Wound dressing comprising polyurethane foam

Details Wound dressing comprising polyurethane foam Wound dressing comprising polyurethane foam

1995-05-08

2001-12-04

Sergent, Rabon (Department: 1711)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Cellular products or processes of preparing a cellular...

C521S117000, C521S137000, C521S159000, C521S174000, C521S914000

Reexamination Certificate

active

06326410

ABSTRACT:

This invention relates to a wound dressing having a wound contact layer formed from a polyurethane foam, and more particularly a conformable, high-density polyurethane foam. The invention also relates to a method for forming such a foam.
Polyurethane foams have been proposed for a number of uses in the prior art, and a wide variety of additives have been included in such foams, depending on the particular end use which is contemplated. For example, U.S. Pat. No. 3,961,629 discloses a hydrophilic polyurethane foam for use as an absorbent pad for body fluids. The cells of the foam have a coating of surfactant, and this is said to accelerate absorption of body fluids into the foam at medically acceptable rates. It is proposed that hygroscopic agents such as glycerol may also be incorporated into the foam, as may germicidal and therapeutic agents.
EP-A-0171268 discloses a dressing for use in deep wounds, which dressing comprises individual pieces of an absorbent hydrophilic foam contained within a porous bag formed from an apertured polymeric film. The absorbent foam is preferably a hydrophilic polyurethane foam which can be made from HYPOL isocyanate-capped polyether prepolymer marketed by W. R. Grace & Co., and non-ionic surfactants. Physiologically active components such as local anaesthetics, antibacterial agents and antifungal agents are proposed as additives which may be included in the foam.
According to EP-A-0171268, the fact that the foam is present in the form of individual pieces confers on the dressing the property of being able to conform to the contours of a wound cavity both on initial application of the dressing and subsequently following absorption of body fluids. It is said that existing commercially available foams, if used as a single piece, have too high a density to possess the required degree of conformability.
U.S. Pat. No. 4,339,550 discloses a hydrophilic foam composition which is prepared by the “in situ” reaction of an isocyanate-capped polyether prepolymer having a functionality of from about 2 to about 8, water, and a chemically compatible, essentially non-polar, volatile organic compound. The foam is stated to be capable of achieving a sustained, controlled release of the volatile materials from the foamed structure. Suitable “control release” ingredients include polyols, such as propylene glycol and glycerine.
EP-A-0335669 discloses a hydrophilic foam composition comprising the “in situ” reaction product of an isocyanate-capped polyether prepolymer, a hydrophilic agent capable of absorbing water, an adjuvant comprising an alcohol, a wetting agent and water. One application which is proposed for the foam composition is in the manufacture of wound dressings. The composition is said to carry the adjuvant releasably, so that at least a portion of the adjuvant is released into an external liquid (e.g. wound exudate) with which the foam composition comes into contact.
A wide range of prepolymers, hydrophilic agents, adjuvants and wetting agents are proposed in EP-0335669. Water soluble monohydric, dihydric and polyhydric alcohols are all said to be suitable adjuvants, but glycerol is preferred.
The present invention is based on the unexpected finding that polyurethane foams having properties which render them particularly suitable for use as wound contact layers may be obtained by including a natural or synthetic rubber in the foam composition. Indeed, the foams of the invention display the very combination of high density and high conformability which EP-A-0171268 indicates to be unattainable in prior art foams.
Accordingly, the present invention provides a wound dressing having a wound-contact layer formed from a polyurethane foam, said foam comprising
a) one part by weight of the reaction product of an isocyanate-capped prepolymer with water and optionally other hydroxy-functional compounds, and
b) from 0.03 to 0.3 parts by weight of a natural or synthetic rubber.
Preferably, the composition comprises one part by weight of the polyurethane reaction product and from 0.05 to 0.2 parts (e.g. from 0.05 to 0.15 parts) by weight of the rubber.
The addition of a natural or synthetic rubber has the effect of increasing the cure time for the polyurethane, and increases extensibility, strength and tack. Most importantly, it substantially reduces shrinkage of the gel on drying, and it also improves bubble formation, producing more regular, smaller bubbles.
If the rubber content is less than 0.03 parts per part of polyurethane reaction product, the effect on shrinkage of the gel may be too small to be of significant benefit. On the other hand, if the rubber content is too high, then the absorbency of the foam may be unacceptably low.
Preferably, the isocyanate-capped prepolymer has an NCO content of from 0.5 to 1.2 meq/g. Isocyanate-capped prepolymers having an isocyanate content within this range have been used in the prior art to produce so-called hydrogels. For this purpose, the prepolymers are mixed with relatively large quantities (eg. a ten-fold excess by weight) of water. The reaction mixture is initially of low viscosity, such that carbon dioxide which is evolved by reaction of the water with isocyanate end groups escapes. In this way, substantially no carbon dioxide is trapped within the hydrogel end product.
In contrast, the use of a relatively small amount of water produces an initial reaction mixture of much higher initial viscosity. Carbon dioxide formed by hydrolysis of isocyanate end groups is therefore trapped, producing a foamed hydrogel.
The prepolymer which is used in the method of the invention is preferably an isocyanate-capped polyether, such as an ethyleneoxy/propyleneoxy copolymer. A particularly suitable prepolymer is that available under Trade Mark HYPOL Hydrogel.
The foams of the invention generally have a density of at least 0.28 g/cm
3
, and preferably at least 0.30 g/cm
3
. Particularly preferred foams have a density in the range 0.32 to 0.48 g/cm
3
, e.g. about 0.35 g/cm
3
.
The foams of the invention generally also have an elongation at break of at least 150%, and more preferably at least 300%. Particularly preferred foams according to the invention have an elongation at break in the range from 500 to 2000%.
Depending principally on the amounts of rubber and other additives which are included, the foams of the invention typically have an absorbency of at least 3 g saline/g, preferably at least 5 g/g, and more preferably from 8 to 20 g/g. The foams are thus highly absorbent, yet conformable.
The preferred foams of the present invention also have the property of swelling and expanding when water is absorbed. This is particularly advantageous in a wound contact layer, because the swelling of the foam causes it to move inwards towards the wound bed, thus filling the wound cavity. This encourages the wound to heal from the base upwards and outwards, and it discourages epithelialization over the wound surface before the bed has been filled with granulation tissue.
The degree of swelling of the foams of the present invention on complete saturation with an aqueous medium is typically at least 100% (expressed in terms of the increase in volume), and preferably at least 200%. Preferred foams swell by 400 to 800%. Despite this high degree of swelling, however, the foams of the invention retain their integrity even after absorption of large quantities of water.
Moreover, the preferred foams according to the invention are found to have a morphology which is particularly appropriate for low adherence wound dressings. The foams are open-celled, the cells being very regular in size and shape, with very smooth edges to the pores in the walls of the cells. Typically, the cells of the foams of the invention have an average diameter in the range 0.1 to 0.6 mm.
In order to increase the softness and conformability of the foam and to facilitate processing, it is preferred to include a minor amount of a hydroxy functional compound in the foaming mixture. Suitable hydroxy functional compounds include lower monohydric alcohols such as methanol, ethanol and

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