred aspect the present invention provides a method of dressing a 
donor site on an animal body which comprises placing over the donor site a 
dressing of the present invention and adhering the dressing to the body 
and leaving in position for a period of from 3 to 20 days. 
Dressings of the present invention are suitable for use on wounds including 
donor sites (split thickness), partial thickness burns, pressure sores and 
leg ulcers, which wounds exude large quantities of liquid and heal by 
reepithelialisation. These wounds will normally be dressed for a period of 
from 3 to 20 days and preferably for 7 to 14 days, when 
reepithelialisation of the wound could be complete. 
Thus in a particularly preferred aspect the present invention provides a 
method of dressing a donor site on an animal body which comprises placing 
over the donor site a dressing of the present invention and adhering the 
dressing to the body and leaving in position for a period of from 7 to 14 
days. 
In a preferred method the dressing will have an intermediate layer between 
the adhesive coated first layer and the continuous film. 
The dressings used in the method of dressing a exuding wound may be any of 
those hereinbefore described. 
The method of dressing a wound using a dressing of the present invention 
may be applied to all exuding wounds such as burns, skin graft donor 
sites,.pressure sores, ulcers, surgical wounds and the like. It is an 
advantage of the dressing of the present invention that they may be left 
in place on the wound for extended periods of time with reduced risk of 
leaking, causing blister formation or adhering to the wound. The time over 
which a dressing may be left in place will vary depending on the type of 
wound and the form of any other treatment which may be given to the wound. 
A dressing may be left covering a skin graft donor site for up to 14 days 
without detrimental effect and can yield a healed wound at the end of that 
time. 
Preferably, the dressing according to this invention is provided in 
sterilised form and, when self-adhesive, is adhered to a removable sterile 
backing sheet. Suitable removable backing sheets or release liners are 
those which are conventionally used in the art, that is release liners 
which are made of, or coated with polyethylene, polypropylene and 
fluorocarbons and silicone-coated release papers or polyester films. A 
preferred release liner is formed from silicone-coated release paper. 
Prior to use, the release liner is stripped from the adhesive coating of 
the dressing so that the dressing may then be applied to the skin. The 
dressing may be packaged in a bacteria-proof package such as a paper, 
plastic or aluminium foil pouch and sterilisation may be achieved in 
conventional manner, e.g. by use of gamma irradiation, heat or ethylene 
oxide. 
Suitable forms of dressing and removable backing sheet(s) include ones 
similar to those described in European Patent Specification No. 51-935. 
Suitable hydrophilic polyurethanes for use include those described in 
European Patent Specification No. 50035. 
In a preferred form of a dressing of the present invention the adhesive 
coated first layer extends beyond the edges of the continuous film on two 
opposite sides. The extended margins of the first layer may then be 
adhered to a non-adhesive polymer film, such as a polyester film, to form 
non-adhesive handles to aid the manipulation of the dressing when 
positioning on the body. 
The polymer which is to form the non-adhesive layer or backing layer of the 
first layer may be extruded or cast (from a solution) onto a silicone 
release paper to give a film of the required thickness and weight when the 
solvent is removed. An adhesive film may be similarly cast on to a release 
paper. The backing layer may be transferred to the adhesive film by 
conventional transfer coating means. Alternatively the first layer may be 
cast onto one surface of a double sided release paper and the adhesive 
layer cast onto the first layer. The combined layers are then transferred 
to the other surface of the release paper so that the adhesive layer is 
covered by the release paper. The handles may be advantageously inserted 
during this transfer process. The combined films may then be formed into 
the first layer by punching holes of the appropriate diameter through the 
backing and adhesive layers and release paper. Alternatively slits of the 
appropriate length and shape may be cut through the backing layer and 
adhesive layer and release paper using a sharp blade or an array of such 
blades which give the correct pattern. 
