Tissue engineering - Biomedical Engineering Desk Reference - page 632

tissues, particularly those comprising large organs such as
the liver, kidney, or heart. To overcome these limitations,
several approaches have been investigated. Vacanti and co-
workers used local delivery of basic fibroblast growth
factor (bFGF) to increase angiogenesis and engraftment of
hepatocytes in tissue-engineered polymer devices ( Lee
et al. , 2002 ). In another study sustained and localized
delivery of vascular endothelial growth factor (VEGF)
combined with the transplantation of human microvas-
cular endothelial cells was used to engineer new vascular
networks ( Peters et al. , 2002 ). Using micromachining
technologies on silicon, Kaihara et al. demonstrated in
vitro generation of branched three-dimensional vascular
networks formed by endothelial cells ( Kaihara et al. ,
2000 ).

or sheet ( Hutmacher et al. ,2001 ), or sponge ( Ta y l o r et al. ,
2002 ), and even as a fiber-based scaffold ( Rothenburger
et al. , 2001 ). It has the disadvantage that it is difficult to
obtain from the patient. Therefore, most of the collagen
scaffolds are of animal origin. Another biological material
displaying good controllable biodegradable properties is
fibrin. Since fibrin gels can be produced from the
patient's blood to serve as autologous scaffold, no toxic
degradation or inflammatory reactions are expected ( Lee
and Mooney, 2001 ).
The use of synthetic materials as scaffolds has already
been broadly demonstrated for cardiovascular tissue
engineering. Initial attempts to create single heart-valve
leaflets were based on synthetic scaffolds, such as poly-
glactin, PGA, PLA, or PLGA (copolymer of PGA and
PLA). To create complete trileaflet heart-valve conduits,
PHA-based materials (polyhydroxyalkanoates) were
used ( Sodian et al. , 2000 ). These materials are thermo-
plastic and can therefore be easily molded into any de-
sired three-dimensional shape. A combined polymer
scaffold consisting of nonwoven PGA and P4HB (poly-4-
hydroxybutyrate) has shown promising in vivo results
( Hoerstrup et al. , 2000a ).
In most cardiovascular tissue-engineering approaches
cells are harvested from donor tissues, e.g., from pe-
ripheral arteries, and mixed vascular cell populations
consisting of myofibroblasts and endothelial cells are
obtained. Out of these, pure viable cell lines can be easily
isolated by cell sorters ( Hoerstrup et al. , 1998 ) and the
subsequent seeding onto the biodegradable scaffold is
undertaken in two steps. First, the myofibroblasts are
seeded and grown in vitro. Second, the endothelial cells
are seeded on top of the generated neotissue, leading to
the formation of a native leaflet-analogous histological
structure ( Zund et al. , 1998 ).
Successful implantation of a single tissue-engineered
valve leaflet has been demonstrated in the animal model
( Shinoka et al., 1996 ) and based on a novel in vitro
conditioning protocol of the tissue-engineered valve
constructs in bioreactor flow systems (pulse-duplicator)
completely autologous, living heart-valves were gener-
ated ( Fig. 7.1.2-4 ). Interestingly, these tissue-engineered
valves showed good in vivo functionality and strongly
resembled native heart valves with regard to biome-
chanical and morphological features ( Hoerstrup et al. ,
2000b ; Rabkin et al. , 2002 ). With regard to clinical ap-
plications, several human cell sources have been in-
vestigated ( Schnell et al. , 2001 ). Recently, cells derived
from bone marrow or umbilical cord have been success-
fully utilized to generate heart valves and conduits in
vitro ( Hoerstrup et al. , 2002a, b ). In contrast to vascular
cells, these cells can be obtained without surgical in-
terventions representing an easy-to-access cell source in
a possible routine clinical scenario. Because of their good
proliferation and progenitor potential, these cells are

Heart valves
For treatment of heart-valve disease, mechanical or bi-
ological valves are currently in use. The drawbacks of
mechanical valves include the need for lifelong anti-
coagulation, the risk of thromboembolic events, pros-
thesis failure, and the inability of the device to grow.
Biological valves (homograft, xenograft, fixed by cryo-
preseration of chemical treatment) have a limited dura-
bility due to their immunogenic potential and the fact that
they represent nonliving tissues without regeneration
capacities. All types of contemporary valve prostheses
basically consist of nonliving, foreign materials, posing
specific problems to pediatric applications when de-
vices with growth potential are required for optimal
treatment.
The basic concept currently used for tissue engineer-
ing of heart valve structures is to transplant autologous
cells onto a biodegradable scaffold, to grow and to con-
dition the cell-seeded scaffold device in vitro, and finally
to implant the tissue-like construct into the donor

patient.
The heart-valve scaffold may be based on either bi-
ological or synthetic materials. Donor heart valves or
animal-derived valves depleted of cellular antigens can be
used as a scaffold material. Removing the cellular com-
ponents results in a material composed of essentially
extracellular matrix proteins that can serve as an intrinsic
template for cell attachment ( Samouillan et al. , 1999 ). In
general, nonfixed acellularized valve leaflets have shown
recellularization by the host, as demonstrated in dogs
( Wilson et al. , 1995 ) and sheep ( Elkins et al. , 2001 ;
Goldstein et al. , 2000 ). However, first clinical applica-
tions of this concept in children resulted in rapid failure
of the heart valves due to severe foreign-body-type
reactions associated with a 75% mortality ( Simon et al. ,
2003 ). In a further approach, specific biological matrix
constituents can be used as scaffold material. Collagen is
one of the materials that show biodegradable properties
and can be used as a foam ( Rothenburger et al. , 2002 ), gel