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United States Patent 3,903,882
Augurt September 9, 1975

Composite Dressing

Abstract

A flexible, tissue surface conformable, water vapor permeable, composite dressing for wounds, particularly major burns, is formed from a knitted fabric of a tissue absorbable polymer, such as polyglycolic acid, having on the non-tissue contacting surface a water vapor permeable elastomeric film, such as polyurethane, silicone, poly(caprolactone) and natural rubber, which excludes microorganisms and dirt, while controlling water vapor permeation to approximately that of the intact human skin. The dressing is tissue surface conformable, minimizes pooling of liquids between the dressing and the wound, minimizes infections, is readily removable to permit a change of dressing, and permits granulation tissue to form under the dressing and subsequent autografting. If any of the tissue absorbable material is trapped within the wound, it is absorbed with no deleterious effects.


Inventors: Augurt; Thomas Anthony (Stamford, CT)
Assignee: American Cyanamid Company (Stamford, CT)
Appl. No.: 05/462,490
Filed: April 19, 1974


Current U.S. Class: 602/43 ; 128/DIG.8; 602/48
Current International Class: A61F 15/00 (20060101); A61F 13/00 (20060101); A61L 15/16 (20060101); A61L 15/26 (20060101); A61F 13/15 (20060101); A61F 013/00 ()
Field of Search: 128/149,153-157,268,165

References Cited

U.S. Patent Documents
2583341 January 1952 Reese
3526224 September 1970 Potts
3739773 June 1973 Schmitt et al.
3800792 April 1974 McKnight
Primary Examiner: Trapp; Lawrence W.
Attorney, Agent or Firm: Walker; Samuel Branch

Claims



I claim:

1. A wound dressing for the surface of living 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 of the order of that of the intact human skin (approximately 2.2 mg/sq. cm./hr.), which comprises: an elastomeric layer of the order of 0.1 to 5 mils thick, and which elastomeric layer has bonded thereto a fabric knitted of continuous fibers of the order of 1/2 to 12 denier of a tissue compatible material which is substantially absorbed by living tissue within about 90 days, and which fibers maintain their integrity for at least about 10 days, and which knitted fabric has a tissue contacting surface free from upstanding loops, and which has interstitial spaces into which granulating tissue can grow, but which readily releases granulating tissue 4 to 10 days after emplacement.

2. The wound dressing of claim 1 in which the tissue compatible material is a polymer subject to hydrolytic degradation to non-toxic, tissue compatible absorbable components, said polymer having glycolic acid ester linkages.

3. The wound dressing of claim 2 in which the tissue compatible material is homopolymeric polyglycolic acid.

4. The wound dressing of claim 2 in which the tissue compatible material is a polymer of 3-methyl-1,4-dioxane-2,5-dione.

5. The wound dressing of claim 1 in which the tissue compatible material is a polymer of N-acetyl-D-glucosamine.

6. An interiorly sterile strippable package having therein in sterile condition a wound dressing for the surface of living 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 of the order of that of the intact human skin (approximately 2.2 mg/sq. cm./hr), which comprises: an elastomeric layer of the order of 0.1 to 5 mils thick, and which elastomeric layer has bonded thereto a fabric knitted of continuous fibers of the order of 1/2 to 12 denier of a tissue compatible material which is substantially absorbed by living tissue within about 90 days, and which fibers maintain their integrity for at least about 10 days, and which knitted fabric has a tissue contacting surface free from upstanding loops, and which has interstitial spaced into which granulating tissues can grow, but which readily releases granulating tissue 4 to 10 days after emplacement.

