referring to the drawings , fig1 to 4 illustrate schematically various different embodiments of wound dressings in accordance with the invention . in all of these drawings , a cross - hatched element represents a sachet of semi - permeable membrane e . g . cellulose acetate or gel slab containing an aqueous solution of glucose and potassium iodide ; an element with bold hatching lines extending from upper left to lower right represents a hydrated hydrogel slab containing glucose oxidase trapped in the gel ; and an element with bold hatching lines extending from upper right to lower left represents a hydrated hydrogel slab containing lactoperoxidase trapped in the gel . filled circles represent beadlets of alginate gel ( typically about 2 mm in diameter ) containing entrapped glucose oxidase . alternatively other gels ( e . g . agarose ) or polymers could be used to form the beadlets . glucose is able to diffuse into these beadlets and hydrogen peroxide is able to diffuse out . empty circles represent beadlets of alginate gel ( typically about 2 mm diameter ) containing entrapped lactoperoxidase . alternatively other gels ( e . g . agarose ) or polymers could be used to form the beadlets . hydrogen peroxide is able to diffuse into these beadlets and reactive oxygen species are able to diffuse out . shaded circles represent beadlets of alginate gel ( typically about 2 mm diameter ) containing entrapped glucose and potassium iodide . alternatively , other gels ( e . g . agarose ) or polymers could be used to form the beadlets . glucose is able to diffuse out of these beadlets . fig1 illustrates one preferred embodiment of wound dressing in accordance with the invention . the dressing is of layered construction and comprises an outer layer or covering 10 in the form of an oxygen - permeable self - adhesive plaster , suitable for adhering to the skin 12 of a subject , so as to cover a wound 14 . covering 10 encloses an upper layer comprising a first moist pad 16 with immobilised glucose oxidase , an intermediate layer comprising a solution of glucose and potassium iodide in a semi - permeable sachet or gel slab 18 and a lower layer comprising a second moist pad 20 with immobilised lactoperoxidase . below pad 20 is a sheet 22 of gauze , for contact with the wound 14 . the pads and sachet may be generally as described below . the dressing is initially supplied as a multi - part system , with the individual components separately packaged in respective sealed , sterile packages . when required for use , the dressing components are removed from the packages and applied to a wound in appropriate manner and order to produce the final dressing as shown . fig2 illustrates another preferred embodiment of wound dressing , generally similar to fig1 , with similar components being identified by similar reference numerals . in this embodiment the upper layer comprises a first moist pad 24 with calcium alginate beads containing entrapped glucose oxidase . the intermediate layer comprises a pad 26 with gel beads containing glucose and potassium iodide . the lower layer comprises a second moist pad 28 with calcium alginate beads containing entrapped lactoperoxidase . fig3 illustrates a further generally similar embodiment , but comprising an upper layer in the form of a pad 30 with gel beads containing glucose and potassium iodide , and a lower layer comprising a moist pad ( or gel slab ) 32 with calcium alginate beads containing entrapped glucose oxidase and calcium alginate beads containing entrapped lactoperoxidase . fig4 is a variant of fig3 in which the upper layer comprises a semi - permeable sachet or gel slab 34 containing glucose and potassium iodide . the enzyme , either lactoperoxidase ( lpo ) ( from sigma , cat no l2005 ) or glucose oxidase ( gox ) ( from boehringer mannheim , cat no . 105147 ) is dissolved in pure water at the rate of 1 microgram per ml lpo ) or 10 micrograms per ml ( gox ). a solution of alginic acid ( manucol dm ( manucol dm is a trade mark ) from c p kelco ) ( 1 gram per 100 ml of water ) is prepared at elevated temperature and cooled . enzyme solution is mixed with the cooled alginic acid at the appropriate rate . the resulting alginic acid / enzyme solution is then pumped through a peristaltic pump into a tube which leads to an exit nozzle conveniently formed by a standard laboratory glass pasteur pipette , placed over a setting bath of calcium chloride solution ( 10 % w / v ). the flow of the pump and height of the exit nozzle are adjusted so that the outflowing stream of alginic acid / enzyme solution forms into discrete