The continuous film may be cast from a solution of the appropriate 
hydrophilic polymer at the required thickness and weight. This film may be 
heat sealed or adhered around its edges to the non-adhesive side of the 
first layer. In other instances the continuous film may be formed by 
extrusion of the appropriate polymer to give a film of the required 
thickness. The continuous film may be sealed using a heated box-section so 
that the holes of the first layer fall within the sealed square so formed. 
Other dressings according to this invention may be prepared by methods 
known to those skilled in the a t. 
The dressings may be formed to any appropriate size using the general 
preparative method described above for example dressings of smaller size 8 
cm.times.12 cm may be used on small wounds while larger sizes, 30 
cm.times.30 cm for 40 cm.times.30 cm may be used on large wounds or donor 
sites.

FIGS. 1 and 2 show a dressing (1) comprising a continuous film (2), first 
layer (3), with an adhesive layer (4) on the wound contacting side and a 
release paper (5) which is removed from the adhesive layer (4) prior to 
use. The continuous film (2) is attached to the non-wound contacting side 
of the first layer (3) by means of a heat seal (6) which extends around 
the periphery of the dressing. The result of this seal line (6) is that a 
reservoir portion (7) is formed between the first layer (3) and the 
continuous film (2) into which wound exudate may flow via the perforations 
(8) which are shown (n this illustration as circular holes. The 
perforations (8) extend through the three layers (3), (4) and (5), for 
ease of manufacture. The dressings are packaged in a bacteria proof pouch 
prior to sterilisation so that the presence of perforations through the 
three layers is not detrimental to overall sterility prior to use. The 
heat seal (6) preferably is arranged to avoid crossing over any 
perforations and hence avoids the risk of bacteria entering the reservoir 
portion. The adhesive layer (4) is also continuous under the seal thereby 
preventing access to bacteria. 
FIG. 3 and 4 show a dressing (11) comprising a continuous film (12), first 
layer (13) with an adhesive layer (14) on the wound contacting side and a 
release paper (15). The continuous film (12) is attached to the first 
layer (13) by means of a heat seal (16). The first layer (13) extends 
beyond the periphery of the continuous film (12) to form an adhesive 
coated margin (19). The first layer (13) in this margin is perforated by 
means of circular holes (18) which help it prevent the underlying skin 
becoming macerated when the dressing is in place. The reservoir (17) is 
formed in a similar manner to that of earlier described dressings. 
FIGS. 5 and 6 show a dressing (21) comprising a continuous film (22) which 
extends beyond the periphery of the first layer (23). The adhesive layer 
(24) is present on both the first layer (23) and on the margin area (28) 
of the continuous film (22). Since the continuous film has a high moisture 
vapour permeability, the margin areas (28) will comprise a continuous film 
and continuous adhesive layer. The continuous film is heat sealed (25) to 
the first layer as previously. 
FIG. 7 shows a dressing of the same type as that of FIG. 1 except that a 
water transmitting non-woven fabric (10) is present between the continuous 
film (2) and the first layer (3). 
In this embodiment the non-woven fabric is sealed into the dressing. The 
non-woven fabric effectively divides the reservoir (7) into two parts but 
does not prevent the free flow of exudate from the wound. 
FIG. 8 shows a dressing similar to that shown in FIG. 7 except that the 
intermediate water transmitting layer (10) is a perforated film. The 
perforations in the film are circular holes and are arranged so as to be 
out of alignment with those (7) of the first layer. 
FIG. 9 shows a dressing similar to that shown in FIG. 1 except that the 
first layer (3) is interrupted by slits (7) instead of circular holes. 
When the dressing is applied under slight tension the slits tend to open 
slightly to aid the flow of wound exudate into the reservoir. 
The following Examples are provided by way of illustration of this 
invention. The hydrophilic polyurethanes may be prepared as described in 
European Patent No 50035. 
Example 1 
Preparation of Adhesive Surgical Dressing 
A film of syndiotactic 1,2-polybutadiene (JSR RB830 available from Japanese 
Synthetic Rubber Company) was formed by extrusion. The film had a 
thickness of 150 .mu.m. A piece of this film was taken and its moisture 
vapour permeability measured using the Payne Cup Method. The film was 
found to have a moisture vapour permeability of 79 gm.sup.-2 24 hr.sup.-1 
at 37.degree. C. when measured at a relative humidity difference of 100% 
to 10%. 
