WO1989012471A1

WO1989012471A1 - Wound dressing

Info

Publication number: WO1989012471A1
Application number: PCT/GB1989/000706
Authority: WO
Inventors: David Charles Wren
Original assignee: Britcair Limited
Priority date: 1988-06-24
Filing date: 1989-06-23
Publication date: 1989-12-28
Also published as: GB8815132D0; AU3854089A; ZA894836B

Abstract

A wound dressing can be prepared by wetting a pad comprising mixed salt alginate fibres which have first and second cations, the first cation (for example calcium) being capable of forming an insoluble alginate salt and the second cation (for example sodium) being capable of forming a soluble alginate salt, the equivalent ratio of the first to second cations being from 30:70 to 70:30, and preferably about 50:50. The wet pad may then be applied to a wound, particularly a dry wound or burn.

Description

WOUND DRESSING

This invention relates to the preparation of dressings for wounds for the human or animal body and to the use of certain material in the treatment of wounds. More particularly, the invention relates to the preparation and use of wound dressings and wound treatment material incorporating alginate fibres. The term "wound dressing" as used in this specification includes surgical dressings. The term "wound" includes burn, cut, sore, blister, rash or any other lesion or area of troubled skin. Dressings prepared and used in accordance with the invention are particularly appropriate for treating dry wounds and burns.

Certain alginate fibres have been known for use in surgical dressings for some time. GB-A-653341 is an example of an early disclosure of the use of calcium alginate materials in surgical dressings. The earliest such materials were calcium alginate fibres, but they suffered from the disadvantage of being quite insoluble in water or wound exudate matter. Accordingly, it was proposed to replace a proportion of the calcium ions in calcium alginate with other cations, whose alginate salts are soluble. Bonniksen in GB-A-653341 therefore proposed that some of the calcium ions be replaced with sodium ions, in a process which has come to be known as "conversion" of calcium alginate to form a mixed salt alginate.

The manner in which conversion took place then received attention from various groups of workers, as can be seen in GB-A-1231506 and GB-A-1394741. It appears that that there was some belief that alginates in which the calcium alginate fibres were converted to a 50:50 Ca:Na mixed salt fibre (on an equivalent basis) promoted haemostasiε. It seems that haemostasis was believed to be closely linked with the property of solubility, which increased with the proportion of solubilising (for example sodium) ions. It is now believed that the property of haemostasis is not wholly attributable to dissolution.

Although the 50:50 Ca:Na mixed salt fibre was popular, it became difficult to handle when in contact with liquid wound exudate material, precisely because of the solubility that made it apparently desirable: the partially soluble 50:50 calcium: sodium mixed salt alginate fibres swell on contact with water and aqueous media to produce a gel-like slab.

Modern theories of wound management are based upon controlling the environment adjacent to the wound while at the same time preventing or reducing the ingress of dirt and bacteria and preventing or reducing the likelihood of physical damage to the healing surfaces.

It has now been found that the above and certain other alginate fibres, having particular ratios of cations, can be used to form particularly useful dressings for the treatment of dry or substantially dry wounds such as burns.

According to a first aspect of the invention, there is provided a method of preparing a dressing for treating a wound, the method comprising wetting a pad comprising mixed salt alginate fibres which have first and second cations, the first cation being capable of forming an insoluble alginate salt and the second cation being capable of forming a soluble alginate salt, the equivalent ratio of the first to second cations being from 30:70 to 70:30.

It is preferred that the ratio of first (insolubiliεing) cation to second (solubilising) cation lie in the range of from 40:60 to 60:40. A 50:50 ratio has been found to be the most preferred.

Calcium is the preferred first cation. Other preferred first cations include zinc. The preferred second cation is sodium, because sodium alginate is readily soluble. However, any other cation whose alginate salt is soluble could be used; examples include potassium, lithium, ammonium and magnesium. It will generally be the case that there will only be two species of cations present in the mixed salt alginate fibre, but this is not necessarily the case. Other cations, such as hydrogen ions, may be present.

The pad will generally be provided in the form of a sheet. Because of the properties of the second (solubilising) cation, the sheet acquires gel-like qualities when it is wetted. Any appropriate (generally aqueous) fluid may be used to wet the pad, as long as it is suitable for being in contact with the wound. Sterile aqueous liquids are preferred. Distilled, deionised and/or pyrogen-free water and physiological or isotonic saline are the aqueous liquids of choice. The sheet may be non-woven, woven or knitted. It is particularly preferred for the sheet to be non-woven, not only for ease of manufacture but also because of the general dimensional stability of non-woven fabrics: they tend not to stretch so easily as knitted fabrics.

