This invention relates to the culturing of mammalian anchorage dependent cells onto a conformable substrate. More particularly the invention relates to the formation of wound dressings suitable for treating for example partial thickness wounds such as burns and skin graft donor sites and to systems for use in the preparation of such dressings.

Mammalian cells that are incapable of proliferating in suspended liquid culture but can be made to proliferate on the surface of a substrate are said to be anchorage-dependent.

Epithelial cells, such as keratinocyteε, are anchorage dependent. Such cells cultured in the presence of a substrate which is non-inhibitory and non-cytotoxic will multiply in stratified colonies and eventually produce a confluent layer. Cell cultures of this type are used to investigate skin growth and have been used as skin grafts. Various technical papers have been published which describe in vitro techniques for growing skin cells and their subsequent use in the

treatment of full-thickness wounds. For example E. Bell et al (J Invest Derm 81; 2s-10ε 1983); E. Bell et al (Science 211; 1052-1054 1981); D. Aεselineau and M. Pruneiras (Br J Derm 1984 III, Supplement 27, 219-222) and J.F. Burke et al (Ann Surg 94; 413-428 1981).

The ability of cells to anchor to a particular substrate is dependent on the properties of the substrate itself as well as the culturing conditions and the componentε of the culture medium. Culturing iε usually carried out in hard plastic flaskε made from a material which iε substantially inert to the growth media and is non-cytotoxic to the cells. Polystyrene is a commonly used material for culture flasks.

One of the problems in using hard plastic flasks for the culture of epithelial cells for use as skin grafts iε that the sheet of cells normally has to reach confluence before they can be harvested. The time taken to reach confluence may be long. Furthermore the layer of cells iε not very strong mechanically and can easily be damaged when the unsupported cells are dislodged and handled unsupported.

It is known that the sheet of cells can be supported after it has been dislodged from the surface in order to facilitate handling and transfer to the wound

surface. However this technique does not overcome the problems associated with hard surface cultures or the risks associated with dislodge ent of the cells.

Epithelial cells have also been grown on natural materials such as collagen which can then be used directly as a skin replacement. The techniqueε are described in the papers referred to hereinbefore. Whilst epithelial cells grow well on collagen there are several diεadvantageε in uεing such a material. Since collagen iε a natural substance it is not well defined and can vary substantially from one batch to another which iε clinically undesirable if it iε to be uεed aε a εkin graft. Collagen is also a difficult material to work with in the laboratory and it is a complex and time consuming process to isolate it, making it expensive to produce. A further disadvantage iε that becauεe collagen iε a protein it cannot be eaεily εteriliεed by methods εuch aε steam penetration as it will be denatured. Accordingly it is difficult to store and to keep sterile.

In an attempt to overcome the problems associated with the use of collagen and hard plastic surfaces as cell culture subεtrates it has been proposed to grow cells on the surface of water-swellable hydrophilic synthetic polymers. The cells are not immersed in the aqueous

medium but are grown at the εubstrate-gaseouε interface. The reεultant εheet is relatively bulky making it poorly conformable and the cell layer haε to be fully confluent before it can be tranεferred to the wound site which can take a considerable time for example 14-21 days. Such a procesε iε deεcribed in PCT Publication No WO88/08448.

We have now found it is possible to grow cells on a readily manageable, flexible, conformable subεtrate wherein the cells are immersed in an aqueous medium and thus avoid the problems aεsociated with cultures grown at the subεtrate - gaseous interface. Moreover, unlike the prior proposalε, it is possible and desirable to transfer a cell layer before it haε reached confluence. Moreover still this procesε iε conεiderably faεter than prior proposals taking less than 7 days to reach the transferable stage. Such a dressing therefore offers considerable advantages over the prior art.

It is an object of the invention to provide a wound dresεing which compriεeε a layer of cultured mammalian cells anchored to one surface of a conformable subεtrate which εubεtrate is a synthetic, polymeric film and which is hydrophobic, non-inhibitory to cell growth, non-cytotoxic.