An acrylic ester copolymer formed by copolymerisation of 47 parts n-butyl 
acrylate, 47 parts 2-ethylhexyl acrylate and 6 parts acrylic acid in 
acetone solution was cast from acetone solution at 35% solids, onto a 
siliconised release paper. The solvent was removed to give a layer of 
polymer at a weight of 30 gsm which had adhesive properties. 
The adhesive layer was transfer coated onto the polybutadiene film to form 
a laminate. A piece of this laminate was taken and after removal of the 
release paper its moisture vapour permeability measured using the Payne 
Cup Method. The laminate was found to have a moisture vapour permeability 
of 52 gm.sup.-2 24 hr.sup.-1 at 37.degree. C. when measured at a relative 
humidity difference of 100% to 10%. A further piece of laminate on its 
release paper, 8 cm.times.12 cm, was taken and perforated with holes 
approximately 1 cm apart, such a size of dressing would contain 
approximately 54 holes. 
A 15% solution of a hydrophilic polyurethane, having a potential water 
content of 25% when hydrated, in industrial methylated spirits was mixed 
with 5% fine silica (Gasil 23, Crossfield Chemical Ltd.) and was cast onto 
silicone release paper to give a film of weight 40 gsm. A piece of this 
film was taken and its moisture vapour permeability measured using the 
Payne Cup Method. The film was found to have a moisture vapour 
permeability of 2200 gm.sup.-2 24 hr.sup.-1 when not in contact with water 
and of more than 13,000 gm.sup.-2 24 hr.sup.-1 when in contact with water 
(when measured at 37.degree. C. and 100% to 10% relative humidity 
difference). A piece of the film was cut to the same size as the laminate 
on its release paper, namely 8cm.times.12cm. 
A piece of spun-bonded polypropylene, 8 cm.times.12 cm, was placed on the 
non-adhesive side of the polyisobutadiene film and the hydrophilic 
polyurethane film placed on top of this. The films were then sealed 
together around their edges using an impulse heat sealer to give the 
surgical dressing of the present invention. 
In use the release paper is removed from the adhesive coated face of the 
perforated polybutadiene film and the dressing adhered around the wound. 
The wound exudate is free to pass through the perforations. Water vapour 
will subsequently evaporate from the surface of the continuous film 
thereby avoiding forming a blister of wound exudate beneath the dressing. 
The dressing may be sealed into a bacteria proof pack and sterilised by 
conventional means prior to use. 
Example 2 
Preparation of Adhesive Surgical Dressing 
A similar dressing to that described in Example 1 was prepared except that 
instead of perforating the adhesive-coated polybutadiene film with holes, 
the perforations took the form of slits approximately 5mm long, spaced 1.4 
cm apart. 
Example 3 
Preparation of Adhesive Surgical Dressing 
A dressing similar to that described in Example 1 was prepared except that 
in place of a film of polybutadiene a film of polyethylene 22 .mu.m in 
thickness was used. The polyethylene film had a moisture vapour 
permeability of 78 gm.sup.-2 24 hr.sup.-1 when measured using the Payne 
Cup Method at 37.degree. C. and a moisture vapour permeability of 32 
gm.sup.-1 24 hr.sup.-1 when coated with adhesive. when coated with 
adhesive. 
Example 4 
Preparation of Adhesive Surgical Dressing 
A dressing similar to that described in Example 1 was prepared except that 
in place of a film of polybutadiene a film of a styrene-butadiene-styrene 
triblock copolymer (Kraton 1101) 30 .mu.m in thickness was used. The 
Kraton 1101 film had a moisture vapour permeability of 260 gm.sup.-2 24 
hr.sup.-1 when measured using the Payne Cup Method at 37.degree. C. and 
100% to 10% relative humidity difference. 
