IL46797A - Synthetic surgical dressing - Google Patents

Description

46797/2 SYNTHETIC SURGICAL DRESSING This invention relates to the treatment of mammals, particularly humans, and, to some extent, animals, which have incurred severe burns or tissue damage which leaves an exposed wound whose skin cover has been damaged. One purpose is to convert a contaminated and open wound to a clean and closed wound in the shortest time possible, protect the wound from outside infection, and maintain it in such condition that the healing process will proceed to a point where coverage by and the taking of autograft skin becomes possible.

It has been customary to use human cadaver skin or porcine skin among other devices to dress and protect wounds and to aid in the development of granulation tissue prior to autografting.

The wound dressing should make the patient more comfortable, reduce protein loss, reduce evaporative 46797/2 heat and water losses from the wound surface and prevent further contamination. It is desirable that the use of the dressing results in decreasing existing^ bacterial growth in the wound. At times a bleeding wound or wound losing ^ plasma rapidly is present as a part of a larger wound and the dressing should aid in minimizing blood or plasma loss. It is desirable that the dressing remain flexible and adherent to the wound at all times and ba sufficiently flexible to permit at least some flexing of the wound surface. The desired degree of flexing to some extent varies with the location of the wound. For example, a wound surface adjacent a joint should preferably have greater flexibility than a wound adjacent a less flexed area such as the chest or back.

In general, the characteristics of a wound dressing, particularly a burn dressing, are known and recognized, but improvements are very much in demand.

United States Patent No.3,800,792 concerns polyurethane foam backed burn dressings having an animal skin surface prepared either from ground cow (bovine) hide or collagen fibers from animal tendons.

The burn dressingsof the present invention are quite distinct from those claimed in the said US Patent 3,800,792. Their synthetic absorbable knit or flocked surface provides a dermal scaffolding which slowly degrades in contact with the granulating tissue.

There is no foreign body reation associated with the fibers left in the granulating tissue following dressing changes. The synthetic polymer merely falls free from the nonabsorbable backing layer and the remains in the living tissue are absorbed.

By providing an open surface into which granulating tissue's body fluids can readily penetrate, greater conformity between the dressing and wound is possible* This minimized the formation of air and liquid pockets between the dressings and the patient's tissue and as a result avoids hypertrophy, infection and fibrosis.

This is not possible with foam rubber or polyurethane foam dressings. They can soak up weeping tissue fluids. However, they cannot present open surfaces to serve as a scaffolding for granulating tissue because upon changing the dressing either fragments of the rubber would be left imbedded in the tissue or more likely would tear loose causing wound disruption.

Finally, from the structural and manufacturing point of view susyn-jketic":absorbable knitted and flocked dressings of the present invention are obviously quite unlike the rubber foams, cadagvr allografts and the porcine and bovine natural and foam backed dressings heretofore employed.

More particularly the present invention relates to a wound dressing, particularly adapted for major burns in humans in which at least a part of the 46797/2 or natural rubber or poly(caprolactone) which has a moisture permeability of the order of that of the intact human skin and to which is adhesively flocked to a layer of fibers or is adhesively united a knitted fabric, said fibers or fabric being composed of a tissue compatible and absorbable material.The fabric is of a close knit stitch. The fibers are preferably predominantly at about right angles to the elastomeric layer. Both the floc¾_fea fibers and knitted fabric are so spaced as to permit slight invasion of granulation tissue, and can be remov-ed from the granulation tissue without pulling off such granulation tissue and with minimum damages to the developing granulation tissue to leave a bed for autograft-ing or other subsequent treatment.

The fibers themselves are absorbable by living tissue so that fibers which are trapped within the wound are absorbed by the living tissue without deleterious effects .