droplets as it enters the calcium chloride solution . each droplet rapidly starts to solidify as the calcium begins to cross - link the alginic acid molecules and , 10 minutes after the delivery of the last droplet , the setting process is complete . all the newly formed beadlets are removed from the calcium chloride solution by pouring the whole through a sieve of suitable mesh size . residual calcium chloride is removed by rinsing with pure water . the beadlets are stored in water or in water - tight containers , or in a physiological buffer solution such as phosphate buffered saline . alternatively , they can be placed in glycerol or glycerol in water solutions to decrease the entrained water content . whatever storage conditions are used , they must not be allowed to dry out or harden in the absence of water or glycerol . the method described above is followed , except that no enzymes are added and glucose is included at the rate of 12 . 5 g per 100 ml in both the pure water and the calcium chloride setting bath . the gel beadlets are washed with calcium - free glucose solution , allowing removal of excess calcium without depleting the glucose . appropriately sized cotton lint is placed on a suitable surface through which water can flow ( e . g . a flat plastic mesh ). a suspension of enzyme - beadlets is poured onto the cotton lint , in such a way that the beadlets become entrapped in the raised nap of the fabric , as the suspending water flows away . a second piece of lint is then placed over the first , so as to sandwich the enzyme beadlets between the two fabric layers . adhesive or stitching or stapling may be used to secure the top layer to the bottom layer . excess fluid is drained away , but the pad is not allowed to dry out . the number of beadlets contained per pad should be determined on the basis that each oxidase pad should carry about 100 mg of gox , and each peroxidase pad should carry about 10 mg of lpo . the method described above is used with glucose - containing beadlets to prepare a pad containing entrapped glucose , except that an extra step is included at the end of the process , in which the pad is soaked in a solution of potassium iodide ( 10 mm ). glucose is dissolved in a 5 mm aqueous solution of potassium iodide at the rate of 12 . 5 g per 100 ml . this solution is then placed in a dialysis bag ( previously placed in boiling water for 10 mins and thoroughly rinsed ) with an accessible area of about 40 × 20 nm , and sealed . a 1 % aqueous solution of agarose is prepared , with potassium iodide added at a concentration of 10 mm and soluble starch at concentration of 1 % w / v . the solution is melted and dispensed into a petri dish to form a continuous layer about 5 mm thick , and allowed to set . once set a peroxidase pad as described above is laid on the surface , followed by a glucose pad or sachet as described above laid on top of that , and finally a glucose oxidase pad as described above is laid on top of them both to form a three - layered stack . the development of a blue colour within the starch agar indicates the oxidative activity of the composite dressing . the pads or sachets for this purpose prepared as described above are sealed in appropriate pouches and subjected to gamma irradiation to ensure microbiological sterility , using techniques known and routinely used in the industry . firstly the wound is covered by a thin sheet of sterile gauze . next the three layers of sterile pads are added as a thin stack , with the peroxidase pad first followed by the glucose pad or sachet next and the glucose oxidase pad last . the pads are cut to a size that just covers the open wound . finally , the composite dressing is preferably held in place by adhesive film , such as normal “ sticking plaster ” or “ micropore ” surgical tape . the enzymes can be immobilised on various types of particle or fibre , using coupling methods known by those skilled in the art . the particles can be made of cellulose , silica or various harmless polymers . alginate can be used in forms other than beadlets , such as slabs or extruded threads , still using calcium as a setting agent . microcapsules , such as those made by known techniques with polyamide , can be used to encapsulate each of the components . loose woven cotton gauze is cut into a number of pieces approximately 100 mm by 100 mm and each piece is laid into a respective suitable flat bottomed container . 