Dehydrated hydrogels

[0002] This application is filed under 35 USC §371 and is based on PCT/GB95/02543, filed Oct. 30, 1995.

[0003] The present invention relates to dehydrated hydrogels which are useful in the treatment of wounds.

[0004] A hydrogel is a cross-linked macromelecular network swollen with water or biological fluids. A dehydrated hydrogel is a cross-linked macro-molecular network that will swell to form a hydrogel upon contact with water or biological fluids. Due to their ‘dehydrated’ condition, dehydrated hydrogels are easy to store and transport. In addition, when applied in the dry state to a wound they behave as superabsorbent materials.

[0005] According to a first aspect of the present invention there is provided a dehydrated hydrogel incorporating a plasticiser and fibres which have provided cations for cross-linking the dehydrated hydrogel.

[0006] According to a second aspect of the present invention there is provided a method of producing a dehydrated hydrogel comprising dispersing fibres into an aqueous solution of a hydrogel precursor material incorporating a plasticiser, the fibres incorporating cations which are capable of cross-linking said precursor material to form a hydrogel, and evaporating water to produce a dehydrated hydrogel which incorporates said fibres, the dehydrated hydrogel being cross-linked by said cations.

[0007] The dehydrated hydrogel may be in the form of a film having a thickness of, for example, 20 microns to 1 mm.

[0008] The dehydrated hydrogels of the invention have a number of advantages. In particular, the presence of the fibres imparts strength and dimensional stability to the dehydrated hydrogel. Furthermore films of the dehydrated hydrogels have the property of swelling in only the thickness dimensions and not in the other two dimensions (as compared to films of conventional dehydrated hydrogels which swell in all three dimensions).

[0009] Typically, dehydrated hydrogels in accordance with the invention will comprise (based on the total weight of the fibres, polymer forming the hydrogel, and plasticiser, i.e. excluding water and other components) 15 to 40% by weight of fibres, 10 to 35% by weight of polymer, and 5 to 75% plasticiser. More preferably the fibres and polymer together provide about 40-60% ideally about 50% by weight on the same weight basis so that correspondingly the plasticiser provides 60-40%, ideally about 50%. Generally the amount of fibres will exceed the amount of polymer. For example the weight ratio may be 1.5-3:1. Typically the dehydrated hydrogel will contain less than 50% by weight of water, ideally less than 20%, based on the total weight of the dehydrated hydrogel.

[0010] Examples of hydrogel precursor material which may be used include sodium alginate, sodium carboxymethyl cellulose, sodium pectinate, sodium O-carboxymethyl chitosan (OCC), sodium N,O-carboxymethyl chitosan (NOCC), sodium polyacrylate, and naturally occurring gums and synthetic polymers containing pendant carboxylic acid groups.

[0011] The hydrogel precursor may consist wholly or partially of acemannan (or other component of Alloe Vera) which is a natural polymer known to accelerate healing of wounds. The acemannan may, for example, provide up to 80% of the matrix. The acemannan may be clinical grade material obtainable from Carrington Laboratories, Dallas, Tex., U.S.A.

[0012] The fibres which are used contain a di- or higher valent cation which is effective for cross-linking the hydrogel. Examples of suitable cations include Ca²⁺, Zn²⁺, and cations which also act as enzyme cofactors. Particular preferred examples of fibres which may be used are calcium alginate fibres. The fibres will generally have a length of 1 to 80 mm and a thickness of 10 to 50 microns.

[0013] The fibres may be such that they absorb water from the aqueous solution of the hydrogel precursor material during manufacture of the dehydrated hydrogel.

[0014] Examples of suitable plasticisers include glycerol, polyethylene glycol, sorbitol and similar sugars, and PLURONIC® brand PEO/PPO polymers.

[0015] In a typical method of preparing a dehydrated hydrogel in accordance with the invention, the fibres, polymer and plasticiser in their relative requisite amounts are admixed with water such that the fibres, polymer and plasticiser together provide less than 5% by weight (e.g. less than 3%, e.g. 2%) of the resultant mixture. After thorough mixing, the dispersion may be cast to an appropriate thickness and water evaporated to give a dehydrated hydrogel product containing less than 50% water, more usually 20% or less.

[0016] Dehydrated hydrogels in accordance with the invention have a number of advantages. In particular when applied to the wounds (e.g. donor sites, abrasions, dermabrasions, surface wounds with high exudate or wide savings in exudate levels) they are capable of absorbing large amounts of exudate, e.g. up to 30 times their own weight, thereby rehydrating to form a hydrogel. If the dehydrated hydrogel is in the form of a film, it is found that the film swells in the thickness dimension without substantial swelling in the other two dimensions. Upon sufficient absorption of exudate, the film is capable of dissolving. The product of the invention is more absorbent than current commercial hydrogels, and is also light and easy to package.

[0017] Dehydrated hydrogels in accordance with the invention may be laminated to hydrophilic films which have an increased breathability in the presence of liquid water as compared to moisture vapour alone. The use of such a film over the dehydrated hydrogel (i.e. on the side remote from the wound) ensures that water is vented from the dehydrated hydrogel through the film. Therefore the dissolution of the hydrogel may be controlled.

[0018] Typically the breathable film will be of a material which, as a 50 micron film, has an Moisture Vapor Transfer Rate in the presence of moisture vapour alone of 6,000 to 10,000 g m⁻²24 hr⁻¹as measured by ASTM E96B and an MVTR in the presence of liquid water (as measured by ASTM E96BW) of 6,000 to 10,000 g m⁻²24 hr⁻¹. Typically the breathable film will have a thickness of 30-70 microns, more preferably 40-60 microns, e.g. about 50 microns.

[0019] The breathable film may for example be of polyurethane. Suitable films are available from Innovative Technologies Limited under the designations IT325, IT425 and IT625.

[0020] If desired, the dehydrated hydrogel may incorporate an active agent (e.g. an antimicrobial material) for delivery to a wound.

[0021] The invention will be further described by the following non-limiting Examples.

[0022] 7 kg of calcium alginate fibres having a length of 3-6 mm were dispersed in 500 liters of water. Separately, a solution of 3 kg sodium alginate in 60 liters of water was prepared and 10 kg glycerol added thereto. The sodium alginate/glycerol solution and the fibre dispersion were thoroughly mixed and the resultant admixture made up to 1,000 liters.

[0023] The mixture was cast at 4 kg/m²to give a thickness of 4 mm. Drying was then effected in an air current under IR at 30° C. so that the film contained less than 50% of water.

[0024] The resultant product was a dehydrated hydrogel which was capable of absorbing at least 10 times its own weight of water.

[0025] 1 g calcium alginate fibre (chopped into 20 mm lengths) was dispersed into an aqueous medium containing 0.5 g NOCC and 1.5 g glycerol. The resultant mixture was then cast into a 16×23 cm stainless steel dish and the solvent evaporated overnight in an oven at 80° C.

[0026] The resultant film had the properties of both NOCC and alginate fibres.

[0027] 1 g of calcium alginate fibre (chopped into 20 mm length) was dispersed into 100 ml distilled water. Separately, 0.5 g of sodium alginate powder (Protanal LF10/60 from Pronova Biopolymers) were mixed with 100 ml of distilled water. A third 100 ml portion of distilled water was mixed with 0.15 g acemannan powder (from Carrington Laboratories).

[0028] The three liquid were then mixed together with 1.65 g of glycerol. After thorough mixing, the mixture was cast into a 16×23 cm stainless steel disc and the solvent evaporated overnight in an oven at 80° C.

[0029] The resultant film comprised a dehydrated hydrogel having the wound healing properties of the acemannan.