The present wound dressing may be used on almost any type of wound in which the skin is broken and body fluids, particularly blood and serum, are released by the wound. It is primarily designed for major burns in which a substantial area of the skin of a human is destroyed leaving underlying tissues which are either contaminated, or subject to contamination by bacteria, or other contaminants, and which wound is of such a size that new skin covering for it may not form spontaneously, or that immediate protection and dressing is required. Decubitus ulcers, sometimes called bedsores, are effectively dressed with the subject dressing. It is desirable to change the wound from a contaminated and open wound to a clean and closed wound in the shortest time possible, to minimize protein loss, control evaporative heat and water losses, and prevent further contamination during the time required for the underlying tissues to build up a granulating surface which will accept a skin autograft.

By skin autograft is meant skin taken from another area of the subject, and which is living so tha¾. when placed on the granulating tissue, the skin will graft to the granulating tissue and grow providing a skin sur-face at the wound area. With rare exceptions, the skin of any other subject will be rejected and not permanently graft to the wounded subject. With major burns, a sufficiently large portion of the subject may be burned so that sites for donor skin are limited and it is necessary to protect burned areas for a prolonged period until graft donor sites can regenerate and be used for a subsequent graft.

The generalized subject of burn treatment of this type is fairly well recognized as are many of the requirements for such a wound dressing. The medical profession recognizes the need for improvements in burn site protection, during burn treatment. Survivability after major burns has been improving — the present invention contributes towards such improvement.

The use of a synthetic tissue absorbable dressing eliminates many disadvantages of prior art dressings, particularly as to availability, and size, and represents a substantial step forward in burn treatment.

The dressing should conform to the surface of the tissue. Conformation comprises an assessment of the suppleness, resiliency, and the dressing's ability to mimic the topography of the wound in such a fashion that there is a minimum gap between the tissue and the dressing which minimizes air gaps and pools of liquid. If pools of liquid build up, whether of serum or blood, such pools become sites for the growth of undesirable microorganisms. If the dressing conforms adequately to thd¾fc. surface of the wound, the body's own defense mechanisms are effective in reducing the bacteria population even in a contaminated wound.

In modern hospital technology, any area not known to be surgically clean must be regarded as contaminated or at least suspect, and treated as if contaminated.

For proper protection of the tissue surface, the loss of body fluids such as serum or blood need to be prevented by the dressing but at the same time just as natural skin is permeable to moisture vapor, a wound dressing must permit the escape of at least some moist-ure to prevent pooling of body fluids which pools become sites for infection. If the rate of moisture loss is too high, there is a cooling effect from the heat required to convert the liquid to vapor. If the wound is not covered so as to reduce moisture loss, electrolytes, pro-teins, and other materials concentrate and crust on the surface, leading to eschar formation. It is desirable that the permeability be of at least approximately that of the intact human skin (which is found to be about 2.2 mg./sq. cm./hr.) and may be several times this value. A permeability of 4.7 mg./sq. cm./hr. has been found effective under many conditions. A permeability of about 0.4 mg./sq. cm./hr. to about 6 mg./sq. cm./hr. gives good water vapor loss control. The values are approximate as different skin areas lose moisture at a different rate and, depending upon the temprature of the subject, the temperature of the atmosphere, the movement of ambient air and under other variables, the natural skin has a 1 considerable range of skin moisture loss. The natural skin is adaptable to a wide range of environmental con-ditions so that if the wound dressing has a moisture loss which is approximately that of the intact human skin and remains within this range of acceptable conditions, the underlying tissues are protected and regenerate. There is a fine and, perhaps, unascertainable line between the regenerative processes of the tissue when protected from adverse effects and the increase that might be caused by dressing characteristics and environmental factors. It is not necessary to ascribe the rate of healing to any special set of factors. It is merely found that by using the present wound dressing, rapid healing is encouraged.

For adequate conformation, the wound dressing must be flexible so that it can conform to the topography of the wound and at the same time it must be sufficently flexible that as the tissues move, the wound dressing can move with them. For instance, if the skin adjacent to a joint is injured, flexing of the joint and movement of the underlying tissues requires that the wound dressing have some flexibility. The minimum required flexibility is exceeded by the present dressing and the dressing is sufficiently flexible for tissue movement over joints and other areas.