1 % w / v alginic acid ( also termed “ alginate ”; e . g . manucol dm ( manucol dm is a trade mark ), from cp kelco ) is prepared by dissolving the gel into an appropriate , heated aqueous solution . after cooling , enzyme is added to the alginate , to give a final concentration of 5 μg / ml glucose oxidase ( gox , from boehringer mannheim , cat . no . : 105147 ) or 10 μg / ml lactoperoxidase ( lpo , from sigma , cat . no . : l2005 ). 10 mls of each enzyme - alginate solution is prepared , and poured evenly onto the individual cotton gauze pads . the gel is set by the addition of excess 10 % w / v calcium chloride ( cacl 2 ) and allowed to stand for 10 minutes . the pads are then washed twice for 5 minutes each , in excess distilled / deionised water to remove the cacl 2 . the pads can then be stored in a humid environment to prevent drying out . loose woven cotton gauze is cut to approximately 100 mm by 100 mm and is laid into a suitable flat bottomed container . 1 % w / v agarose and 40 % w / v glucose is dissolved into an appropriate aqueous solution and poured onto the gauze while still molten . the gel is allowed to set by cooling . the gel pad can then be stored in a humid environment to prevent drying out . to produce a pad containing glucose and potassium iodide , a similar procedure is followed , but after the glucose and agarose have been dissolved , potassium iodide ( ki ) is added to a final concentration of 10 mm . the solution can then be poured , allowed to set and stored as above . using standard practices , a 1 . 5 % agarose microbial growth plate is prepared , but with the glucose replaced by fructose as the sole carbon source . the plates are typically in the region of 5 mm thick . pseudomonas aeruginosa was spread over the surface of the plate in an even “ lawn ”. a pad containing lpo - alginate prepared as described immediately above ) of approximately 20 mm 2 was placed onto the surface of the plate . placed directly onto this pad is a 20 mm 2 pad containing glucose - agar ( prepared as described immediately above ) ( with or without ki ). finally , a 20 mm 2 pad containing gox - alginate ( prepared as described immediately above ) was layered onto the glucose - agar pad . a clearance zone around the pad stack can be clearly seen after 24 hours , showing the production and diffusion of active anti - microbial species , preventing the growth of the applied bacteria . removal of any one of the 3 pads ( control experiments ) results in no clearance zone around the pad stack , showing that both enzymes and glucose all need to be present for the cascade to progress effectively . alternatively , to show the production of reactive oxidative species via the stacked enzyme system , a 1 % w / v agar plate is cast , that includes 1 % soluble starch ( e . g . arcos cat . no . : 177132500 ) and 10 mm ki . the plate is allowed to set by cooling . the lpo - alginate pad , followed by the glucose - agar pad and finally the gox - alginate pad can then be stacked as described above in example 1 . the production of the reactive oxidative species can be then visualised by the intense blue coloured chromogen produced by the well documented interaction of elemental iodine ( the oxidative species oxidise the iodide to iodine ) and starch . this coloration can be clearly seen after 5 minutes , with the intensity and spread building over time . after 30 minutes , the colour intensity has built to become a deep blue , indicating continued product formation . this shows that both reactive oxidative species and iodine are produced , both of which aid in the anti - microbial activity of the composite dressing . a variation on example 2 is to include a low level of gox ( 0 . 25 μg / ml ) in the lpo - alginate pad , to promote an initial production of reactive oxidative species . this utilises oxygen available in the gel , and is initiated by the glucose that is diffusing into the lpo - alginate pad . this reaction is limited , due to the availability of oxygen and will cease when the oxygen is depleted . to show the production of reactive oxidative species via the stacked enzyme system , a 1 % w / v agar plate is cast , that includes 1 % soluble starch ( e . g . arcos cat . no . : 177132500 ) and 10 mm ki . the plate is allowed to set by cooling . the combined lpo and gox - alginate pad , followed by the glucose - agar pad and finally the gox - alginate pad can the be stacked as described above . the accelerated production of the reactive oxidative species can be then visualised by the intense blue coloured chromogen produced by the well documented interaction of elemental iodine ( the oxidative species oxidize the iodide to iodine ) and starch . this coloration can be clearly seen after as little