Polymeric Sheets as Synthetic Medical Dressings or Coverings for Wounds

This invention relates to synthetic medical dressings or coverings for wounds, and to processing techniques for preparing such dressings or coverings. More particularly, it relates to thin, drapable sheets of polymeric material which have physiologic properties similar to human skin plus other desirable properties which make the sheets especially useful in the treatment of thermal wounds conventionally classified as "burns". The medical problems posed by burn wounds and the general requirements for the successful treatment of such wounds are known to those skilled in the art. See for example, the discussion in U.S. Pat. No. 3,648,692 entitled "Medical-Surgical Dressing For Burns And The Like", particularly that at column 1, lines 10-69.

The major problem with a burn wound is that the protective layer of skin is either missing or badly damaged at the wound site so that the normal physiologic functions of the skin are absent or at best materially impaired. Two important physiologic functions of the skin are to serve as an anti-microbial barrier layer to prevent infection, and to prevent the undue loss of body fluids, proteins and electrolytes. Once the skin can no longer adequately perform these functions, the body fluids, proteins and electrolytes are continuously lost and the invasion of harmful micro-organisms and other harmful agents into the body can proceed with predictable adverse results. For example, normal human skin has a water vapor phase transfer rate of about 2 mg./hr.-cm..sup.2 whereas the rate for burned skin can be 25 to 45 mg./hr.-cm..sup.2, or even higher during the first few hours after the burn. To alleviate these problems, the standard medical treatment for burns involves a combination of therapy and dressing to cover the burn site as soon as possible with a protective layer whose properties resemble the burned-away skin. While the use of topical and systemic antibacterial agents can reduce the extent of infection in a burn patient, coverage of the wound site with a skin-like dressing before the onset of infection remains a major factor in burn management.

At the present time, most of the burn dressings used by the medical profession are either human or animal skin. The more common of these dressings are generally referred to as "autografts", "allografts" (also sometimes called "homografts") or "xenografts" (also sometimes called "heterografts"). An autograft is a portion of the burn victim's own skin taken from an uninjured part of the body. The limitations of this technique are apparent, especially in cases where the victim has suffered extensive dermal destruction. A homograft is skin taken from a cadaver. A xenograft is skin taken from a different species. Pigskin is the most commonly used xenograft. Autografts are generally preferred to homografts and pigskin xenografts.

The various human and animal skins are expensive, and are difficult to store for prolonged periods of time. Perhaps the most serious disadvantage of such materials, especially of homografts, is their limited availability. Over 100,000 burn cases are hospitalized in the United States alone every year. While not all patients require a covering for the burn site, it is estimated that 50 to 75% of those hospitalized would benefit from such a covering.

Recently, efforts have been made to develop synthetic burn dressings having physiologic properties similar to human skin which could be inexpensively prepared in large quantity and stored for long periods of time without degradation. See, for example, the polyurethane foam burn dressing described in U.S. Pat. No. 3,648,692. Kornberg and his coworkers have described a synthetic burn dressing composed of an ultra-thin (0.5 to 2 mils), pinhole-free silicone rubber membrane to one surface of which a sheet of spun-bonded nylon, open-weave nylon or double-knit dacron is laminated, See "Ultra Thin Silicone Polymer Membrane: A New Synthetic Skin Substitute", Kornberg, et al. Transactions of the American Society of Artifical Internal Organs, Vol. 18, pp. 39-44 (1972). The Kornberg, et al. dressing is impervious to bacteria, inert, non-antigenic, has a water vapor permeability similar to intact human skin, and is transparent and relatively inexpensive to produce.

Much of the success of the Kornberg, et al. dressing is attributed to the work of Nora E. Burns on techniques for mass-producing ultra-thin, pinhole-free silicone rubber membranes for use in membrane oxygenators. This work is described in "Production of Silicone Rubber Film for the Membrane Lung", N. Burns, Biomedical Engineering, Vol. 4, pp. 356-359 (1969). Briefly, Burns prepares her ultra-thin silicone membranes by applying an extremely thick dispersion of silicone rubber to a moving horizontal surface and spreading the dispersion into a film of uniform thickness using an accurately ground doctoring blade. Major processing problems generally arise because of the extreme thinness of the silicone rubber membrane, which makes it very difficult to handle without damaging it, and the need for uniform thickness in order to obtain uniform properties throughout the membrane and establish satisfactory quality control.

Among the desired properties of a synthetic burn dressing are that it have water vapor phase transfer rates and anti-microbial barrier layer properties approximating those of human skin, that it adhere well to the wound and preferably have voids in the surface applied to the wound for fibroblastic ingrowth of tissue and to serve as a debris reservoir for necrotic tissue and other debris from the wound so that this material is removed from the wound. The dressing should also be transparent or translucent so the progress of the wound can be observed without removing the dressing. Two dimensional elasticity is another desirable property because it permits the dressing to expand and contract if applied to an elbow, knee or other body location where it is likely the dressing will be flexed. The dressing should be drapable and readily conform to the shape of the body. It should possess a sufficiently high tensile and tear strength so that the dressing can be handled and stretched without damage to the dressing. The uniformity throughout the dressing of such properties as the water vapor phase transfer rate and the anti-microbial barrier layer properties is also important, as is control of the characteristics of the dressing which effect these properties. Preferably, the dermal surface has controlled wicking characteristics to remove some but not all the fluid from the wound site so as to maintain a medically acceptable, balanced fluid level on the wound site. And, of course, the dressing must be constructed from biologically innocuous and inert materials which are acceptable to the medical profession for use with human beings.

It is an object of this invention to provide improved synthetic polymeric wound dressings having the above properties and characteristics.

It is another object of this invention to provide processing techniques for preparing these improved synthetic dressings by which the properties and characteristics of the dressing can be varied in accordance with the contemplated end use of the dressing.

It is another object of this invention to provide improved silicone rubber burn dressings and processing techniques for preparing such dressings.

These and other objects of the invention will be apparent to those skilled in the art upon a consideration of the specification and attached drawings, taken in their entirety.

The above objectives are accomplished, in accordance with the invention, by providing an integral, continuous, non-foamed, drapable polymeric sheet which can be partly or completely non-woven, non-fibrous and non-filamentary, one or both sides of which have a fabric texture. In one embodiment, the sheet comprices a plurality of interbonded continuous polymeric ribs extending between the thin edges of the sheet which define between them recessed portions in one of both sides of the sheet. These ribs contain a plurality of elongated continuous channels of a filamentary configuration interiorly located therein which also extend between the thin edges of the sheet to form a network of voids within the ribs. In another embodiment, the sheet comprises an ultra-thin polymeric membrane layer one side of which has a fabric texture.

The sheets of the invention have a water vapor phase transfer rate of about 2 to 20 mg./hr.-cm..sup.2, two dimensional elongation of at least about 100% in each direction, an open area of zero to about 60%, and numerous other properties desirable in a wound or burn dressing. All water vapor phase transfer rate data herein is at 37.degree. C. and 46% relative humidity.

Sheets having a fabric texture on both sides are prepared by applying a polymer to the surfaces of a skeleton sheet having a fabric texture on each surface, the polymer being different than the material of the skeleton sheet, curing the polymer on the skeleton sheet, and treating the composite of polymer and skeleton sheet with a solvent in which the skeleton sheet but not the polymer is soluble. The solvent selectively leaches the skeleton sheet out of the composite leaving only a polymeric sheet having substantially the surface contour of the skeleton sheet and a containing network of voids conforming to the configuration of the solids portion of the leached skeleton sheet.