In the case of the knitted fabrics continuous fibers of about 1/2 to 12 denier per filament in a yarn which is neither spun nor bulked nor textured are con- veniently used.

It is desirable that the fabrics or flocked^-fibers be absorbable by living tissue as some of the fibers may remain in the wound, and non-absorbable ma-terials can be the source of undesirable side effects.

A preferred tissue absorbable material is a polyglycolic acid polymer, such as a homopolymer of gly-colide or a copolymer containing small quantities of other materials, such as dl-lactic acid, its optically active forms, homologs and analogs. Any polymers in which tissue absorption results from the hydrolytic degradation of glycolic acid ester linkages can be used with good results. Because strength of the fibers is not a major requirement, a copolymer containing considerable lactic acid makes a good dressing.

The important thing about the tissue absorbable polymer is that it be of a material which is not deleterious to living human tissue, and that it be spinn-able as a fiber, and it has sufficient strength to main-tain its integrity during the manufacture of the wound dressing, that it maintains its integrity in contact with the tissue long enough for granulation tissue to form and the dressing removed, about 4 to 10 days and yet is absorbable within a reasonable length of time, for in-stance, about 90 days, so that no fragments remain as a foreign body trapped in tissue after the wound has healed. The dressing may be changed after 24 hours, or less, but mature granulation tissue does not form in such a short time.

The fibers which conveniently are spun at about 1/2 to about 12 denier per filament, are in a non-textured continuous yarn which is knitted as described be¾ w. The knitted fabric may be adhesively united to the elastomeric layer as it is formed. More conveniently, when the elastomeric layer is formed, a separate adhesive layer is placed thereon and the knitted fabric is placed on such adhesive layer. Evaporation of the solvent in the polyurethane layer unites the knitted absorbable fabric to the moisture-controlling polyurethane layer .

The elastomeric layer may be any material which is permeable to moisture vapor, has a low Young's modulus, and is thus readily conformable to the wound surface.

Elastomers with a Young's modulus of between about 200 pounds per square inch per inch and about 40,000 pounds per square inch per inch give acceptable results. A particularly useful material is polyurethane. A soft, flexible elastomeric layer of polyurethane of the order of 1 mil thick (0.1 to 5 mil) provides a barrier to excessive water loss, is permeable to moisture within the range of that of the intact human skin, which is approximately 2.2 mg./sq. cm./hr., to two to four times this value, which permits moisture vapor to pass through the backing layer at such a rate that formation of liquid pools under the dressing is minimized, and yet the water loss is within a desirable range as regards heat loss and concentration of dissolved components in the body fluids which are underneath the dressing.

The flocked fibers are conveniently spun in about 1 to about 12 denier and then cut and adhesively flocked to the elastomeric backing layer. By using an electrostatic charge, as is conventional in flocking systems, the fibers are caused to stand on end and are p¾?-dominantly at right angles to the elastomeric layer.

The fibers may be adhesively united to the elastomeric layer by using the adhesive characteristics of the elastomeric layer as it is formed. More conveniently , when the elastomeric layer if formed, a separate adhesive layer is placed thereon and the flocked fibers are placed on such adhesive layer and caused to stand on end elec-trostatically .

At the time of use, the wound should be de-brided in accordance with the usual techniques. Some surgeons prefer to use antimicrobial agents such as silver nitrate or alpha-amino-p-toluenesulfonamide acetate. The wound should preferably be substantially dried so that there is no standing liquid on the surface of the wound. Then, the wound dressing is placed in contact with the wound, being sure that it conforms to the topography of the wound surface so that there are no gaps or spaces between the tissue and the dressing; with the dressing being held in place by adhesive tape, additional bandages, or suturing. A low pressure bandage may be used to hold the dressing in place, depending upon the location of the dressing and the condition of the patient. The dressing is permitted to remain in place for up to approximately seven days. There are times when many dressing changes are required before the wound is in condition for autografting or before skin for autograft-ing becomes available. For major burns or traumatic tissue damage, prolonged treatment may be required be- fore the wound is ready for final grafting.