The preferred sheet of alginate fibres which goes to form the pad or fabric is generally formed by converting a sheet formed from insoluble fibres. However, insoluble fibres could first be converted and then the converted fibres could be formed into a sheet or other pad. For ease of handling during sheet or pad formation, it is generally preferred to leave the conversion until afterwards.

Insoluble alginate fibres, in the form of calcium alginate fibres, may be prepared by the method of Bonniksen as disclosed in GB-A-415042 and the method of Tallis as disclosed in GB-A-568117. The calcium (or other insolubilising cation) fibres thus prepared can then be converted or, for preference, formed into a sheet or other piece of fabric. If a non-woven fabric is to be prepared, and this is the fabric of choice, a cotton card may be used to form a web, which may then be cross-lapped, for example with a Garnet Bywater cross-lapper and then needle punched in a Garnet Bywater needle loom.

If a woven fabric is to be prepared, the calcium (or other insoluble) alginate fibres may be carded and then spun into a yarn, which can be woven in a conventional loom. Alternatively, the fibres may be collected in a spinning box, according to the method of Tallis (GB-A-568177) and woven. If a knitted fabric is to be prepared, the fibres can be prepared as a continuous filament yarn (again according to the method of Tallis (GB-A-568177) ) , which is then knitted on a conventional knitting machine.

Various conversion methods, to replace some of the calcium or other insolubilising cations with sodium or other solubilising cations, can be used either on a tow of fibres prior to fabric preparation or on a fabric prepared from such fibres. First, the technology as developed by Miller and Caldwell (GB-A-1231506) may be used. In one embodiment of this technique, fibres or fabric of calcium (or other insolubilising cation) alginate and water are put into a dyeing machine. Sufficient hydrochloric acid to remove the desired percentage of insolubilising cation is introduced and circulated for 30 minutes. The liquid is drained and the fibres or fabric are then washed with water until no more acidity is detectable in the wash water. The dyeing machine is then charged with 50% w/w industrial methylated spirits (IMS) in water. The hydroxide of the desired solubilising cation (for example caustic soda) is then added. In the case of caustic soda, this should be done very slowly and with circulation. Care should be taken that the pH never exceeds 10. The amount of hydroxide added should be sufficient to neutralise the acid groups on the yarn; the system is recirculated until all the hydroxide has been used. The machine is then drained and the fabric or fibres washed in 50% IMS in water. The fabric or fibres are then transferred to a centrifuge and spun dry, and then air dried at a temperature of from 30° to 40°C. A different conversion method that can be used is the method disclosed in GB-A-1394741 (Medical Alginates) , an embodiment of whiph can also use a dyeing machine or even a stainless steel washing machine. Example 12 of GB-A-1394741 illustrates a suitable embodiment in general terms for converting calcium alginate fibres to calcium:sodium mixed salt fibres. The quantity of sodium acetate trihydrate used is varied according to the desired resulting proportion of sodium ions. 12% w/v sodium acetate, calculated as the anhydride, would be used to obtain 50:50 calcium:sodium alginate; this would approximate to about 50 kg of sodium acetate trihydrate in 100 dm³ of liquor.

Alternative conversion methods that can be used are the subject of UK patent applications nos 8820292.4 and 8820293.2, both filed on 26th August 1988.

As with the fabric preparation methods, the conversion methods given above are purely illustrative, and it is to be emphasised that any suitable conversion method can be used.

The pad may include one or more antimicrobial (for example antibacterial or antifungal) agents and/or one or more local anaesthetics (for example novocain) and additionally or alternatively one or more pharmaceutical agents.

According to a second aspect of the invention, there is provided a method of dressing or treating a wound, the method comprising wetting a pad comprising mixed salt alginate fibres which have first and second cations, the first cation being capable of forming an insoluble alginate salt and the second cation being capable of forming a soluble alginate salt, the equivalent ratio of the first to second cations being from 30:70 tc 70:30 and applying the wetted pad to the wound.

According to a third aspect of the invention, there is provided the use of a pad comprising mixed salt alginate fibres which have first and second cations, the first cation being capable of forming an insoluble alginate salt and the second cation being capable of forming a soluble alginate salt, the equivalent ratio of the first to second cations being from 30:70 to 70:30 in the preparation of material for use in a method of treating a wound comprising wetting the pad and applying the wetted pad to the wound.

