Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Cellular products or processes of preparing a cellular...
Reexamination Certificate
2001-11-15
2003-01-21
Cooney, Jr., John M. (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Cellular products or processes of preparing a cellular...
C521S067000, C521S099000, C521S102000, C521S130000, C521S137000, C521S170000, C521S174000
Reexamination Certificate
active
06509388
ABSTRACT:
This invention relates to polyurethane foams, and more particularly to a method of making conformable, high-density polyurethane foams for use in wound dressings. The invention also relates to a wound dressing having a wound-contacting layer formed from such a foam.
Polyurethane foams have been proposed for a number of uses in the prior art. For example, U.S. Pat. No. 3,903,232 discloses hydrophilic cross-linked polyurethane foams which are said to be useful for external body cleaning, for internal body usage and as absorptive products such as diapers. The foams are prepared by reacting particular isocyanate-capped polyoxyethylene polyols having an isocyanate functionality greater than 2 with large amounts of an aqueous reactant, preferably water.
EP-A-0171268 discloses a dressing for use in deep wounds, which dressing comprises individual pieces of 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 (Registered Trade Mark) isocyanate-capped polyether prepolymer marketed by W.R. Grace & Co., and non-ionic surfactants.
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 prepolymer, hydrophilic agents, adjuvants and wetting agents are proposed in EP-0335669. Suitable prepolymers are said to include prepolymers having an NCO content as high as 2.55 meq/g or as low as 0.5 to 0.9 meq/g. Water soluble monohydric, dihydric and polyhydric alcohols are all said to be suitable adjuvants, but glycerol is preferred, and the majority of the examples involve the use of glycerol. The sole example in which a monohydric alcohol is employed as the adjuvant involves the use of a prepolymer having an NCO content of 1.6 meq/g. The resulting product is said not to be acceptable because of “gross porosity”.
EP-A-0541391 describes a method of forming a polyurethane foam suitable for use as a wound-contacting layer, the method comprising mixing 1 part by weight of an isocyanate-capped prepolymer having from 0.5 to 1.2 meq NCO groups/g with from 0.4 to 1.0 parts by weight of water in the presence of from 0.05 to 0.4 parts by weight of a C
1
to C
3
monohydric alcohol, and then drying the product such that the amount of water-soluble alcohol remaining in the product is less than 1% by weight. The resulting foam has high density and conformability.
GB-A-1417962 discloses a non-reticulated polyurethane foam material, the foam cells adjacent to at least one surface of which are irreversibly partially collapsed relative to foam cells remote from said surface. The specification notes that polyurethane foam can be reversibly deformed or compressed to reduce the thickness up to a certain extent, but the original thickness will be recovered upon washing or steam heating. In contrast, the “irreversibly partially collapsed” cells are formed by applying a greater degree of compression, but not so great a degree of compression as to cause the cells to collapse completely or fuse.
According to GB-A-2253628, the foams disclosed in GB-A-1417962 are suitable for the treatment of moderately exuding wounds, but they may not have sufficient absorbency to absorb all the exudate from heavily exuding wounds. GB-A-2253628 further reports that an attempt to increase the absorbency of the known foam by incorporating an alginate during manufacture of the foam was unsuccessful. The alginate was said to be degraded, with consequent impairment of its absorbent and haemostatic properties. A deterioration in the appearance of the foam was also noted. However, GB-A-2253628 teaches that alginate may be used to enhance the absorbency of a polyurethane foam, after the foam has already been formed. Thus, the specification discloses a polyurethane foam which is impregnated with alginate, for example by spreading an alginate gel onto the surface of the foam, or immersing the foam in a solution of alginate.
In contrast to the teaching of GB-A-2253628, we have found that alginates can advantageously be incorporated during the production of polyurethane foams, if the foam is formed by mixing an isocyanate-capped prepolymer with a relatively small amount of water. Although the addition of alginate does not substantially increase the absorbency of the polyurethane foam (as measured on a weight per weight basis), it has been found that polyurethane foams containing alginate generally have smaller, more uniform cells, and higher densities.
Accordingly, the present invention provides a method of forming a polyurethane foam suitable for use as a wound-contacting layer, the method comprising mixing one part by weight of an isocyanate-capped prepolymer having from 0.5 to 4.8 meq NCO groups/g with from 0.4 to 1.0 parts by weight of water, characterised in that the mixture further contains from 0.2 to 20% by weight of alginic acid or a salt thereof Preferably, the alginic acid or alginate salt is present in an amount of from 0.5 to 10% by weight, more preferably 0.5 to 5% by weight, e.g. from 0.75 to 2.5% by weight.
In particularly preferred embodiments, the isocyanate-capped prepolymer has from 0.5 to 1.2 meq. NCO groups/g.
Preferably, the step of mixing is carried out in the presence of from 0.05 to 0.4 parts by weight of a C
1
to C
3
monohydric alcohol. Subsequent drying of the foam preferably results in a product containing less than 1% by weight of the monohydric alcohol.
The use of a relatively small amount of water in accordance with the present invention produces an initial reaction mixture of relatively high initial viscosity. Carbon dioxide formed by hydrolysis of isocyanate end groups is therefore trapped, producing a foamed hydrogel.
The present invention also provides a polyurethane foam which is suitable for use as a wound-contacting layer, the foam having a density of at least 0.28 g/cm
3
, and containing from 0.2 to 20% by weight of alginic acid or a pharmaceutically acceptable salt thereof, generally uniformly distributed throughout said foam.
Foams produced according to the method of the invention preferably have a density of at least 0.3 g/cm
3
. Particularly preferred foams have a density in the range 0.3 to 1.0 g/cm
3
, e.g. about 0.5 g/cm
3
.
The foams of the invention also preferably 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 on the proportions of other additives, the foam s of the invention have an absorbency of at least 3 g saline/g, preferably at least 5 g/g, and mo
Cooney Jr. John M.
Johnson and Johnson Medical Limited
Woodcock & Washburn LLP
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