If an automobile or appliance part is damaged or malfunctions, replacing it is fairly simple. Not so for the human body, because the immune system presents a formidable barrier to introducing foreign tissue. Immunosuppressant drugs help, but they raise the risk of infection and cancer by dampening aspects of the immune response other than transplant rejection.

A solution to the challenge of replacing body parts is tissue engineering, which combines synthetic materials with cells. The basic recipe for a bioengineer tissue is to place cells in or on a scaffolding sculpted from a synthetic material that is accepted in the body. The cells secrete substances as they normally would, or they may be genetically altered to overproduce their natural secreted products or supply entirely different ones with therapeutic benefit, such as growth factors that might make the implant more acceptable to the body. Bioengineered tissues are just beginning to be tested in humans.

Replacement Blood Vessels

A GraftArtery as one company calls its product, is a flexible yet strong tube built of collagen, a natural connective tissue. An external layer of porcine collagen provides strength and handling characteristics, while an inner layer of bovine collagen provides a smooth flow surface. This biosynthetic construct does not evoke an immune response and serves as a scaffold for remodeling by the body's own cells. These bioengineer blood vessels can be manufactured to any length or width, and they withstand pressures comparable to that of the blood racing through the circulatory system. The vessels can be stitched next to natural blood vessels so smoothly that blood clots do not form nearly as easily as they do in completely synthetic grafts. If testing goes well, the vessels will be used in cardiac bypass surgery, to replace damaged arteries in the legs, and to replace brain arteries damaged by a stroke.

New Skin and More

Burn patients may soon be helped by a bioengineer skin consisting of the patient's epidermal cells placed in sheets over dermal cells grown in culture, both layers supported by a nylon mesh framework. This semisynthetic skin may also be useful for patients who have lost a great deal of skin in surgery to remove tattoos, cancers, and moles. Bioengineer skin is already available for in vitro toxicity testing, where in many laboratories it has replaced live animals in testing cosmetic ingredients.

Other tissues on the bioengineering drawing board include liver, connective tissue, and bone marrow. A replacement cartilage similar to the skin recipe, consisting of chondrocytes in collagen, may be helpful in replacing joints destroyed by arthritis.

A scaled-down version of an engineered tissue, called a cell implant, offers a new route to drug delivery, placing cells that naturally manufacture needed substances precisely where a patient needs them. The cells are immunoisolated because their packaging enables them to secrete without being detected by the immune system. This is done by surrounding cells with a polymer membrane with holes small enough to allow nutrients in and the therapeutic biochemicals out, while excluding the larger molecules responsible for immune rejection.