Biomedical Engineering Reference
In-Depth Information
cized whole-heart prostheses, from the Jarvik-7 in 1982 to the AbioCor in 2001
(47), and the less-publicized but much more common ventricular assist devices
and hepatic assist devices (53).
While many methods of organ replacement have been developed and are
used successfully every day, most of these techniques are imperfect, as they
cannot permanently restore full organ function. Four major methods for organ
replacement have been conceived: taking tissue from a donor (transplantation),
moving tissue from one location on a patient to another (autografting), fabricat-
ing a synthetic organ (prosthesis), or growing living replacement tissue (synthe-
sis). Transplantation of major organs has been spectacularly successful, but
requires a donor who is a close genetic match to the patient. More than 65,000
people are on transplant waiting lists in the Unites States alone, and many will
die before a suitable organ becomes available (76). Autografting is useful in
replacing damaged skin (11) and nerves (58) but has limited application else-
where. External (Figure 3) and dental prostheses have been widely successful,
and implanted prostheses generally can only provide temporary function.
An implanted prosthesis, such as a hip replacement, may function for fifteen
years or more before damaging the implant site or becoming sufficiently dam-
aged to lead to failure, as is often seen in hip prostheses (80). Additionally, im-
plantation of synthetic materials may cause clotting, calcification, or infection
(74), and synthetic materials are unable to grow with a growing patient. An im-
plant constructed from living tissue will not have these problems and, once im-
planted, can grow to operate seamlessly with the rest of the body.
As technological advances have always led to development of new organ-
replacement techniques, modern technologies in cell biology and genetic ma-
nipulation allow now for the development of living tissue implants. This field,
known as "tissue engineering," has already shown promising successes in a vari-
ety of physiological systems. The most widely researched and most successful
engineered tissue thus far has probably been the effort to grow replacement skin
for burn victims. A number of groups have produced "skin-equivalents" in the
lab (3), and other researchers have had success regenerating skin at the site of
the injury in human patients (79). The tissue engineering of heart valves has also
been widely researched (30,70,73), with successful aortic valve replacement in
sheep reported (70) (Figure 4). Promising progress has been reported in regen-
eration of nerves (26), and in fabrication of tissue-engineered bladders (16),
stomach (54), trachea (42), cartilage (43), and other tissues. These tissues are all
relatively simple, and researchers are now developing more complex tissue-
engineered organs like the liver and kidney (47).
To construct a tissue-engineered organ, the researcher must understand the
components of the organ and how they interact. All organs, regardless of com-
plexity or location in the body, consist of four components: cells, scaffolds, sig-
nals, and nutrients. The cells are the familiar living building block of a tissue.
They provide any active behavior or functionality of a tissue, such as the con-
