Biomaterials in cardiac restoration therapy - Biomaterials and Devices for the Circulatory System - page 269

implanted into the infarcted rat myocardium. The cardiomyocytes main-
tained viability within the scaffold and formed beating aggregates after
24 h in vitro . The implanted patch was examined after 9 weeks, at which
time the histological evaluation showed massive neovascularization at the
site of implantation. This angiogenic response was thought to contribute
to the high cell viability in the graft. Although the fetal cardiomyocytes
differentiated into a mature cardiac phenotype in situ , only a small portion
of the graft was occupied by cardiomyocytes whereas most of the scaffold
was fi lled with collagen fi bers and scattered fi broblasts. Moreover, the
alginate was nearly completely degraded after nine weeks and was
replaced almost entirely by the collagen fi bers. The authors suggested that
the implantation improved cardiac function by altering the remodeling
process and preventing the imminent deterioration of the cardiac wall fol-
lowing the MI. Because the cardiomyocyte population in the graft was
very small and not completely integrated with the host muscle fi bers, it
was not likely that the improvement in cardiac function was due to the
contribution of the implanted cells to the contractility of the heart muscle.
It was further suggested that the changes in mechanical properties induced
from the scaffold implantation and the neovascularization could have
been the more relevant factor in maintaining improved cardiac
performance.
A cardiac tissue patch can also be created using the cell self-assembly
approach pioneered by Okano and coworkers. In this approach, cell sheets
are cultivated and detached from their culture substrate without the use of
a scaffold material. A temperature-responsive polymer substrate made
from poly( N -isopropylacrylamide) (PIPAAm) is used instead to remove
the cell sheets without causing damage to their integrity. The PIPAAm is
hydrophobic at 37 °C and allows the cultivation of cells under normal con-
ditions. By lowering the temperature to 32 °C, the polymer quickly becomes
more hydrophilic and non-adhesive to the cell sheets. This property enables
the detachment of a confl uent uniform monolayer of cardiomyocytes
without the use of destructive enzymatic harvesting techniques. The mono-
layer is maintained intact, including cell-cell connections and ECM pro-
teins which were structured during the in vitro culture, so that when the
3-D functional tissue is created from stacked monolayer sheets, the tissue-
like integrity is preserved. 35,36 Unfortunately, only a very limited number of
the 50

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m thick layers can be stacked together before the cells require
additional perfusion to survive (no more than three cell sheets). To
overcome this limitation, a method of sequential transplantations of
multi-sheet constructs was proposed whereby the fi rst implant would
be vascularized in vivo before new sheets would be grafted on. Using
this multistep transplantation procedure, the authors reported grafting a
1 mm thick cardiac tissue onto an infarcted adult rat myocardium. The

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