165
compared to unstretched controls, while 1 Hz and 2 Hz stimulation did not produce
statistically significant changes [106].
Mechanical forces also play a powerful role in regulating endothelial cell behavior and vascularization. In vitro and in vivo studies have confirmed that the shear
stress caused by blood flow strongly affects the phenotype of vascular endothelial
cells [118]. However, in engineered cardiac tissues, where there is generally no
blood flow during in vitro culture, the field has relied on tissue-level strain instead.
Like cardiomyocytes, endothelial cells in engineered scaffolds have been shown to
respond positively to uniaxial static strain, displaying improved angiogenesis and
vascular alignment [23, 57]. However, the effect of cyclic strain on angiogenesis is
more controversial. Some early reports suggested that cyclic uniaxial strain can
actually disrupt endothelial networks and result in more random vascular organization [129]. Other studies present evidence that cyclic strain promotes endothelial
cell and vascular alignment parallel to the strain [14, 38, 57, 108], and others have
suggested that the strain induced by surrounding cell contraction is sufficient to
induce that effect [121]. Again, the frequency of this strain is an important factor, as
experiments using human coronary artery endothelial cells show that a minimum
threshold frequency of 0.1 Hz is required to elicit an alignment effect and that the
alignment develops more robustly and on a shorter time scale when the strain was
applied at 1 Hz [38].
Experiments culturing endothelial cells and cardiomyocytes together, along
with a variety of vascular support cell types (detailed above), have thus far been
conducted using either cyclic strain conditions or no induced strain at all. These
co-culture and tri-culture experiments have produced significant advancements in
generating vascularized cardiac tissue. Very little work, however, has been done to
evaluate whether using these conditions is optimal for promoting both cardiomyocyte development and endothelial cell survival and vascular network formation. It
is very possible that conditions chosen to promote cardiomyocyte maturation are
impeding angiogenesis in the tissues. Careful optimization of mechanical stimulation will be important in the continued development of these systems.
6.6.7 Combined Electrical and Mechanical Stimulation
Applying strain conditions and electrical stimulation together has proven a particular engineering challenge, and several bioreactors have been designed with the aim
of incorporating these stimuli into hydrogel cultures [20, 72, 78, 81, 126]. These
devices take advantage of the full diversity of mechanical strain modalities detailed
above and have been configured to apply uniaxial and circumferential strain [20, 29,
72, 81, 126]. Across these devices, though, the vast majority generate electrical
stimulation using the parallel electrode setup. A notable exception is the bioreactor
reported by Cook and colleagues, which applies electrical impulses via the integration of a direct contact in the clamping mechanism that secures the tissue in place
[20]. The characteristics that vary widely between these devices, including substrate
6 Strategies for Tissue Engineering Vascularized Cardiac Patches to Treat Myocardial…
compared to unstretched controls, while 1 Hz and 2 Hz stimulation did not produce
statistically significant changes [106].
Mechanical forces also play a powerful role in regulating endothelial cell behavior and vascularization. In vitro and in vivo studies have confirmed that the shear
stress caused by blood flow strongly affects the phenotype of vascular endothelial
cells [118]. However, in engineered cardiac tissues, where there is generally no
blood flow during in vitro culture, the field has relied on tissue-level strain instead.
Like cardiomyocytes, endothelial cells in engineered scaffolds have been shown to
respond positively to uniaxial static strain, displaying improved angiogenesis and
vascular alignment [23, 57]. However, the effect of cyclic strain on angiogenesis is
more controversial. Some early reports suggested that cyclic uniaxial strain can
actually disrupt endothelial networks and result in more random vascular organization [129]. Other studies present evidence that cyclic strain promotes endothelial
cell and vascular alignment parallel to the strain [14, 38, 57, 108], and others have
suggested that the strain induced by surrounding cell contraction is sufficient to
induce that effect [121]. Again, the frequency of this strain is an important factor, as
experiments using human coronary artery endothelial cells show that a minimum
threshold frequency of 0.1 Hz is required to elicit an alignment effect and that the
alignment develops more robustly and on a shorter time scale when the strain was
applied at 1 Hz [38].
Experiments culturing endothelial cells and cardiomyocytes together, along
with a variety of vascular support cell types (detailed above), have thus far been
conducted using either cyclic strain conditions or no induced strain at all. These
co-culture and tri-culture experiments have produced significant advancements in
generating vascularized cardiac tissue. Very little work, however, has been done to
evaluate whether using these conditions is optimal for promoting both cardiomyocyte development and endothelial cell survival and vascular network formation. It
is very possible that conditions chosen to promote cardiomyocyte maturation are
impeding angiogenesis in the tissues. Careful optimization of mechanical stimulation will be important in the continued development of these systems.
6.6.7 Combined Electrical and Mechanical Stimulation
Applying strain conditions and electrical stimulation together has proven a particular engineering challenge, and several bioreactors have been designed with the aim
of incorporating these stimuli into hydrogel cultures [20, 72, 78, 81, 126]. These
devices take advantage of the full diversity of mechanical strain modalities detailed
above and have been configured to apply uniaxial and circumferential strain [20, 29,
72, 81, 126]. Across these devices, though, the vast majority generate electrical
stimulation using the parallel electrode setup. A notable exception is the bioreactor
reported by Cook and colleagues, which applies electrical impulses via the integration of a direct contact in the clamping mechanism that secures the tissue in place
[20]. The characteristics that vary widely between these devices, including substrate
6 Strategies for Tissue Engineering Vascularized Cardiac Patches to Treat Myocardial…