In other instances, a wound may be fairly m^-imal and the present wound dressing may be applied even though a less adequate wound dressing could have been used, and a single application of the wound dressing permits substantial healing.

The present wound dressing in particular functions as a temporary synthetic skin, giving a protective covering to any area of a body from which the skin is re-moved and th- underlying tissues are exposed. The covering does not interfere with normal physiological processes conducive to wound healing and autograft conditioning.

The composite structure provides both a good moisture control layer and bacteria barrier. The con-formability resulting from the elastomeric backing permits conformation to nearly any wound topography.

A polyurethane such as described in United States Patent 3,582,423 is cast on a release coated paper to form a film with a thickness of 1-2 mils is useful in the present invention. The material is a reaction product of a hydroxy terminated polyester such as diethylene glycol adipate and ρ,ρ' -methylene dianiline and a mixture of 2,4- and 2,6-toluene diisocyanates , at about 25% solids. The polyester type segmented polyurethane is permitted to harden to form a water vapor permeable elastomer layer, and then coated with an additional half a mil thickness of the same polyurethane on which is placed the knit fabric of absorbable polymer.

The polyurethane structure may be graphically described as: where , R = -CH2-GH2-E- 0-C-CH2-CH2-CH2-CH2-C-0-CH2-CH2 -3" (V) m or 0 0 It II or -CH2-CH2-CH2-CH2 -E- 0-CH2-CH2-GH2-CH2 ~_h (VII) The polyurethane film is cast by pouring trie resin dissolved in solvent on a release paper such as a silicone coated release paper, which is pulled through rollers set to leave about 4 mils of 25% solids solution on the release paper, so that when dry a 1-2 mil layer remains. After a second pass to leave an additional adhesive coat which dries to about 0.5 mil thickness, the knitted fabric is placed with the loop side in contact with the still tacky adhesive layer and pressed into place.

The knitted structure embodied in the wound dressing may be a tricot knit, as is formed in a conven- tional flat warp knitting machine. Preferably, the fabric comprises a system of yarns which is fully threaded on a 28 gauge knitting machine so as to knit on all of the needles of the machine and which is controlled to knit in a pattern 1-2/1-0. A second system of yarns is likewise fully threaded so as to knit on every needle and is controlled to knit 0-1/4-3. The first system of yarns are knit to provide knitted loops as alternately placed in adjacent wales. The second system of yarns are knit alternately to provide knitted loops in laterally-spaced wales with underlaps spanning between these wales and across the intermediate wales. Because of the tension in the yarns and the overfeeding of the second system of yarns, the knitted loops are canted alternately left and right along each wale. Furthermore, the overfeeding of the yarn causes the loops to project from the surface to provide a relatively loose looped pile effect. The overfeed of the second system yarns is at least twice the feed of the first system of yarns so that overall coverage is provided on the looped side of the fabric by the projecting loops.

The knitted base fabric may take other forms, and other knitting mechanisms may be used for making the fabric. For example, the base fabric structure may be knitted on a warp knitter having a terry attachment or other pile-forming mechanism, or on a weft knitter with a fleece or other attachment. Of prime importance is the location of projecting yarn positions of one yarn system to substantially cover the interknitted structure with a degree of freedom to be displaced relative thereto.

A preferred embodiment of the fabric comprises a base fabric structure knitted on a 28 gauge tricot "¾ftrp knitter, with two bars fully threaded. The front bar is operated to knit 1-2/1-0 and the back bar is operated to knit 0-1/4-3. Both bars of the macnine were threaded with 25 denier 12 filament non-textured polyglycolic acid having a tenacity of 6.8 grams per denier. The feed ratio between the front and back bars was set to 0.345:1, and the knitting density was 80 courses on the machine with a quality of 6 inches.

The knitted fabric is pressed into the polyurethane while still containing enough solvent to be soft and adhesive. The looped face of the fabric is embedded in the moisture control layer.

The laminate should have a delamination pull of at least about 1 pound per inch of width. The delam-ination resistance may drop somewhat during use, but when the wound dressing is used, should be high enough to pull the knit fabric from the wound.

