GB2313338A - Perforated film for wound dressings - Google Patents

Abstract

A low-adherence film comprises a perforated membrane where the perforations are defined by regions of the membrane receding under surface tension force when the regions are rendered molten by localised hot-gas heating. The hot-gas heating may comprise a plurality of hot-gas jets or flames. Preferably the edges of the perforations have a circular tapered shape and define regions of lower softening temperature material than the rest of the membrane. The film may be a biaxially oriented polyester material having a thermally activated adhesive coating on one side for bonding to an absorbent layer in a wound dressing.

Description

A LOW-ADHERENCE FILM FOR WOUND DRESSINGS The present invention relates to a low-adherence film for wound dressings, and more particularly such films incorporating perforations in order to allow passage of moisture from a wound site into an absorbent wound dressing material.

It will be understood that wound sites are generally covered to protect the wound site from ingress of foreign material, i.e. keep it clean, and also to collect exudate from the wound site. Furthermore, a good wound dressing will stimulate healing within the wound by providing a moist warm environment about the wound site. Thus, a traditional wound dressing is a simple cotton bundle arranged to absorb the exudate.

Recently, more sophisticated non-woven fibrous materials have been used incorporating polyester fibres and viscose fibres, specifically mechanically constructed in order to facilitate exudate collection and ensure a good wound site regenerative performance. However, even though such non- woven materials have enhanced absorbency in terms of weight of exudate absorbed as a proportion of their own weight, it is possible that the wound site will become adhered to the absorbed material through exudate penetration into the absorbent material and fibre strand migration into the wound site. Thus, when the wound dressing is removed a top layer of regenerated tissue is removed from the wound site; this is a source of trauma and can diminish the speed of patient recovery. Finally, removal of such regenerated skin tissue leaves nerve ends exposed with a possible reduction in sensitivity and numbness.

In order to prevent such detrimental effect upon a regenerating wound site, it has become commonplace to add a low-adherence film to the contact surface of the wound dressing. Thus, the low-adherent film acts as a barrier to prevent migration of absorbed material fibres into the wound site and to prevent regenerated skin tissue strands penetrating the absorbent layer. Unfortunately, if the barrier presented by the low-adherent film was complete, it would also prevent exudate passing through it to be absorbed by the absorbent layer, and so the benefits of such an absorbent layer would be nullified. Generally, the lowadherent film has perforations to allow localised passage of exudate through to the absorbent layer.

In the past, these perforations in the low-adherence layer have been achieved using known hot-needle technology.

Typically, a roller of heated hot needles is presented to the low-adherent film such that the tips of the needles contact the film and penetrate it to achieve the necessary perforations. Figure 1 illustrates a typical perforation produced by such hot-needle technology. A low-adherent film 1 outside of an area defined between points A and B is unaffected by penetration by the hot needle and thus maintains a barrier of low adherence to the wound site.

Within the area defined by points A to B, the low-adherent film has shrunk back due to the thermal effect of the hot needle upon the film, in order to present a rim, or gob, of melted film 1 material. This rim 2 is, in effect, rolled back by the hot needle travelling in the direction of arrowhead N. The dimensions of a perforation 3 are substantially set by the thickness of the hot needle penetrating the film 1. Furthermore, this hot needle penetrating the film 1 will, over time, become coated with previous film material and, due to the fact that the roller is fed in contact with the film, the perforation 3 tends to be distorted into an oval shape due to the relative motion of the hot-needle roller relative to the film 1. The previous melted film 1 retained on the hot needle can degrade over time and thus segments of burnt film can be embedded in the rim 2. Finally, the rim 2 tends to be thermally transformed into a more brittle structure than the original low-adherence film 1.

As indicated above, the film 1 is penetrated by the hot needle and this needle protrudes through the film 1 with a proportion of such film molten as a coating on the needle.

Thus, when the needle is removed, a thin sheath of film may remain to close, or partially close, the perforation.

Alternatively, the edge or rim 2 may be made ragged by the molten film coating to the needle. There may be some crystallisation due to the progressive heating and stretching of the film 1.

The rim 2 can cause problems in subsequent manufacture of a wound dressing in that, an absorbent layer is adhered to the film 1 generally by a hot compression procedure to activate an adhesive coating to one side of the film 1 in order to secure the film 1 to the absorbent layer. This compression adhesion may fracture a brittle rim 2 and thus present fissures, etc., which can be penetrated by regenerating skin tissue strands. Furthermore, contamination of the rim 2 surface with burnt or degraded molten film, as described above, from the hot needle can additionally cause adherence problems and may be detrimental to the wound site sterility. Finally, being substantially bulbous the rim 2 has a shoulder 4 which acts as a recess where there is generally lower compression, or abutment force to secure the low-adherent film 1 to the absorbent layer. Thus, there may be an area of low bond strength between the film 1 and any absorbent layer behind the rim 2 and this can be exploited by ingress of regenerated skin tissue strands and/or exudate. Furthermore, if the wound dressing is lifted to inspect the wound site and replaced, this area of low bond strength behind the rim 2 may become detached from the absorbent layer to allow further ingress of exudate and regenerated skin tissue, with the result that when the dressing is again lifted or removed the wound site is traumatised.