It is also an object of the invention to provide a system for producing a wound dressing by culturing anchorage dependent mammalian cells comprising a conformable synthetic polymeric film which is hydrophobic, non-inhibitory to cell growth, non-cytotoxic, together with a meanε for maintaining an aqueouε culture mechaniεm containing εaid cells in contact with one surface of said film.

In accordance with the preεent invention there iε provided a conformable wound dreεεing comprising a layer of cultured mammalian cells anchored to one εurface of a subεtrate comprising a film of synthetic polymer and which substrate is hydrophobic, non-inhibitory to cell growth and non-cytotoxic.

The present invention also provides a system for the manufacture of wound dressings comprising a substrate for culturing anchorage dependent mammalian cells and means for maintaining a culture medium in contact with said substrate wherein said subεtrate is a conformable film of a synthetic polymeric material which is hydrophobic, non-inhibitory to cell growth and non-cytotoxic.

The present invention further provides a method for producing a wound dressing by culturing anchorage

dependent mammalian cells onto a substrate contained within the εyεtem.

The mammalian cellε employed in the preεent invention are those cellε which are anchorage-dependent i.e. they require a subεtrate onto which they can bind before they are able to proliferate.

By the term 'conformable' we mean that the dreεεing will conform to changeε in dimenεionε of the body portion to which the dreεεing iε attached.

In order for the εubεtrate to have deεirable εurface propertieε which allows the anchorage of the cells the subεtrate should preferably be a corona-diεcharge treated film. Corona diεcharge treatment increases the surface energy of a material and provides improved conditions for cell anchorage. The effect of corona-discharge treatment can be asseεεed by measuring the contact angle of water on the treated material. A method for measuring contact angle will be hereinafter described. Aptly the contact angle should be reduced by at least 10%, favourably by at least 15% and preferably by at least 20%.

The substrate is a conformable synthetic polymeric film. In order for the subεtrate to have the

- 7 - flexibility and conformability required for the purposes of the present invention it εhould εuitably have a thickneεε not exceeding 0.075mm. More εuitably the substrate will have a thickness not exceeding 0.05mm, and favourably not exceeding 0.04mm. More favourably the substrate will have a thicknesε between 0.005mm - 0.03mm and preferably between 0.010mm and 0.025mm for example, 0.015mm or 0.020mm.

The substrate is formed from a hydrophobic material. By 'hydrophobic' iε meant that the water-uptake does not exceed 15% by weight of the material. More aptly the water uptake should not exceed 10% and favourably it should not exceed 7% by weight of the material. More favourably the water uptake of the substrate should not exceed 5% by weight of the material. Preferably the εubεtrate should not εwell at all when in contact with aqueous media.

Percentage water uptake of the substrate iε aεεeεsed by the following method. A 5 cm x 5 cm piece of the substrate is weighed in its dry εtate. Thiε iε then immersed in exceεε distilled water (at least 100 mis) and left for a period of 24 hours at 20 C. The piece of the substrate is then removed, excess water is allowed to drain and the substrate is re-weighed. The percentage increase in weight obtained is the

percentage water uptake.

The subεtrate should not be inhibitory to cell growth. A measure of such inhibition can be expresεed aε a percentage cell growth reduction aε meaεured against cells allowed to grow in the absence of a test εubεtrate aε hereinafter described. Aptly the εubεtrate εhould not reεult in more than 50% reduction in cell growth. More aptly it εhould not reεult in more than a 40% reduction in cell growth. Favourably it should not result in more than a 30% reduction in cell growth and preferably not reεult in more than a 20% reduction in cell growth.

The subεtrate εhould not be cytotoxic to cell growth. Cytotoxicity can be meaεured againεt a non-cytotoxic control material by a method aε hereinafter described. Aptly the cytoxicity of the substrate will not exceed 30%. More aptly the cytotoxicity of the subεtrate will not exceed 20%. Favourably the cytotoxicity of the subεtrate will not exceed 20% and preferably it will not exceed 15%.

The subεtrate may be in the form of a continuouε film or an apertured film, for example, formed into a net. Preferably however the substrate is a continuous film which is adapted to be perforated after the cell layer

has formed on the subεtrate.