Preferred skeleton sheets include weaves, knits or meshes of continuous multifilament strands of varying percent open area. The polymer impregnates the voids between the individual filments in each strand and builds up a coating of polymer around each strand until the skeleton sheet is thoroughly impregnated with, and substantially encased within a shell of, polymer. This shell becomes the sheet of the invention upon removal of the skeleton sheet.

Sheets having a fabric texture on one side only are prepared by joining one surface of the polymeric sheets just described to a suitable substrate layer at a point in the preparation of the polymeric sheet either before or after leaching out the skeleton sheet. If done before the skeleton sheet is leached, the polymer on the skeleton sheet can be cured before or after joining to the substrate. An ultra-thin membrane (0.2 to 2 mils) of silicone rubber is one example of a suitable substrate layer.

Another embodiment of a sheet having a fabric texture on only one side comprises an ultra-thin, pinhole-free substantially non-porous and voids-free membrane layer of silicone rubber, which preferably is a composite of (1) a non-sticking, high-release silicone rubber and (2) a high strength silicone rubber, to one surface of which is joined a sheet having a fabric texture. The sheet having the fabric texture may optionally contain elongated channels and voids located in the interior of its structural framework and can be of silicone rubber or different materials.

Sheets having an ultra-thin silicone rubber membrane on one side only are prepared by applying one or more successive layers of silicone rubber to a smooth forming surface, embedding one surface of a sheet having a fabric texture in the silicone rubber layer, curing the silicone rubber to form a composite of the sheet and silicone rubber, and removing the composite from the support surface. If the embedded sheet contains one or more materials other than the silicone rubber, as many of these materials as desired can be leached out of the composite by appropriate selection of a leaching solvent at any convenient point in the processing, as described above.

A still further embodiment of a sheet having a fabric texture on only one side comprises a non-laminated continuous sheet of polymer having on one side thereof an ultra-thin, pinhole-free membrane layer and on the other side a fabric texture formed by portions of the polymer which project from the membrane layer. These sheets also contain embedded therein a skeleton sheet of a material other than the polymer which has a fabric texture conforming to that of the sheet. If desired, this skeleton sheet can be removed by treatment with a solvent which dissolves the skeleton sheet but not the polymer, to thereby form a plurality of elongated continuous channels interiorly located therein which extend in a direction generally parallel to the plane of the sheet to form a network of voids within the sheet.

These non-laminated sheets are prepared by applying a skeleton sheet having a fabric texture on at least one surface to a backing surface to form a composite of the sheet and backing surface. In a preferred embodiment, for example, a sleeve of multifilament yarn fabric is placed over the outer surface of a smooth cylindrical drum, and then circumferentially stretched on the drum. A liquid containing a polymer such as silicone rubber is then applied to the skeleton sheet while it adjoins the backing surface. The polymer coats the surfaces of the skeleton sheet and forms an ultra-thin layer between the skeleton sheet and backing surface. The polymer is then cured and the sheet of cured polymeric material containing the embedded skeleton sheet removed from the backing surface. The skeleton sheet can be removed, if desired, as described above by treatment with a selective solvent for the skeleton sheet. A principal advantage of this mode of sheet preparation is that the final product is entirely non-laminated, being formed of a continuous polymer phase. One surface of the polymer sheet is an ultra-thin membrane while the other surface is formed by portions of polymer projecting from the membrane layer. These projecting portions, however, are continuous with the membrane and thus cannot delaminate or otherwise separate from the membrane.

The sheets of the invention can be fabricated with substantially uniform physiologic properties approximating those of human skin. They have excellent strength and two-dimensional elongation characteristics, and can be fabricated from biologically inert and medically acceptable materials such as silicone rubber. The texture of the surface adheres well to a wound site and provides a reservoir for wound debris so that the debris does not remain in place on the wound site. The sheets are transparent or translucent, easy to handle, and can be stably stored for prolonged periods of time. Their drapability and conformability characteristics are excellent. Because the sheets are prepared from readily available synthetic materials, they can be made available in abundant supply.

The sheets of the invention and the processing techniques for preparing them are described in greater detail below, in conjunction with the accompanying drawings and the description of the preferred embodiments of the invention.

FIG. 1 is a schematic flow sheet of a preferred embodiment of the invention, showing the preparation of a sheet having a fabric texture on each surface, and containing elongated continuous channels in the interior of the sheet.

FIGS. 1A-1D are enlarged sectional views taken generally along the lines 1A-1D in FIG. 1, with background structure eliminated for clarity of illustration.

FIGS. 1B', 1B", 1C', 1C", 1D' and 1D" are enlarged views similar to FIGS. 1B-1D illustrating variations in the extent of polymer build-up on the multifilament strands of FIGS. 1A.

FIG. 2 is an enlarged fragmentary plan view taken generally along the line 2--2 of FIG. 1D.

FIGS. 3 and 4 are enlarged fragmentary sectional views taken generally along the lines 3--3 and 4--4 of FIGS. 1D' and 2, respectively.

FIG. 5 is a schematic flow sheet of another preferred embodiment of the invention, showing the preparation of a sheet having a coarse, fabric texture on one surface only and, optionally, containing elongated channels in the interior of the sheet at the fabric side.

FIGS. 5A-5I are enlarged sectional views taken generally along the lines 5A-5I of FIG. 5, with background structure eliminated for clarity of illustration. FIGS. 5F-5I are even more enlarged than FIGS. 5A-5E.

FIG. 6 is a schematic flowsheet of still another preferred embodiment of the invention, showing the preparation of a non-laminated sheet having a coarse, fabric texture on one surface only and, optionally, containing elongated channels in the interior of the sheet at the fabric side.

FIGS. 6A-6D are enlarged sectional views taken generally along the lines 6A-6D of FIG. 6, with background structure eliminated for clarity of illustration. FIGS. 6C-6D are even more enlarged than FIGS. 6A-6B.

None of the drawings is drawn to scale or blueprint specification. This is particularly true of FIGS. 5C to 5I and 6A to 6D where the thickness of the membrane layers 54, 54' and 111 is greatly enlarged for clarity of illustration.

FIG. 5 occupies sheets 5 and 6 of the drawings and is best viewed by placing sheets 5 and 6 end-to-end along their short dimension, with sheet 6 to the right of sheet 5.

FIGS. 5A to 5I occupy sheets 7 and 8 of the drawings and are best viewed by placing sheets 7 and 8 end-to-end along their short dimension, with sheet 8 to the right of sheet 7.

FIG. 6 occupies sheets 9 and 10 of the drawings and is best viewed by placing sheets 9 and 10 end-to-end along their short dimension, with sheet 10 to the right of sheet 9.

FIGS. 6A to 6D occupy sheets 11 and 12 of the drawings and are best viewed by placing sheets 11 and 12 end-to-end along their short dimension, with sheet 12 to the right of sheet 11.

The sheets of the invention are readily understood by a consideration of the process by which they are prepared.

Referring to FIG. 1, the initial starting material is a sheet 10 preferably having a coarse, uneven, fabric texture on both sides thereof. Sheet 10 serves as a skeleton or superstructure to carry the polymer from which the sheet of the invention is ultimately formed until such time as the polymer on the skeleton sheet can be cured, rigidified, hardened or otherwise rendered self-supporting. At this point, sheet 10 is removed by solvent leaching or other suitable techniques leaving an integral, self-supporting polymeric shell generally conforming in shape to the leached skeleton sheet and containing elongated channels or voids formed by the removal of sheet 10.