as 1 minute , with the intensity and spread building over time . the coloration will slow down noticeably after 15 minutes if the top gox - alginate pad is not used . if the gox - alginate pad is used as described above , the coloration will continue strongly , as seen in example 2 above . after 30 minutes , the colour intensity has built to become a deep blue , indicating continued product formation . this shows that both reactive oxidative species and iodine are produced , both of which aid in the anti - microbial activity of the composite dressing . further work was carried out using relatively high concentration xanthan gum gels . a series of solutions of xanthan gum ( keltrol ) were prepared at different concentrations ( 5 %, 10 % and 20 % by weight ) by dissolving appropriate amounts of keltrol into distilled / de - ionised water at room temperature ( about 20 ° c .). a series of solutions of alginic acid ( manucol dm ( manucol dm is a trade mark ) from cp kelco ) at different concentrations ( 2 % and 4 % by weight ) were also prepared by dissolving appropriate amounts of manucol dm into distilled / de - ionised water at appropriate elevated temperature . the resulting alginic acid solutions were cooled . a series of xanthan gum / alginate gels including the two materials in different proportions were prepared by mixing the solutions in appropriate quantities until a homogeneous mixture was obtained . for example , gels were prepared having a weight ratio of xanthan gum to alginate of 5 : 1 ( e . g . by mixing equal amounts of 5 % and 1 % solutions or 10 % and 2 % solutions ), 5 : 2 , 10 : 1 etc . as required . enzyme - containing gels were prepared by the addition of the appropriate quantity of glucose oxidase ( gox ) or lactoperoxidase ( lpo ) enzymes to the gel samples . enzyme solution may be added either to a xanthan gum or alginate solution prior to mixing , or to a xanthan gum / alginate gel after mixing . experiments were carried out using lpo from sigma , ( catalogue number l2005 ) dissolved in pure water at the rate of 1 microgram per millilitre and gox from boehringer mannheim ( catalogue number 105147 ) dissolved in pure water at the rate of 10 micrograms per millilitre . construction of cross - linked xanthan gum / alginate gel pads supported by cotton gauze a 10 % xanthan gum 2 % alginic acid gel mixture was prepared by mixing equal volumes of stock 20 % xanthan gum solution and 4 % alginic acid solution , prepared as described above . lpo and gox were added to separate samples of this gel mixture to produce an lpo - containing gel and a gox - containing gel . enzyme levels can be varied according to the required level of activity . in this case enzyme levels used were 100 μg / ml for lpo and 50 μg / ml for gox . approximately 5 ml of the lpo - containing gel was smoothed onto a strip of cotton gauze approximately 40 mm × 50 mm . a second layer of gauze of similar size was placed on top of the lpo gel with light pressure to ensure an even distribution of the gel . approximately 5 ml of the gox - containing gel was then dispensed onto the upper face of the second layer of cotton gauze , with a third layer of gauze of similar size applied on top of the gox - containing gel , again with light pressure to ensure an even distribution of the gox - containing gel . this produced a pad or slab of sandwich construction comprising three sheets of cotton gauze separated , respectively , by a layer of lpo - containing gel and a layer of gox - containing gel . the resulting pads were placed in a bath of calcium chloride solution ( 10 % w / v ) for 10 minutes . the pads were removed from the bath and then washed to remove residual calcium chloride by two 10 minute washes in distilled / de - ionised water . excess water was removed by drying the pads for several minutes in absorbent tissue . the starch / iodine complex reaction was used to visualise the enzymatic production of reactive oxidative species ( ros ) from within the cross - linked xanthan gum / alginate gel pad , prepared as described previously . in the presence of ros , iodide is oxidised to produce elemental iodine , which complexes with starch to produce an intense blue chromogen . experiments were carried out using a 1 % w / v agar plate that includes 1 % soluble starch ( arcos catalogue number 177132500 ) and 10 mm potassium iodide . a slice of the cross - linked xanthan gum / alginate pad as described above ( approximately 20 mm × 10 mm × 4 mm ) was placed on the starch / iodide plate , with the gox layer uppermost . on top of this was placed a pad of gel comprising 40 % glucose in 1 % agar to initiate the reaction . after about 30 minutes the lower pad had turned yellow , clearly indicating the production of elemental iodine ( iodide being present due to diffusion from the plate into the gel ). after about 1 hour , blue staining was visible beneath the pad , indicating the presence of ros within the starch / iodide plate . 