The use of apertured dresεingε in accordance with the invention haε the advantage that wound exudate, found in a wound covered by a dreεεing of the invention, can readily escape through the apertures and not build up under the dreεεingε.

Perforation of the film, either before or after cell culture can be carried out by any suitable conventional method such as hot pin perforation or slitting. Perforation after cell growth should be carried with minimum cell disruption and loss. Aptly perforation should give at least a 5% open area and preferably at least 20% open area. Aptly perforation should give lesε than 50% open area and preferably less than 40% open area. 25% open area iε an example.

An especially preferred form of substrate which is adapted to be perforated after cell culture is a continuous film which has been biaxially orientated, for example by drawing and stretching in the machine or transverεe direction during manufacture such that when the films are subsequently stretched in the other direction, aperatures are formed therein. Such films are described, for example, in UK-914489, UK-1055963, EP-0141592.

The filmε forming the substrate may be flat or contoured, for example by embossing. Suitably contoured filmε may also have apertures. Such contoured materials are described in WO90/00398, for uεe as cove'rεtockε.

Aptly the continuouε filmε comprising the subεtrate εhould be permeable to moisture vapour, oxygen and carbon dioxide. In this way a dresεing when in place on the wound will provide moiεt conditions allowing for the cellε to remain viable while the wound healε. The continuous films substrate, whilst desirably being impervious to liquid water may have a moisture vapour transmission rate (MVTR) not exceeding 2500 gm

24 hrs~ . More aptly the εubεtrate εhould have an MVTR not exceeding 1500gm -2 24 hrε-1. Favourably the

_2 εubεtrate εhould have an MVTR not exceeding 1200gm 24 hrε~ and preferably not exceeding lOOOgm ^" 24 hrs .

A method for determining the MVTR of a subεtrate iε given as follows:

The moisture vapour transmiεεion rate (MVTR) may be meaεured by the Payne Cup method. Thiε method uses a cup 1.5cm deep which has a flanged top. The inner

2 diameter of the flange provides an area of 10cm of material through which moisture vapour may pasε. In

thiε method 10ml of distilled water is added to the cup and a sample of the material under test, large enough to completely cover the flange, is clamped over the cup. When the test material has an adhesive surface it is clamped with the adhesive surface facing into the cup. The complete assembly is then weighed and placed in a fan assisted electric oven where the temperature and relative humidity are maintained at 37 C and 10% respectively. The relative humidity within the over is maintained at 10% by placing 1kg of anhydrous 3-8 mesh calcium chloride on the floor of the oven. After a suitable period of time, for example 17 hours, the cup is removed from the oven and allowed to cool for 20 minutes to reach room temperature. After reweighing,the maεε of water loεt by vapour tranεmisεion iε calculated. The moisture vapour permeability iε expressed in unit of gm -2 24 hrε-1 at

37°C, 100% to 10% relative humidity difference, that iε it iε the maεε of water transmitted through a square metre of material in a 24 hour period when maintained at 37°C and there are differences of relative humidity at the two surfaces of the material at 100% inside the cup and 10% outside. This iε the moisture vapour transmission rate when the film or dressing is in contact with moisture vapour. The moiεrure vapour transmiεsion rate when the adhesive is in contact with water may be measured using the same apparatus. When

- 12 - the cup iε placed in the oven the cup iε inverted εo that liquid water (and not moisture vapcur) iε in contact with the teεt material.

In a modification of the invention the εubεtrate could be formed from knitted or woven polymers to form a tight web of εmall mesh εize. After cell culture the web centre can be stretched to form a web having a larger mesh size without loss of cellε.

In a further modification still the subεtrate could comprise two interlocking nets such that the apertureε in one net correspond to the strand portions of the other net. The two netε could then be peeled apart poεt-culture and since both would carry cells they could both be applied to the patient.