Sheet 10 can be of virtually any type of construction, e.g., woven, knit, molded, mesh, gauze, netting, expanded, etc., provided it has a fabric texture on both sides. Thus, for example, sheet 10 may be woven or knit fabric formed of monofilament or multifilament yarn strands. The mesh weave shown schematically in FIG. 1 is for purposes of illustration only. Other conventional weaves and knits such as, for example, the weft knit construction shown in FIG. 9 of U.S. Pat. No. 3,463,158, also provide suitable skeleton sheets.

Preferably, sheet 10 is composed of a plurality of continuous multifilament yarn strands 14 (see FIG. 1) in which the individual continuous filaments 15 (see FIG. 1A) forming each strand can be twisted, braided, plaited, laid parallel, or in any other suitable construction. Generally, the axes of strands 14 and filaments 15 are substantially parallel to the plane of sheet 10, with the strands 14 and filaments 15 extending from edge to edge of sheet 10.

The thickness of the skeleton sheet can vary widely, depending largely upon the thickness and other properties and characteristics, such as the water vapor phase transfer rate, desired in the finished polymeric sheet. Illustrative thicknesses of sheet 10 are about 2 to 50 mils, preferably about 5 to 30 mils, and even more preferably about 10 to 20 mils.

The amount of open area in the skeleton sheet can also vary widely depending on the extent of open area and other properties and characteristics, such as the water vapor phase transfer rate, desired in the finished polymeric sheet. Illustratively, sheet 10 has an open area of zero to about 60%, and preferably about 10 to 50%.

The thickness of the multifilament strands 14 making up sheet 10, and the number and size of the individual filaments 15 in each strand, can also vary considerably depending largely upon the overall thickness of the finished sheet, and the number and size of the elongated channels desired in the sheet. For example, the multifilament strands 14 can vary from about 2 to 100 or more individual filaments per strand having diameters of about 0.1 to 5 mils or more. Preferably, the multifilament strands 14 are less than about 20 mils in diameter, contain 10 to 50 individual filaments 15 with diameters ranging from 0.5 to 2 mils. When sheet 10 is composed of monofilament strands, each monofilament is illustratively about 1 to 50 mils in diameter, and preferably less than about 20 mils in diameter.

The choice of material from which the skeleton sheet is made is based on the ability of the material to dissolve in a solvent in which the polymer applied to it is substantially insoluble. Consequently, numerous materials can serve as sheet 10. Some illustrative materials for sheet 10 include cellulose; cotton; rayon; silk; linen; polyamides such as nylon, including nylon velours, polyesters such as Dacron; polyacrylonitriles such as Orlon or Creslan; halogenated polyalkylenes such as tetrafluoroethylene, Teflon, Kel-F, and FEP; polyalkylenes such as polyethylene and polypropylene; polyvinylalcohols; polyvinylacetates; polyglycolic acid; polylactic acid; metals such as stainless steel; and the like. Sheet 10 is preferably fabricated from a polymeric, medically-acceptable material such as nylon.

Sheet 10 is mounted on a suitable frame (not shown). If sheet 10 is a biaxially stretchable fabric such as a woven or knit mesh, it is desirable to biaxially stretch it somewhat on the frame to the extent of about 5 to 75% in each direction. Stretching sheet 10, while preferred, is not mandatory.

The stretched sheet 10 is then immersed one or more times in a bath 12 which can be a solution, dispersion or other suitable form of the polymer from which the finished sheet is to be formed, for a time sufficient to cover sheet 10 with polymer to the extent desired. FIG. 1A is a greatly enlarged cross-sectional view of the individual multifilament strands 14 in sheet 10 prior to their immersion in bath 12. Strand 14 is composed of a plurality of individual monofilaments 15 which are separated from each other by void spaces 16. As sheet 10 is immersed in the bath 12, the solution or dispersion of polymer impregnates, penetrates and fills the voids 16 and covers or coats the surfaces of the individual filaments 15 as shown in FIGS. 1B to 1B". The extent to which the strands 14 are also covered or coated by polymer depends upon such factors as the concentration of polymer in the bath, and the residence time and number of dips of sheet 10 in the bath. Thus, the amount of polymer build-up on sheet 10 can be controlled to produce as thick or as thin an encapsulating coating 12' of polymer dispersion or solution 12 as desired on strands 14, as shown in FIGS. 1B to 1B". This allows the open area of sheet 10 to be reduced as much as desired, or even totally eliminated (see FIG. 1B"), depending on how much polymer is applied to sheet 10. FIGS. 1B to 1B" show the progressive reduction of the open space 18 between strands 14 until in FIG. 1B", space 18 is totally eliminated and replaced by a bridge 12" of solution or dispersion between adjoining strands 14.

The particular polymer which is selected for application to sheet 10 depends largely upon the characteristics and properties desired in the finished sheet. Any of the numerous polymers which have heretofore been used in, or disclosed for use in, medical and surgical dressings, bandages or other medical products and applications can be employed as coating polymers for sheet 10. These polymers are well known to those skilled in the art and need not be repeated in detail herein. Some illustrative polymers include polurethanes, polyethylenes, polypropylenes, natural rubber, polybutadiene, silicone rubber and other synthetic and natural elastomeric polymers such as those of isoprene, neoprene, chloroprene, styrene-butadiene and various copolymers of the above.

Silicone rubbers are preferred polymers for application to sheet 10. Silicone rubber is composed of high molecular weight linear polysiloxanes such as polydimethylsiloxane and other polysiloxanes in which the methyl groups are replaced by groups such as ethyl, phenyl, vinyl and others. A wide variety of useful silicone rubbers of widely varying properties, chemical compositions and cure properties and characteristics are available commercially from suppliers such as General Electric Co., Dow Corning Corp. and Union Carbide Corp. See, for example, the silicone rubbers disclosed in "The Science and Technology of Silicone Rubber" by F. M. Lewis, Rubber Chemistry and Technology, Vol. XXXV, No. 5, December 1962 (pp. 1222-1275), said publication incorporated herein by reference. Other silicone rubbers are disclosed in U.S. Pat. Nos. 3,334,067, 3,592,795 and 3,708,467, and in the numerous patents on silicone rubbers assigned to the above mentioned three suppliers, said patents incorporated herein by reference.

A dispersion of silicone rubber in hexane available from the General Electric Co., Schenectady, N.Y. under the trade name RTV-7000 has been found quite suitable for use as bath 12. RTV-7000 is believed to be a dimethyl type silicone rubber containing about 5% phenyl type silicone rubber. Preferred silicone rubbers are those which cure at or close to room temperature to produce transparent films and which, when cured, have a tensile strength and a tear strength for a 25 mil thick film of at least about 750 pounds per square inch and 75 pounds per inch, respectively.

The concentration of polymer in bath 12 can vary widely, e.g., from about 10 to 60%, depending upon factors such as the extent of polymer pick-up desired and the type of polymer.

Once sheet 10 has picked up a sufficient amount of bath 12 to permeate the voids 16 between the individual filaments 15 in each strand of sheet 10 (see FIGS. 1A-1B) and build up as thick a coating 12' as desired on strand 14 (contrast FIGS. 1B-1B"), the polymer treated sheet 17 is then removed from the bath 12 and preferably air-dried for 15 minutes to two hours to remove part of the bath liquid and increase the polymer concentration on sheet 17. The air-drying step is optional and could be omitted if desired.

If the skeleton sheet is dipped in bath 12 more than one time, it is preferable to air-dry the dipped sheet as described above, or by other drying techniques, prior to each subsequent dip and again after the final dip.