24 hours after initiation , a large area of the plate had turned blue , indicating continued production of ros . two similar experiments were carried out using an arrangement generally as described above in example 4 but with the glucose pad below the cross - linked xanthan gum / alginate pad . two experiments were carried out , with the xanthan gum / alginate gel pads in different orientations , in one case with the gox layer uppermost and with the other with the lpo layer uppermost . with the pad positioned with the gox layer lowermost , next to the glucose pad , after approximately 1 hour blue staining could be seen under the pad . with the other experiment , with the lpo layer lowermost , next to the glucose pad , after about 2 hours blue staining could be seen under the pad . this shows that even with an arrangement using a pad with enzymes in non - optimal sequence , ros can still be produced in sufficient quantity to produce an oxidative effect in the indicator plate . in both experiments , yellow coloration was seen in both gels , in the lpo layers , showing the presence of iodine production . after 24 hours , in both cases much of the starch / iodide plate had turned blue , again showing the production of ros . a 10 % by weight preparation of keltrol in water was prepared as described above . 1 . 06 g of the 10 % gel was added to 10 . 22 g of distilled / de - ionised water , maintained at 21 ° c . and sealed to minimise evaporation effects . the water was decanted off and measured by weight to assess the degree of absorbency . after 15 minutes , 8 . 34 g of water remained ; after 45 minutes , 7 . 56 g remained ; after 2 hours 6 . 51 g remained ; after 18 hours , 2 . 1 g remained . this experiment shows that the 10 % keltrol gel is able to absorb at least 8 times its weight in water . demonstration of the moisture absorbency of a cross - linked xanthan gum / alginate gel mixture a mixture of 5 % by weight keltrol and 1 % by weight alginic acid was prepared as described above . two layers of gel were cast between two layers of cotton gauze to produce a pad that was cross - linked using a 10 % calcium chloride solution and washed twice for 10 minutes in water , as described above . the pad was blotted dry using absorbent tissue paper . a piece of the pad 10 mm × 10 mm × 3 mm was weighed to establish the initial weight ( 0 . 28 g ). 1 ml of water was added to the pad . the pad was removed and weighed after various times to assess the increase in weight ( due to absorption of water ). after one 1 hour the weight of the pad was 0 . 37 g ; after 2 hours 0 . 44 g ; after 3 hours 0 . 49 g and after 6 hours 0 . 5 g . this experiment shows that the cross - linked xanthan gum / alginate gel is still able to absorb water . compared with the example above using 10 % keltrol , use of a lower percentage of keltrol and cross - linked alginate results in less water absorbency . cross - linked xanthan gum / alginate gels prepared as described above were used in the production of a wound dressing in accordance with the invention , as shown schematically in fig1 . such gels were used together with a glucose / iodide containing sachet prepared as described above . in particular , glucose was dissolved in a 5 mm aqueous solution of potassium iodide at the rate of 12 . 5 g per 100 ml . the solution was placed in a dialysis bag ( previously placed in boiling water for 10 minutes and thoroughly rinsed ) with an accessible area of about 40 × 20 mm , and sealed . the components were assembled to form a dressing of the construction shown in fig1 . the dressing is of layered construction and comprises an outer layer or covering 10 in the form of an oxygen - permeable self - adhesive plaster , suitable for adhering to the skin 12 of a subjects so as to cover a wound . covering 10 encloses an upper layer comprising a pad of glucose oxidase - containing cross - linked xanthan gum / alginate gel , prepared as described above ; an intermediate layer comprising a solution of glucose and potassium iodide in a semi - permeable sachet 18 , prepared as described above ; and a lower layer comprising a slab of lactoperoxidase - containing cross - linked xanthan gum / alginate gel prepared as described above . below pad 20 is a sheet 22 of gauze , for contact with the wound 14 . the dressing is initially supplied as a multi - part system , with the individual components separately packaged in respective sealed , sterile packages . when required for use the dressing components are removed from the packages and applied to a wound in appropriate manner in order to produce the final dressing as shown . fig5 illustrates schematically a further embodiment of dressing in accordance with the invention . this form of dressing is currently favoured . the dressing has major dimensions of 100 mm × 100 mm , in the form of a square . the illustrated dressing comprises a soft , hydrated hydrogel slab 50 composed of polyamps ( as described in wo 01 / 96422 supplied by first water ltd ). the hydrogel contains up to 23 % glucose ( which acts as a substrate for oxidoreductase enzyme ) and iodide salts e . g . 1 . 6 % w / v potassium iodide ( which are a precursor to iodine ). the hydrogel may also contain up to 20 % w / v glycerol ( which acts as a humectant and moisturiser ). the hydrogel slab 50 forms the lower layer of the dressing . the dressing also comprises an upper layer constituted by a cellulose woven gauze 52 to which are irreversibly ( covalently ) bound glucose oxidase and lactoperoxidase . the gauze 52 is prepared as follows . the gauze is cut to a suitable size and shape ( a square 100 mm × 100 mm ), and is washed in water to remove any solutes or particulates . excess fluid is removed . the gauze is then soaked in a solution of 10 mm sodium meta - periodate for 60 minutes at 25 ° c . after this oxidation step , where reactive aldehyde groups are formed , the gauze is extensively washed in water to remove the periodate . after the wash step , the gauze is soaked in a solution of glucose oxidase ( biocatalysts — code g638p ), at 100 μg powder per ml 50 mm sodium hydrogen carbonate at ph 9 . 0 . this is equivalent to 7000 u / ml . lactoperoxidase ( dmv international ) is also incorporated at 100 μl powder per ml 50 mm sodium hydrogen carbonate at ph 9 . 0 . these doses can be reduced , since they represent an excessive amount of activity . the gauze is left to react with the enzyme solution for 4 hours at about 20 ° c ., after which the gauze is removed and washed extensively between low ionic strength solution ( deionised water ) and high ionic strength buffer ( 50 mm nah 2 co 3 ph 9 . 0 + 1m nacl ) to remove loosely bound enzyme . the gauze is then coated in a preservative , for example 5 % pva , 10 % sucrose or 2 % gelatin and dried either at room temperature ( approximately 21 ° c .) or , preferably , at 40 ° c . the lower and upper layers are assembled with each other in a nitrogen atmosphere ( to prevent premature reaction ), and are packaged together by being sealed in an oxygen - impermeable pouch or enclosure , e . g . made of laminated aluminium foil pouches as supplied by sigma ( code z183407 ). the interaction of the glucose in the gel with the immobilised glucose oxidase is limited by the rate at which the glucose can diffuse into the immobilised - enzyme layer . this delay is sufficient to allow the two layers to be assembled together in the presence of oxygen , and then placed in an oxygen - free package , before any substantial reaction can take place . once oxygen is excluded from the assembled product , the reaction is arrested anyway , and it can only resume when the oxygen supply is renewed ( e . g . when the product is removed from the packaging for use on a wound ). this oxygen deprivation within the packaging prevents the glucose from being used up in premature peroxide production . the enzymes , while initially in dry form on the gauze on assembly , become hydrated with water from the hydrogel slab 52 and are maintained in hydrated condition by water from the hydrogel slab 52 while sealed in the pouch or enclosure prior to use . in use , the dressing is removed from the pouch or enclosure and placed on the skin of a patient over a wound site , shown schematically at 54 , with the lower hydrogel layer 50 in contact with the skin . an oxygen - permeable and moisture - permeable covering or overlayer 56 ( which may or may not form part of the dressing ) is located over the upper layer 52 and is adhered to the skin surrounding the wound site by means of suitable adhesive provided on the lower face of overlayer 56 . in this way the dressing is retained in position on the skin , covering the wound site . the glucose oxidase in the upper layer ( which is in hydrated condition ) catalyses reaction of the glucose in the lower