Suitable corona-discharge treated polymeric filmε may have a thickneεε not exceeding 0.075mm, a water uptake not exceeding 15%, a percentage reduction in cell growth of not more than 50%, a cytotoxicity of not more than 30% and a moisture vapour permeability not exceeding 2500gm -2 24hrε-1

The polymerε employed in the preεent invention are εynthetic and do not comprise any naturally occuring polymeric materials or residues. Such synthetic

polymerε can therefore be produced to a high degree of conformity and conεiεtency. Suitable polymerε having for uεe in the manufacture of the εubstrates employed in the dressings of invention include copolymerε, block copoly ers and polymer blends.

Apt copolymerε are thoεe containing vinyl acetate reεidueε such aε the ethylene-vinyl acetate copolymerε. Suitable ethylene-vinyl acetate copolymerε are those containing not more than 20% vinyl acetate. A preferred material, known aε EVA 538/539 containε 16% vinyl acetate.

Other εuitable polymerε include essentially hydrocarbon based materials such as the polybutadienes, polypropylene and polystyrene. Preferred grades of polystyrene include the high impact polystyrenes such as that sold under the trade name STYRON.

Suitable polymerε for uεe in the invention may include block copolymerε having hard end blocks and softer mid blocks. Apt block copolymerε include styrene based rubbers such aε εtyrene-butadiene-εtyrene (manufactured by Shell Chemical Co under the trade name CARIFLEX or KRATO ) .

Another claεs of polymers suitable for the purposes of

the invention are polyeεterε. A suitable member of this claεε iε polyethylene pterephthalate (manufactured and εold by ICI under the trade name MELINEX).

Polymer blendε may alεo be employed for the εubεtrateε of the invention. Preferred materialε are blends of ethylene-vinyl acetate with hydrocarbons such aε a polyolefin or an aromatic hydrocarbon. A preferred material iε a blend of 90% EVA and 10% high impact polyεtyrene.

Preferably the polymeric film subεtrate should desirably be transparent in order to allow visualisation of the wound through the dresεing.

It iε preferred to use autologous cultivated epithelial cells since these have little or no imsunological rejection problems when applied to the host (patient). Preferably the cells are keratinocyteε. We have found that it iε deεirable not to allow the cell layer to reach confluence before tranεferring the wound dreεεing onto the wound εite. Aptly the cellε εhould have reached at leaεt 30% confluence before tranεfer, favourably at leaεt 40% and preferably at leaεt 50%. It iε poεεible that a εuitable wound dressing could be produced within a few hours making the dressing particularly suitable for use as an emergency field

dreεεing when rapid treatment is required.

Preferably the dresεing compriεeε a cell layer which iε not more than 2 cells thick, more preferably the cell layer iε a monolayer.

The polymeric film substrates can be εteriliεed by either ethylene oxide (allowing the required time for de-gaεεing) or by gamma-irradiation. It iε important that the polymeric filmε are waεhed to remove any low molecular weight contaminants, for example unpolymerised monomer. Such monomers can be cytotoxic and for the reasons given above the substrate should be subεtantially non-cytotoxic. The waεhing proceεε may comprise several sequential washes using sterile de-ionised water in sequential εtepε.

In the system of the invention the substrate on which the cellε are grown εhould preferably be easily removable from the other parts of the system, namely the means for maintaining the aqueous culture medium containing said cellε in contact with one εurface of the film. For example, where the εubεtrate forms one of the walls of the vessel containing the culture medium, it should be readily removable from the other parts of the vessel. The polymeric film substrate may form a part or all of the container in which the cell

culture iε grown. The other partε of the container or culture veεsel may be formed from suitable materials conventionally used for the manufacture of tiεεue culture veεεelε. High impact polystyrene is preferred.

In an alternative embodiment the εubεtrate may be laid down within a flask of an appropriate design adapted to allow the removal of the subεtrate.

Where the εubεtrate is an integral part of the culture flask it may be removably sealed to the other parts of the flask for example by heat sealing or by meanε of an adhesive. Preferably the subεtrate will form a flat εurface and will aptly form the wall of the flask.

Where the subεtrate is to be contained within the flask, the flask will be provided with a closeable opening having dimensions sufficient to enable the subεtrate to be readily removed without disruption of the cells anchored thereto.