7. A method of protecting the surface of living tissue during a healing process which comprises: during a preliminary period, covering and protecting the surface of damaged tissue by emplacing thereon a wound dressing for the surface of living 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 of the order of that of the intact human skin (approximately 2.2 mg/sq. cm./hr.), which comprises: an elastomeric layer of the order of 0.1 to 5 mils thick, and which elastomeric layer has bonded thereto a fabric knitted of continuous fibers of the order of 1/2 to 12 denier of a tissue compatible material which is substantially absorbed by living tissue within about 90 days, and which fibers maintain their integrity for at least about 10 days, and which knitted fabric has a tissue contacting surface free from upstanding loops, and which has interstitial spaces into which granulating tissue can grow, but which readily releases granulating tissue 4 to 10 days after emplacement, permitting granulation tissue to grow thereunder to give a clean wound closed by said dressing, stripping off said dressing, to expose granulation tissue, then autografting living skin to said exposed granulation tissue.

8. The process of claim 7 in which the tissue compatible material is homopolymeric polyglycolic acid.
Description



BACKGROUND OF THE INVENTION

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 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 be 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.

DESCRIPTION OF THE PRIOR ART

The use of polyglycolic acid is disclosed in a series of patents and applications to Schmitt, et al:

U.S. Pat. No. 3,297,033, Schmitt and Polistina, Jan. 10, 1967, SURGICAL SUTURES, discloses polyhydroxyacetic ester absorbable sutures. The material is also called polyglycolic acid, and is disclosed as permitting small quantities of comonomers to be present, such as dl-lactic acid, its optically active forms, homologs and analogs. A small quantity is recognized by the art as up to 15 percent, as shown by U.S. Pat. No. 2,668,162, Lowe, Feb. 2, 1954, PREPARATION OF HIGH MOLECULAR WEIGHT POLYHYDROXY-ACETIC ESTER.

U.S. Pat. No. 3,463,158, Schmitt and Polistina, Aug. 26, 1969, POLYGLYCOLIC ACID PROSTHETIC DEVICES, discloses surgical uses of polyglycolic acid, and incorporates definitions of some terms.

U.S. Pat. No. 3,620,218, Schmitt and Polistina, Nov. 16, 1971, CYLINDRICAL PROSTHETIC DEVICES OF POLYGLYCOLIC ACID, lists many uses of polyglycolic acid.

U.S. Pat. No. 3,736,646, Schmitt and Epstein, June 5, 1973, METHOD OF ATTACHING SURGICAL NEEDLES TO MULTIFILAMENT POLYGLYCOLIC ACID ABSORBABLE SUTURES, discloses surgical elements of a copolymer containing from 15 to 85 mol percent glycolic acid and 85 to 15 percent lactic acid.

U.S. Pat. No. 3,739,773, Schmitt and Polistina, June 19, 1973, POLYGLYCOLIC ACID PROSTHETIC DEVICES, claims particularly bone pins, plates, nails and screws of polyglycolic acid.

U.S. Application Ser. No. 365,656, Schmitt and Polistina, May 31, 1973, SURGICAL DRESSINGS OF ABSORBABLE POLYMERS, now U.S. Pat. No. 3,875,937, Apr. 8, 1945, discloses additional subject matter on surgical dressings of polyglycolic acid.

U.S. Pat. No. 3,739,773, supra, lists a number of U.S. patents on methods for preparing polyglycolic acid and starting materials therefor.

In U.S. Pat. No. 3,620,218, supra, in Column 2 are listed a number of medical uses of polyglycolic acids, including in Column 2; line 52, knitted or woven fibrillar products, including velours, and mentioning specifically in line 53, burn dressings; line 57, felt or sponge for liver hemostasis; line 63, foam as absorbable prosthesis; and in lines 74 and 75, burn dressings (in combination with other polymeric films).

The use of gauzes, felts, and knitted fabrics as a wound dressing is quite conventional. The use of collagenous products as a sponge or pad has been disclosed. The requirements for surgical dressings are varied and more satisfactory dressings than presently available are constantly in demand.