layer with oxygen that passes through the overlayer 56 from the surroundings , producing hydrogen peroxide as discussed above . the hydrogen peroxide itself has beneficial antimicrobial effects , as discussed above . the hydrogen peroxide also undergoes a further reaction catalysed by the lactoperoxidase to produce a variety of species with antimicrobial properties , as discussed above . in addition , the iodide salts in the lower layer react to produce elemental iodine and possibly also hypoiodous acid , further powerful antimicrobial agents , as discussed above . the dressing thus has a number of very effective mechanisms for in - situ production of antimicrobial agents that can be very effective in promoting wound healing . further , the hydrogel of the lower layer is highly fluid absorbent , allowing the take - up of high volumes of exudate from the wound . fluid exuded from the wound , including potentially harmful bacteria etc , can thus be absorbed into the dressing and killed by the antimicrobial species generated therein . the dressing can thus be self - sterilising . the hydrogel is also able to donate fluid , should the wound require it , thus allowing the wound site to be kept moist at all times aiding the healing process . in a modification of the fig5 embodiment described above , no lactoperoxidase is present . in this case , although the hydrogen peroxide produced by the reaction catalysed by the glucose oxidase does not undergo lactoperoxidase - catalysed reaction , nevertheless useful amounts of the hydrogen peroxide spontaneously undergo non - catalysed reaction with iodide ions to generate molecular iodine and valuable antimicrobial effects are nevertheless still obtained . fig6 illustrates schematically yet a further embodiment of dressing in accordance with the invention . this form of dressing is currently most favoured . the dressing has major dimensions of 100 mm × 100 mm , in the form of a square . the illustrated anti - bacterial and anti - fungal wound dressing comprises a glucose - containing hydrogel slab 60 as the lower layer of the dressing . cast onto the upper surface of slab 60 is a film 62 of pva ( polyvinyl alcohol ) that incorporates glucose oxidase . the hydrogel lower layer 60 was formulated to include the following reagents by weight : 20 % sodium amps ( 2 - acrylamido - 2 - methylpropanesulfonic acid , sodium salt ( lubrizol , code 2405 )), 20 % glucose ( fisher , analytical grade ), 10 % glycerol ( fisher , analytical grade ), 50 % deionised water , 0 . 1 % poly ethylene glycol 400 diacrylate ( ucb chemicals ) and 0 . 01 % photoinitiator ( 1 - hydroxycyclohexyl phenyl ketone ( aldrich )). the mixture was dispensed into casting trays , to a depth of 2 - 3 mm . the hydrogel was then set , by irradiation under a uv lamp , for up to 60 seconds and a power rating of approx . 100 mw / cm 2 . the hydrogel was then allowed to cool to 30 ° c . or below . the enzyme - containing pva film 62 was prepared by dissolving high molecular weight pva ( 124 , 000 - 186 , 000 mw , aldrich ) in water by heating the mixture . the pva was incorporated to a final concentration of 6 % w / v . once dissolved , the solution was allowed to cool to 30 ° c . or below , before enzyme ( glucose oxidase ( gox , biocatalysts g638p )) was added , to a concentration of 100 μg / μl ( weight freeze dried powder per volume ). 50 - 100 μl of the pva / gox mixture was then added to a 20 mm 2 surface of the cooled hydrogel , and allowed to set . a thin film was formed after approximately 30 mins . to prevent enzyme activation , the addition of the pva / gox solution to the glucose hydrogel may be carried out in an oxygen - free atmosphere ( e . g . under nitrogen ). when film 62 contacts the hydrogel 60 , most of the water from the film is drawn into the hydrogel , leaving the pva as a moist membrane containing the enzyme in hydrated condition . sufficient water remains in the film for the pva to be hydrated and to remain flexible . the resulting dressing is packaged in an oxygen - impermeable pouch or enclosure , e . g . made of laminated aluminium foil pouches as supplied by sigma ( code z183407 ). the interaction of the glucose in the gel with the immobilised glucose oxidase is limited by the rate at which the glucose can diffuse into the immobilised - enzyme layer . this delay is sufficient to allow the two layers to be assembled together in the presence of oxygen , and then placed in an oxygen - free package , before any substantial reaction can take place . once oxygen is excluded from the assembled product , the reaction is arrested