Example 5 
Preparation of Adhesive Surgical Dressing 
A dressing similar to that described in Example 1 was prepared except that 
in place of a film of polybutadiene a film of polyisobutylene 60 .mu.m in 
thickness was used. 
Example 6 
Preparation of Adhesive Surgical Dressing 
A dressing using the same materials as those described in Example 1 was 
prepared except that the dimensions chosen for the laminate of release 
paper, adhesive layer and polybutadiene was 12 cm.times.12 cm. The 
laminate was perforated over its entire surface as before. The dimensions 
of the spun bonded polypropylene and hydrophilic polyurethane film were 
however restricted to 8 cm.times.8 cm. The films and laminate were sealed 
together using an impulse heat sealer around the edges of the 
polypropylene non-woven and polyurethane. Care was taken to avoid sealing 
over a perforation as this might result in an incomplete seal around the 
polyurethane film which would allow exudate to flow out of the dressing 
and consequently could allow bacteria in. The resultant dressing therefore 
had a sealed central portion 8cm.times.8 cm and a 2 cm margin of adhesive 
coated perforated film. 
Example 7 
Preparation of Adhesive Surgical Dressing 
An acrylic ester copolymer formed by the copolymerisation of 47 parts 
n-butyl acrylate, 47 parts 2-ethylhexyl acrylate and 6 parts acrylic acid 
in acetone solution, was cast from acetone solution at 35% solids onto a 
siliconised release paper. The solvent was removed to give a layer of 
adhesive at a weight per unit area of 30 gsm. 
The adhesive layer was transfer coated onto a styrene-butadiene-styrene 
triblock copolymer film (Kraton 1101 film) 60 .mu.m in thickness by 
passage between a pair of rollers. This laminate on its release paper was 
perforated with knives to provide slits which were 4mm long, arranged in 
rows in which the distance between each row was 1 cm and the distance 
between two slits in any one row was 0.2 cm. The perforated laminate was 
cut into pieces 8 cm.times.12 cm. A portion of the adhesive-coated Kraton 
film was taken prior to perforation and its moisture vapour permeability 
measured by the Payne Cup method, was 237 cm.sup.-2 24 hr.sup.-1. 
A 15% w/w solution of a hydrophilic polyurethane, having a potential water 
content of 25% when hydrated, in industrial methylated spirits was mixed 
with 5% fine silica (Gasil 23, Crossfield Chemical Ltd.) and was cast onto 
silicone release paper to give a film of weight per unit area of 40 gsm. 
The hydrophilic polyurethane film was also cut into pieces 8 cm.times.12 
cm. 
Pieces of spun-bonded polypropylene 7 cm.times.10 cm were placed on the 
non-adhesive side of the Kraton film and the hydrophilic polyurethane film 
placed on top of this. The Kraton and polyurethane films were then sealed 
together around their edges and around the polypropylene non-woven fabric 
using an impulse heat sealer to give the surgical dressing of the present 
invention. 
The dressing may be sealed into a bacteria proof pack and sterilised using 
ethylene oxide. 
Example 8 
Preparation of Adhesive Surgical Dressing 
An adhesive coated Kraton film was prepared using the method described in 
Example 7 to give an adhesive coated first layer which was 40 cm wide. 
During the process of transfer coating of the adhesive onto the Kraton 
film two strips of polyester film, each approximately 4 cm wide were 
adhered to each edge of the adhesive coated Kraton film to provide opposed 
non-adherent handles. The central 30 cm portion of the adhesive coated 
film, that is the portion between the handles, was then perforated using 
an array of knife points to form a multiplicity of slits arranged in rows 
1 cm apart. The perforated laminate was then cut across its width to give 
pieces 40 cm.times.30 cm. 
A piece of non-woven polypropylene material, approximately 27 cm.times.27 
cm was then placed on the non-adherent perforated surface of the Kraton 
film so as to leave an eve margin around the non-woven material. 