Sheet 17, air-dried or otherwise, is then subjected to conditions which will cure the polymer. The term "cure" as used herein means the conversion of the polymer from a low viscosity form such as a solution or dispersion wherein the polymer is not self-supporting to a significantly more viscous or solid form in which it is self-supporting. Curing is generally a "time at temperature" phenomenon with shorter times required at higher temperatures and longer times at lower temperatures. Conditions for curing the common polymers are well known to those skilled in the art and need not be repeated in detail herein. In general, most polymer curing conditions involve treatment at room temperature to about 200.degree. C. for about 15 minutes to 48 hours. A 10 mil thick film of RTV-7000 silicone rubber, for example, will cure in 24 hours at room temperature or after 30 minutes at room temperature and 60 minutes at 95.degree. C. Thicker films take longer to cure than thin films.

If the skeleton sheet is dipped into bath 12 more than one time, the polymer can be optionally dried and/or cured or partially cured after each dip, as well as after the final dip. Any combination of drying alone or drying plus curing could also be employed after each dip.

FIGS. 1C-1C" show the condition of the multifilament strands 14 in the cured sheet 20 for the three different loadings of polymer depicted in FIGS. 1B-1B". The cured or hardened polymer 21 is located in the void spaces 16 (see FIG. 1A) between the individual monofilaments 15 and forms a shell 21' around the yarns 14 whose thickness depends upon how much polymer was applied to strands 14 in bath 12. FIG. 1C" depicts the condition where the skeleton sheet picked up enough polymer on adjoining strands 14 to form a bridge 21" of cured polymer between the adjoining strands 14 which totally fills in the open space 18 which existed between the strands 14 prior to the immersion of sheet 10 in bath 12. At this point in the processing, the sheet 20 comprises a plurality of polymeric ribs or struts 24 interbonded at points 26 (see FIG. 1) which may or may not also be interbonded along substantially their entire length, and which obtain a multifilament core strand 14 of the skeleton material.

The sheet of cured polymer is then immersed in a bath 30 of a material which is a solvent for the material from which the skeleton sheet is made but not for the polymer 21, 21', 21". Solvents for the normal materials from which sheet 10 is fabricated are well known to those skilled in the art. For example, formic acid, hydrochloric acid and phenol are well known solvents for nylon, a preferred material for the skeleton sheet. The solvent dissolves out the individual filaments 15 which formed the skeleton or super-structure of the cured sheet 20, leaving a plurality of elongated continuous channels or voids 32 interiorly located in the structural framework of the finished polymeric sheet 33, as shown in FIGS. 1D to 1D".

In FIGS. 1D' and 1D", the elongated channels 32 are located within the interior of the polymeric framework of sheet 33 which is composed of a plurality of polymeric ribs or struts 33' interbonded at points 34 (FIG. 1) which may (FIG. 1D") or may not (FIGS. 1D and 1D') also be interbonded along substantially their entire lengths. In FIG. 1D", the ribs 33' are connected by the bridge 21" of cured polymer which is of a thinner dimension than ribs 33'. Channels 32 are interiorly located because all the filaments 15 which provided the channels 32 were completely enclosed with a shell 21, 21', 21" of cured polymer prior to leaching out the individual filaments 15 (see FIGS. 1C' and 1C"). However, in FIG. 1D, the surface of sheet 33 also contains elongated channels 32' similar to channels 32 but which are located exteriorly on ribs 33' instead of interiorly like channels 32. This is because not all the filaments 15 which provided the channels 32 were completely enclosed within a shell of cured polymer prior to leaching out the filaments 15. Thus note in FIG. 1C that certain of the filaments, designated 15', were still at or close to the surface of the ribs 24 after curing of the polymer. When strands 15' were leached out, they created the elongated voids 32' (see FIG. 1D) in the surfaces of sheets 33.

The elongated channels both on the surface (channels 32') and in the interior (channels 32) of sheet 33 generally have the same configuration as the leached solid material from which they were formed. Thus the network of voids created in sheet 33 by the leaching of the skeleton sheet is an approximate image of sheet 10. Illustratively, channels 32' and 32 are of a continuous filamentary configuration and have a length to diameter ratio of at least 100 and diameters ranging from about 0.1 to 5, and preferably from about 0.5 to 2, mils.

The number of channels 32 and 32' present in each rib 33', as well as their total length, internal surface area and volume can vary considerably, depending on such factors as the size and quantity of the precursor filaments 15 or strands 14 from which they were formed. For example, the number of internal channels 32 found in a given rib 33' is illustratively about 10 to 40. The total length of internal channels 32 present in a sheet can vary widely, e.g., from about 1 to 8 miles of such channels per square foot of sheet. The internal surface area of channels 32 is illustratively about 2 to 10 square feet per square foot of sheet. The volume of channels 32 can range from about 1 to 6 cubic centimeters per square foot of sheet, and can occupy about 20 to 60% of the total volume of the sheet. Preferably, the sheets 33 contain about 12 to 30 channels per rib, have a total channel length of about 2.5 to 6.5 miles per square foot, an internal surface area of 3 to 8.5 square feet per square foot, a volume of about 1.5 to 4.75 cubic centimeters per square foot and occupy about 30 to 53% of the total volume of the sheet.

Channels 32 and 32' can be essentially parallel to each other as shown in FIGS. 2-4 or they can extend in random or haphazard fashion as shown in FIGS. 2'-4'. It is not uncommon, for example, for the surface channels 32' to descend into the interior of the polymeric framework as shown in FIGS. 2' and 2'. Nor is it uncommon for the surface channels 32' or the interior channels 32 to intercommunicate as at locations 36 in FIGS. 3' and 4'. However, the channels 32 and 32' normally extend in a direction generally parallel to the plane of sheet 33, with most of the channels extending between the thin edges of the sheets and not between the two sides or faces of the sheet. This produces a network of voids 32, 32' which is oriented to provide continuous communication between the edges instead of between the faces of the sheet.

As will now be apparent, virtually any type of a network of voids can be built into the polymeric framework of the sheets by appropriate selection of the skeleton sheet 10. For example, the number and size of channels provided is readily controlled by the number and size of the filaments which make-up sheet 10. The ability to vary the voids or porosity of sheet 33 in this fashion can also be utilized to vary properties of the sheet which are related to its porosity such as its water vapor phase transfer rate and anti-microbial barrier layer properties.

The elongated channels 32, 32' also provides a reservoir for one or more medicating agents which can be released to a wound covered by the sheet over a period of time. The channels can be loaded with a medicating agent by immersing sheet 33 in a solution or dispersion of the agent until the desired loading is achieved. The sheet can then be removed from the solution or dispersion and dried if necessary. Illustrative medicating agents include medicines, antibiotics, antiseptics, germicides, antimicrobial agents, and other materials useful in treating wounds or burns. For example, the elongated channels of sheet 33 can be impregnated with PVP-Iodine (polyvinyl-pyrrolidone-iodine) complex by immersing sheet 33 in a 10% aqueous solution of the PVP-Iodine solids. PVP-Iodine is a water soluble, non-irritating microbiocide with broad spectrum activity and is available from the General Aniline and Film Corp.

Sheet 33 has a fabric texture on each of its surfaces similar to the texture of the leached skeleton sheet on which it was constructed. This type of surface provides pockets or reservoir spaces to accept the necrotic tissue and other debris from the wound site.

Water vapor phase transfer rates through sheet 33 can be controlled by varying the thickness of sheet 33, the extent of the open area in the sheet and the porosity imparted to the sheet by the elongated channels. Thickness and open area are largely controlled by the thickness and open area of sheet 10 and the extent of polymer build-up on sheet 10, while porosity is largely controlled by the nature of the skeleton sheet as previously discussed.