anyway , and it can only resume when the oxygen supply is renewed ( e . g . when the product is removed from the packaging for use on a wound ). this oxygen deprivation within the packaging prevents the glucose from being used up in premature peroxide production . the enzyme is maintained in hydrated condition while sealed in the pouch or enclosure prior to use . in use , the dressing is removed from the pouch or enclosure and placed on the skin of a patient over a wound site , shown schematically at 64 , with the lower hydrogel layer 60 in contact with the skin . an oxygen - permeable and moisture - permeable covering or overlayer 66 ( which may or may not form part of the dressing ) is located over the film 62 and is adhered to the skin surrounding the wound site by means of suitable adhesive provided on the lower face of overlayer 66 . in this way the dressing is retained in position on the skin , covering the wound site . the glucose oxidase in the film 62 ( which is in hydrated condition ) catalyses reaction of the glucose in the lower layer with oxygen that passes through the overlayer 66 from the surroundings , producing hydrogen peroxide as discussed above . the hydrogen peroxide has beneficial antimicrobial effects , as discussed above , and the oxygen released when it is decomposed by endogenous catalase aids the healing process by supporting cellular metabolism , potentiating amino acid hydroxylation and inhibiting the growth of anaerobic bacteria . to demonstrate the generation of oxidative species , an indicator plate consisting of 1 % starch ( aldrich ), 100 mm potassium iodide ( fisher ) and 1 % agar ( sigma ) was used . the dressing comprising hydrogel 60 with pva / gox film 62 was placed onto the indicator plate , in air , with the gox thus being activated through the available oxygen . hydrogen peroxide is produced in sufficient quantity to be able to diffuse through the hydrogel and reach the indicator plate below . the oxidative power of the hydrogen peroxide then oxidises iodide to iodine , which complexes with the starch to form an intense dark blue complex . by removing the activated hydrogel dressing and placing it onto new indicator plates at 24 hour intervals , sustained hydrogen peroxide release can be demonstrated over a period of at least 5 days . in addition , to demonstrate further the stability of the gox enzyme in the pva film , the film was removed from a hydrogel after 4 days use , and placed onto fresh 20 mm 2 glucose hydrogel and placed onto an indicator plate . after 24 hours , the intense blue starch / iodide complex was clearly visible , indicating enzyme activity was still present in the pva film . to demonstrate the localised gox activity in a glucose - free hydrogel , a pva / gox film was produced as described above . the hydrogel was placed onto a bed of pbs ( phosphate buffered saline ) saturated cotton gauze , and allowed to swell slowly over 24 hours . the hydrogel was then sliced into two , through the horizontal plane , and bathed in a solution of 1 % starch + 100 mm potassium iodide + 1 % w / v glucose + 5 mm edta + 50 μg / ml lactoperoxidase . very quickly , the presence of gox can be localised , by the detection of hydrogen peroxide . the gox activity is clearly localised to the pva film and the contact surface of the hydrogel where the pva film was cast . beneath this , there was no colour generation , thus showing that gox is not mobile in the amps hydrogel , even in a swollen state . the hydrogel slab 60 is highly fluid - absorbent , and so has the properties and benefits describes above in connection with the fig5 embodiment .
a skin dressing , particularly a wound dressing , comprises oxidoreductase enzyme and , optionally , peroxidase enzyme , wherein the enzyme are present in hydrated condition , e . g . being present in one or more hydrated hydrogels . the dressing is used by being located on the skin of a human or animal e . g . over a wound . the oxidoreductase enzyme catalyses a reaction that produces hydrogen peroxide from an appropriate substrate , the substrate either being naturally present in body fluids and / or being supplied separately and / or being incorporated into the dressing . the currently preferred oxidoreductase enzyme is glucose oxidase . the catalyses reaction of β - d - glucose substrate to give hydrogen peroxide and gluconic acid . a mixture of oxidoreductase enzyme can undergo reaction to produce a variety of species including reactive oxygen intermediates that have antimicrobial properties and that can therefore assist in promoting wound healing .