It is an objective of the present invention to provide a low-adherent film for wound dressings, which substantially relieves the above-mentioned problems.

In accordance with the present invention there is provided a low-adherence film for wound dressings, the film comprising a polyester membrane including a plurality of perforations having a circular tapered edge defined by surface tension force when the membrane is subjected to localised hot-gas heating until molten.

Preferably, the hot gas is air.

Further in accordance with the present invention there is provided a method of making a low-adherence film for wound dressings comprising presenting a polyester membrane to a plurality of hot-gas jets in order that said jets locally melt said membrane where incident thereupon, said molten segments of said membrane being allowed to recede due to surface tension to provide perforations having a tapered edge.

An embodiment of the present invention will now be described by way of example with reference to the accompanying drawings in which: Figure 2 is a schematic cross-section of a lowadherence film having a perforation indicative of the multiplicity of perforations present in a low-adherence film and, Figure 3 is a schematic cross-section of the film illustrated in Figure 2 attached to a non-woven absorbent layer.

Polyester films are freely available in a range of gauges and including surface coatings to provide adhesion and other functional requirements. Polyester films of the required type are available from Smith & Nephew Ltd.

In the present invention a polyester film 21 is perforated using a hot-gas or flame-gas technique. In such a technique, typically, a film 21 is presented to and over a rotating drum having a plurality of hot-gas jets. These hot-gas jets are incident upon the film 21 in order to locally melt sections of the film 21. These melted sections of the film 21 are molten and, due to natural surface tension and if necessary after some form of nucleation, the molten material recedes to form a perforation with a tapered edge 22. This edge 22 has hardly any rim 2 as present and shown in Figure 1 when a hot needle is used to provide perforation. Thus, the edge 22 is generally indistinguishable from the flat film 21 as compared to the situation with the rim 2 produced by a hot-needle perforation technique and illustrated in Figure 1. This facilitates bonding of the film 21 closely with an absorbent layer as described later.

Generally, the perforation, has a dimension of less than 1 mm. However, the actual size is dependent upon expected macro transmission rates of exudate and other liquids to be absorbed by an absorbent layer.

The roller as indicated previously presents the hot jets of gas towards the film 21 on one side as illustrated in Figure 2 by arrowhead G. Each hot-gas jet is well defined and thus specifically heats until molten a segment of the film 21. An area segment of the film 21 is heated from the outset rather than from a progressive point using a hot needle. Obviously, when molten, the gas may balloon to nucleate a breach of the film 21 and thus allow by surface tension expansion and retraction of the film 21 molten mass- to form the tapered edge 22. Generally, the film 21 remains smoother on the gas-entry side 23, as compared to the exit side 24 of the film 21. Smoothness is important in achieving low adherence and thus it is surface 23 which is usually presented to the wound site, whilst the rougher surface 24 assists with bond adhesion between the film 21 and an absorbent layer when attached. Furthermore, if the film 21 has an adhesive coating to facilitate bonding between the film 21 and an absorbent layer, this coating will most conveniently be on the gas-exit side of the film 21.

Typically, each gas jet presented to the film 21 will be columnar and have a diameter marginally greater than the size of the perforation desired. However, the diameter will generally be less than 1 mm. As hot gas is used, rather than a hot needle, there is no potential contamination of the film 21 from a penetrating needle, i.e. degraded molten film 21 material carried by the needle is not passed to each perforation for incorporation in its edge.

It will be understood that the film 21 and any rotating hot-gas jet drum will not be stationary; there will be relative rotary or lateral movement with respect to each other. As indicated previously with hot-needle perforation techniques, this generally results in an oval, or irregular shape, to each perforation. However, in the present hot-gas perforation technique it will be understood that a localised segment of the film 21 is made molten and then surface tension forces are used to develop the perforation. These surface tension forces in the molten film 21 are equally applied and thus typically the perforation created is substantially regular and circular. Such regular perforations again limit the ragged nature of the edge of the perforation and so reduces further any detrimental effect upon adhesion to a wound site.