A continuous film, 30 cm.times.30 cm, of a hydrophilic polyurethane was 
formed by casting a solution of the polymer using the method described in 
Example 7. This film was placed over the non-woven material and then heat 
sealed around its edges to the Kraton film. The dressing so formed had 
non-adhesive handles on two opposite sides which may be used for 
manipulating the dressing during application and a central area of 
approximate area 30 cm.times.30 cm in which the adhesive coated first 
layer is perforated. 
The dressings may be packaged into bacteria proof packs, sealed and 
sterilised using ethylene oxide. 
The dressings of this particular size are useful in the treatment of skin 
graft donor sites. 
Demonstration of Effectiveness 
Moisture Vapour Permeability of Surgical Dressing Determination 
A surgical dressing of the invention is applied adhesive face down, to an 
aluminium hot plate maintained at 35.degree. C. The dressing covers a 2mm 
deep, 7.5 cm in diameter recess in the plate. The recess is filled by a 
disc of polyurethane foam pre-soaked in horse serum. A hole in the bottom 
of the recess allows serum to be injected into the foam beneath the 
dressing. 3 ml of serum are injected beneath the dressing to form a 
blister. The serum penetrates through the holes in the first layer so as 
to wet the surface of the continuous film and evaporate therefrom. 
The moisture vapour transmission rate from the top of the dressing is 
measured using a Servo-Med Evaporimeter EPI (available from Servo-Med AB 
of Stockholm, Sweden) placed adjacent to the top of the dressing. The rate 
is measured when the 3 ml of serum is injected and at specific intervals 
thereafter to follow changes in the rate as the volume of excess liquid 
diminishes. 
A dressing of the invention formed according to the method described in 
Example 7 was tested in this manner. The dressing when placed on the metal 
plate was observed to adhere to the metal surface but did not adhere to 
the wet foam surface. The serum was observed to very rapidly penetrate the 
slits and to contact the underside of the upper, hydrophilic layer. 
On removal of the dressing 48 hours after the injection of the serum it was 
observed that the surface of the foam was still moist but at no stage was 
a blister formed. 
A comparative test with a conventional thin film moisture vapour permeable 
dressing showed a large blister of serum was still present beneath the 
dressing 48 hours after the injection of serum. 
Measurements of the variation of moisture vapour transmission rate with 
time were performed using the Servo-Med Evaporimeter and gave the 
following results: 
______________________________________ 
Time after injection 
Evaporimeter reading 
Stops (Hours) (g/m.sup.2 /hr) 
______________________________________ 
0 140 
0.5 100 
1.0 75 
2.0 62 
3.0 42 
5.0 23 
24.0 14 
48.0 15 
______________________________________ 
The results show a high moisture vapour transmission rate in the presence 
of serum which decreases to a low steady value as the volume of serum 
decreases. The top layer of the dressing at the end of the test was dry 
but the foam surface was still wet. 
This illustrates the effectiveness of the dressings of the present 
invention in preventing blister formation whilst also preventing a drying 
out of the underlying surface. 
A clinical trial was carried out on 9 patients involving 10 split skin 
graft donor sites. The dressings used were formed from a 
styrene-butadiene-styrene first layer coated with acrylic ester copolymer 
pressure sensitive adhesive and containing slits, an intermediate layer of 
non-woven polypropylene and a top layer of a hydrophilic polyurethane 
polymer which contained 25% by weight of water when hydrated. The size of 
the dressings was 40 cm.times.30 cm or 30 cm.times.30 cm and they were 
sterilised by ethylene oxide. The dressings were applied to the donor 
sites and left in position for 14 days. During this period it was observed 
that very little fluid collected under the dressings; water loss was high 
in the presence of fluid produced by the wound and lower in the absence of 
free fluid; the dressings were comfortable and easily removed and did not 
adhere to the wound. Seven of the 10 donor sites were observed to have 
healed within the 14 days of the test. 