Where apertured εubεtrateε which are used to form an integral part of the culture flask these may be overlain by continuous film to keep the vessel water tight and to maintain sterility. If the substrate is laid down within the flask or vessel it may be retained by, for example, a pre-sterilised stainless steel ring or, alternatively, by coating the subεtrate on one εide

- 17 - with a layer of non-cytotoxic adheεive which iε capable of maintaining tack in the presence of tissue culture medium.

Nutrients, growth factors or medicaments such as antibiotics or antiflammatories may be incorporated into the aqueous medium in which the cell culture iε grown. The nature and weight and/or volume of such additional ingredients are conventionally well known.

The wound dressing of this invention is particularly suitable for treating partial-thickness wounds that iε thoεe where only the epidermiε and poεεibly part of the dermis is loεt. Such wounds include for example skin graft donor siteε, first or posεibly second degree burns, shallow leg ulcers or pressure sores. Continuous polymeric film subεtrate aptly act aε barrierε to bacteria whilst being sufficiently permeable to moiεture vapour, oxygen and carbon dioxide to allow wound healing to occur at a desirable rate. If the εubεtrate iε perforated, a εecondary dressing could be applied to maintain the desired degrees of moisture vapour, oxygen and carbon dioxide permeabilities. A suitable material is a polyurethane film dressing such aε OPSITE (Trade Mark) to create the εame conditionε. The dressing can suitably be left in place on the wound for a period of up to 7 days allow

the wound to become from 30-90% re-epithelialiεed (healed) depending on the nature of the particular wound and the condition of the patient. At thiε time the dreεεing can be removed and replaced with conventional wound dressings.

The dresεingε of the preεent invention offer many advantageε ovr prior art arrangementε. Hitherto it haε been neceεεary to culture the layerε to a thickneεε of εeveral cellε in order for the cell layer to be handled and manipulated. It waε alεo neceεεary to grow cell layerε to a larger area than required for the dresεing since during the conventional enzymatic harvesting techniques the sheet tended to shrink. In addition to the time required to produce a cell εheet which waε both large enough to cover the wound and εtrong enough to be handled, there are additional diεadvantages to the use of multi-layer cell sheets. In order to obtain rapid assimilation of the donor sheet into the wound it iε highly deεirable that the baεal εurface (which contains actively growing cells) of the cell sheet be in contact with the wound surface. During manipulation of known multi layer cell sheets it is possible for the non-basal layer (where the cells are terminally differentiated) to be the surface in contact with the wound.

With the dreεεingε of the invention it iε not necessary, or even desirable to achieve confluence. The dressings of the present invention which comprise a sub-confluent monolayer of cells can be readily produced in a matter of hours, can be readily handled and when presented to the wound will bring actively growing basal cells directly into contact with the host (patient) substrate.

In addition to the foregoing advantages the systemε of the invention may utilise cell feed techniques without the disadvantages hitherto associated with these techniques.

In 1975 Green et al proposed the use of transformed cell line 3T3 cells derived from the mouse as a feeder layer syεtem in order to expand εkin cultureε. 3T3 cellε synthesized factors which were essential for the growth of keratinocytes which were εeeded at very low denεititeε and 3T3 cellε inhibit fibroblaεt growth, which are unwanted in skin cell preparation. Skin cell preparation containing 3T3 cells first have to be γ-irradiated (typically about 6000 rads) to inhibit cell division yet not kill the cells. Such cells will survive for several days and during that time will synthesize and supply materials for the host keratinocyte cells. Eventually the 3T3 cells are

expelled for the εkin cell layer, but inevitably εome mouεe cellε remain and will be grafted with the other hoεt keratinocyteε.

In the dreεεings of the present invention the feeder cells such as 3T3 may be εeeded into a medium in contact with the reverse side of the substrate. These cellε will then εyntheεize materialε for the host cells. Since the feeder cells do not come into contact with the host cells there iε no need to irradiate them. When the εubεtrate is removed from the culture falεk the reverse side may be washed to reverse the free floating feeder cells.

Where a feeder cell layer is employed with an apertured εubεtrate the apertureε εhould be large enough to allow free exchange of the culture media but not large enough to allow cellε to paεs through. Aptly the aperture size should not be larger than Sμ acroεε itε largest dimenions. Suitably the aperture εize will be from 0.5 - 2μ .