U.S. Pat. No. 3,526,224, R. M. Potts, Sept. 1, 1970, DRESSING, discloses an occlusive dressing designed to act as a synthetic skin which has an elastomeric polyurethane film having a water vapor transmission rate of 150 to 500 g/m.sup.2 /24 hrs. laminated to a knitted velour fabric. A tricot fabric of 30 denier textured 6.6 nylon yarn is suggested. The pile or nap side of the knitted velour forms the wound-contacting side of the dressing.

U.S. Pat. No. 3,648,692, L. M. Wheeler, Mar. 14, 1972, MEDICAL-SURGICAL DRESSING FOR BURNS AND THE LIKE, discloses a thrombogenic open-cell foam, such as a reticulated open-cell polyurethane foam backed by a microporous polypropylene film, but also disclosing natural rubber and silicone rubber of specified pore size. A permeability of a 5 mil thick silicone rubber of 6.2 grams of liquid water per square foot per hour is disclosed and oxygen penetration of 20 liters per 24 hours per 100 square inches per 5 mil thickness per atmosphere. This patent shows a clear recognition of many of the problems of burn dressings.

Studies are being made by and for governmental agencies on burn protection. One such "Development of a Synthetic Polymer Burn Covering" prepared by Dynateck R/D Company, for the Office of Naval Research, June 1973, National Technical Information Service AD-761,631 discloses the use of polymeric films, including lactic acid polymers, and 75/25 lactic/glycolic copolymers, and the disadvantages of flat, solvent cast films. A normal insensible water loss from the human body is given as 2.2 mg/hr cm.sup.2, quoting Treger, "Physical Functions of the Skin," Academic Press, New York.

The complete disclosures of the above patents and articles are hereby herein incorporated by this reference thereto.

SUMMARY OF THE INVENTION

This invention relates to a wound dressing, particularly adapted for major burns in humans in which at least a part of the skin has been destroyed or removed and which underlying tissue is subject to moisture and liquid loss and bacterial contamination which dressing comprises an exterior surface of an elastomeric material such as polyurethane or silicone 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 united a knitted fabric of a tissue compatible and absorbable material having a close knit stitch so spaced as to permit slight invasion of granulation tissue, and which can be removed from the granulation tissue without pulling off such granulation tissue and with minimum damages to the developing granulation tissue to leave a bed for autografting 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 covered 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 that when placed on the granulating tissue, the skin will graft to the granulating tissue and grow providing a skin surface 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 the 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 needs 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 moisture 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, proteins, 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. as measured by ASTM method E96 gives good water vapor loss control. The values are approximate as different skin areas lose moisture at a different rate and, depending upon the temperature of the subject, the temperature of the atmosphere, the movement of ambient air and under other variables, the natural skin has a considerable range of skin moisture loss. The natural skin is adaptable to a wide range of environmental conditions 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 sufficiently 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.

Continuous fibers of about 1/2 to 12 denier per filament in a yarn which is neither spun nor bulked nor textured are conveniently used.

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

A preferred tissue absorbable polymer is polyglycolic acid, such as described in the Schmitt patents above and which is meeting with commercial success as a suture. Polymers in which tissue absorption results from the hydrolytic degradation of glycolic acid ester linkages give good results. Because strength of the fibers is not a major requirement, a copolymer containing considerable lactic acid makes a good dressing. Such polymers are disclosed in U.S. Pat. No. 3,736,646, supra.

A polymer having an ordered configuration of glycolic acid units and lactic acid units which is tissue absorbable is described at length in Ser. No. 435,365, Jan. 24, 1974, Augurt, Rosensaft, and Perciaccante, UNSYMMETRICALLY SUBSTITUTED 1,4-DIOXANE-2,5-DIONES.

Another absorbable polymer which may be used for the graft is poly(N-acetyl-D-glucosamine) such as described in U.S. Ser. No. 441,717 filed Feb. 11, 1974, Richard Carl Capozza, POLY(N-ACETYL-D-GLUCOSAMINE) PRODUCTS.

