Wound dressings and materials suitable for use therein

Details Wound dressings and materials suitable for use therein Wound dressings and materials suitable for use therein

2000-12-20

2003-04-15

Lucchesi, Nicholas D. (Department: 3764)

Surgery: splint, brace, or bandage

Bandage structure

Skin laceration or wound cover

C602S041000, C602S042000, C602S054000

Reexamination Certificate

active

06548730

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to wound dressings, in particular to dressings in fabric form and other forms suitable as contact layers for exuding wounds, and to methods for the manufacture of such dressings and of materials for use therein.
Commonly-used wound dressings include foams, sponges and fibre-based materials such as gauzes and waddings, for example of cotton or viscose rayon. Such fibre-based materials tend to adhere to the wound surface and are accordingly difficult to remove, after use, in one niece and without causing trauma to the patient. Known wound dressings also include advanced dressings based for example on alginates or on various kinds of hydrocolloids or hydrogels, but such dressings are relatively expensive and are accordingly used in general only when clinical needs so recommend. There exists a desire for wound dressings which possess at least some degree of absorbency, but which are sufficiently non-adherent that they are capable of being removed from a wound in a single piece without shedding fibre fragments and without trauma to the patient, and which are inexpensive in comparison with advanced dressings.
BACKGROUND ART
WO-A-94/16746 discloses a wound dressing in which the wound-contacting surface comprises carboxymethylcellulose (CMC) filaments capable of absorbing at least 15 times, preferably at least 25 times, their own weight of 0.9% by weight aqueous saline solution (as measured by a defined free-swell absorbency test) to form a swollen transparent gel, the thusly-swollen dressing retaining sufficient fibrous character to be removed as a coherent dressing from a wound. The degree of substitution (D.S.) of the CMC filaments is preferably at least 0.15, more preferably from 0.2 to 0.5, although it may be up to for example 1.0. The CMC filaments may generally be prepared by reacting cellulose filaments in the presence of strong alkali With chloracetic acid or a salt thereof. The cellulose filaments may be viscose rayon, cuprammonium rayon or cotton, but they are preferably solvent-spun and may accordingly be lyocell.
WO-A-95/19795 discloses a wound dressing which comprises a blend of textile fibres and gel-forming fibres. The textile fibres may be natural or synthetic but are preferably cellulosic fibres such as viscose rayon or cotton. The gel-forming fibres may for example be carboxymethylcellulose or alginate fibres. The gel-forming fibres can be of the type which retain their structural integrity on absorption of exudate or can be of the type which lose their fibrous form and become a structureless gel or a solution on absorption of exudate. The absorbency of the gel-forming fibre (measured by the free-swell method) is desirably at least 2 g/g of 0.9% saline solution, preferably at least 15 g/g, more preferably between 25 and 50 g/g. The D.S. of a carboxymethylcellulose fibre is desirably at least 0.05, preferably at least 0.2, more preferably between 0.3 and 0.5. Such dressings are said to have the advantage that the fibres they contain are not engulfed by new tissue formed during the healing process, so that they can he removed without causing wound injury.
DISCLOSURE OF THE INVENTION
According to a first aspect of the invention, there is provided a process for the manufacture of a wound dressing comprising the step of carboxymethylating a cellulosic fabric such that the absorbency (as defined hereinafter) of the fabric after carboxymethylation is no more than 3 g/g, preferably no more than 2.5 g/g, further preferably no more than 2 g/g or 1 g/g, greater than the absorbency (as defined hereinafter) of the fabric before carboxymethylation and such that the degree of substitution of cellulose by carboxymethyl groups in the carboxymethylated fabric measured by IR spectroscopy (as defined hereinafter) is in the range from 0.12 to 0.35, preferably from 0.2 to 0.3. The invention also includes wound dressings which comprise such carboxymethylated fabric.
By the absorbency of the fabric, before and after treatment, we mean the absorbency figures assessed by the method described in British Pharmacopoeia 1993, Addendum 1995, page 1706 for Alginate Dressings, but with substitution of the fabric under test for the alginate specified therein, which method yields absorbency in terms of weight per unit area, and then converted to absorbency in terms of weight ratio (g/g).
The cellulosic fabric preferably consists solely cellulosic fibre but may contain a proportion of non-cellulosic textile fibre or of gel-forming fibre. The cellulosic fibre is of known kind and may be a natural fibre such as cotton or a manmade fibre such as lyocell or viscose rayon; blends of such fibres may be used. We have observed that dressings according to the invention made from cellulosic fibres of low wet strength such as viscose rayon may tend to shed fragments when wetted, and accordingly use of such fibres is generally less preferred. The cellulosic fabric may comprise continuous filament yarn and/or staple fibre. A fabric of continuous filament yarn many be preferred, on the ground that such fabrics are less liable to shed fibre fragments on handling or on removal from a wound. We have nevertheless surprisingly found that dressings according to the invention composed of staple fibres have a low tendency to shed fragments.
The cellulosic fabric is of known kind and may be made in known manner. The basis weight of the fabric is generally in the range from 30 to 250 g/m
2
. The cellulosic fabric may be a woven or knitted fabric or a nonwoven fabric such as a hydroentangled fabric or a needlefelt. A nonwoven fabric should be of sufficiently robust construction—for example, having sufficiently high fibre entanglement—that the carboxymethylated fabric after absorption of aqueous liquors possesses sufficient mechanical integrity to permit it to be removed from a wound in a single piece.
The carboxymethylation step is generally performed by contacting the fabric with strong alkali, for example sodium hydroxide, and a carboxymethylating agent such as chloracetic acid or a salt thereof such as the sodium salt. These reagents may be applied to the fabric separately or together. The reaction is conveniently performed in an aqueous system. This system preferably comprises a water-miscible organic solvent such as ethanol or industrial methylated spirit, in order to suppress swelling and dissolution of carboxymethylated cellulose. Reference may be made to WO-A-94/16746 for general discussion of the carboxymethylation reaction.
A preferred method of performing such a carboxymethylation step is as follows, and such a method of carboxymethylating a fibre of cellulose II forms a second aspect of the invention. Regenerated or reconstituted cellulose fibre (cellulose II) is contacted with a solution containing from 4 to 8 percent by weight sodium hydroxide, the amount of sodium chloracetate necessary to achieve the desired degree of substitution, from 50 to 60 percent by weight water, and the balance ethanol. For calculation purposes, the solution is considered to consist of the substances applied to the fibre and any moisture introduced with the fibre and/or ethanol. The solution may contain a small proportion of methanol if industrial methylated spirit is used as the source of ethanol. We have found that the presence of lower proportions of water than the specified minimum tends to result in uniform carboxymethylation, which is undesirable in the manufacture of dressings according to the invention. We have found that the presence of greater proportions of water than the specified maximum tends to result in too low a degree of reaction and too low a degree of slipperiness of the carboxymethylated fibre when wetted. We have further found that lesser proportions of water than those specified above are generally required for satisfactory results on fibres of cellulose I such as cotton. The method is preferably performed at from 40 to 80° C., more preferably from 50 to 60° C. The method is preferably performed for from 20 to 90 min., more preferably from 30 to 60 min. Ethanol may b

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