Sheets 33 are integral, continuous, non-laminated, non-woven, non-fibrous, non-filamentary, non-foamed, gauze or mesh-like sheets preferably formed from a single polymeric entity, and having a fabric texture on each side thereof. They have a structural framework comprising a plurality of interconnecting polymeric ribs or struts 33' which are spaced from each other along their length either by open area 18 (FIGS. 1D-1D') or a bridging layer 21" (FIG. 1D") which is thinner than ribs 33'. The ribs 33' define between them recessed portions 18 in (FIGS. 1D-1D') and 18' (in FIG. 1D") in both sides of sheet 33. The ribs 33' contain the elongated, continuous filamentary channels 32, 32' in their interior, and, optionally, on their surface, with the axis of most of the ribs and elongated channels being generally parallel to the plane of the sheet and extending between the edges of the sheet.

The size of the ribs 33' can vary widely depending on how they were formed. Illustratively, ribs 33' have a diameter of about 4 to 30, and preferably about 5 to 10, mils. The length of the ribs 33' is typically about 650 to 2600, and preferably about 975 to 2300, feet per square foot of sheet. The outer surface area of the ribs 33 is typically about 1 to 4 and preferably about 1.5 to 3, square feet per square foot of sheet.

A number of sheets were prepared using the procedures illustrated in FIG. 1. For example, multifilament nylon (nylon 66) mesh of sizes 1 .times. 40 (denier per strand) / 13 (filaments per strand 1 .times. 50/13 and 1 .times. 50/17 obtained from Hanes Corporation were stretched on 91/4 inch by 11 inch on stainless steel frames and then immersed in a silicon rubber-hexane dispersion (RTV-7000, General Electric Co.). The number of dips in the silicone rubber dispersion and the concentration of solids in the dispersion were varied. Each dip was carried out by immersing the stretched nylon mesh sleeves in the silicone rubber-hexane dispersion, and then immediately removing the sleeves from the dispersion at a constant withdrawal rate of about 38 inches per minute. Each sample required about 15 seconds before it was completely withdrawn from the dispersion. Between dips, the coated nylon was air dried for about 10 minutes. The silicone rubber on the nylon was then cured at about 75.degree. C. for 11/2 hours. The coated nylon was then immersed in formic acid at room temperature for about 16 hours or longer to dissolve out the nylon. The silicone rubber sheets which remained were dried and various physical properties of the finished sheets were then measured. The following results were obtained.

As the data in Table 1 show, sheets with a wide variation of properties are obtainable. The open area of the sheets can be varied from 0 to about 60%, as desired, and water vapor phase transfer rates of about 2 to 20 mg./hr.-cm..sup.2 or more are obtainable. Preferably, the percent open area is about 10 to 50% and the water vapor phase transfer rate is about 2 to 10 mg./hr.-cm..sup.2 in order to prevent excessive drying of the wound surface. By varying the thickness and/or extent of open area, varying degrees of transparency can be imparted to the sheet. The sheets have excellent drapability and conformability characteristics and have illustrative thicknesses of about 5 to 30 mils. The sheets elongate easily in both directions, as much as 100%, and preferably as much as 200% or more. Illustratively, a 1 .times. 3 inch strip will stretch 100% in a direction parallel to the 3 inch dimension at a force of less than about 0.5 pound per inch, and will stretch 200% at a force of less than about 1 pound per inch. Typically, a force of about 0.1 to 0.3 pounds is required for 100% stretch and a force of about 0.2 to 0.5 for 200% stretch.

The particular properties of the sheet are normally selected in accordance with the desired end use of the sheet. For example, if the sheet is to be used as a burn dressing, a low water vapor phase transfer rate approaching that of human skin would be desired.

Additional sheets were prepared in a manner similar to that just described for the sheets of Table 1 using a variety of different nylon meshes as the extractable component. The nylon mesh samples used were obtained from Hanes Corporation, Amtex, Inc. and Finetex Elastic Corporation. The Hanes sample was identified as 1 .times. 40/13 (see Example 1). The Amtex sample was identified as Style No. 224-2. The three Finetex samples were identified as 2, 9, and 15, respectively.

Each sample of nylon was immersed in a 22% solid dispersion of General Electric RTV-7000 silicone rubber in hexane and then removed from the bath as in Example 1. The sample was air-dried for 20 minutes and reimmersed in the 22% RTV-7000 silicone rubber dispersion as before. The sample was then air-dried for 20 minutes and cured at 75.degree. C. for 2 hours.

The resulting products were then leached with 88% formic acid for at least 8 hours, after which the leached products were rinsed in water for 10 minutes and dried at room temperature.

The following data on the microchannels 32 and the ribs 33' was obtained on the various samples:

The process illustrated in FIG. 1 could be readily performed on a continuous basis by advancing a web or continuous strand of the skeleton sheet 10 sequentially through (1) a bath 12 of polymer to coat it with polymer, (2) a curing chamber or oven to cure the polymer on sheet 10, and (3) a leach tank containing a solvent to selectively dissolve sheet 10 but not the cured polymer, and finally (4) a drying chamber to remove solvent from the finished product.

FIG. 5 depicts a process for preparing polymeric sheets having a coarse fabric texture on one side only. The other side, or non-fabric side, is a pinhole-free, ultra-thin silicone rubber membrane which is an anti-microbial barrier layer but remains sufficiently permeable to water vapor to permit its usage as a wound dressing. A plurality of filamentary micro-channels can be optionally provided at the fabric side of the sheet.

Briefly, sheets of this type are prepared by forming the ultra-thin membrane layer of uncured silicone rubber and embedding in one side of the silicone rubber layer a material which will impart a fabric texture to the one surface. The silicone rubber is then cured to produce a sheet having one surface which is smooth and one which has a fabric texture. The fabric embedded in the membrane layer can be a gauze or mesh or other equivalent, either coated or uncoated with a polymer, or it can be the fabric-textured sheets prepared by the process of FIG. 1. If the fabric sheet is coated with a polymer, the fabric sheet is optionally leached out to create a network of voids at one side of the sheet corresponding in configuration to the leached fabric sheet.

Referring to FIG. 5, a supporting or forming surface such as plate 50 is immersed in a bath 52 of silicone rubber to coat each surface of the plate with a layer 53 of silicone rubber (see FIG. 5B). Since a main concern is the eventual separation of the finished polymeric sheet intact from plate 50, it is desirable to employ a non-sticking, high-release silicone rubber in bath 52 and a plate 50 having a smooth surface with good release or non-stick properties. Removal of the silicone rubber from the plate becomes a serious problem because normally the silicone rubber layer must be kept very thin, e.g., about 0.2 to 2 mils, in order to obtain desirable water vapor phase transfer rates. The thinness of the sheet detracts from its strength and unless care is taken the sheet can tear or otherwise rupture as it is being separated from the support plate. This tendency is greatly minimized by using a combination of a high-release silicone rubber as the layer in contact with plate 50 and a plate with a non-sticking surface.

Plates having a smooth, highly polished or mirror surface are preferred. For example, metal plates surface-coated with chrome are quite satisfactory, as is highly polished metal foil such as aluminum foil. In addition, any smooth surface of low surface tension can be used including, teflon, polyethylene, polypropylene, teflon coated metal, and the like. Release agents or non-stick agents such as various polyvinylchlorides, soaps and other fatty materials can also be advantageously applied to the surface of plate 50 before it is immersed in bath 52.