In the case of the flocked fibers, the fibers are preferably cut to be 25 mils long. They are flocked on to the adhesive coat and then subjected to a strong electrostatic field, in accordance with conventional procedures, to cause the fibers to stand at approximately right angles to the field and, hence, at right angles to the backing, while the polyurethane adhesive layer cures. The thus-formed flock is shaken and then blown with air to remove any loose fibers, after which the flocked sheet is cut into 2" x 3" rectangles for individual, dressings .

As a surgical device, it is obviously desir- able, almost mandatory, that the wound dressing be sterile at the time of use. The wound dressing may be sterilized by an appropriate sterilizing cycle using ethylene oxide as a sterilizing agent. Radiation sterilization may be used, as may heat sterilization.

If ethylene oxide is used to sterilize, it is convenient that the ethylene oxide be diluted with carbon dioxide or a chlorofluoroalkane to such an extent that the sterilizing gas is non-explosive. For storage stability, it is desirable that the wound dressing be protected from atmospheric influences. Because the wound dressing contains hydrolyzable polyglycolic acid ester linkages, the linkages can be hydrolyzed by ambient moisture under room storage . The wound dressing requires that the knitted fabric retain sufficient strength to be separated from the wound when the dressing is changed. Preferably, it should retain strength at least 10 days to two weeks. It is desirable that such storage conditions be used as to maintain the wound dressing in a dry environment so that whether used immediately after packaging or after a storage period of several years, the wound dressing has the same characteristics and, hence, has known predictable attributes as far as the using surgeon is concerned.

A good method of sterilizing and storage is the same as is used for polyglycolic acid sutures on a commercial scale and as disclosed in United States Patent 3,728,839. As there described, the polyglycolic acid product is stored in a moisture proof envelope in which conveniently the product is packaged except for one open side and sterilized using ethylene oxide diluted so as to be non-explosive, and then while protecting sterility, the product is vacuum dried and the envelope sealed. By having the foil envelope hermetically sealed, as there taught, the wound dressing may be maintained in a usable form with consistent characteristics for a period of at least several years. Conveniently, but not necessarily, the wound dressing may be placed between two sheets of paper, or a single sheet of paper with a fold, so that the wound dressing is held in flat condition between the sheets during storage and service to the using surgeon.

The polyglycolic acid-polyurethane laminate wound dressing can be placed in a folded sheet of paper, which is sealed in an inner envelope, which is then in turn sealed in an outer strippable envelope. The package is similar to those used for sutures.

A single envelope can be used, which, on stripping, releases the wound dressing folded in a sheet of paper .

For large sheets, the wound dressing may be folded, but for sheets up to 3" x 5" it is conveniently placed in an envelope large enough to hold the sheet flat. A plurality of sheets may be packaged in a single envelope if desired. Single sheets of about 3" x 5" are a surgically acceptable size, with the wound dressing being cut. to size if necessary by the surgeon, or an assistant, at the time of use. For many surgical procedures, a single sheet is all that is required. For major burned areas, either a number of smaller sheets or a single layer sheet of the wound dressing gives good re- suits. A series of smaller dressings gives more conformity on irregular areas. A single large sheet reduces ^ie number of seam lines. Conveniently, the wound dressing is made available in sterile form in sheets 3" x 5", 3" x 12", 3" x 18", 3" x 24" and continuous rolls, about 12" wide. Other sizes and shapes can be provided to supply the using surgeon with a choice of sizes, consistent with reasonable inventory demands.

Because it is not practical to purposely infect humans, tests were conducted on rats which had wounds created by surgically excising 20% to 30% of the body 5 surface. These wounds were seeded with dosages of 10 Pseudomonas Aeruginosa. After a 48 hour test period, all of the control animals, that is, those in which the wound was left bare, showed greater than 10' colony forming units per gram of tissue of Pseudomonas Aeruginosa. Porcineskin dressed animals, that is, those in which procineskin was used as a dressing, had counts between 3 7 10 and 10 Pseudomonas per gram of tissue whereas ani-mals using the wound dressing of this invention showed a marked reduction. Seven of the dressings showed less 2 3 5 than 10 , two had 10 , and one had 10 . These results are extremely encouraging and show that the present wound dressing aids dramatically in reducing contamina-tion of wounds. Usually, rats with values above 10^ do not survive .