According to a fourth aspect of the invention, there is provided a wet pad comprising mixed salt alginate fibres which have first and second cations, the first cation being capable of forming an insoluble alginate salt and the second cation being capable of forming a soluble alginate salt, the equivalent ratio of the first to second cations being from 30:70 to 70:30.

According to a fifth aspect of the invention, there is provided a kit for treating a wound, the kit comprising a pad comprising mixed salt alginate fibres which have first and second cations, the first cation being capable of forming an insoluble alginate salt and the second cation being capable of forming a soluble alginate salt, the equivalent ratio of the first to second cations being from 30:70 to 70:30 and a container for liquid for wetting the pad.

Various exemplary embodiments of the invention will now be described.

PREPARATION 1 Manufacture of Calcium Alcrinate Fibre

6.6 kg (6.0 kg bone dry) of sodium alginate powder was dissolved in 100 dm of demineralised water containing sodium hypochlorite (60 g available chlorine) using a high speed stirrer. The excess chlorine is reduced to 25 ppm by the addition of sodium sulphite and the resultant solution was filtered to remove incompletely dissolved material. The solution was then spun through a viscose type spinning jet into a spin bath containing 1% calcium chloride and a sufficient quantity of cetyl pyridinium chloride (a quaternary ammonium wetting agent) to prevent fibre adhesion. After stretching the fibres in a steam chamber, the yarn is washed free from spin bath liquors in a conventional wash bath system, dried and collected either in a box or on a cheese winder.

EXAMPLE 1 Manufacture of non-woven 50:50 Ca:Na Fabric

26.875 kg of calcium alginate tow from the spinning machine (equivalent to 21.5 kg bone dry alginate; defined as 100 equivalents) as prepared in Preparation 1 was placed in a stainless steel centrifuge equipped with a sump tank and a pump to recirculate the liquors through the fibres. 190 dm³ of water were added to the sump tank and pump started with the centrifuge slowly revolving. 5 dm of concentrated hydrochloric acid were slowly added to the water which were circulated for 10 minutes. The liquor was titrated against EDTA to measure its calcium content. Small ad itions of hydrochloric acid were made until the calcium content of the liquors corresponded to 50% removal of the calcium originally present in the fibres (50 equivalents) . The liquor is then run to drain and the fibre washed with fresh water until no acidity is detectable in the runnings.

The sump tank is filled with 200 dm³ of 50/50 v/v industrial methylated spirits (IMS)/water. Neutralisation is effected by circulating this liquor through the fibres and slowly adding 2 kg of sodium hydroxide dissolved in 50/50 v/v IMS/water, taking care that the pH does not rise above 9.0. On completion of neutralisation a sample of the yarn gives no residual acidity.

The tow is washed in 80% v/v IMS/water and then in pure IMS; it is centrifuged as dry as possible and then dried in air at 40°C. The tow is then crimped, staple cut and converted to non-woven fabric by a conventional carding, cross-lapping and needle punching technique. EXAMPLE 2 Alternative Manufacture of non-woven 50:50 Ca:Na Fabric

Calcium alginate tow as prepared n Preparation 1, is crimped, staple cut and converted to non-woven fabric by a conventional carding cross-lapping needle punching technique. 4 kg of calcium alginate fabric (3.2 kg bone dry) were placed in a stainless steel dyeing machine having a perforated mandrel, around which a roll of calcium alginate fabric is placed. The vessel was filled with the solution consisting of 5.9 kg sodium hydroxide, 14 dm³ glacial acetic acid, 20 dm³ of IMS and made up to 100 dm³ with water. After operation for 30 minutes the liquor is drained, and the fabric washed free of acetate liquor with successive amounts of 8% v/v IMS/water. The fabric is then air dried.

EXAMPLE 3

Calcium alginate tow as prepared in Preparation 1, is crimped, staple cut and converted to non-woven fabric by a conventional carding cross-lapping needle punching technique. The fabric is fed into a bath containing sodium carbonate solution at approximately 1.5 N (79.5g/dm³ as anhydrous Na₂C0₃) . The bath is well agitated by means of a recirculation pump, and the fabric is allowed to lie in the bath for about 30 seconds. To prevent fibre adhesion, acetone (10% w/v) is present in the bath. The fabric is then washed in increasing concentrations of acetone/water before being squeezed dry and dried in a through-air drier. The resulting fibres contained calcium and sodium ions in an 50:50 equivalent basis. Calcium carbonate precipitated in the course of the reaction finds its way into a sump tank in the agitation pump circuit and is easily removed at the completion of the reaction .