Embodimentε of the dreεεing εystems of the invention will be illustrated by reference to the accompanying drawings. Refering to Figure 1, one wall of a culture flask 1 compriseε a laminate of an apertured film 2 upon a confcinuouε carrier layer 3. The edgeε of the

carrier layer are removably bonded to the other wall portions 5 of the flask 3. Culture media 4 and donor cellε can be introduced into flask 3 through neck 6 and sealed therein by stopper 7.

After cell culture, the laminate 2, 3 can be removed by peeling from flask 1. An apertured dreεεing compriεing cellε anchored to εheet 2 can be separated from the carrier layer 3 and applied to a patient.

In figure 2, the laminate 2, 3 is formed by casting the εubεtrate film 2 over a carrier layer 3 being a plurality of raiεed portions 8. The cast substrate layer 2 has a plurality of thin 9 and thickened 10 areas. When the substrate is separated from the carrier layer, the thin areaε 9 rupture to form apertureε 12 aε shown in Figure 3.

Referring to Figure 4, the culture flaεk 1 iε divided into compartments 21, 31 by a perforated film 2 comprising the substrate. Nutrient media 4 is introduced into the flask and occupies both compartments 21, 31. Skin cells are εeeded into compartment 21 through neck 6 whilεt feeder cellε εuch aε 3T3 cells are seeded into compartment 31 through access port 11.

After the layer of εkin cellε 11 haε reached the required degree of confluence it iε detached from the wall portion 5 of the flaεk and removed from the acceεε port 13.

Nutrients, growth factors or medicaments εuch aε antibioticε or antiinfla matorieε may be incorporated into the aqueous medium in which the cell culture is grown. The nature of weight and/or volume of εuch additional ingredientε are conventionally well known.

The invention will now be illuεtrated by the following Exa pleε wherein the contact angle, cell growth reduction and cytotoxicity were determined aε followε:

Measurement, of contact angleε - surface energy determination

Contact angleε are meaεured uεing a contact angle goniometer aε follows:

Each film subεtrate iε layed flat on the goniometer εtage. The two contact solutions used are distilled water, dekalin and glycerol. The micrometer screw syringe is filled with distilled water and one drop is allowed to contact the film surface to be measured. At time - 0 a photograph of the droplet shape is taken via

a camera mounted to the eyepiece of the goniometer. Further photographs are taken periodically (every minute) for approximately ten minutes. The procedure iε then repeated uεing glycerol and dekalin aε the contacting medium.

The reεultant photographs are then developed. The angle the droplet makes to the horizontal is then meaεured uεing a protractor (meaεured both sides).

Graphε of contact angle versus time are then plotted and the angle at time - 0 determined according to the following formula:

surface tension of a liquid « surface free energy (Y) Total surface energy ( _γ ) « + γ ^p γ - dispersive component γP - polar component

1 ₊ Cosθ - 2[(γ*( { <Y Ϊ|]

^Y l

A method for measuring percentage cell growth reduction

Human epithelial cells (keratinocytes) are seeded onto

a substrate or tisεue culture plate at a density of 8 x

10 5 or 1.5 x 105 respectively per well in 6 well plates. Cells are cultured in 3mlε of the appropriate media containing 10% foetal calf εerum and incubated at

37 C until they reach approximately 50% confluence.

The culture media iε then aεpirated and the cellε re-fed with 3mlε of media containing 0.66 Ci ml ^"

(εpecific activity 5.0 Cimmol ^"" ) of tritiated thymidine εupplied by Amerεham International pic. After 18 hours incubation, the media iε again aεpirated and the εubstrateε removed from their wells. The εubεtrateε are then washed extensively with phosphate buffered saline to remove excess tritiated thymidine. The subεtrateε are extracted and then the radioactivity preεent in trichloroacetic acid insoluble precipitate iε measured in a liquid scintillation counter.

The results are expressed as a percentage reduction in tritiated thymidine uptake compared to values obtained in control wells containing no subεtrate.