Test Methods 
Measurement of Moisture Vapour Permeability 
The moisture vapour permeability (MVP) of self-supporting materials, for 
example the continuous film and the first layer when unperforated (i.e. 
uninterrupted) may be measured by the Payne Cup method. This method uses a 
cup 1.5 cm deep which has a flanged top. The inner diameter of the flange 
provides an area of 10 cm.sup.2 of material through which moisture vapour 
may pass. In this method 10 ml of distilled water is added to the cup and 
a sample of the material under test, large enough to completely cover the 
flange, is clamped over the cup. When the test material has an adhesive 
surface it is clamped with the adhesive surface facing into the cup. The 
complete assembly is then weighed and placed in a fan assisted electric 
oven where the temperature and relative humidity are maintained at 
37.degree. C. and 10% respectively. The relative humidity within the oven 
is maintained at 10% by placing 1 Kg of anhydrous 3-8 mesh calcium 
chloride on the floor of the oven. After 24 hours the cup is removed from 
the oven and allowed to cool for 20 minutes to reach room temperature. 
After reweighing, the mass of water lost by vapour transmission is 
calculated. The moisture vapour permeability is expressed in units of 
gm.sup.-2 24 hr.sup.-1 at 37.degree. C., 100% to 10% relative humidity 
difference, that is it is the mass of water transmitted through a square 
meter of material in a 24 hour period when maintained at 37.degree. C. and 
there are differences of relative humidity at the two surfaces of the 
material of 100% inside the cup and 10% outside. The MVP is abbreviated to 
g/m.sup.2 /24 hr or gm.sup.-2 24 hr.sup.-1 when used herein. This is the 
moisture vapour permeability referred to herein unless otherwise stated. 
However, this permeability may also be referred to as the "upright-MVP" or 
"dry-MVP" to distinguish it from the "inverted-MVP" or "wet-MVP". The 
inverted-MVP is measured using the same apparatus as the upright-MVP 
except that when the cup is placed in the oven the cup is inverted so that 
liquid water (and not water vapour) is in contact with the test material 
and the test period is 24 hours. 
The upright-MVP values of an adhesive film may be measured in the manner 
described above. The MVP of the adhesive layer can not usually be measured 
in this way because of the difficulty of preparing a self-supporting film 
of adhesive. However, the MVP of the adhesive may be obtained by coating a 
layer of adhesive onto a standard backing material of known thickness and 
MVP and using the relationship P.sup.-1 =A.sup.-1 +B.sup.-1 in which P is 
the upright-MVP of the adhesive coated film, A is the upright-MVP of the 
adhesive layer and B is the upright-MVP of the standard backing layer. The 
backing material used as a standard is a continuous film of polyurethane 
having a thickness of 25 m, a weight per unit area of 30 gsm, a water 
content when fully hydrated of 2.5% by weight and an MVP of 1900g/m.sup.2 
/24hr. Thus since the value of P is measured and the value of B is known, 
the value of A may be calculated. 
The MVP of the adhesive coated first layer may be measured on uninterrupted 
(unperforated) samples of the materials. However, if none are available a 
guide to the moisture vapour permeability of the adhesive coated first 
layer when unperforated (i.e. uninterrupted) may be determined from a 
sample of the perforated layer in the following way using the Payne Cup 
method. Two pieces of the adhesive-coated first layer of suitable size and 
possessing holes are taken and adhered together so that the holes in each 
sheet are offset from each other. In this way every hole is covered by an 
adhesive-coated layer of film that is the laminate is continuous. The 
moisture vapour permeability of the combined film is then measured using a 
Payne Cup. The desired MVP is taken as twice the measured value. If the 
holes form an appreciable area of the surface of the first layer then the 
multiplying factor is reduced in proportion to their area. For example an 
adhesive coated layer prepared using the process described in in Example 7 
had a moisture vapour permeability when measured as an imperforate film of 
237 gm.sup.-2 24 hr.sup.-1 whilst an offset double layer of the layer 
containing slits had a moisture vapour permeability of 121 gm.sup.-1, thus 
leading to a calculated value of 242 gm.sup.-2 24 hr-1 for a single layer 
which corresponds within experimental error to the actual measured value.