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 spinnable as a fiber, and it has sufficient strength to maintain 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 instance, 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 above two patent applications, the disclosures of which are herein incorporated by this reference thereto, give examples of such materials.

Inasmuch as the useful characteristics of such materials are largely as a function of size, shape and structure, these materials may be substituted for the polyglycolic acid fibers described in more detail in the following examples.

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 below. 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.

At the time of use, the wound should be debrided in accordance with the usual techniques. Some surgeons prefer to use antimicrobial agents such as silver nitrate or Sulfamylon (alpha-amino-p-toluenesulfonamide acetate). Then, the knitted surface of 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 7 days. There are times when many dressing changes are required before the wound is in condition for autografting or before skin for autografting becomes available. For major burns or traumatic tissue damage, prolonged treatment may be required before the wound is ready for final grafting.

In other instances, a wound may be fairly minimal 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 removed and the 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 conformability resulting from the elastomeric backing permits conformation to nearly any wound topography.

DRAWINGS

FIG. 1 shows a fabric face view of a polyglycolic acid knit wound dressing at about 30 diameters magnification, using a scanning electron microscope.

FIG. 2 is part of the same structure as claim 1, at about 60 diameters magnification.

FIG. 3 is a lateral cross section view of the wound dressing of FIG. 1, at 50 diameters magnification, using reflected light on a section in an epoxy matrix.

FIG. 4 is a longitudinal cross section of the wound dressing of FIG. 1.

FIG. 5 is a drawing showing the wound dressing in a double envelope package.

FIG. 6 is a drawing in a pictorial, partly broken away, showing the sterile wound dressing in a strippable envelope.

As shown in the drawings, a polyurethane such as described in U.S. Pat. 3,582,423, Wang, June 1, 1971, PROCESS FOR COATING POROUS SUBSTRATES, and sold as Helastic 13141 polyurethane, is cast on a release coated paper to form a film with a thickness of 1 mil. The material is a reaction product of a hydroxy terminated polyester such as diethylene glycol adipate and p,p'-methylene dianiline and a mixture of 2,4- and 2,6-toluene diisocyanates, at about 25 percent 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:

NH.sub.2 -R.sub.1 --NH-CO-NH-R.sub.2 -NH-CO-O-R.sub.3 -O-CO-NH-R.sub.2 -NH---CO-NH-R.sub.1 ].sub.n NH.sub.2

where, ##SPC1##

or

--CH.sub.2 -CH.sub.2 -CH.sub.2 -CH.sub.2 -- O-CH.sub.2 -CH.sub.2 -CH.sub.2 -CH.sub.2 ].sub.m

The polyurethane film is cast by pouring the 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 percent solids solution on the release paper, so that when dry a 1 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 conventional flat warp knitting machine. As shown in FIGS. 1 and 2, 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. Thus, as shown in FIGS. 1 and 2, 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. In the illustrated embodiment, 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 portions of one yarn system to substantially cover the interknitted structure with a degree of freedom to be displaced relative thereto.

The illustrated embodiment of the fabric comprises a base fabric structure knitted on a 28 gauge tricot warp knitter, with two bars fully threaded. The front bar was operated to knit 1-2/1-0 and the back bar was operated to knit 0-1/4-3. Both bars of the machine 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.

FIG. 1 shows the smoother surface of the knit fabric which is the face contacting the wound. This figure is at 30 diameters magnification and shows the wound contacting face. FIG. 1 is from a scanning electron microscope, which gives a great depth of field to a photomicrograph.

FIG. 2 is similar but at 60 diameters magnification. The individual fibers are clearly seen, as in the knit construction. The structure of the knit permits granulation tissue to grow closely to the knit, but separate smoothly and readily when the wound dressing is removed.

FIG. 3 is a cross section of the same wound dressing as in FIGS. 1 and 2. The wound dressing was embedded in an epoxy matrix, which was permitted to harden, then sliced and polished. The epoxy resin tends to distort the smooth surface of the polyurethane film 16. The embedment of the polyglycolic acid fibers 17 in the knit loops in the polyurethane film keeps the laminate together. The picture is a photomicrograph by reflected light from a polished surface at 50 diameters magnification. Most of the individual fibers are cut at nearly right angles.