A method for meaεuring percentage cellular cytotoxicity of the εubεtrateε

1. Collection of εubεtrate supernatants

Each substrate is εet up in duplicate in a 6 well

culture plate containing 5mlε of either εerum free media or media containing 10% foetal calf εerum. After one week'ε incubation at 37°C the "substrate supernatants" are collected and frozen at -20°C until required.

2. Cytotoxicity Asεay

Human epithelial cellε are εeeded at a density of 2 x

4 10 cells per well (100 1 volume) into a 96 well culture plate, and incubated at 37°C until the cellε reach confluence. The culture media is then aspirated and media containing 10 Ci ml -1 of 51Cr aqueous sodium dichro ate added to the cells. After 24 hours further incubation, the media is aspirated and the cellε waεhed three timeε with calcium-magneεium free Hankε Balanced

Salt Solution to remove exceεε Cr not taken up by viable cellε. Each "Substrate Supernatant" iε added to each of four wells in either serum free media or media containing 10% foetal calf serum. 10% foetal calf serum is also added to substrate supernatants which were previously incubated in serum free media. After

24 hours incubation at 37 C the supernatants are collected and Cr released from dead or damaged cells is measured in a gamma counter.

51 Baseline measurement of release of Cr from viable

cellε iε meaεured in εerum free media and media containing 10% εerum and maximum releaεe of Cr from cellε lapεed in 1% sodium dodecyl sulphate. The percentage cellular cytoxicity iε meaεured according to the following formula:

%Cytotoxicity -

Experimental releaεe - baseline releaεe x 100

Maximum releaεe - baεeline releaεe

Example 1

Human Epithelial cellε (keratinocyteε) were cultured to 50% confluence onto corona-diεcharge treated discε of 15 cm x 12 cm MELINEX film 23 _/ / thick.

Corona diεcharge waε carried out on the film substrate prior to culture using a Sherman C-treater. The C-treater waε used manually and set to deliver greater than 50 dynes/cm to the εubεtrate surface. The subεtrate iε placed under the head of the machine and removed slowly in order to achieve this level of treatment.

The culture medium uεed waε baεed on Green'ε method aε εtated in Barlow & Pye (1990) Methods in Molecular Biology Chapter 5 (Humana Presε). It consisted of 3 parts DMEM: 1 part F12 containing 10% foetal calf serum

and EGF (10 ng ml ^" ), insulin (5 _/ g ml ^" ), hydrocortisone (0.2 μg ml -1), cholera toxin (10-9M),

—1 —8 tranεferrin/triiodothyronine ( 5 g ml ), (2 x 10 M) and adenine (1.8 x 10 -4M) final concentration.

Alternative but equally acceptable media include Keratinocyte Basal Media available from Clonetics & BIORICH Media available from Flow Laboratories. Both of these media are εerum free.

The percentage cell growth reduction was found to be 28%.

The percentage cellular cytotoxicity was found to be 7%.

Example 2

Example 1 waε repeated uεing corona-discharge treated ethyl vinyl acetate film at 15// thickneεε (EVA 538/539).

The percentage cell growth reduction was found to be 41.5%.

The percentage cellular cytotoxicity was found to be 0%.

The contact angle waε found to be 78° on untreated film and 55.5° on corona-diεcharge treated film.

Example 3

Example 1 waε repeated uεing corona-diεcharge treated polyiεobutadiene film at 20// thicknesε.

The percentage cell growth reduction was found to be 33%.

The percentage cellular cytotoxicity was found to be 1%.

The contact angle was found to be 98° on untreated film and 79.5% on corona-diεcharge treated film.

Example 4

Example 1 waε repeated using corona-diεcharge treated ethyl vinyl acetate/high impact polyεtyrene blends at 20 _/ thicknesε. The following blendε gave the reεultε εhown.

EVA(28-05)/HIPS 80:20 23% cell growth reduction and

13% cellular cytotoxicity EVA539/HIPS 90:10 8% cell growth reduction and

29% cellular cytotoxicity

Example 5

Example 1 was repeated uεing corona-discharge treated polypropylene film at 20 _/ / thickneεε.

