Biomedical Engineering Reference
In-Depth Information
biocompatibility and epithelial ingrowth ( Griffith et al. ,
2002 ).
corrosivity, and other testing ( Fentem et al. , 2001 ;
Portes et al. , 2002 ). Gene therapy for genodermatoses
( Spirito et al. , 2001 ), junctional epidermolysis bullosa
( Robbins et al. , 2001 ), and ichthyosis ( Jensen et al. ,
1993 ) remains a topic of great interest using either
transgenic fibroblasts or keratinocytes.
Skin
Several new types of tissue transplants are being studied
for treatment of burns, skin ulcers, deep wounds, and
other injuries. One approach to skin grafts involves the
in vitro culture of epidermal cells (keratinocytes). Small
skin biopsies are harvested from burn patients and ex-
panded up to 10,000-fold. This expansion is achieved,
e.g., by cultivating keratinocytes on a feeder layer of ir-
radiated NIH 3T3 fibroblasts, which, in conjunction with
certain added media components, stimulates rapid cell
growth. This approach allows coverage of extremely large
wounds. A disadvantage is the 3- to 4-week period re-
quired for cell expansion, which may be too long for
a severely burned patient. Cryopreserved allografts may
help to circumvent this problem ( Nave, 1992 ). Another
promising approach uses human neonatal dermal fibro-
blasts grown on PGA mesh. In deep injuries involving all
layers of skin the grafts are placed onto the wound bed
and a skin graft is placed on top followed by vasculari-
zation of the graft. This results in formation of an orga-
nized tissue resembling dermis. Clinical trials have
shown good graft acceptance without evidence of
immune rejection ( Hansbrough et al. , 1992 ). Fibroblasts
have also been placed on hydrated collagen gel. Upon
implantation, the cells migrate through the gel by enzy-
matic digestion of collagen, which results in reorganization
of collagen fibrils ( Bell et al. , 1979 ). ApliGraf, formerly
known as Graftskin, is a commercially available two-
layered tissue-engineered skin product composed of type
I bovine collagen that contains living human dermal fi-
broblasts and an overlying cornified epidermal layer of
living human keratinocytes. Both cell types are derived
from neonatal foreskin and grow in a special mold that
limits lateral contraction ( Bell et al. , 1991a , b). ApliGraf
has been investigated in a multicenter study after exci-
sional surgery for skin cancer with good results (Eaglstein
et al. ,1999).
The artificial skin developed by Burke and Yannas
( Burke et al. , 1981 ), now called Integra, consists of
collagen-chondritin 6-sulfate fibers obtained from bovine
hide (collagen) and shark cartilage (chondritin 6-sulfate).
It has been engineered into an open matrix of uniform
porosity and thickness and covered with a uniformly
thick (0.1-mm) silicone sheet. This artificial skin has
been studied extensively in humans ( Heimbach et al. ,
1988 ; Sheridan et al. , 1994 ) and was approved for use in
burn patients in 1997.
Besides clinical use of artificial skin, several companies
have explored the possibilities of dermal substitutes for
diagnostic purposes. There is particular interest in min-
imizing the use of animals for topological irritation,
Endoderm
Liver
Liver transplantation is a routine treatment for end-stage
liver disease, but donor organ shortage remains a serious
problem. Many patients die while waiting for a transplant
and those with chronic disease often deteriorate resulting
in low survival rates after transplantation. Therefore
a ''bridging'' device that would support liver function
until a donor liver became available or the patient's liver
recovered is of great interest. Most liver support systems
remove toxins normally metabolized by the liver through
dialysis, charcoal hemoperfusion, immobilized enzymes,
or exchange transfusion. However, none of these systems
can offer the full functional spectrum performed by
a healthy liver. Hepatocyte systems aiming at re-
placement of liver function by transplantation of isolated
cells are being studied for both extracorporeal and im-
plantable applications. Extracorporeal systems can be
used when the patient's own organ is recovering or as
a bridge to transplantation. These systems provide a good
control of the medium surrounding the cell system and
a minimized risk of immune rejection. Their design is
primarily based on hollow-fiber, spouted-bed, or flat-bed
devices ( Bader et al. , 1995 ). Implantable hepatocyte
systems, on the other hand, offer the possibility of
permanent liver replacement ( Yarmush et al. , 1992a ).
Successful hepatocyte transplantation depends on
a number of critical steps. After cell harvest the hepa-
tocytes must be cultured and expanded in vitro prior to
transplantation. Hepatocyte morphology can be main-
tained by cultivating the cells on three-dimensional
structures, such as sandwiching them between two hy-
drated collagen layers. Under these conditions the he-
patocytes have been shown to secrete functional markers
at physiological levels (Dunn et al. , 1991). Moreover the
hepatocytes must be attached to the polymer substrata
so that they maintain their differentiated function and
viability. A sufficient mass of hepatocytes must become
engrafted and remain functional to achieve metabolic
replacement and vascularization, which is critical for
graft survival ( Yarmush et al. , 1992b ). Finally, hepatocyte
transplantation per se provides neither all cell types nor
the delicate and complex structural features of the liver.
Products normally excreted through bile may accumulate
because of the difficulty in reconstructing the biliary tree
solely from hepatocytes. However, hepatocytes placed
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