The silicone rubber in bath 52 is preferably one which is characterized by high-release or non-stick properties. For example, a preferred silicone rubber for bath 52 is one which after curing can be separated intact from plate 50 by the application of a 180.degree. peel strength of less than about 25, and preferably less than 15, grams per inch. As used herein, 180.degree. peel strength is the maximum force required to peel away intact an ultra-thin (i.e., less than about 2 mils in thickness) strip of material at an angle of 180.degree. divided by the width of the strip of material. For example, if the strip dimension was 1 inch by 3 inch, and the direction of peel was substantially perpendicular to the 1 inch dimension, the strip width to be used in calculating the peel strength would be the one inch dimension, not the three inch dimension.

If peel strengths become too excessive, the silicone rubber sheet, because of its thinness and relatively low strength, tends to tear or rupture in the effort to remove it from plate 50. The problem cannot be overcome by resort to increased thickness because once the thickness exceeds the ultra-thin level of about 2 mils or less, the water vapor phase transfer rate declines sharply to values which are too low to permit usage of the sheet as a burn dressing.

General Electric's RTV-615 silicone rubber is one example of a high release silicone rubber suitable for use in bath 52. It is a dimethyl type silicone rubber with a relatively low molecular weight.

Of course, other materials besides silicone rubber can be used provided they are capable of providing water vapor phase transfer rates comparable to silicone rubber. The problem with many materials is that they have much lower permeability to water vapor than silicone rubber making it necessary to use such thin films of the material in order to obtain acceptable water vapor phase transfer rate that the strength of the film is too low for practical usage. The polymer in bath 52 could be any non-stick or high release polymer which could be separated from plate 50 with a 180.degree. peel strength of less than about 25 grams per inch and which would have water vapor phase permeability characteristics essentially equivalent to those of silicone rubber.

The coated forming plate 54 is preferably, but not necessarily, air-dried at room temperature for about 15 minutes to 2 hours to evaporate liquid from the dispersion and increase the concentration of silicone rubber on the plate.

The dried plate is immersed in a second bath 58 of a silicone rubber of a different type than is present in bath 52 in order to add to each side thereof a second layer 60 of silicone rubber 58 (see FIG. 5C). The silicone rubber in bath 58 is selected for its high tensile and tear strength characteristics. The high release silicone rubbers used in bath 52 generally tend to have poor tensile and tear strength. To compensate for this, bath 58 builds onto the first silicone rubber layer 53 a second layer to add tensile and tear strength to the finished sheet. In general, any silicone rubber can be used in bath 58 which upon curing has a tensile strength and tear strength for a 25 mil thick cured film of at least about 750 pounds per square inch and 75 pounds per inch, respectively.

Polymers other than silicone rubber could be used in bath 58 provided they have the above strength characteristics and water vapor phase transfer rates equivalent to those of silicone rubber.

The plate 70 from the second dip tank is then preferably, although not necessarily, air-dried in the same manner as described above for plate 54.

General Electric's RTV-7000 silicone rubber discussed above, is one example of a silicone rubber which has been found suitable for use in bath 58. RTV-7000 has a molecular weight after cure which is substantially greater than that of the RTV-615 silicone rubber of bath 52.

The amount of silicone rubber applied to the plate in baths 52 and 58 can be varied as desired by altering such variables as the concentration of solids in the baths, the residence time in the baths and the number of times the plate is dipped into the baths. Illustratively, the concentration of solids in the baths is about 10 to 60% and anywhere from 1 to 5 dips of a few seconds duration, e.g. 5 to 60 seconds, normally suffices.

Illustratively, only enough of the high-release silicone rubber is applied in bath 52 to form a uniform layer 53 (see FIG. 5B) of sufficient thickness to cover the surface of support plate 50 and thus facilitate easy release of the finished sheet from the plate. Normally, only enough silicone rubber is applied to plate 50 in bath 52 to form a layer of less than about 1 mil in the finished sheet, and preferably about 0.1 to 0.3 mils. This thin layer 53 forms one part of a transparent, pinhole-free, anti-microbial barrier layer 54, while the layer 60 of silicone rubber applied in bath 58 forms the other half.

The high tensile strength, high tear strength silicone rubber used in bath 58 is applied to the plate in sufficient amounts to provide an overall membrane thickness (which includes the thickness of the high-release silicone rubber layer 53) of less than about 2 mils in the finished sheet, and preferably about 0.5 to 1.5 mils. Illustratively, the thickness of the high strength layer 60 can vary from about 0.2 to 2 mils in the finished membrane and preferably is about 0.5 to 1.5 mils.

Because the water vapor phase transfer rate is strongly dependent on the overall thickness of the pinhole-free membrane 54 formed by the two layers 53, 60 of silicone rubber picked up in baths 52 and 58, respectively, it is important that the overall thickness of the membrane portion of the finished sheet be kept highly uniform, e.g., .+-. about 0.2 mil. The vertical dip-coating techniques shown in FIG. 5 have been especially useful in producing sheets whose membrane component is of substantially uniform thickness throughout, at least for sheets whose vertical dimension does not exceed about 12 inches. This results in finished sheets of highly uniform water vapor phase transfer rate characteristics, an important consideration in a burn dressing.

Although FIG. 5 depicts the application of two silicone rubbers having different characteristics to plate 50 in two separate steps, it is to be understood that FIG. 5 represents only a preferred embodiment of the invention. Acceptable products could also be produced by fabricating the pinhole-free membrane layer 54 from a single material which would involve the immersion of plate 50 in but a single bath instead of two. For example, the pinhole free membrane 54 could be formed solely from RTV-615 silicone rubber, especially in applications where water vapor phase transfer rates were not particularly important so that strength could be increased by increasing the overall thickness of the RTV-615 rubber. Similarly, the pinhole free membrane could be formed solely from RTV-7000 silicone rubber in applications where water vapor phase transfer rates were not particularly important so that the thickness could be increased to avoid tearing or rupturing the sheet when it was removed from plate 50.

Baths 52 and 58 comprise a liquid which contains a polymer. The baths can be a solution or dispersion of the polymer, a latex, or any other form of the polymer which will coat the surface of plate 50 with polymer.

If plate 50 is immersed more than one time in baths 52 or 58, the dipped plates can be optionally dried and/or cured or partially cured between each dip, as well as after the final dip. Any combination of drying alone or drying plus curing could be employed after each dip.

Returning now to FIG. 5, the next step following buildup of the membrane layer 54 on plate 50 is to apply to the outermost layer 60 of the silicone rubber membrane the material which will provide the fabric texture at one side of the finished sheet. Normally, this is done by applying to layers 60 on the air dried plate 70 a sheet 71 having a coarse, fabric texture or other type of coarse or rough surface (see FIG. 5D). Sheets 10, 17, 20 and 33 discussed above in connection with FIG. 1 are examples of the sheet material which can be used as sheet 71. The nature of sheets 10, 17, 20 and 33 have already been explained in detail. Sheets 10 and 17 are preferred sources of sheet 71 for this embodiment of the invention. Although FIGS. 5D-5E show sheet 71 as being composed of a plurality of strands 71' separated by open spaces 72, sheets 17, 20 and 33 (from FIGS. 1B", 1C" and 1D") which have no open spaces between the strands could also be employed as sheet 71.

For the case where sheet 10 from FIG. 1 is used, the sheet preferably will stretch or elongate at least about 100% and preferably about 100 to 300% in each direction. For example, sheet 10 preferably will stretch 100% in a given direction by the application of a force less than about 0.5 pound (for a 1 .times. 3 inch strip), and will stretch 200% in a given direction by the application of a force less than about 3 pounds on the same basis. Sheet 10 preferably also has good wicking and liquid absorption characteristics to aid in removing liquids from a wound site at medically acceptable rates. Sheet 71 is preferably formed from a plurality of multifilament yarn strands.