The percentage cell growth reduction waε found to be 47%.

The percentage cellular cytotoxicity was found to be 8%.

Example 6

Example 1 was repeated uεing corona-discharge treated high impact polyεtyrene film (STYRON) at 20// thickneεε.

The percentage cell growth reduction was found to be 32%.

The percentage cellular cytotoxicity was found to be

8% .

Example 7

Example 1 was repeated using corona-discharge treated styrene-butadiene-styrene rubber film (CARIFLEX 1101) at 20 _/ / thicknesε.

The percentage cell growth reduction waε found to be 41.5%.

The percentage cellular cytotoxicity waε found to be 1%.

Example 8

Example 1 waε repeated using corona-discharge treated polyvinylidene chloride (PVDC) film at 20// thickneεε.

The percentage cell growth reduction waε found to be 27%.

The percentage cellular cytotoxicity was found to be 25%.

Exampleε 9 - 16

Exampleε 1 - 8 were repeated but the filmε were perforated prior to culture uεing hot-pin perforation techniqueε to give an open area of 25%. Good cell growth waε achieved in all caεeε with minimal cell loss or perforation.

Exampleε 17 - 24

A system according to Figure 1 waε uεed to produce a wound dresεing. The pre-perforated films used were thoεe aε deεcribed in Exampleε 9 - 16. The cells were grown to 20% confluence (approximately 24-36 hourε). The carrier (2) and perforated substrate carrying the cells (1) was detached from the culture vesεel (3) and the εubεtrate waε then peeled from the carrier. Good cell growth waε achieved in all caεes.

Examples 25 - 32

Exampleε 17 - 24 were repeated but instead of pre-perforating the films they were cast onto the base of a flaεk containing 1mm stumps according to Figure 2, After culture the substrate was peeled from the base according to Figure 3 and was successfully perforated

aε well aε achieving good cell growth in all caεeε. Cell loεε waε minimal.

Example 33

Example 4 waε repeated and the εubstrate was perforated by stretching according to the method described in European Patent No 0141592. Good cell growth waε achieved with no loεε of cellε upon perforation.

Example 34

Example 20 waε repeated but the εubεtrate waε not pre-perforated. Inεtead it waε εtretched according to Example 33 and εpontaneouεly perforated. Good cell growth waε achieved with no loss of cellε upon perforation.

Example 35

A εyεtem according to Figure 1 waε uεed to produce a wound dreεεing. The net used was as described in PCT Publication No GB090/00398. The cells were grown for approximately 24 hours and the carrier (3) and net (2) carrying the cells were detached from the culture vesεel (5). The net was then peeled from the carrier and good cell growth was observed on the net.

- 33 -

Example 36

Example 35 waε repeated but two sheets of the net were aligned such that the apertureε of one net coincided with the continuouε portions of the other net. The whole waε then sealed to the bottom of the culture veεεel (5) and the cellε cultured aε before. Upon removal from the veεεel after 24 hourε the two netε were εeparated and both εhowed good cell growth. Both netε were then tranεferred to a patient succeεεfully. Good heating rateε and goods host compatibility were observed.

Example 37

Exampleε 1 and 2 were repeated and the resultant dresεingε were uεed εimultaneouεly in the treatment of a 3 year old female burnε patient. The patient waε burned to approximately 40% of body surface area covering the head, one arm, one leg and one εide of her body. The patient waε grafted on four separate occaεions using a combination of split skin grafting and the wound dressing containing the culture cells. The cell culture was derived from an autologous biopsy taken during debridement of the burns patient. The initial seeding denεity waε between 2.5 x 10 and 6 x

104 cells per cm2. The culture waε grafted approximately 80 hourε after the cellε were εeeded.

The εubεtrate film waε covered with a secondary dresεing comprising a liquid paraffin tulle gras

JELONET (Trade Mark) which in turn waε covered with crepe bandaging. After 5 dayε the secondary dresεingε were removed, the substrate films fell-off with no noticeable adherency and no cells remained on the film.

The wounds beneath had healed. No significant difference was obεerved between the two types of dresεing.