FIG. 4 is a similar section at right angles to FIG. 3, and shows a number of fibers 17 cut nearly lengthwise. Again the barrier layer of polyurethane is nearly flat, but is distorted by the epoxy resin used as a matrix for the cross section.

The laminate should have a delamination pull of at least about 1 pound per inch of width. The delamination 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.

As a surgical device, it is obviously desirable, 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 U.S. Pat. No. 3,728,839, Arthur Glick, Apr. 24, 1973, STORAGE STABLE SURGICALLY ABSORBABLE POLYGLYCOLIC ACID PRODUCTS. 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.

As shown in FIGS. 5 and 6, the polyglycolic acid-polyurethane laminate wound dressing 11 is placed in a folded sheet of paper 12, which is sealed in an inner envelope 13, which is sealed in an outer strippable envelope 14. 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 12. Such a single envelope package is shown in U.S. Pat. 3,017,990, Singerman, Jan. 23, 1962, STERILE PACKAGE FOR SURGICAL FABRIC.

For large sheets, the wound dressing may be folded, but for sheets up to 3 .times. 5 inches 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 .times. 5 inches 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 results. A series of smaller dressings gives more conformity on irregular areas. A single large sheet reduces the number of seam lines. Conveniently, the wound dressing is made available in sterile form in sheets 3 .times. 5 inches, 3 .times. 12 inches, 3 .times. 18 inches, 3 .times. 24 inches and continuous rolls, about 12 inches wide. Other sizes and shapes can be provided to supply the using surgeon with a choice of sizes, consistent with reasonable inventory commands.

Because it is not practical to purposely infect humans, tests were conducted on rats which had wounds created by surgically excising 20 to 30 percent of the body surface. These wounds were seeded with dosages of 10.sup.5 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.sup.7 colony forming units per gram of tissue of Pseudomonas Aeruginosa. Porcineskin dressed animals, that is, those in which porcineskin was used as a dressing, had counts between 10.sup.3 and 10.sup.7 Pseudomonas per gram of tissue whereas animals using the wound dressing of this invention showed a marked reduction. Seven of the dressings showed less than 10.sup.2, two had 10.sup.3, and one had 10.sup.5. These results are extremely encouraging and show that the present wound dressing aids dramatically in reducing contamination of wounds. Usually, rats with values above 10.sup.5 do not survive.

An article Saymen, Nathan, Holder, Hill and MacMillan, Control of Surface Wounds Infections: Skin versus Synthetic Grafts, Applied Microbiology, June 1973 (V. 25, No. 6, pages 421 to 434), gives considerable detail from others who have been experimenting on surface wound infection. This article indicates that grafts may lower bacterial levels in an established infection by modifying the host response to the surface contamination.

In summary, the present wound dressing remains flexible and adherent to a wound, has 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 percent or more of the wound. Granulation tissue is a young vascularized connective tissue formed in the process of healing of wounds.

When applied to contaminated wounds, the subject wound dressing reduced 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 percent of the grafted area was viable 21 days after transplantation.

In an additional preparation, polyglycolic acid yarn, from a suture manufacturing run, was spun into fibers of about 2 denier per filament, using a multiple 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 percent. 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 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 percent solids, in dimethylformamide, and a viscosity range of 6,000 to 15,000 centipoises. A film 0.6 mils dry thickness was cast as a skin coat, on a release paper, dried at 150.degree.C. for four minutes then followed by a 0.4 mil dry thickness tie coat, to which, dried at 85.degree.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.degree.C. for three minutes. The peel strength was 6 pounds per linear inch.

The laminate was cut to 3 .times. 5 inches 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 host defense mechanisms aid in controlling infection.

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