Once sheet 71 has been placed on the silicone rubber layer 60, it is forced into layer 60 deep enough to form a good bond therewith but not deep enough to pierce through to the surface of the support plate 50 or to totally embed sheet 71 in the silicone rubber (see FIG. 5E). It is important that one side of sheet 71 remain exposed to provide the coarse, fabric-like surface in the finished sheet so as to be available for adherence to the wound area.

The silicone rubber in layers 53 and 60 is then cured while one side of sheet 71 is embedded in the rubber to bond sheet 71 to the pinhole-free silicone rubber membrane portion 54. This is conveniently done by sandwiching plate 70 with its adhering sheets 71 between two sheets 73 of cured silicone rubber whose surfaces adjacent sheet 71 are preferably covered by a material such as aluminum foil or a relatively thick layer of nylon fabric. The resulting composite is then placed between two platens 75 of a conventional press, which may be heated by electrical resistance wires 77. Pressure is applied to the composite while it is being heated by the platens to cure the silicone rubber. Enough pressure is used to embed sheet 71 to the desired level in the silicone rubber. The pressure selected can depend on a number of factors such as the viscosity of the silicone rubber and the percent open area of sheet 71. Illustratively, pressures of about 10 to 200 psi suffice for many applications. Curing temperatures and times for silicone rubber are well known to those skilled in the art. Illustrative cure cycles involve treatment at room temperatures to about 200.degree. C. for about 15 to 48 hours.

As the rubber cures, it securely bonds to the individual strands 71' of sheet 71, thereby anchoring sheet 71 to the pinhole-free cured silicone rubber membrane portion 54' composed of cured rubber layers 53' and 60', as best seen in FIG. 5E. The curing step will also cure any polymer on the surfaces of sheet 71. For example, if sheet 71 originates from sheet 17 of FIG. 1, the polymer 12, 12' and 12" on sheet 71 will cure in the curing step.

After curing, the composite 80 of membrane 54' and sheet 71 can take one of two routes depending upon the nature of sheet 71 and the desired end product. If sheet 71 originated from sheets 10, 17, 20 or 33 of FIG. 1, and if there is no need to remove the skeleton formed by the presence of sheet 10 in sheets 17 and 20, the composite 80 of membrane 54' and sheet 71 is separated from plate 50 to produce the finished polymeric sheet 81 having on one side a generally smooth even surface 82 and on the other a coarse, uneven, fabric-like surface 83, as best seen in FIGS. 5F and 5G. FIG. 5F exemplifies the situation where sheet 71 was a sheet of multifilament strands 14 each containing a plurality of individual monofilaments 15 (such as sheet 10 in FIGS. 1 and 1A) which had not been coated with a polymer prior to its application to the silicone rubber layer 60. It is evident that if sheets 17 or 20 from FIG. 1 were used instead of sheet 10, the voids 16 between the individual filaments 15 in each yarn strand 14 (see FIG. 5F) would be filled with cured polymer 21 (as shown in FIGS. 1C to 1C" and 5G), and each multifilament strand 14 could be encased within a sheath of cured rubber 21' (as also shown in FIG. 5G) whose thickness would depend on the degree of polymer coating on sheets 17 or 20. In short, the finished sheet would be the same as the sheet shown in FIG. 5F except that the voids 16 in FIG. 5F would be filled with cured polymer 21, 21', as shown in FIG. 5G. If sheet 33 from FIG. 1 was used as sheet 71, the filaments 15 would have already been leached out so that the finished sheet, after removal from plate 50, would be as shown in FIG. 5I, with the voids 32 replacing the former location of the filaments 15.

On the other hand, if sheet 71 originated from sheets 10, 17 or 20 in FIG. 1, and it was desired to remove the skeleton sheet 10 contained therein to provide a network of elongated continuous voids in the sheet similar to voids 32 and 32' in FIGS. 1D to 1D", the composite sheets 80 (see FIG. 5E) are immersed in a solvent 90 in which the skeleton sheet 10 is soluble but in which the cured polymer coating on the skeleton sheet is substantially insoluble. The solvent 90 leaches or dissolves out the skeleton sheet 10 creating a plurality of elongated channels or voids 32, 32' in the finished sheets 93 (see FIGS. 5H-5I) as described above in connection with FIGS. 1D to 1D". The nature of the coarse surface 83 of the finished sheet 93 and the voids 32, 32' in sheet 93 depends on the nature of the skeleton sheet 10. If sheet 10 was composed of a multifilament yarn which has not been coated with a polymer prior to its application to the silicone rubber membrane layer 54, the finished product would appear as in FIG. 5H, with a combination of internal voids 32 and surface voids 32' where filaments 15 had once been (compare FIGS. 5F and 5H). If sheet 10 had been coated with a polymer prior to its application to membrane layer 54, the finished product would appear as in FIG. 5I. It can be appreciated that the coarse surface 83 of sheet 93 in FIG. 5I is essentially sheet 33 from FIG. 1, and accordingly can be varied as desired in the manner discussed above for sheet 33 and as described in FIGS. 1D to 1D".

The skeleton sheet 10 can, of course, be leached from the composite 80 either before or after its separation from plate 50. Preferably the sheet is leached prior to separation from plate 50, provided plate 50 is substantially insoluble in the leaching solvent, and is then air-dried to reduce its moisture content and separated from plate 50 to produce the finished polymeric sheet 93.

As shown in FIGS. 5H and 5I, sheet 93 has a smooth, uniform surface 82 on one side and a coarse, fabric-textured surface 83 on the other. The coarse surface 83 in FIG. 5I comprises a plurality of cured rubber ribs or struts 33' each containing a plurality of filamentary voids 32 therein where the leached skeleton filaments 15 were at one time located. As pointed out in connection with FIGS. 1D, 2 and 2', ribs 33' could also contain elongated surface channels such as channels 32' in FIGS. 1D, 2 and 2' depending upon the extent to which the leached skeleton strands 14 were coated with polymer in the process of preparation.

The finished sheets 81 and 93 have a smooth-surfaced, thin, pinhole-free, substantially non-porous and voids-free, non-foamed membrane layer 54' on one side thereof which has antimicrobial barrier layer properties and desirable water vapor phase transfer rates. The other side of sheets 81 and 93 has a fabric texture imparted to it by the ribs 14 (FIG. 5F), 24 (FIG. 5G) or 33' (FIG. 5I) joined to membrane 54' or by the elongated voids 32' (FIG. 5H). Sheets 81 and 93 can be formed from one or more polymeric entities. The axis of most of the elongated channels 32, 32' and of the ribs is generally parallel to the plane of sheets 81 and 91, as discussed above in connection with sheets 33. The sheets 81 and 93 are very thin and drapable.

The finished polymeric sheets 81 and 93 have properties similar to those discussed above for the sheets 33 produced by the process of FIG. 1. The water vapor phase transfer rate is a function of the thickness of the pinhole-free, anti-microbial barrier or membrane layer 54' and the open area of the fabric-side 83 of the respective sheets. Generally, it is necessary to keep the thickness of the membrane layer 54' quite thin, e.g., less than 2 mils, in order to obtain water vapor phase transfer rates which approximate those of human skin. The ribs 14, 21' and 33' in sheets 81 and 93 are normally significantly thicker than the membrane 54', and illustratively are about 10 to 6 mils in thickness.

