Mary F. McNulty
Skin, the human body's largest organ, protects the body from disease
and physical damage, and helps to regulate body temperature. It is composed
of two major layers: the epidermis and the dermis. The epidermis, or outer,
layer is composed primarily of cells: keratinocytes, melanocytes, and
langerhans. The dermis, composed primarily of connective tissue fibers
such as collagen, supplies nourishment to the epidermis.
When the skin has been seriously damaged through disease or burns, the
body cannot act fast enough to manufacture the necessary replacement cells.
Wounds, such as skin ulcers suffered by diabetics, may not heal and limbs
must be amputated. Burn victims may die from infection and the loss of
plasma. Skin grafts were developed as a way to prevent such dire consequences
as well to correct deformities. As early as the sixth century B.C., Hindu
surgeons were involved in nose reconstruction, grafting skin flaps from
the patient's nose. Gaspare Tagliacozzo, an Italian physician, brought
the technique to Western medicine in the sixteenth century. Until the
late twentieth century, skin grafts were constructed from the patient's
own skin (autografts) or cadaver skin (allografts). Infection or, in the
case of cadaver skin, rejection were primary concerns. While skin grafted
from one part of a patient's body to another is immune to rejection, skin
grafts from a donor to a recipient are rejected more aggressively than
any other tissue graft or transplant. Although cadaver skin can provide
protection from infection and loss of fluids during a burn victim's initial
healing period, a subsequent graft of the patient's own skin is often
required. The physician is restricted to what the patient has available,
a decided disadvantage in the case of severe burn victims. In the mid-1980s,
medical researchers and chemical engineers, working in the fields of cell
biology and plastics manufacturing, joined forces to develop tissue engineering
to reduce the incidences of infection and rejection. One of the catalysts
for tissue engineering was the growing shortage of organs available for
transplantation. In 1984, a Harvard Medical School surgeon, Joseph Vacanti,
shared his frustration over the lack of available livers with his colleague
Robert Langer, a chemical engineer at the Massachusetts Institute of Technology.
Together, they pondered whether new organs could be grown in the laboratory.
The first step was to duplicate the body's production of tissue. Langer
came up with the idea of constructing a biodegradable scaffolding on which
skin cells could be grown using fibroblasts, cells extracted from donated
neonatal foreskins removed during circumcision. In a variation of this
technique developed by other researchers, the extracted fibroblasts are
added to collagen, a fibrous protein found in connective tissue. When
the compound is heated, the collagen gels and traps the fibroblasts, which
in turn arrange themselves around the collagen, becoming compact, dense,
and fibrous. After several weeks, keratinocytes, also extracted from the
donated foreskins, are seeded onto the new dermal tissue, where they create
an epidermal layer. An artificial skin graft offers several advantages
over those derived from the patient and cadavers. It eliminates the need
for tissue typing. Artificial skin can be made in large quantities and
frozen for storage and shipping, making it available as needed. Each culture
is screened for pathogens, severely curtailing the chance of infection.
Because artificial skin does not contain immunogenic cells such as dendritic
cells and capillary endothelial cells, it is not rejected by the body.
Finally, rehabilitation time is significantly reduced.
The raw materials needed for the production of artificial skin fall into
two categories: the biological components and the necessary laboratory
equipment. Most of the donated skin tissue comes from neonatal foreskins
removed during circumcision. One foreskin can yield enough cells to make
four acres of grafting material. Fibroblasts are separated from the dermal
layer of the donated tissue. The fibroblasts are quarantined while they
are tested for viruses and other infectious pathogens such as HIV, Hepatitis
B and C, and Mycoplasma. The mother's medical history is recorded. The
fibroblasts are stored in glass vials and frozen in liquid nitrogen at
-94°F (-70°C). Vials are kept frozen until the fibroblasts are
needed to grow cultures. In the collagen method, keratinocytes are also
extracted from the foreskin, tested, and frozen. If the fibroblasts are
to be grown on mesh scaffolding, a polymer is created by combining molecules
of lactic acid and glycolic acid, the same elements used to make dissolving
sutures. The compound undergoes a chemical reaction resulting in a larger
molecule that consists of repeating structural units. In the collagen
method, a small amount of bovine collagen is extracted from the extensor
tendon of young calves. The collagen is mixed with an acidic nutrient,
and stored in a refrigerator at 39.2°F (4°C). Laboratory equipment
includes glass vials, tubing, roller bottles, grafting cartridges, molds,
The Manufacturing Process
The manufacturing process is deceptively simple. Its main function is
to trick the extracted fibroblasts into believing that they are in the
human body so that they communicate with each other in the natural way
to create new skin.
1. Mesh scaffolding method
Fibroblasts are thawed and expanded. The fibroblasts are transferred from
the vials into roller bottles, which resemble liter soda bottles. The
bottles are rotated on their sides for three to four weeks. The rolling
action allows the circulation of oxygen, essential to the growth process.
2. Cells are transferred to a culture system. The cells are removed from
the roller bottles, combined with a nutrient-rich media and flowed through
tubes into thin, cassette-like bioreactors housing the biodegradable mesh
scaffolding and sterilized with e-beam radiation. As the cells flow into
the cassettes, they adhere to the mesh and begin to grow. The cells are
flowed back and forth for three to four weeks. Each day, leftover cells
suspension is removed and fresh nutrient is added. Oxygen, pH, nutrient
flow and temperature are controlled by the culture system. As the new
cells create a layer of dermal skin, the polymer disintegrates.
3. Growth cycle completed. When cell growth on the mesh is completed,
the tissue is rinsed with more nutrient-rich media. A cryoprotectant is
added. Cassettes are stored individually, labeled, and frozen.
4. Collagen method
Cells are transferred to a culture system. A small amount of the cold
collagen and nutrient media, approximately 1–2% of the combined solution,
is added to the fibroblasts. The mixture is dispensed into molds and allowed
to come to room temperature. As the collagen warms, it gels, trapping
the fibroblasts and generating the growth of new skin cells. 5Keratinocytes
added. Two weeks after the collagen is added to the fibroblasts, the extracted
keratinocytes are thawed and seeded onto the new dermal skin. They are
allowed to grow for several days and then exposed to air, inducing the
keratinocytes to form epidermal layers. 6Growth cycle completed. The new
skin is stored in sterile containers until needed.
The medical profession is using artificial skin technology to pioneer
organ reconstruction. It is hoped that this so-called engineered structural
tissue will, for example, someday replace plastic and metal prostheses
currently used to replace damaged joints and bones. Ears and noses will
be reconstructed by seeding cartilage cells on polymer mesh. The regeneration
of breast and urethral tissues is currently under study in the laboratory.
Through this technology, it is possible that one day, livers, kidneys,
and even hearts, will be grown from human tissues.
Where to Learn More
Langer, Robert, and Joseph P. Vacanti. "Artificial Organs."
Scientific American (September 1995): 130-133.
Langer, Robert, and Joseph P. Vacanti. "Tissue Engineering."
Science (14 May 1993): 920-921.
McCarthy, Michael. "Bio-engineered Tissues Move Towards the Clinic."
The Lancet (17 August 1996): 466.
Rundle, Rhonda L. "Cells 'Tricked' To Make Skin For Burn Cases."
The Wall Street Journal (17 March 1994).