EXAMPLE 4

25.8kg of calcium alginate fabric are placed in a centrifuge the outlet of which runs into a tank fitted with a pump which recirculates the liquor through the fibre in the drum. 100 litres of water are added to the tank, the pump is started and the drum slowly rotated. Sufficient hydrochloric acid is added to the liquor to remove 50% of the calcium present in the fibre. Since the initial calcium alginate in the fabric was 21.5kg, bone dry, i.e. 100 equivalents, it is necessary to remove 50 equivalents of calcium. The theoretical quantity of hydrochloric acid is 5 litres, but in practice, due to the reverse reaction of the calcium ions in the liquors considerably more than this quantity of acid is required (about 15 litres) . The calcium in the liquor is determined by EDTA titration. When the desired quantity has been removed to the liquor, the liquors are drained and the fibre washed with water until free of acid. The centrifuge is then spun to full speed to remove as much water as possible. The yarn is then neutralised using a solution of 100 litres of 20% w.w. acetone/water containing 1550g of anhydrous sodium carbonate which is circulated by means of the pump through the fibre in the slowly rotating drum. These liquors are displaced by 50% w.w. acetone water and f inal ly washed at ful l speed o f the centrifuge with pure acetone . The fabric obtained consists of 50 : 50 calcium sodium alginate .

EXAMPLE 5

A pad of calcium: sodium alginate fabric as produced in Example 1 was immersed in sterile distilled water, whereupon it gelled and thickened into a slab. The wet slab was applied to a burn on human skin and afforded excellent protection.

EXAMPLE 6

A pad of calcium: sodium alginate fabric as produced in Example 4 was immersed in sterile distilled water, whereupon it gelled and thickened into a slab. The wet slab was applied to a burn on human skin and afforded excellent protection.

Claims

l. • A method of preparing a wound dressing, the method comprising wetting a, pad comprising mixed salt alginate fibres which have first and second cations, the first cation being capable of forming an insoluble alginate salt and the second cation being capable of forming a soluble alginate salt, the equivalent ratio of the first to second cations being from 30:70 to 70:30.

2. A method as claimed in claim 1 or 2 , wherein the ratio of first (insolubilising) cation to second (solubilising) cation lies in the range of from 40:60 to 60:40.

3. A method as claimed in claim 2, wherein the ratio of first (insolubilising) cation to second (solubilising) cation is about 50:50.

4. A method as claimed in claim 1, wherein the first cation is calcium.

5. A method as claimed in claim 1, wherein the second cation is sodium.

6. A method as claimed in claim 1, wherein the pad is wetted with a sterile aqueous liquid.

7. A method as claimed in claim 6 wherein the Distilled, deionised and/or pyrogen-free water and physiological or isotonic saline are the aqueous liquids of choice.

8. A method as claimed in claim 1, wherein the pad comprises a non-woven sheet of fibres.

9. A method as claimed in claim 1, wherein the pad includes one or more antimicrobial agents and/or one or more local anaesthetics.

10. A wet pad comprising mixed salt alginate fibres which have first and second cations, the first cation being capable of forming an insoluble alginate salt and the second cation being capable of forming a soluble alginate salt, the equivalent ratio of the first to second cations being from 30:70 to 70:30.

11. A method of dressing a wound, the method comprising preparing a wound dressing as claimed in claim 1 and applying the wetted pad to the wound.

12. The use of a pad comprising mixed salt alginate fibres which have first and second cations, the first cation being capable of forming an insoluble alginate salt and the second cation being capable of forming a soluble alginate salt, the equivalent ratio of the first to second cations being from 30:70 to 70:30 in the preparation of material for use in a method of treating a wound comprising wetting the pad and applying the wetted pad to the wound.

13. A kit for treating a wound, the kit comprising a pad comprising mixed salt alginate fibres which have first and second cations, the first cation being capable of forming an insoluble alginate salt and the second cation being capable of forming a soluble alginate salt, the equivalent ratio of the first to second cations being from 30:70 to 70:30 and a container for liquid for wetting the pad.