In any one wound dressing, and in particular in the low-adherence film used therein, it will be appreciated that there will be many hundreds, if not thousands, of perforations presented to the wound site. Thus, although the advantages of the present low-adherence film with regard to an individual perforation profile are relatively small compared to the previous hot-needle technique, the benefits of the present film will be multiplied by that number of perforations in a wound dressing. Thus, the multiplied effect of a better perforation for a whole wound dressing is significant.

Generally, the hot gas used in the hot-gas jets will be air at a temperature in excess of that necessary to rapidly turn the segments of the film 21 molten. If a polyester film is used the temperature necessary is in excess of 1200C. The hot-gas technique can be considered as a gas flame with implicit high temperatures and in any event temperatures in the film 21 are significantly greater than developed by a hot needle.

In Figure 3 there is illustrated a schematic crosssection of the film 21 bonded to an absorbent layer 25. As indicated above, the absorbent layer 25 is present in order to absorb exudate from a wound site, schematically illustrated as W in Figure 3. Thus, the absorbent layer 25 spreads the exudate throughout its volume. Typically, the film 21 is bonded to the absorbent layer 25 by applying a heat-activatable coating to the film 21 and then either laminating the film 21 onto the layer 25, or by using a heated mangle to compress the film 21 and layer 25 such that the adhesive coating to the film 21 is activated to cause a bond between the film 21 and the layer 25. In some instances, a mangle technique is preferred as it may result in less distortion of the film 21, i.e. a laminating technique involves heating the film and bringing it into contact with the layer 25. It will be understood that heating a film such as polyester evidently increases its flaccidity, and consequently any pull of that film in a direction will distort the film. These distortions may cause stress between the film 21 and the layer 25. These stresses may result in unacceptable curling, or possibly failure of the bond between the film 21 and the layer 25.

An example of an absorbent layer 25 is given in European Patent No. 0 388 062 . This is a so-called graduated density felt where a central portion of the layer 25 is adapted to accommodate exudate. The whole object of the wound dressing, in addition to being a protective barrier to foreign body contamination of the wound site, is to ensure the wound site remains moist and warm. Thus, by appropriate removal of exudate the wound site is kept in the best conditions for healing.

Generally, the polyester film will have a thickness of less than 1 mm and the perforations will be arranged in an appropriate pattern, size and distribution in order to best achieve wound exudate collection and absorption. Thus, the low-adherent film may be designed to have a closer perforation pattern at the centre of any eventual wound dressing, as compared to the peripheral edges of any wound dressing. However, such choices are determined by desired performance and necessity, and can be varied as required.

An example of a film 21 is a 12 zm thick PET film perforated with 330 perforations per square inch. The base polyester membrane is a biaxially oriented polyester film.

Each perforation is produced by hot gas, i.e. gas flame technique with a perforation spacing of 1.3 mm in one direction and 1.5 mm in another perpendicular direction.

Furthermore, each perforation will have a diameter of 0.60.7 mm to give a perforation density of 322-323 perforations per square metre.

As indicated previously, relatively minor variations in each perforations performance when multiplied by the number of such perforations in a wound dressing can have appreciable benefits. A further particular advantage of the present invention is a slight reduction in the softening or melting temperature of the film after molten perforation along with a tapered edge to each perforation. Thus, upon combination of the film with an absorbent layer under heat and pressure, these edge portions of the perforations tend to preferentially soften compared to the rest of the film.

However, and more importantly, it will be appreciated that such softened edge portions of the film are more susceptible to fibres becoming embedded into them increasing the adhesive bond strength between the perforated film and the absorbent layer.

Claims (7)

Claims:

1. A low-adherence film for wound dressings, the film comprising a membrane including a plurality of perforations having a circular tapered edge defined by surface tension force when the membrane is subjected to localised hot-gas heating until molten.

2. A film as claimed in Claim 1 wherein the membrane is a biaxially oriented polyester film.

3. A film as claimed in Claim 1 or Claim 2 wherein upon one side of the film there is a thermally-activated adhesive coating to facilitate bonding of the film to an absorbent layer in a wound dressing.

4. A wound dressing comprising an absorbent layer secured to a low-adherence film as claimed in any of Claims 1, 2 or 3.

5. A method of making a low-adherence film for wound dressings comprising presenting a membrane to a plurality of hot-gas jets in order that said jets locally melt a segment of said membrane where incident thereupon, said molten segments of said membrane being allowed to recede under surface tension force to provide a plurality of perforations in the film.

6. A method as claimed in Claim 5 wherein the membrane and the hot-gas jets are presented with the respect to each other such that there is relative lateral or rotary motion between said hot-gas jets and said membrane.

7. A low-adherence film for wound dressings, the film comprising a membrane including a plurality of perforations having a tapered edge and said edge defining a margin of relatively lower softening temperature material compared to said remainder of the membrane.