The present wound dressings remain flexible and adherent to a wound, have excellent conformity, including suppleness, resiliency, and ability to mimic the wound topography, controls the loss of water, prevents the loss of protein, and protects against contamination of the wound. When removed after about seven days, ~' wounds showed granulation tissue covering 80% or more of the wound. Granulation tissue is a young vascularized connective tissue formed in the process of healing wounds When applied to contaminated wounds, the subject wound dressings reduce bacterial growth, allowing the host's own defense mechanisms to deal with the surface infection effectively. When removed after seven days, and autografted, 80% to 90% of the grafted area was viable twenty-one days after transplantation.

In an additional preparation, a polyglycolic acid yarn, from a suture manufacturing run, was spun into fibers of about 2 denier per filament, using a mult-iple orifice extrusion head yielding a 56 denier yarn with 28 filaments, an average tenacity of 6.8 grams per denier, and an average elongation of 22%. The knit scaffolding layer was knit on a tricot machine, of 28 needles per inch, with a warp of 14 ends per inch, using a front bar pattern of 2-3/1-0, a runner of 57.5 inches, and a threading of 1 in, 1 out, and a back bar pattern of 1-0/-7-8, a runner of 134 inches, and a threading of 1 in, 1 out, take-up gears 108-104, a quality of 8.5 inches with a density of 3.0 0.2 ounces per yard being obtained.

The moisture control layer was cast of Helastic 13141 polyurethane resin, having 25% solids, in dimethyl-formamide, and a viscosity range of 6,000 to 15,000 cen-tipoises. A film 0.6 mils dry thickness was cast as a skin coat, on a release paper, dried at 150°C. for four minutes then followed by a 0.4 mil dry thickness tie coat to which, dried at 85°C. for about 10 seconds, and while still soft, was applied the knit layer, with the loop^ side toward the polyurethane film.

The polyglycolic acid knit was pressed into the still soft polyurethane film. The laminate was cured at 150°C. for three minutes. The peel strength was 6 pounds per linear inch.

The laminate was cut to 3" x 5" size and sterilized as above, in strippable packages.

The knit surface presented to the wound readily conforms to wound topography. The knit absorbs some fluids from the wound, and rapidly adheres by capillarity. The interstices fill with body fluids, and are so close to the wound that hose defense mechanisms aid in controlling infection.

Claims (5)

1. A sterile wound dressing for the surfacj^ of living mammalian tissue from which at least part of the skin has been removed and which tissue is subject to moisture loss, and bacterial contamination, which dressing is sufficiently flexible that it will conform and adapt to the surface of the tissue to minimize pools of fluid or air between the dressing and the tissue surface, which pools generate problems of infection, and which has a permeability to moisture vapor of the order of that of the intact human skin, characterized as an elastomeric layer, which elastomeric layer has bonded thereto a knitted fabric or layer of flocked fibers, said fibers or fabric being of a tissue compatible material which is substantially absorbed by living tissue within about 90 days, and maintain their integrity for at least about 10 days, and have a tissue contacting surface which has interstitial spaces into which granulating tissue can grow, but which can be readily removed from the wound without removing the bed of granulation "tissue and are composed of a polymer having glycolic acid ester linkages .

2. A dressing according to Claim 1 wherein the elastomeric layer is bonded to a knitted fabric.

3. A dressing according to Claim 2 wherein the polymer is homopolymeric polyglycolic acid and the elastomeric layer is polyurethane.

4. A dressing according to Claim 1 wherein the elastomeric layer is bonded to a layer of flocked fibers.

5. A dressing according to Claim 4 wherein the polymer is homopolymeric polyglycolic acid and the elastomeric layer is polyurethane . AGENTS FOR APPLICANTS