Wettability of the fabric-textured side of sheets 81 and 93 is important in many medical applications. Wettability is defined and compared herein by applying to a given sheet of the invention a 0.006 cubic centimeter drop of 1% by weight Congo Red dye in water for 10 seconds. Excess dye solution is then removed by wicking with tissue paper. The diameter of the spread of the dye solution on the sheet surface is then measured and the area of the spread computed from the measured diameter. Higher area values mean increased wettability characteristics. Using this test procedure, illustrative spread diameters and spread areas of anywhere from about 2 to 20 millimeters and about 3 to 300 square millimeters, respectively, have been obtained with the sheets of the invention whose fabric-textured side is formed by a sheet of multifilament yarn. These variations were largely attributable to differences in the materials and structures used to form the fabric-textured side of the sheets. The sheets can be fabricated to yield a wider range of desired wettability characteristics on the fabric side.

Medicating agents of the type previously discussed can be incorporated into the coarse or fabric-textured side of sheets 81 and 93 by impregnation of the elongated voids 32, 32', or of voids 16 in the case of the embodiment of FIG. 5F.

FIG. 6 depicts another process for preparing polymeric sheets having a smooth membrane layer on one side and a coarse fabric texture on the other side. The advantage of the processing technique of FIG. 6 as compared to that of FIG. 5 is that it results in a completely non-laminated product thereby eliminating the possibility of the various layers of the composite experiencing undesirable delamination.

Referring to FIG. 6, a sleeve 100 of fabric material of the same type as described above in connection with skeleton sheet 10 is fitted over a cylindrical drum 101 in such manner that sleeve 100 is stretched on drum 101, thereby causing it to be uniformly pressed toward the external surface of the drum. Illustratively, sleeve 100 can be circumferentially stretched anywhere from 10 to 200% on drum 101.

Sleeve 100 serves as a skeleton on which the polymeric sheets of the invention are constructed. Drum 101 serves as a supporting surface for sleeve 100 during the build-up of polymer on sleeve 100. Drum 101 also serves as a surface on which an ultra-thin membrane layer of polymer can build-up in such a way that it forms a continuous phase with the polymer which coats the surfaces of the sleeve 100.

Drum 101 preferably has a smooth uniform surface, although a coarse surface can be used if a coarse rather than smooth texture is desired for the membrane side of the finished sheet. Teflon and polypropylene are two examples of materials which can be used to fabricate drum 101.

The composite 102 of sleeve 100 and drum 101 is then immersed in a bath 103 of a polymer using the handles 104 which are provided at the top of the drum. Preferably, bath 103 is a solution or dispersion of a silicone rubber. A preferred silicone rubber is General Electric's RTV-7000 discussed above. The amount of silicone rubber applied in bath 103 can be varied as desired by altering such parameters as the concentration of solids in the bath, the residence time of sleeve 100 in the bath and the number of times the composite 103 is dipped into the bath. Illustratively, the concentration of rubber solids in the bath is about 10 to 60% and anywhere from 1 to 5 dips of a few seconds duration, e.g., 5 to 60 seconds, normally suffices. Different type silicone rubbers may be used in successive baths if desired.

Sleeve 100 is preferably composed of a plurality of multifilament strands 14, each composed of a plurality of individual filaments 15 with void spaces 16 between them, as best seen in FIG. 6A. The polymer bath 103 permeates the void spaces 16 to the degree desired and forms an ultra-thin layer 106 of polymer on the surface 107 of drum 101, between surface 107 and the strands 14, as best seen in FIG. 6B. Layer 106 extends between the individual strands 14 and to form a sheet of polymer to which each of strands 14 is joined. It is this ultra-thin layer 106 which, upon curing, provides the thin smooth membrane layer of the finished sheet.

After the stretched sleeve 100 has been treated with bath 103 for a sufficient time, the composite 102 of drum 101 and sleeve 100 is removed from bath 103. The polymer is then cured, as previously described, and removed from drum 101 to produce the finished sheet 110 (see FIG. 6C). Sheet 110 has a membrane layer 111 of cured polymer and a coarse fabric-textured layer 112 also formed of cured polymer. The yarn strands 14 which make up sleeve 100 are encapsulated in cured polymer layer 112 and also partly in layer 111. Layer 111 has a smooth surface 113 whereas layer 112 has a coarse fabric-textured surface. This coarse surface is formed by the portions 114 of cured polymer which project from membrane layer 111 and form a continuous polymeric phase with layer 111. In contrast to the sheets shown in FIGS. 5F to 5I, the smooth and coarse layers 111, 112 of sheet 110 are formed from a continuous phase of cured polymer not from two separate pieces of which are laminated together to provide the smooth and coarse sides of the sheet, respectively. This avoids delamination problems by providing a sheet which is formed of a single integral, unlaminated piece of polymer.

If desired sheet 110 can be immersed in a solvent 115 (see FIG. 6) which dissolves the sleeve 100 but not the cured polymer. As sleeve 100 dissolves it creates a sheet 120 (see FIG. 6D) containing elongated channels 32 in coarse side of the sheet. Channels 32 are of the same type as those previously discussed in connection with other embodiments of the invention.

Illustratively, the thickness of the ultra-thin membrane layer 111 in sheets 110 or 120 is about 0.5 to 2 mils, while the thickness of layer 112 can vary from about 10 to 60 mils.

The sheets 110 and 120 shown in FIGS. 6C and 6D have essentially the same characteristics as those of FIGS. 5G and 5I, respectively.

Although sheets 110 and 120 were prepared by stretching the sleeve 100 on a cyclindrical surface such as drum 101, sheets 110 or 120 could also be prepared by mounting a suitable skeleton sheet, such as sheet 10 of FIG. 1, against an appropriate surface, e.g., a flat surface, to form a composite of the sheet and surface, and then immersing the composite in bath 103. In such case, the skeleton sheet would not necessarily have to have a sleeve configuration and could be for example, a piece of knit or woven cloth formed from a plurality of multifilament yarn strands.

It can be appreciated that the sheets of this invention are particularly useful as dressings or coverings for wounds and, in particular, for burns because of their antimicrobial barrier layer properties and water vapor phase transfer rates. The sheets are applied to wound sites in such manner that the fabric-textured side is toward the wound. If a non-sticking dressing is desired the fabric-textured side should be formed from a low surface energy material such as silicone rubber, teflon, polyethylene, polypropylene or the like whereas if an adherent dressing is desired, the fabric-textured side should be formed from a high surface energy material such as nylon, dacron, cotton or polymethane.

The sheets can be sutured to the wound site or adhesively applied. The voids or open area in the coarse surface provides a reservoir for wound debris. The sheets have excellent conformability or drapability characteristics and the desirable ability to stretch significantly in two directions because of their elastomeric nature.

The presence of the elongated voids 32, 32' is important for several reasons. For example, they provide a means for varying the water vapor phase transfer rate through the sheet by imparting more or less voids to the sheet. They also provide reservoirs for medications which can be released to the wound site and a reservoir for wound debris. Finally, the voids can remove liquid from the wound site by capillary action.

When the sheets are ultra-thin, as is preferred when they are to be used as burn dressings, they are also transparent or translucent thus permitting the doctor to examine the progress of the wound without having to remove the dressing.

The sheets of the invention can be readily sterilized using ethylene oxide. For example, exposure to ethylene oxide vapors at a pressure of about 9 psi for 4 hours at 130.degree. F. will suffice for most applications.

The specific and detailed information presented above was for purposes of illustration only, and such alterations, modifications and equivalents thereof as would suggest themselves to those skilled in the art are deemed to fall within the scope and spirit of the invention, bearing in mind that the invention is defined by the following claims.