Skin: Artificial Skin and Types of Wound Dressings

by Food and Drug Administration (FDA)

Last spring, a 68-year-old Northern California man suffered deep, third-degree burns when he dropped a cigarette and his pants leg caught fire. Unfortunately, such injuries are all too common. What's unusual is that this man became the first patient treated outside clinical trials with a new artificial skin that the Food and Drug Administration had just approved for marketing the month before.

A serious burn is one of the most horrendous traumas the body can suffer. Every year, about 51,000 Americans are hospitalized for burn treatment, according to the American Burn Association, and 5,500 die. The good news is that the incidence and severity of burn injuries have declined significantly over the past 20 years. And patient survival keeps improving.

"This is a very exciting area," says Charles Durfor, Ph.D., in FDA's division of general and restorative devices. "Thirty to forty years ago, many burn patients didn't live. Advances in treatment have created a whole new patient population that not only lives, but has an improving quality of life."

The first great strides were in getting patients through the initial shock, and preventing fluid loss. Controlling infection, a serious threat to burn patients, also improved. Specialized nutritional support has helped. Another leap occurred when doctors began surgically removing, or excising, all burned tissue from the wound as soon as possible. After stabilizing the patient and cleaning out the wound, the next step is to cover it.

"The sooner you close the wound, the sooner the patient gets better," says Robert Klein, M.D., medical director of the regional burn center at Children's Hospital Medical Center of Akron, Ohio.

"The problem is, we've never had an optimal way to do it," says Jerold Kaplan, M.D., director of the burn centers at Alta Bates Hospital in Berkeley, Calif., and at Children's Hospital in nearby Oakland. The need to cover wounds as quickly as possible while minimizing scarring and additional trauma has driven development of advanced wound dressings and skin substitutes. Kaplan treated the 68-year-old California man's wounds with Integra Artificial Skin Dermal Regeneration Template, from Integra LifeSciences Corp., Plainsboro, N.J. "Integra is a significant addition to the armamentarium of the burn surgeon," Kaplan says, and other surgeons agree.

Skin Deep

Surgeons also agree that no single product or technique is right for every burn situation. And so far, there's no true replacement for healthy, intact skin, which is the body's largest organ, and one of the most complex. It's the first line of defense against infection and dehydration, but it's more than just a physical barrier. Skin also helps control temperature, through adjustments of blood flow and evaporation of sweat. It's an important sensory organ, too.

Skin thickness varies with age and body location, but averages only 1 to 2 millimeters (0.04 to 0.08 inches) thick. Thick or thin, it has two layers. The thin outer epidermis is nourished from the thicker, more sensitive dermis below. The outermost surface is a tough, protective coating of dead, flat cells resembling paving stones. As these cells wear away, they're replaced from beneath. The innermost part of the epidermis consists of rapidly dividing cells, called keratinocytes, which produce keratin, a tough protein. Epidermis also contains a unique fatty substance that makes skin waterproof.

The skin's blood vessels, lymph vessels, and nerves are in the dermis. Hair follicles, sweat glands, and oil glands also reside deep in this layer, which is mainly connective tissue. A network of collagen, the most common protein in the body, gives flexibility and structural support to the skin. Fibroblasts are the dominant cell type. Dermis plays a role in preventing wound contraction and scarring.

Treatment of burns depends on how deep and extensive they are, and the overall health of the patient. First-degree burns (such as sunburns) affect only the epidermis; they may peel but generally heal quickly. Second-degree burns damage the skin more deeply, causing blisters but sparing some of the dermal layer. Unless they're extensive, these burns usually heal without serious scarring. Third-degree burns destroy the full skin thickness, sometimes exposing muscle or bone, and require specialized treatment and skin grafts to obtain complete wound healing and reduce scarring. Left alone, the body tries to close wounds quickly by contraction, which results in serious scarring that is not only disfiguring, but can also be disabling.

Currently, the best wound covering most often is the patient's own skin. Healthy skin from another body site can be transplanted, which is called an autograft (autos means self). Sometimes little slits are cut so the resulting meshed graft can be stretched to cover more area. A split-thickness graft takes only the upper skin layer, and the donor site usually heals within several days. The thinner the graft, the faster the donor site heals. Surgeons may even take additional thin grafts from healed sites. Full-thickness grafts usually give a better-looking final result, but sometimes they don't adhere and survive. Donor sites are limited and autografting isn't always possible.

"People with great big burns don't have enough of their own skin, so you have to have some other way of covering them," says David M. Heimbach, M.D., director of the University of Washington Burn Center at Harborview, Seattle. Some patients can't withstand the additional trauma of a donor site wound. Older patients heal slowly and have thinner skin to begin with. And grafting creates another scar.

Doctors often use temporary coverings while patients get stronger, or while donor sites heal for additional harvesting. Two traditional possibilities are an allograft (allos means other) of human skin, usually cadaver skin, or a xenograft (xenos means stranger, in this case from another species) of pig skin. Cadaver skin is preferable, but as with other donated organs, sometimes it's in short supply and transmission of infectious agents is a concern. Human skin is regulated under FDA's Human Tissue Program, which requires donor screening for HIV (the AIDS virus) and hepatitis. In any case, the immune system rejects allo- and xenografts in a matter of days or weeks, and they must be removed and replaced. To avoid such problems, researchers and manufacturers are developing better wound dressings.

About the Author

www.fda.gov
FDA is A United States government body that oversees medical devices, including contact lenses, intraocular lenses, excimer lasers and eyedrops. In the US, these products must be approved by the FDA before they can be marketed.